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New Light Cast on Sewage

It takes all of 15 seconds for ultra-violet light at a wavelength of 254 nanometers to kill the bacteria and viruses in 15,000 gallons of wastewater at Central Contra Costa Sanitary District's newly converted plant in Martinez. The UV-treated effluent discharged into the Bay is so clean that the plant can meet and even exceed a stringent fecal coliform discharge criteria of 200 mpn per 100 milliliters.

"We usually come out with a count of less than 2," says Central San's Lynne Putnam. Vallejo and the Mountain View Sanitation District have also switched to UV disinfection but the technology is not for everyone. UV is not recommended for plants that treat effluent from certain heavy industries, as metals in some effluents can absorb UV light and undermine treatment. Textile manufacturing effluent is also not a good candidate for UV, as some dyes block out UV, much like sunscreens.

Central San's Martinez plant was well-suited for UV conversion, having originally been built as a water reclamation facility with existing denitrification channels suitable for a UV system. Wastewater now flows into six below-surface channels for UV disinfection after primary and secondary treatment. Three banks of lights - in grids of 26 lamps across by 16 down-are lowered into each channel and plugged in. The UV light prevents any algal growth, but occasional maintenance of the lamps is required to clean off precipitated salts.

Although the capital costs equal those of traditional treatment, Central San's Chuck Batts feels that UV will save the district money in the long run. Cleaning and electricity costs are somewhat higher, but training and reporting costs are much lower than when dangerous chlorine gas is used. "There's no health risk of chlorine gas on site and no sulfur dioxide needed to reverse the free chlorine," says Batts. "UV is as good as chlorine at eliminating pathogens and better than chlorine at removing viruses, and it doesn't have the chlorine byproducts-like dioxins."

Contact: Chuck Batts (510) 689-3890

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