Experimental and modelling evaluations of sulfide formation in a mega-sized deep tunnel sewer system and implications for sewer management.

Mega-sized deep tunnel sewer systems are indispensable infrastructures to convey the sewage and/or stormwater to the centralized sewage treatment works in large cities with dense populations and limited land. The rapid urbanization in China and other countries is boosting the construction of the deep tunnel sewer systems. However, the formation of sulfide, which induces serious odor nuisance and sewer corrosion, has not been investigated in such sewer systems. Taking a real Sewage Conveyance System (SCS) with 23.3 km-long and 70-160 m-deep interconnected tunnels in Hong Kong as a representative example, this study conducted experimental and modelling investigations to evaluate the sulfide formation in the mega-sized deep tunnel sewer systems. The field investigation revealed that the daily sulfide production rate in the SCS was up to 1410 kg S/d, suggesting the substantial sulfide production during the long-distance and long-time sewage conveyance. Using a validated Biofilm-Initiated Sewer Process Model (BISM), the sulfide formation in the SCS under the influences of various factors, which are relevant to the situations in China and other countries, were simulated. The simulation results showed that 89% of the total sulfide production in the SCS was generated in the two tunnels with long hydraulic retention times (HRT) and large flowrates. The specific sulfide formation rates exhibited a linear relationship with HRT (R = 0.61), but the linear relationship with the sewer diameter was weak. The sulfide production rate increased with increasing temperature (12 °C-32 °C) by 3.5 times, and it only decreased by 50% when the sulfate concentration decreased from 309 to 17 mg S/L, indicating that serious sulfide pollution could still happen in the sewers with a low concertation of sulfate in sewage. Increasing the organic levels in sewage would also promote the sulfide production in sewers. The flowrate would not influence the sulfide production rate significantly, but a storm event could remarkably reduce the sulfide production in rainy days. The findings unveil the potential serious sulfide problems in the mega-sized deep tunnel sewer systems, which are being increasingly constructed in China and other countries. To mitigate the odor and corrosion problems in the deep tunnel sewer systems, the sulfide control strategies should be considered during the sewer design and management.