Hydraulic and Oxygen Transfer Characteristics of an Industrial Roughing Filter

Roughing trickling filters are used to reduce organic loadings ahead of the activated sludge process at one of Merck &Co.'s manufacturing sites. The site is subject to the Pharmaceutical MACT (Maximum Achievable Control Technology) Rule, and must demonstrate required reductions of hazardous air pollutants (HAPs). TOXCHEM+ predictive fate software is used at the site to estimate the reduction and removal of the HAPs during wastewater treatment. Site-specific biodegradation rates of the HAPs can be demonstrated as part of an overall strategy to devise the best approach for achieving compliance with the MACT Rule. Concurrent with a testing program, implemented for determining site-specific biorate coefficients, the hydraulic and oxygen mass transfer characteristics of one of two parallel trickling filters were determined to calibrate the process for the fate model. Tracer studies using fluorescent dye provided an estimated hydraulic retention time of approximately 2 minutes. Dissolved oxygen measurements in the trickling filter feed and effluent revealed the filters as effective oxygenating devices, with an increase in D.O. of 1.0 to 1.5 mg/L through the filter. Off-gas measurements, collected in conjunction with analyses of HAPs, provide an opportunity to determine the oxygen mass transfer characteristics of the full-scale trickling filter. Estimated values of the mass transfer coefficient αKL ranged from 1.92 to 6.59 m/d. The wastewater flow rate (excluding recycle) was found to have a significant effect on the oxygen mass transfer coefficient. Determination of the hydraulic flow characteristics and oxygen mass transfer coefficient in the trickling filter aided the calibration of the TOXCHEM+ fate model for the site, which could then be used for estimating site-specific biodegradation rate coefficients for the HAPs in the trickling filter.