Aerated Stablilization Basin Improvements Study, 1998 Environmental Conference Proceedings

Gilman Paper Company (Gilman) owns and operates a bleached kraft pulp and paper mill in St. Marys, Georgia. Gilman retained BE&K Environmental to perform a study of the existing aerated stabilization basin (ASB) system to enhance its operation under varying influent biological oxygen demand-five day (BOD5) loading conditions. An important component of the work was a conservative (recoverable) tracer study which defines the actual average retention time of the existing ASB system. The tracer study also aids in defining the wastewater flow patterns within the ASB. Computer modeling was also used to predict ASB performance under varying loadings.

The effluent discharge from the ASB to the North River is programmed to operate on a tidal cycle during the three hours before and the three hours after the local high tides. Occasionally, process upsets within the mill cause an overload condition regarding the influent BOD5 to the ASB. Also, EPAs cluster rule will impose stringent limitations on the ASB. Gilman intends to optimize the wastewater flow and aerator configuration of the existing ASB to better address and respond to upset conditions and to the future cluster rule requirements.

Based on the field study and modeling calculations the following conclusions were developed:

The actual average retention time of the ASB system is 7.5 days based on the results of lithium tracer study (103% lithium recovery) but short circuiting is occurring.

The theoretical retention time of the ASB system is 7.2 days based on a total surface area of 53 acres, a lagoon depth of 13 feet and an average influent flow rate of 31 mgd.

Approximately 36 percent of the ASB wastewater flow is by-passing some or all of the aerators and exits the lagoon in approximately 3 days on the outside of the basin away from the aerators. The remainder of the wastewater flow passes through the aerators and exits the lagoon after 7 or 14 days.

ASB modeling results indicate that the BOD5 removal efficiency of the by-pass wastewater flow is between 60 to 65 percent. The BOD5 removal efficiency for the no by-pass flow is between 80 to 90 percent. ASB partitions may be beneficial to minimize by-passing and maximize the BOD5 removal efficiency.

Future work is planned for this study including the waste loading changes to the ASB resulting from future foul condensate treatment and from the implementation of future BMP measures for black liquor, turpentine, and soap spill control.