Biosolids (a.k.a. Sewer Sludge)

Gardeners and health conscious consumers may be shocked to learn that the organic sounding ingredient “biosolids” found in some potting and compost mixes is actually municipal sewage sludge.

“Biosolid” is a misleading public relations term for semi-treated municipal sewage sludge. The sludge is sometimes sold or given to consumers and farms for use as a free or low-cost fertilizer. It is also used in some low grade potting and compost mixes. While biosolids do contain about 4% nitrogen, and several micronutrients, they also contain elevated levels of harmful bacteria, heavy metals, medical waste, and other hazardous materials. An estimated 3-5 million dry tons of biosolids fertilizer is in use annually on farms, in landscapes, and by unwitting homeowners.

Biosolids are a problem by-product of sewage treatment. Since the ban on ocean dumping went into effect, waste management companies have increased efforts to sneak more and more of the toxic substance into the consumer food chain. Waste products enter the sewage system through toilets, sewers, industrial and medical drains, and then collected at sewage facilities.

Untreated sewage contains everything dumped into the sewer, not only human waste products, but anything else that gets poured down a drain. Considered to be a dangerous biohazard, untreated sewage generally contains human pathogens, and at least some form of medical waste. It can also be a chemical hazard depending on the amount of drain cleaners, solvents, heavy metals, radioactive materials, or other potentially toxic chemicals are present.

Biosolids Contain High Levels of: harmful pathogens, medical waste, heavy metals, chemical pollutants and more.

Biosolids Contain High Levels of: harmful pathogens, medical waste, heavy metals, chemical pollutants and more.

The EPA defines sewage sludge as a solid, semi-solid, or liquid residue generated from processing domestic sewage in a treatment works.

“Sewage sludge includes scum or solids removed in primary, secondary, or advanced wastewater treatment processes and any material derived from sewage sludge” — The Standards for the Use or Disposal of Sewage Sludge (Part 503) – US Environmental Protection Agency

It also mentions that material containing 99% industrial waste, and 1% domestic waste can still be considered “biosolids” after treatment, since it contains some domestic wastewater product.

The details of treatment vary, but they generally start with using a bar screen to remove large objects, such as shopping carts, that could damage the processing equipment. The waste is then allowed to settle. The portion that sinks to the bottom is the sewage sludge, and the portion that floats is known as scum.

The water between the two layers is then removed, filtered and treated, and released into rivers or oceans. The scum from the top is sometimes collected and reintroduced to the collected sewage sludge from the bottom. Most studies on sewage treatment effectiveness have centered around the removed wastewater effluent layer, and not the leftover sewage sludge.

Treatment to convert the sewage sludge into “Class A biosolids” can be accomplished by aerobic composting, anerobic composting, heat drying, or pasteurization. These methods reduce the amount of active live pathogens present in the material. Class B biosolids have less stringent requirements and use restrictions. It can only be applied to areas where public access is “limited”, such as commercial food crop production. Some studies suggest that at least some of the inactive pathogens in the sludge may reactivate when exposed to wet conditions (such as watering of plants treated with sludge).

Biosolids may be spread on agricultural fields, which they consider as “surface disposal sites.” Cattle and other animals meant for human consumption are allowed to graze on surface disposal sites, as long as there is documentation that steps are taken to:

ensure protection of public health and the environment from any reasonably anticipated adverse effects of certain pollutants that can be present in biosolids.” — The Standards for the Use or Disposal of Sewage Sludge (Part 503) – US Environmental Protection Agency

Class B biosolids may be applied to food crops or grazing land as long as the edible portions do not touch the surface of the soil, and harvest is at least 30 days after the last application. If the edible portions come in contact with the soil, the last application must have been over 14 months before harvest. Grazing animals cannot be allowed back on the pasture for the first 30 days after application. People without protective suits are also restricted from the site for 30 days on a farm, or 1 year for public access (although these restrictions are lower for the actual food produced in such fields).

“EQ” biosolids show lower (but measurable) levels of heavy metals and pathogens, and are exempt from these restrictions. These biosolids can be sold or given away in bulk, mixed with soil amendments and sold to consumers, or bagged for direct consumer use.

99% Industrial Waste + 1% Domestic Waste is Still Considered BIOSOLIDS

99% Industrial Waste + 1% Domestic Waste is Still Considered BIOSOLIDS

The United States Geological Survey (USGS) purchased nine off the shelf samples of biosolids available to consumers for study. They tested the samples for 87 different pollutant chemicals selected for their ability to pass through current treatments essentially intact. Out of the 87 chemicals tested for, 55 were present in at least one of the samples. Every sample had 25 chemicals in common, with one sample having measurable levels of 45. Some of the organic wastewater contaminants (OWCs) they all had in common include an antimicrobial disinfectant, an antihistamine, an antiepileptic drug, and steroids. These contaminants were found to comprise up to 1,800 ppm of the biosolids tested. The study concluded that:

the results indicate that biosolids have high concentrations of these emerging contaminants compared to treated liquid wastewater effluent.” — Household Chemicals and Drugs Found in Biosolids from Wastewater Treatment Plants – United States Geological Survey

Another study found elevated levels of these types of chemicals found in earthworms taken from areas of repeated biosolid use. (Environmental Science and Technology – Colorado State University of Pueblo, USGS)

Although the information on long-term effects of the use of biosolids is inconclusive at this time, a USGS study of long-term intensive land disposal of treated sewage in Cape Cod has shown the damage caused to a nearby aquifer will take at least decades to return to pre-contamination conditions. (Toxic Substances Hydrology Program – United States Geological Survey).

We don’t have to spread sewage sludge on our food crops and home gardens. Alternative uses include using it as a fuel source for bioreactors, or used to help restore landscapes already damaged from mining and landfills. The intentional misrepresentation of the product to the
consumer is a cause for concern. Amendment products that contain biosolids may or may not be labeled clearly as such, and the meaning of the organic sounding word “biosolid” is not commonly understood by the public. Foods grown under conditions where they are exposed to biosolids, even under direct contact, are not required to bear any indication to allow the consumer to make an informed choice in making purchasing selections.

When in doubt as to whether a product contains biosolids, contact the manufacturer. If they do not use sewage sludge in their products, they will likely be quick to say so. If they do use biosolids, be prepared for an obtuse or confusing answer as they try to cover up the reality of their ingredients.


[alert type=white ]This article by Grubbycup was originally published in Garden Culture Magazine, Issue 3 under the same title.[/alert]

Leave a Comment

Your email address will not be published. Required fields are marked *


Grubbycup was raised on a family-operated organic dairy farm in central California.