Population dynamics by methanol addition in denitrifying wastewater treatment plants

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Abstract
The population dynamics were tracked in the system ‘denitrification with methanol’. Experiments with both anoxic and aerobic cultivation were carried out in a lab plant comprising a sequencing batch reactor. Three cultivation periods were tracked: an anoxic 10°C phase, which is not discussed here, then an anoxic 20°C phase and finally a mixed aerobic/anoxic 20°C phase. Batch tests were performed to characterise the adaptation period of the methanol degraders and to determine kinetic and stoichiometric parameters as well as the effects of temperature and of inhibitors. A mathematical model for denitrifying methanol degraders was developed and calibrated with the measured data. The verification was performed by full-scale experiments at the wastewater treatment plant of Zürich-Werdhölzli. The results show that it can be modelled by two types of microorganisms, mainly involved in the process of methanol degradation. Which type dominates in the system depends on the mode of cultivation. In order to optimise the process with respect to cost and performance, it is important to cultivate the beneficial population. This would be the one that produces less sludge and grows anoxically faster than others. A highly selective criterion is the available supply of soluble oxygen. So optimising nitrate elimination by denitrification with methanol means minimising the simultaneous exposure of the biomass to oxygen and methanol.

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