Understanding the effect of changing urban water management on sewer emissions

Abstract

Sewer emissions, such as hydrogen sulfide, methane and volatile organic sulfur compounds (VOSCs) can cause severe problems to sewer systems and the overall environment, including sewer pipe corrosion, malodor nuisance and greenhouse effect. In the past few decades, various studies have been carried out to understand, to model and to mitigate the effects of sulfide and methane emitted from sewers. However, with the perceived implementation of new urban water management practices, like water demand management or decentralized water management, the sewer conditions will change accordingly. These changes may affect existing in-sewer biochemical processes as well as the sewer emissions. Therefore, the overall aim of this thesis is to understand the effect of changing urban water management on three major sewer emissions, i.e. hydrogen sulfide, methane and VOSCs. The effect of reduced water consumption (RWC), achieved by water demand management, on sulfide and methane production in rising main sewers was investigated through laboratory tests and mathematical modeling. Under RWC conditions, both sulfide and methane concentrations increased in rising main sewers. The increase of sulfide concentration was mainly due to the longer hydraulic retention time (HRT), as the sulfide-producing activity of sewer biofilms was not significantly affected. Whereas, the higher methane concentration under RWC condition was caused by both enhanced methanogenic activity and the longer HRT. The mathematical modeling revealed that the volumetric chemical dosing rate for sulfide mitigation would increase; however, due to the lower flow rate, the daily chemical dosing cost would decrease. The microbial community structure and activities of sewer biofilms under RWC conditions was investigated using a combination of microelectrode measurements, molecular techniques and mathematical modeling. It was seen that sulfide was mainly produced in the outer layer of the biofilm, between 0 - 300 mm, which was in good agreement with the distribution of sulfate reducing bacteria (SRB). SRB had a higher relative abundance of 20% on the surface layer, which decreased gradually to below 3% at the depth of 400 mm. In contrast, methanogenic archaea (MA) mainly inhabited in the inner layer of the biofilm, with their relative abundances increasing from 10% at a depth of 200 mm to 75% at a depth of 700 mm, into the biofilm. The biofilm modeling indicated that the coexistence and spatial structure of SRB and MA in the biofilm were due to differences in the microbial types, their proposed metabolic transformations and substrate utilization. The effect of iron-rich coagulation sludge, discharged from decentralized systems, on sulfide and methane production in rising main sewers was investigated through laboratory studies. It was observed that the application of the iron-rich coagulation sludge significantly reduced the total dissolved sulfide concentration in sewers. The decrease of dissolved sulfide concentration was mainly due to the precipitation between iron and sulfide, but other reactions might be also involved. The results indicated that iron-rich coagulation sludge could be used to control sulfide levels in sewer systems. The addition of sludge slightly increased the total chemical oxidation demand (tCOD) concentration (by approximately 12%), but slightly decreased the soluble chemical oxidation demand (sCOD) and methane formation by 7% and 20%, respectively. In order to understanding the transformation of VOSCs under different sewer conditions, an efficient method was developed to measure dissolved VOSCs in wastewater. This method used gas chromatography with a sulfur chemiluminescence detector (GC-SCD) and a static headspace technique. The method is simple and rapid, as it requires no pre-concentration treatment of samples. It has low detection limits (l1.0 ppb) and good linearity (g0.999). The recovery ratio tests and real wastewater sample analysis demonstrated that this method was suitable for routine VOSCs measurement in wastewater. In addition, sample preservation tests showed that VOSCs in wastewater samples could be preserved for at least 24 hours by acidification (pH ~1.1). Thus, this method can be used for both laboratory studies and field measurements. With use of the GC-SCD method, the degradation of methanethiol (MT), a predominant VOSC in rising main sewers, was investigated under different biofilm development conditions. MT degradation was found to be strongly dependent on the methanogenic activity of sewer biofilms. The MT degradation rate accelerated with the increase of methanogenic activity of sewer biofilms, resulting in MT accumulation in sewers with relatively low methanogenic activities, and MT removal with higher methanogenic activities. A modified Monod-type kinetic expression was developed to describe MT degradation kinetics in anaerobic sewers, in which the maximum degradation rate was correlated to the maximum methane production rate through a power function. It was also found that the MT concentration had a linear relationship with the acetate concentration, which might be used for preliminary assessment of MT presence in anaerobic sewers. The research outcomes of this thesis indicated that changes in urban water management practices would affect the in-sewer processes and sewer emissions. These unintended impacts should be considered in sewer management in future.