The High Rate Algal Pond (HRAP) is an efficient treatment system for controlling wastewater pollution by reducing the organic matter and the inorganic nutrient content. Deterministic modelling of algal growth could provide a rational basis for pond design and management. This study compares two deterministic models for algal growth in HRAP s. The Montpellier model predicts the chlorophyll a concentration in the HRAP s, based on daily observed irradiance, calculated temperature, observed chlorophyll a content, zooplankton and grazable phytoplankton numbers. The Auchincruive model predicts algal dry matter and oxygen production at 1 cm intervals throughout the pond depth, based on daily irradiance, day length, temperature minima and maxima and the chlorophyll a or dry matter content. Both models correlate well with weekly measurements of chlorophyll a contents in HRAP s treating rural sewage at Mèze, France. The Montpellier model also correlated well with daily measurements of chlorophyll a contents in an HRAP treating an artificial sewage at Auchincruive, Scotland. The correlation was less good for the Auchincruive model. Reason for these discrepancies and the relative advantage and disadvantage of each model are discussed.

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