H2S Control Put to the Test in Orange County Sanitation District

Orange County Sanitation District (OCSD) is the third largest wastewater treatment system west of the Mississippi River. OCSD serves 2.2 million people in 21 cities covering 471 square miles. The system includes 650 miles of trunk sewers and collection lines ranging from 6 to 96 inches in diameter. There are 22 pumping stations and the entire collection system transports 250 million gallons of wastewater per day. Flow is split between two treatment plants. 

Since the mid-1980s, OCSD has employed caustic shock dosing as a means to control H2S gas generation within the system. In July of 1998 OCSD embarked on phase one of a “Sewer Trunk System Odor Characterization Study” to evaluate its odor strategies. This was followed by the completion of phase two in May of 2000. 

Results of the study characterize portions of the OCSD system not previously examined, together with data verification of areas already monitored for H2S gas. In addition, OCSD examined the current state-of-the-art technologies in wastewater odor and corrosion control. Over 70 odor and corrosion minimization technologies were considered. Of these, strategies still under consideration by OCSD include five major chemical categories: caustic soda (shock), or continuous addition using hydrogen peroxide, nitrates, iron salts or Thioguard magnesium hydroxide.

During the second half of 2000, OCSD is conducting large-scale field testing under specification C-146, “Collection System Pilot Demonstration Testing For Sulfide Control.” This report presents data collected during field demonstration of Thioguard magnesium hydroxide on the 56.5 mgd Knott interceptor.

Field Test Team
Twelve field personnel from Hill Brothers and Premier worked 24 hours per day to collect over 23,000 H2S data points together with 1,440 manually collected pH readings. Data was collected prior to and during the test period. Instrumentation included eight Industrial Scientific STX-70s, one Gastec sampling system, K&E pH meters, and Sensidyne dissolved sulfide tubes.

Feed Site
From Sept. 6 to Sept.18, 2000, the Knott gravity main flowed an average of 56.6 mgd. This flow was treated using a daily average of 5,419 gallons of Thioguard  (95.73 gallons per mgd). Thioguard was fed at the Seal Beach pump station located on the southwest corner of Westminster and Seal Beach Boulevards.  The Seal Beach pump station consists of two force mains (east and west) discharging 2.6 miles downstream. The west force main is 42 inches in diameter with a VFD pump, while the east force main is 30 inches in diameter with a duplex pump for peak periods.

A diagram of the treated system follows:

Objectives
The primary objective of the test was to reduce H2S gas at the control point located at Second and Talbert, 10.2 miles from the feed site. In addition, this configuration was chosen by OCSD to examine concerns regarding potential H2S gas flare-off, as pH decreases with dilution from the Plant 1 diversion line. Flow of 56.6 mgd from the Knott is combined with 60 mgd from the Plant 1 diversion. The combined flow then travels approximately 5 miles (one hour) to treatment plant No. 2 located in Huntington Beach. If dilution caused flare-off, it would result in a substantial increase in H2S gas at Plant 2.

A further objective of the test was to evaluate the impact of chemical addition on treatment plant performance. 

Monitoring
Monitoring was performed by CH2M Hill and OCSD at several points along the system including: the West Side interceptor, Casa Camino Restaurant, Main and Hoover, Second and Talbert, the Plant 1 diversion line, as well as the plant headworks and digesters.

Hill Brothers and Premier Chemicals also collected data at: the West Side Interceptor, Casa Camino Restaurant just downstream of the Seal Beach force main discharge, the Bolsa gravity main, the Slater force main, Second and Talbert and other sites necessary to verify pH and H2S readings. Plant Vapex readings and flow data were provided by OCSD.

Between the feed site and the control point there are thousands of connections including two major side flows. None of these flows are treated and large amounts of H2S gas produced in these lines discharge into the Knott system. A complication peculiar to this trial is the location of the Slater force main shown in Figure 1. This line produces an average of 74.2 ppm H2S gas, peaking as high as 165 ppm. The Slater discharges into the Knott gravity main less than 1 mile before Second and Talbert, the targeted control point.

Results
Summary results presented here include peak and average headspace H2S gas for the pretreated conditions (baseline) and the treated conditions (results). Industrial Scientific STX-70 recorders sampled continuously reporting five-minute averages. The STX-70s were calibrated with 25 ppm H2S span gas and field readings were cross-verified with Sensidyne samples taken at the monitoring points to insure instrument accuracy.


Discussion
The development of Thioguard offers the prospect for revolutionizing systemwide H2S gas reduction in terms of effectiveness, economics and safety. First, at a pH of 8.2, roughly 90 percent of the H2S gas can be eliminated from the headspace compared with wastewater at pH 7.0. This property is a result of the equilibrium shift that keeps liquid phase sulfides in solution. Because Thioguard moderates pH rather than destroying sulfides, headspace H2S gas levels are greatly reduced at constant dissolved sulfide levels. 

As a result of the independence from sulfide concentration, the dosing and cost of Thioguard is more closely related to volumetric flow rates and retention times, rather than baseline sulfide production. Therefore, unlike alternative chemistries, which require direct reaction with sulfide, Thioguard requirements do not increase as sulfide levels and retention times increase. 

Thioguard continued to control headspace H2S gas even when diluted more than 1-1. Headspace readings at Plant 2 were reduced by more than can be accounted for from the reduction in the Knott system alone. That is, Thioguard accounted for an 86 percent reduction in 56.5 mgd from the Knott, and a 77 percent reduction in the 116.5 mgd arriving at Plant 2. This demonstrates its effectiveness both over long distances and when diluted.  

To find out how you can harness the power of technical-grade magnesium hydroxide for your wastewater treatment plant or collection system, please visit www.thioguard.com and locate your regional representative on the contact page, or call 800/227-4287.