Reduced Sludge Production in BNR Systems: Reality or Myth?

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This paper compares the performance of a pilot Modified Ludzack Ettinger (MLE) process treating degritted municipal wastewater, with full-scale activated sludge plants at two towns in Southern Ontario, namely Listowel and St. Marys. At Listowel, the MLE process achieved superior effluent quality compared to the full-scale extended aeration system at a total bioreactor HRT of 20 hours versus 48 hours for the full-scale plant. Furthermore, the overall sludge yield of the MLE process of 0.59 gTSS/gBOD was 40% lower than the full-scale plant at the same SRT of 17 days. St. Marys wastewater treatment plant employed a conventional activated sludge system with a primary clarification, and the MLE pilot process achieved a comparable effluent quality, albeit with much lower total nitrogen concentrations compared to the full-scale system at a similar bioreactor HRT of 13-14 hours and a comparable SRT of 8 days (versus 6.5 days at the full scale plant). The overall yield of biological sludges at St. Marys pilot plant of 0.83 gTSS/gBOD was 45% lower than the full-scale yield. In both studies, performance variability as reflected by changes in effluent characteristics was less pronounced in the MLE system than the full-scale activated sludge plants, emphasizing the process ability to dampen the wide influent loading fluctuations. Interestingly, due to relatively high raw wastewater COD of 700-1000 mg/L, both MLE pilot plants achieved enhanced biological phosphorous removal, matching the full-scale effluent soluble phosphorous concentrations achieved with alum addition.
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