Relationships between metal concentrations and crown corrosion in Los Angeles County sewers

Sulfide generation and crown corrosion rates increased sharply in reinforced concrete trunk sewers in Los Angeles County during the years following implementation of categorical industrial waste pretreatment requirements. Measured rates of crown deterioration were several times larger than predictions developed using accepted engineering methods and many times faster than rates observed in the same sewers before 1990. To investigate the discontinuous record of crown corrosion in Los Angeles County sewers, relationships between sulfide generation and wastewater quality characteristics were examined for statistical significance. Wastewater concentrations of dissolved and/or total sulfide were correlated with concentrations of several transition metals, including Ni, Fe, Pb, Cd, and Cr. A series of experiments was then undertaken to examine directly the effect of transition metal supplements, alone and in combination on the activities of sulfate-reducing bacteria (SRB) in batch reactors and two types of continuous-flow reactors. None of the metals tested individually, at concentrations approaching those in Los Angeles County wastewater before implementation of federally mandated industrial waste pretreatment measures, inhibited sulfide generation. When added together at concentrations that approximated their 1971-1974 levels in county wastewater, Cr, Cu, Ni, Zn, and CN^- inhibited SRB activity in each reactor configuration tested. Results suggest that wastewater quality was an important determinant of sulfide generation and crown corrosion rates in Los Angeles County sewerage.