Sludge samples from the primary settling tank and the first stage digester of a four-million-gallon-per-day sewage treatment plant were prepared for atomic absorption spectrometry by four methods: dry ashing, digestion with nitric acid, digestion with nitric acid and hydrogen peroxide, and digestion with nitric acid in pressure decomposition vessels. The prepared samples were analyzed for cadmium, chromium, copper, iron, lead, nickel and zinc. Typical results were: 50#x2013;75 ppm Cd, 15000#x2013;20000 ppm Cr, 2500#x2013;3000 ppm Cu, 10000#x2013;15000 ppm Fe, 3000#x2013;3500 ppm Pb, 50#x2013;75 ppm Ni and 8000#x2013;8300 ppm Zn. In the cases of copper and zinc, these values were confirmed by instrumental neutron activation analysis. In terms of metal recovery, dry ashing appears to be less efficient than digestion with nitric acid in decomposition bombs, digestion with nitric acid and hydrogen peroxide or digestion with nitric acid.
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