USP Technologies was featured in the September 2017 issue of Water Environment & Technology (WE&T) magazine, in the “Problem Solvers” section. Following is the article in its entirety, you can also view a portion of it online at the WE&T website here.
Fully Automating Hydrogen Peroxide Odor Control to Treat Biosolids
Problem: Variable industrial feeds led to high hydrogen sulfide in biosolids.
Solution: Full-service, automated hydrogen peroxide dosing system handles unpredictable sulfide loadings.
Since its original construction in the late 1950s, the Norfolk (Neb.) Water Pollution Control Plant has undergone several capacity expansions. The city, which is located about 161 km (100 mi) northwest of Omaha, has reached a population of approximately 24,210 because of population growth and a growing industrial base. The water resource recovery facility (WRRF) currently has a 57-ML/d (15-mgd) peak capacity with flows averaging 13 ML/d (3.5 mgd). About 69% of the loading at the facility comes from industrial sources.
The WRRF receives wastewater with high biochemical oxygen demand and variable solids loading. The facility began struggling with hydrogen sulfide levels at two points related to biosolids processing. To protect worker safety, prevent corrosion of electronics and concrete, meet regulatory limits, and control odor, the facility invited USP Technologies (Atlanta) to help determine a more effective hydrogen sulfide strategy. The company recommended implementing a full-service hydrogen peroxide solution with fully automated dosing control to handle unpredictable loadings and reduce hydrogen sulfide to acceptable levels.
Biosolids holding tank experiences volatilization of hydrogen sulfide
A holding tank receives all biosolids awaiting dewatering at the WRRF. The tank generally is emptied every 1 to 3 days depending on biosolids generation rates and the volume of industrial discharges. Biosolids entering the tank, especially those from gravity thickeners, contain a significant sulfide load. Mechanical mixing and aeration in the tank minimized further sulfide generation, but, high loadings could lead to extreme volatilization of hydrogen sulfide, exceeding 500 ppm.
Such previous control methods as adding iron salt and in situ scrubbing using a hydroxyl ion generating system only provided partial success. Radicals generated interfered with proper measurement of hydrogen sulfide, compounding the challenge, said Michael Nelson, regional program manager for USP Technologies.
As the hydroxyl ion generating unit for the biosolids holding tank wore out, the WRRF asked USP to provide a second hydrogen peroxide system to oxidize the sulfide within the holding tank. Since multiple lines feed the holding tank with solids, none run constantly. The only viable injection point for this system was directly into the holding tank. The system was wired to run only when mixing occurs to ensure the most cost-effective treatment. An initial dosing rate of 23 L/h (6 gal/h) of 50% hydrogen sulfide essentially eliminated hydrogen sulfide within the holding tank.
Testing to determine an ideal solution
Biosolids are pumped from the holding tank to a dewatering building that houses two belt filter presses. The turbulence of these presses causes most of the hydrogen sulfide, present to volatize. The facility installed a wet scrubber to control the hydrogen sulfide, but by 2014 it had reached the end of its serviceable life and the hydrogen sulfide levels were too high above the press and in the dewatering building.
USP Technologies conducted benchscale dose response tests to determine the hydrogen peroxide needs and reaction times for the belt filter press from several injection points. They found the optimal injection point to be at the biosolids pump outlet. This location provided approximately 3 minutes of reaction time before the solids reach the presses.
With sufficient contact times of 2 to 5 minutes, hydrogen peroxide offers a solution for removing hydrogen sulfide from wastewater biosolids. Efficiency of treatment depends on available reaction time, the initial level of hydrogen sulfide, and non-hydrogen sulfide odors. Dose ratios for nearly complete prevention of hydrogen sulfide volatilization are about five-parts hydrogen peroxide to one-part aqueous sulfide.
The facility installed a full-scale hydrogen peroxide storage and dosing system. The system connects dosing pumps, by relays, to the biosolids pumps to ensure that the hydrogen peroxide is dosed while the biosolids pumps were running, Nelson said.
Within the first hour of operation, the system reduced hydrogen sulfide levels to below 1 ppm both above the belt filter presses and within the dewatering building’s main room. Dosing rates during the first 6 months ranged from 6 to 23 L/h (1.5 to 6 gal/h) of 50% hydrogen peroxide while the presses were running. Operators received training on how to set dosing rates based on the observed hydrogen sulfide levels at the belt filter presses.
Automated hydrogen peroxide dosing produces big savings
The constantly changing industrial loading, which made sulfide loadings into the holding tank and belt filter presses unpredictable, posed a challenge for optimizing ongoing operation. Hydrogen peroxide dosing needs fluctuated greatly day to day and week to week; this required constant monitoring and changes to dosing rates to ensure cost-effectiveness. USP automated variable hydrogen peroxide dosing for the holding tank and both belt filter presses using a 4-20 mA-output from Odalog® hydrogen sulfide analyzers equipped with OdaTrak® fiber optic units.
With extensive mixing and aeration within the holding tank, hydrogen sulfide levels responded quickly to hydrogen peroxide dosing. With most of the sulfide eliminated in the holding tank, hydrogen peroxide feed rates at the belt filter presses dropped substantially, Nelson said. Fully automating hydrogen peroxide dosing for each control point minimized overall chemical use and achieved overall better results.
“USP and their personnel have been good to work with,” said Todd Boling, wastewater superintendent for the City of Norfolk Water Pollution Control Plant. “Since inception, our costs have been decreased by half.”
The two-step treatment solution meant that a new hydroxyl ion generating unit was not needed; this reduced costs. After the program was fully implemented, the WRRF saw a 50% reduction in total chemical use, which provided significant savings. Removing the hydrogen sulfide from the dewatering facility eliminated to need for a costly replacement of the wet scrubber.
>> Download the article in pdf format here [pdf: 2 MB]