On-site mixed oxidants demonstrate benefits in Puerto Rico

The U.S. Environmental Protection Agency’s (USEPA) new limits on total trihalomethanes (TTHMs) and the five haloacetic acids (HAA5) are a challenge for potable water system operators in Puerto Rico, a small island in the Caribbean Sea wedged between Cuba and the Virgin Islands. A significant portion of the 125 filtration plants in this tropical paradise, which produce close to 1.9 million cubic meters per day (500 million gallons per day, mgd) of drinking water, receive raw water from lakes laden with organic TTHM precursors. The Stage 1 Disinfectants/Disinfection By-Products (D/DBP) Rule promulgates a reduced limit of 80 micrograms per liter (mg/L), or parts per billion (ppb), for TTHMs and only 60 mg/L for HAA5. The effective dates of enforcement were last January for large drinking water systems serving more than 10,000 people, and January 2004 for small drinking water and groundwater systems.

Due to its challenging raw water characteristics, the Coto Laurel water treatment plant in Puerto Rico presented an ideal opportunity to evaluate potential treatment solutions. The 7,570 cubic meter (2 mgd) plant, with two parallel processing modules, receives water primarily from the Toa Vaca Lake with a demand of up to 68 kilograms (150 pounds) of chlorine per day. From time to time, operators are forced to increase the chlorine injection as a pretreatment strategy to control incoming manganese levels and taste and odor problems. Even though this strategy has proven to be effective at not only controlling manganese levels—but also reducing customer complaints due to bad odor and taste— the chlorine pretreatment results in increased formation of TTHMs, putting the plant on the border line of the Stage 1 DBP limits.

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