Waste activated sludge high‐rate treatment of septage: Biodegradability studies and contact phase trials towards a cleaner environment

Abstract The waste activated sludge high‐rate (WASHR) process, developed in our previous study, is used for septic wastewater treatment. This high‐rate contact stabilization pre‐treatment uses typical waste streams found in wastewater treatment plants to reduce a portion of the loadings on the main treatment trains. This batch process starts with a contact phase where wastewater is added to mixed liquor and left to aerate over a short residence time, allowing for the physical adsorption of organics onto the biological floc while limiting the oxidation of contaminants. After clarification, the settled biomass is sent to a stabilization tank for the oxidation of biodegradable contaminants. The biodegradability assessments of three actual septage samples along with a dilution series are studied to observe the toxicity of the samples towards aerobic microorganisms at various concentrations. The BOD 5 /COD ratios of the samples were 0.22, 0.40, and 0.44 with the distinction between septage from septage tanks and from portable toilets being a major source of variability. An analysis of the WASHR contact phase for septage treatment is also presented, where recommendations for the septage pH, contact time, and septage loadings are given for maximizing supernatant COD removals while achieving appropriate sludge settleability characteristics. It was determined that the septage pH was a non‐significant factor as the mixed liquor used for these trials had strong buffering capabilities, and the optimal loading rate was 2 mgCOD/mgMLVSS achieving a maximum supernatant COD removal of 74.3% and a sludge volume index (SVI) of 48.2 mL/g after 5 h of reaction time.