Influence of Ultrasonic Disintegration on Sludge Growth Reduction and Its Estimation by Respirometry CHOCKALINGAM LAJAPATHI RAI,* ,† GEORG STRUENKMANN, ‡ JOHANNES MUELLER, ‡ AND PARUCHURI GANGADHAR RAO § Chem ical Engineering Division, Central Leather Research Institute, Adyar, Chennai 6000 20, India, Regional Research Laboratory, P.O Jorhat 7850 06, India, and Institute of Mechanical Process Engineering, Volkm aroder strasse 4/5, Technical University Braunschweig, 38104 Braunschweig, Germ any Excess biomass generation during wastewater treatment is a great threat to the ecology if it is not properly disposed. Sludge growth reduction is one of the best ways to alleviate the problem. The present study focused in this direction employs ultrasound to disintegrate the floc without causing cell disruption. It has been found that a major size reduction that takes place at a lower energy input (3000 kJ/kg) and a further increase in energy input results only in minor changes. Sludge growth reduction is found to be of the order of 29% and energy efficient under flow- through conditions. The effect of disintegration is substantiated by respirometric studies, microscopic examination, degree of inactivation, and degree of COD release. Introduction The biological method of wastewater treatment is found to be cost-effective, and efficient methods currently are opted for (1). In this, the treatment is done by biological digestion, defined as a decomposition process of the biodegradable organic compounds by microorganisms. Excess sludge production seems to be an unavoidable byproduct in a biological wastewater treatment plant, which is produced by the growth of microorganisms in the aeration tank while removingthe pollution.Due to the stringent effluent criteria and restrictions to landfill the excess sludge, processing and disposalis becominga more difficult and complexproblem. Moreover, the cost of excess sludge treatment and disposal can account up to 60% of the total operating cost (2). In this context, there is an increased interest in the reduction of excess sludge production while retainingthe effluent quality criteria. Therefore, efforts have been undertaken to reduce the excess sludge production in the activated sludge process. In the present work, disintegration is added as a new additionalunit operation in the wastewater treatment plant, and the main objective is to reduce the amount of excess sludge. A respirometric procedure has been introduced to estimate the heterotrophicconversion yield (YH)ofthesludge. The method analyzes the amount of oxygen uptake rate (OUR) versus the amount of substrate. By using the respiro- metric technique in sequenced batch respirometers, YH for both treated and untreated sludge were estimated. Past Studies on Sludge Reduction in Biological Waste- water Treatment. Anew approach was developed to reduce the excess sludge by ozonation to achieve complete elimina- tion of sludge production (3, 4). Studies (5) carried out to investigate the maintenance and cryptic growth phenomena of Pseudom onasfluorescens showed that viabilityand sludge production yield decreased with agingofthe sludge.Variation on cell content ratio was also observed after discontinuous thermal treatment (5). The effect of maintenance energy requirements on biomass production revealed possible methods to reduce the excess biomass during the treatment process (6). Comparisonsofdifferent strategiesfor reducingexcesssludge production and its merits and demerits were clearlypresented (7). Another new approach (8) by increasing the oxygen concentration in activated sludge floc led to minimize the excess sludge production. The reduction of sewage sludge production wasfound possible byintegratingthe mechanical disintegration of sludge into the activated sludge process (9). Studies on mechanical disintegration of excess sludge showed accelerated anaerobic digestion (10). The use of ultrasound in sludge disintegration (11-13) showed that it affects the physical, chemical, and biological characteristics of the activated sludge and also enhances the anaerobic sludge digestion. To study the effect of the anaerobic biological process, for the removal of phosphorus, a respiro- metric method was used to determine the readily biode- gradable COD concentration (14).Similarly,the respirometric method wasproposed to estimate slowlybiodegradable COD and the active heterotrophic biomass present in wastewater (15).The respirometrytechnique was also employed to assess the storage yield for different substrates and to predict and control the activated sludge process as well as for the measurement ofsubstrate concentration in wastewater (16). Furthermore, this technique was also used for the biological characterization as it assesses the biological behavior of the pollution and provides a rapid tool for wastewater and activated sludge characterization (17). To promote the conversion oforganicpollutantsto respiration productswith the concomitant increase in aeration requirements (18), a respirometric procedure was introduced to estimate the heterotrophic conversion yield (Y H) of the sludge. Thus, the literature studies summarized above clearly indicates that there exists a gap in providinga complete study for reducing the excess biomass production by employing an ultrasonic homogenizer (UH) coupled with the respiro- metric studies. Therefore, the present study is designed to fulfill the knowledge gap, in addition to generating data to compare the disintegrability batch experiments with flow- through methods. Further, the focus of the present study is to devise a mechanism and methodology to reduce the biomass growth, in which neither the disruption of bacteria nor the biosludge would take place but a demand for maintenance energy would increase, which in turn will facilitate the consumption ofmore substrates,i.e.,to continue the digestion process without the cell growth. Materials and Methods The sewage sludge used in this study is collected from the outlet of the aeration tank of the municipal wastewater *Corresponding author phone: +91 (0) 44 249 16 706; fax: +91 (0) 44 249 11 589; e-mail: clrai@hotmail.com. † Central Leather Research Institute. ‡ Technical University Braunschweig. § Regional Research Laboratory. Environ. Sci. Technol. 2004, 38, 5779-5785 10.1021/es049775o CCC: $27.50  2004 American Chemical Society VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5779 Published on Web 09/28/2004