Heavy Metals the Encore – Because You Asked for More

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My last post began the discussion of heavy metals in biosolids, a topic that can provoke a noisy reaction. So let’s keep rockin’! Heavy metals in compost don’t seem to get the same response as heavy metals in biosolids. For some reason, heavy metals are associated primarily with biosolids.  That is incorrect.  This post continues the analysis.

Simple reasons why heavy metals are present in both compost and biosolids

Though it’s often thought that compost is the purest of substances, it too has metals, and these are also subject to regulatory limits.

A review of research bore out that compost has regulatory limits for heavy metals similar to biosolids. This should not be a surprise because both biosolids and compost from food scraps are composed of many of the same organic materials, such as proteins and fats.

With biosolids, the difference is that the food was eaten first.

Municipalities are required to submit information on biosolids quality, which enabled me to access information. I accessed a land application report from the Water Pollution Control Department in Lafayette, IN – the plant where I started my career, as well as a land application report from Angola, IN where I spent six years as the superintendent. (Thanks to Brenda Stephanoff from IDEM for her assistance.) Data on commercial compost is publicly available. For comparison I found compost information in separate reports from a solid waste facility in Lincoln, NE and a food compost site in State College, PA.

Comparison 1Comparison 2

It’s worth noting that it appears some biosolids have much higher levels of some metals, but this is relative. Not only do the charts show all metals are below EPA standards, all the values for biosolids are certainly within an order of magnitude of the compost values. The North East Biosolids & Residuals Association has compiled a much more comprehensive evaluation on metals in compost and biosolids. Thanks to Ned Beecher.

It’s a fact that compost and biosolids have the same regulatory limits for heavy metals.

For more information on this same subject, San Francisco Public Utilities compared commercial compost against composted biosolids in 2010 to evaluate compliance with heavy metal limits.  The utility determined that pollutant metals results were lower than the pollutant limits listed in Part 503 Rule Table 2-1.

Lorraine Herity’s master’s thesis from Ireland  showed sewage sludge met limits for heavy metals more often than compost.

Comparison 3

Number and Percentage of Samples in Compliance with Irish EPA Guidelines. Lorraine Herity. 2003. A Study of the Quality of Waste Derived Compost In Ireland. Queens University of Belfast.

The conclusion – you will find that the same heavy metals of concern are present in both compost and biosolids. Presence does not equate to risk or hazard.


Heavy Metal Town

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Wacken Open Air, Wacken, Germany

Heavy metal from my era has inundated TV commercials over the past few years. Classic rock and roll by Led Zeppelin, ACDC, and Judas Priest is the background music for selling cars and cologne, but heavy metal is not so popular when it comes to organics management.

I recently joined in a Facebook conversation in the Solar Cities group regarding food waste recovery using anaerobic digestion at the Reedy Creek Wastewater Treatment Plant in Orlando. It specifically focused on the concept of codigestion. Concern arose over mixing food waste with sewage sludge – it was thought that the value of food scraps was lost due to contamination by or with heavy metals.

Heavy metals are found in the soil, and are also present in food. Arsenic is found in apples and Mercury is found in fish. (In fact, some of us even take vitamins with heavy metal ingredients.)  But did you know compost has heavy metals too? It should be no surprise that both biosolids and compost are regulated to control the levels of heavy metals. Composting is not the only way to return carbon and nutrients to the soil – land application of residuals from wastewater treatment digesters (biosolids) is also a viable means of recycling carbon and nutrients.

Reluctance to see food waste codigestion as resource recovery is throwing the baby out with the bathwater or, put another way — “throwing out the baby to save the bathwater.” Environmentally conscientious people should embrace the concept of recycling both food scraps and biosolids together because both are resources and neither should be discarded.