This article presents a mathematical model to describe High-Rate Algal Ponds (HRAPs). The hydrodynamic behavior of the reactor is described as completely mixed tanks in series with recirculation. The hydrodynamic pattern is combined with a subset of River Water Quality Model 1 (RWQM1), including the main processes in liquid phase. Our aim is to develop models for WSPs and aerated lagoons, too, but we focused on HRAPs first for several reasons:

  • • Sediments are usually less abundant in HRAP and can be neglected

  • • Stratification is not observed and state variables are constant in a reactor cross section

  • • Due to the system's geometry, the reactor is quite similar to a plugflow type reactor with recirculation, with a simple advection term.

The model is based on mass balances and includes the following processes:

  • Phytoplankton growth with NO3-, NO2- and death

  • • Aerobic growth of heterotrophs with NO3-, NH4+ and respiration

  • • Anoxic growth of heterotrophs with NO3-, NO2- and anoxic respiration

  • • Growth of nitrifiers (two stages) and respiration.

The differences with regard to RWQM1 are that we included a limiting term associated with inorganic carbon on the growth rate of algae and nitrifiers, gas transfers are taken into account by the familiar Adeney equation, and a subroutine calculates light intensity at the water surface. This article presents our first simulations.

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