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Each day on my way to work in the San Francisco Bay Area, I drive with thousands of other commuters through Walnut Creek. Located in the shadow of nearby Mt. Diablo, Walnut Creek is one of the commercial centers of the East Bay, complete with shopping, recreation, arts and leisure. It does not, however, have much of a creek. Years ago, Walnut Creek was diverted under and around part of the city of the same name, and since then the downtown has grown from a quaint crossroads to a lively shopping and residential hub with a focus on high-end retail and easy access to commerce centers.
In 2006, construction began in downtown Walnut Creek for a luxury condominium complex. Excavation for a two-level underground parking garage exposed the water table and necessitated long-term dewatering of the building. The dewatering flow ranges from about 35 gal per minute (gpm) to around 75 gpm in the wet season with minor surface water content expected from the garage.
Unfortunately, the groundwater in this region contains a small amount of dissolved hydrogen sulfide (H2S)—a colorless gas responsible for the smell of rotten eggs. Workers in the excavation for the subsurface parking garage were first to notice the smell.
“Sulfide can occur naturally in groundwater that is oxygen-deficient,” said Amy Wilson of TRC, an engineering and construction management firm contracted by the developer to consult on the sulfide matter. This is the case in several areas of Walnut Creek.
“Sulfur-containing minerals are naturally present in the rock of the Mount Diablo region,” Wilson said. “The minerals are leached from rock and soil by infiltrating rainwater and groundwater. Hydrogen sulfide is produced metabolically by naturally occurring subsurface bacteria.”
Hydrogen sulfide in the groundwater presented a rather unsavory condition in the neighborhood. Through the storm sewer, this water drains into the creek, near the suburb’s City Hall and down the street from the high-end retail stores. For several reasons the city asked the developer to abate the issue.
The city required the developer to pretreat the water containing sulfide because in addition to the need to resolve the odor, they were concerned about potential corrosion in the city storm sewer pipes and water quality in the creek.
“We chose ozone because the only other feasible option was activated carbon filtration, which is recommended only for low sulfide levels,” Wilson said. According to Wilson, other alternatives included oxidation by chlorine, permanganate or peroxide, but chlorine is better for higher concentrations (>6mg/L) and the other two are potentially hazardous chemicals that require special handling.
The sulfide levels weren’t too high—about 1 to 2 parts per million (ppm), according to TRC, but they opted to ensure sufficient treatment even in a worst-case scenario.
“We couldn’t guarantee that sulfide levels would stay low long term, as subsurface conditions can change, especially when you are pumping,” Wilson said. “Additionally, as this will be an ongoing system for the life of the building, we felt that ozone would be the least expensive as far as long-term operations and maintenance. Plus, we didn’t want to have noisy and potentially messy carbon change-out events going on in this high-end building.”
TRC contracted Clean Water Systems for a solution to the H2S treatment issue, who recommended Pacific Ozone Technology, with whom both TRC and Clean Water Systems had worked before.
“We have used Pacific Ozone generators in other applications for hydrogen sulfide with great results,” said Gerry Bulfin of Clean Water Systems. “Unlike sodium hypochlorite, ozone adds no additional sodium to the water and is easier for the client to work with.
“To verify the effectiveness for this project for the city engineers, we conducted a field pilot test using a Pacific Ozone generator. After injecting ozone at a rate of 2 mg/L, and a brief contact time of three to four minutes, there was no hydrogen sulfide gas detected,” Bulfin added.
Ozone oxidizes H2S into elemental sulfur, which then precipitates out as bright yellow particles. Some will accumulate in the tank for occasional flushing, and some will flush out in low levels to the storm line. The excess sulfur can then be drained off and disposed of normally. Based on pH fluctuations, sulfite and sulfate can also be created in the water stream at levels low enough for standard storm drain release. The reactions are outlined in Figure 1.
The first stage of sulfide oxidation—aqueous sulfide ion changing to insoluble free sulfur—is very rapid. Studies have shown that as little as 2 g per day of ozone at a rate of 3 liters per min is effective in removing the smell and taste of hydrogen sulfide from well water.2 Ozone must be added to the tank-stored water continuously.
Ozone can also oxidize the subsequent sulfur forms when given enough contact time, but the Pacific Ozone system is designed to control the ozone generator’s power supply to maintain a small, set dissolved ozone concentration. The specific stoichiometric amount of ozone required for complete oxidation of the aqueous sulfur (as well as marginal ancillary disinfection) is dosed and consumed, leaving only a marginal residual. This residual is measured by a dissolved ozone monitor with single loop Proportional Integral Derivative feedback to the generator’s power supply, which then ramps up or down, modulating to reach the necessary residual level.
Pacific Ozone provided an IOCS15-A63 Integrated Ozone Contact System comprising a 145-gal contact tank and SGA63 ozone generator that produces 10 lb of ozone per day, though it need not run at full power. In addition to the variable treatment of the IOCS system, Pacific Ozone supplied a Kaeser Rotary Screw Air Compressor to feed the ozone system across a wide flow range for maximum efficiency, reducing excess energy costs given the fluctuating seasonal ozone demand.
The system features a D412 ozone destruct unit and an ambient ozone monitor to prevent the accumulation of ozone gas—especially important in this residential installation. Because this ozone system is addressing a water issue that is tangential to the day-to-day operation of the building, it is important that the system be stable, trouble-free and able to regulate itself. The integrated contacting and controls of the Pacific Ozone IOCS system ensured that as water flows and contaminant concentration fluctuates, the sulfide will still be eliminated from the water outflow.
TRC redesigned the piping to the treatment room and added an influent tank, a transfer pump to the system and an effluent tank to pump up to the street-level storm lines.
After treatment with ozone, the water was suitable for discharging into the storm drain or sewer, having received oxidation and disinfection of the target sulfide and bacteria or other contaminants that may have entered in excess surface water or storm water runoff.
The Walnut Creek condominium project is drawing nearer to completion, with over 40% of the units already sold. The city, developer and residents alike must be relieved that a solution to such a “stinky” problem was found and executed by TRC, Clean Water Systems and Pacific Ozone Technology.
The ozone system will operate continuously, removing sulfide and other trace contaminants from the groundwater and leaving it much higher in quality. With an IOCS ozone system similar to those used in many water bottling lines, one could almost think about bottling a private label for the condominium’s on-the-go suburban residents. You probably won’t see it appearing on shelves in today’s growing bottled water market, but Walnut Creek’s water will never have smelled so good.