Effect of Small‐Scale Composting of Sewage Sludge on Heavy Metal Availability to Plants

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Abstract
The effect of small‐scale composting of sewage sludge on heavy metal availability to plants was investigated. Sand‐bed dried, anaerobically digested sewage sludge was mixed with wood chips in a 1:3 volumetric ratio, placed in a 1.4‐m3 box, and amended with sucrose as an available C source. The mixture was composted for about 90 d. A greenhouse study was conducted to evaluate the differences in sludge due to composting. Lettuce (Lactuca sativa) and oats (Avena sativa) were grown in pots containing composted and uncomposted sludge additions at rates equivalent to 0, 60, 120, and 240 Mg/ha to an acid Redfeather loamy sand (Lithic Cryoboralfs) and a neutral Nunn clay loam (Aridic Argiustolls). After 7 weeks, dry‐matter yields were measured and soil and plant samples were analyzed for total content of Cd, Cu, and Zn. Determinations were also made of soil pH, electrical conductivity (EC), and cation exchange capacity (CEC). An organic matter fractionation was carried out on both sludges to determine changes in organic matter and metal distribution brought about by composting.Additions of compost produced greater yields of both species than additions of sludge due to the lower soluble salt content and Zn availability of the compost. The higher application rates of compost maintained a higher pH on the loamy sand, which possibly decreased Cd and Zn availability to plants. Leaching of soluble Cu complexed during composting might have contributed to a 40% decrease in sludge Cu content, yet no change was noted in Cu availability to crops. Composting increased the humic acid, decreased the β‐humus, and changed Cu and Cd distribution in sludge organic matter.