H2S CONTROL
collection systems intro
Basis of Control
H2O2 controls odors and corrosion within wastewater collection
systems by the following mechanisms.
- Direct oxidation of H2S 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, H2O2 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, H2O2 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.
2H2O2 ----> O2 + 2H2O
In the prevention mode, H2O2 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, H2O2 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).
Practical Considerations
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.
Evaluation Process
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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