Titre de série : | Thèse en Sciences et Technologies de l'Eau, de l'Energie et de l’Environnement | Titre : | Optimization of two-stage high-rate anaerobic reactors coupled with baffled pond and wet-dry sand filters for domestic wastewater treatment in a warm-dry climate (Ouagadougou, Burkina Faso) | Type de document : | texte imprimé | Auteurs : | Diafarou Ali MOUMOUNI | Année de publication : | 2016 | Importance : | 156p. | Langues : | Français (fre) | Résumé : | Au cours des dernières décennies, les techniques de traitement des eaux usées par les procédés du système extensif, et particulièrement le lagunage à microphytes, ont connu une véritable évolution dans les pays tropicaux où le climat est favorable. Ces technologies de traitement des eaux usées à faible coût sont non seulement fiables, efficaces, durables mais aussi adaptées, aux populations à faible revenu vivant dans les zones urbaines et périurbaines de l’Afrique Subsaharienne. Les déficits hydriques sont récurrents dans ces zones où la rareté des ressources en eau a des répercussions importantes sur l’économie, l’alimentation et la santé des populations. Pour pallier ce manque, les eaux usées sont utilisées et réutilisées en agriculture avec ou sans traitement préalable entraînant ainsi des problèmes de santé publique. Pour réduire les risques dus à la réutilisation de ces eaux, plusieurs techniques innovantes de traitement des rejets domestiques et urbains ont été développées. La méconnaissance des conditions d’usage et de maintenance de ces systèmes remet en cause leur viabilité. Il est donc nécessaire de proposer des systèmes de traitement qui tiennent compte au mieux les réalités de la zone d’étude. Ces systèmes à moindre coût de conception et de maintenance, ne doivent pas avoir une forte emprise sur le sol. C’est dans cette optique que cette étude a été menée sur la conception, la mise en oeuvre, l'évaluation et l'optimisation de deux options de traitement des eaux usées domestiques sous le climat sahélien de Ouagadougou au Burkina Faso. Le choix de ces deux options est basé sur les concepts de technologies extensives à faible coût, tant au niveau de la collecte qu’à celui de l’épuration des eaux usées.
La première option comporte deux Réacteurs Anaérobies à haut rendement connectés en série, puis suivis par un Bassin Lamellé avec des bouchons en plastique fixés aux chicanes (RA-BL). Le principe de cette option est basé sur le fonctionnement à trois étages de bassins de lagunage à microphytes. Le premier bassin qui est le bassin anaérobie a été modifié pour former deux réacteurs anaérobies, où le biogaz est collecté. Le second bassin, dit bassin facultatif a été omis afin de minimiser la zone d’emprise du système de traitement. Enfin, pour optimiser l’efficacité hydraulique et augmenter la biomasse épuratrice, trois chicanes munies des bouchons de bouteilles en plastique usagées ont été introduites verticalement à contre-courant dans le troisième bassin dit bassin de maturation, formant ainsi quatre cloisons. Cette configuration a été prévue pour améliorer l'efficacité d'élimination des matières organiques, des nutriments (azote et phosphore), des agents pathogènes et permettre une valorisation de la production d’énergie (biogaz).
Abstract : Over recent decades, there is renewed interest in optimizing and innovating wastewater treatment technologies (WTTs) in sub-Saharan Africa, to reduce the impact of domestic and industrial sewage on the environment. However, poor city-dwellers need low-cost, reliable WTTs that allow for the safe reuse of the effluent in water scare context. This research focuses on the design, implementation, evaluation and optimization of two options for domestic wastewater treatment in the warm, dry sub-Saharan Africa climate of Ouagadougou, Burkina Faso. The first option consisted of two-stage high-rate Anaerobic Reactors followed by a Baffled Pond (AR-BP) with recycled plastic media as a medium for attached growth. The three vertical plastic baffles (with plastic bottle caps affixed to them to increase their surface area) formed four compartments in the baffled pond (BP). The second option included the same two-stage high-rate Anaerobic Reactors but followed them with wet-dry Sand Filters (AR-SF). The research was conducted on the pilot scale, by applying a design flow of 1 m3/day, which was later increased to 1.5 m3/ day. A peristaltic pump was used to provide an intermittent flow three times a day (at 8:00 am, 1:00 pm and 5:00 pm) from the buffer tank to the system.
After two years of operation, COD, BOD5 and TSS mean removal efficiencies were achieved by significant difference in both systems : 79%, 81% and 72% for AR-BP; 84%, 88% and 88% for AR-SF respectively. It was also found out that high pathogen removal efficiencies were achieved in both treatment options with 6 and 5 log units for AR-BP and AR-SF respectively. In addition, the AR-SF option presented a high rate of nitrification, while the BP was more efficient in removing ammonia nitrogen (84%) and E. coli (6 log units). Furthermore, no E.coli were ever detected in the BP effluent, nor did clogging occur in the SF, during the entire study. E-coli were, however, found in the effluent of a control pond (CP) that had no baffles. In fact, it was found that E. coli concentrations were lower in the upper layers of all four compartments of the BP, with an undetectable level in the last compartment down to a depth of 0.60 m. A tracer test with salt results showed actual mean hydraulic retention times of 4.1 and 3.2 days for BP and CP respectively. Also, it was found that the volume of the pond was more efficiently used for wastewater treatment in the BP, since more half of the volume of the CP was estimated to be inactive. The tracer experiment also showed that there was better mixing in the BP, thus treatment would be more predictable. Consequently, incorporating three verticals baffles in a pond, under Sahelian climate, not only improved the hydrodynamics and the performance of the pond, but also reduced costs and the amount of land that is required.
Another important aspect revealed by this research was the dense and rich biodiversity on both the attached media and in the water column of the BP. The biofilm was thick and green on the upper parts of both sides of all three at the top of the two sides of the baffles (on both the plastic sheets that form the baffles and the plastic bottles caps affixed to it). The biomass attached on the media constituted 35.5 times of that in the water column. Three major groups of diverse zooplankton were found in the water column at 15-90 cm depth, which included Cladocera, Copepoda and rotifers. The latter group was dominant with 13 identified species, which are attracted to a wide spectrum of food items. In addition, the Principal Components Analysis (PCA) carried out to examine the interactions between biotic and abiotic components of BP further revealed the symbiotic algal-bacterial activity and abiotic parameters, such as pH, dissolved oxygen and temperature interdependences in the course of organic matter degradation in the top layer of the BP. Furthermore, the very strong negative correlation between zooplankton and phytoplankton associated with abiotic parameters corroborates their predatory relationship. As a result, the predatory symbiosis distributions of phytoplankton and zooplankton have shown that the baffles had an effect on water quality which in turn has affected the ecology of the BP. Moreover, this dense and abundant presence of the zooplankton community could play an important role in the control of bacterial and algal populations in BP.
Lastly, the two-stage high-rate anaerobic reactors (R1 and R2) produced ample amounts of valuable biogas under the warm Sahelian climate, with 9.7 L/m2 per day of biogas and with methane content of 54%. More importantly, very low sludge yields were recorded in R1, R2, and BP (0.0006, 0.0002 and 0.0014 m3/capita/year respectively), thus reducing the cost of its extraction and management.
Both treatment options can be recommended as an alternative low-cost wastewater treatment technologies, separately or in tandem, for African cities, with the final effluent being used for restricted irrigation in periurban agriculture. To contribute even more to the alleviation of hunger in poor neighborhoods, further investigations may look at the use of this effluent in aquaculture, before its use in irrigation. |
Thèse en Sciences et Technologies de l'Eau, de l'Energie et de l’Environnement. Optimization of two-stage high-rate anaerobic reactors coupled with baffled pond and wet-dry sand filters for domestic wastewater treatment in a warm-dry climate (Ouagadougou, Burkina Faso) [texte imprimé] / Diafarou Ali MOUMOUNI . - 2016 . - 156p. Langues : Français ( fre) Résumé : | Au cours des dernières décennies, les techniques de traitement des eaux usées par les procédés du système extensif, et particulièrement le lagunage à microphytes, ont connu une véritable évolution dans les pays tropicaux où le climat est favorable. Ces technologies de traitement des eaux usées à faible coût sont non seulement fiables, efficaces, durables mais aussi adaptées, aux populations à faible revenu vivant dans les zones urbaines et périurbaines de l’Afrique Subsaharienne. Les déficits hydriques sont récurrents dans ces zones où la rareté des ressources en eau a des répercussions importantes sur l’économie, l’alimentation et la santé des populations. Pour pallier ce manque, les eaux usées sont utilisées et réutilisées en agriculture avec ou sans traitement préalable entraînant ainsi des problèmes de santé publique. Pour réduire les risques dus à la réutilisation de ces eaux, plusieurs techniques innovantes de traitement des rejets domestiques et urbains ont été développées. La méconnaissance des conditions d’usage et de maintenance de ces systèmes remet en cause leur viabilité. Il est donc nécessaire de proposer des systèmes de traitement qui tiennent compte au mieux les réalités de la zone d’étude. Ces systèmes à moindre coût de conception et de maintenance, ne doivent pas avoir une forte emprise sur le sol. C’est dans cette optique que cette étude a été menée sur la conception, la mise en oeuvre, l'évaluation et l'optimisation de deux options de traitement des eaux usées domestiques sous le climat sahélien de Ouagadougou au Burkina Faso. Le choix de ces deux options est basé sur les concepts de technologies extensives à faible coût, tant au niveau de la collecte qu’à celui de l’épuration des eaux usées.
La première option comporte deux Réacteurs Anaérobies à haut rendement connectés en série, puis suivis par un Bassin Lamellé avec des bouchons en plastique fixés aux chicanes (RA-BL). Le principe de cette option est basé sur le fonctionnement à trois étages de bassins de lagunage à microphytes. Le premier bassin qui est le bassin anaérobie a été modifié pour former deux réacteurs anaérobies, où le biogaz est collecté. Le second bassin, dit bassin facultatif a été omis afin de minimiser la zone d’emprise du système de traitement. Enfin, pour optimiser l’efficacité hydraulique et augmenter la biomasse épuratrice, trois chicanes munies des bouchons de bouteilles en plastique usagées ont été introduites verticalement à contre-courant dans le troisième bassin dit bassin de maturation, formant ainsi quatre cloisons. Cette configuration a été prévue pour améliorer l'efficacité d'élimination des matières organiques, des nutriments (azote et phosphore), des agents pathogènes et permettre une valorisation de la production d’énergie (biogaz).
Abstract : Over recent decades, there is renewed interest in optimizing and innovating wastewater treatment technologies (WTTs) in sub-Saharan Africa, to reduce the impact of domestic and industrial sewage on the environment. However, poor city-dwellers need low-cost, reliable WTTs that allow for the safe reuse of the effluent in water scare context. This research focuses on the design, implementation, evaluation and optimization of two options for domestic wastewater treatment in the warm, dry sub-Saharan Africa climate of Ouagadougou, Burkina Faso. The first option consisted of two-stage high-rate Anaerobic Reactors followed by a Baffled Pond (AR-BP) with recycled plastic media as a medium for attached growth. The three vertical plastic baffles (with plastic bottle caps affixed to them to increase their surface area) formed four compartments in the baffled pond (BP). The second option included the same two-stage high-rate Anaerobic Reactors but followed them with wet-dry Sand Filters (AR-SF). The research was conducted on the pilot scale, by applying a design flow of 1 m3/day, which was later increased to 1.5 m3/ day. A peristaltic pump was used to provide an intermittent flow three times a day (at 8:00 am, 1:00 pm and 5:00 pm) from the buffer tank to the system.
After two years of operation, COD, BOD5 and TSS mean removal efficiencies were achieved by significant difference in both systems : 79%, 81% and 72% for AR-BP; 84%, 88% and 88% for AR-SF respectively. It was also found out that high pathogen removal efficiencies were achieved in both treatment options with 6 and 5 log units for AR-BP and AR-SF respectively. In addition, the AR-SF option presented a high rate of nitrification, while the BP was more efficient in removing ammonia nitrogen (84%) and E. coli (6 log units). Furthermore, no E.coli were ever detected in the BP effluent, nor did clogging occur in the SF, during the entire study. E-coli were, however, found in the effluent of a control pond (CP) that had no baffles. In fact, it was found that E. coli concentrations were lower in the upper layers of all four compartments of the BP, with an undetectable level in the last compartment down to a depth of 0.60 m. A tracer test with salt results showed actual mean hydraulic retention times of 4.1 and 3.2 days for BP and CP respectively. Also, it was found that the volume of the pond was more efficiently used for wastewater treatment in the BP, since more half of the volume of the CP was estimated to be inactive. The tracer experiment also showed that there was better mixing in the BP, thus treatment would be more predictable. Consequently, incorporating three verticals baffles in a pond, under Sahelian climate, not only improved the hydrodynamics and the performance of the pond, but also reduced costs and the amount of land that is required.
Another important aspect revealed by this research was the dense and rich biodiversity on both the attached media and in the water column of the BP. The biofilm was thick and green on the upper parts of both sides of all three at the top of the two sides of the baffles (on both the plastic sheets that form the baffles and the plastic bottles caps affixed to it). The biomass attached on the media constituted 35.5 times of that in the water column. Three major groups of diverse zooplankton were found in the water column at 15-90 cm depth, which included Cladocera, Copepoda and rotifers. The latter group was dominant with 13 identified species, which are attracted to a wide spectrum of food items. In addition, the Principal Components Analysis (PCA) carried out to examine the interactions between biotic and abiotic components of BP further revealed the symbiotic algal-bacterial activity and abiotic parameters, such as pH, dissolved oxygen and temperature interdependences in the course of organic matter degradation in the top layer of the BP. Furthermore, the very strong negative correlation between zooplankton and phytoplankton associated with abiotic parameters corroborates their predatory relationship. As a result, the predatory symbiosis distributions of phytoplankton and zooplankton have shown that the baffles had an effect on water quality which in turn has affected the ecology of the BP. Moreover, this dense and abundant presence of the zooplankton community could play an important role in the control of bacterial and algal populations in BP.
Lastly, the two-stage high-rate anaerobic reactors (R1 and R2) produced ample amounts of valuable biogas under the warm Sahelian climate, with 9.7 L/m2 per day of biogas and with methane content of 54%. More importantly, very low sludge yields were recorded in R1, R2, and BP (0.0006, 0.0002 and 0.0014 m3/capita/year respectively), thus reducing the cost of its extraction and management.
Both treatment options can be recommended as an alternative low-cost wastewater treatment technologies, separately or in tandem, for African cities, with the final effluent being used for restricted irrigation in periurban agriculture. To contribute even more to the alleviation of hunger in poor neighborhoods, further investigations may look at the use of this effluent in aquaculture, before its use in irrigation. |
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