Imhoff Ta nkThe Imhoff tank was developed to correct the two main defects of the septic tank. 1. It preve nts the so lids on ce remov ed from th e sewage fro m again b eing mixed wi th it, bu t still provides for the decomposition of these solids in the same unit 2. It pr ovide s an efflu ent a menabl e to furth er tre atment . Contact between the waste stream and the anaerobic digesting sludge is practically eliminated and the holding period in primary settling compartment at the tank is reduced. The Imhoff tank may be either circular or rectangular and is divided into three compartments1. the upper se ction or sed imentation compartment 2. the lower se cti on known as t he digest ion com partment and !. th e gas ven t a nd scum sect io n It is desirable to be able to reverse the direction of flow to prevent excessive deposition of solids at one end of the sedimentation compartment. "eriodically reversing the flow will result in an even accumulation of sludge across the bottom of the tank. In operation, all of the wastewater flows through the upper compartment. #olids settle to the bottom of this sloped compartment, slide down and pass through an o pening or slot to the digestion compartment. $ne of the bottom slopes extends at least six inches bey ond the slot. This forms a trap to prevent gas or d igesting sludge particles in the lower section from entering the waste stream in the upper section. The gas and any rising sludge particles are diverted to the gas vent and scum section. Imhoff Tank OperationThere are no mechanical parts in an Imhoff tank. %ttention should, h owever, be g iven to the following1
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a. &aily removal of grease, scum and floating solids from the sedimentation compartment.
b. 'eekly scraping of the sides and sloping bottoms of the sedimentation compartment by a
rubber s(ueegee to remove adhering solids which may decompose.
c. 'eekly cleaning the slot at the bottom of the sedimentation compartment. This can be
done by use of a chain drag.
d. "eriodic reversal of flow where provided for in the design of the tank.
e. Control of the scum in the scum chamber, by breaking it up, hosing with water under
pressure, keeping it wet with supernatant from the digestion compartment and removal if
the depth approaches two to three feet.
f. )emoval of sludge should be done before the sludge depth approaches within 1* inches
of the slot in the sedimentation compartment. It is better to remove small amounts
fre(uently than large amounts at long intervals. #ludge should be removed at a slow
regular rate to avoid coning +i.e. the formation of a channel through the sludge whichwould permit partially digested sludge and li(uid held in storage above the digested sludge
to be withdrawn from the tank. -efore winter temperatures are expected, most of the
digested sludge except that necessary for seeding +about 2 percent should be removed to
provide space for winter accumulations when digestion is very slow. The height of the
sludge in the sludge compartment should be determined at inlet and outlet end of the tank
at least once a month.
g. %fter each time that sludge is removed, the sludge pipes should be flushed and drained to
prevent sludge from hardening in and clogging the pipes.
h. "revention of /0oaming/. very effort should be made to prevent /foaming/ because
correction after the condition arises is sometimes difficult. /0oaming/ is usually
associated with an acid condition of the sludge and in such cases may be prevented or
corrected by treatment with lime or sodium bicarbonate to counteract the acidity of the
sludge. There are a few simple measures which may, under certain circumstances, remedy
or improve the condition.
1. The use of hydrated lime or sodium bicarbonate added to the gas
vents will usually aid in correction. The p value of the resulting sludge and lime
mixture in the digestion compartment should not exceed 3.4.
2. )emoving the tank from service where possible for a few days and
allowing it to rest will sometimes improve conditions.
!. %gitation of the gas vent area with a water hose or paddles will
sometimes help.
The Imhoff tank has no mechanical parts and is relatively easy and economical to operate. It
provides sedimentation and sludge digestion in one unit and should produce a satisfactory
primary effluent with a suspended solids removal of 5 to 4 percent and a -$& reduction of 16
to !6 percent. The two7story design re(uires a deep over7all tank. "rimary tanks with separate
digesters have largely replaced the Imhoff tank for large municipal installations. The Imhofftanks is best suited to small municipalities and large institutions where the tributary population is
6, or less, and a greater degree of treatment is not needed.
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Imhoff tank
The Imhoff tank , named for German engineer 9arl Imhoff +1*34:1;46, is a chamber suitablefor the reception and processing of sewage. It may be used for the clarification of sewage by
simple settling and sedimentation, along with anaerobic digestion of the extracted sludge. It
consists of an upper chamber in which sedimentation takes place, from which collected solids
slide down inclined bottom slopes to an entrance into a lower chamber in which the sludge is
collected and digested. The two chambers are otherwise unconnected, with sewage flowing only
through the upper sedimentation chamber and no flow of sewage in the lower digestion chamber.
The lower chamber re(uires separate biogas vents and pipes for the removal of digested sludge,
typically after 47; months of digestion. The Imhoff tank is in effect a two7story septic tank and
retains the septic tank<s simplicity while eliminating many of its drawbacks, which largely result
from the mixing of fresh sewage and septic sludge in the same chamber.
Imhoff tanks are being superseded in sewage treatment by plain sedimentation tanks using
mechanical methods for continuously collecting the sludge, which is moved to separate digestion
tanks. This arrangement permits both improved sedimentation results and better temperature
control in the digestion process, leading to a more rapid and complete digestion of the sludge.
% test for settleable solids in water, wastewater and stormwater uses an Imhoff cone, with or
without stopcock . The volume of solids is measured after a specified time period at the bottom
"rimary treatment is designed to remove organic and inorganic solids by the physical processes
of sedimentation and flotation. "rimary treatment devices reduce the velocity and disperse the
flow of wastewater. In primary treatment the velocity of flow is reduced to 1 to 2 feet per
minute to maintain a (uiescent condition so that the material denser than water will settle out and
material less dense than water will float to the surface. %pproximately 5 to 4 percent of the
suspended solids are removed from the waste stream +26 7 !6? -$& reduction. The solids that
remain in suspension as well as dissolved solids will usually be biochemically treated insubse(uent processes for physical separation and removal in the final +secondary settling tanks.
The si@e and number of primary tanks is dependent on the estimated wastewater flow and the
design detention time. Aenerally, a detention time of 2 to ! hours will provide a sufficient time
period for most particles to settle out. 0urther, the settling rate of a particle depends on the
strength and freshness of the wastewater being treated, the weight of the solid compared to the
specific gravity of water, the si@e and shape of the solid and the temperature of the water. 'ater
is more dense at lower temperaturesB therefore, the re(uired settling time increases. %s the
temperatures of the water increases, the re(uired settling time decreases. (ual distribution of
flow throughout the tank is critical. The greater the velocity in one area, the less the actual
detention time. #olids not having sufficient time to settle out will be discharged in the effluent.
"rinciple primary treatment devices are referred to as sedimentation tanks, primary tanks,
primary clarifiers or primary settling tanks, some of which have the further function of providing
an additional compartment for the decomposition of settled organic solids which is known as
sludge digestion. There are several types of primary tanks in use.
Septic Tanks
The septic tank was one of the earliest treatment devices developed. Currently, septic tanks
provide wastewater treatment for small populations, such as individual residences, small
institutions, schools, etc.
They are designed to hold wastewater at low velocity, under anaerobic conditions for minimum
detention time of !4 hours. &uring this period, a high removal of settleable solids is achieved.
These solids decompose in the bottom of the tank with the formation of gas which, entrained in
the solids, causes them to rise through the wastewater to the surface and lie as a scum layer until
the gas has escaped, after which the solids settle again. This continual flotation and resettling of
solids carries some of them in a current toward the outlet to be discharged with the effluent. The
final effluent disposal occurs by subsurface methods. The effectiveness of this method isdependent on the leaching ability of the soil.
These primary type units re(uire a minimum of attention which involves an annual inspection
and the periodic +! 7 6 years removal of sludge and scum accumulations.
Imhoff Tank
The Imhoff tank was developed to correct the two main defects of the septic tank.
1. It prevents the solids once removed from the sewage from again being mixed with it, but
still provides for the decomposition of these solids in the same unit
2. It provides an effluent amenable to further treatment.
Contact between the waste stream and the anaerobic digesting sludge is practically eliminated
and the holding period in primary settling compartment at the tank is reduced. The Imhoff tank
may be either circular or rectangular and is divided into three compartments
1. the upper section or sedimentation compartment
2. the lower section known as the digestion compartment and
!. the gas vent and scum section
It is desirable to be able to reverse the direction of flow to prevent excessive deposition of solids
at one end of the sedimentation compartment. "eriodically reversing the flow will result in an
even accumulation of sludge across the bottom of the tank. In operation, all of the wastewater
flows through the upper compartment. #olids settle to the bottom of this sloped compartment,
slide down and pass through an opening or slot to the digestion compartment. $ne of the bottom
slopes extends at least six inches beyond the slot. This forms a trap to prevent gas or digestingsludge particles in the lower section from entering the waste stream in the upper section. The
gas and any rising sludge particles are diverted to the gas vent and scum section.
Imhoff Tank Operation
There are no mechanical parts in an Imhoff tank. %ttention should, however, be given to the
following
a. &aily removal of grease, scum and floating solids from the sedimentation compartment.
b. 'eekly scraping of the sides and sloping bottoms of the sedimentation compartment by a
rubber s(ueegee to remove adhering solids which may decompose.
These tanks may be rectangular, circular or s(uare, but all operate on the same principle of
collecting the settled solids by slow7moving scrapers to the point of removal.
In the rectangular tanks the wastewater enters at one end and flows hori@ontally to the other
end. The scrapers +flights are attached by their ends to two parallel chains which pass oversprockets. The flights move slowly along the tank floor, pushing the settled solids to a sludge
hopper at the end of the tank. %t the same time, the partially submerged flights, in their return
path, push the floating solids, grease and oils +scum to a trough at the end of tank.
In the centerfeed circular tanks the wastewater enters at the center and flows radially and
generally hori@ontally to the periphery. The circular tanks have scraper arms attached to a
central motor7driven shaft. The bottom of the tanks are sloped toward the center and the
scrapers move the settled solids to a sludge hopper at the center. #kimmer arms, if present, are
attached tot he central shaft at the surface for the collection of floating solids, grease and oils
+scum.
In square tanks, the wastewater enters at the center and flows to the four sides. The scraper
mechanism is similar to that in the circular tanks. The maDor difference is that the rigid arms of
the scraper mechanism are e(uipped with pivoted corner blades which reach out into the four
corners of the tank and move the solids in these areas to the path of the circular mechanism.
Operation
Collection and removal of sludge from the sedimentation tank, as well as correct maintenance
procedures, are important factors in successful plant operation. The mechanical collectione(uipment can be run intermittently but is most often run on a continual basis. This prevents the
excess accumulation of solids on the tank bottom and reduces the load on the collector
mechanisms and thereby helps to prevent e(uipment damage. #olids left in the tank bottom too
long will gasify and rise to the surface, therefore, sludge should be removed from the tank as
often as necessary. This, in turn, is dependent on waste stream characteristics, volume of flow
and sludge (uality. The sludge removal schedule must be worked out for each plant by
observations and tests, keeping in mind that the obDective is to remove sludge at the proper rate,
at the proper rate, at the proper concentration and with the proper (uality for the receiving
process unit. Concentrating the sludge reduces the volume of water being pumped and
contributes to pumping efficiency. There are, however, pump design limitations to consider.
#easonal revision of this schedule will probably be necessary. In addition, scum and grease
1. Correct flow obstructions in the collection systemB
2. Godify, pretreat, reduce, or eliminate organic wastes, such as cannery, milk
processing, brewery, tannery, and organic textiles wastesB
!. "re7Chlorinate in the sewer systemB or ahead of sedimentation tank to reduce
decomposition of wastewaterB this is particularly effective when time of travel in sewer isunusually long or wastewater temperatures are high, or when certain industrial wastes are
presentB
5. "reaerate wastes of types listed in Ho. 2 aboveB or
6. Improve flow of wastewater system to reduce accumulations of solids.
0or strong recycled digester supernatant
1. Correct or improve sludge digestion to produce improved (uality supernatantB
2. )educe rate of withdrawal of supernatant to sedimentation tankB
!. #elect better (uality supernatant from another @one or delay withdrawal until (uality
improves, if that is possible.
5. ntil (uality improves, discharge part or all of supernatant to other points such as lagoon,
sludge drying bed, or aeration tank until (uality improves.
6. "resettle the supernatant.
C. %&cessi!e Sedimentation in Inlet Channels
Cause: elocity too low for cross7sectional area of channel at existing flow rates.
Prevention and Cure:
1. )educe cross7sectional area.
2. %gitate with air, water, or other means to prevent deposits.
&. %&cessi!e #ouling of Surfaces and 'eirs (ith 'aste(ater Solids or )ro(ths
Cause: %ccumulation of wastewater solids and resultant plant growth.
Prevention and Cure:
1. Gore fre(uent and thorough scrubbing of all surfaces in contact with wastewater.
. Intermittent Surging of #lo(
Cause: igh intermittent pumping rates.
Prevention and Cure:
1. %dDust controls for pump se(uence, pump combinations, andautomatic cut7in and cutoff water surface levels to maintain pumping rates close to rates of
0. $roken Scraper Chains and #requent Shear Pin #ailure
Cause: xcessive load on mechanical scraper.
Prevention and Cure:
1. "eriodically empty tank and examine all metal parts for defects and wearB
2. )eplace defective and worn parts, particularly hinge pins, chain links, and wear shoes, and
badly worn, broken, or warped scrapers +flightsB
!. Install grit chamber or otherwise reduce entrance of grit to the primary tanks if grit
accumulation is evidentB or
5. $perate collector mechanism for longer period andFor pump sludge more often.
A. Sludge *ard to +emo!e from *opper
Cause:
1. igh content of grit, clay, or other heavy compacted materialB or
2. 'ithdrawal lines too small.
Prevention and Cure:
1. )educe grit content by installing or modifying grit chamber, or locate and correct sources of
grit entering the wastewater systemB
2. Eoosen compacted material manually or by pressure Detting with air or water hoseB
!. -ack7flush clogged pipelinesB
5. "ump sludge more fre(uentlyB or
6. Godify sludge piping
Solids Accumulation
1. 0ollowing biological treatment +and, as an integral part of the secondary treatment process, is the secondary clarifier. The primary and secondary clarifier differ as to
physical location and to the density of the solids handled, as well as in the (uality of the
tank effluent. #econdary solids are usually less dense and secondary effluents are
normally of a higher (uality than those of primary clarifiers.
2. In both the primary and secondary clarifiers, the sludge +settled solids is scraped or
drawn to a hopper or sump for removal, treatment and disposal. The sludge removal can
occur separately and independently in each unit, or the sludge from the secondary
clarifier can be pumped to the primary clarifier or to a separate thickener unit and mixed
with the primary sludge. This combination of primary clarifier sludge and secondary
clarifier sludge is collectively referred to as waste sludge and will usually undergo further treatment for ultimate disposal. 0re(uency of solids removal is dependent on sludge
(uality as well as the design of the related sludge wasting e(uipment devices.
!. The actual sludge removal and disposal system +for the various types of clarifiers varies
from plant to plant. In addition, the floatable solids must be skimmed andFor pumped to
be disposed of separately or with the settled solids.
Clarifier %fficiency
1. To calculate the efficiency of any treatment unit it is necessary to measure influent andeffluent parameters. 0re(uently, the determination of unit efficiency is appraised in terms
of -$& and suspended solids removal.
2. The operator should also be aware that recycled loadings from digesters, vacuum filters,
etc. can impose severe organic and suspended solids loadings on primary clarifiers. The
strength and volume of these recycled waste streams must be added to the influent
wastewater loadings in order to properly calculate unit process efficiencies.
Typical Clarifier fficiencies
1. Eisted below are some range values for primary clarifier efficiencies
+emo!al %fficiency
-iochemical $xygen &emand 26? 7 !6?
#ettleable #olids ;? 7 ;6?
#uspended #olids 5? 7 4?
Total #olids 1? 7 16?
2. Clarifier efficiencies vary depending on influent flow rate and wastewater characteristics
as well as in7plant conditions. The most important structural elements effecting clarifier
efficiency are inlet flow distribution and effluent weir placement. %lthough effluent weir
configurations are often beyond the control of the operator, there are some minor
alterations that can be made to improve the influent flow patterns so that the affect ofshort7circuiting is minimi@ed. These remedial alterations would include the installation
of target baffles or the installation or relocation of a maDor flow baffle.
!eir "#erflo$ %ate
1. $utlet weirs vary greatly in design. They provide for uniformed removal of clarifier
Chemical treatment is included under primary treatment because it involves chemical and
physical processes as distinct from the biological processes which are the basis of secondary
treatment. Chemical addition has application +1 in the treatment of industrial wastes which
are sometimes difficult to decompose by biological means, +2 where discharge to receiving
waters re(uire a higher degree of treatment than primary units can achieve, but secondary
treatment is not warrantedB +! after secondary treatment when a higher degree of treatment is
demanded +often referred to as tertiary treatment.
The exact point of chemical application is dependent on design considerations as well as permit
re(uirements. 'e are concerned, here, with chemical treatment and its application to primary
treatment.
Chemical treatment involves the addition of one or more chemicals to wastewater to produce
floc, which is an insoluble chemical compound that adsorbs colloidal matter, enmeshes non7
settleable suspended solids and settles more readily. The precipitating chemical also dissociates
or ioni@es in the wastewater and neutrali@es the electric charges held by colloidal particles
causing them to coagulate to form larger readily settleable solids. The chemicals most widely
used are aluminum sulfate +alum, ferric chloride, lime and polymers.
% chemical treatment plant usually has the following features
1. "reliminary &evices 7 screens, grit chambers, etc.
2. Chemical 0eeders
!. 0lash Gixing nits
5. 0locculation Tanks
6. #edimentation Tanks such as have already been described
4. Increased 0acilities for the Treatment and &isposal of #ludge
Chemical #eeders
% large variety of units to feed chemicals, either dry or in solution, in controlled amounts, are
made by a number of manufacturers.
Mi&ing ,nits
The chemicals, when added to sewage, must be thoroughly and (uickly mixed with it to provide
complete and uniform reactions. This is accomplished by violent agitation for a short period of
time either by mechanical or hydraulic methods. This agitation is carried on in special tanks, insections of other tanks, or in the piping system. #uch mixing devices are made by a number of
%fter the chemical is mixed with sewage it is gently agitated for 16 to ! minutes to foster the
coagulation of particles. If -$& reduction is desired, the agitation time may be increased to 56minutes. The colloidal and suspended solids meet and adhere together in large flocculant masses
which settle readily in the sedimentation tank. &ifferent types of e(uipment to accomplish this
purpose are made by a number of manufacturers and must be designed to operate at varying
speeds as determined by the actual process characteristics.
Sludge
The volume of sludge obtained by chemical treatment is greater than with standard primary
treatment, necessitating a comparable increase in the capacity of the sludge handling facilitiesand in the cost of sludge treatment and disposal.
%fficiency
Chemical treatment can effect a reduction up to ; percent in suspended solids and up to 3
percent in the -$&. It is well adapted to intermittent operation and has value in sewage
treatment to reduce pollution of streams during periods of low flow or to lessen pollution of
bathing beaches and recreational waters during months when these facilities are in use. It is of
value also for the treatment of sewage containing high concentrations of industrial wastes which
will inhibit biological life and interfere with secondary treatment processes. $perational costs
are high due to increase in operator<s time, chemical usage and the (uantities of sludge to be
treated and disposed of.
"essons Inde&
0or lessons 4 : 2, answer the ven number &iscussion and )eview (uestions in your textbooks,
'astewater Treatment "lant $perations ols. I and II. "lease note that there are no &iscussionand )eview (uestions for Chapter 1; but your assignment for this chapter is to write a Gonthly
)eport. )efer to page 435 ol. II, as a reference. 'hen you have completed the assignment for
each chapter, email, mail or fax them assignment to your instructor.