Wastewater treatment processes modelling for anaerobic digestion effluents

The general topic of this research project is the modelling of wastewater treatment processes for anaerobic digestion effluents, which are characterized by high nitrogen concentrations. In order to avoid eutrophication of lakes and rivers caused by nutrient enrichment as it happened in Switzerland a few decades ago, the implementation of efficient nitrogen removal processes for those effluents remains nowadays of importance. Moreover nitrogen removal will even become a pre-requisite for efficient micropollutants techniques to be implemented. The first objective of this master project is the modelling of the biological nitrogen removal process Anammox in the software gPROMS. This process converts ammonium and nitrate (close to 1:1 influent concentration ratio) into dinitrogen and nitrate by Anammox bacteria. In order to obtain an influent with the proper concentration ratio, this process can be coupled to a Sharon reactor operated for partial nitrification (two stages operation). In this study, the Anammox biological developed model has been integrated into a continuous stirred tank reactor (CSTR) combined with a particles separator. The two modeled reactors, Sharon and Anammox, have then been run in series and steady-state results have been obtained. Through a comparison with literature results (lab experience and pilot-plant), the steady-state simulation results in terms of nitrogen removal efficiency and effluent concentrations have been considered as reliable. However, a calibration of the model with real data still needs to be investigated. The second objective of the project is the study of key influencing parameters (pH and temperature) on the operation of the two stages Sharon-Anammox process. A sensitivity analysis showed that temperature and pH are influencing the nitrite to ammonium ratio Sharon effluents and consequently affecting the efficiency of the Anammox reactor; the growth and selection of bacteria in the Sharon reactor are determined by those two parameters (chemostat principle). The optimal range of operation for the Sharon reactor in order to maximize the efficiency of the Anammox reactor are found to be between 306K and 308K and between 7.8 and 8.0 for the pH according to the steady-state simulations; those results are coherent with literature findings and operation of full-scale Sharon reactors. The final objective of the project is the comparison of the Sharon-Anammox process with an activated sludge system in terms of energy consumption and nitrogen removal efficiency. By minimizing in both cases the mmonium effluent concentration and playing with the aeration needs, savings of 75% in benefit of the Sharon-Anammox system have been observed; the order of magnitude obtained by optimization is coherent with the stoichiometry implemented in the model. The biological Anammox model itself will be implemented in other types of reactors models (one stage technologies involving biofilms) and the models predictions will be compared with operational data (at pilot or full-scale). Modelling wastewater treatment processes is a help to better understand the processes themselves, their interactions, their optimal operation conditions both by taking into consideration the energy consumption and the water quality through optimization techniques. The challenge of modelling will however still remain the modelling of bacteria interaction and inhibition, as well as a deep understanding and quantification of various mass transfer processes involved. This will need a close collaboration with expert scientists in order to better understand these phenomena. Modelling is a good support for experimental studies, as it can help to plan efficient experimental campaign by minimizing the required experimentations run.

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

Doulaye Koné

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.

EPFL2002

Bacterial Resource Management for Nutrient Removal in Aerobic Granular Sludge Wastewater Treatment Systems

David Gregory Weissbrodt

The approach of wastewater treatment has shifted towards a holistic view in order to achieve sustainability in addition to environmental protection. The aerobic granular sludge (AGS) technology, which relies on the use of fast-settling granular biofilms called granules, is progressively becoming a new standard for high-rate biological nutrient removal (BNR) and secondary clarification in single sequencing batch reactors (SBR). The AGS intensive process has been related with definite savings in land area, construction, and operation costs. In an economical analysis, theoretical savings of CHF 0.45 per m3 (1 Swiss franc CHF ≈ 0.83 €) were computed for a Swiss wastewater treatment plant (WWTP) of 200’000 capita removing all nutrients biologically. Besides scale-up, fundamental research was required to understand and tailor the structure of underlying bacterial communities that form granules and remove nutrients. Mechanisms of bacterial selection were investigated in a systems approach to propose strategic axes for optimal management of the bacterial resource for efficient process performances. This research led to the following advances. After having designed a flexible reactor infrastructure for AGS research, a mathematical modeling methodology was developed to understand the hydraulic and biological processes involved during plug-flow transport of wastewater across the settled AGS bed during the feeding phase. BNR relies on the preferential selection of polyphosphate-accumulating organisms (PAO) by proper uncoupled supply of electron donors and electron acceptors. A bed height to diameter ratio of 6 was deduced to be optimal for full bed loading and efficient anaerobic acetate uptake under reference simulation conditions (20°C, pH 7.0). A bioinformatics methodology called PyroTRF-ID was developed for the identification of bacterial relatives involved in BNR processes by combination of terminal-restriction fragment length polymorphism (T-RFLP) and pyrosequencing data. This procedure provided high resolution of bacterial community structures and dynamics of AGS systems. Electrical conductivity and polyphosphatase assays were proposed for rapid and low-cost assessment of the fractions of active PAO and of the dephosphatating potential of activated sludge and granular sludge. Positive linear correlations were obtained between the fraction of PAO, the biomass specific rate of conductivity evolution measured in anaerobic metabolic batch tests, and the polyphosphate-hydrolyzing enzymatic activity of cell extracts. Wash-out conditions used to stimulate granule formation were shown to exert a selection pressure not only on physical properties of early-stage granules, but also on the underlying bacterial community composition. Operation with constant volumetric organic loading rates (OLR) during wash-out resulted in the formation of slow-settling fluffy granules dominated by filamentous Burkholderiales affiliates formed under low aeration (< 2 cm s-1) and high mesophilic temperature (30°C), and of fast-settling dense granules dominated by Zoogloea spp. under high aeration (4 cm s-1) and with an inoculum originating from a BNR-WWTP. These exopolysaccharide producers were considered as model organisms for granulation. However, their proliferation over PAO and nitrifiers correlated with poor BNR. PAO estab-lished at mature stage as soon as sufficient AGS was present to fully remove volatile fatty acids (VFA) during the anaerobic feeding phase. Without purge of excess sludge, glycogen-accumulating organisms (GAO) competed with PAO at a later stage of reactor operation. Granulation was shown to occur in stirred-tank SBRs operated under steady-state condi¬tions to cultivate PAO and GAO enrichments. The Accumulibacter- and Competibacter-dominated communities displayed granulation potential without involvement of Zoogloea spp. Fast-settling nuclei formed despite initial operation with a settling time as high as 60 min, and evolved towards granules after decrease of settling time to 10 min. Dynamic control of the OLR, anaerobic contact time, and sludge retention time (SRT) selected for active PAO in early-stage granules. Confocal laser scanning microscopy (CLSM) revealed that granula¬tion mechanisms depend on the predominant organisms involved. Fast-growing Zoogloea spp. formed smooth biofilm continuum matrices. The slower-growing PAO and GAO formed het¬erogeneous granular structures by proliferation in compact colonies around flocs. Fluctuations in operation variables were shown by multivariate statistics to impact on process performance and bacterial community structures in anaerobic-aerobic AGS-SBRs. The size of granules impacted on nitrification and dephosphatation by affecting oxygen mass transfer. Efficient BNR was obtained with at least 500 mgCOD L-1of acetate, 30 gCOD gP-1, and 10 gCOD gN-1. Although clades of PAO can denitrify, nitrogen removal was thus not restricted only to PAO. A broad denitrifying community was identified. The bacterial community continuum comprised two major opponent clusters, composed of members of the core microbiome of full-scale BNR-WWTPs. The first mainly comprised Accumulibacter, Nitrospira, Xanthomonadaceae, and Aminobacter affiliates selected by conditions of efficient BNR. The second mainly comprised Competibacter, Sphingobacteriales, Cytophaga, and Tetrasphaera affiliates correlating with periods of low BNR. Bacterial selection in AGS systems was mainly impacted by pH, according to multifactorial experiments. Accumulibacter and BNR were favored with alkaline pH, low mesophilic temperatures, and in the presence of propionate. Competibacter proliferated with acidic pH, temperature close to 30°C, and only acetate. Tetrasphaera spp., potential major PAO of certain full-scale plants, withstood Competibacter-selective conditions. Enhanced BNR was obtained under non-limiting conditions with food-to-microorganism ratios above 25 mgCODs gCODx-1. With fixed 2-h starvation, dephosphatation was only complete with full aeration. Since alternating aerobic-anoxic starvation conditions led to partial conversions, the length of starvation phases should be controlled in function of the redox conditions applied. Overall, optimal bacterial resource management in AGS systems requires full control of system behavior by taking advantage of the flexibility of the SBR technology. A methodology is proposed to this end, comprising milestones for wastewater characterization, SBR design, “anaerobic selector” design, control of the starvation phase length, shaving of fluctuations in operation variables, purge of excess sludge, proper selection for a stable, active and cooperative bacterial community, as well as efficient BNR. As key research perspective, investigations with real wastewater should validate the knowledge gained on bacterial behaviors under low-complexity conditions. Significant differences in predominant bacterial relatives were detected between lab-scale and full-scale BNR sludge. Metagenomics will provide a broader view on key organisms and functions of BNR and AGS microbiomes.

EPFL2013

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