Water Treatment > Quality Water Solutions > Treatment Issues > Variable Source Water

Utilities typically test for variability in turbidity and temperature because they are easy to measure. These are often used as surrogates to indicate changes in other water parameters. Water treatment plants can deal with sharp changes in water quality in a number of ways.

Many facilities add one or more pretreatment steps, e.g., lamellar plate separators, reservoirs or high-rate clarifiers. These methods tend to remove larger particles, so a finer fraction enters the plant. This fraction is harder to remove and generally calls for additional coagulant.

They also can use preoxidation, especially in dealing with seasonal raw water variability, to remove some organics and aid coagulation. This step generally involves alternative oxidation systems such as those using chlorine dioxide, potassium permanganate or ozone.

Plants also find that use of polyaluminum hydroxychloride (PACl) provides an extra edge in controlling source water excursions. PACl has a high level of positive charge so it has a built-in reserve that often can meet a surge in demand for charge neutralization and flocculation if turbidity, temperature, color or total organic carbon changes suddenly.

PACl is a family of polymeric compounds that has sufficient diversity so they can treat water across the range of conditions found in North America. Low basicity PACls (below 30%) are typically used for phosphorus removal, waters high in color and TOC, and where fast reaction rates are needed. Medium basicity PACls (30% to 50%) are often applied for cold water, low turbidity and waters with only slightly variable raw water quality.

High basicity PACls (above 70%) are especially useful where water quality is variable and in some raw waters high in color and TOC. They are also useful as co-coagulants with alum or iron coagulants because they can stop excursions before they get problematic, which can avoid overdosing the traditional coagulant to bring an excursion back in line. PACls can also eliminate the need for alkali addition if the excursion occurs along with low-alkalinity rain water.