MACT 1 Alternative: A Five-Fold Improvement in HAP Control, 2003 Environmental Conference Proceedings

On April 15, 1998, U.S. EPA promulgated 40 CFR Part 63, Subpart S requiring the installation of Maximum Achievable Control Technology (MACT) to control the emissions of hazardous air pollutants (HAP) from select semi-chemical pulp mill process sources. Specifically, the rule required that low volume high concentration (LVHC) gases vented from spent liquor evaporators, hotwells and pulp digesters be collected and thermally destroyed. Based in part by a study conducted by the National Council of Air and Stream Improvement, MACT implementation was expected to achieve a level of HAP control approximating 1.9 lb HAP/pulp ton at stand-alone semi-chem mills subject to Subpart S requirements.

During its first phase of MACT implementation, the mill installed a digester blow heat recovery system (BHRS) featuring two indirect contact condensers connected in series. BHRS diagnostic emissions tests determined that the chief HAP of concern, methanol, was partitioning to the unregulated foul condensate stream at a rate >2 lb methanol/pulp ton while the LVHC stream contained ~0.4 lb methanol/pulp ton. The mill presented this finding to the Agency and proposed an alternative approach to the MACT involving foul condensation collection and anaerobic biotreatment in lieu of the MACT prescription. In August 1999, EPA granted conditional approval to pursue the project under a regulatory flexibility program established with the State of Wisconsin.

A pilot-scale treatability test conducted in July 2000 predicted that methanol biodegradation would exceed 99%. Subsequent to full-scale project implementation in May 2001, performance tests demonstrate that the alternative MACT approach achieves a methanol control rate of >2.5 lb/pulp ton or a >500% improvement over what would have occurred if the specified MACT had been installed.