Epuration des eaux usées par lagunage à microphytes et à macrophytes en Afrique de l'Ouest et du Centre

Stabilization ponds and water lettuce-based systems in West and Central Africa - State of the art, removal performances and design criteria Despite several projects undertaken so far, sanitation remains a huge challenge in West and Central Africa. Big cities are growing without sustainable solution for sanitation planning. Sewage systems and wastewater treatment plants are underdeveloped or absent. Stabilization ponds have been introduced 30 years ago, but this low-cost technology remains marginal in wastewater treatment systems. The climate context is favourable to develop a set of low-cost wastewater treatment plants, particularly macrophyte-based systems. Pilot scale studies show the enormous potentialities of such sustainable technologies for water pollution control and treatment. None of the full scale applications works and this is due to the low economic level and the lack of political support. Another important reason is the lack of training and research. This situation requires the development of a collaborative network in which African researchers can share knowledge and promote sustainable wastewater treatment plants, as sanitation demand is growing exponentially in conjunction with the population growth in the cities. Stabilization ponds account for only 7% of the whole wastewater treatment technologies built in West and Central Africa, against 75% of activated sludge processes that in most cases failed. In spite of their bad quality, the effluents are widely reused in urban agriculture. Experimental data showed a maximum removal rate of 30% and 60% for COD and BOD5, respectively, and a maximum loading rate of 500 kg BOD/ha/d. Macrophytes-based systems are even more scarce (3%). The link of this technology with potential risks of malaria has slowed down its development. Nevertheless, data from studies in Niamey and Yaounde, and from rice culture irrigation zones showed that this correlation is not significant. Little research is done on wastewater treatment technologies for local application. This is one more reason why this study was based at EIER in Ouagadougou (Burkina Faso). The aim of this work was to determine the parameters for optimal removal performance and to elucidate design criteria of water lettuce-based wastewater treatment systems, possibly in combination with stabilization ponds to promote restrictive irrigation in market gardening. In secondary treatment, the maximum admissible loading rate was found to be 500 kg BOD5/ha/d (400 mg O2/l). Above this value, sludge accumulation at the pond surface caused by intensive methanogenic degradation of the sediments lead to plants die-off. COD and BOD5 removal rate was not correlated to redox state or dissolved oxygen concentrations. This indicated that the removal was mainly due to settlement and trapping of suspended solids in roots and sediments. Aerobic degradation remains low because the extent of oxygen release by plants cannot explain the yield obtained. COD and BOD5 removal performance reaches an optimum rate of 75% and 85% in two weeks hydraulic retention time, respectively. Regression equations between applied (λappl) and removed (λrem) loads is expressed by λrem (COD) = 0.75λappl (COD) – 10.4, (r2 = 0.99). The first order kinetic constant for BOD5 removal, kT = 0.14 d-1 (or kT = 0.11 d-1 for the DCO) can be used for designing of treatment ponds with the equation from the International Water Association (IWA). From the total nitrogen entering the treatment ponds, depending on the nitrogen load, between 44% and 60% were removed in the configuration with three ponds and a HRT of 18 days. Nitrogen can be removed by plant uptake with the harvest, by settlement or trapping with the suspended solid or by elementary nitrogen formation which occurs by the combination of nitrification, denitrification, and anaerobic ammonium oxidation. Nitrogen removal by plants was estimated to account for 10% to 39% in total nitrogen removed from ponds with a HRT of 18 days based on the steady growing and nitrogen uptake rate found. With a weekly harvesting rate of 50% of total plant biomass in the ponds, the water lettuce growing rate and the nitrogen uptake were 50 ± 1 g dry weight/m2 (182.5 t dry weight/ha/year) and 0.36 g N/m2/d (1314 kg N/ha/year), respectively. Trapping and settling of organic nitrogen present mainly in the suspended solids fraction removed about 25-48% of total nitrogen. It was found that water lettuce-based treatment systems conserved quite large amounts of ammonium in the effluents, a fact that is favourable for irrigation water reuse. Nevertheless, nitrogen loss by nitrification-denitrification occurred in water lettuce ponds, and perhaps even anaerobic ammonium oxidation. Nitrification became visible when COD dropped below 140 mg O2/l. With nitrogen loading varying from 31 to 97 kg N-NH4+/ha/d, the ammonium removal rate varied from 15-40% with a HRT of 18 days and from 20-60% with a HRT of 21 days. Hence, 40-80% of the ammonium was conserved in the water lettuce-based systems. In addition to conserve ammonium, water lettuce ponds also perform better than facultative ponds in organic pollutant removal, 75% against 50% in DCO reduction, respectively. When discharging the facultative pond effluent in a water lettuce pond, removal performance of the combined (upgraded) system is equal to that of a water lettuce pond. Faecal coliform removal in the two systems is very close, three weeks were necessary to ensure a reduction of three logarithmic units in faecal coliform concentration. To reach the objective of irrigation reuse, water lettuce-based treatment plants should not significantly affect the availability of the treated water. It was shown that the additional water loss due to the presence of Pistia stratiotes is lower than 10% of the average evaporation rate of microphytes ponds. Based on the results obtained during this study the following recommendations can be made. If the surface occupied by a plant is a limiting parameter, the footprint of the ponds can be decreased by designing a water lettuce based system with ponds depth bigger than the recommended 70 cm. This depth has been proposed to promote oxygen release in ponds, which was supposed to support aerobic oxidation. According to our results, organic matter removal is not correlated to oxygen availability. In addition, increasing the pond depth could also increase the ammonium conserved in the effluent. For restrictive irrigation, in market gardening, a hybrid system combining facultative ponds with water lettuce ponds is suitable, not only to secure the effluent quality, but also to reduce constrains related to plant management. During this thesis, the beginning of a network for collaboration has been created between members of different research teams including the Abdou-Moumouni University in Niamey, the Cheick Anta Diop University of Dakar, the University of Yaounde and the Ecole Inter-Etats d'Ingénieurs de l'Equipement Rural (Ouagadougou) in Burkina Faso. Several topics investigated during this study should be studied in more detail, and this best within the framework of this network of collaboration in West and Central Africa.

Recyclage des eaux usées en irrigation

Yeli Mariam Sou

The present study takes place in Ouagadougou (Burkina Faso), a typical Sub-Saharan city where water shortage and food crisis necessitate wastewater reuse in urban agriculture. Different issues linked to this practice were studied in two components separated in two experimental sites. The first part treats about agronomic and sanitary aspects and take place in the 2iE (International Institute for Water and Environmental Engineering) experimental site of Ouagadougou. The second component was emphasised on industrial effluents impacts on irrigated soil quality and takes place in Kossodo, an industrial suburb of Ouagadougou. The 2iE site is supply with treated wastewater from domestic origin. The effluent was used to realise the following objectives: (1) managed wastewater fertilizers to fit the plant needs; (2) assess different crops sanitary quality and the effectiveness of methods used for this assessment. Three treatments were tested: (i) irrigation with the treated wastewater (ii) irrigation with the treated wastewater and fertilizers management, adjusted to plant needs and (iii) conventional treatment i.e. fresh water irrigation and total doses of commercial fertilizers. Each treatment was tested on three different crops: lettuce (Lactuca sativa), carrot (Daucus carota) and eggplant (Solanum melongena). The first objective was achieved by comparing crops yields (on basis of wet and dry matter) obtained with the different treatments. The three crops, irrigated with only wastewater, led to yield reduction between the first and the second year. At the contrary, the adjusted treatment showed higher yield for lettuce, with comparable values to conventional treatment yield. However, the effects on carrot and eggplant did not demonstrate better results; indicating that further investigations must be completed for crops with longer growing periods. Most likely, the fertilizers management should better correspond to the crop period demand. The sanitary assessment (second objective) was performed by measuring the heavy metals concentration (Cd, Cu, Pb, Ni, Cr) and the faecal levels (faecal coliform, Escherichia coli and four pathogens among which Salmonella typhimirium and Vibrio cholerae). Chromocult Coliform Agar medium was used for simultaneous detection of faecal coliform and Escherichia coli whereas pathogens were analysed by Polymerase Chain Reaction (PCR). From methodological point of view, faecal coliform were not sufficiently discriminatory in the case of this study since they were naturally present in the soil as well as in the fresh water. At the contrary, Escherichia coli were better linked to wastewater contamination. However, some "false positive" results were detected meaning that each presumptive colony of Escherichia coli must be confirmed. Nevertheless, microbial quality assessment indicated no Escherichia coli on edible parts of carrot or eggplant irrigated with wastewater. Carrots were expected to have the poorest microbiological quality. Therefore carrot root bacterial quality must be further examined. Results on eggplant are not surprising as these vegetables are somewhat far from the ground and they receive significant solar radiation which acts like a disinfectant. Lettuce from wastewater treatment plots, however, exhibited presence of Escherichia coli, meaning that lettuce is likely to be contaminated by pathogens. However, no pathogens were detected neither on crops irrigated with fresh water nor on those irrigated with wastewater. Heavy metals results demonstrated that the crop's consumption did not involve higher risk for consumers when the vegetables were irrigated with domestic wastewater, in comparison with fresh water irrigation. Kossodo treatment plant receives mainly industrial wastewater, which are characterized by high levels of sodium and bicarbonates. The treated wastewater supplies a market garden in which space has been defined for the study of this second component. The main objective was to assess the impact of treated wastewater on soil quality during two years of irrigation. Early in the second year, the presence of black spots, suspected to be black alkali (dissolved organic matter), appeared at the surface of plots irrigated with treated wastewater. These indications led to put forward the hypothesis of alkalinisation/sodisation on soil receiving wastewater. The methodological approach used was based on in situ measurement of infiltration at the end of the second year of irrigation, supplemented by shrinkage curve modelling and chemical analysis. The hypothesis of an alkalinization/sodisation and dissolved organic matter (black alkali) presence in plots irrigated with treated wastewater were validated by the chemical analysis. Shrinkage curve modelling showed a reduction of the structural porosity on wastewater irrigated plots. The reduction was more marked in the subsurface horizons than in surface, which is probably caused by dispersed clay leached in depth. These structural changes were confirmed in situ by an infiltration capacity greatly reduced on plots irrigated with treated wastewater, compared to control plots (irrigated with fresh water) where the infiltration capacity is higher and comparable to that of non-irrigated soil. The study confirmed the fertilizing value of domestic wastewater while emphasizing the need to concentrate future investigations on sustainable effluent/fertilizers management. The sanitary assessment with different crops regarding their respective distances to the soil was very useful since it showed the strong interaction of soil microbial community on the crop microbial quality. It also revealed the importance to determine a faecal indicator for the crop, specifically related to wastewater contamination. Soil quality assessment reiterated the necessity to verify wastewater quality before its reuse in agriculture. In the case of this study, the sodic and alkaline properties of the industrial effluents make them definitively unfit for irrigation.

EPFL2009

Origines et flux de biocides et de filtres UV dans les stations d'épuration des eaux usées

Cécile Plagellat

Adverse effects on the environment have become a matter of increasing concern considering the increase of industrialization and of the amount of commercialized chemicals. At present, approximately 100'000 substances are on the market, 3000 are sold at high volumes. In Europe, 200 to 300 new substances are authorized every year (in Switzerland, approximately 50). Contamination is induced by consumption of numerous products frequently applied by private or professional users. Persistence and ecotoxicity of certain organic compounds contained in these products might have impacts on the quality of our environment. Knowledge on potential sources of these substances is the first step in order to limit their release. The project developed within the present PhD thesis aims at determining the sources of certain compounds by applying material flux analysis on the basis of sewage sludge. Presently, a large number of anthropogenic organic compounds originating from private households, craft industry, industry and stormwater is disposed of into the collector system and ends up in sewage sludge. Carbendazim, diuron, Irgarol 1051®, octhilinone, permethrin, tributyltin (TBT) and triphenyltin (TPT) comprise numerous applications as biocides and pesticides whilst methylbenzylidene camphor (4-MBC), octyl methoxy cinnamate (OMC), octocrylene (OC) and octyl triazone (OT) are used as UV-screens (type UVB) in a large number of cosmetic products. Their toxic effects on non-target organisms and their potential for bioaccumulation give rise to environmental concern. These substances are used as ingredients in products of daily use and are disposed of into wastewater. Due to their physico-chemical properties they are more or less lipophilic and are likely to persist in sewage sludge. On the basis of the analysis of sewage sludge samples from well characterized sites (catchments and wastewater treatment plants, WWTPs) it is possible to determine the sources (private households, craft industry, industry and stormwater). The first step of this study was the development of analytical methods which allow for quantification of these compounds in sewage sludge. Due to their different physico-chemical properties, four different methods were required. These methods have recovery rates ranging between 75 and 106 % and limits of detection in the order of μg/kg dry matter. The analysis of sludge from 12 sites obtained within 2 sampling periods show that most of the substances are present in all samples. Mean concentrations were 6,8 μg/kg dm (dry matter) for carbendazim, 9,5 μg/ kg d.m. for diuron, 98,1 μg/kg d.m. for permethrin, 148 μg/kg d.m. for TBT, 1777 μg/kg d.m.for 4-MBC, 110 μg/kg d.m. for OMC, 4834 μg/kg d.m. for OC and 5517 μg/kg d.m. for OT. Octhilinone was not detected whilst Irgarol 1051® and TPT were found in 7 and 11 samples, respectively. The transfer of these compounds from wastewater into sludge is an important parameter for material flux analysis on the basis of sewage sludge. A study on the fate of carbendazime and permethrin during wastewater treatment has been carried out on two WWTPs. Whilst removal of permethrin was high (>94%) up to 70% of carbendazime present in sewage were found in treatment plant effluents. Adsorption is an important process for removal of permethrin. However, only 4 to 15 % of the load in wastewater was transferred to sewage sludge. Carbendazim is mainly associated to the aqueous phase and thus, a very small quantity ends up in sludge (approximately 1%). Therefore, the loads found in sewage sludge are more or less representative for consumption of the compounds in the catchments according to their physico-chemical properties. Additionally, it has been shown that the part of permethrine ending up in sludge depends on the treatment techniques of the WWTPs. Thus, for comparison of the pollutant loads in sewage sludge between different sites their fate in the WWTPs has to be known. The loads per inhabitant of the three well characterized types of sites have been calculated in order to determine the sources of the substances. Type A comprises sites with a separate sewer system and a catchment where mainly private households are present. Type B has a combined sewer system collecting sewage from private households, craft industry and stormwater whilst type C comprises a combined sewer system which includes effluents from private households and industry as well as stormwater. The results are as follows: Stromwater is considered as main source of carbendazim in wastewater whilst diuron originates mainly from stormwater and from industrial effluents. Private households contribute significantly to the loads of permethrine and TBT in sewage. For UV-screens the loads from private households are also important but stormwater and industrial effluents are likely to be potential sources as well. A difference in consumption between rural (type A) and more urban sites (type B and C) is possible which might hamper a proper estimate of the sources of UV-screens.

EPFL2004

Treatment of micropollutants in municipal wastewater: study of a sequential batch biofilter

Andreu Contijoch Culleré

A fungal biofilter composed of woodchips was designed in order to remove micropollutants from the effluents of waste water treatment plants. Two fungi were tested: Pleurotus ostreatus and Trametes versicolor in order to evaluate their efficiency for the removal of two micropollutants: the anti-inflammatory drug naproxen and the antibiotic sulfamethoxazole. Although Trametes versicolor was able to degrade quickly naproxen, this fungus was not any more active after one week of operation in the filter. Pleurotus ostreatus was, on contrary, able to survive more than 3 months in the filter, showing good removal efficiencies of naproxen and sulfamethoxazole during all this period, in tap water but also in real treated municipal wastewater. Several other experiments have provided insight on the removal mechanisms of these micropollutants in the fungal biofilter (degradation and adsorption) and also allowed to model the removal trend. Fungal treatment with Pleurotus ostreatus grown on wood substrates appeared to be a promising solution to improve micropollutants removal in wastewater.