N-removal optimization by various aeration strategies

In order to improve the purification capacity of an aerobic granular sludge, two aeration strategies were designed and tested. The goal was to optimize the nitrogen removal performances via various oxygen patterns in the aeration phase of the reactor cycle. The main process driving theses patterns is that a difference between oxygen concentrations in the reactor will modify the size of the anoxic inner zone of a granule. The first aeration was based on a succession of oxygen peaks (50%DO) and semi anoxic valleys (5%DO). The second was based on a short pulse of oxygen followed by the hermetical isolation of the reactor and a continuous recirculation of the pulse. The objective was to get an n-removal superior to 82%, results of a previous article, and if possible better than 88%, results of a previous experiment in the lab. The succession of oxygenated peaks strategy worked well and gave good results, as the n-removal was increasing with the sludge bed height and time. However, due to laboratories problem and to the shortness of the time-slots available to experiment, the system could not have the time to stabilize and no constant removal value could be found. Therefore, the average n-removal was equal to 69% for 40 days of experimentation, with some peak over 80%. The highest value observed was 88% of n-removal. The recirculation strategy gave unsatisfactory results, actually unexplained. The first recirculation led to the disappearance of high nitrification capacity, giving an n-removal percentage of 48%. The second attempt was worse, as the oxygen concentration started to behave incoherently and over-oxygenated the system. This behavior still lacks a proper explanation and only hypotheses can actually be proposed. Besides these mains results, the surveillance of the sludge in the reactors allowed the discovery of a bias in the evaluation of the suspended biomass measurement. It was explained by the fact that heavier granules tend not to mix well, and therefore the system could not be considered homogenous. The bias was created because measurements were made at different height between the two reactors. In order to remove it, a linear transformation was developed and tested. The first results seem to be consistent, but due to the lack of experimenting time, the working hypothesis could not be deepened. Another bias that was spotted is the possible influence of the operator during the aeration cycle measurement. Three of four off-trend points in the first experiment originated from a cycle measurement. The data set was too small to draw strong conclusions upon the bias. An extended survey of the cycle measurement and their supposed off-trend consequences in previous and future experiments could clarify the situation.

Identification of parameters affecting the outcome of biochemical methane potential (BMP) tests

Jonas Stöckli

Anaerobic digestion of organic waste to methane gas (CH4) is an attractive method to produce renewable energy. Due to the trend towards the reuse of waste and away from fossil energy sources, this technology saw a worldwide development in the recent years. A fundamental parameter in this domain is the Biochemical Methane Potential (BMP), which defines the amount of CH4 which can be produced out of a certain organic substrate. Such information is essential in order to plan and optimise anaerobic digestion plants, and to evaluate the reactor feed with a new substrate or a new substrate combination (co-digestion). BMPs are assessed in batch test, which can be regarded as the simulation of a full-scale biogas plant. Two experiments are required, the test in which the test substrate is digested on the inoculum, and the blank, in which the inoculum is digested alone. The part of the inoculum which can be digested is called endogenous substrate. Since the test experiment produced not only on the test substrate but only on the endogenous substrate, the CH4 production on the test substrate alone is obtained by the subtraction of the test production minus the blank production. The units of a BMP are generally indicated in litre of CH4 per gram of volatile solids (VS) test substrate [L/gVS]. Despite the importance and the wide application of this parameter, its exact determination remains a challenging issue and results are often not consistent in between (inter-) and within (intra-) laboratories. Experimental protocols exist but do not lead to a satisfactory BMP tests consistency neither. The aim of this study was to identify parameters affecting the outcome of BMP tests, based on the investigation of two different data sets. An inter-laboratory study of BMP tests providing the final BMP values of 327 experiments and information about 40 related experimental parameters, and a second data set containing the complete CH4 production curves of 136 BMP experiments provided by one single laboratory. The method consisted in graphical and statistical analysis (Mixed Effect Modelling), using [R] programming language and software environment. This study found out, that a significant part of the inter-laboratory BMP inconsistency can be explained by an imprecise assessment of the VS, which was not expected. As the VS of the substrate are directly implied in the computation of the BMP, the impact was significant and therefore the BMP were corrected regarding the VS imprecision. With these data, up to 70% of the inconsistency of BMPs could be explained by inter-laboratory effects. The statistical analysis led to the conclusion that the concentration of the endogenous substrate and the moisture content in the digestate would be the principal factors affecting the outcome of BMP tests. An effort was made to identify and correct errors contained in CH4 production curves, which turned out to be delicate for certain cases. Also indications regarding the precision and the reliability of BMPs were formulated. Further, the investigation of the CH4 production curves led to the development of a new method in order to compute the BMP result. This method was based on the fact, that a certain inoculum reaches always the same slope toward the end of the experiment, no matter what was digested before. The advantage of this method would be that the experiment end-point could be set clearly and that the concentration of the endogenous substrate would not have an impact on the outcome of a BMP test. According to the findings of this report, it was proposed to add the following requirements into experimental protocols for BMP tests: Tests, blanks and the analysis of VS should be carried out in triplicates and their standard deviation should be indicated for each of them, together with the BMP result If triplicates contain set-up errors, these experiments must be repeated, including the corresponding blank/test In the annex: An experimental protocol for the TS and VS analyses An indication of a required moisture content of the digestate (mechanism to investigate in detail first) An indication of the range of required VS concentrations or a maximal production rate for blank tests according to certain experimental conditions (mechanism to investigate in detail first). This report identified several parameters, which contribute to the inter-laboratory inconsistency of BMPs. These findings should be investigated further in order to prove and quantify their impact. An effort should be made to demonstrate the newly developed BMP-computation-method, which could eventually lead to more consistent results. The limitation of this study was, that a relatively low amount of data was available compared to their characteristics. Consequently, the findings could only be proven on a few examples and should therefore only be seen as evidences. Further, this led also to the risk of overfitting, since a relative high amount of parameters needed to be included into the statistical model.

2012

Wastewater treatment processes modelling for anaerobic digestion effluents

Miriel Steudler

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.

2014

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