ELSEVIER 0960-8524(94)00027-1 Bioresource Technology 49 (1994) 203-207 @ 1994 Elsevier Science Limited Printed in Great Britain. All rights reserved 0260-8774/94/$7.00 MICROBIOLOGICAL STABILITY OF WASTEWATER SLUDGES FROM ACTIVATED SLUDGE SYSTEMS R. Y. Surampalli United States Environmental Protection Agency, PO Box 17-2141, Kansas City, KS 6611Z USA S. K. Banerji & J. C. Chen Department ofCivilEngineering, University of Missouri, Columbia, MO 65211, USA (Received 28 March 1994; revised version received and accepted 31 May 1994) Abstract Proposed United States Environmental Protection Agency (EPA) regulations require that processed sludge, prior to disposal, must meet certain pathogen-reduction requirements. The present study was conducted to evaluate reductions in pathogen-indicator bacteria in the slugdes of six wastewater plants. Five of the plants use extended aeration and oxidation ditches; the other is a trickling filter plant. Fecal coliform, fecal streptococcus, and Salmonella were measured on all sludge samples. Temperature, pH, dissolved oxygen, volatile suspended solids (VSS), and total suspended solids were also measured. The fecal coliform log reductions/g VSS for all the extended aeration and oxidation ditch plants were greater than 1"76. They varied from a log reduction of 1.76 to 4"21. The fecal streptococci reductions for the same sludge were generally lower than the fecal coliform reductions. The variations in the fecal streptococci log reductions in different plant sludges were from 0"87 to 2"73. The Salmonella log reductions varied from 1"39 to 4"7 and paralleled the fecal coliform reductions. The bacterial reductions were related to the plant design parameters. A longer storage of sludge usually produced a larger log reduction of the indicator bacteria. Gener- ally, the extended aeration and oxidation ditch plants were meeting the proposed EPA class B Pathogen reduction criteria, a two-order reduction. Key words: Sludge, pathogen reduction, coliforms, Salmonella, streptococcus, regulation. INTRODUCTION Land application of municipal wastewater sludge is gaining popularity in the United States because of economic considerations. The Environmental Protec- tion Agency (EPA) estimates that some 17% of the publicly owned treatment works use the land applica- tion option for disposing of sewage sludge (EPA, 1989). The use of sludge for land application takes advantage of its nutritive value and can help revegetate lands destroyed by mining, dredging and construction activities. However, proposed United States EPA sewage sludge technical regulations (40 CFR Part 503) for the land application of sewage sludge require that the possible risks of the pollutants in sewage sludge be evaluated and reduced, if necessary. Some of the pollutants of concern are pathogenic organisms such as bacteria, viruses, helminths, etc. Before land application, the processed sludge must meet certain pathogen reduction requirements. The most stringent, class A pathogen reduction, would be achieved if the density of fecal coliform and fecal streptococci bacteria/g VSS is equal to or less than 100 (Federal Register, 1989). For class B pathogen reduc- tion, the average density of pathogenic bacteria and viruses/unit mass of VSS in the sludge should be two orders of magnitude less than the densities in the incoming wastewater, or a facility must demonstrate that the densities of each of the fecal indicator organ- isms is 6 logl0/g VSS, or less (EPA, 1989). Class C pathogen reduction is achieved when sludge treatment processes reduce the density of bacteria and viruses/ unit of VSS in the sludge by 1"5 orders of magnitude lower than those in the influent wastewaster, or demonstrate that the density of fecal coliforms is 6-3 log~0/g VSS, or less, and the density of fecal strepto- cocci in the sludge is 6.7 lOgl0/g VSS, or less (EPA, 1989). No restrictions on access to or use of the land are imposed where sludge with class A pathogen reduction is applied. For class B and class C sludges, additional restrictions are imposed. The EPA proposal also includes requirements for reducing the attraction of vectors to sewage sludge (EPA, 1989). There are many small municipal wastewater treat- ment plants that may not be able to achieve class A-type pathogen reduction and are looking at relatively simple processes to achieve class B or class C pathogen reductions. The open anaerobic storage of sludge (without mixing) will certainly be a more economical means of processing compared to aerobic or anaerobic 203