STUDIES ON DAIRY WASTEWATER TREATMENT BY CARROUSEL SYSTEM COMBINED WITH SUBMERGED TURBINE AERATOR

Abstract

Since the turn of the century, wastewater treatment plants whatever their size, have become increasingly important to local communities. The activated sludge process and its modifications have been popular options for treating biodegradable wastewaters for the last several decades. Extended aeration is a low load modification of the activated sludge process, characterized by long retention times of mixed liquor in aeration tanks and high contact of activated sludge suspended solids in mixed liquor. The extended aeration modification of the activated sludge process often employs oxidation ditch as biological reactor. Oxidation ditch system was devised as a low maintenance option for the treatment of municipal wastewaters. The design of an endless channel with surface aerators providing aeration, mixing and propulsion of the liquors around the channel has undergone several modifications since its inception. Carrousel system is the latest modification in practice which seeks to solve some design problems encountered in the development of large oxidation ditches [1,2]. There are about 200 carrousel ditches existing and several others under construction. Basically, carrousel ditches are deep oxidation ditches to economise in land requirements. However, proper aeration to ensure sufficient turbulence for maintaining high rate of oxygen transfer while still retaining high treatment efficiency constitute various problems faced in the design of these ditches. In the carrousel system, kessener brush or TNO type commonly used in smaller ditches, could not supply sufficient turbulence at depths greater than 1.5 meters [3]. Therefore, one or more vertical shaft aerators located centrally at the end(s) of the looped channel are normally provided. Vertical shaft surface aerators impart a spiral flow mixing pattern, create angular momentum to the liquid and deflect into the channel with a common dividing wall, thus, providing required turbulence over the entire cross section of the channel [43. ill Successful carrousel system requires sufficient contact of active microorganism in the obligate aerobic condition for a period during which the biochemical oxygen demand (BOD) of the wastewater is almost fully satisfied. Maintenance of dissolved oxygen in the mixed liquor at an anticipated level is based upon the performance of the aeration system adopted. Alsjo, the success^ * * of the biokinetic activity in the reactor is reflected by' settleabi1ity of bioflocs in suitably sized settlement tanks [5]. In fact, proper design of carrousel system involves critical study of various design parameters viz. geometry of aeration basin, proper design of aeration system including specification of mechanical configuration, speed of rotation, oxygen transfer efficiency achieved, possibility of combination of mechanical and compressed aeration for better efficiency; and power requirements to run the system while accomplishing the maximum removal efficiency of the system. These aspects have been studied in this research. The thesis presents an experimental study on aforementioned aspects and recommends a modified aeration system by introducing submerged turbine aeration along with the existing mechanical vertical aeration and through a highly efficient design of carrousel ditches. These ditches can be aptly taken as energy and land saving treatment systems. Experimental work was carried out on a laboratory scale model of carrousel system in various phases as below : Phase I : First phase deals with the laboratory study on aeration performance of twelve different impeller configurations employed on a laboratory model of common carrousel system.- Significant design considerations taken into account include rotational speed and impeller submergence which have significant bearing on oxygen transfer efficiency. Power consumption and oxygen transfer efficiency have been evaluated for each of the impeller types employed in the study to arrive at a relatively better one . On the basis of this phase, best three impeller designs out of twelve types tried were chosen. For all the impellers tried, the maximum values of oxygenation capacity and oxygen w transfer efficiency correspond to full immersion depth of blade of impeller at 150 rpm. The chosen three mechanical configurations of impeller are capable of providing higher oxygen transfer efficiency value at relatively lower specific power. The aeration efficiency achieved in each case is comparatively higher than reported values else where. Phase II :The second phase deals with a study on the performance of the three best selected impellers earlier on the modified carrousel system laboratory model. The investigations include determination of mass transfer coefficient, oxygenation capacity, oxygen transfer efficiency with respect to different speeds of rotation and air flow rates in each case and to arrive at relatively superior impeller configuration at a specified speed of rotation and air flow rate. For the three impellers, and five different air flow rates at a full submergence, it was found that the oxygen transfer efficiency of the modified carrousel system is about 1.25 times higher than the normal carrousel design. Also, with the increase in the speed of rotation values, oxygen transfer efficiency increased, whereas, by increasing the air flow rate, oxygen transfer efficiency increases up to some level and then shows decreasing trend. On specific power consideration, it is found that one of the impellers chosen provides better oxygen transfer efficiency with 100 rpm and air flow rate of 2.3 litre per hour. These values provide aeration system with maximum transfer efficiency and therefore, adopted for treatment studies subsequently. Phase III : The third phase deals with the study of treatment kinetics on modified carrousel system for synthetic dairy wastewater. The study was carried out on continuous input-output basis. After acclimatization, eight values of hydraulic detention time with raw dairy wastewater characteristics viz. B0D=s = 810 mg/1 and COD = 1300 ing/1 were employed. The organic loading rate and volumetric loading rate applied to the unit varied by varying the hydraulic detention time. Various observations taken include determination of BODr; and COD, pH, temperature, MLSS, MLVSS, SVI, sludge yield, etc as per Standard Methods. Parallel experimentals were carried out on common carrousel system for a logical comparison. Kinetic coefficients for the wastewaters have also been evaluated for scanning the modified carrousel system. The observations reveals that the system efficiency is higher than the efficiency reported in existing carrousel system. Modified carrousel system accomplished maximum of 97.1 percent B0D= removal with an organic loading rate (F/M) of 0.07 kg. BOD/kg MLVSS. day and volumetric loading rate of 0.286 kg B0D/ma day rate. By increasing the loading rate to 0.083 kg. BOD/kg MUSS, day it is found that B0D=> removal decreases to 95 percent only, whereas, at higher loading rate than 0.2 kg. BOD/kg MLIISS. day the B0D= removal is reduced to 82 percent only. It is observed that B0D= removal increases by increasing the hydraulic detention time to some level and then remains constant. In the case of common carrousel system, maximum B0D= removal of 94.1 percent at F/M value of 0.85 kg BOD/kg MLVSS. day and volumetric loading Of 0.247 kg BOD/cum. day was achieved. Regarding mean cell residence time, it is seen that percentage BODs removal increased sharply up to a sludge age of 16 days. However, sludge age of 29.9 days provides the maximum removal efficiency of 97.1 percent. Further, the sludge received was fully stabilized and nicely settleable with a SVI value range of 71 to 75. In conclusion, the thesis has brought out with the help of laboratory study, the efficacy of modified carrousel system for the treatment of dairy wastewater. It has compared the system with a parallel laboratory study on a commonly designed carrousel system model and has adequately made out enough merits for the modified carrousel system in comparison to other extended aeration systems in vogue from the point view of energy, land and treatment efficiency requirements. The thesis stresses the need for carrying out field studies to confirm the findings of this laboratory study, so that relevant design data is available for providing an optimal land and energy saving extended aeration system design highly modified carrousel system to treat dairy wastewaters. The system can be extended as a feasible treatment method for other biodegradable industrial wastewater and municipal wastewaters.