Life cycle energy and greenhouse gas assessment of the co-production of biosolids and biochar for land application

Abstract

The co-production and co-application to land of biochar and biosolids may make conventional wastewater treatment more sustainable; therefore, this study provides a first life cycle evaluation of the co-location of a pyrolysis (biochar production) plant with a municipal wastewater treatment plant. Life cycle energy use and greenhouse gas emissions are evaluated for the co-production of these two materials. Three US national scenarios are included: biosolids only produced and disposed of as is the current common practice in the US (landfilled, incinerated, and land applied); biosolids only produced but not land applied; and co-production of biosolids and biochar and their disposal as currently practiced. A case study is also presented with slightly different disposal practices. Large contributors to life cycle greenhouse gas emissions are from the disposal of biosolids in landfills and incinerators, making alternatives like land application attractive for mitigating greenhouse gas emissions. The addition of biochar production adds little to the overall energy use but provides substantial (26%) reduction in greenhouse gas emissions for the national case, largely due to the recalcitrant carbon storage in biochar. Because biochar production is an energy positive process, incorporation of the energy co-products into the plant is explored for producing high quality Class A biosolids. The addition of biochar to biosolids is shown to be more sustainable from a co-production standpoint. This study contributes a benchmark analysis in evaluating the co-production of biochar and biosolids for potential environmental, economic, and health benefits and the results are useful for utility planning.