OPTIMIZATION OF PACKED BED SCRUBBERS TO CONTROL ODORS FROM SOLIDS HANDLING PROCESSES

Many designers and operators of odor control systems at wastewater treatment plants assume that because odorous air is passing through an operating wet scrubber, effective odor control is being achieved. With a packed bed scrubber, many design and operating parameters affect odor removal efficiency. These include packing depth, detention time, scrubbant recycle rates, overflow rates, pH and oxidation-reduction potential (ORP) of the scrubbant solution, constituents in the odorous air and their mass transfer coefficients, loading patterns, and the type of chemical control system.Odors from solids processing operations such as storage, thickening, blending, and dewatering are particularly difficult to treat due to the presence of non-H2S reduced sulfur compounds like dimethyl disulfide. Unfortunately, failure to consider these other reduced sulfur compounds can result in poor odor removal efficiency in a scrubber designed to remove only hydrogen sulfide.Studies conducted by Bowker & Associates, Inc. and Camp Dresser & McKee evaluated the effect of various operating parameters such as pH and ORP, overflow rate, and type of control (proportional only, proportional plus integral) on scrubber performance at two large wastewater treatment plants in Massachusetts. Data are presented on removal of odor (as measured by dilutions to threshold) and reduced sulfur compounds through packed tower scrubbers operated with caustic soda only and with caustic soda and sodium hypochlorite. Conclusions are presented regarding the appropriate control set-points to maximize odor removal efficiency, and the importance of properly controlling the injection of sodium hypochlorite to maintain consistent performance and improve operational reliability.