Modelling of modified activated sludge systems for excess sludge reduction

Nowadays active sludge process is one of the most widely used processes among biological wastewater treatment systems not only for domestic but also for industrial wastewater treatment. The activated sludge process has undergone many changes from its discovery to today and has been modified and used for certain wastewater characterizations. In the activated sludge process, a portion of the organic matter in the wastewater is removed from the medium by conversion to water and carbon dioxide while the rest is turned into a by-product and formed so called "sewage sludge". Due to the high organic matter and water content in the formed sewage sludge, it is forced to be treated and disposed rather than be removed directly from the system. Sludge treatment and disposal is a costly process that must be carried out in accordance with the environmental obligations. The cost of sludge treatment and disposal is about 50-60% of total operating cost of biological treatment systems. The amount of sludge is related to the configuration of the activated sludge system and the sludge retention time. The generally applied sludge treatment procedure is based on collecting the sludge at the outlet of the activated sludge system and reducing the amount of water by thickening reactor. After thickening, the stabilization process is applied in order to reduce the content of organic matter and the sewage sludge is finally adapted to the its ultimate final disposal alternative after dewatering process. Stabilization can be carried out in either aerobic or anaerobic conditions. Stabilization is described as one of the separate or post-process sludge treatments. The use of technological approaches to reduce the amount of sludge "in process" without significantly increasing operating costs has great importance from an operational point of view. The sludge reduction which is carried out in aeration tank is one of the implementations of "in process sludge reduction mechanisms" in the small activated sludge systems. Contact stabilization (CS) has been applied mainly in-process sludge treatment system and oxic-settling-anaerobic (OSA) systems is currently used in-process sludge treatment and disposal alternative. Unlike the activated sludge systems where the sludge treatment is made out of the process, in these systems the sludge stabilization is ensured in the same volume and the system is operated more efficiently and less sludge production is ensured. The purpose of this thesis is to determine the effect of in-process sludge stabilization on the sludge formation. In this context, OSA and CS systems which are fed with domestic wastewater evaluated within the framework of modern environmental biotechnology modeling approach by using AQUASIM software. The OSA system is a modification of the conventional activated sludge system. The wastewater is primarily aerated in an aerobic tank and then settled. While the upper phase of the sedimentation tank is being discharged from the system in the OSA process, a portion of the sewage sludge is directly recycled to the aeration tank to form the recycle of the activated sludge system. The rest of the sludge portion is then anaerobically stabilized. The stabilized sludge in the anaerobic stabilization tank is fed back into the aeration tank. The OSA system is operated as a system without discharging the sewage sludge. In the modelling approach, the anaerobic stabilization system is excluded and the biomass from OSA system is indicated as the input as a recycle aerobic reactor. Contact stabilization is a modification of the rapidly operated activated sludge system. The wastewater is first fed to the contact reactor which is aerated very shortly, and then it is transmitted to the settling tank. While the upper phase of the sedimentation tank leaves the system, the sludge that has been sedimented is transferred to the stabilization reactor for re-aeration. The stabilized sludge is fed back into the contact reactor. The basic principle of the contact stabilization process is to reduce the amount of sewage sludge by achieving rapid biological treatment and carrying out adsorption in the reactor via transferring sewage sludge to the stabilization reactor including particulate organic matter. In the context of this thesis, three different activated sludge configurations, CAS, OSA and CS, were evaluated in terms of sludge production by conducting a modeling study using AQUASIM software for carbon removal. All systems were scrutinized under different operational conditions to identify the effect of system configuration on the production of excess sludge. CAS was considered as a control system to predict the improvement in excess sludge production. The model for the CAS system was first run for an HRT of 8 hours (3350 m3 volume) to characterize the common operational conditions of a CAS. Then HRT was reduced to 5 hours (2100 m3 volume) to demonstrate the limitation of system operation due to the settling conditions. As expected from a CAS process, the sludge generation was decreased enormously when the sludge age was increased from 6 to 15 days. This was actually the reason for operating the activated sludge systems at an extended mode. It should be noted that high SRT, as 15 days, ended up with the lowest active biomass concentration as a result of the dominant endogenous respiration. Results showed that the increase of the sludge age from 6 days to 15 days reduced the total sludge production approximately 25%, and the meaningful reduction was in the active biomass with a level of 50%, where a remarkable stabilization of organic matter was observed for both 8 hours and 5 hours HRT. It is obvious that increasing the SRT and decreasing HRT as an operational parameter have a limitation due to the feasibility reasons. It seems only applicable to small treatment plants, where the sludge is aerobically stabilized within the activated sludge system. Modeling for OSA system reflected the features of the classical activated sludge system with an initial active biomass. The volume of the reactor was selected as 3350 m3 yielding an HRT of 8 hours (0.335 days). In OSA process, an input of 50 gr cell COD/m3 in the influent was sufficient to yield a 75% reduction, whereas a 100 gr cell COD/m3 ended up approximately with a 100% reduction. In this case, the wasted sludge consisted of only the initial biomass load (Q XH1). The results of the simulation proved that enhanced endogenous decay due to higher active biomass level sustained in the OSA reactor should be regarded as the major cause of excess sludge reduction in the OSA system. In summary, for the studied OSA system, the simulations show that 8 d SRT and 200 gr cell COD/m3 XH1 influx would be enough to stop biomass generation which is not comparable level achieved in a CAS process that operated at 8 days. CS system was designed to have a CR with a very short HRT at a very small volume aiming only the removal of soluble substrate and an aerobic reactor added in the recirculation line to examine the possible effect on the sludge reduction. First, the system was designed to be compared to the operational conditions of CAS by selecting a similar total HRT of 8 hours, where a very short HRT of 45 minutes was allocated to CR. The volumes were adjusted to 300 m3 for CR and 3000 m3 for SR, as a total of 3300 m3. In the second run, the total HRT was reduces to 5 hours to outline the effect of system behaviour on the sludge production. The HRT was divided as 30 minutes and 4.5 hours to CR and SR, respectively. In this case the total volume was reduced to 2100 m3, shared as 200 m3 by CR, 1900 m3 by SR. First of all, the CS process with two different HRTs were compared in terms of the sludge production. They were compared with CAS to evaluate the extent of sludge reduction of both configurations. CS process with the total HRTs of 5 hours and 8 hours were compared with CAS with an HRT of 8 hours in terms of the sludge production. HRT for 5 hours was not considered in the evaluation with the fact that the produced mass in the reactor cannot be settled in the conventional settling tank due to the solid flux limitation. The total sludge production, in other words the excess sludge to be further treated in a sludge treatment facility was found approximately the same for CAS with an HRT of 8 hours compared to CS with HRTs of 8 and 5 hours. The reason of having mostly the same sludge amount in CAS and CS araised from the fact that the amount of sludge remained the same in CS as a result of the raised concentration in the decreased volume. Namely, decreasing the HRT was increasing the concentration of the particulates yielding the generation of the same amount of sludge at significantly lower volumes. As a future perspective, the modifications of activated sludge systems for the sludge reduction may be developed/improved in conducting experimental studies within the view of modelling studies.