Molecular weight distributions in activated sludge effluents

Abstract

Bench scale activated sludge reactors with a solids retention time of 9 days were operated at all combinations of two levels of pH, dissolved oxygen (DO) concentration and feed type (pH 6 and 8; DO 1 and 7 mgl −1; simple and complex feed). Long-term composite samples were collected and adjusted to neutral pH and equal concentrations of alkalinity and ammonia nitrogen. The molecular weight distributions of the soluble organic carbon (SOC) in the samples were determined by ultrafiltration through membranes with nominal rejection values of 1000, 10,000, 25,000 and 100,000. The distributions were generally bimodal with the bulk of the SOC in the largest and smallest fractions. Following ozonation to a residual of 0.30–0.35 mg 1 −1 after 5 min contact, the distributions were shifted so that more of the SOC appeared in the lowest molecular weight fraction although significant amounts remained in the other fractions. When samples were breakpoint chlorinated to a residual of 5–10 mg l −1 free available chlorine after 2 h contact, very little high molecular weight material remained and almost all of the SOC appeared in the low molecular weight fraction. By using 36Cl during breakpoint chlorination it was also possible to evaluate the distribution of organically bound chlorine among the molecular weight fractions and this revealed that the bulk of it was associated with organics having molecular weight less than 1000.