Meat Packing Wastewater Disinfection Difficulties Leads to a State of the Art Control System and Consistent Regulatory Compliance

John Morrell & Co. is a meat packing plant located in Sioux Falls, South Dakota. The facility also maintains a tertiary wastewater treatment facility with a direct discharge following treatment. The treatment facility had good effluent compliance history until the spring of 2001, when sporadic elevated levels of fecal coliform and infrequent spikes in chlorine residual were experienced. Facility staff undertook exhaustive measures to determine the cause or causes for these infrequent, but problematic elevated conditions. HDR was contracted to assist in this evaluation and to propose corrective measures.Elevated coliform levels correlated closely with dewatering operations. The dewatering is accomplished using a belt filter press, which happens to be located one floor above and directly over the post aeration basin. Investigation of this area indicated that hairline cracks in the concrete were apparently allowing some of the pressate to weep through the floor, migrating to the post aeration basin, where the effluent fecal coliform samples were taken. This condition was corrected by sealing the cracks and installing an impervious liner below the belt filter press.A second potential contributor to the coliform spikes was due to the chlorine feed control system. While the system was flow paced and capable of adjusting the feed rate to account for flow variations, it was not able to account for fluctuations in chlorine demand. To correct this situation, a compound-loop type control system was recommended to be implemented. This type of system provides the ability to flow pace flow and to “trim” the feed rate to maintain a desired disinfectant level. To accomplish this, a state-of-the-art redox, or oxygen reduction potential (ORP) control system was installed.The causes of the spikes in effluent chlorine residual were determined to be a result of the type of chemical feed system being used for dechlorination. Dechlorination is accomplished by feeding sulfur dioxide to remove remaining chlorine residual at the end of the chlorine contact basin. The chemical feed was manually adjusted and relied on manual operator adjustment to account for flow variations and did not have a means of accounting for variations in the chlorine residual levels at the end of the basin. To improve the reliability of the dechlorination process and eliminate the effluent chlorine spikes, it was recommended that the sulfur dioxide feed system also be controlled by the proposed ORP control system. This allows for flow paced control and also monitors the oxidant level remaining prior to dechlorinating to set a sulfur dioxide dose. The system further allows for “trimming” the feed rate by verifying the ORP of the effluent is at or below the desired effluent limit.These changes were implemented and were successful in elimination of the spikes in the effluent fecal coliform and chlorine residual levels. Since implementation in 2004, the facility has had no excursion of either of these parameters and has been 100% compliant. The facility received an award for Excellence in Operation and Maintenance of an Industrial WWTP for 2005, from the South Dakota Department of Environment and Natural Resources.