Biosolids Odor Control at Holding Tank and Belt Filter Presses via Hydrogen Peroxide

Project Scope

A Midwest wastewater treatment plant was struggling with problematic hydrogen sulfide (H₂S) levels at two points within their facility arising from biosolids processing. The factors driving the need for H₂S control included worker safety, corrosion of electronics and concrete, implied regulatory limits and biosolids odor control. Along with normal residential and commercial flows, the facility also receives unpredictable slugs of high biological oxygen demand (BOD) wastewater from agricultural industries and handles septage hauler loads from a wide radius. USP Technologies (USP) implemented a full-service hydrogen peroxide (H₂O₂) solution with operator-adjustable dosing options to account for the unpredictable sulfide loadings and to lower H₂S to acceptable levels at both their biosolids holding tank and dewatering building.

Technology

H₂O₂ is ideally suited to remove H₂S from wastewater solids, provided that sufficient contact times are available – typically 2-5 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. The chemical reaction is as follows:

H₂S + H₂O₂ → S0 + 2H₂O

Background

A biosolids holding tank receives all biosolids awaiting dewatering at the plant’s two belt filter presses. The tank is generally emptied every one to three days depending on biosolids generation rates correlated with industrial discharge volumes. The biosolids entering the tank, especially from the gravity thickeners, contain a significant sulfide load throughout the entire year. The holding tank employs mechanical mixing and aeration to minimize further sulfide generation, however, this causes extreme volatilization of H₂S, exceeding 500 ppm. Previous control methods, including iron salt addition and “in-situ scrubbing” using a hydroxyl ion generating system only provided limited success in achieving desired targets.

Compounding the challenge, the radicals generated interfered with the proper measurement of H₂S. From the holding tank, biosolids are pumped to a dewatering building housing two belt filter presses. The turbulence of these belt filter presses causes most of the sulfide present to volatilize. A wet scrubber was installed and used for years to control the H₂S, however, by 2014 it was at the end of its serviceable lifespan.

Solution

USP was invited to visit the facility to determine a more effective treatment strategy. Bench scale dose response tests utilizing shake tests were then conducted to determine theoretical hydrogen peroxide dosing needs and possible reaction times from several injection points. The optimal injection point was determined to be at the biosolids pump outlet, which provided approximately 3 minutes of reaction time before reaching the belt filter presses.

Shortly afterwards, a full-scale H₂O₂ storage and dosing equipment system was installed. The H₂O₂ pumps were connected by relays to the biosolids pumps to ensure that H₂O₂ only dosed while the biosolids pumps were running. The program was demonstrated to be effective within the first hour of operation, with H₂S levels brought down below 1 PPM both above the belt filter presses and within the dewatering building’s main room. The program continued with dosing rates throughout the first six months ranging from 1.5 to 6 gallons/hour.

In addition, the operators were trained on how to both raise and lower dosing rates based on the observed H₂S levels to mitigate variable industrial loadings. The removal of H₂S from the dewatering facility eliminated the need for a costly replacement of their wet scrubber, and the customer invited USP to co-present our success story at their state conference.

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Biosolids Odor Control at Holding Tank and Belt Filter Presses via Hydrogen Peroxide