Minimization of excess sludge production

Large amounts of waste sludge are produced with the removal of biodegradable compounds and organic or inorganic matter in municipal and industrial wastewater treatment plants for disposal. It is expected that, in near future, sludge production and the regulation limits for disposal alternatives cause an increase in costs for sludge disposal. The current approach to sludge minimization is the reduction of volume of wet sludge and the reduction of dry mass of sludge. The volume of wet sludge for disposal is reduced significantly with the increase of the sludge solid content by dewatering. The reduction of dry mass of sludge leads to the reduction of solid content and volume and this strategy should be fovoured, beause it allows the immediate reduction of sludge dry mass during its production in the biological treatment stage.The fundamental aim of this thesis is the reduction of sludge production during the biological treatment. All of the techniques for sludge reduction described in this thesis suitable for implementation during the wastewater treatment plants, not after the sludge production, often by retrofitting the specific additional equipment. Two of these techniques were carried out experimentally and evaluated within the scope of this thesis. The effectiveness of ozonation integrated in activated sludge process and oxic-settling anaerobic (OSA) process were investigated on sludge reduction, especially.In the first stage of the study, Box-Behnken Statistical Design Program was used in order to determine the optimum operational conditions. Optimum hydraulic retention time (HRT), solid retention time (SRT) and initial COD (CODi) values for maximum COD removal were determined as 25 h, 25 d and 400 mg/L, respectively. After the determination of optimum operational conditions of activated sludge process, two systems were operated in parallel under these optimum conditions during 45 days until the steady state conditions and the effluent quality and sludge properties of the systems were monitored. When the steady-state conditions were reached, specific batch tests were carried out in order to investigate the biodegradability of disintegrated sludge and determine optimum ozone dose prior to the modification with the addition of ozonation unit to one of the activated sludge process. Optimum ozone dose for return activated sludge disintegration was determined as 0.05 gO3/gTS in terms of disintegration degree with 56.2 %. The higher doses improved disintegration degree, slightly.After the ozone dose optimization study, the effectiveness of partial ozonation of return activated sludge was investigated for the minimization of excess sludge production using ozonation coupled with activated sludge process in two stages. In the first stage of the ozonation, in ozonated system, 10 % of return activated sludge (0.1 QR) was ozonated every day during a month with optimum ozone dose and recirculated to the aeration tank. No excess sludge was withdrawn from the ozonation coupled with activated sludge process. The other activated sludge system was remained unmodified and operated as a control run with optimum operational conditions. Excess sludge was withdrawn every day due to SRT. 43 % observed sludge yield (Yobs) reduction could be achieved in ozone run during the operation period. At he the end of the operation, the 56 % of Yobs reduction was observed in ozone system compared to control system. In ozone run, the effluent quality was weakened slightly in term of COD and NH4-N removal. Sludge settling properties represented by SVI was improved in ozone run. Ozone run had lower dewaterability characteristics with higher CST values. The filterability of sludge was lower in ozone run than control run in terms of SRF.In the second stage of the ozonation 800 mL of the return sludge corresponded to 20 % of of the return activated sludge (0.2 QR) was ozonated with optimum ozone dose. 73 % Yobs reduction could be achieved in ozone run during the operation period. At the end of the operation, the 62 % of Yobs reduction was observed in ozone run compared to control run. The effluent quality was weakned slightly in term of COD and NH4-N removal in ozone run. Sludge settling properties was improved in ozone run in terms of SVI. Lower dewaterability characteristics in ozone run were proved with CST values. The filterability of ozonated sludge was lower than control run in terms of SRF.After the application of ozonation to activated sludge system, one of the system was modified by inserting anoxic tank to comprise OSA system. The other system was remained unmodified as a control system. A specific volume of return activated sludge of OSA system was subjected anaerobic conditions (-250 mV of low ORP level every day. OSA and control system were operated during 45 days. The sludge reduction efficiency and effluent quality were evaluated. After the completion of continuous operation, some batch experiments were conducted in order to investigate sludge reduction mechanisms in OSA process.In OSA system, 62 % reduction efficiency was obtained in yield production in OSA system during the operation. OSA system was also achieved 58 % reduction efficiency of Yobs compared to control run at the end of the operation. It was found that COD concentrations in the effluent in the OSA system were lower than that in the effluent of the control run due to the additional substrates from the anaerobic tank. The NH4-N removal efficiency in OSA system was lower compared to control run during the operation period due to denitrification. The setteability characteristics of sludge was also found better than that of the control system in terms of SVI. Higher CST values were obtained in control system compared to OSA system during the long term operation. It was revealed that the OSA system enhanced the filterability of sludge.In order to investigate the sludge decay theory in OSA sistem, two batch experiments with sludge taken from aeration tank of control and OSA systems were carried out. The aim of the first batch experiment was to investigate the relationship among sludge anaerobic reaction time, sludge lysis and sludge yield. The sludge reduction in terms of Yobs caused by sludge decay in OSA batch reactor was 40 % compared to control batch reactor for the first batch experiment. In the second batch test, the investigation of the energy uncoupling theory was aimed. The sludge reduction in terms of Yobs caused by energy uncoupling in OSA batch reactor was 5 % compared to control batch reactor.When the sludge reduction techniques used in this thesis were compared as modified activated sludge processes; the activated sludge process ozonated of 0.2 QR showed better Yobs reduction efficiency than that the fist stage of ozonation process (ozonation with 0.1QR) and OSA system during the operation period. The effluent quality of control systems were better than that modificated systems in terms of COD and nitrogen removal capacity however, the effluent quality of the modificated systems were at satisfactory level. The sludge characteristics of activated sludge were changed even slightly for some paramaters after the application of sludge minimization techniques.Consequently, the ozonation based on cell lysis-cryptic growth mechanism is an applicable and effective technique integrated to the activated sludge process for the minimization of excess sludge production. Besides, it was revealed that the OSA process especially based on sludge decay and energy uncoupling metabolism can be used as another simple effective method for sludge reduction.Keywords: excess sludge minimization, modification of activated sludge process, ozonation, OSA process.