Influence of Intermittence and Pressure Differentials in Hydrogen Sulfide Concentration in a Gravity Sewer

Abstract

The work presented herein was carried out to assess the effect of intermittent pumping events in sewer headspace pressure differentials, as well as their relationship with hydrogen sulfide gas concentration. A full scale gravity sewer in Portugal, located downstream of several pumping stations, was used as the guiding case study. Under normal system operation, pressure difference between the outside atmosphere and the sewer headspace seemed to influence the in and out-gassing of gas pollutants emitted through the venting stack. Wastewater pumping cycles generated maximum pressure differentials of roughly 100 Pa, which in turn originated maximum air velocities of 1.76 m s−1 exiting the venting stack. Each pumping event was followed by a pressure drop of about 50 Pa, quickly attaining null concentrations of H2S at the venting stack. A statistically significant relationship between pressure differentials and air exit velocity was observed, which allowed obtaining an empirical equation for expedite prediction of airflows emitted to the outside atmosphere (R2 = 0.77). Conversely, the same effect was not observed along the length of the sewer pipe, unlike the findings of other studies. The effect of a full flowing pipe at the downstream end of the gravity trunk sewer was also noticeable in downstream sewer pressurization and gas build-up. It was concluded that the magnitude of the gas pollutant emissions may heavily depend on the impacts of hydraulic flows and pumping characteristics in headspace pressure differences, denoting the need for better approaches when designing and installing venting stacks.

Model-Driven Strategies for Sulfide Control in a Regional Wastewater System Receiving Tannery Effluents in Portugal

“…Nonetheless, most field studies and model applications found in the literature refer to either domestic wastewater conveyance networks [10][11][12][13], or lab/pilot scale abatement strategies for source control, for varied industrial sectors [42,43]. Industrial effluents with significant pollutant loads, in terms of sulfide and other relevant parameters for the sulfur cycle, may require higher reagent dosage to control the fate of sulfide and H 2 S along the sewer system.…”

“…This indicates that wastewater discharges may have occurred where sulfide was not present, and H 2 S in the system can be expected to correlate heavily with the timing of tanning industry discharges. In addition, it is known that headspace pollutant concentrations are also heavily influenced by ventilation parameters, such as wastewater drag, turbulence, wind speed, fluctuations in wastewater levels, buoyancy, temperature and headspace pressurisation [13], which illustrate the complexity in addressing and accurately modelling these processes.…”

“…While it is known that severe corrosion issues in sewers may occur with sulfide concentrations over 2.0 mg/L [3], ranges of dissolved sulfide contents between 3.5 and 12 mg/L have been reported for domestic effluents, originating peak H 2 S headspace concentrations of about 100-260 ppm [10][11][12][13]. Some industrial effluents contain considerably higher sulfide contents, thus presenting different characteristics than typical domestic wastewater systems.…”

Prediction of hydrogen sulfide emission from an energy dissipation chamber and assessment of its distribution in the ambient air

“…Some of the previously mentioned studies [20,21,24] allow us to estimate the amount of H2S formed inside single sewer sites and on sites consisting of a complex of networks and structures. However, up to date, there are no methods to determine the amount of gases that are released in specific sewerage facilities.…”

“…Work is also under way to study the factors affecting the gases emission within networks and their distribution. [20][21][22], the type of transportation networks (combined, separate) and hydraulic characteristics of the flow in them (slope, filling, length) [23,24], the velocity of air masses movement in the headspace of structures and sewers [25], changes in pressure within the network [22], changes in properties and composition of wastewater [26][27][28].…”