Project Abstract

<p>                 <b>ABSTRACT&nbsp;</b><br></p><p> Constructed Wetlands (Cws) are a wastewater treatment technology that benefits from the inherent water-purification potential of natural wetlands and optimizes their performance to comply with regulations for treated discharges. These systems have evolved to become a real and equally performing alternative to conventional wastewater treatment technologies for small communities (up to 2000PE) with significantly lower energy and maintenance costs. Despite their great potential, CWs still lack reliability, which holds back their full deployment in the territory. This fact results from the lack of understanding on their internal functioning and also because they are prone to clogging. The enormous diversity of CWs typologies and operation strategies, and the fact that they operate at the mercy of the environmental conditions, makes each CW unique on its kind, and experimental studies are usually only representative of the studied system. This fact makes mathematical models an essential tool to evaluate the relative impact of each parameter on the internal functioning of CWs. Several mathematical models for CWs have proliferated in the last dozen years to simulate the behaviour of these systems and to provide supporting tools for their design and operation as well as more insight into the treatment processes. However, until the begining of this research, and compared to models utilized in similar disciplines, CWs models development was still in an embrionary stage. Accordingly, the main objectives of the current work were, on the one hand, to develop a CWs mathematical model able to describe the most common processes taking place within these systems. And, on the other hand, to use this model to shed some light on the internal functioning of these systems in the long-term. 7 The model, named BIO PORE, was built in COMSOL MultiphysicsTM and includes equations to simulate subsurface flow and pollutants transport in porous media. It also implements the biokinetic model Constructed Wetlands Model number 1 (CWM1) to describe the fate of organic matter, nitrogen and sulphur and the growth of the most common functional bacterial groups found within these systems. The model was calibrated with experimental data for an entire year of operation of a pilot system. Two empirical parameters (Mcap and Mbio max) were used to improve the description of bacterial growth obtained with the original formulation of CWM1 and to include the effects of solids accumulation on bacterial communities. The effect of these two parameters was evaluated by means of a local sensitivity analysis. The model was later used to unveil the dynamics of bacterial communities within CWs. In addition, a theory was derived from simulation results, which aimed at describing the most basic functioning patterns of horizontal subsuface flow CWs based on the interaction between bacterial communities and accumulated solids. At the end of the document a mathematical formulation is presented to describe bioclogging in CWs and a numerical experiment is carried out to showcase its impact on simulation results. The main outcome of the current work was the BIO PORE model. This model was able to reproduce effluent pollutant concentrations measured during an entire year of operation of the pilot system. Empirical parameters Mcap and Mbio max proved essential to prevent unlimited bacterial growth predicted by the original Monod equations of CWM1 near the inlet section of CWs. These two parameters were in great part responsible for the good fitting with experimental data. This was confirmed with the results from the sensitivity analysis, which helped demonstrate that they have a major impact on the model predictions for effluent COD and ammonia and ammonium nitrogen. The theory derived from simulation results, named The Cartridge Theory, indicated that bacterial communities are not static, but move towards the outlet with time, following the progressive accumulation of inert solids from inlet to outlet. This result may prove that CWs have a limited life-span, corresponding to the time after which bacterial communities are pushed as much towards the outlet that their total biomass is not sufficient to provide effluents with acceptable quality. The inclusion of bioclogging effects on the hydrodynamics of the granular media was seen to be a requisite in order to properly reproduce the bacterial distribution, fluid flow and pollutants transport within CWs. Finally, results of this work also showed that more work on the BIO PORE model is required and more experimental data is necessary to calibrate its results. Keywords Constructed Wetlands, modelling, finite elements, porous media, flow, reactive transport, biokinetic reactions, bacterial growth, clogging. <br></p>

Project Overview

<p> <b>1.1 Global water context&nbsp;</b></p><p>Water is indispensable for all forms of life on Earth. According to the World Health Organization (WHO) a single human being requires between 50 and 100 litres of water to ensure that its most basic needs are met. This fact was officially recognised by the United Nations General Assembly on July 28th 2010, which declared the human right to water and sanitation through Resolution 64/292 (UN-OHCHUNHabitat-WHO, 2010). This resolution acknowledges the importance of equitable access to safe and clean drinking water and sanitation as an integral component of the realization of all human rights. Moreover, among The Millennium Development Goals (MDG), established in year 2000 following the Millennium Summit of the United Nations, goal number seven was dedicated to Ensure environmental sustainability. Section three of goal number seven was dedicated to Halve, by 2015, the proportion of the population without sustainable access to safe drinking water and basic sanitation. Although this goal was achieved 5 years before schedule, there are still lots of challenges ahead regarding sanitation and water accessibility and quality. In fact, at current time approximately 884 million people still lack access to safe drinking water and more than 2.6 billion do not have access to basic sanitation. It is also estimated that approximately 1.5 million children under 5 years of age die each year as a result of water and sanitation-related diseases (UN-OHCH-UNHabitatWHO, 2010) and according to WHO, 88% of the diarrhoeal deaths are due to unsafe water, inappropriate sanitation and lack of hygiene. In fact, an estimated 1 billion people (15% of the world population) still practice open defecation. This is of special concern, since just a small number of people practicing open defecation can threaten the quality of water resources, which will in turn infringe the right to water and the right to health (Alburquerque, 2013). Moreover the majority (71%) of those without sanitation live in rural areas and 90% of all open defecation takes place in rural areas. Adding to all that, water is a limited resource and, most importantly, it is badly distributed geographically. This fact combined with the ever increasing population living on Earth is expected to be the cause of many conflicts in the future. Figure 1.1 helps the purpose of showing how water is a scarce resource. In this figure, the largest sphere represents all of Earth’s water (estimated diameter of 1384.0 km), the intermediate corresponds to Earth’s liquid fresh water (i.e. groundwater, lakes, swamp water and rivers) and the smallest one is the water in lakes and rivers. <br></p><p> In 2010, nearly all megacities (&gt;10 million inhabitants) were facing water scarcity and as the world population continues to grow, so has the demand for water (Alburquerque, 2013). In 2011 world population hit the 7 billion figure, which is twofold the population on the earth on the 1970. This pace of growth is expected to continue at least for the next 40 years, and predictions indicate that the population in 2050 will reach 9.6 billion (United Nations, 2013). Accordingly water withdrawals tripled over the last 50 years and demand for water for food production is projected to double by 2050 (Alburquerque, 2013). Moreover, global warming is expected to put water resources even more at stake. Moreover, the increasing world population also contributes to the deterioration of water quality (UNEP-UNWATER-FAOWATER, 2010). Deterioration of water quality occurs when existing water treatment and/or sanitation infrastructures are overloaded or when there are no water treatment facilities at all. In those cases wastewater is discharged directly into the environment and the receiving water body, and possibly subsurface water are contaminated (UNEP-UNWATER-FAOWATER, 2010). Enhancing and expanding infrastructure can be very costly and therefore in general is not keeping up with rapid development. Wastewater management therefore is emerging as a major global challenge (Alburquerque, 2013). According to the World Water Development Report of 2012, over 80% of wastewater worldwide is not collected or treated, and urban settlements are the main source of pollution. And in developing countries up to 90 % of wastewater is released untreated into the receiving bodies, which in many cases pollute potable water sources. However, this is not only a problem concerning developing countries. For instance, and according to the World factbook from the Central Intelligence Agency of the USA, among the main environmental issues affecting Spain are the pollution of the Mediterranean Sea from different sources and the water quality and quantity nationwide. To this regard Alburquerque (2013) states that in times of financial and economic crisis, retrogressive measures are more common and their impacts often exacerbated by austerity measures. Indeed, in the current context of economical crisis, the treatment of wastewater is less of a priority for administrations. For the sake of example, in Catalonia the lack of financial resources is letting the water management at the hands of private companies. Nowadays c.a. 80% of wastewater treatment plants in Catalonia are run by private capital (Garriga Riu, 2013), which may condition the treatment of water to net profit. For all the reasons stated before, it is clear that among many other things, the world needs sustainable, economical and reliable wastewater treatment technologies to tackle the challenges of the future regarding water quality and accessibility. <br></p><p> According to Alburquerque (2013), choosing the right technology is essential to achieving sustainability of water and sanitation services. To this regard, in this work we focus on Constructed Wetlands technology; these systems are cheap, low in energy requirements and thus, a sustainable treatment technology. However, as we will expose in following sections <b>o</b>f this work, they suffer from certain issues, derived from the combined effect of their internal complexity and our lack of understanding of their functioning, which can hinder their full deployment in the territory.&nbsp;<b></b></p><p><b>1.2 Constructed Wetlands for wastewater treatment&nbsp;</b></p><p><b>1.2.1 General background&nbsp;</b></p><p>Constructed Wetlands (CWs) are engineered systems, based on the principles of natural wetlands, that are used to treat wastewater originated from different sources (urban runoff, municipal, industrial, agricultural and acid mine drainage) (Figure 1.2). The most common application of these systems is for the treatment of municipal wastewater. CWs are designed to simulate the conditions that allow the development of the processes occurring in natural wetlands, but in a controlled environment (Garc´Ä±a et al., 2010). Figure 1.2: Treatment plant based on Constructed Wetlands technology (Verd´u, Catalonia). The treatment line consisted on three septic tanks in parallel followed by four parallel horizontal subsurface flow Constructed Wetlands and two surface flow wetlands. This technology is recognized to have low energy and maintenance requirements and to be easy to operate. These facts make it suitable for wastewater treatment where land availability and land prices are not limiting factors (Garc´Ä±a et al., 2001; Puigagut et al., 2007). CWs have been actively used around the world since the early 70’s as an alternative to intensive treatment technologies for the sanitation of small communities (Puigagut et al., 2007).&nbsp;</p><p><b>1.2.2 Constructed Wetlands typologies</b>&nbsp;</p><p>CWs consist of impermeable excavated basins, which use engineered structures to control the flow direction, liquid retention time and water level (USEPA, 2000). Water is fed and retained during a specified time in these systems, which depends on the inflow rate and the volume of the basin. CWs are planted with aquatic macrophytes, typical from natural wetland areas. According to the way water circulates through the basins, they can be classified as either Subsurface Flow Wetlands (SSF CWs) or Surface Flow Wetlands (SF CWs). In the first case, water circulates underground through the porosity of a granular medium, whereas in SF CWs water circulates in contact with the atmosphere (Kadlec and Wallace, 2008). SSF CWs can also be subdivided in horizontal flow or vertical flow systems (Kadlec and Wallace, 2008). In horizontal flow wetlands (HSSF CWs) (Figure 1.3), wastewater is maintained at a constant depth and flows horizontally below the surface of the granular medium. Figure 1.3: Schematic representation of a horizontal subsurface flow CW. Flow circulates from left to right. In vertical flow systems (VSSF CWs) (Figure 1.4), wastewater is distributed over the surface of the wetland and trickles downward through the granular medium (Brix and Arias, 2005).&nbsp;<br></p><p> Combinations of these two types of systems can be used in certain cases, benefiting from the advantages of the two. However, from now onwards, only HSSF CWs will be considered, although most of the criteria that applies for HSSF CWs does also apply for VSSF CWs and any combination of the two.&nbsp;</p><p><b>1.2.3 Applications&nbsp;</b></p><p>CWs are designed to eliminate pollutants from wastewater such as: suspended solids, organic matter, nutrients and faecal bacteria indicators. Other pollutants that are also removed but that are not commonly targeted when designing municipal wastewater treatment systems are heavy metals, surfactants, pharmaceuticals and personal care products (PPCPs) as well as other emerging pollutants. In particular, SSF CWs are one of the most common types of extensive wastewater systems used throughout the world (Garc´Ä±a et al., 2010). Traditionally, wastewater treatment plants (WWTPs) based on HSSF CWs consist of several in-series treatment stages. First, the pre-treatment and primary treatment are combined to eliminate solids, while subsequent stages consist of CWs sometimes combined with other extensive technologies (Rousseau et al., 2004; Vera et al., 2011; Vymazal, 2005). Thus, SSF CWs are mainly designed to treat primary settled wastewater, although they are also used to improve the quality of secondary treated effluents (Garc´Ä±a et al., 2010).&nbsp;</p><p>&nbsp; <b>1.2.4 Design criteria&nbsp;</b></p><p>In engineering practice, the design of SSF CWs is often carried out using the black box concept. Hence, important design factors such as areal organic loading rate (AOLR), hydraulic loading rate (HLR), aspect ratio, granular medium size and water depth are defined mostly from previous experience (Garc´Ä±a et al., 2005). Other equally important parameters are the selection of the pretreatment, inflow and collection systems, as well as the plant species to be planted. Several experimental studies have focused on studying different systems under different hydraulic loading rates (HLR) (Garc´Ä±a et al., 2004a), organic loading rates (OLR) and other design parameters.&nbsp;</p><p>Results of these studies have been used to propose recommended ranges for the different factors. The main operational problem associated with SSF CWs is the clogging of the granular media (Knowles et al., 2011; Pedescoll et al., 2011; Rousseau et al., 2005). Clogging development reduces the infiltration capacity (Caselles-Osorio et al., 2007; Ruiz et al., 2010) thus causing the hydraulic malfunctioning of wetlands and, in some cases, the decrease of treatment efficiency (Rousseau et al., 2005).&nbsp;</p><p>Among the most important factors contributing to clogging are the retention and accumulation of wastewater solids, biofilm and plant growth and the accumulation of plant litter and chemical precipitates (Knowles et al., 2011). Several authors (Langergraber, 2003; Nguyen, 2000; Pedescoll et al., 2011) have reported that correct operation and maintenance is of great importance to avoid rapid clogging of SSF CWs. The organic and suspended solids loads are the main operation factors affecting clogging development (Alvarez et al. ´ , 2008; Chazarenc et al., 2007; Winter and Goetz, 2003). In this regard, some authors have suggested that the OLR in horizontal SSF CWs should not exceed 6 gBOD · m−2d −1 (Garc´Ä±a et al., 2005; USEPA, 2000). Organic loads between 8 and 12 gBOD·m−2d −1 are generally associated with a total suspended solids loading of 110 gT SS·m−2d −1 (Kadlec and Wallace, 2008).&nbsp;</p><p>However, as reported by Alvarez et al. ´ (2008), little information is available on the maximum acceptable total suspended solid (TSS) loading rates and only some recommendations are prescribed for vertical SSF CWs. It is well known that the reduction of suspended solids and organic content in wastewater achieved by using a good previous treatment, is essential to delay clogging development and thus to extend life-span of SSF CWs (Alvarez et al. ´ , 2008; Caselles-Osorio et al., 2007; Pedescoll et al., 2011; Winter and Goetz, 2003). The life-span of CWs depends directly on solids retention (organic and inorganic) and on the intrinsic organic matter turn-over rate (decomposition and cycling) (Nguyen, 2000). If the accumulation rate is higher than the turn-over rate then it is likely that clogging will develop at a rapid peace. <br></p><p> The HLR applied to these systems is dependent on the allowed OLR, but it is also considered in the hydraulic design of a wetland.&nbsp;</p><p><b>1.2.5 Contaminant removal processes</b>&nbsp;</p><p>The two major mechanisms at work in most treatment systems are liquid/solid separations and constituent degradations and transformations (USEPA, 2000). SSF CWs are essentially fixed-biofilm reactors in which organic matter is removed through interactions between complex physical, chemical, and biochemical processes. Many studies have shown that organic matter removal rates are not clearly related to changes in water temperature, which suggests that the principal removal mechanisms are physic-chemical and subsequently biological (McNevin et al., 2000). The relative importance of the different biochemical pathways for removing organic matter depends primarily on the redox conditions (Garc´Ä±a et al., 2010). The main mechanisms for influent particulate organic mater (POM) (and in general TSS) retention in SSF CWs are those of physic-chemical nature, and include impact and retention encouraged by path variations of water flow owing to the grains of the medium, settling due to low speed movement, and adhesion owing to superficial interaction forces. Retained POM either accumulates or disintegrates and undergoes hydrolysis, which generates dissolved organic compounds that can be degraded by different pathways that occur simultaneously in a given wetland (Garc´Ä±a et al., 2010). POM accumulation in the granular medium is a typical feature of SSF CWs (Nguyen, 2001). Most of the POM is removed close to the inlet, and the remaining dissolved organic matter is removed more slowly along the entire length of the beds (Garc´Ä±a et al., 2010). The removal rates of influent TSS in SSF CWs are usually very high (&gt;90%). During the hydrolysis step, which occurs after disintegration, a defined particulate or macromolecular substrate is degraded into its soluble compounds.&nbsp;</p><p>Disintegration and hydrolysis are processes that occur either under aerobic, anoxic, or anaerobic conditions. The hydrolysis reaction is one of the processes that most restricts the removal of organic matter in wastewater treatment plants (Garc´Ä±a et al., 2010). Inflow dissolved organic matter (DOM) and that produced after disintegration and hydrolysis processes can be removed by aerobic respiration through the metabolism of a large number of heterotrophic bacteria that use oxygen as a final electron acceptor. Aerobic respiration requires oxygen, which can be the most limiting substrate for this reaction in SSF CWs, as the amount of oxygen transported from air to water in a horizontal SSF CW is insignificant in comparison to the oxygen demand of standard urban wastewater (Garc´Ä±a et al., 2010). <br></p><p> Studies carried out in the last 10 years have shown that aerobic respiration is not the only reaction to exert a significant influence on organic matter removal in horizontal SSF CWs (Aguirre et al., 2005; Baptista et al., 2003). In fact, aerobic respiration is not the most important reaction involved in organic matter removal. In the next few lines, a description of other reactions occurring in constructed wetlands that also lead to the removal of organic matter is made. Denitrification is the biochemical reduction of nitrate and nitrite to nitrogen gas. This process links the C and N cycles in CWs because it enables denitrifying bacteria to obtain energy from organic compounds at the same time that nitrate is used as an electron acceptor.&nbsp;</p><p>Denitrification is conducted by a wide range of heterotrophic aerobic facultative bacteria that are able to use nitrate as electron acceptor under anoxic conditions. These bacteria groups use oxygen preferentially over nitrate as electron acceptor when it is available in the surrounding environment. Consequently, significant denitrification rates are only observed in depleted oxygen environments (Garc´Ä±a et al., 2010). Fermentation is a multi-stage biochemical process in which the soluble organic monomers present in wastewater and those generated through hydrolysis are converted into volatile short-chain fatty acids (VFAs). A large number of heterotrophic bacteria groups are involved in fermentation reactions. Fermentation occurs under anaerobic conditions and is therefore an important reaction in horizontal SSF CWs. Interest in sulphate reduction in SSF CWs has grown in recent years because research has shown that it can contribute significantly to the removal of organic matter in horizontal SSF CWs (Aguirre et al., 2005). Sulphate is a normal constituent of many types of wastewater and can be used as an electron acceptor in the absence of oxygen by a large group of strictly anaerobic heterotrophic microorganisms called sulphate-reducing bacteria. These microorganisms can grow by using a wide range of fermentation products as electron donors (e.g., acetate, lactate, butyrate) (Lloyd et al., 2004; Stein et al., 2007). Reduced sulphur compounds such as sulphide are released by the activity of sulphate-reducing bacteria which are known to be potent inhibitors of plant growth and certain microbial activities (Gonzalias et al., 2007; Wiessner et al., 2005). Methanogens are strictly anaerobic bacteria that produce methane as an end product of metabolism. Methanogens and sulphate-reducing bacteria require environments with similar redox potential levels and use the same types of electron donors (i.e., hydrogen, acetic acid). When these two groups of bacteria grow together and the COD:sulfate ratio (expressed as COD:S) is lower than 1.5, sulfate-reducing bacteria are able to outcompete methanogens. When the ratio is greater than 6, methanogens predominate over sulfate-reducing bacteria (Stein et al., 2007). Methanogenesis can remove significant quantities of organic matter from wastewater and has been studied more extensively in recent years (Garc´Ä±a et al., 2010).&nbsp;<br></p><p> Despite the extensive literature available on CWs, and all the efforts dedicated over the years to improve their understanding, there are still some knowledge gaps which prevent us from being able to optimize their functioning and reduce their known weaknesses (i.e. clogging development). Over recent decades, several mathematical and numerical models have been developed to help improve the understanding of these systems. The following section is dedicated to update the state of the art regarding these models. 1.3 Constructed Wetland models This section is based on the book chapter: • Sams´o, R., Meyer, D., Garc´Ä±a, J., 2014. Subsurface flow constructed wetlands models: review and prospects, in: Vymazal, J. (Ed.), The Role of Natural and Constructed Wetlands in Nutrient Cycling and Retention on the Landscape. Springer, Dordrecht, The Netherlands (in press). A mathematical model can be simply described as an attempt to translate the conceptual understanding of a real-world system or process (conceptual model) into mathematical terms (Eberl et al., 2006).&nbsp;</p><p>Therefore, mathematical models for Constructed Wetlands are a set of mathematical expressions (algebraic or differential equations), each describing a process that is known to take place within them. Among the many processes taking place within wetlands, those originated from microbial metabolism are key to describe their global functioning (Sams´o and Garc´Ä±a, 2013b). The mathematical models describing the rates at which microbial processes take place are called biokinetic models. On the other hand, numerical models involve the use of some sort of spacial and/or temporal discretization techniques to obtain approximate solutions to mathematical equations. Numerical modelling is an interesting tool as it allows to observe the outcome of complex conceptual models in various experimental conditions and to test their validity and how they enable a better understanding of the involved processes (Oberkampf and Trucano, 2002). For several years now, numerical models for CWs have been considered a promising tool to increase the understanding of the simultaneous physic-chemical and biological processes involved in the treatment of wastewater with this technology. This belief has translated in an increase on the number of publications on the development and utilization of these models over time. In fact, there exist arguably as many numerical models for CWs as there are types of wetlands, water pollutants and processes that take place within these systems. Indeed, a general search for the words constructed wetland model on common databases of scientific papers brings a limitless number of publications on this topic.&nbsp;</p><p>1<b>.3. Constructed Wetland models</b>&nbsp;</p><p>Among them, a very general distinction can be made: those focusing on the simulation of the hydraulics (Arias et al., 2014; Dittmer et al., 2005; Fan et al., 2008; Galv˜ao et al., 2010; Korkusuz et al., 2007; Kotti et al., 2013),the hydrodynamics and clogging (or any of them individually) (Brovelli et al., 2009b; Giraldi et al., 2009, 2010; Hua et al., 2013; Knowles et al., 2011; Suliman et al., 2006) and those focusing on the removal of a specific pollutant or a set of pollutants (which generally also include hydraulic and hydrodynamic models of diverse complexity). Among the latter, the most commonly targeted pollutants are organic compounds (Akratos et al., 2008; Henrichs et al., 2007; Liolios et al., 2012; Toscano et al., 2009), nitrogen (Akratos et al., 2009; Henrichs et al., 2009; Mayo and Bigambo, 2005; Mcbride and Tanner, 2000; Meyer et al., 2006; Meyer, 2011; Moutsopoulos et al., 2011; Toscano et al., 2009) sulphur (Lloyd et al., 2004), phosphorous (Hafner and Jewell, 2006), heavy metals and mine drainage (Goulet, 2001; Lee et al., 2006; Mitsch and Wise, 1998), arsenic (Llorens et al., 2013) pesticides (Krone-Davis et al., 2013) and emerging pollutants (Hijosa-Valsero et al., 2011). At least 7 review papers have been published in recent times to summarize the state of the art of CWs numerical models (Garc´Ä±a et al., 2010; Kumar and Zhao, 2011; Langergraber et al., 2009b; Langergraber, 2010, 2008; Meyer et al., 2014; Rousseau et al., 2004). These reviews mostly consist of descriptions of the models features and no critical in-depth comparison is made between them (Sams´o et al., 2014b). In this section only the most recent numerical codes, applied to simulate the treatment of urban wastewater and those able to provide new insight into the functioning of CWs are reviewed. The 5 models selected for review are: PHWAT (Brovelli et al., 2009a,b,c, 2007), FITOVERT (Giraldi et al., 2009, 2010), HYDRUS-2DCW2D (Langergraber, 2005), HYDRUS-2D-CWM1 (Langergraber and Simunek, 2012), CWM1-RETRASO (Llorens et al., 2011a,b) and AQUASIM-CWM1 (Mburu et al., 2012). From the selected numerical codes, the only one that does not use either the biokinetics models CW2D or CWM1 is FITOVERT model. Provided the importance of the biokinetic equations within CWs models, the description of the selected models will be preceded by a brief description of CW2D and CWM1. CW2D Constructed Wetlands 2D (CW2D) (Langergraber, 2001) is a biokinetic model, based on the mathematical formulation of the Activated Sludge Model series (ASM) (Henze et al., 2000). This model was specifically conceived to simulate the most common biokinetic processes taking place in VF CWs. The components defined in CW2D include dissolved oxygen (O2), three fractions of organic matter (CR, CS, and CI), four nitrogen compounds (NH4, NO2, NO3 and N2N), inorganic phosphorus (IP), and heterotrophic and autotrophic microorganisms. Organic nitrogen and organic phosphorous are modelled as part of the COD. Heterotrophic bacteria (XH) are assumed to be responsible for hydrolysis, mineralization of organic matter (aerobic growth) and denitrification (anoxic growth). On the other hand, autotrophic bacteria (XANs and XAN b) are assumed to be responsible for nitrification, which is modelled as a two-step process. Microorganisms are assumed to be immobile. Lysis is considered to be the sum of all decay and loss processes. The temperature dependence of all process rates and diffusion coefficients is described using the Arrhenius equation. <br></p>