Water Treatment > Quality Water Solutions > Treatment Issues > Turbidity Considerations

Turbidity is more than an optical parameter. Although it is defined by light scattering, it relates broadly to the nature of the particles present. The form of turbidity present affects coagulant selection and dosage. Information on particle number and size distribution and charge are often needed to optimize turbidity control.

Plants that have trouble removing turbidity must determine if the cause rests with the form of turbidity or with plant factors. This can involve a degree of detective work.

Consider a common problem in which turbidity that is easily treated at one coagulant dose at one time does not respond well to multiple doses of the same coagulant at another time. This can occur if particle size grows smaller because smaller particles can demand more coagulant or a different coagulant. Another cause can involve a shift in charge if different components enter the raw water. A profound shift in coagulant dose can be caused by increases in dissolved organic carbon, which may not cause a meaningful change in turbidity.

Raw water turbidities that do not exceed 2 NTU can be harder to deal with than those with 10 or even 100 times more turbidity. Lower turbidity water often contains small, highly charged particles that are hard to remove. High turbidity, which tends to occur as large sand and clays particles, is generally easy to treat.

Plants are exploring many strategies in improving turbidity control. Many are looking beyond traditional metal coagulants to remove turbidity and improve economics. Several Midwestern utilities are using high-basicity, high-concentration PACls as co-coagulants with alum or iron salts in place of or along with polyelectrolytes. Others are experimenting with multiple coagulation feed points to allow for different mixing environments as water quality changes.

Operational practices can cause some turbidity problems. Chemical addition is a good example. If lime or a fluoridation chemical is added with the coagulant, the coagulant will react with them and be unavailable for turbidity removal. Moving the feed lines can resolve this problem.

Dilution water used to carry aluminum and iron coagulants to the raw water can also crate turbidity problems. If its pH is much above 3.5 to 4.0, the coagulant it contains will hydrolyze and floc with the carrier water. The solution added to the raw water will be partially spent and will lose a part of its ability to remove turbidity.