Water Treatment > Quality Water Solutions > Operational Issues > Residual Reduction

Strict regulations for solids disposal and the closing of many waste sites are forcing water systems to cut residues to a minimum. Three factors contribute to solids generation and sludge bulk: chemically generated solids from chemical coagulants, flocculants and lime; the water that comprises most of a sludge's weight and bulk; and solids from the raw water source, which can only be affected by source water control.

Treatment facilities can control chemically generated solids to some degree because different iron- and aluminum-based coagulants yield different amounts of solids depending on the quantity of metal they add. One strategy being adopted to limit sludge is the use of polyaluminum hydroxychloride (PACl) coagulants rather than traditional alum and iron salt coagulants to reduce chemical sludge as much as 80%. Reasons for this reduction include:

• They work at lower dosages, which creates less aluminum and other metal hydroxides than those from other inorganic coagulants.
• They often produce a higher pH in settled or clarified water, so less lime is needed for pH adjustment, which also cuts sludge amounts.
• They are normally effective at low raw water temperatures. Traditonal inorganic coagulants are often used at elevated dosages in cold water, increasing sludge volume.
• They are effective to a pH of 9.4, while alum loses effectiveness above 8.0. Some plants add excess alum to lower pH enough so coagulation will be effective. This practice can greatly increase residue. Use of acid alum will lower pH without increasing residue unnecessarily.

In terms of bulk due to water content, a Midwestern plant found that as it changed from ferric sulfate to an alum/polymer blend and then to PACl, the dewatered solids portion of the residue increased from 25% to between 35% and 45%. The plant not only decreased chemical solids, but the amount of water present in the processed solid waste as well. Other plants have had similar results.

Water treatment facilities have been focusing on lowering chemical solids and increasing percent solids in the final waste. They frequently do this by switching to coagulants that more effectively remove solids from the source water, resulting in a minimal amount of solids produced.