ABSTRACT
University of Washington, College of Forest Resources, AR-10, Seattle, WA 98195
The nature of Cd, Cr, Cu, Ni, Pb, and Zn in sewage sludge compost was investigated, including studies on the solubility, retention, and uptake of metals by plants. Compost used in the studies was produced from a mixture of 3:1 (v/ v) wood chips and 25% solids sewage sludge composted in large static piles for a period of 1 year. A laboratory study was conducted using sequential extractions with distilled water, MgCl2, and pyrophosphate to establish the strength of trace metal bonding within the mature and two-year-old field-aged compost. After 9 months, compost was sampled by depth and analyzed for change in metal concentration. PVC tubes installed in field sites were removed and analyzed after 2, 6 , 12 and 24 month incubations. Measurements of weight loss were used to assess decomposition, and, together with change of metals concentration, total loss of metals from the compost. Additionally, grass, lettuce, carrots, and tomatoes were field grown in 100% compost, 50% compost, soil fertilized with agronomic levels of NPK, and control soil. Vegetation samples were collected and analyzed for trace metal concentrations for two consecutive growing seasons. The bulk of trace metals in composted sewage sludge were not available either for plant uptake or leaching through the soil. Distilled water extracts 195
accounted for < 1 % of the total for all metals in newly finished compost and less than 1% of total for Cd, Cr, Cu, and Pb in two-year-old compost. Exchangeable trace metals varied considerably, measured in the following order: Cu (0.80.9%) < Cr (3.5-7.7%) < Pb (6.8-7.4%) < Ni (12-16%) < Zn (15-52%) < Cd (32^48%). Residual metals (not leached by distilled water, MgCl2, or Na4 P2 0 7) is nearly at or well over 50% for all metals, and for Cd, Cu, and Pb greater than 80%. Those metals that showed little water or MgCl2 leaching (Cr and Cu) were similarly conservative after 9 months in the field. Metal concentrations increased in the compost due to reduction of compost mass by decomposition. Small loss of Cd and Zn and relatively large loss of Ni from the compost occurred after 9 months in the field. Uptake slopes of plants grown in sludge compost are in the order of Cd > Zn > Ni > Cu > Cr. In general, Cd, Cr, and Cu are similarly available for plant uptake in compost (at a significantly lower pH) as compared to the soil, while Ni and Zn are slightly more available. Uptake rates of trace metals by the plants in this study are generally in the order of lettuce > grass > carrots > tomatoes.
