Reject water comprised of anaerobic digester supernatant and the liquid reject from sludge dewatering operations (filtrate and centrate) is a problematic recycle stream for municipal biological wastewater treatment facilities. The high ammonia loadings exert a substantial oxygen demand to the front end of nitrification facilities. The situation is compounded by imported nitrogen to centralized sludge processing facilities and intermittent operation of dewatering facilities. A number of side-stream reject water treatment configurations have emerged in recent years. In addition to reducing the impact of recycle loadings, a number of the configurations tout the potential for cultivating a stable source of nitrifying microorganisms to bioaugment the main-stream.
This research investigated the impact of engineering decisions, specifically the choice of a plug flow reactor (PFR) versus a continuously stirred tank reactor (CSTR) configuration of the side-stream reject water treatment bioreactor, on the structure and function of ammonia-oxidizing bacteria (AOB) communities. The objective of this research was to assess the extent and impact from bioaugmentation of side-stream biomass into the main-stream process. A two-sludge configuration in which the side-stream biomass is isolated from the main-stream biomass was selected to facilitate the development of unique populations in the side-stream bioreactor in order to further allow evaluation of the impact of biodiversity on nitrification function. The goal was to show that input biomass from side-stream reactors allows main-stream reactors to maintain performance under conditions that cause failure in control reactors not receiving input biomass. Chemical analysis of bioreactor effluent was used to characterize bioreactor function. Biomass samples collected from the experimental bioreactors were used in molecular biology assays to characterize microbial structure and to use in respirometric assays to further characterize nitrification function.
Molecular analysis of experimental and full-scale systems revealed that the AOB community structure in side-stream reject water treatment bioreactors matches the AOB community structure in the main-stream bioreactors in which the side-stream biomass is input. This was true for either a two-sludge configuration in which side-stream biomass is isolated from the main-stream or for a single-sludge system with continuous re-inoculation of the side-stream bioreactor from the main-stream RAS. It is also evident that bioaugmentation of main-stream bioreactors with biomass from side-stream reject water treatment increases AOB diversity in the main-stream. Furthermore, a main-stream bioreactor with biomass input from a PFR side-stream exhibited greater AOB diversity than a main-stream bioreactor with biomass input from a CSTR side-stream. A potential benefit of increased AOB diversity due to bioaugmentation from the side-stream system is increased system reliability in the face of short-term toxicity events. Furthermore, side-stream bioreactors would be beneficial as a source of nitrifiers to re-seed the main-stream bioreactor following a seasonal transition to cooler wastewater temperatures or short-term peak flows during wet weather conditions. In this research, bioaugmentation may have prevented washout of critical populations in the main-stream bioreactor coupled to a PFR side-stream bioreactor thereby stabilizing nitrification.