Use of alum water treatment sludge to stabilize C and immobilize P and metals in composts

Alum water treatment sludge is composed of amorphous hydroxyl-Al, which has variable charge surfaces with a large Brunauer-Emmett-Teller (BET) surface area (103 m⁻² g⁻¹) capable of specific adsorption of organic matter molecules, phosphate, and heavy metals. The effects of adding dried, ground, alum water treatment sludge (10 % w/w) to the feedstock for composting municipal green waste alone, green waste plus poultry manure, or green waste plus biosolids were determined. Addition of water treatment sludge reduced water soluble C, microbial biomass C, CO₂ evolution, extractable P, and extractable heavy metals during composting. The decrease in CO₂ evolution (i.e., C sequestration) was greatest for poultry manure and least for biosolid composts. The effects of addition of water treatment sludge to mature green waste-based poultry manure and biosolid composts were also determined in a 24-week incubation experiment. The composts were either incubated alone or after addition to a soil. Extractable P and heavy metal concentrations were decreased by additions of water treatment sludge in all treatments, and CO₂ evolution was also reduced from the poultry manure compost over the first 16–18 weeks. However, for biosolid compost, addition of water treatment sludge increased microbial biomass C and CO₂ evolution rate over the entire 24-week incubation period. This was attributed to the greatly reduced extractable heavy metal concentrations (As, Cr, Cu, Pb, and Zn) present following addition of water treatment sludge, and thus increased microbial activity. It was concluded that addition of water treatment sludge reduces concentrations of extractable P and heavy metals in composts and that its effect on organic matter stabilization is much greater during the composting process than for mature compost because levels of easily decomposable organic matter are initially much higher in the feedstock than those in matured composts.