Water Treatment > Quality Water Solutions > Operational Issues > Beneficial Reuse of Solid Residuals

Many water utilities have come to see their residuals as a raw material rather than a waste. This shift in perspective is due, to a great degree, to rising landfill costs and increasing regulatory control. The ongoing search for practical ways to use water treatment plant residuals has led to a number of interesting applications. Here are some of the more promising and successful ones:

• Landscaping to fill in or build features. Residuals are being used to create hills in parks and golf courses, as well as to fill in small quarries.
• Remediating poor or spent soils by improving nutrient content.
• Enhancing a soil's water holding and release capacity. Since most residues retain some flocculant capacity, they enable soils to absorb moisture which can retard cracking in dry weather and help prevent muddy conditions in light rains.
• Preventing phosphorus run-off. Soils laden with soluble phosphorus, usually due to overfertilization, allow phosphorus to run off. This is a major cause of surface water eutrophication. Residuals from water plants using aluminum-based coagulants are being applied to fields to convert soluble phosphorus to an insoluble form.
• Blending with wastewater treatment plant solids to tie up monophosphorus. This is especially important in states that regulate biosolids based on phosphorus level rather than nitrogen content.
• Applying them in tree farms and other silvacultural applications to promote growth by acidifying the soil and providing some nutrients and organics.
• Mixing it with compost as a source of nutrients and bulk. The mixture can then be used as a top soil or a potting soil.
• Placing it beneath road beds as a grading material.
• Using it in cinder block or mixing it with fly ash to make aquarium gravel.

Residuals from plants using aluminum-based coagulants tends to have lower heavy metal levels than those from plants using iron-based coagulants. The presence of heavy metals may restrict use of a residual in some applications.

In reusing residual solids, water treatment plants generally have to reduce water content well below that of residuals placed in landfills. Use of polyaluminum hydroxychloride coagulants reduces the level of aluminum in the residual, improving drainage and making it much easier to dewater.