Reduced Sludge Production in BNR Systems: Reality or Myth?

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 0.83gTSS/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.

Denitrification and enhanced biological phosphorous removal are integral to biological nutrient removal processes. As regulations for biosolids become increasingly stringent, and the cost of sludge processing escalates, it is critical to evaluate sludge production from biological nutrient removal (BNR) plants. There are controversial reports on the impact of anoxic conditions on sludge yields. IWA models such as ASM1, ASM2, etc. use a similar true yield under aerobic and anoxic conditions although selected commercial software packages such as Biowin (Envirosim Associates, Burlington, ON) use an anoxic yield that is 15% lower than the aerobic yield. Ferrer et al. (2004) successfully modeled two treatment plants in Spain with ASM2d using identical anoxic and aerobic yields while Brdjanovic et al. (2000) modeled a medium-size wastewater treatment plant in the Netherlands using same anoxic and aerobic yields with the Delft bio-P model. Lishman et al. (2000) in a study of sequencing batch reactors treating a mixture of raw municipal wastewater and a synthetic protein solution observed anoxic yields higher than aerobic ones, while Spérandio et al. (1999) conducted respirometric studies on synthetic substrates and soluble municipal wastewater and observed 15-20% lower anoxic yields compared to aerobic 359 yields.

Similarly, Muller et al. (2003) used oxygen uptake rates and nitrate uptake rates in batch studies of municipal wastewater by sludges from a Modified-Ludzack Ettinger process to assess anoxic and aerobic yields and concluded that anoxic yields are 19% lower than aerobic yields. Chuang and Ouynag (2000) observed during a bench-scale BNR study of synthetic municipal wastewater that about 48% of the total sludge yield occurs anoxically, implying that overall sludge production would be reduced by a mere 8-10% in BNR processes. Invariably, all previous studies on anoxic yields have been conducted in the lab, and predominantly with synthetic substrates. In real life, several factors complicate the delineation of the impact of BNR processes on sludge production. First, the fraction of readily biodegradable COD (RBCOD) in the wastewater varies not only from plant to plant but also from time to time depending on industrial contribution. The extent of denitrification is also time- and site-dependent, thus affecting both RBCOD consumption and sludge yield. Additionally, the accumulation of slowly biodegradable particulate substrates and inert particulates in the system will vary with sludge retention time (SRT) and hydrolysis kinetics. In plants that employ enhanced biological phosphorus removal (EBPR), phosphorus accumulating organisms (PAOs) introduce another complexity that is difficult to quantify. Furthermore, the inability to provide site-specific comparison with conventional systems emphasizes the need for pilot studies.

The objective of this work is to shed light on the experiences derived from comprehensive pilot BNR studies at two municipal plants in Southern Ontario, with particular emphasis on a comparative assessment of sludge production. Both plants, Listowel WWTP (Town of Listowel) and St. Marys WWTP (Town of St. Marys) receive highly variable industrial wastewaters.

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