Simple tools for improved management of small wastewater treatment plants

Abstract

The treatment performance of small WWTPs (< 250 PE) in England is not well understood and their ecological impact may be underestimated. However, the critical role such systems play in ensuring sustainable wastewater management, means they can no longer be neglected. The aim of this thesis, therefore, was to provide new data, understanding and analytical approaches to improve the management of existing, small WWTPs. Firstly, through an extensive sampling campaign, we found a significant difference (p < 0.05) between the effluent quality discharged from twelve small and three larger WWTPs across a range of abiotic parameters. Specifically, mean removal rates at the small plants were 67.3 ± 20.4%, 80 ± 33.9% and 55.5 ± 30.4% for sCOD, TSS and NH4-N (± standard deviation), respectively, whereas equivalent rates for larger plants were 73.3 ± 17.6%, 91.7 ± 4.6% and 92.9 ± 3.7%. A Random Forest classification model accurately predicted the likelihood of a small WWTP becoming unreliable. Among the important predictors was population equivalence, suggesting the smallest WWTPs may require particularly stringent management. Quantifying, in the raw and treated wastewater samples, three genetic faecal markers targeting Bacteroides and two targeting E. coli, revealed that humanassociated Bacteroides markers have the greatest potential as alternative performance metrics at small WWTPs, however, all markers were influenced by seasonality. Next, the problem of predicting flows at small scales was overcome using an inverse approach to solve a linear reservoir function (NSE = 0.77 – 0.93). The model was combined with the field data to generate pollutant loads and investigate the effect of influent peak loading of COD on the final effluent quality at small discharges. Simple tools developed, here, provide wastewater managers with new techniques to improve the operation and increase the understanding of small WWTPs. Growing awareness of the need for sustainable wastewater and water resources management makes the work both timely and of global relevance.