atv-dvwk-a-281-e

GERMAN ATV-DVWK RULES AND STANDARDS

Standard ATV-DVWK-A 281E

Dimensioning of Trickling filters and Rotating Biological Contactors

September 2001 ISBN 3-937758-36-4

Publisher/marketing: ATV-DVWK German Association for Water, Wastewater and Waste, Theodor-Heuss-Allee 17 D-53773 Hennef Tel. ++49-22 42 / 8 72-120 Fax:++49 22 42 / 8 72-100 E-Mail: [email protected] Internet: www.atv-dvwk.de

ATV-DVWK-A 281E

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The main fields of activity of the ATV-DVWK are technical-scientific subjects and the economic as well as the legal concerns of environmental protection. The politically and economically independent association works nationally and internationally in the fields of pollution control, wastewater, water-hazardous sub-stances, waste, hydraulic engineering, hydraulic power, hydrology, soil protection and contaminated sites. The ca. 16,000 members are active in municipalities, engineer offices, authorities, firms and associations and also in universities. Of these there are 10,000 specialists with personal membership; these are engi-neers, scientists, lawyers, business persons, operating personnel and technicians. Via the corporate mem-bership in the ATV-DVWK there is access to ca. 160,000 specialists.

All rights, in particular those of translation into other languages, are reserved. No part of this Standard may be reproduced in any form - by photocopy, microfilm or any other process - or transferred into a language usable in machines, in particular data processing machines, without the written approval of the publisher.

Publisher: ATV-DVWK Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.V., Theodor-Heuss-Allee 17, D-53773 Hennef

Marketing: GFA Gesellschaft zur Förderung der Abwassertechnik e.V., Hennef

Setting and printing (German original): DCM, Meckenheim

© GFA Gesellschaft zur Förderung der Abwassertechnik e. V., Hennef 2001

Die Deutsche Bibliothek [The German Library] – CIP-Einheitsaufnahme

ATV-DVWK, German Association for Water, Wastewater and Waste: ATV-DVWK Rules and Standards [Media combination] / ATV-DVWK, Water Management, Wastewater, Waste. -Hennef : GFA, Ges. zur Förderung der Abwassertechnik Previously under the title of: “Abwassertechnische Vereinigung”: ATV Set of Rules and Standards

Standard A 281E. Dimensioning of Trickling Filters and Rotating Biological Contactors.

ISBN 3-937758-36-4

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Foreword

The revision of ATV Standard ATV-A 135 (now ATV-DVWK-A 281) has become necessary as it no longer corresponds with the status of technology.

Compared with the issue of ATV Standard ATV-A 135 dated March 1989 the following important amend-ments have been made:

• Basic validity for trickling filters and rotating biological contactors without limitation of the capacity (previ-ously ≥ 500 PT).

• Removal of the determination of loading principles; a separate ATV-DVWK standard for all types of wastewater treatment processes is being prepared.

• The addition of a dimensioning approach for denitrification using trickling filters. • Increase of tank surface area and reduction of tank depth of the secondary settling stage due to new tri-

als results.

The biological stage of wastewater treatment plants, employing trickling filters and rotating biological con-tactors without sludge return feed, is dealt with in this Standard. The standard applies only for rotating bio-logical contactors without artificial aeration for the supply of the biofilm with the required oxygen.

A detailed description of the theoretical basic elements and practical application of both the fixed bed proc-esses is contained in the ATV Handbook „Biologische und weitergehende Abwasserreinigung“ [“Biological and Advanced Wastewater Treatment”] and „Mechanische Abwasserreinigung“ [“Mechanical Wastewater Treatment”]. The development of the trickling filter process and the rotating biological contactor as well as the factors on their treatment efficiency are covered in advanced literature.

As with all aerobic processes for biological wastewater treatment, the contact between biomass and wastewater is to be established and the biomass is to be supplied with oxygen. With the trickling filter proc-ess the wastewater is spray irrigated over the filter material so that, during the dripping process, the contact between biomass and wastewater is established. In general, aeration is without application of further en-ergy. With rotating biological contactors the partially submerged filter material is rotated about its longitudi-nal axis with the application of energy. During the emergent phase of the material the biofilm can take up oxygen from the surrounding air and in the submerged phase the pollutants from the wastewater.

The following are to be mentioned as favourable characteristics of trickling filters and rotating biological contactors:

• in general they are simple and stable to operate. • no activated sludge return is necessary. • trickling filter and rotating biological contactor facilities enable the colonisation of micro-organisms which

have long generation times. Thus even compounds which are difficult to degrade can be eliminated with little loading.

• in general the energy requirement is small.

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Authors

This standard has been prepared by the ATV-DVWK Working Group KA-6.3 “Trickling filters and contac-tors”, within the ATV-DVWK Specialist Committee KA-6 “Aerobic biological wastewater treatment proc-esses”, and the ATV-DVWK Specialist Committee KA-5 “Settling processes”.

The ATV-DVWK Working Group KA-6.3 “Trickling filters and biological contactors” has the following mem-bers:

Dr.-Ing. Jürgen Bever, Oberhausen (Chairman) Dr.-Ing. Georg Mehlhart, Darmstadt Prof. Dr.-Ing. Harro Bode, Essen Dr.-Ing. Manfred Roth, Stuttgart Dr.-Ing. Bernd Dorias, Stuttgart Dr.-Ing. Sigurd Schlegel, Essen Prof. Dr.-Ing. Werner Gebert, Planegg Dipl.-Ing. Gert Schwentner, Sindelfingen Dr.-Ing. Hans-Dieter Kruse, Bad Zwischenahn Dr.-Ing Gerald A. Steinmann, Weißenburg

The members of the ATV-DVWK Specialist Committee KA-5 “Settling processes” are:

Prof. Dr.-Ing. Ernst Billmeier, München Dr.-Ing. Helmut Resch, Weissenburg (Chairman) Dipl.-Ing. Winfried Born, Kassel Prof. Dr.-Ing. Karl-Heinz Rosenwinkel, Hannover Dr.-Ing. Andrea Deininger, Weyarn Dr.-Ing. Reinhold Rölle, Stuttgart Dr.-Ing. Thomas Grünebaum, Essen Dr.-Ing. Andreas Schulz, Essen Prof. Dr.-Ing. F. Wolfgang Günthert, Neubiberg Prof. Dr.-Ing. Carl Franz Seyfried, Hannover Dr.-Ing. Karl-Heinz Kalbskopf, Dinslaken Dr.-Ing. Andreas Stein, Emsdetten Prof. Dr. Peter Krebs, Dresden

The ATV-DVWK Specialist Committee KA-6 “Aerobic biological wastewater treatment processes” has the following members:

Dipl.-Ing. Reinhard Beer, Cottbus Dr. Dipl.-Biol. Hilde Lemmer, München Dr.-Ing. Jürgen Bever, Oberhausen Prof. Dr.-Ing. Jörg Londong, Wuppertal Prof. Dr.-Ing. Harro Bode, Essen Prof. Dr.-Ing. Norbert Matsché, Wien/Österreich Dr.-Ing. Reiner Boll, Hannover Dipl.-Ing. Anton Peter-Fröhlich, Berlin Prof. Dr. Lothar Huber, Neubiberg Prof. Dr.-Ing. Karl-Heinz Rosenwinkel, Hannover Prof. Dr.-Ing. Dr. Rolf Kayser, Braunschweig Dipl.-Ing. Peter Schleypen, München (Chairman) Dr.-Ing. Burkhard Teichgräber, Essen Prof. Dr.-Ing. Karlheinz Krauth, Stuttgart Dipl.-Ing. Volker Ziess, Haan Dr. rer. nat. Joachim Richard Lemke, Leverkusen

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Contents

Foreword .................................................................................................................................................. 3

Authors ................................................................................................................................................... 4

User notes ................................................................................................................................................ 7

1 Area of application................................................................................................................... 7

1.1 Preamble.................................................................................................................................... 7 1.2 Objective .................................................................................................................................... 7 1.3 Scope ......................................................................................................................................... 8

2 Symbols...................................................................................................................................... 8

3 Basic elements of dimensioning .............................................................................................. 10

3.1 Loading with wastewater............................................................................................................ 9 3.2 Loading from sludge liquor and external sludge. ....................................................................... 11

4 Pre-treatment ............................................................................................................................ 11

5 Trickling filters ......................................................................................................................... 11

5.1 Description of the the process ................................................................................................... 11 5.1.1 General....................................................................................................................................... 11 5.1.2 Filter material ............................................................................................................................. 12 5.2 Dimensioning ............................................................................................................................. 13 5.2.1 General details on dimensioning................................................................................................ 13 5.2.2 Wastewater treatment without nitrification ................................................................................. 14 5.2.3 Wastewater treatment with nitrification ...................................................................................... 14 5.2.4 Wastewater treatment with nitrification and denitrification......................................................... 15

6 Rotating biological contactors ............................................................................................... 16

6.1 Description of the process.......................................................................................................... 16 6.1.1 General....................................................................................................................................... 16 6.1.2 Material and types...................................................................................................................... 17 6.2 Dimensioning ............................................................................................................................. 18 6.2.1 General details on dimensioning................................................................................................ 18 6.2.2 Wastewater treatment without nitrification ................................................................................. 19 6.2.3 Wastewater treatment with nitrification ...................................................................................... 19

7 Phosphorus removal ............................................................................................................... 20

8 Waste sludge production ........................................................................................................ 20

9 Secondary settling tanks......................................................................................................... 20

9.1 General....................................................................................................................................... 20 9.2 Dimensioning of the secondary settling tank of single-stage trickling filters and rotating

biological contactors................................................................................................................... 21 9.3 Notes on tank shape and design ............................................................................................... 21

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10 Costs and environmental effects........................................................................................... 22

11 Relevant regulations, directives and standard specifications ........................................... 22

Literature ................................................................................................................................................. 23

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User Notes

This Standard is the result of honorary, technical-scientific/economic collaboration which has been achieved in accordance with the principles appli-cable therefor (statutes, rules of procedure of the ATV-DVWK and the Standard ATV-DVWK-A 400E). For this, according to precedents, there e-xists an actual presumption that it is textually and technically correct and also generally recognised.

The application of this Standard is open to everyo-ne. However, an obligation for application can ari-se from legal or administrative regulations, a contract or other legal reason.

This Standard is an important, however, not the sole source of information for correct solutions. With its application no one avoids responsibility for his own action or for the correct application in spe-cific cases; this applies in particular for the correct handling of the margins described in the Standard.

1 Area of Application

1.1 Preamble

The treatment of the stormwater in the sewer net-work and of wastewater in the wastewater treat-ment plant form one unit for the protection of sur-face waters. For the dimensioning of the wastewater treatment plant and the stormwater overflows the planning periods are to be matched to each other. The planning period should com-prise not more than 25 years.

In the case of special conditions the dimensioning can often be carried out more correctly with the aid of trials and operating results of existing plants. Under certain circumstances costs can be saved through this. The trials plants for this are to be es-tablished at least on a semi-industrial scale and operated for not less than half a year under practi-cal operating conditions with the inclusion of the cold season.

1.2 Objective

Using the dimensioning values recommended in this standard for municipal wastewater one can

meet or undercut the achievable minimum effluent requirements which correspond with the require-ments of the German Wastewater Ordinance dated 09.02.1999, Appendix 1, and the associated sam-pling specifications.

It is pointed out that short-term ammonium dis-charge peaks, in particular with combined waste-water in plants with large preliminary settling tanks, are unavoidable and more marked than with acti-vated sludge plants.

If commercial or industrial wastewater with high fractions of slowly biodegradable and/or inert or-ganic substances is discharged, a higher residual COD than with domestic wastewater can arise. The same applies for areas with low water con-sumption and low infiltration rate, as then the inert COD concentration increases.

In this Standard technical regulations are drawn up for the dimensioning both for

• carbon removal as well as the nitrification and denitrification using trickling filters as well as for

• carbon removal and nitrification using rotating biological contactors.

In addition, information is given for phosphorus removal.

In accordance with the requirements under [Ger-man] water law, the structural and operating re-quirements and the sensitivity of the surface wa-ters through parallel units, reserve equipment etc. is to be oriented towards an appropriately high op-erational safety.

A prerequisite for the secure function of the plant planned in accordance with this standard is that sufficient qualified, trained and permanently tech-nically supported operating personnel are em-ployed and are involved in the planning process, comp. ATV Advisory Leaflet ATV-M 271 „Person-albedarf für den Betrieb kommunaler Kläranlagen“ [Personnel requirement for the operation of mu-nicipal wastewater treatment plants, – currently not available in English].

With the systems dealt with in this Standard one is concerned with fixed bed reactors with very differ-ent types of construction and process technology. Therefore, in this Standard, trickling filters (Chapter

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5) and rotating biological contactors (Chapter 6) are dealt with separately. The subjects “Basic ele-ments of dimensioning” (Chapter 3), “Pre-treatment” (Chapter 4), “Phosphorus removal” (Chapter 7), “Waste sludge production” (Chapter 8) and “Secondary settling tanks” (Chapter 9) are presented jointly.

1.3 Scope

This Standard applies basically for the dimension-ing of single-stage trickling filter and rotating bio-logical contactors and for pre-anoxic denitrification trickling filters. Some advice is given for trickling fil-ters and rotating biological contactors in the sec-ond stage. Attention is drawn to the ATV Report “Multi-stage biological wastewater treatment plants” [3] [currently not available in English] with regard to multi-stage facilities. ATV-A 257E applies for wastewater lagoons with intermediate trickling filters and rotating biological contactors.

Due to the peculiarities of small wastewater treat-ment plants attention is drawn to the ATV Standard ATV-A 122E. For small scale wastewater treatment plants with a wastewater inflow up to 8 m3/d, DIN 4261 applies. For hospital wastewater treatment plants DIN 19250 is to be additionally taken into account. ATV Standard ATV-A 129 [currently not available in English] applies for the disposal of wastewater from recreation and tourist facilities.

The Standard applies for wastewater which origi-nates from households or from facilities which serve commercial or agricultural purposes insofar as the harmfulness of this wastewater can be re-duced by means of biological processes with the same success as with wastewater from house-holds.

2 Symbols

[Translator’s note: the symbols/indices below in English are in line with the general rules of ATV-DVWK Standard A 198E. Where these differ from the original German the latter are shown in square brackets.]

a - number of rotary distributor arms

ASST [ANB] m2 surface area of the secon-dary settling tank

ARC [ART] m2 theoretical surface area of the rotating biological contac-tor (sum of the surfaces of the trickling material)

ARC,C m2 theoretical surface area of [ART,C] the rotating biological contactor for carbon removal ARC,N m2 theoretical surface area of [ART,N] the rotating biological con- tactor for nitrification ATF [ATK] m2 surface area of the trickling

filter BA,BOD g/(m2.d) BOD5 surface loading of the [BA,BSB] rotating biological contactor BA,TKN g/(m2.d) TKN surface loading of the

rotating biological contactor Bd,BOD,InB kg/d daily BOD5 load in the [Bd,BBS,ZB] influent to the biological reac tor Bd,NO3,D kg/d daily nitrate-nitrogen load to

be denitrified Bd,N,WS kg/d daily load of nitrogen which [Bd,N,ÜS] is removed through the waste sludge from the trick- ling filter or rotating biological contactor facility Bd,TKN,InB kg/d daily TKN load in the influent [Bd,TKN,ZB] to the biological reactor Bd,inorgN,InB kg/d daily load of inorganic nitro- [Bd,anorgN,AN] gen in the effluent of the secondary settling stage Bd,orgN,SST kg/d daily load of organic nitrogen [Bd,orgN,AN] in the effluent of the second dary settling stage BR,BOD kg/(m3.d) BOD5 volumetric loading of [BR,BSB] the trickling filter BR,TKN kg/(m3.d) TKN volumetric loading of

the trickling filter DSST m diameter of the secondary [DNB] settling tank PTXXX I total number of inhabitants [EWXXX] E and population equivalents

referred to the parameter XXX, e.g. BOD5

hSST [hNB] m depth of the secondary set-tling tank

hTF [hTK] m height of the trickling filter filler material

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n 1/h rotationsrevolutions per hour of the rotary distributor

qA,SST m/h surface loading rate of the [qA,NB] secondary settling tank qA,TF m/h surface overflow rate of the [qA,TK] trickling filter qWO [qÜ] m3/(m.h) weir overflow rate QDW,d m3/d daily wastewater inflow with [QT,d] dry weather QComb,h m3/h dimensioning peak flow from [QM,h] combined or separate sys tems QSST [QNB] m3/h dimensioning inflow of the

secondary settling tank QRF m3/h recirculation flow QDW,2h m3/h maximum dry weather flow [Qt] rate as 2 hourly mean QTF [QTK] m3/h influent to the trickling filter:

QDW+QRF RRm [RVm] - recirculation ratio QRF to QDim,In RRDW [RVt] - recirculation ratio QRF to

QDW,2h FF [SK] mm/arm flushing force tSST [tNB] h retention period in the sec-

ondary settling tank VSST [VNB] m3 volume of the secondary set-

tling tank VTF [VTK] m3 volume of the trickling filter VTF,C m3 volume of the trickling filter [VTK,C] for carbon removal VTF,N m3 volume of the trickling filter [VTK,N] for nitrification VTF,D m3 volume of the trickling filter [VTK,D] for denitrification

Pollution parameters and concentrations: CXXX mg/l concentration of the parame-

ter XXX, in the homogenised sample

SXXX mg/l concentration of the parame-ters XXX, in the filtered sam-ple (0.45µm membrane filter)

XXXX mg/l concentration of the filter residue, XXXX = CXXX – SXXX

Indices for the location or purpose of the sampling (always last): In [Z] sample from influent to the

wastewater treatment plant InB [ZB] sample from influent to bio-

logical reactor

EB [AB] sample from the effluent of biological ractor

ESST [AN] sample from the effluent of the secondary settling tank

WS [ÜS] sample from the waste sludge

RF sample from the recirculation flow

MV [ÜW] monitoring value [Authors’ af-ternote: here, effluent re-quirement with defined sam-pling procedure]

Frequently used parameters: CBOD,InB mg/l average BOD5 concentration [CBSB,ZB] with dry weather from daily inflow Qd without recirculation c flow in the influent to the biological reactor CBOD,InB,RF mg/l average BOD5 combined [CBSB,ZB,RF] concentration with dry weather from daily inflow Qd and recirculation flow at the rotary distributor CN,InB mg/l concentration of the total [CN,ZB] nitrogen in the homogenised sample in the influent to the biological reactor SinorgN,MV mg/l monitoring value for inor- [SanorgN,ÜW] ganic nitrogen in the effluent sample as N SNH4,ESST mg/l concentration of the ammo- [SNH4,AN] nium in the effluent sample as N SNO3,ESST mg/l concentration of the nitrate in [SNO3,AN] the effluent sample as N SNO3,D mg/l concentration of nitrate-nitro-

gen to be denitrified SorgN,ESST mg/l concentration of the organic [SorgN,AN] nitrogen in the effluent of the secondary settling tank XorgN,BM mg/l organic nitrogen incorporated

in the biomass XSS,ESST mg/l concentration of suspended [XTS,AN] solids in the effluent of the secondary settling tank

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3 Basic Elements of Dimensioning

3.1 Loading with Wastewater

The BOD5 load (Bd,BOD,In in kg/d), undercut on 85 % of the dry weather days in the influent to the wastewater treatment plant plus a planned capac-ity reserve, is to be used for the classification into the Size Class in accordance with Appendix 1 of the [German] Wastewater Ordinance and for the determination of the dimensioning capacity of the plant the assessment under water law. If the di-mensioning capacity is determined on the basis of the number of connected inhabitants, the inhabi-tant-specific BOD5 load for raw wastewater from Table 1 is to be used.

In principle it applies that the sewer system and wastewater treatment plant are operated for the same wastewater effluent and influent.

For dimensioning, the following important numeri-cal values are required from the influent to the bio-logical reactor, if applicablewith the inclusion of the return flows from sludge treatment (comp. 3.2):

• Relevant organic load (Bd,BOD) for the calculation of the required trickling filter volume or the nec-essary surface area of rotating biological con-tactors for wastewater treatment without nitrifi-cation as well as for the determination of the waste sludge production.

• Relevant organic load (Bd,BOD) and nitrogen load (Bd,TKN) for the calculation of the necessary trick-ling filter volume or the required surface area of rotating biological contactors for wastewater treatment with nitrification.

• Relevant concentration of nitrogen (CN) and the associated concentration of organic matter (CBOD) for the determination of the nitrate to be denitrified with the dimensioning of trickling fil-ters for denitrification.

• Relevant daily wastewater inflow Qd and maxi-mum inflow with dry weather QDW for the dimen-sioning of trickling filters.

• Maximum inflow with dry weather QDW,2h and maximum dimensioning inflow QComb,In for the design of the secondary settling tank.

Daily loads can only be calculated on the basis ofvolumetric- or flow-proportional 24 hour compos-ite samples and the related daily inflow. The rele-vant loads are to be determined on the basis of

measurements on arbitrary days, i.e. with the in-clusion of wet weather days. Relevant are those loads which are undercut on 85 % of the days. At least 40 load values are to be included for the de-termination of the values. The relevant concentra-tions are to be determined using relevant loads and the associated daily wastewater inflows.

If the daytime and weekly courses of the concen-trations and inflows of the wastewater deviate from the variations with predominantly domestically pro-duced wastewater, for example through the indus-trial wastewater component, then this is to be taken into account with the determination of the dimensioning quantities.

Arrangements should be made to balance peaks if the daily curves of the nitrogen loads show up in 2-hourly atypically high loading peaks (greater than 2 times the daily average), whereby the loading from sludge treatment must also be taken into account.

If the data are insufficient or the expense for inves-tigation, for example with small plants, are in no re-lation to the use, loads and concentrations can be determined on the basis of connected inhabitants plus industrial/commercial and other loads.

Details on the determination of relevant loads and concentrations are to be taken from the Standard ATV-DVWK-A 198E “Dimensioning Principles for Wastewater Facilities” [4].

If the relevant loads have to be estimated based on the connected inhabitants the values in Table 1 can be used. The estimation of the associated wastewater inflow is to be undertaken in accor-dance with the ATV-DVWK Standard [4]. Until this standard is published determination of the waste-water flow can be determined in accordance with Standard ATV-A 131E (1991). [Translator’s note: Standard ATV-DVWK-A 198 was published in April 2003 and was translated into English in 2004].

Table 1: Inhabitant-specific loads in g/(I.d), which are undercut on 85 % of the days, without taking into account the sludge liquor

Parameter Raw waste-water

Retention time in the primary settling stage with QDW

0.5 to 1.0 h 1.5 to 2.0 h BOD5 60 45 40 TKN 11 10 10 P 1.8 1.6 1.6

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Deliberate investigations of wastewater and de-termination of loads over two to four weeks cannot, as a rule, be used directly for dimensioning, as one cannot be certain of having considered the rele-vant period of time. They are, however, practical for the supplementing of the existing database. The loading of internal return flows, for example from sludge treatment should also be recorded within the scope of such investigations.

3.2 Loading from Sludge Liquor and External Sludge

Water from the thickening and dewatering of (an-aerobic) digested sludge contains ammonium in high concentrations. It can be assumed that 50 % of the organic nitrogen introduced into the sludge digester is released as ammonium nitrogen. If sludge liquor is produced for a few hours daily only, or on odd days weekly, an intermediate stor-age for dosed input is necessary.

Return loading with phosphorus and organic mat-ter (BOD5 and COD) is, as a rule, small from dewa-tering of digested sludge. Therefore a return load-ing may not be added, for example, globally as a percentage to all loads from the wastewater.

As a rule, more or less anaerobic processes occur in sludge silos for aerobic stabilised sludge. With this, ammonium can be released and rerisolution of phosphorus is possible, if excess biological phosphorus removal is applied. In order to mini-mise impairment of the biological treatment

- sludge liquor should be drawn off in small quan-tities

– when dewatering the silo content filtrate or cen-trate should be collected in silos of a similar size and be fed to the inlet over a long period of ti-me.

If external sludge (sludge from other wastewater treatment plants, faecal sludge or similar) is dis-charged, then an intermediate storage can be practical in order to make a dosed input possible.

Further information on the determination of sludge liquor quantities and characteristics are to be taken from [7].

4 Pre-treatment

The wastewater flowing into the trickling filters and rotating biological contactors must be as free as possible of disturbing substances and settleable solids in order to avoid blockages. Therefore a pre-treatment and primary settling of the inflowing wastewater before the biological reactor is indis-pensable. With denitrification trickling filters this is particularly important as the removal of faults there is very expensive. Normally primary settling tanks, possibly also fine sieves, are employed for this.

Depending on the treatment requirements the pri-mary settling tanks should be dimensioned differ-ently. With pure carbon removal and nitrification (without denitrification) the retention time with dry weather should not be less than 1.5 to 2.0 hours. With pre-anoxic denitrification and a lack of an or-ganic carbon compound the retention time can be reduced from 0.5 to 1.0 hours with dry weather.

With high wet weather inflow peaks the primary settling tanks should be so dimensioned for a re-tention time shorter than 0.5 hours with wet weather inflow QComb. This applies above all for small wastewater treatment plants with a capacity below 1,000 PT.

Sufficient sludge storage volume is also to be taken into account with small plants. This can, for example, be arranged as separate tanks or in an Imhoff tank combined with the primary settling tank.

5 Trickling filters

5.1 Description of the Process

5.1.1 General

The treatment of wastewater in trickling filters as fixed bed reactors is effected by micro-organisms, which settle on the filter material as biofilm. In trick-ling filters the treatment process proceeds from top to bottom. In the various treatment zones there are respectively biocoenoses of different composition involved. Depending on the loading condition of the trickling filter, the influence of nitrifying bacteria

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is, for example, first completely effective if the deg-radation of the organic loading is completed to a large extent.

The following prerequisites are to be met for an assured efficiency of the trickling filter:

• The filter material concerned must be flawless in constitution and installation; filter materials must correspond with DIN 19557.

• Attention is to be paid to an even, surface-proportional distribution of the wastewater over the trickling filter surface. The rotating distributor must be suitably designed for this. An as even as possible complete wetting of the filter mate-rial surface with wastewater is to be ensured. Here a sufficient minimum surface loading and fine distribution are significant.

• A sufficient flushing force for the removal of waste sludge is to be ensured, i.e. for the re-spective loading there is a minimum hydraulic load which, if necessary, is to be ensured using return pumps.

• An unhindered percolation of the wastewater through the filter material must be avoided at all costs.

• It is recommended not to install differently struc-tured material in a trickling filter. To secure the removal of sludge a sufficient transmissibility is to be ensured in the vertical direction.

• The feed and return pumps are to be graded according to the different inflows taking into ac-count the minimum surface loading rate. A con-tinuous feed is to be sought.

• The air access from outside to the hollow floor of the trickling filter and into the filter material (exception denitrification trickling filter) must be ensured via supply air openings. In order to avoid too heavy a cooling in winter the air open-ings should be constructed so that they are ca-pable of being reduced.

• A too heavy cooling in locations with severe winter climate is to be countered through insula-tion of the walls, an enclosed construction and differential pump operation in comparison with the warm season; the forced ventilation recom-mended under circumstances in such a case can, together with a treatment of exhaust air, contribute to the prevention of odour nuisances with very highly loaded trickling filters. As a rule, a treatment of exhaust air can, however, be dis-pensed with as long as the trickling filter is suffi-

ciently ventilated and the filter material in this way itself acts as a filter.

5.1.2 Filter material

Most important component of the trickling filter are the filler materials used which can be roughly di-vided into mineral material and material made from plastic. With the selection of the filler material it is to be ensured that the wastewater sprayed over the trickling filter and, with aerobically operated trickling filters, the air flowing through have overall free access to the biofilm and that waste sludge can be removed with the wastewater. A blockage of the hollow spaces can thus limit the treatment efficiency or even reduce it completely to zero.

DIN 19557 differentiates between the theoretical surface, the effective surface (growth area) of the filter material and the biologically active surface of the growth. The effective surface is the surface of the filter material wetted in operation. The theoreti-cal surface deviates from this. The ratio of effective surface to theoretical surface is defined by the utilisation factor. There is no doubt that the biologi-cally active surface of the growth would represent the correct reference parameter for the description of the metabolic efficiency. The biologically active surface is, as a rule, not to be determined. The di-mensioning details for trickling filters in this stan-dard therefore essentially concern the volumetric loading.

As a rule, lumps of rock or slag sized from 40 to 80 mm, which are placed over the hollow base on a supporting layer sized from 80 to 150 mm, serve as mineral filler material for trickling filters. The re-quirements on characteristics, testing and installa-tion of the mineral filling are summarised in DIN 19557. The sizes 40 to 80 mm correspond with specific theoretical surfaces of ca. 90 m2/m3 and a hollow space share of ca. 50 %. Under operating conditions ca. 2/3 of this can be assumed to be biologically active.

Plastic filter materials have very different struc-tures. From this result in part considerable differ-ences between the theoretical, the effective and the biologically active surface. Suppliers of plastic filler materials should therefore present retraceable calculations for the theoretical surface.

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So far as no reliable transferable experience is available for the filler and for the characteristics of the wastewater, trials should be carried out at least on a semi-industrial scale.

5.2 Dimensioning

5.2.1 General Details on Dimensioning

Dependent on the level of treatment sought the BOD5 volumetric loading and, in the case of nitrifi-cation, in addition the TKN volumetric loading in kg/(m3

• d), are relevant for the dimensioning of the trickling filter volume. The filler material planned for the trickling filter contents, according to the permit-ted volumetric loading, results as

VTF,C = Bd,BOD,InB /BR,BOD [m3] (1)

In addition with nitrification:

VTF,N = Bd,TKN,InB/BR,TKN [m3] (2)

Thus the total volume is:

VTF = VTF,C + VTF,N [m3] (3)

As a rule the BOD5 concentration at the rotary dis-tributor CBOD,InB,RF is to be set at less than 150 mg/l by return pump operation. For this, as also for a partial balance of large variations of the inflow, a recirculation ratio RRDW ≤ 1 is sufficient with BOD5 concentrations in the influent ≤ 400 mg/l. The trick-ling filter surface and the biological filler height re-sult as:

ATF = QDW • (1+RRDW)/qA,TF [m2] (4)

hTF = VTF/ATF [m] (5)

Trickling filter filler heights of about 4 m for mineral filled trickling filters have proved their worth. With the employment of plastic filler material with a high vertical transmissibility a larger filler height is rec-ommended.

The surface loading rate qA,TF with mineral filled trickling filters, related to QDW • (1+RRDW), should be 0.4 m/h, with trickling filters with plastic filler ma-terial at least 0.8 m/h. Smaller filler heights up to a minimum of 2 m require a particularly even, finely distributed filter dosing and careful selection of the filler material, and enable a reduction of the sur-face loading rate of up to 0.4 m/h. Plastic filler ma-

terial with good transverse distribution is to be used with smaller filler heights.

In addition to the surface loading rate the design of the rotary distributor also has an effect on the flushing force FF. For this the following relationship applies

FF = qA,TF • 1000/(a • n) [mm/arm] (6)

Values for FF of 4 to 8 mm have proved their worth in order to ensure a satisfactory sludge removal. The higher the trickling filter the stronger is the re-quired flushing force in order to be able to prevent blockages due to heavy growth in the upper part of the trickling filter. Equally, for plastic filler materials with increasing specific theoretical surface, in-creasing values for FF are to be planned. Further-more, if the talk is of surface loading rate, here a loading rate by normally rotating rotary distributors with a flushing force FF within this range is as-sumed.

In practical dimensioning the following procedure has proved its worth:

a) Determination of the necessary trickling filter volume VTF in m3 in accordance with Sections 5.2.2 or 5.2.3 and 5.2.4 dependent on the treatment target.

b) Calculation of the mean average concentration

at the rotary distributor without recirculation flow CBOD,InB = Bd,BOD,InB • 1,000/Qd in mg/l.

c) Determination of the required recirculation ratio

for the achievement of the desired mean con-centration CBOD,InB,RF at the rotary distributor (CBOD,InB,RF ≤150 mg/l): RRDW≥(CBOD,InB/CBOD,InB,RF) - 1.

d) Determination of the maximum hydraulic filter

loading of the trickling filter from the maximum inflow with dry weather to the trickling filter QDW in m3/h and the recirculation ratio QTF = QDW

• (1+RRDW) in m3/h.

e) Selection of a trickling filter filler height hTF

in m. f) Determination of the required surface of the

trickling filter ATF = VTF / hTF in m2.

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g) Examination of the surface loading rate of the trickling filter with maximum inflow with dry weather including recirculation flow qA,TF = QDW • (1+RRDW)/ATF in m/h. This sur-face loading rate should be at least 0.4 to 0.8 m/h whereby, with trickling filters filled with plastic filler, the upper value is to be main-tained. If the given values are not met then the calculation is to be repeated with modified height or modified recirculation ratio.

h) Determination of the number of the rotary dis-

tributor arms and the rate of rotation under consideration of the flushing force FF.

i) It is to be ensured that, during the night, a con-

tinuous operation of the rotary distributor is guaranteed and the complete surface of the trickling filter is evenly wetted.

5.2.2 Wastewater Treatment without Nitrification

For the dimensioning of mineral filled trickling filters and trickling filters with plastic filler material with a specific theoretical surface of a minimum of 100 m2/m3 the following is recommended:

BOD5 volumetric loading BR,BOD ≤ 0.4 kg/(m3•d)

With trickling filters with plastic filler material with a specific theoretical surface of more than 100 m2/m3 BOD5 volumetric loadings of more than 0.4 kg/(m3

•d) are possible. These should, however, be substantiated through trials (see Chapter 1.1) or references. Specific theoretical surfaces of more than 150 m2/m3 and BOD5 volumetric loading of more than 0.6 kg/(m3

•d) are not effective for further improvement of performance. It is pointed out that, blockages can already occur with specific theoreti-cal surfaces of about 150 m2/m3.

With small wastewater treatment plants, due to marked inflow or loading peaks, it is recommended to reduce linearly the BOD5 volumetric loading from 0.4 kg/(m3

•d) to 0.2 kg/(m3•d) with capacities

between 1,000 and 50 PT.

5.2.3 Wastewater Treatment with Nitrification

With the dimensioning of trickling filters with nitrifi-cation the volumetric content planned for the filter material is determined separately for the carbon removal and for the nitrogen oxidation.

For the dimensioning of mineral filled trickling filters and of trickling filters with plastic filler material with a specific theoretical surface of a minimum of 100 m2/m3 the following is recommended:

For carbon removal:

BOD5 volumetric loading BR,BOD ≤ 0.4 kg/(m3.d)

For nitrification:

TKN volumetric loading BR,TKN ≤ 0.1 kg/(m3.d)

This value takes into account a nitrification already started in the carbon removal zone. The permitted volumetric loading BR,TKN for the dimensioning is not identical with the volumetric efficiency of deg-radation.

With trickling filters with plastic filler material, BOD5 volumetric loading of more than 0.4 kg/ (m3

•d) for carbon removal and TKN volumetric loading of more than 0.1 kg/(m3

•d) for nitrification are possible. These should be substantiated through trials (see Chapter 1.1) or references. Specific theoretical surfaces of more than 150 m2/m3 as well as BOD5-volumetric loading of more than 0.6 kg/(m3

•d) and TKN volumetric loading of more than 0.15 kg/(m3

•d) are not effective for fur-ther improvement of performance.

With small wastewater treatment plants, due to marked inflow or loading peaks, it is recommended to reduce linearly the BOD5 volumetric loading from 0.4 kg/(m3.d) to 0.2 kg/(m3

•d) and the TKN volumetric loading from 0.1 kg/(m3

•d) to 0.05 kg/(m3

•d) with capacities between 1,000 and 50 PT.

Note: If nitrification takes place in a second trick-ling filter following extensive carbon removal in a first stage with intermediate treatment, the follow-ing loading values are recommended: TKN volu-metric loading up to 0.1 kg/(m3

•d) with mineral filled trickling filters and up to 0.2 kg/(m3

•d) with trickling filters with plastic filler material. For this,

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plastic filler material with a specific theoretical sur-face of up to 200 m2/m3 can be employed. For rea-sons of safety it is recommended that the nitrogen incorporation is not to be taken into account in the first stage.

Note on alkalinity: the remaining alkalinity in the effluent from nitrifying trickling filters should, if nec-essary taking into account precipitants, not under-cut 0.5 mmol/l, in order to exclude possible inhibi-tion of the nitrification. This is to be ensured when necessary through suitable measures. It is recom-mended that such measures are first carried out following presentation of pertinent operational ex-perience.

5.2.4 Wastewater Treatment with Nitrifi-cation and Denitrification

The following remarks apply both for mineral filled trickling filters as well as trickling filters with plastic filler material.

For procedural integration of denitrification with trickling filter plants there are basically three possi-bilities:

• simultaneous denitrification in the trickling filter with recirculation of wastewater containing ni-trate

• pre-anoxic denitrification in an anoxically oper-ated

a) fixed bed reactor (e.g. trickling filter) b) activated sludge tanks with intermediate set-

tling tanks • post denitrification process with addition of ex-

ternal carbon sources in a a) fixed bed reactor b) activated sludge tank

Attention is also drawn for these process tech-niques to two ATV Reports [neither available in English]: „Umgestaltung zweistufiger biologischer Kläranlagen zur Stickstoffelimination“ [“Conversion of two-stage biological wastewater treatment plants for phosphorus removal”] [5] and ATV Re-port „Denitrifikation bei Tropfkörperanlagen“ [Deni-trification with trickling filter facilities”] [6]. Below, only pre-anoxic denitrification itself is covered in an anoxically operated trickling filter.

To use existing trickling filters for targeted denitrifi-cation as a rule only small conversion measures and an appropriate operation are required.

Through the prevention of the inflow of air (cover-ing of the trickling filter and prevention of the air in-flow through the outlet and lower air openings, usually already achievable through impounding of the outlet channels around the trickling filter) it is possible to set anoxic conditions on the inside of trickling filters, if recirculated effluent containing ni-trate of a downstream nitrifying treatment unit to-gether with the mechanically treated wastewater is applied to the trickling filter.

An impounding of the trickling filter filler material involves the danger of blockage and would, in most cases, lead to static problems; it therefore should not take place. The partially treated effluent from upstream denitrifying trickling filters is fed via an intermediate settling tank or directly to a subse-quent aerated nitrifying treatment unit. As a rule these are trickling filters or activated sludge plants.

The following dimensioning values are given for denitrification in trickling filters:

• The achievable denitrification capacity is de-pendent on the BOD5 volumetric loading and can be determined using the values in Table 2. With this, the BOD5 removal in addition to the BOD5-loading is also dependent on whether the effluent of the denitrification trickling filter follows an intermediate settling tank.

• The daily average nitrate concentration to be denitrified results as follows:

SNO3,D = CN,InB – SorgN,ESST – SNH4,ESST – SNO3,ESST – XorgN,BM [mg/l] (7)

• As influent nitrogen concentration (CN,InB) the relevant value determined for T = 12° C is to be applied. If, during the year, at times of higher temperatures, higher CN,InB : CBOD,InB ratios have been determined several load cases are, if necessary, to be considered.

The influent nitrate concentrations (SNO3,InB) is, in general, negligibly small. With greater infiltration rates (groundwater containing nitrate) or with in-flows from certain commercial and industrial plants it can be necessary to take account of SNO3,InB in CN,InB.

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At plants with anaerobic sludge digestion and me-chanical dewatering at the site, the nitrogen of the sludge liquor must be contained in the inflow con-centration (CN,InB) if no separate sludge liquor treatment takes place.

• The concentration of organic nitrogen in the ef-fluent can be set as SorgN,ESST = 2 mg/l. With the inflow of certain commercial wastewater the concentration can be higher. To be on the safe side, the ammonium content in the effluent for dimensioning is, as a rule, assumed as SNH4,ESST = 0. The nitrogen incorporated in the biomass is taken into account simplified as XorgN,BM = 0.03 • CBOD,InB.

• The relevant effluent concentration of nitrate

nitrogen is to be applied as daily average. If, as in Germany, the monitoring takes place as grab or 2 hour composite samples, a significantly smaller concentration than the monitoring value [effluent requirement for inorganic nitrogen (SinorgN,MV)]. It is practical to set SNO3,ESST = 0.8 to 0.6 SinorgN,MV, whereby the smaller value applies for plants with greater variations in the influent load.

• Surface loading rate qA,BF < 3 m/h. • Too high oxygen transfer in the trickling filter is

to be avoided; therefore the recirculation ratio should always be optimised and RRDW = 3, re-lated to QDW, should not be exceeded.

Table 2: Recommended values for the dimensioning of the necessary denitrification volume VTF,D

BOD5-

volumetric loading

Denitrification capacity

BOD5-removal BOD5-removal

without with intermediate

settling intermediate

settling kg/(m3.d) SNO3,D/CBOD,InB % %

0.2 0.14 60 80 0.6 0.10 45 65 1.0 0.08 40 60

The values of Table 2 are valid for ≥ 12 °C and a nitrate concentration in the effluent of denitrification trickling filters ≥ 2 mg/l N. Intermediate values are to be interpolated.

The dimensioning of downstream treatment units for nitrification and carbon oxidation can take place taking into account previous treatment steps in ac-cordance with Sections 5.2.2 and 5.2.3 for trickling

filters and in accordance with ATV-DVWK Stan-dard ATV-DVWK-A 131E for activated sludge plants.

With downstream trickling filters for nitrification the recirculation should be taken directly from the ef-fluent of the trickling filter to relieve the hydraulic load the settling tank.

It can be an advantage to feed the internal recircu-lation via the primary settling tank. With this, an additional denitrification can be achieved, to a lesser degree however and not capable of estima-tion. The additional hydraulic loading of the primary settling tank is to be taken into account here.

With downstream nitrifying activated sludge plants the recirculation should, in general, be taken from the effluent of the secondary settling tank. This is to be taken into account with the dimensioning of the secondary settling tank. With the employment of a suitable filler material (plastic), however, in principle a recirculation with activated sludge is also capable of being carried out after successfully executed pre-trials.

With the design and with the operation of trickling filters for denitrification attention is to be paid that, following an opening of the trickling filter for the cleaning of the rotary distributor or similar, denitrifi-cation is not possible or not possible to the full ex-tent until the oxygen has again been fully depleted following closure.

6 Rotating biological contactors

6.1 Description of the Process

6.1.1 General

With the treatment of wastewater using rotating biological contactors the biomass required for the biological wastewater treatment is firmly attached to the rotating growth surfaces. Rotating biological contactors are partially submerged and slowly ro-tate in a trough through which the wastewater flows. The biofilm attached to the growth surfaces

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is subjected alternately to air and wastewater dur-ing rotation.

With the rotation of the rollers, following contact with the wastewater on immersion, there is respec-tively an aeration phase above the water level. The oxygen supply taken up with this must be sufficient to cover the depletion process during the sub-merged stage and for the maintenance of the aerobic conditions in the trough. This and the maintenance of a thin biofilm require a minimum rotation rate of the roller. The oxygen transfer through the rotation is sufficient and is not a limit-ing factor for the nitrification, if at least 40 % of the discs/roller surface is permanently out of the water.

The activated sludge, which is held suspended in the water in the trough, plays only a small part in the treatment performance and, from the technical aspect of dimensioning, remains unconsidered.

Trough and biological contactor are to be so de-signed and/or the revolution rate is to be so se-lected, that a sufficient turbulence is guaranteed in order that the settling of sludge in the biological contactor and in the trough is prevented.

Biological contactors should be covered as prob-lems with ice formation can occur in winter with open biological contactors. The resultant gaseous metabolic products also escape into the air space of the cover. In order to prevent a hazardous accu-mulation of the gaseous products of wastewater treatment and always to provide sufficient atmos-pheric oxygen, an uninterrupted exchange of air in the space above the rotation trickling filter is to be ensured.

Favourable volumetric degradation performanc can be achieved through the compact construction of rotating biological contactors. Very high volumetric loadings and thus short retention times lead, how-ever, to only a slight equalisation of loading peaks.

Rotating biological contactors usually consist of 2 to 4 sequentially arranged rollers in separate troughs (cascade arrangement). The cascade type of construction enables the realisation of various surface or volumetric loadings and carrier material packed to different densities. Then, on each roller, there is another growth to be found corresponding with the degree of pollution of the wastewater. In

addition, cascades reduce the effects of loading peaks.

It can be an advantage to feed back from the efflu-ent of the last rotating biological contactor into the influent to the primary settling tank. With this an evening out of the hydraulic loading, a reduction of the danger of blockage and a reduction of peak loadings can be achieved. The additional hydraulic loading is to be taken into account.

With rotating biological contactors precautions are to be taken that the rollers, also after long idle pe-riods, can be taken into operation again without additional measures.

6.1.2 Material and Types

Today, in addition to discs, rotating biological con-tactors made from profiled, wrapped foils as well as made from corrugated or grid-shaped pipes are employed. In any case attention is to be paid that the selected structure permits a secure removal of waste sludge, the oxygen transfer to the biofilm is guaranteed and, at the same time, the energy con-sumption of the system is kept as small as possi-ble.

► Disc biological contactors

Disc biological contactors have discs mounted on a shaft with a non-structured, smooth or rough-ened surface which, following overgrowth by the active biofilm, has a smooth appearance. The discs can also be made up from segments fitted to the shaft by a girder construction. The discs nor-mally are flat, but can also have a shape which satisfies the demands of a flat disc, (e.g. roughly corrugated discs). In particular the distance be-tween discs may not be altered by the shape and, apart from the spacers, the distance may not have any further bridging. The discs are separated from each other by spacers. The distance between discs depends on the organic surface loading rate of the respective stage of a cascade. With disc bio-logical contactors the disc surface corresponds approximately with the biologically active surface.

► Roller biological contactors

In a roller body which is permeable to water there are shaped, fixed or also movable filler material on

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whose surface the biomass grows. They can con-currently serve for oxygen transfer and the genera-tion of turbulence. With roller biological contactors the biologically active surface cannot be derived from the theoretical surface (for this comp. Section 5.1.2). The biologically active surface is, as a rule, smaller than the theoretical surface of the sfiller material. It is subjected to seasonal variations and often first forms after longer operating periods. As long as no other data can be supplied a reduction of the permitted surface loading rate by 30 %, re-lated to the values for the permitted surface load-ing rate of disc biological contactors, is first as-sumed generally for the dimensioning in accordance with Sections 6.2.2 and 6.2.3.

Depending on the degree of loading of the treat-ment stage (cascade), materials with varyingly large surfaces can be employed. With high organic surface loading rates or large specific theoretical surfaces there is a danger of blockage. This in-creases with unfavourable structural formation of the material which prejudices the removal of the solids formed.

Suitable precautions are to be taken which prevent blockage such as, for example, flushing facilities.

6.2 Dimensioning

6.2.1 General Details on Dimensioning

Concerning the comparability of the dimensioning proposals there is a similar problem with regard to the biologically active surface as with trickling filter filler material.

The dimensioning values presented below have been determined for disc biological contactors. In the meantime there are numerous other rotating biological contactors functioning satisfactorily, so that the dimensioning rules are transferable if the treatment performance can be derived from refer-ence plants operated over long periods.

In order to prevent the effects of peak loadings a trough volume of some 4 l per m2 theoretical sur-face should not be undercut.

In the first place the following listed dimensions are to be determined or specified for the dimensioning:

• number of stages (roller, cascades) and • specific theoretical surface of the roller material

used or the minimum separation of the discs.

Odour nuisances can occur with this process with high loadings, so that it is advisable to limit the BOD5 surface loading rate of the first stage with domestic wastewater to a value of 40 g/(m2

•d).

From BOD5 surface loading rates ≥ 20 g/(m2•d) in

one stage, a minimum separation of discs of ≥ 18 mm is recommended for disc biological con-tactors. With this loading roller biological contac-tors are to be designed for specific theoretical sur-face of ≤ 100 m2/m3, unless it can be proved that a higher surface is permanently available.

With BOD5 surface loading < 20 g/(m2•d) in one

stage, a disc separation of 15 mm and a specific theoretical surface of ≤ 150 m2/m3 is recom-mended.

It can be an advantage to install an intermediate settling tank between the units for carbon removal and nitrification. In this way the waste sludge can be separated which increases the performance of the nitrification units. In such a nitrification unit the separation of the discs can be reduced to 10 mm and the specific theoretical surface of the rollers increased to a maximum of 200 m2/m3.

Subsequently the BOD5 surface loading rate BA, to be determined dependent on the treatment target, is relevant for the determination of the required theoretical surface.

The necessary theoretical surface ARC is deter-mined as follows:

Bd,BOD,InB • 1000 ARC,C = ___________________

[m2] (8) BA,BOD

In addition with nitrification:

Bd,TKN,InB • 1000 ARC,N = ___________________ [m2] (9) BA,TKN

The overall necessary theoretical surface is calcu-lated from the BOD5 daily load flowing into the re-actor and the TKN daily load flowing into the reac-

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tor as well as from the permitted surface loading rates BA,BOD and BA,TKN:

ARC = ARC,C + ARC,N [m2] (10)

6.2.2 Wastewater Treatment without Nitrification

2 to 4 stages are to be assumed for the dimension-ing of the theoretical surface. With this the follow-ing dimensioning values are recommended:

For disc biological contactors:

– 2 stage cascades: BA,BOD ≤ 8 g/(m2

• d)

– 3 and 4 stage cascades: BA,BOD ≤ 10 g/(m2

• d)

With small wastewater treatment plants, due to the marked inflow and loading peaks, it is recom-mended to reduce linearly the BOD5 surface load-ing rate from 8 respectively 10 g/(m2

•d) to 4 g/(m2

•d) between capacities of 1,000 and 50 PT.

For other rotating biological contactors:

– 2 stage cascades: BA,BOD ≤ 5.6 g/(m2

• d)

– 3 and 4 stage cascades: BA,BOD ≤ 7 g/(m2

• d)

With small wastewater treatment plants, due to the marked inflow and loading peaks, it is recom-mended to reduce linearly the BOD5 surface load-ing rate to 3 g/(m2

• d) between connection capaci-ties of 1,000 and 50 PT.

6.2.3 Wastewater Treatment with Nitrifi-cation

If nitrification is necessary then a three or four stage cascade plant is advisable for the dimen-sioning of the theoretical surface. The following dimensioning values are recommended:

For disc biological contactors:

– 3 stage cascades: BA,BOD ≤ 8 g/(m2

•d) and BA,TKN ≤ 1.6 g/(m2•d)

– 4 stage cascades: BA,BOD ≤ 10 g/(m2

•d) and BA,TKN ≤ 2 g/(m2•d)

With small wastewater treatment plants, due to the marked inflow and loading peaks, it is recom-mended to reduce linearly the BOD5 surface load-ing rate from 8 respectively 10 g/(m2

•d) to 4 g/(m2

•d) and the TKN surface loading rate to 1.6 respectively 2 g/(m2

•d) to 1.2 g/(m2•d) between

capacities of 1,000 and 50 PT.

For other rotating biological contactors:

– 3 stage cascades: BA,BOD ≤ 5.6 g/(m2

•d) und BA,TKN ≤ 1.1 g/(m2•d)

– 4 stage cascades: BA,BOD ≤ 7 g/(m2

•d) and BA,TKN ≤ 1.4 g/(m2•d)

With small wastewater treatment plants, due to the marked inflow and loading peaks, it is recom-mended to reduce linearly the BOD5 surface load-ing rate to 3 g/(m2

•d) and the TKN surface loading rate from 1.1 respectively 1.4 g/(m2

•d) to 0.85 g/(m2

•d) between capacities of 1,000 and 50 PT.

If, for the individual growth materials, it is verified that the specific biologically active surface is per-manently more than 70 % of the specific theoreti-cal surface, the dimensioning values can be raised correspondingly to a maximum of the values valid for disc biological contactors.

The values for BA,TKN take into account a nitrifica-tion already started in the carbon removal zone. The permitted surface loading rate BA,TKN for the dimensioning is not identical with the rate of nitrification.

A biological denitrification as with trickling filters is also possible with rotating biological contactors. Technical testing is, however, still outstanding.

Note on alkalinity: the remaining alkalinity in the ef-fluent of nitrifying rotating biological contactors should, if necessary taking into account precipi-tants, not undercut 0.5 mmol/l in order to exclude possible inhibition of the nitrification. If required this is to be ensured through suitable measures. It is recommended that such measures are imple-mented following availability of relevant operational experience.

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7 Phosphorus Removal

With fixed bed systems the P-removal can be achieved reliably only through chemical precipita-tion. For this the addition of precipitating chemicals in the effluent of reactors before the secondary set-tling and/or preliminary precipitation is advisable. If necessary the optimum dosing point is to be de-termined through trials. With preliminary precipita-tion an undersupply of the fixed bed biology with phosphorus due to the precipitation is to be pre-vented. ATV Standard ATV-A 202 [currently not available in English] is to be observed.

Due to the slight sensitivity to low pH values, measures to raise the alkalinity as a rule are not necessary with trickling filters and rotating biologi-cal contactors. In particular, with the application of acidic precipitants, however, attention is to be paid that a residual alkalinity is retained in the effluent of the secondary settling tank. Possibly, the addi-tion of alkaline precipitant or basic neutralisation agent is necessary

8 Waste Sludge Production

The size of the possible range of variation with waste sludge production is, based on biology, de-pendent on the type of wastewater, the loading and the hydraulic conditions. The biological waste sludge production, related to the loading values recommended here, in the absence of measured results with eliminated BOD5 incl. stormwater treatment, can be assumed to be 0.75 kg SS per kg. The calculation of the precipitation sludge pro-duced can take place in accordance with ATV Standard ATV-A 202. A simultaneous sludge stabi-lisation is not possible as primary and secondary sludge is produced separately.

9 Secondary Settling Tanks

9.1 General

Secondary settling tanks of trickling filters and ro-tating biological contactors have the task of sepa-rating the waste sludge, removed from the biologi-cal reactor with the treated wastewater, from the wastewater. In comparison with activated sludge plants the secondary settling tanks of trickling fil-ters and rotating biological contactors are fed with significantly smaller quantities of sludge with set-tling characteristics which are normally without problem. Sedimentation of the individual small par-ticles here is of decisive significance for the reten-tion performance.

Due to the small particle concentration, coagula-tion and precipitation with mixed liquor suspended solids contents of ca. 30 to 100 mg/l SS in the ef-fluent of single-stage trickling filters and rotating biological contactors have no relevant effect. They are to be taken into account only with very exten-sive requirements on solids removal.

Particle concentration and the coagulation effect can be increased significantly through sludge re-circulation or addition of precipitants in the influent of the secondary settling setting tankstage or into an upstream precipitation chamber.

Through this the retention of solids can be im-proved (see also ATV Advisory Leaflet ATV-M 274 „Einsatz organischer Polymere in der Abwasserre-inigung“ [“Application of Organic Polymers in wastewater Treatment” - currently not available in English]).

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September 2001 21

9.2 Dimensioning of the Secondary Settling Tank of Single-stage Trickling filters and Rotating biological contactors

If trickling filters and rotating biological contactors are employed with normal municipal conditions as single-stage plants for biological pre-treatment or nitrification, then the secondary settling tank can be dimensioned simply according to pure hydraulic aspects with the surface loading rate qA and flow time tSST.

The surface loading rate of the secondary settling tank is made up from the quotients of the maxi-mum hourly inflow to the secondary settling tank QSST including all return flows which flow into the secondary settling tank, and the surface area of the secondary settling tank ASST. The wet weather case is also to be taken into account and, in most cases, decisive for dimensioning.

Thus, with the surface loading rate

qA,SST ≤ QSST / ASST [m3/(m2•h) or m/h] (11)

the respectively greater value is to be applied for

QSST = QDW • (1+RRDW) or QSST = Qm • (1 + RRm)

Through the performance of the (trickling filter-) feed pumps and an appropriate regulation (e.g. float valve or recirculation pumps), it is to be ensured that the recirculation ratio does not exceed the se-lected dimensioning value.

With trickling filters or rotating biological contactors the surface loading rate of the secondary settling tank may not exceed 0.80 m/h if effluent limiting values of SSe < 20 mg/l are to be maintained [2].

The required tank surface area results as

ASST,nec = QSST/ qA,SST, perm [m2] (12)

The flow time can be defined as theoretical flow time in the secondary settling tank as

tSST = VSST/QSST [h] (13)

It should not be less than 2.5 h.

The required tank volume thus results as

VSST,nec = tSST • QSST [m3] (14)

The minimum depth of water hSST is 2.0 m (in circu-lar tanks 2/3 of the radius).

If flocculation is carried out through dosing of phosphate precipitants or polymers into the influent to the secondary settling tank, the surface loading rate can be increased to 1.00 m/h, if the secondary settling tank maintains a minimum depth of water of hSST ≥ 2.50 m (in circular tanks 2/3 of the ra-dius).

In the case of intermediate settling tanks or with short retention times using downstream ponds, surface loading rates of 1.5 to 2.0 m/h can be se-lected with appropriately reduced flow times.

With constraints which lead to increased loading of the trickling filter or rotating biological contactor and unfavourable conditions for the settling proc-ess in the secondary settling tank (e.g. preliminary settling with tSST < 0.75 h with Qm, higher combined wastewater inflow greater than 2.2 • QDW, small re-circulation ratio with QDW), the permitted surface loading rate of the secondary settling tank should be reduced by up to 20 %.

With existing secondary settling tanks the dimen-sioning values can be determined through full-scale loading trials.

9.3 Notes on Tank Shape and Design

The structural aspects which effect the dimension-ing or which are assumed for the dimensioning are dealt with within the scope of this standard. Further planning aspects of construction and design, for example due to space and underground condi-tions, progress of construction, traffic safety or similar are not listed here explicitly; for this see the ATV Manual [2].

The tank shape is not decisive for the settling effi-ciency and sludge collection in secondary settling tanks after trickling filters and rotating biological contactors. Even with vertical flow secondary set-tling tanks no better effluent values can be achieved than with secondary settling tanks, so long as with these the minimum retention time is maintained. The reason for this lies in that, due to the small particle concentrations no floc filter can be installed. With trough-shaped tanks with steep

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slopes (at least 60°), however, no mechanical sludge removal is necessary.

Compared with rectangular tanks circular tanks of-fer the advantage of a smaller weir overflow rate and are frequently somewhat more cost effective, on the other hand, however, there is the greater sensitivity to wind and the greater space require-ment. As a continuous sludge recirculation is not necessary with trickling filters and rotating biologi-cal contactors, at most simple sludge scrapers sur-fice even with rectangular tanks.

With rectangular tanks the ratio of the depth of the tank to the length of the tank should be ca. 1:15 to 1:25. For the width of the tank values up to 7.0 m have proved themselves in practice.

An as even as possible distribution of the inflow over the cross-section of the flow is to be sought. The weir overflow rate qWO must be smaller than 15 m3/(m • h).

As only the waste sludge is to be separated out in the secondary settling stage tank trickling filters and with rotating biological contactors, cloth filters or microstrainers can be considered in place of the secondary settling tanks. References and appro-priate functional verifications are to be requested, operating safety and maintenance expense are to be observed.

So far as further treatment stages are added downstream the employment of lamella separators is also possible to reduce the space requirement, if the increased maintenance expense is accepted. With regard to the surface loading rate these are to be dimensioned precisely as for secondary settling tanks. For this see the ATV Manual [2].

10 Costs and Environmental Effects

With this Standard planners and examiners receive a differentiated working basis for the dimensioning of trickling filters and rotating biological contactors. From this they can, from the process technical as-pect, develop the most sustainable and most eco-nomical solution with regards to the required envi-ronmental protection.

The requirements on the quality of the water to be discharged into surface waters are not established in this Standard; they are legally prescribed either ([German] Wastewater Ordinance) or are specified by the authorities. This Standard is aimed at a se-cure and economical observance of these specifi-cations.

11 Relevant Regulations, Directives and Standard Specifications

[Translator’s note: those references available in English are shown as such. Otherwise a courtesy translation is provided in square brackets.]

• Abwasserverordnung [(German) Wastewater Ordinance]

Ordinance on the requirement on the discharge of wastewater into surface waters (AbwV). Bundes-gesetzblatt 1999, Part 1, No. 6 dated 18.02.1999

• ATV-DVWK Standards

ATV-A 122E Principles for Dimensioning, Construction and Op-eration of Small Sewage Treatment Plants with Aerobic Biological Purification Stage for Connec-tion Values between 50 and 500 Inhabitants and Population Equivalents, Issue 6/91

ATV-A 129 Abwasserbeseitigung aus Erholungs- und Frem-denverkehrseinrichtungen [Wastewater Disposal From Recreation and Tourist Facilities], Issue 5/1979

ATV-DVWK-A 131E Dimensioning of Single-Stage Activated Sludge Plants, 5/2000

ATV-A 257E Principles for the Dimensioning of Wastewater La-goons and Intermediate Trickling Filters or Biologi-cal Contactors, Issue 10/1989

ATV-A 202 Verfahren zur Elimination von Phosphor aus Ab-wasser [Processes for the Removal of Phosphorus from Wastewater], Issue 10/1992

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ATV-M 271 Personalbedarf für den Betrieb kommunaler Kläranlagen [Personnel Requirement for the Op-eration of Municipal Sewage Treatment Plants], Is-sue 9/1998

ATV-M 274 Einsatz organischer Polymere in der Abwasserre-inigung [Employment of Organic Polymers in Wastewater Treatment], Issue 11/1999

• Standard specifications

EN 1085 Wastewater treatment - Vocabulary

DIN 4045 Wastewater engineering - Vocabulary

DIN 4261-2 Small sewage treatment plants – Plants with sew-age aeration; application, design; construction and testing

DIN 18202 Dimensional tolerances in building construction - Buildings

DIN 19553 Kläranlagen – Tropfkörper mit Drehsprenger, Hauptmaße [Sewage treatment plants – Trickling filters with rotating sprinklers, main dimensions]

DIN 19557-1 Sewage treatment plants – mineral filter media for percolating filters – requirements, testing, delivery, placing

DIN 19557-2 Kläranlagen – Füllstoffe aus Kunststoff für Tropf-körper – Anforderungen, Prüfungen [Sewage treatment plants – filter materials for trickling filters – requirements, tests]

DIN 19558 Überfallwehr mit Tauchwand, getauchte Ablaufroh-re in Becken; Baugrundsätze, Hauptmaße, An-wendungsbeispiele [Overflow weir with scumbo-ard, submerged outflow pipes in tanks; construction principles, main dimensions, e-xamples of application]

DIN 19569-1 Principles for the design of structures and technical equipment for sewage treatment plants; general principles

DIN 19569-2 Principles for the design of structures and equip-ment for sewage treatment plants; installations for separating and thickening solids

DIN 19569-3 Baugrundsätze für Bauwerke und technische Aus-rüstung; Besondere Baugrundsätze für Einrichtun-gen zur aeroben biologischen Abwasserreinigung [Principles for the design of structures and techni-cal equipment for sewage treatment plants; special construction principles for installations for aerobic biological wastewater treatment]

Literature

[1] ATV (Publ.): ATV-Handbuch „Biologische und weiterge-hende Abwasserreinigung“ [ATV Manual “Bio-logical and advanced wastewater treatment”]. 4th Edition, Berlin: Ernst & Sohn, 1997

[2] ATV (Publ.): ATV-Handbuch „Mechanische Abwasserreini-gung [ATV Manual “Mechanical wastewater treatment”] 4th Edition, Berlin: Ernst & Sohn, 1997

[3] ATV Report „Mehrstufige biologische Kläranlagen“ [Multi-

stage biological wastewater treatment plants”] Korrespondenz Abwasser 2/1989, p. 181-189

[4] Standard ATV-DVWK-A 198E “Standardisation and Derivation of Dimensioning Values for Wastewater facilities”, 2003

[5] ATV Report „Umgestaltung zweistufiger biologischer

Kläranlagen zur Stickstoffelimination“ [“Conversion of two-stage biological wastewater treatment plants for phosphorus removal”] Korrespondenz Abwasser 1/1994, p. 95-100

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[6] ATV Report „Denitrifikation bei Tropfkörperanlagen“ [De-

nitrification with trickling filter facilities”] Kor-respondenz Abwasser 11/1994, p. 2077-2081

[7] ATV-DVWK Report „Rückbelastung aus der Schlammbehandlung

– Menge und Beschaffenheit der Rückläufe“ [“Return loading from sludge treatment – quantity and characteristics of the return flows”] KA-Wasserwirtschaft, Abwasser, Abfall 8/2000, p. 1181-1187

The above Standard ATV-DVWK-A 281E replaces the draft of ATV Standard ATV-A 135 and contains up-dated approaches to dimensioning for trickling filters and rotating biological contactors with secondary set-tling. The advantages of the treatment of wastewater in fixed bed reactors in general lie in the small energy con-sumption and the simple and stable method of operation. With the trickling filter process the wastewater is sprinkled over the filler material. With this the necessary oxygen is taken up passively. An active aeration using energy is, as a rule, not required. On the other hand, with rotating biological contactors, the disks or rollers up to a half submerged in a wastewater trough, are rotated about their longitudinal axis using en-ergy. Aeration also takes place passively during the emerged phase. Trickling filter and rotating biological contactor facilities enable the colonisation with micro-organisms which have long generation times. Thus even compounds which are difficult to degrade can be eliminated with small loading. The standard in addition contains details fort he dimensioning of trickling filters with denitrification. It should be emphasised, with the dimensioning of secondary settling tanks for trickling filters and rotating biological contactors that, based on the results of new investigations, the necessary tank surface has been increased and the tank depth reduced. With trickling filters the dimensioning depends on the filler material used. The Standard shows how to take into account adequately the characteristics of the various obtainable filler materials.

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