A different approach for predicting H₂S_(g) emission rates in gravity sewers

All detrimental phenomena (malodors, metal corrosion, concrete disintegration, health hazard) associated with hydrogen sulfide in gravity sewers depend on the rate of H₂S emission from the aqueous phase to the gas phase of the pipe. In this paper a different approach for predicting H₂S_(g) emission rates from gravity sewers is presented, using concepts adapted from mixing theory. The mean velocity gradient (G=γSV/μ; S is the slope, V the mean velocity), representing mixing conditions in gravity flow, was used to quantify the rate of H₂S _(g) emission in part-full gravity sewers. Based on this approach an emission equation was developed. The equation was verified and calibrated by performing 20 experiments in a 27-m gravity-flow experimental-sewer (D=0.16m) at various hydraulic conditions. Results indicate a clear dependency of the sulfide stripping-rate on G1 (R2=0.94) with the following overall emission equation:-d[ST]dt=8×10-7γSV/μwAcs1.024(T-20)(ST1+Ks110- pH+Ks1Ks210-2pH-PpH2SKH), where S_T is the total sulfide concentration in the aqueous phase, mg/L; w the flow surface width, m; A_cs the cross-sectional area, m²; T the temperature,°C; K_H the Henry's constant, mol L^-1 atm^-1; and P_pH2S the partial pressure of H₂S_(g) in the sewer atmosphere, atm.