Mathematical modelling of the removal of organic micropollutants in the activated sludge process: a linear biodegradation model

Mark Ian Nelson, Rubayyi Turki Alqahtani, Faisal I. Hai

Abstract


Before wastewaters can be released into the environment, they must be treated to reduce
the concentration of organic pollutants in the effluent stream. There is a growing
concern as to whether wastewater treatment plants are able to effectively reduce the
concentration of micropollutants that are also contained in their influent streams. We
investigate the removal of micropollutants in treatment plants by analysing a model
that includes biodegradation and sorption as the main mechanisms of micropollutant
removal. For the latter a linear adsorption model is used in which adsorption only occurs
onto particulates.
The steady-state solutions of the model are found and their stability is determined as
a function of the residence time. In the limit of infinite residence time, we show
that the removal of biodegradable micropollutants is independent of the processes of
adsorption and desorption. The limiting concentration can be decreased by increasing
the concentration of growth-related macropollutants. Although, in principle, it is
possible that the concentration of micropollutants is minimized at a finite value of
the residence time, this was found not to be the case for the particular biodegradable
micropollutants considered.
For nonbiodegradable pollutants, we show that their removal is always optimized at a
finite value of the residence time. For finite values of the residence time, we obtain a
simple condition which identifies whether biodegradation is more or less efficient than
adsorption as a removal mechanism. Surprisingly, we find that, for the micropollutants
considered, adsorption is always more important than biodegradation, even when the
micropollutant is classified as being highly biodegradable with low adsorption.

doi:10.1017/S1446181118000226

Keywords


activated sludge, biodegradation, mathematical modelling, micropollutants, wastewater treatment.



DOI: http://dx.doi.org/10.21914/anziamj.v60i0.12206



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ANZIAM Journal, ISSN 1446-8735, copyright Australian Mathematical Society.