“Doing the Two-Step” – Reduced Energy Consumption Sparks Renewed Interest in Multistage Biological Treatment

Historically, the implementation of two-stage treatment (i.e. carbonaceous removal followed by nitrification) has been implemented at wastewater treatment plants that originally were designed with the carbonaceous removal step first. Some of the primary reasons for implementing two-stage treatment included ease of implementation (i.e. challenging to de-rate existing plants to allow for nitrification) and to reduce or mitigate the impact of toxic substances on the nitrifying organisms. Another key benefit of two-stage treatment that is gaining greater prominence in recent years is the improved overall sustainability of these processes compared to single stage treatment in terms of aeration energy and biogas production. This paper examines, through a generalized modeling study, the difference in aeration energy and biogas production of single stage activated sludge compared to two-stage treatment processes (i.e. conventional two-stage treatment and the A/B Process). The results from this study suggest that overall aeration energy savings could be in the order of 15-20% less with two stage treatment processes, while biogas production could be 25-38% higher with two-stage treatment. These two factors are the primary reasons why some treatment facilities (e.g. Strass, Austria) are demonstrating that net energy positive wastewater treatment is indeed possible.