In-sewer treatment of domestic wastewater

Abstract

Urban sewerage systems, which are normally used for the transport of wastewater from its origin to a wastewater treatment plant (WWTP), could be used as a treatment facility because (i) they contain heterotrophic bacteria capable of oxidising organic matter both suspended within the body of the flowing wastewater and attached to the surface of the wetted perimeter, and (ii) they provide retention times which are often comparable to those in a conventional activated sludge aeration tank and which, in some cases, may be equal to the hydraulic retention time in a WWTP. Using sewers as a treatment facility could be an economical method of alleviating the load on an existing WWTP or reducing the size of the proposed WWTP. The current study was undertaken to investigate the feasibility of using urban sewerage systems as suspended growth biological reactors for the treatment of domestic wastewater. The flow in a linear gravity sewer was simulated using a batch reactor fed with raw domestic wastewater. A comparison of simulated aerobic and anaerobic gravity transport indicated that aerobic treatment would be the most favoured method of in-sewer biological treatment. The soluble COD (SCOD) removal efficiencies over a retention period of 8 hours averaged 36 and 6% under aerobic and anaerobic conditions, respectively, at an average temperature of 22°C. The corresponding total COD removal averaged 8 and 11%, respectively. When the effluent samples, taken from the batch reactors after a retention period of 6 hours, were settled in a bench-scale settling column for one hour, the average suspended solids removal under aerobic conditions was 29% greater than those under anaerobic conditions. Under aerobic conditions, the removal of soluble organic matter during simulated gravity transport was found to be strongly influenced by the strength of the incoming wastewater. To investigate the effect of wastewater influent soluble COD (SCOD 0) and influent suspended solids (SS ()) on in-sewer aerobic treatment, 27 individual wastewaters collected from the inlets to three wastewater treatment plants were subjected to batch tests at 20°C. The SCOD over a retention period of 8 hours at 20°C averaged 48, 40 and 61% for wastewaters having low SCODo and low SS 0, high SCOD0 and low SS 0, and high SCOD0 and high SSo, respectively . The corresponding soluble BOD5 removal efficiencies averaged 64, 59, and 81%. A statistical analysis of soluble COD data revealed that, over a retention period of one to three hours, the soluble COD removal is only significantly influenced by SSo. At higher retention periods, the soluble COD removal was found to be significantly affected by both SCOD 0 and SSo. Soluble COD removal was found to follow first-order kinetics with respect to time. The oxygen uptake rate of the individual wastewaters varied widely and did not appear to show any clear relation with the SCOD0 or SSo. An increase in the suspended biomass of the wastewater by the addition of activated sludge, at a concentration as low as 100 mg VSS/1, at the inlet of the simulated aerobic gravity sewer resulted in a significant increase in the removal of soluble organic matter. The soluble COD removal in the seeded wastewater was found to increase almost linearly with the increase in seed concentration in the range of 100-1000 mg/l. The effect of seed concentration on soluble COD removal however, appeared to diminish with the increase in retention time. SCOD removal in the seeded wastewater appeared to follow secondorder kinetics with respect to time. At an initial seed concentration of 100-1000 mg/1, the batch reactor's effluent after a retention period of 6 hours showed satisfactory settling characteristics. The oxygen uptake rate of the seeded wastewater did not show any specific trend over time at seed concentrations of 100 and 250 mg/1, while at higher seed concentrations it was similar to that observed in a typical plug flow activated sludge aeration tank. The results of the case study in which the wastewater collected from the inlet of the Greater Amman Siphon (GAS) was maintained aerobic in a batch reactor, showed that by maintaining aerobic conditions in the GAS, average SCUD and soluble BOD 5 removal efficiencies of 60 and 78%, respectively, could be achieved over 8 hours at an average temperature of 25°C. The average oxygen demand of the wastewater was estimated to be 30 mg/l.h. The result of the current study suggest a strong possibility of using urban sewerage systems as an aerobic biological reactor for the removal of soluble organic matter during transit.