Methods for Quantifying Lime Incorporation into Dewatered Sludge. I: Bench-Scale Evaluation Jason M. North 1 ; Jennifer G. Becker 2 ; Eric A. Seagren, A.M.ASCE 3 ; Mark Ramirez 4 ; and Christopher Peot, P.E. 5 Abstract: The addition of alkaline material usually lime to treated municipal sludge can be used to raise the pH to 12 and generate Class A or B biosolids. When lime is added to dewatered sludge, it must first be made into a slurry before the pH can be measured to demonstrate regulatory compliance. In this study, pH 12 was achieved in slurries prepared from lime-amended dewatered sludge, even when the lime was poorly incorporated and relatively high fecal coliform levels were detected. Thus, quantitative indicators of lime incorporation are needed to complement slurry pH measurements and ensure that sufficient contact occurs between lime and sludge particles to achieve adequate stabilization. In this study, the usefulness of several potential measures of lime incorporation—pH, CO 2 consumption, distribution of calcium, fecal coliforms, NH 3 and reduced sulfur compound production, and ATP—was systematically evaluated using a bench-scale system. Sludge pH and CO 2 consumption were not influenced by the extent of lime incorporation. The distribution of calcium and fecal coliform levels appear to be useful measures of lime incorporation. NH 3 and reduced sulfur compound emissions and ATP levels can also be used to assess lime incorporation provided recommended experimental techniques are used. DOI: 10.1061/ASCE0733-93722008134:9750 CE Database subject headings: Sludge; Lime; Mixing; Full-scale tests; Dewatering; pH; Laboratory tests. Introduction Incorporation of alkaline material, especially lime, into mechani- cally dewatered municipal wastewater sludge postlime stabiliza- tion can be used to produce materials that meet the U.S. Environmental Protection Agency USEPA minimum criteria for Class B or A biosolids USEPA 1993. To meet the Class B Pro- cesses to Significantly Reduce Pathogens PSRP requirement for lime-stabilized biosolids, levels of indicator organisms in these materials must fall within maximum allowable limits, and the pH of the biosolids must be maintained at 12 for 2 h and at 11.5 for an additional 22 h. Achievement of the pH criterion is demon- strated by making measurements on a slurry prepared by adding water to the lime-amended sludge and subjecting it to additional mixing. Unfortunately, the addition of water to sludge–lime mixtures and the subsequent homogenization step make it difficult to infer any information about the extent of lime incorporation in the original undiluted sample. The pH levels that meet regulatory requirements for classification as biosolids could conceivably be measured in slurries derived from sludge samples that contain poorly incorporated lime. In the micromorphology analyses shown in Fig. 1, differences in the extent of lime incorporation into biological sludges that were generated at the same wastewa- ter treatment plant District of Columbia Water and Sewer Au- thority’s DC WASA’s Blue Plains advanced wastewater treatment plant Blue Plains WWTP, but were treated in distinct dewatering and post-lime-stabilization systems, are visually obvi- ous. However, the pH levels measured in the two systems were generally very similar. The apparent inability of slurry pH mea- surements to distinguish significant differences in the extent of lime incorporation is important because poor incorporation of lime can result in production of malodors, which in turn can lead to decreased public acceptance of biosolids processing and land application programs. As a result, the potential for beneficial re- cycling of plant nutrients and organic in biosolids may not be fully realized. It is generally recognized that adequate contact between lime and sludge particles must occur in order to achieve adequate in- corporation of the lime and a stable residuals product Metcalf & Eddy 1991. As a result, most laboratory studies of postlime sta- bilization used extended mixing times on the order of hours e.g., Riehl et al. 1952; Wong et al. 2001. Counts and Shuckrow 1975 used short mixing times 20–30 s to examine the effects of two different bench-scale mixing types on the pH of lime- amended sludge. However, their study involved liquid sludge, rather than dewatered cake. The extended mixing times used in most bench-scale studies may not be reflective of the mixing that occurs in full-scale systems. Consequently, there is little infor- 1 Regional Manager, Natgun Corporation, 4500 Black Rock Rd., Hampstead, MD 21074. E-mail: jnorth@Natgun.com 2 Associate Professor, Dept. of Environmental Science and Technology, Univ. of Maryland, College Park, MD 20742 corresponding author. E-mail: jgbecker@umd.edu 3 Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of Maryland, College Park, MD 20742. E-mail: eseagren@ eng.umd.edu 4 Biosolids Process Engineer, District of Columbia Water and Sewer Authority DC WASA, 5000 OverlookAve. SW, Washington, DC 20032. E-mail: mark_ramirez@dcwasa.com 5 Biosolids Division Manager, District of Columbia Water and Sewer Authority DC WASA, 5000 OverlookAve. SW, Washington, DC 20032. E-mail: chris_peot@dcwasa.com Note. Discussion open until February 1, 2009. Separate discussions must be submitted for individual papers. The manuscript for this paper was submitted for review and possible publication on May 21, 2007; approved on August 27, 2007. This paper is part of the Journal of Environmental Engineering, Vol. 134, No. 9, September 1, 2008. ©ASCE, ISSN 0733-9372/2008/9-750–761/$25.00. 750 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / SEPTEMBER 2008 J. Environ. Eng. 2008.134:750-761. Downloaded from ascelibrary.org by NDSU LIBRARY on 02/28/14. Copyright ASCE. For personal use only; all rights reserved.