An experimental and mathematical simulation of biological processes in a sewerage system

Sewer systems may often operate as bioreactors causing considerable wastewater quality
changes and in some cases reducing the pollution load conveyed to the treatment plant.
Conceptually, transformation of organic matter takes place both in the bulk water (by
suspended micro-organisms) and by the sewer biofilm, under aerobic and anaerobic
conditions. The most important processes are the hydrolysis of hydrolysable substrate to
readily biodegradable substrate and the uptake of the latter for the growth of the biomass (in
the form of biofilm and suspended in the liquid) and for maintenance purposes. Depending on
whether the critical parameter is the hydrolysis rate or the growth rate of biomass there is a
significant increase (former) or decrease (latter) of readily biodegradable substrate.
The scope of the work presented herein was to study the processes governing the fate of
readily biodegradable substrates in sewer systems in an attempt to verify the mathematical
model of a sewerage system developed by Gavalakis et al. (2003) [5]. The specific objectives
were to evaluate the effect of (1) biofilm growth, (2) suspended biomass and (3) dissolved
oxygen (DO) concentration on readily biodegradable substrate removal.
The experimental results obtained showed that high soluble substrate uptake rates can be
observed in sewer systems indicating that sewer systems can act as biological reactors where
substantial changes in sewage quality characteristics can occur. Especially for soluble
substrates like acetate these changes that take place in the sewer may significantly affect
enhanced biological phosphorus removal and denitrification in the wastewater treatment.
The experimental data on soluble substrate removal in sewers showed that the previously
developed mathematical model describes in a satisfactory way the kinetics of acetate
removal. Acetate removal rates due to biofilm growth did not appear to be influenced by
biofilm mass or biofilm thickness. Acetate removal as described by the model appeared to
follow a linear correlation between pipe surface to liquid volume (A/V) ratio and soluble COD
removal was obtained. In addition acetate uptake rate appeared to increase significantly with
increasing DO concentration. The kinetics describing the effect of DO on substrate removal
appeared to follow a half order reaction rate throughout the range of DO concentrations
studied.