Limits of Credibility

Posted on

Four Points About Biosolids You May Not Want to Believe

This month, I will try to persuade you that we should not outlaw or discourage use of a valuable resource, biosolids, on the basis of a problem not proven to exist. See if you agree.

A Necessary Resource

During the time that I worked at a wastewater treatment plant where we distributed biosolids, I met many farmers. Their demand for biosolids often exceeded our supply because of the results farmers came to expect. This picture is proof of the statement that biosolids are a valuable resource in crop growth.


This is corn grown without biosolids, compared to corn grown with biosolids (AKA sludge.) Both ears are from Burlington, Wisconsin and the display itself was at the local wastewater treatment plant.

Biosolids are also necessary for the infrastructure of modern civilization because they are a means of reducing the volume of the byproducts of wastewater treatment, something we’d be hard-pressed to live without.

The Limits of Credibility

Also this month, the Pennsylvania and Tennessee state legislatures drafted language allowing communities to stop the land application of biosolids. Why? It would be no surprise to hear it was because they believed some combination of the four points that are frequently used to make a case against the land application of biosolids: They are:

  1. Odors
  2. Pathogens
  3. Heavy metals
  4. Toxics

I contend that these points alone have not made a credible case against the use of biosolids and in particular, points number three and four fail to do so. Here is why.

The first point is easily rebuffed as somewhat unserious. While it’s true that biosolids can have an odor, it’s not dissimilar to that of spreading manure. How practical would it be to outlaw fertilizing of crops just because of an unpleasant smell once or twice a year? The next two points about pathogens and heavy metals are false arguments because the pathogens and metals already must be reduced in order to meet the EPA 503 regulatory limits. Non-reduction is not an option if biosolids are to be distributed as either Class A or Class B.

This leaves point number 4, toxics, a term that includes organic compounds such as polychlorinated biphenyls (PCBs) and personal care and pharmaceutical products (PCPPs.) PCBs and PCPPs have been identified in biosolids but at very low levels. Their presence alone does not automatically equate to harmful effects. Just because a lab identified the existence of something as a “contaminant” when found at 1 part per million or per billion, does not mean that a cause-and-effect relationship exists. For example, we know low-level nitrates (45 ppm) in drinking water can cause methemoglobinemia (blue baby syndrome), but even lower levels are tolerable. Identification of a chemical is not enough. It is essential to establish a cause and effect relationship as part of a hazard evaluation before pursuing control or prohibition.

According to the Water Environment Federation, the EPA assessed 200 compounds and elements from an original list of thousands before looking closer at 22 specific constituents. They ultimately decided to set limits for nine: Arsenic, Cadmium, Copper, Lead, Mercury, Molybdenum, Nickel, Selenium and Zinc.   So let’s look at one of these metals, Cadmium.

Assuming a typical concentration in biosolids of 7 mg/kg Cadmium (found mostly in batteries), to reach the annual limit for Cadmium on an acre of ground, a farmer would have to apply 121 dry tons of biosolids per acre. To reach the lifetime limit, a farmer would need to apply 2,500 dry tons per acre.  For reference, farms growing corn would typically limit biosolids application to only around 6 dry tons of biosolids per acre per year to limit Nitrogen, so they would never even come close to the limit allowed for Cadmium anyway!

This reminds me of the artificial sweetener debate from a long time ago when consumption of so much sweetener over the course of a lifetime was needed to make the FDA’s case about harm that the argument became a laughing stock with the public and ultimately went away on its own.

Do you agree that the usual arguments against the valuable resource that is biosolids need a whole lot more documentation and research before they deserve serious consideration?








4 Replies to “Limits of Credibility

  • Eric Martin on said:

    Agreed! Biosolids are a great resource and it be foolish to not utilize them.

  • TH Culhane on said:

    I wholeheartedly agree, and was using your arguments in my workshop today in Turkish Universities. Good stuff!

  • Jeffrey Ling on said:

    Every farm boy knows: “excrement” makes good corn!!

    While I agree and support your proposition, the issues of “harvest concentrations” are real!

    Clearly, Cations don’t depreciate in the digestive tract or compost pile. If the plant absorbs the metallic atom, it’s transported. This lowers the concentration on-site,(phyto-mitigation) but “spreads” it around.

    I faced this on a municipal project. The biosolids were “hotter” than the slag!! For processing operations with low metal cation content, this is a great concept.

    • kelemanm on said:

      Thanks for the input Jeffery! Clearly, every case is unique, but by and large biosolids have low concentrations of heavy metals. If a particular city has trouble meeting 503 regulatory limits for heavy metals, they should not be beneficially reused for land application.

Leave a Reply

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