Microbial Biomass Grown on Primary Treated Wastewater

Abstract

This preliminary research examined microbial biomass growth in sequence batch reactors fed primary treated wastewater from Christchurch Wastewater Treatment Plant (CWTP), New Zealand. Reactors were inoculated with indigenous microalgae (and bacteria) from oxidation ponds at CWTP. Microalgal-bacterial flocs were developed by systematically discarding the non-settleable material (supernatant) and retaining settleable solids within the reactors. Treated ammonia concentrations averaged 5.5 mg/L (as N) with a 9-day hydraulic retention time, representing a reduction of feed water concentrations by 46-100%. This ability is of significance to municipal wastewater treatment plants who struggle to remove ammonia. Activated sludge was then added to improve biomass settleability, but resulting anaerobic conditions caused a loss of aerobic bacterial activity. Bacterial community analysis using 16S ribosomal RNA gene sequencing confirmed the dominance of anaerobic genera within the settleable biomass. Highly reducing conditions within the reactors inhibited nitrification, the pH (i.e., typically < 8.0) prevented N2 volatilisation, and some ammonification occurred, so ammonia levels decreased slowly. Activated sludge addition also caused a decrease in microalgae growth and diversity possibly due to ammonia toxicity, higher organic loading, and/or shading by excess bacteria hindering photosynthesis. Despite these effects, settleability improved with activated sludge addition. Bioflocculation and incorporation of microalgae into the activated sludge flocs were the primary mechanisms affecting biomass settleability.