Optimization of the effluent treatment system (anaerobic/aerobic) for rubber industry by kinetec modeling

Abstract

Raw rubber processing factories generate large amounts of wastewater containing organic pollutants & various process chemicals. Factory effluents exhibit high BOD (Biochemical Oxygen Demand) & COD (Chemical Oxygen Demand) concentrations, ammonia & suspended solids that are amenable to biological treatment methods. Rubber Research Institute (RRI) of Sri Lanka developed a novel & cost effective biological effluent treatment technique for rubber-processing effluents discharged by crepe rubber & centrifuged latex factories. Treatment system, based on high rate anaerobic digestion coupled with aerobic stabilization also consists of settling & sand filtration. The main feature of this technique is the use of a low cost, septic tank type anaerobic digester filled up with coir fibres for the attachment of useful microorganisms for effective biological conversion. Biological kinetic expressions have been derived for the design & control of effluent treatment plants where aerobic digestion is used commonly as the only treatment method. The kinetic coefficients in these expressions are widely used in design calculations. For a specified waste, a given biological community & a particular set of operating conditions the kinetic coefficients are fixed, Kinetic coefficients used for the design of domestic effluent treatment plants cannot be applied for the design of industrial effluent treatment plants as the waste composition & biological communities involved are different. Also kinetic coefficients for the anaerobically pretreated wastewater could be very different to those of the raw wastewater even for the same type of waste. No kinetic study has been carried out yet for the RRI developed treatment process for making possible improvements & modifications for optimal operation & performance of the aerobic treatment system to reduce, capital, operational & maintenance costs under low loading conditions. The objective of this study is to find out the kinetic coefficients required for the design of activated sludge process from anaerobically pretreated rubber industry wastewater. The obtained values of kinetic coefficients were used to model an existing treatment system. A pilot-scale continuously aerated stirred tank was used as a model reactor. Reactor was operated without a recycle stream & fed with a steady flow of anaerobically pretreated wastewater obtained from a full-scale rubber industry effluent treatment plant. Samples were taken for five different runs at five different mean-cell residence times (). BOD & MLVSS (Mixed Liquor Volatile Suspended Solids) of each sample for each run were measured according to Standard Methods for the Examination of Water & Wastewater. The following kinetic coefficients were estimated by a graphical method using measured data & the standard kinetic expressions. • Y = cell yield coefficient • kd= cell decay coefficient • Ks = half-velocity constant • k = maximum substrate concentration per unit mass of microorganisms • μm = maximum specific growth rate). The obtained kinetic.coefficients show significant differences to those of domestic wastewater reported in literature. Maximum substrate concentration per unit mass of microorganisms (k) is less than one-half of the corresponding value for domestic wastewater. This implies more than double the concentration of microorganisms is required to be maintained in the aeration tank than that for domestic wastewater. Half-velocity constant (Ks) is more than double the concentration of the corresponding value for domestic wastewater. It implies that the microorganisms have high affinity to anaerobically digested substrate. This could be expected because most anaerobically digested intermediate products & end products are considered good substrates for heterotrophic organisms. The cell yield coefficient (Y) is comparatively higher & the cell decay coefficient (kd) relatively lower than those for domestic wastewater leading to a higher μm. maximum specific growth rate. Therefore a richer microorganism concentration could be expected in the aeration tank. Obtained kinetic coefficients were used to model an existing activated sludge treatment system. The minimum mean-cell residence time calculated with the obtained kinetic coefficients lead to a value of O. 9 (d) with a safety factor of 3.33 & is within the accepted range for plant operation (2 - 20). Sludge washouts are very unlikely due to the fulfillment of the condition >1/ μm indicating a good waste stabilization. Calculations revealed significant difference between the predicted & operated condition of the plant. The obtained kinetic coefficients were used to optimize the plant operation by estimating sludge recirculation rate, aeration rate & sludge production rate. The findings will help improve the treatment system design & reduce the associated costs.