The currently high sludge production and increasing processing costs call for waste-water treatment plants with high purification efficiency and low biomass production. We studied the latter issue through two-stage chemostat cascades to assess the overall biomass reduction due to ciliate grazing. The bacteria were cultured in the first chemostat whereas the ciliates, grazing on the bacteria from the first chemostat, were cultured in the second chemostat. Mathematical modelling was used to describe the bacteria/ciliate dynamics and some of the growth parameters were fitted. In the second chemostat 22-44% of the carbon originating from the first chemostat was mineralized to CO2. An extra biomass reduction of 12-43% was possible due to grazing by the ciliates. At lower growth rates of the ciliates the extra biomass reduction was higher than at high growth rates. This finding is auspicious, suggesting that predator organisms indeed can reduce sludge production.

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