Biosolids Management and Resource Recovery

Biosolids, the semi-solid by-product of the wastewater treatment process, represent a valuable, albeit problematic, resource. My entry into the relatively niche but endlessly fascinating world of biosolids management was driven by an interest in both recycling and sustainable agriculture. The idea of utilizing the by-product of the process we use to reclaim wastewater as a soil amendment to then help improve soil fertility and soil health has always been appealing to me. In 1972, the year in which the Clean Water Act was enacted, my family moved to Cleveland, OH, which at that time due to regular fires on the Cuyahoga River and the eutrophication of Lake Erie was a poster child for the necessity of the Clean Water Act; this regulatory act is near and dear to my heart.

The concept of adding manure and human waste back to soils to increase soil fertility and soil health is as old as agriculture itself. With the increased reliance on commercial fertilizer in the past century, we have lost some of the motivation to return our waste materials to soil and, with that shift, we have squandered opportunities to build rather than deplete the organic matter content of our soils. One bright light related to the Clean Water Act and the need to add organic matter back to our soils is that the material captured during the wastewater treatment process (a lot of the material that would otherwise pollute surface water) is what our soils need. 

From a global context, soils and biomass combined contain approximately four times as much carbon as the atmosphere

With the climate heating up quickly, we know that sequestering as much carbon as we can in the soil is an important tool in mitigating the trend. Returning biosolids (and other organic matter–based wastes) back to the soil to not only increase soil carbon levels but also increase biomass production is imperative

Additionally, there is inherent energy embodied in biosolids, the capture and use of which can further improve the positive carbon benefits associated with biosolids recycling. While deriving energy from the oxidation of organic carbon in biosolids can mean less carbon being returned to the soil, greenhouse gas emissions accounting models have demonstrated that capturing and utilizing energy from the anaerobic digestion of biosolids followed by returning the digestate to the soil typically provides the greatest carbon benefit of the potential biosolids management pathways. 

There are risks inherent in returning biosolids to soil, primarily related to organic pollutants that are present in wastewater. The chemical makeup of wastewater mirrors chemical use in our consumer products and in our environment. In some cases, the wastewater treatment process volatilizes or otherwise destroys/metabolizes organic pollutants. In other cases, such as with flame retardants, plasticizers and dioxin, the trace levels of these compounds found in wastewater are not destroyed but are captured in the biosolids. Most of these compounds are relatively immobile in soils and consequently not prone to significant leaching and/or plant uptake after being applied to the soil in small amounts in the form of biosolids.But as we have seen recently with perfluoroalkyl and polyfluoroalkyl substances (PFAS), there are some compounds that both survive the wastewater treatment process and are mobile in the soil environment. 

One critical element of following the principal of continuing improvement in the wastewater and biosolids management fields is developing a two-pronged approach to addressing contaminants while continuing the critical work of improving the global carbon balance. First, we need to improve the upstream control of wastewater quality by both further improving existing industrial wastewater pretreatment programs already in place, but more importantly reducing our reliance on chemicals in our consumer products that significantly influence the chemical quality of solids captured in the wastewater treatment process. Second, we need to thoroughly assess processes that eliminate organic pollutants in biosolids. For some of these compounds, anaerobic digestion and/or composting are not effective. Some thermally driven processes, including pyrolysis, have been demonstrated to destroy some of the more recalcitrant compounds while preserving much of the carbon in biosolids. 

In the end, I don’t advocate for any one method of biosolids processing, but I am interested in continuing to work on improving current solutions and/or finding new ones that focus on taking advantage of the opportunities while limiting any negative impacts related to recycling biosolids. Solutions to the current climate crisis are more critical than ever, and as we strive for cleaner water globally, we will generate more biosolids and we need to be deliberate in our approach to managing them.