THERMODYNAMIC APPROACH FOR ESTIMATION OF VOC EMISSIONS FROM A . WASTE WATER TREATMENT PLANT

Abstract

The present work deals with the estimation of release of VOCs from an open tank and covered tank treatment basin for the refinery wastewater. The wastewater from a refinery has a maximum of 1mg/1 of various components with the exception of phenol whose concentration is 200 mg/l. It has been assumed that the two- film theory is applicable to volatilization rate (mass transfer), the tank contents are well mixed and that the removal of VOCs by adsorption, biodegradation, seepage or any other form of degradation is negligible and that all the VOCs are miscible with water and are in liquid solution phase. The equations for the determination of the Henry's law constant and mass transfer coefficients, etc. as are available in literature were used and the estimations were made for an open tank and a covered tank using a computer code written in MATLAB. The temperature was varied from 5°C to 80oC, wind velocity from 0.25 m/s to 4 m/s, and the wastewater flow velocity was varied from 0.1 m/s to 0.5 m/s. for covered tank, the head space was taken as 2 m high and the vent flow rate was taken in the range of 0.25 m/s to 6 m/s. the tank dimensions were assumed to be 50 m(L)x 10m(w)xlm(d). The estimated emission rates are compared with those obtained from the Water 9 model developed by USEPA. It is found that there is a wide difference between the estimations made in the present study and those obtained from Water 9. This is because of the fact that water 9 takes default values of several properties which are not available in literature and because of the effect of phenol which is present in large concentrations as compared to other components in wastewater. The group interaction parameters for phenol, methanthiol and ethanthiol are not available and the estimation of activity coefficients made in the present study is based on values for similar compounds and the available parameters. Temperature, wind velocity/ vent velocity and the waste water velocity in the tank have profound impact on the VOCs emission.