Duncan Mara_Morocco Mission Report_19-24January2010 Anglais

THE WORLD BANKWater Unit

From: Duncan Mara (Consultant)To: Mr. Alex Bakalian (Middle East and North Africa Region), Mr. Peter Kolsky (Energy, Transport and Water Department)cc: Mr. Augustin Maria (Middle East and North Africa Region), Ms Maryanne Leblanc (Energy, Transport and Water Department)Date: January 30, 2010

Subject: MOROCCO : Technical Support for the Management and Design of Wastewater Treatment Works (Waste Stabilization Ponds)

INTRODUCTIONIn accordance with my Terms of Reference dated January 6, 2010, I visited Morocco during January 19−24 for discussions with the Direction de l’Assainissement et de l’Environnement of the Organisation National de l’Eau Potable (ONEP) in Rabat on the waste stabilization pond (WSP) component of the World Bank-funded Oum Er Rbia Sanitation Project (P098459). A list of persons met is given in Annex I.

MISSION ACTIVITIES

1. Mission briefing and preliminary discussions

ONEP confirmed that its two major problems were (a) to eliminate odor release from anaerobic ponds, and (b) to identify the most appropriate means of designing WSP systems which would operate satisfactorily in all parts of the country.

2. Field visits

Visits to the WSP systems at Ouarzazate and Kalaa M’Gouna were made on January 21.

►OuarzazateThe WSP system at Ouarzazate comprises three anaerobic ponds and six facultative ponds, with provision for an additional anaerobic pond and two additional facultative ponds. The anaerobic pond effluents are mixed together and then spilt into equal parts for discharge into the facultative ponds. The pond system was constructed to a very high standard and its current performance is very satisfactory in terms of BOD removal, but it reportedly suffers from serious odor (H2S) problems. However, at the times of our visits (around 2 pm and 8 pm on January 21) there were no discernable odors – nor at the airport (immediately adjacent to the ponds) at 6 am the following day.

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It was clear at the times of our visits that the ‘anaerobic’ ponds were not fully anaerobic as they were green in color (Figure 1). This suggested that they were receiving well below their design flow and BOD load and were consequentially operating as overloaded facultative ponds, rather than strictly anaerobic ponds (see section 3 below). In contrast, the facultative ponds, which were gray/brown in color (Figure 2), appeared to be working satisfactorily with a healthy algal population − which was apparent only after throwing a small stone into the pond: the green color, characteristic of algae, immediately appeared (Figure 3).

Figure 1: Close-up of one of the six ‘anaerobic’ ponds at Ouarzazate showing the high concentration of algae present.

Figure 2: One of the six facultative ponds at Ouarz-azate characterized by an absence of green coloration.

Figure 3: Close-up of the facultative pond shown in Fig. 2 after a small stone had been thrown into the pond, showing that the algae, now clearly visible, had been located below the pond surface.

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►Kalaa M’GounaThe WSP system at Kalaa M’Gouna comprises two anaerobic ponds and two facultative ponds (Figures 4 and 5). The anaerobic pond effluents are mixed together and then spilt into equal parts for discharge into the facultative ponds. As at Ouarzazate, the Kalaa M’Gouna pond system was constructed to a very high standard and its current performance is also very satisfactory in terms of BOD removal, but it also reportedly suffers from serious odor (H2S) problems, especially in the summer (June−September). However, at the times of our visits (around 4 pm on January 21) there was no discernable H2S odor, only a slight, but inoffensive, ‘wastewater’ odor. Biogas bubbles were clearly seen rising to the surface of the two anaerobic ponds (and these were not green in color).

The WSP system at Kalaa M’Gouna is adjacent to housing areas and two schools. We asked a group of half a dozen 14−16 year old schoolgirls if they noticed any odor from the treatment works and they replied that there was an odor of “bad eggs” (i.e., H2S) in summer during the early mornings and late afternoons.

Figure 4: One of the two anaerobic ponds at Kalaa M’Gouna.

Figure 5: One of the two facultative ponds at Kalaa M’Gouna.

3. Post-visit analyses

►Ouarzazate WSP

(a) Design flow and load:The design flow and BOD load for the existing WSP at Ouarzazate are 9011 m3/day and 3545 kg/day, respectively, giving a BOD concentration in the raw wastewater of 393 mg/L. The

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volume of each of the three anaerobic ponds is 15,900 m3, so the volumetric BOD loading on the anaerobic ponds at the design flow and load (λV) is:

= 74 g/m3 day

This value is too low for a design (January) temperature of 9°C: a value of 100 g/m 3 day is used for design temperatures of ≤10°C.

(b) Current flow and load:The current flow to the Ouarzazate WSP is ~4600m3/day and the BOD concentration in the raw wastewater is ~276 mg/L. Thus the volumetric BOD loading on the anaerobic ponds is currently:

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3

V m15,9003

/daym4600 mg/L276λ

= 27 g/m3 day

which is a very low value. The corresponding surface BOD loading (λS) is:

λS =

m3.4

m159003

/daym4600mg/L276103

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= 900 kg/ha day

where 3.4 m is the wastewater depth in the ponds. The surface loading of 900 kg/ha day is very high: a facultative pond would be designed to receive a loading of only 90 kg/ha day for a design temperature of 9°C.

(c) Only one anaerobic pond in operation:The volumetric loading on only one anaerobic pond would currently be:

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3

V m15,900

/daym4600 mg/L276λ

= 80 g/m3 day

This is a much more satisfactory value.

It is worth noting that it is stated in the 1998 Design Manual for Waste Stabilization Ponds in Mediterranean Countries (details in Annex II), “if, due to an initially low rate of sewer connections in newly sewered towns the sewage is weak or its flow low, it is best to by-pass the anaerobic ponds until the sewage strength and flow is such that a loading of at least 50 g/m 3 d can be applied to them”. In the case of the Ouarzazate WSP it would be better to by-pass only two of the three anaerobic ponds as the current flow and load are such that the one remaining anaerobic pond would receive a loading of >50 g/m3 day.

►Kalaa M’Gouna WSP

(a) Design flow and load:The design flow and BOD load for the existing WSP at Kalaa M’Gouna are 613 m3/day and 376 kg/day, respectively, giving a BOD concentration in the raw wastewater of 613 mg/L. The

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combined volume of the two anaerobic ponds is 3814 m3, so the volumetric BOD loading on the anaerobic ponds at the design flow and load is:

3V m8143

g/day376,000λ = 100 g/m3 day

which is perfectly satisfactory for a design temperature of 9°C.

(b) Current flow and load:The current flow to the Kalaa M’Gouna WSP is ~350m3/day and the BOD concentration in the raw wastewater is ~467 mg/L. Thus the volumetric BOD loading on the anaerobic ponds is currently:

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3

V m8143

/daym350 mg/L467λ

= 42 g/m3 day

which is low (but not as low as that on the Ouarzazate ponds).

(c) No anaerobic pond in operation:If both anaerobic ponds were by-passed, the surface BOD loading on the two facultative ponds, each of which has an area of 5152 m2, would currently be:

λS = 2

3

m15252

/daym350mg/L46710

= ~160 kg/ha day

This is too high for a design temperature of 9°C, so the best option would be to leave the two anaerobic ponds in operation (despite the volumetric BOD loading on them currently being 42 g/m3 day, rather than being >50 g/m3 day).

►Odor releaseOdor release from anaerobic ponds, also from overloaded facultative ponds, is principally due to the release of hydrogen sulfide gas (H2S) formed from the reduction of sulfates in the raw wastewater under anaerobic conditions by the obligately anaerobic sulfate-reducing bacteria (such as Desulfovibrio spp.). The sulfides so produced co-exist in two forms in aqueous solutions: dissolved H2S gas and the bisulfide ion (HS−), with their proportions being pH-dependent. Odor is released as some of the dissolved H2S molecules escape from the pond to increase the partial pressure of the H2S gas in the atmosphere above the ponds (Henry’s law). Values of pH lower than 7 favor H2S rather than HS−, but the pH of anaerobic ponds treating domestic/municipal wastewater is usually in the range 7.2−7.8.

The sources of sulfates in wastewater are (a) the sulfates in the local drinking water and (b) domestic detergents which can contain up to 40% by weight of sodium sulfate. In Ouarzazate, for example, the sulfate concentration in the local drinking water is 320 mg SO 4/L, which is lower than the 400 mg/L permitted in Morocco (whereas the US EPA, EU and WHO maximum permitted levels are all 250 mg/L). However, early work in Texas found that odor release was

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not a problem if the raw wastewater contained <500 mg SO4/L,1 so it is always recommended that wastewater sulfate concentrations are measured, especially if the sulfate concentrations in the local drinking water are high.

As no odor release occurred at the times of the field visits , it is difficult to say more at this stage into the reasons for the reported odor releases from the WSP at Ouarzazate and Kalaa M’Gouna – see ‘Plans for Next Mission’ below.

►WSP effluent reuseONEP explained that its position was that it should only treat wastewater to current discharge standards (presently a BOD of ≤120 mg/L); treatment to reuse standards was not its responsibility, although it would work with other agencies who wished to pay for additional treatment for reuse (e.g., maturation ponds). However, due to the large differences between winter and summer temperatures in Morocco, the alternative strategy shown in Figure 6 is especially attractive: essentially one of a pair of facultative ponds is used as a maturation pond in summer; costs are reduced and the effluent is suitable for restricted irrigation (i.e., the irrigation of all crops that are not eaten uncooked).

4. Presentation of field-visit findings and preliminary recommendations to ONEP

On January 23 a presentation on the mission findings and preliminary recommendations were made to ONEP. These were, together with additional recommendations, were as follows:

presentation of field-visit findings and post-visit analyses, as detailed above; temporary decommissioning of two of the three anaerobic ponds at Ouarzazate; measurement of the sulfate concentration in raw wastewaters; and discussion of options for odor control – for example, covering the anaerobic pond,2 and

using high-rate anaerobic ponds;3

A further recommendation is that ONEP should measure H2S concentrations in the air near reportedly odorous anaerobic ponds using a hand-held H2S monitor.4

[The opportunity was taken at this meeting on January 23 to make a short presentation on Brazilian condominial sewerage to familiarize ONEP with this wastewater-collection technology prior to the forthcoming study visit to Brazil.]

1Gloyna, E. F. and Espino, E. (1969). Sulfide production in waste stabilization ponds. Journal of the Sanitary Engineering Division, American Society of Civil Engineers 95 (SA3), 607–628.

2As, for example, at the WSP systems in Melbourne (details in the Water Science and Technology paper by DeGarie and others listed in Annex II).

3High-rate anaerobic ponds combine the advantages of upflow anaerobic sludge blanket (UASB) reactors and the simplicity of anaerobic ponds (details in Annex III).

4Several models are available – see, for example, http://www.professionalequipment.com/hydrogen-sulfide/.

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Winter operation: two series of anaerobic and facultative ponds.

Summer operation: one of the two facultative ponds receives the effluent of both anaerobic ponds and the other receives the effluent of this facultative pond, so acting as a maturation pond. Note: the elevation of the base of the maturation pond must be below that of that of the facultative pond to permit gravity flow between the two in summer.

Figure 6: Alternative strategy for the production of effluents suitable for restricted irrigation.

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Anaerobic ponds

Anaerobic ponds

Facultative ponds

Facultative pondMaturation pond

PLANS FOR NEXT MISSION

It was agreed with Mme Bourziza (Chef de la Division Suivi des Etudes Décentralisées, Direction de l’Assainissment et de l’Environnement, ONEP) that my next mission should be after the ONEP condominial-sewerage study visit to Brazil which is currently scheduled (but not yet confirmed) for 20 February – 2 March; thus the weeks of 22 and 29 March may be possible (to be confirmed with World Bank and ONEP) (earlier dates in March are not possible due to other commitments).

Prior to this second mission I would: review the report Capitalisation de l’Expérience ONEP dans le Domaine de l’Epuration

− Cas de 5 STEP (detailed in Annex II) ; and prepare, and send to World Bank and ONEP for comment, a draft document setting out

the WSP design and commissioning procedures to be followed by consulting engineers preparing WSP designs for medium-sized cities and towns in Morocco under contract to ONEP. This would essentially be a “to do” and “how to do” list of WSP design and commissioning procedures which ONEP’s consulting engineers would have to demonstrate that they had followed and would follow, or justify to ONEP why they had not or would (or could) not. This document would also explain in detail the reuse strategy presented in Figure 6 and other reuse options.

Also prior to the second mission it is recommended that ONEP purchase a hand-held H2S monitor, so that H2S concentrations in the air near odorous anaerobic ponds can be ascertained.

During the second mission (of ~10 days’/2 weeks’ duration) I would: complete the review of the report Capitalisation de l’Expérience ONEP dans le Domaine

de l’Epuration − Cas de 5 STEP; finalize the WSP design and commissioning procedures document and ensure that ONEP

fully comprehends its contents so that it can be properly translated into French; investigate further the anaerobic-pond odor problem and control options – and, if

anaerobic ponds are to be covered, means of measuring the biogas collected and burnt so that ONEP can apply for carbon credits;

visit additional WSP sites as required by ONEP and using, as appropriate, the hand-held H2S monitor;

discuss further WSP effluent reuse options; and review alternative wastewater treatment technologies for small communities in Morocco

– for example, constructed wetlands and other low-energy options.

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ANNEX IPage 1 of 1

LIST OF PERSONS MET

ONEP, Rabat:

Mme. Fadila Ben Osman (Directeur, Direction de l’Assainissement et de l’Environnement)

Mme. Mahjouba Bourziza (Chef de la Division Suivi des Etudes Décentralisées, Direction de l’Assainissement et de l’Environnement)

M. Rachid Lahnine (Chef, Etudes Spécifiques, Direction de l’Assainissement et de l’Environne-ment)

M. Yassir Lambarki Dae/sn (Chef, Etudes Décentralisées Nord, Direction de l’Assainissement et de l’Environnement)

M. Stili Abdelaziz (Chef, Etudes Décentralisées Sud, Direction de l’Assainissement et de l’Environnement)

M. Mrhari Saad (Ingénieur Génie Rural, Direction de l’Assainissement et de l’Environnement)

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ANNEX IIPage 1 of 1

A. ONEP REPORTS RECEIVED

Assistance Technique pour la Réalisation du Projet d’Assainissement de la Ville d’Ouarzazate – Mission 1, Actualisation de l’APD: Lot – Station d’Epuration (EWI-Maroc, 2003)

Capitalisation de l’Expérience ONEP dans le Domaine de l’Epuration − Cas de 5 STEP: Tâche 1, Diagnostic du Fonctionnement des STEP et Recommandations pour l’Amélioration, Parties I−III (Fichter Water & Transportation, Freiburg, 2008) (as pdf files). [Note: this updates the 1994 CEREMHER report.]

Schéma Directeur National d’Assainissement Liquide – Document Final SDNAL Synthèse (Rapport Minute) (Ministère de l’Intérieur, 1997)

B. ELECTRONIC DOCUMENTS GIVEN TO ONEP

Guidelines for the Hydraulic Design of Waste Stabilisation Ponds (A. Shilton & J. Harrison, Massey University, New Zealand, 2003)

Design Manual for Waste Stabilization Ponds in Mediterranean Countries (D. Mara & H. Pearson, Lagoon Technology International Ltd, Leeds, 1998)

Thematic Overview Paper: Waste Stabilisation Ponds (M. Peña Varón & D. Mara, IRC International Water and Sanitation Centre, Den Haag, 2004)

Domestic Wastewater Treatment in Developing Countries (D. Mara, Earthscan Publications, London, 2004)

Floating geomembrane covers for odour control and biogas collection and utilization in municipal lagoons (C. J. DeGarie and others, Water Science and Technology 2000: 42 (10–11), 291–298)

►ONEP was also given the URL of my personal website (which is essentially an online library on wastewater treatment, especially in WSP, and reuse and many other aspects of environmental

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health engineering in developing countries, including inter alia condominial sewerage): www.personal.leeds.ac.uk/~cen6ddm.

ANNEX IIIPage 1 of 1

HIGH-RATE ANAEROBIC PONDS

High-rate anaerobic ponds (HRAnP) were developed in Colombia by Professor Miguel Peña Varón.* They are in two parts: a deep digestion pit where the untreated wastewater enters at the base, and a shallower quiescent settlement zone to allow any suspended solids to settle; the two parts are separated by a plastic mesh to minimize solids carry-over from the digestion pit to the settlement zone. They are covered, so the biogas can be collected. They thus combine the advantages of UASB reactors and the simplicity of anaerobic ponds, and in warm climates they can achieve 80% BOD removal at a hydraulic retention time of 12 hours.

Currently the only full-scale HRAnP is at Cerrito (population: ~50,000), near Cali in southwest Colombia (Figures A1 and A2).

Figure A1: Cover over the digestion pit of the HRAnP at Cerrito, southwest Colombia.

Figure A2: The settlement zone (left) and the digestion pit (right) of the HRAnP at Cerrito.

_________________________*(a) M. R. Peña Varón (2002). Advanced Primary Treatment of Domestic Wastewater in Tropical Countries: Development of High-rate Anaerobic Ponds (PhD thesis). Leeds: University of Leeds (available at: http://www.personal.leeds.ac.uk/~cen6ddm/ThesisPenaVaron.html).

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(b) M. R. Peña Varón and D. D. Mara (2006). High-rate anaerobic pond concept for domestic wastewater treatment: results from pilot-scale experience. In Aqua 2003: Multiple Uses of Water for Life and Sustainable Development (ed. M. R. Peña Varón, I. Restrepo, D. D. Mara and H. Gijzen), pp. 137−142. London: IWA Publishing.

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