BIOSOLIDS – The soil-like residue of materials removed from sewage during the treatment process. During treatment, bacteria and other tiny organisms break sewage down into simpler, harmless organic matter. This organic matter, combined with bacterial cell masses, settles out to form biosolids. Biosolids in their liquid from look like muddy water and contains 1 to 10% solids. Biosolids may be “dewatered” in a second step of the treatment process, which turns it into a “cake” with the texture of a wet sponge. In this stage it contains 11-40% solids.

SEWAGE – Wastewater that is carried by sewer lines into sewage treatment facilities. Such wastewater typically includes all wastes poured down drains, sinks and flushed down toilets in your home, and wastes from local industries and businesses.

SLUDGE – A term also used to refer to the by product of the sewage and water treatment process. This term also has other meanings and is being replaced with the term biosolids which more accurately describes the material.


Once the wastewater is treated it is returned to the environment. The remaining biosolids can now be treated in a number of ways:

CHEMICAL FIXATION – This is the treatment of biosolids with alkaline chemicals such as lime. This process kills additional pathogens (harmful microorganisms) that may have survived the sewage treatment process by generating heat and creating an alkaline environment that kills these organisms. It produces a stable, soil like material that can be used as a source of lime by farmers and as landfill cover. This process is also known as Advanced Alkaline Additive Stabilization (AAAS).

COMPOSTING – This is the process in which solid organic materials are decomposed in the presence of oxygen through the action of bacteria and other microorganisms. Dewatered biosolids are mixed with sawdust or wood chips that act as bulking agent by absorbing water and increasing the porosity of the mixture. This mix is then composted and cured for a minimum of six weeks. High temperatures (103 degrees to 150 degrees Fahrenheit) generated during this process pasteurize the mixture. A peat like product is the end result. Composted biosolids are easier to handle, store and use than liquid and dewatered biosolids.

PELLETIZATION – In this process biosolids are first stabilized (see above definition), then completely dried and pressed into small pellets. The pellets are then used as fertilizer since they are high in nitrogen.

STABILIZATION – This is the process used to reduce harmful bacteria and odors in biosolids. Typically, stabilization is accomplished through aerobic (with oxygen) or anaerobic (without oxygen) digestion. Digestion refers to the breakdown of complex organic substances through the action of bacteria and other microorganisms.


AS FERTILIZER – Wet, dewatered and pelletized biosolids provide a significant amount of the basic nutrients (nitrogen, potassium and phosphorus) needed by soil and plants.

AS A SOIL AMENDMENT – All biosolids products add organic matter that improves the texture and water retention of soil. Chemically fixed biosolids can be used as a liming agent to alter the pH, or acidity of soil.

AS MULCH – Composted biosolids can be used as mulch, which prevents weed growth and adds nutrients and texture to the soil as it decomposes.

FOR LAND RECLAMATION – Biosolids products can be used to reclaim disturbed lands, including land that has been mined or quarried for sand and gravel. Such sites often have environmental problems such as runoff and erosion. Following the application of composted biosolids, these areas can again support the native ecosystem.

AS LANDFILL COVER – Biosolids which are chemically fixed resemble natural soil in texture and can be used as daily cover at landfills (landfills are required to cover wastes on a daily basis for sanitary reasons).