Metals in Wastewater Treatment

A number of structural metals were exposed at two locations in the flow stream of a municipal wastewater treatment plant. Test metals included several aluminum alloys, mild steel, low alloy steel, Type 304 stainless steel, titanium, and zinc. Exposures for a 1-yr period were made at the raw sewage intake point and at the post-treatment effluent discharge. The lowest measured corrosion rate in raw sewage was 0.002 mpy for the stainless steel and the highest rate was 3.345 mpy for the mild steel. The aluminums went from 0.007 mpy for the super purity metal to 0.181 mpy for the copper-bearing 2036 alloy. Low alloy steel showed a rate of 2.055 mpy and zinc a rate of 0.016 mpy. No material showed any substantial loss in tensile properties as a result of the exposure. The aluminums generally were unpitted after 1 yr in raw sewage. The 1-yr exposure in the heavily chlorinated effluent was more aggressive toward the aluminums than the raw sewage.