Introduction

The County Sanitation Districts of Orange County (Districts) own and operate two wastewater treatment plants. Reclamation Plant No. 1, in Fountain Valley, treats approximately 80 million gallons per day (MGD) of sewage. Treatment Plant No.2, in Huntington Beach, treats about 175 MGD.

The Districts use chlorine for influent sewer odor control, plant water disinfection, odor control scrubbers, emergency effluent disinfection and activated sludge filamentous growth control. The Districts have a bulk chlorine tank and three one-ton chlorine stations at Plant No. 1. The bulk tank station at Plant No. 1 feed chlorine to the raw sewage for hydrogen sulfide control (H2S). Two one-ton stations back up the bulk station. The bulk station and one of the one-ton stations are in a ventilated room of the scrubber building. The other one-ton station at Plant No.1 is for use in the activated sludge plant.

Plant No. 2 has a central bulk chlorine station with a one-ton chlorine tank back-up that is not in a building. All chlorine uses are fed from the central station. Chlorine is used for raw sewage hydrogen sulfide control, scrubber sulfide compound oxidation, plant water chlorination and emergency outfall disinfection, if ever required.

The Districts used a total of 310 tons of chlorine for raw sewage odor control and 20 tons of chlorine for other uses per month. Chlorine feed at the plants has historically averaged 14 ppm at Plant No.1 and 11 ppm at Plant No. 2 for summer dosing, and 8 ppm at Plant No.1 and 8 ppm at Plant No. 2 for winter dosing.

In 1982, the Districts trialed hydrogen peroxide (H2O2) and ferric chloride as alternative chemicals to oxidize hydrogen sulfide coming to the treatment plants in the trunk sewers. Chlorine was found to be the most cost effective.

Recent concerns regarding chlorine toxic emissions including safety, hazardous by-products, and new regulations regarding the storage and transportation of chlorine have mandated that the Districts evaluate other control alternatives.

In 1992, staff again initiated studies to find alternative chemicals to chlorine that was being used to reduce hydrogen sulfide in the incoming trunk sewers of the treatment plant. To date studies to evaluate H2O2 as an alternative chemical to influent raw sewage chlorination and scrubber odor control application have been successfully completed.

Influent Trunklines Hydrogen Peroxide Treatment

In April 1993, the Districts initiated a study to evaluate the possibility of using H2O2 as an alternative chemical for trunk sewer odor control. Plant No. 2 was chosen as the site of the trial. The plant processed an average of 175 MGD of influent wastewater. Five major trunklines collected wastewater, entered Plant No. 2, and combined their flows at the Headworks. The trial consisted of dosing all five trunklines with 50% H2O2, monitoring the process and evaluating the economics of chlorine and H2O2.

Hydrogen Peroxide is a proven environmentally safe chemical that destroys sulfide as shown by the following chemical reaction:

By weight, one part H2O2 oxidizes one part H2S. At higher dosing ratios oxygen is added to the system to prevent regeneration of sulfide downstream from the injection point as shown by the following reaction:

Note that the reaction byproducts, oxygen and water, are non-toxic and non polluting; elemental sulfur is inert. In the event of a spill, H2O2 will decompose immediately into water and oxygen when it comes into contact with the soil.

H2O2 dosing modules were mounted on aluminum skids, and equipped with a 2,500 gallon polyethylene tank with special ultraviolet inhibitors, metering pumps, water injection line, timers and control panel. H2O2 was directly injected into the manhole and sprayed above the wastewater level as shown in Figure 1 below.

Prior to selection of a suitable injection point, an initial test was carried out to estimate the kinetic rates of the H2O2 reaction with sulfides. Ten minutes after addition of H2O2 at the ratio of 1.5:1 peroxide to sulfide, the aqueous sulfide concentration in the wastewater for all five trunklines was below 0.5 mg/L as shown in Figure 2.

The dosing units were installed at 5 different locations all situated inside Plant No. 2 at the boundaries to maximize the H2O2 contact time with hydrogen sulfide in the trunk sewer. Dosing units for Miller-Holder lines were located 700 feet upstream of the Headworks with retention time of 6 minutes. The Interplant line was located 1,000 feet upstream with 8 minutes retention time, District 5&6 line was 1,100 feet upstream with 9 minutes retention time and Coast line was 2,000 feet upstream with 17 minutes retention time. The reaction is estimated to be 90% to 95% complete at the Districts' Headworks.

From the plant Headworks, through the wetwell, grit chamber and distribution boxes to the primary basins, the retention time averaged 15 minutes. This provided enough time for the reaction to be carried out to completion.

To properly dose the trunklines, diurnal flow rate and fluctuations in mass of sulfides in the system were characterized. Grab samples were collected at each of the five influent trunklines and analyzed for aqueous sulfides. The result combined with flow data tabulated in Figure 3 were used to determine the mass sulfides profiles. The mass sulfide profiles are shown in Figure 4. During the trial, the dosage was optimized from a peroxide to sulfide ratio of 2.4:1 at the beginning of the trial to 1.5:1 ratio.

Grab samples were collected at each of the five influent trunklines before and after dosing locations, Headworks, influent and effluent of the primary basins, and analyzed for total aqueous sulfides, pH, temperature, dissolved oxygen and peroxide residual. Results were used to evaluate performance of H2O2 as compared to chlorine. The following table summarizes the result of aqueous sulfides testing at different locations throughout the plant for hydrogen sulfide and chlorine.

The Districts have implemented full scale use of H2O2 for raw sewage hydrogen sulfide control. Recent cost for bulk chlorine have increased by 78% making H2O2 use more economical. Preliminary further full-scale use has been at a ratio of 1:1 H2O2 to sulfide which made H2O2 use one-third less than trialed. If this ratio can be maintained, the cost of H2O2 at Plant No. 2 would be two-thirds of the previous cost of chlorine ($250.00/ton), hence chlorine would have to be below $167.00/ton to be cost competitive.

Hydrogen sulfide concentrations in the air to the inlet of the scrubbers, in working spaces of the primary basins and Headworks, were monitored to evaluate performance of H2O2 in the air and its ability to maintain a safe working environment. The following table summarizes the result of air sampling at different locations throughout the plant.

The trial has demonstrated that the H2O2 was able to satisfy the following conditions:

• Maintain total sulfide concentration in wastewater below 0.5 ppm.
• Maintain inlet concentration to the primary scrubber below 5 ppm.
• Maintain H2S concentration in the air in working areas below 2 ppm.
• The chemical usage economic study compared influent treatment with chlorine versus H2O2 at today's market price.

After five weeks of trial, the study concluded that H2O2 was proven to be effective in controlling sulfides in influent wastewater at equal cost to chlorine. Performance of H2O2 at 1.5:1 ratio H2O2 to sulfide was similar to chlorine. The peroxide system is simple, inexpensive and easy to maintain and operate.

Scrubber "T" Peroxide Trial

The Districts practiced wet scrubbing with packed towers for treatment of foul air. Each of the scrubbers has been designed for removal of H2S and to use either caustic, chlorine or hypochlorite as the scrubbing liquid. A study was undertaken to demonstrate that H2O2 can effectively replace chlorine and/or hypochlorite and meet treatment goal.

The influent polishing Scrubber "T" at Plant No. 2 was chosen for the demonstration. The scrubber was operated at the rate of 27,000 scfm with a recirculation rate of approximately 500 gpm. A skid mounted H2O2 storage/dosing module consisting of a storage tank and metering pumps was installed beside Scrubber "T" to add H2O2 to the scrubber sump. 50% H2O2 was used throughout this demonstration.

Following the start-up of the H2O2 system, grab samples were collected from the sump, influent and effluent air sampling ports and analyzed on site for total sulfide, peroxide residual and H2S in the air. The result is used to determine scrubber performance. H2O2 dosing rate were adjusted to minimize overall chemical usage without compromising treatment goal. Optimum treatment levels used in the demonstration were as follows:

Overall, the reduction in H2S in air level was 98.9%. Air analysis indicated that H2S, volatile organic compounds and sulfur compounds removal was comparable to bleach and chlorine. As usual the scrubber was brought down after 300 hours for acid washing. One acid wash was sufficient to descale the packing.

H2O2 proved to be effective in removing H2S in the scrubber. The chemical usage cost for H2O2 is higher than chlorine. However, the cost of maintaining the H2O2 dosing system was much lower than the chlorine system.

[Back to Municipal Wastewater Applications] | [Forward to Next Article]

Copyright © H2O2.com