Technical Resources

The need to dispose of human-generated waste streams is growing in line with the expansion of urban population centers. This is particularly true for the byproducts of wastewater treatment. According to the US EPA, there are over 7 million dry tons of biosolids (stabilized sewage sludge) produced per year in the US. In 2004, 49% of biosolids were beneficially used—primarily for agricultural land application—with most of the remainder either landfilled or incinerated (NEBRA 2007). Because biosolids have a high nutrient content, land application as a fertilizer substitute is an appealing management strategy. Yet concerns around nutrient run-off and contamination of waterways have led to tighter environmental controls making land application increasingly tenuous. Promising alternate management strategies exist but are in early stages of development. Pyrolysis and gasification—a continuum of thermochemical conversion processes—have been shown to minimize harmful air emissions, while producing energy and biochar, a carbon-rich solid material with beneficial soil health properties. This white paper briefly explores experiences of pyrolysis and gasification of biosolids as a waste management strategy, and research into biosolids biochar (BSB) as a soil amendment.

Purpose Biochars are a by-product of the biofuel processing of lignocellulosic and manure feedstocks. Because biochars contain an assemblage of organic and inorganic compounds, they can be used as an amendment for C sequestration and soil quality improvement. However, not all biochars are viable soil amendments; this is because their physical and chemical properties vary due to feedstock elemental composition, biofuel processing, and particle size differences. Biochar could deliver a more effective service as a soil amendment if its chemistry was designed ex ante with characteristics that target specific soil quality issues. In this study, we demonstrate how biochars can be designed with relevant properties as successful soil amendments through feedstock selection, pyrolysis conditions, and particle size choices.

Background Biochar’s role as a carbon sequestration agent, while simultaneously providing soil fertility improvements when used as an amendment, has been receiving significant attention across all sectors of society, ranging from academia, industry, government, as well as the general public. This has lead to some exaggeration and possible confusion regarding biochar’s actual effectiveness as a soil amendment. One sparsely explored area where biochar appears to have real potential for significant impact is the soil nitrogen cycle. Scope Taghizadeh-Toosi et al. (this issue) examined ammonia sorption on biochar as a means of providing a nitrogen-enriched soil amendment. The longevity of the trapped ammonia was particularly remarkable; it was sequestered in a stable form for at least 12 days under laboratory air flow. Furthermore, the authors observed increased 15N uptake by plants grown in soil amended with the 15N-enriched biochar, indicating that the 15N was not irreversibly bound, but, was plant-available.