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Case Study

Hydrogen peroxide was foundto be an environmentally

friendly, cost-effective solution

Hydrogen Peroxide Solves

Bio-Solids Headache

When the San Antonio Water System (SAWS) decided toconsolidate and centralize all sludge dewatering activitiesfor their Water Recycling Centers, they encounteredseveral perplexing problems:

Increased hydrogen sulfide (H2S) odors;A decrease in dewatering efficiency (lower

percentage filter cake solids);

And no increase in methane production, which was

expected with the increased sludge volumes.

These issues resulted in a search to identify the causes and find acost-effective, environmentally friendly solution, which turned out to bethe application of hydrogen peroxide (H2O2).

Problems and Issues

Located in the nations eighth largest city, SAWS meets thewastewater treatment needs of over 1.2 million people through fourmajor Water Recycling Centers (WRCs) with a combined capacity of225 MGD. In April of 2003, their largest facility, the Dos Rios WRC,began receiving pipeline transfers of up to 400,000 gallons per day ofprimary and waste activated sludge from their second largest facility,the Leon Creek WRC. Several negative consequences were

immediately observed including a significant increase in sulfide odorsfrom the belt filter presses and an unrealized expected increase inmethane gas production.

Frederic Winter, SAWS Environmental Services Manager, recalls Wehad been expecting that increased digester gas volumes would makeusing the gases for electrical generation economically feasible, so wehad to move to find out what was going wrong or the electricalgeneration project would be in jeopardy. We couldnt afford a delay."

From operational experience, the problems were thought to be relatedto higher than normal amounts of poorly biodegradable fibrous wastes,such as lignin and cellulose that historically were received at the LeonCreek WRC. These poorly digestible materials typically demandeddigester detention times of up to 40 days. When the solids began to

be transferred to Dos Rios, the long pipelineretention times in combination with the primarysludge mixture resulted in significant H2Sproduction within the force main. This H2S wassubsequently released to the air in the dissolved aiflotation (DAF) units at the Dos Rios WRC. Thenext step was to investigate and implement asolution that would address these issues.

Technology Selection

Winter describes the technology selection processstating, After conducting research and weighing ouoptions, we decided to evaluate H2O2as a solutionto the problem. We use H2O2in other areas of theDos Rios WRC, so we have experience using it andour operators are comfortable with it. We knew itwould address the H2S odors at the DAFs and wealso felt there was a good possibility of the H2O2giving us other process benefits.

Past experience with H2O2use in sludge treatmentsuggested that H2O2might help to reduce the DAFpolymer usage and improve sludge dewateringefficiency. Another potential process benefit wasan increase in digester gas (methane) production.

Similar wastewater industry research anddevelopment has shown that treatment of bio-solids with H2O2and catalytic amounts of iron(ferrous or ferric) prior to digestion can improvedigester performance, resulting in improved volatile

solids reduction and methane production. Underthe right conditions, hydrogen peroxide in thepresence of iron salts will generate the hydroxylradical (OH*), an extremely potent oxidizer with anoxidation potential of 2.8 electron volts. It wassuspected that since iron used for odor control inthe Leon Creek collection system is present in thesludge, this chemistry might facilitate theoxidation/modification of some of the large organiclignin and cellulose compounds, resulting in betterdigestion. Essentially, it was thought that theoxidative pre-treatment would partially break downsome of the refractory organic compounds, makingthem more amenable to anaerobic digestion.

Two of San Antonios Four Major Water Recycling Centers:Leon Creek and Dos Rios

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Commercial Office500 Bishop St. NW, Suite C3

Atlanta, GA 30318info@h2o2.comwww.h2o2.com

Inquiries call 404.352.6070

Bio-solids Treatment with H2O2 Case Study Page 2

Economic benefitsoutweighed the cost of the

turn-key H2O2program

H2O2Trial and Results

SAWS initiated a full-service trial involving injection of 50% H2O2,equipment and technical applications assistance. The trial began onFebruary 20, 2004 and was initially set to run 60 to 90 days. It wasthought that 2-3 digester sludge ages would be sufficient time tomeasure any trends in digester performance, belt filter press

dewatering efficiency and to monitor H2S levels in the DAF area. TheH2O2was injected into the Leon Creek force main, several minutesahead of the DAF units at the Dos Rios WRC. Impacts to be quantifiedwere DAF H2S concentrations, DAF polymer usage, digester gasproduction, and dewatering efficiency.

The use of H2O2 had immediate positive effects. Data collected using

an Odalog H2S data logger placed next to the underflow weir of the #3DAF unit demonstrated that DAF vapor phase H2S was reduced frompeaks of over 150 ppm to non-detectable levels within 90 minutes ofstarting H2O2dosing. The H2O2dose rate to achieve these resultswas approximately 175 mg/L based on 300,000 GPD Leon Creeksludge flow.

In addition to an almost immediate control of hydrogen sulfide, an

improvement in digester methane gas production was realized afterabout two sludge ages. When observing the H2O2impact ondewatering efficiency, the Dos Rios facility appears to have taken in arecovery of about 2% TS (from about 19% up to 21% cake solids) inits belt filter press sludges since the addition of H2O2 . The economic

benefit of the increased methane production andpercent solids increase in dewatered sludge offsetthe cost of the turn-key hydrogen peroxide odorcontrol program. Finally, the increase in gasproduction was an important consideration in

evaluating the feasibility of electrical cogeneration.Regarding the impact of H2O2 on polymer usage,additional data and further work are required toevaluate its full impact, however, initial data andoperator interviews provided supporting evidencethat H2O2was reducing polymer usage at the DosRios DAF.

Conclusion

The use of hydrogen peroxide at the Dos RiosWRC has helped to solve San Antonio WaterSystems problems in consolidating andcentralizing sludge dewatering activities for theirWater Recycling Centers. Today, the full-servicehydrogen peroxide program consistently controlshydrogen sulfide in and around the Dos Rios DAFunits and blend tank. Significantly, this H2O2program has also resulted in improved digester gasproduction (methane), as well as improveddewatering efficiency.

After dosing H2O2, hydrogen sulfide rapidly fell to non-detectable levels

H2O2dosing location: Dos Rios, San Antonio