Microbial Biomass Grown on Primary Treated Wastewater

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

Municipal Wastewater Selection for Microbial Biodiesel Production

This research compared the effects of municipal wastewaters (i.e., primary and secondary treated) from the Christchurch Wastewater Treatment Plant (CWTP) in Christchurch, New Zealand (NZ) on microbial (microalgal-bacterial) biomass production, settleability, and quality as biodiesel feedstock. Inoculums consisted of native, mixed cultures from an oxidation pond and an activated sludge process. Growth of settleable biomass was encouraged by recycling settleable solids within laboratory-scale sequencing batch reactors (SBRs) operated using a 24-hr cycle, 8-day hydraulic residence time (HRT), and controlled climate conditions. Generally, biomass concentrations of reactors fed with primary wastewater (i.e., 200/400 mg/L final mean for Cold/Warm conditions) were at least double those of secondary wastewater reactors (i.e., 70/210 mg/L final mean for Cold/Warm conditions) due to greater nutrient loading and microbial growth. Furthermore, primary wastewater reactors demonstrated much greater settling (i.e., 76 vs. 22% on average) indicating more efficient biomass harvesting. Lipid contents and types were comparable for all microbial cultures. The benefits of high carbon and bacterial concentrations in primary wastewater appeared to outweigh any disadvantage of reduced light penetration to microalgae from shading

Growth of microalgal-bacterial biomass on primary treated wastewater

The use of microalgae as an economically viable feedstock for biofuel production requires development of efficient methods for growth and harvest of biomass. Here we describe a preliminary investigation of the growth and settling characteristics of microalgal-bacterial biomass using primary treated wastewater from the Christchurch Wastewater Treatment Plant (CWTP) as a nutrient source. Sequencing batch reactors (SBRs) were established using a mixture of oxidation pond and primary treated wastewater from CWTP. Microalgal-bacterial flocs were developed by systematically discarding the non-settleable material (supernatant) and retaining settleable solids within the reactors. Subsequent addition of activated sludge (AS) improved the settleability of the biomass but resulted in development of anaerobic conditions and increased the ammonia, COD, and TSS concentrations in reactor supernatants. Analysis of 16S ribosomal RNA (rRNA) gene clone libraries prepared from settleable and suspended (supernatant) fractions revealed distinct differences in the bacterial community structure. Settled biomass was dominated by Firmicutes (45%) of which 80% were members of the Clostridia. Proteobacteria were also abundant (39%) and included, in order of dominance, the γ- (20%) β- (10%), and α- (8%) classes. In contrast, the supernatant community was dominated by β- Proteobacteria (60%) followed by Firmicutes (26%). Further studies are planned to verify these results and determine the possible bioflocculation role of Proteobacteria in this context

Microbial (Microalgal-Bacterial) Biomass Grown on Municipal Wastewater for Sustainable Biofuel Production

High biomass productivity and efficient harvesting are currently recognised challenges in microbial biofuel applications that were addressed by this research using ecological engineering principles and an integrated systems approach. Microbial (microalgal-bacterial) biomass was grown in laboratory reactors using municipal wastewaters from the Christchurch Wastewater Treatment Plant (CWTP) in New Zealand. Reactors were inoculated with native microbes, fed with primary and secondary treated wastewaters, and subjected to various hydraulic and solids retention times (i.e., 1.4- to 9-d HRT and 4- to 80-d SRT, respectively) under cold, warm, and ambient climate conditions. Biomass settleability and productivity (i.e., settleable productivity) were sequentially improved over the course of experiments to optimise settleable productivity at 21 g/m2/d on average using primary treated wastewater, 2-d HRT, 12-d SRT, and warm climatic conditions. Secondary treated wastewater was a poor substrate most likely because of low C, elevated pH, and supersaturated oxygen levels limiting growth. Biomass recycling generally improved settleable productivity of primary treated wastewater cultures since productivity increased at short HRT and settleability increased at longer SRT. No overriding trends were found relating productivity or settleability to biomass ecology or biochemistry. Growth rate modelling of warm climate cultures indicated that heterotrophy was mostly C limited at long (≥ 4-d) HRT and DO limited at short (≤ 2-d) HRT of primary treated wastewater while photoautotrophy was probably always light limited. Nevertheless, almost 50% greater C fixation was achieved using these systems compared to conventional activated sludge systems. Cold climate cultures, with up to 66% less biomass than warm climate cultures, were limited by lower light and/or temperature (i.e., 13 °C mean water temperature with 410 μmol/m2/s photosynthetically active radiation [PAR] for 9.6 h/d vs. 21 °C mean water temperature with 925 μmol/m2/s PAR for 14.7 h/d). Biomass settleability was facilitated by microbial aggregation into stable, compact flocs over time and also by bioflocculation during 1-h sedimentation periods. These mechanisms were largely influenced by wastewater loading and microbial growth rate, but also to a lesser extent by monitoring methods (i.e., light, duration, and sedimentation container). Settleability of primary treated wastewater cultures was mainly greater than 70% and more consistent when operated at longer SRT and shorter HRT compared to only 22% on average for secondary treated wastewater cultures. Symbiotic growth of native microalgae and bacteria promoted efficient O2/CO2 exchange to improve productivity and enhanced natural floc formation to improve settleability while requiring low energy inputs and providing some wastewater treatment. These capabilities greatly increased the biomass’ sustainability for biofuel production compared to other feedstocks. This research demonstrated the value of biomass recycling to concurrently achieve greatest productivity and settleability to maximise harvestable yield since the overall growth rate of more total biomass was reduced at longer SRT which thereby facilitated excellent floc formation and sedimentation at shorter HRT. The resulting biomass was best suited for biofuel conversion pathways such as anaerobic digestion or thermochemical liquefaction. Potential other uses included animal feed and fertiliser since biomass was harvested without additional chemicals