Optimizing the Predator/Prey Relationship in a Treatment System to Minimize Operating Costs

Elliott, A. and Mahmood, T., Paprican, Canada


With sludge management costs as high as 60% of the total wastewater treatment plant operating costs, it is economically
advantageous for industrial applications such as with the pulp and paper sector, as well as the municipal sector to reduce their
biosolids production. A reduction of excess biological sludge generated can substantially reduce these operating costs.
Through the manipulation of the aeration stages for activated sludge systems, a scenario has been developed to reduce the
generation aerobic sludge. This process is known as the low sludge production (LSP) process. In this process the return
activated sludge by-passes the first (dispersed growth) stage to be received only by the second (predatory) stage. The
resulting once-through operation of the first stage makes it potentially susceptible to bacterial washout under hydraulic shock
conditions, which is an important consideration in full-scale implementation of this process. We have performed a sensitivity
analysis of the first stage operation by varying its HRT time. The experimental data revealed that the optimal first stage
hydraulic residence is between three to five hours with bacterial washout likely to be initiated below two hours.
A dual-train activated sludge pilot treatment system was installed at pulp and paper mill site to evaluate the potential benefits
of the LSP process. The treatment performance, including aeration demand and sludge dewatering, of the LSP train was
compared with that of a conventional activated sludge (CAS) train. The LSP system produced 31% less sludge. The LSP
system required 19% more aeration. Treatment performance in terms of BOD and COD removals remained unchanged
compared to the CAS system. Ammonia release in the predatory stage was exhibited. The LSP system consistently exhibited
lower settling indexes than those from the CAS system. A 40% reduction in dewatering cost was calculated for the mill if an
LSP system were to be implemented.
Based on these findings, as well as data from other researchers, the process has been implemented at several pulp and paper
installations in Canada. There is potential for the transferability of this process to the municipal sector but several factors must
be considered.

Low Sludge Process, LSP, Sludge Reduction, Dispersed Growth, RAS, Pulp and Paper

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