SEWAGE SLUDGES IN HAWAII: CHEMICAL COMPOSITION AND
REACTIONS WITH SOILS AND PLANTS
N. V. Hue and S. A. Ranjith
Water, Air, and Soil Pollution 72:265-283, 1994.
Sewage sludges from six wastewater treatment plants in Oahu, Hawaii, were periodically sampled to determine the seasonal variation of their elemental composition. The Sand Island and Honouliuli treatment plants produced primary sludges averaging 1.6% and 2.0% total N,
respectively. The Kailua, Kaneohe, Waimanalo, and Waianae treatment plants produced
secondary sludges averaging 5.4, 5.1, 6.1, and 3.5% N, respectively. All the sludges
tested were virtually devoid of K with concentration ranging from 0.01 to 0.15%, which was
less than half of the 0.30% K considered typical for a US sewage sludge. Mean
concentrations of Cd, Cu, Fe, Mn, Ni, and Zn in the Hawaii sludges were 5.9, 373, 12343,
218, 36.7 and 817 mg kg-1, respectively, which were within the norms for sludge heavy metals as reported by the US Environmental Protection Agency. Seasonal variations in
elemental concentration were small and only statistically significant for Ca and Zn.
Sludges from the three treatment plants with highest annual production (Sand Island,
Honouliuli, and Kailua) were then mixed at 5, 50, and 250 g kg-1 with three representative tropical soils (a Mollisol, and Oxisol, and an Ultisol) to study sludge-soil reactions and plant responses. Soil-solution data indicated that chemical properties of a sludge-soil mixture depended not only on the soil, sludge, and its application rate, but also on sludge-soil interactions. At an agricultural rate of 5 g kg-1 (10 Mg ha-1), the anaerobically digested Kailua sludge increased corn (Zea mays L.) biomass, whereas the two undigested sludges reduced it. At higher rates, Mn phytotoxicity resulted from sludge applications to the Mollisol and Oxisol, both of which contained reducible Mn nodules. Significant growth reductions would be expected when corn seedling contained 200 mg Mn kg-1 or 0.30% Ca; and, adequate supplies of Ca and Zn seemed to lessen Mn phytotoxicity.
CHANGES OF METAL FORMS BY ORGANIC AMENDMENTS TO HAWAII SOILS
Mengbo Li, N.V. Hue, and S.K.G. Hussain
Commun. Soil Sci. Plant Anal. 1997 (in press).
Forms of metals in soils control their availability to plants and animals and
affect the environment differently. To evaluate shifts of metal forms as affected by organic
amendments, a sequential extraction procedure was used to fractionate Ca, Fe, Mg, Mn and
Zn in two Hawaii soils amended with three organic wastes. The designated forms are
water-soluble, exchangeable, sorbed, organically bound, carbonate, and residual fractions.
The soils, a Mollisol (Waimanalo series) and an Ultisol (Paaloa series), were incubated at
25øC ñ 2øC at field moisture capacity with either chicken manure, sewage sludge, or green
manure (cowpea leaves) at 0, 5 and 20 Mg ha-1 for one or five months. Organically bound
metals decreased with time because of organic matter decomposition. Iron was mostly
residual, but water-soluble Fe also increased in the acid Paaloa soil. Unlike Fe, most forms
of Ca and Mg were transformed to the exchangeable form in 5 months. There was no
significant change of Mn forms during the 5-month incubation. Virtually all organically
bound Zn shifted to carbonate and residual forms in the neutral Mollisol (pH 6.2), but
shifted to carbonate and exchangeable forms in the acid Ultisol (pH 4.5). The solubilities
andexchangeabilities of the five metals in the two soils treated with sewage sludge were not
significantly different from those treated with cowpea green manure or chicken manure
during the 5-month incubation. The results suggest that the additions of sewage sludge,
chicken manure, or cowpea green manure to Hawaii soils at 20 Mg ha-1 do not have
environmentally significant impacts in terms of Ca, Mg, Fe, Mn and Zn. On the other hand,
the amendments may decrease Ca and Mg deficiencies in highly weathered, nutrient-poor
soils such as Ultisols and Oxisols of the tropics.
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