Headworks Primary Wastewater Treatment for Odor and Corrosion Using Hydrogen Peroxide
Hydrogen Peroxide typically
controls odors and corrosion at primary watsewater treatment plant
headworks by direct oxidation of hydrogen sulfide (H2S) within 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, generally as the wastewater line enters the plant boundary.
The
efficiency of hydrogen peroxide 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
odor. 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.
H2O2 + H2S → S0 + 2H2O
Frequently,
control of odors through the primary clarifiers is wanted. In such
case, the mechanism of control is both direct oxidation of H2S (as it
rises from the solids blanket), and prevention of odor generation (by
supplying dissolved oxygen). Control is typically achieved with a
booster dose of 1-2 mg/L H2O2 added to the clarifier influent. Higher
doses or alternate modes of addition may be required in cases where: 1)
hydraulic retention times are > 2-3 hours; 2) solids blanket depths
are > 1-2 feet; 3) soluble BOD levels are > 200-300 mg/L; or 4)
waste activated sludge is co-settled with the primary solids.
2H2O2 → O2 + 2H2O
Practical Considerations for Odor Control with Hydrogen Peroxide
The
most frequent obstacle to implementing H2O2 for headworks odor control
involves providing sufficient reaction time. Although a minimum of 5
minutes is recommended, there are certain instances (where iron levels
are > 2-3 mg/L) where 60-70% completion is achieved within two
minutes.
It is also important to carefully define treatment needs
hourly and seasonally, as this will greatly affect chemical demands.
This has two aspects: changing sulfide loadings and changing levels of
control. Because of reduced wastewater flows in the early AM hours and
reduced temperatures in the Winter season, sulfide loadings during these
times can be half of those during Summer afternoons. Similarly,
different levels of control may be needed for different times of the day
and year. Factors related to this issue include: 1) the relation
between aqueous levels and atmospheric release (i.e., effect of
wastewater flow on turbulence); 2) the relation between atmospheric
release and ambient levels (i.e., effect of wind speed); and 3) the
relation between ambient levels and odor complaints (i.e., effect of
wind direction and outside activities).
The availability of
supplemental odor treatment (e.g., odor scrubbers) will affect the
economics of treatment in two ways. First, it will lessen the need to
control sulfides to very low levels; and second, significant credits
will be afforded by the reduced operating cost (chemical use) within the
supplemental treatment device.
Evaluation Process for Headworks Odor Control with Hydrogen Peroxide
The
evaluation process for headworks odor control is straightforward and
reasonably accurate estimates can be made before progressing to a field
study. The preliminary assessment involves two aspects: a review of
facilities design, plant operating records and metereological data,
supplemented with diurnal sulfide profiling; and a beaker study of the
H2O2 - sulfide reaction within the wastewater matrix to determine the
extent of removal at varying reaction times and dose ratios. This may be
followed by a field pilot test to confirm the preliminary findings and
to establish variable compliance criteria.
The following information will greatly assist in completing a preliminary assessment:
- Schematic
of the headworks area, including the plant perimeter, influent junction
structures, bar screens / grit removal facilities and, if appropriate,
plant recycle streams and primary clarifiers;
- Approximate distances between these structures, including the nature and sizing of transfer piping;
- Average daily flows of influent interceptors and force mains;
- 24 hour profile of total plant influent flow;
- Current
and seasonal levels of aqueous sulfide, total iron, wastewater
temperature and BOD for both the individual and combined influents;
- Locations where odor release and corrosion is most severe;
- Treatment objectives (target H2S-in-Air and/or aqueous sulfide levels, if known);
- Availability
of off-site chemical feed locations (e.g., pump stations) which provide
10 - 30 minutes reaction time to the headworks facility;
- Existence
of supplemental vapor treatment such as odor scrubbers, including
information regarding scope of coverage, gas flow rate,
influent/effluent H2S concentrations, and chemical use-rates and costs;
- If appropriate, estimated hydraulic retention times and solids blanket depths within the primary clarifiers; and
- Current
(and abandoned) chemical injection points, if any, and type of
chemical, dosing rates, costs, and any comments on effectiveness;
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