2. Model development
2.1. System boundaries
The present model focuses on hydraulics (flow pattern, velocity and related transport of substances due to advection and diffusion), biochemical processes (algal and bacterial growth and decay) and the physical process of re-aeration. Both aerobic (autotrophic and heterotrophic bacteria), anoxic (heterotrophic bacteria) and anaerobic (fermenting, acetotrophic sulphate reducing and acetotrophic methanogenic bacteria) bacterial metabolism was included. In addition, the environmental factors that are light related (solar radiation and photosynthetic active radiation), air temperature, humidity and wind were implemented as forcing functions.
Sedimentation, processes in sediments and sediment–water interactions were not included in this model. Since SFPs receive pre-treated wastewater, a major part of the settleable solids is already removed. Therefore, it can be assumed that sedimentation, sediment related processes and resuspension do not play a major role, especially in a newly started pond. Such a model could thus be more suitable in describing the processes and predicting effluent quality in new ponds. Authors such as Kayombo et al. (2000) and Dochain et al. (2003) have also excluded sediments in their models for similar reasons. This will have several implications on model results such as all waste water components will be present in the water phase and available to suspended microorganisms. Moreover, the total (=unfiltered) effluent COD (TCOD) as predicted by the model can be higher than in reality as the sedimentation process is not included. We however anticipated that ignoring these processes should not cause a too high deviation from reality since we specifically aimed to model secondary facultative ponds where a large part of the particles are already removed during the preceding treatment step. In addition, the model does not include ammonia volatilization because it was considered negligible as compared to uptake by organisms (Senzia et al., 2002). This might result in a slightly higher prediction of ammonia in the effluent concentrations. Also, phosphate removal was not a part of the model because according to Shilton (2005), the major pathway for phosphorus removal is by physical and chemical processes and not by biological processes.