Abstract
● Content of heavy metals in hydrochar varies considerably, from 50% to 100%. ● Concentrations of heavy metals in hydrochar can be higher than those in the dairy manure. ● Concentrations of heavy metals in hydrochar are far below the regulatory level.
Hydrochar produced from dairy manure is a regulated biosolid if being promoted for agricultural applications thus must have the properties that comply with all environmental standards and government regulations, including the levels of heavy metals (HMs). In this study, systematic research was conducted on HM levels in hydrochar from dairy manure and on the effects of processing conditions, including processing temperature (180-255 °C), holding time (30 -120 min) and solid content of manure slurry (2% -15%), through a central composite design and statistical analyses. It was found that HMs can be retained in hydrochar, ranging from 40% to 100%. The processing temperature and solid content in the feed were the most influential process parameters that affected HMs retention in hydrochar. Statistical analysis showed that there was no single optimal point to minimize HMs retained in hydrochar, but there were minimization points at given processing time and solid content. Most HMs concentrations were higher in hydrochar than those initially in dairy manure but were greatly below the thresholds as set by the US government regulations. Thus, hydrochar is feasible for use as a phosphorus-enriched organic fertilizer and/or soil amendment for agricultural applications without serious concerns about HMs it might contain.