Measuring Sludge Network Strength Using Rheology and Relation to Dewaterability, Filtration, and Thickening— Laboratory and Full-Scale Experiments Mohammad M. Abu-Orf 1 and Banu Örmeci 2 Abstract: Researchers Örmeci and Abu-Orf used rheology; a fundamental character of sludge, to arrive at a standard protocol for measuring network strength in terms of energy required to break up the structure of a certain volume of sludge. A mathematical derivation showed that the area under the rheograms indicated energy dissipation within the sludge system, which was related to the network strength. The research described in this paper investigates the use of this protocol for measuring sludge network strength at different polymer doses and relates the results to filtration, thickening, and dewatering. Laboratory tests used anaerobically digested sludge and both capillary suction time and filtration tests to indicate dewaterability and filtration. Network strength measurements used a torque rheometer. At full scale, dose response testing was used to correlate the measured network strength of the conditioned sludge to both centrifugation and gravity belt thickening performance as indicated by solids output. Both laboratory and full-scale testing showed that the network strength could be used to identify the optimum polymer conditioning to achieve good water removal from the sludge. The network strength increased with increasing the polymer dose, however, within the optimum dose range, a “drop” in the network strength occurred. This paper also discusses how to use the sludge network strength information to achieve the desired dryness from a dewatering device, and ultimately automate conditioning and dewatering processes. DOI: 10.1061/~ASCE!0733-9372~2005!131:8~1139! CE Database subject headings: Sludge; Rheology; Dewatering; Filtration; Laboratory tests. Introduction Previous research by Örmeci and Abu-Orf ~2005! developed a protocol for measuring network strength of sludges using rheo- logical measurements. The researchers used the term network to describe conditioned and unconditioned sludges instead of floc and followed the model offered by Michaels and Bolger ~1962!, who referred to the floc as the basic unit of flow under shear, not the single particle. The flocs tend to cluster together forming weak aggregates, which in turn grow by collision and break down by shear, thus forming extended networks that give the suspension its structural properties. The ability to measure sludge network strength is an important aspect in sludge conditioning and dewatering processes. The ability of a network to withstand shear forces during dewatering depends upon the chemical and physical bonds that hold it together. Effective dewatering of sludges depends on the strength of the aggregates formed following polymer conditioning, of which different strengths are required for different dewatering devices ~e.g., Hannah et al. 1967; Wu and Huang 2003!. It is expected that centrifuges require higher network strength than belt filter presses. Hannah et al. ~1967! observed that the highest attainable strength is not necessarily desirable for particle separa- tion by sedimentation or filtration. Wu and Huang ~2003! mea- sured flocculated sludge strength, following the method devel- oped by Leentvaar and Rebhun ~1983!, and used different laboratory dewatering assessment techniques @capillary suction time ~CST!, time to filter ~TTF!, specific resistance to filtration ~SRF!, and centrifugation# to determine optimum doses for good dewatering. They concluded that different floc strengths matched the optimum polymer dose obtained from each dewatering test— minimum floc strength was needed for filtration while high centrifugation forces required stronger floc structure. However, it was noted by Glasgow and Hsu ~1982! that stronger networks would be more difficult to dewater. The above discussion strongly suggests that practitioners need to design flocculated sludges with different network strengths depending on the type of dewatering device to be used. Several advances must be made to achieve this. First, we need to develop technologies and methods able to precisely measure and determine the sludge network strength. Second, we need to be able to determine and define the required network strength value for producing a good dewaterable sludge with a specific dewatering device. Third, in order to accommodate daily and hourly changes in sludge properties, continuous measurement of the strength of the flocculated material prior to entering the dewatering device must be conducted. Finally, we must develop 1 Director, Biosolids Processing, Central Technical Services, Veolia Water North America Operating Services, Pitman, NJ 08071; formerly, Director, Biosolids R&D Program, North American Technology Center, Vivendi Water/USFilter, 1901 West Garden Rd., Vineland, NJ 08360. E-mail: mohammad.abu-orf@veoliawaterna.com 2 Assistant Research Professor, Dept. of Civil and Environmental Engineering, Duke Univ., Box 90287, Durham, NC 27708. E-mail: banu@duke.edu. Note. Discussion open until January 1, 2006. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on January 7, 2004; approved on February 3, 2005. This paper is part of the Journal of Environmental Engineering, Vol. 131, No. 8, August 1, 2005. ©ASCE, ISSN 0733-9372/2005/8-1139–1146/ $25.00. JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2005 / 1139