Abstract:
Increasing legislation and rising treatment and disposal costs have promoted
optimisation of the activated sludge process to encompass reduction of waste biomass.
Manipulation of process control such as increasing sludge age and decreasing food to
microorganism ratio can lower waste sludge production, but capital works as well as
increased operating costs in the form of power requirement for oxygen supply may be
required. The need for a cost effective method of biomass reduction without capital
expenditure has prompted research into methods beyond process control. The use of
chemicals capable of disrupting microorganism metabolic pathways can theoretically
allow continuation of catabolic (degradative) paths whilst halting some or all of the
anabolic (growth) pathways. This project explored the use of metabolic inhibitors
(uncouplers, tricarboxylic acid cycle inhibitors and antibiotics) to reduce the yield of the
activated sludge process. Initial respirometric studies identified many chemicals
capable of interacting with the activated sludge microorganisms. Increased oxygen
uptake rate was taken as an indication of a good uncoupler, and tests highlighted 4
chemicals with significant potential for achieving biomass reduction (trypan blue,
rotenone, 2,4 DNP and 4 NP). These chemicals were then tested at a laboratory scale
and at bench scale in both batch and continuous simulations. Simulations were carried
out using activated sludge and settled sewage feed to obtain as realistic conditions as
possible. In batch tests, trypan blue, rotenone and 2,4 DNP successfully reduced mixed
liquor suspended solids accumulation with little effect on COD removal compared to
controls. In continuous simulations, 2,4 DNP and 4 NP both lowered yield with respect
to their relative controls. Rotenone addition did not result in lowered yield. In all cases,
any yield reduction was not at the expense of process efficiency in terms of COD and
BOD removal. At pilot scale, 2,4 DNP almost halved the observed yield compared to
the control whilst having no significant effect on BOD, COD or ammonia removal,
nitrite and nitrate production, SVI or CST. Addition of chemical uncouplers had little
effect on the species diversity of the activated sludge though a reduction in the floc size
was observed in treated samples. Selection of a suitable chemical can result in reduced
yield without detrimental effect to process efficiency in the activated sludge process. An
increase in oxygen consumption occurred which has an associated cost implication, but
this was not found to be significant compared to the savings made by reducing the yield.