Water Treatment > Quality Water Solutions > Treatment Issues > Particulate Considerations

Water treatment plants across the U.S. have come to see particle control as an important consideration. This is reinforced by the strict particle control standards proposed by the U.S. EPA for removing microorganisms in its Enhanced Surface Water Treatment Rule and the 0.1 NTU finished water particle count target set by The Partnership for Safe Drinking Water.

Since coagulation can improve particle counts in clarified and filtered water, its role as a reliable barrier against microorganisms should become more important. Many see enhanced coagulation, which uses very high dosages, as the best way to sweep colloid-size pathogens from the water column. Doing so would lessen reliance on disinfection, especially for hard to kill organisms like Cryptosporidium and Giardia. But a focus on particle reduction through coagulation can affect other treatment goals, such as controlling turbidity, TOC, chemical residue and pH. Utilities must make careful choices in embracing an enhanced coagulation strategy.

In studies at dozens of water treatment plants, General Chemical has found that utilities often can have the best of both worlds — they can lower particle counts via enhanced coagulation while meeting or exceeding traditional goals for preclarification, clarification and overall treatment efficiency. This was observed most often with a one-product approach using either a formulated aluminum-based coagulant or polyaluminum hydroxychloride (PACl). These products tended to reduce particulate and turbidity burdens to filters, effectively eliminating the impact of wide excursions in raw water quality. As a result, they gave superior filter run lengths, lower headloss buildup, extended time to breakthrough and lower effluent particle count.

In one study, an Ohio River utility that preclarified influent water replaced an alum/polyelectrolyte combination with a formulated aluminum-based coagulant. It found that settled, preclarified, raw-water particle counts dropped 62% and 53% in the 2 to 5 mm and 5 to 10 mm ranges, respectively. In a second study, a Lake Michigan utility added PACl in increments and boosted particle removal 57% to 67% versus its normal coagulant. A second test with PACl lowered particle counts another 40% to 69%.

Water treatment plants can expect that PACl and other advanced coagulant technologies can help them meet the evolving pressures to reduce particles of all sizes without degrading their ability to meet other treatment goals. Intelligent use of these materials in preclarification and clarification can dampen swings in water quality, protecting filters from inordinately high solids loading and high particulate levels in effluent.