Reference Library/Applications:Municipal Wastewater

H₂S CONTROL

solids processing intro

Basis of Control

H2O2 controls odors and corrosion within gravity thickeners by direct oxidation of H2S (as it rises from the solids blanket) and, to a lesser degree, by prevention of odor generation (by supplying dissolved oxygen). Control is typically achieved by adding 10-20 mg/L H2O2 to the influent to the thickeners. 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 > 2-4 feet; or 3) waste activated sludge is co-thickened with the primary solids. It is generally not practical to eliminate odors within the thickened sludge - supplemental post-treatment will be needed if this is important (see below).

H₂O₂ + H₂S ----> S₀ + 2H₂O

For solids dewatering (e.g., by centrifuge or belt filter press), the mechanism of control is direct oxidation of H2S within the feed solids. The range of suitable reaction times is 2-30 minutes prior to the dewatering device. The efficiency of treatment depends upon the available reaction time, the initial level of H2S, and the relevance of non-H2S odors. Under optimal conditions, effective dose ratios are about 5 parts H2O2 per part aqueous sulfide, and can be reliably estimated through beaker tests.

Practical Considerations

The most frequent obstacle to implementing H2O2 for solids dewatering involves providing sufficient reaction time. Although a minimum of 2-3 minutes is recommended, there are certain instances (where iron levels are > 10-20 mg/L) where 60-70% completion is achieved within one minute.

In some cases, a secondary issue may arise with regard to non-H2S (organic) odors. In such cases, longer reaction times should be considered. This will allow the slower reaction of H2O2 with these organics to proceed, while encouraging the bio-mediated oxidation of these odors through supply of dissolved oxygen.

The effect of this treatment on thickening/dewatering efficiencies is neutral (if H2O2 is replacing an existing chemical treatment) to positive (if no prior sulfide treatment exists). At the doses used in these applications, H2O2 will not interfere with the performance of most polymers.

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

The evaluation process for odor control in solids dewatering 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 and plant operating records; and an on-site beaker study of the H2O2 - sulfide reaction within the sludge 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.

The evaluation process for gravity thickeners can be more complicated but still, reasonable estimates can be made by reviewing the facilities design and plant operating records; and performing on-site beaker tests.

The following information will greatly assist in completing a preliminary assessment:

Schematic of unit processes showing: the number, location and sizing of clarifiers, thickeners, holding tanks, and dewatering devices, including the sizing and length of the interconnecting piping;
Average daily flows to and from these units;
Description of the dewatering system (number, type and capacity of devices);
Operating records showing: the amount and type of sludge processed; the schedule/frequency of operation; and daily or seasonal variations on these practices;
If appropriate, estimated hydraulic retention times and solids blanket depths within the thickeners;
Current (and abandoned) chemical injection points, if any, and type of chemical, dosing rates, costs, and any comments on effectiveness;
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;

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