Effects of lignocellulosic biomass type on nutrient recovery and heavy metal removal from digested sludge by hydrothermal treatment

Sludge is a nutrient-rich organic waste generated from wastewater treatment plants. However, the application of sludge as a nutrient source is limited by its high contents of water and pollutants. In this study, the effects of biomass type on nutrient recovery and heavy metal removal from digested sludge by hydrothermal treatment (HTT) were investigated. Blending biomass with digested sludge for HTT at 180–240 °C increased the recovery of nitrogen in the treated solids. At the HTT temperature of 240 °C, HTT with hardwood sawdust led to the highest nitrogen recovery of 70.6%, compared to the lowest nitrogen recovery of 36.5% without biomass. Blending biomass slightly decreased the recovery of phosphorus compared to those without biomass. Nevertheless, the lowest phosphorus recovery of 91.3% with the use of hardwood sawdust at the HTT temperature of 240 °C was only ∼7.0% less than that without biomass. Blending biomass reduced the contents of macro-metals such as Ca, Fe, Mg and Al in treated solids but the metal contents varied with different biomasses. Regarding the heavy metals, the use of rice husk did not decrease the contents of Ni and Co while blending bagasse did not decrease the content of Cr at HTT temperatures of 210 °C and 240 °C compared to the use of other biomasses. The different effects of biomass type on nutrient recovery and heavy metals were likely related to the types and abundances of organic acids such as acetic acid, oxygen-containing functional groups such as C–OH and COOH, oxide minerals such as silica from biomasses and the overall effects of these factors. This study provides very useful information in selection of lignocellulosic biomass for HTT of sludge for nutrient recovery and heavy metal removal. •Biomass type affected recoveries of nitrogen and phosphorus in treated sludge.•Hardwood sawdust led to the highest nitrogen recovery of 70.6%.•Biomass-derived organic acids had leaching effects on nutrients and metal contents.•Biomass-derived functional groups and silica interacted with nutrients and metals.•The contents of nutrients and metal ions were determined by the overall effects.