Effect of varying food to biomass ratios on the performance of sludge bioreactors for wastewater treatment

Abstract

In response to the world’s growing population, engineers are seeking for better wastewater treatment alternatives to meet the inevitable growing demand of water. Amongst the various types of wastewater treatment technologies, the activated sludge system has been proven to be one of the most promising technologies due to its high treatment efficiency albeit its low cost. Sequencing Batch Reactors (SBRs) minimize the carbon footprint required as well as the overall operational cost for the system as they eliminate the need for clarification tanks. They can be deemed as ‘three-in-one’ reactors in which all three treatment processes; equalisation, aeration and clarification can be achieved in a single reactor. The fluctuations in the daily municipal wastewater influent have been of rising concern since they affect the treatment performance of wastewater treatment units. Therefore, perturbations in the wastewater treatment process caused by the organic ‘shock loading’ in the influent quality should be properly studied to minimize their impact on large-scale wastewater treatment plants (WWTPs). The Food to Biomass ratio (F: M ratio) represents the concentration of organics present in the sludge relative to the biomass concentrations, and is thus related to the organic loading into the biological system. Studying F: M ratio pulse-press disturbance effects on wastewater treatment is relevant and essential, as too much organics could promote the growth of filamentous bacteria causing foaming and bulking problems. On the other hand, if organics in the treatment unit is too low, the microbes are unable to carry out its respective functions in the treatment units. In this project, a bench-scale microcosm experiment was set up using 50mL tubes as SBRs filled to 25mL mark with sludge from a full-scale WWTP in Singapore. To test the effects of food to biomass ratio (F: M) and carbon to nitrogen (C: N) ratio on microbial performance using a complex synthetic feed, two batches of experiment were conducted. The first batch employed different frequencies of F: M ratios (0.2 and 0.4) with a constant C: N ratio (6.8), and involved quadruplicate reactors at eight levels (n= 32) and only ran for seven days. The second batch operated six sets of five replicate SBRs (n= 30) for 42 days, in which both F: M (0.2 and 0.4) and C: N (6.7 and 12.9) ratios were imposed at varying frequencies. During this period, several indicators of wastewater performance were monitored on a weekly basis, and sludge samples were collected for DNA extraction and next generation sequencing. The process performance data obtained from this experiment generally indicated that reactors operated at lower F: M ratios had better ammonia removal and nitrifying activity. However, there were also indications of higher denitrifying activity at high F: M ratios. Chemical Oxygen Demand (COD) removal efficiency across the disturbance levels was lower in the initial phase of the experiment but was later observed to remain constant in the subsequent weeks of the experiment. Sludge Volume Index (SVI) were in the range of 40 to 90 mL/g, which indicated that the activated sludge could settle quickly within the reactors.Phosphate (PO4-P) had better removal efficiency in the reactors receiving lower F: M ratios. The comparison between the results yielded on Day 7 for phase 1 and 2 of the experiment revealed that varying the C: N ratios in the synthetic feed generally resulted in a better microbial treatment performance in the microcosm reactors. Altogether, the observed differences in ecosystem function suggest that reactors operated at different F: M ratio regimes would harbour different microbial communities. However, microbial DNA sequencing of these samples is still ongoing and hence microbial community analysis is not covered in this report, but will be carried out as part of future research. Altogether, these results will aid our understanding of how F: M ratios affect the wastewater treatment process from a functional and microbiological perspective, which will in turn enhance the management of organic ‘shock loadings’ entering WWTPs. Managing the organic “shock loadings “entering the wastewater treatment plants is of paramount importance as too much organics could promote the growth of filamentous bacteria whereas if organics in the treatment unit is too low, the microbes are unable to carry out its respective functions in the treatment units.