INNOVATIVE TREATMENT TECHNOLOGY FOR RECYCLE/REUSE OF COOLING TOWER BLOWDOWN TO ACHIEVE ZERO LIQUID DISCHARGE

Since the makeup water contained 70 to 100 mg/L silica, the existing cooling towers had an efficiency of only 50 percent. Water allocation limits and an anticipated plant expansion demanded an increase in the efficiency of the cooling towers. To address that objective, a three month pilot study was made to evaluate cooling tower recycle using a novel treatment technology. The pilot study evaluated a combination of silica precipitation chemistry and cross flow microfiltration to reduce the silica concentration of the blowdown so that it could be recycled increasing the efficiency of the cooling towers and conserving water. Treatment of the blowdown rather than of the influent saved chemical and capital costs. Incorporating cross flow microfiltration into the system eliminated the need of both clarifiers and filters. This not only conserved space, but made the system more reliable.The treatment system was not only to reduce the silica so that the water could be recycled, but the water had to be non scale forming and non corrosive. Because of that, the pilot system included the equipment for the treatment system as well as a pilot cooling tower and heat exchanger to indicate the potential scaling and corrosion effects. One constraint of treating and recycling the cooling tower water, however, was a NPDES total dissolved solids limitation for any cooling tower discharge. Because of the discharge limitation, the pilot study also evaluated the treatment of the blowdown using reverse osmosis (RO) and evaporation to obtain zero liquid discharge.Once the pilot system proved the treatment scheme to be a success using potable water makeup, on-site secondary sewage effluent was also evaluated as the makeup source to further conserve water. Although that source had similar inorganic constituents as the potable water, the increased organic and biological characteristics had to be addressed.