Abstract:
The wastewater generated by industry, municipal and household contains a complex mixture of both organic and inorganic substances. The treatment of this wastewater is highly important before discharging into the ecosystem to avoid pollution of water-bodies. Industrial wastewater contains a diverse range of toxic compounds that cannot be released directly into the environment. Their degradation is mostly carried out by the activated sludge process (ASP) either at Effluent Treatment Plants (ETPs) for wastewater generated by large-scale industries or Common Effluent Treatment Plants (CETPs) that treat wastewater generated by a cluster of small- scale industries. The wastewater treatment processes carried out in ETPs are relatively economical and widely accepted methods for the degradation and detoxification of domestic as well as industrial wastewaters. The wastewater can be treated by physical, chemical and/or biological methods. The cost of chemical treatment is too high for large wastewater quantities. The ASP involves the transformation of dissolved and suspended organic contaminants to biomass and gases such as CO2, CH4, N2 and SO2. The effectiveness of ASP depends upon the development of an adequate microbial community and dissolved oxygen and usually operates in the mixed liquor suspended solids (MLSS) range of 4000 to 9000 mg/L. The high levels of MLSS may become attractive if the corresponding need of dissolved oxygen can be met. However, scant information is available in the published literature regarding the kinetics of ASP in the high range of MLSS (15000 to 40000 mg/L) and a systematic study was undertaken to enhance the understanding of associated processes.
In the present work, waste from an agrochemical plant was used. For wastewater treatment oxygen was supplied by either sparging air from bottom of the sparged reactor (BCR) or by surface aeration in stirred tank reactor (STR). The mass transfer coefficient ( kLa ) for activated sludge in STR and BC were experimentally measured for agrochemical industry wastewater. The MLSS concentration was varied in the range of 6000 to 40000 mg/L. In STR, the superficial gas velocity and the impeller speed were varied in the range of 0.188 to 0.5 mms-1 and 5 to 11.67 rps, respectively. In BCR, the superficial gas velocity was varied from 0.6 to 2 mms-1. For the measurementof kLa , the physical method of transient oxygen uptake wasemployed. In both the reactors, the value of kLa was found to decrease with an increase in the MLSS concentration covered in this work. However, the extent of reduction was found to be a factor of 2.5 to 4.5 in BCR as compared with a factor of only 1.6 to 1.8 in STR. In both reactors, the reduction was found to vary because of increase in viscosity with an increase in MLSS. However, the extent of such decrease was nominal in STR as compared with substantial decrease in BCR.
In addition to mass transfer characteristics, the kinetics of reduction of COD was investigated over a wide a range of MLSS (6000 to 40000 mg/L). Experiments were performed in surface aerators as well as in classical STRs with air sparging. The agitator speed was varied from 3 to 15 rps. Two modes of operations were investigated: (a) constant dissolved oxygen [1 and 2.5 mg/L] (DO) at different levels of impeller speed with respect to time and (b) variable DO [0.5 to 7 mg/L] at a constant speed with respect to time. The kinetics was investigated with and without nutrients.
Aerobic digestion, an alternative method for treatment of waste activated sludge is carried out in the endogenous respiration phase of aerobic bacteria. As the supply of available food (substrate) is depleted, microorganisms begin to consume their own protoplasm to obtain energy for cell maintenance reactions thus achieving endogenous oxidation of cell tissue. This in turn stabilizes the sludge with substantial reduction in its volume. Some of the cell material, utilized at a negligible rate, is non-biodegradable. The rate of destruction of activated sludge during its endogenous respiration phase can be expressed as a first order function of the concentration of the degradable biomass present, expressed as degradable volatile suspended solids (VSS). The endogenous decay rate was found to depend on the type of wastewater on which the sludge was grown, sludge concentration, sludge age (the time for which the sludge is aerated prior to digestion) and on the environmental factors such as pH and digestion temperature.
The excess activated sludge degradability was studied from an agrochemical industry wastewater treatment plant. The objective was to understand the kinetics of sludge digestion using air and oxygen. Experiments were performed in 2 L stirred tank reactor. The impeller speed was varied in the range of 1.67 to 11.67 r/s. The superficial air and oxygen velocities were maintained at 0.39 mm/s. The value of MLSS was also varied in the range of 6 to 90 g/L. The rate of digestion of MLSS/MLVSS was found to be of the first order.