Land Application of Chemically Treated Sewage Sludge: III. Effects on Soil and Plant Heavy Metal Content

Abstract

Anaerobically digested sewage sludges resulting from treatment of sewage with Ca(OH)₂, Al₂(SO₄)₃, or FeCl₃ for phosphorus precipitation were applied to corn (Zea mays L.) and bromegruss (Bromus inermis Leyess) grown on a soil having an initial pH of 7.3. Rates of sludge supplied 200, 400, 800, and 1,600 kg N/ha each year for 5 years. Treatments with NH₄NO₃ supplying 0, 100, 200, and 400 kg N/ha were included for comparison. Plant tissue was analyzed for Cu, Zn, Mn, Cd, Ni, Cr, and Pb. No toxicity or deficiency symptoms were noted. Soil Zn, Cd, and Ni extracted by NTA (nitrilotriacetic acid) were increased by continued sludge application. The NTA‐extractable Zn and Cd were positively correlated with the Zn and Cd concentrations in corn stover. Soil pH was reduced by the Fe‐sludge application, slightly affected by the Al‐sludge, and increased by the Ca‐sludge. Increases in Cu concentrations in bromegrass and corn stover were associated with increases in the N content rather than the source of N, and plant Cu concentrations remained relatively constant across years. Sewage sludge application increased Zn, Cd, and Ni concentrations in bromegrass and corn stover, and Zn and Ni concentrations in corn grain, particularly at the higher metal loadings from sludge application. Zinc and Cd concentrations, especially in corn stover, increased with continued sludge application during the 5‐year period. The inclusion of soil pH as a factor, in addition to cumulative amounts of Zn or Cd added as a constituent of sludge, improved the regression equations predicting Zn or Cd uptake. Manganese concentrutions in bromegrass were reduced by the sludges used in this study, but did not change from one year to the next. Chromium and Pb concentrations in corn and bromegrass were not significantly increased by the application of 680 kg Cr/ha and 310 kg Pb/ha from sludge.