After my first post listed three procedures banned in organic food production, I wrote a full post on radiation, and two on genetic engineering. Now it’s time to dive into biosolids and sewage sludge.

As usual, I’ll start with defining terms. Sewage sludge is residue, typically solid, generated during treatment of sewage that does not include the ash, grit, or screenings during treatment. This is the stuff that’s leftover when wastewater is treated. Biosolids are sewage sludge that has been treated to remove odors and pathogens.

Biosolids are federally regulated by the EPA through the Clean Water Act, which establishes levels of quality for these waste materials to be applied to land. Class B has been treated to remove pathogens to less than 2 million MPN/g. MPN stands for Most Probable Number, because measuring is difficult, for real. Class A has been treated to remove pathogens to less than 1,000 MPN/g. And to add an extra layer, Class A, EQ (exceptional quality) meets pathogen requirements AND has a low concentration of heavy metals.

Class A, EQ can be used anywhere, including home gardens without additional regulations. Class A is restricted from things like being use on flooded or frozen land, being used 10 meters from water, adversely threatening endangered species, and it faces labeling requirements. Class B has all of the same restrictions as Class A and more. Food crops must be harvested 14-38 months after the application of Class B biosolids, depending on what parts of the plant typically come into contact with soil. All crops, regardless of use, must wait at least 30 days for harvest after application, and animals are not allowed to graze during this 30 day window either. Public access is restricted for 30 days to 1 year, depending on the potential for exposure.

With the myriad of restrictions, use of Class B biosolids has been decreasing in favor of higher quality levels. No surprise there.

So what are the benefits of using biosolids? They can be used as fertilizer to stimulate plant growth and improve soil, reducing the need for chemical fertilizers. This offers environmental benefits by recycling carbon to the soil. If not used in this way, sewage sludge from wastewater treatment facilities is typically incinerated or buried in a landfill.

What are the risks of using biosolids? Risks include exposure to pathogens and the addition of heavy metals including arsenic, lead, and mercury to soil. There is also the risk of emerging contaminants, which include antibiotics, hormones, and industrial and household chemicals. Basically anything that is commonly found in sewage whose effects are not well understood.

How common is the use of biosolids on land? About 55% of biosolids in the U.S. are applied to land. It is a common practice in many European countries, Australia, and New Zealand. It is not used in Switzerland, Austria, and parts of the world that don’t have developed wastewater treatment systems.

How do I know if biosolids were used as fertilizer to grow my food? As usual, it’s easier to know when it’s not used. Biosolids are not allowed in USDA Organic products. Regardless, the EPA estimates that less than 1% of farmland in the country has biosolids applied.

Image by: Ivan Bandura

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