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 G1G1 (R2=0.94R2=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 ST is the total sulfide concentration in the aqueous phase, mg/L; w the flow surface width, m; Acs the cross-sectional area, m2; T the temperature, °C; KH the Henry's constant, mol L−1 atm−1; and PpH2S the partial pressure of H2S(g) in the sewer atmosphere, atm.