Biosolids

A recycled organic fertilizer

Biosolids: A recycled organic fertilizer

Using biosolids in your yard: Health and safety questions and answers

Stewardship gardening links outside WSU

Biosolids are the treated solid material left over from the wastewater treatment process. Biosolids, also called sewage sludge, contains water, sand, nutrients used as a fertilizer, a small number of microorganisms and trace amounts of metals and chemicals.

Much of the biosolids is recycled on land in agriculture, forestry, as a soil improver and in composting projects. Biosolids are a valuable source of fertilizer nutrients and are used in a wide variety of crops, including wheat, barley, hops, rangeland or are composted and used in yards and gardens. Biosolids contain significant amounts of all nutrients required by plants, including nitrogen, phosphorus, potassium, and micronutrients. Because biosolids vary by source and treatment process, different biosolids will contain different amounts of these nutrients.

Biosolids are soil conditioners and in a short time after being added to the soil, will resemble a rich, organic topsoil. The organic matter increases water retention and retains nutrients in the soil, similar to the effects of peat moss, and helps plants withstand drought. In addition, the organic matter improves soil tilth, making the soil easier to work and permits easier root penetration. Adding biosolids to the soil can both improve water retention and accelerate plant establishment, potentially reducing storm-water runoff and erosion.

The Environmental Protection Agency has defined two classes of biosolids as they relate to pathogens, organisms that are capable of causing diseases in humans. A designation of "Class A" means that all pathogens have been destroyed in the treatment process. Class A can be achieved through a digestion which heats biosolids to 140 ^o F, through composting the biosolids, or with heat drying to produce a pellet or granular product. Class B means that there has been a significant reduction of pathogens in the biosolids but have not been eliminated. Both Class A and Class B materials are safe for land application, however, additional management practices are required of Class B products. Class A products may be used in any setting, including home lawn and gardens. Examples of Class A products include Tagro from the City of Tacoma, Washington; GroCo from King County, Washington; and Milorganite from Milwaukee, Wisconsin.

Biosolids can be composted with several different materials. The most common in Washington is sawdust or wood chips. Currently, yard waste has become a common bulking agent in composting biosolids. Composting, the biological decomposition of organic matter, produces a stable, drier biosolids product with about half water and half solids. During decomposition, some materials in the biosolids are converted to gas, water and heat. This dries the compost, while the heat generated increases the efficiency of the composting process and kill pathogens.

Using biosolids in your yard: Health and safety questions and answers

More information about this topic is available through your local Cooperative Extension Office.

Source: James A. Kropf, Extension Faculty, Horticulture, Small Farms and Farm Marketing. (1998)