Collection Systems Sulfide Odor and Corrosion Control using Hydrogen Peroxide
Basis of Control
Hydrogen Peroxide controls sulfide odors and corrosion within wastewater collection systems by the following mechanisms:
- Direct oxidation of hydrogen sulfide within the wastewater
- Bio-mediated oxidation of complex septic odors within the wastewater
- Prevention of septic odor formation (by providing dissolved oxygen to the wastewater)
In the direct oxidation mode, hydrogen peroxide is applied to the wastewater 5-30 minutes prior to the point where the odors are being released. Typically, this is upstream of sensitive pump stations or force main discharges.
The efficiency of treatment depends upon the available reaction time, the level of iron in the wastewater (reaction catalyst), wastewater pH and temperature, and the initial and target levels of H2S. Under optimal conditions, effective dose ratios are 1.2 - 1.5 parts H2O2 per part dissolved sulfide, and can be reliably estimated through beaker tests. This (end-of-the-pipe) direct oxidation mode has the advantages of lowest cost and ease of control -- H2O2 feed rates may be controlled online per local H2S-in-Air readings.
|H2O2 + H2S ----> S0 + 2H2O |
In the bio-mediated oxidation mode, hydrogen peroxide is applied to the wastewater 30-180 minutes prior to the point where the odors are being released. The efficiency of treatment depends upon the wastewater retention time, temperature, and BOD, and the amount of biomass available to affect the transformation (correlated to wastewater velocity). H2O2 doses of 5 - 10 mg/L are typical for domestic wastewater, but a field demonstration is recommended -- Laboratory modeling is not practical due to the subjectivity of measuring complex odors, and the uniqueness of the pipe biology. The bio-mediated approach has the advantage of controlling complex organic odors which may be difficult to oxidize chemically.
In the prevention mode, hydrogen peroxide is applied to the wastewater to prevent the formation of odors downstream. The efficiency of treatment depends upon the retention time, wastewater temperature, wastewater BOD, and whether a gravity main or force main is involved. For typical domestic sewers, hydrogen peroxide is cost-effective for gravity sewers with retention times < 3-4 hours and force mains with retention times < 2-3 hours. Within these parameters, dose ratios of 2-3 parts H2O2 per part dissolved sulfide are typical. Again, laboratory modeling is not practical due to the difficulty in mimicking pipe biofilms. The prevention approach has the advantage of protecting downstream piping from H2S-induced corrosion and, if applied widely within the collection system, substantially lowering BOD loadings to the treatment plant (by culturing aerobic biofilms within the collection lines).
In actual practice, wastewater collection systems are complex structures with dynamic hydraulics. Consequently, more than one control mechanism is often at work, and a number of factors should be considered in designing a cost-effective control program. These include:
- The presence of syphons, pipe depressions/surcharges, low velocity lines, or other aspects which contribute to solids deposition (and hence biofilm mass) within the collection system. These features increase the chemical demands of the prevention mode.
- The contribution of commercial or industrial discharges which encourage odor generation through increasing temperature, sulfate levels, suspended solids levels, biomass or BOD. These factors increase the chemical demands of the prevention mode.
- The effect of diurnal flow variations, manifolded force mains, or intermittent industrial discharges which make reliable prediction of retention times (and hence sulfide loadings) impractical. This affects the degree and frequency of overdosing and underdosing of chemical in the prevention mode.
The complexity of the evaluation will depend on the need. The most simple is a single end-of-the-pipe (direct oxidation) application. The most complex is a mixed force main - gravity main collection system with multiple flows and control points. In all cases, field demonstrations are recommended. Depending on the complexity, the following information may be useful in preparing a preliminary assessment.
- Maps of the pertinent section(s) of the collection system, including areas that are located upstream of the odor complaint (or corrosion control) area;
- Locations of all transitions from force mains to gravity mains;
- Pipe diameter, pipe geometry, pipe material, and design slopes of gravity mains;
- Average daily flows of relevant interceptors and force mains;
- Current (and abandoned) chemical injection points, if any, and type of chemical, dosing rates, costs, and any comments on effectiveness;
- Locations of any industrial discharges, and the nature and flow of such discharges;
- Monitoring locations and any points subject to regulatory compliance;
- Current and historical levels of H2S-in-Air, aqueous sulfide, and wastewater temperature;
- Locations where corrosion has been observed and comments on its severity;
- Pump station flow rates, pumping mode (intermittent or variable drive), and diurnal flow variation;
- Treatment objectives (target H2S-in-Air and/or aqueous sulfide levels, if known).
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