Converting sludge into a valuable product – biochar

Abstract

Production of sludge during wastewater treatment processes poses a major challenge due to its large production volume and presence of hazardous substances, like heavy metals (HMs) and pathogens. It is conventionally dewatered to be incinerated and the ashes are disposed of in landfills. However, rather than being a waste product, sludge is a valuable resource that has uses in energy and resource recovery. Sludge is generally high in organic matters and rich in nutrients and trace minerals. One sustainable method of utilising sludge is through pyrolysis to convert it into biochar which is a potential soil amendment. Biochar can undergo steam activation and be converted to activated carbon (AC) with higher surface area and pore volume which is a potential low-cost adsorbent.

In this study, the feasibility of using sludge to produce biochar and AC was evaluated with 5 different sludge samples under varying pyrolysis temperatures and steam activation conditions respectively. The relationship between pyrolysis temperatures and steam activation conditions and characteristics of produced biochar and AC were investigated. The results showed that as pyrolysis temperatures increased, properties such as leachate pH, HMs content, specific surface area and pore size generally increased, while leachate HMs concentration, leachate nutrients, macronutrients and trace elements decreased. Steam activation proved to be successful in producing AC of high surface area and pore volume. Generally, biochar from low pyrolysis temperatures are optimal for soil conditioning, while AC from steam activation are promising as a low-cost contaminant adsorbent. This sludge treatment method carries huge potential for a wide range of applications in soil remediation and contaminant adsorption.