OPTIMIZING DISINFECTION THROUGH MIXING AND ORP CONTROL: A CASE STUDY OF THE ASPLUND WWTF IN ANCHORAGE, ALASKA.

In the United States, there remain wastewater treatment facilities that employ only primary waste treatment processes – solids removal followed by disinfection – to meet conditions established in an EPA NPDES permit for effluent discharges to receiving waters. The objective of this project is to enhance the chlorine delivery to and the disinfection process control of the effluent at a Primary WWTF owned and operated by the Anchorage Water and Wastewater Utility (AWWU). The subject facility processes an average of 30 mgd of domestic wastewater. Discharge is to the marine waters of Cook Inlet, Alaska, which are influenced by significant glacial runoff and extreme tidal fluctuations.Prior to this project, operators injected a 120-gpm chlorine-water solution into the effluent stream through a 6” PVC pipe diffuser installed across a 15' wide channel to affect pathogen kill. This system was particularly inefficient and resulted in the escape of chlorine gas to the surrounding environment. Ferrous and aluminum items in the area were subjected to corrosive attack, increased maintenance, and shortened working lives; and landscaping features suffered under the chemical attack. Further, plant operators manually monitored chlorine residual concentrations each hour, resulting in disruptions to other plant operations.Plant operators worked with a selected vendor to plan installation of an induction mixing unit and an oxidation-reduction potential (ORP) control unit on a trial basis. This facility's effluent works are not ideally suited to the use of this vendor's ORP control technology due to a lack of latency in the flow stream. Further, system configuration is such that the “best placement” of the induction mixer (axial to flow stream) was not chosen for the trial installation for practical and cost reasons. Therefore, plant operators and their project management consultant devised a program to install, operate, and performance test the mixing equipment and ORP control, basing final system acceptance on appropriate performance criteria.During the course of the operational test period, several events conspired to thwart the full-scale trial: a gas feed hose froze twice during the winter, a leak in the vacuum piping allowed air into the disinfection stream and led to the inability to maintain vacuum at high feed rates, the lack of flow stream latency, and faults in the controller's data record all impacted the process. Most significantly, the Utility learned that chlorine injection transverse to flow does NOT perform as anticipated by the equipment provider.Initial installation of the induction mixer was completed in early November 2001 and the ORP control unit was placed on-line in early-December of that year. A process of investigation, discovery, refinement, and assessment was undertaken by operators and the authors during the next six months leading to the determination that a) the induction mixer was not developing the injection frustum anticipated, meaning that transverse injection would not provide mixing across the entire flow stream and b) the ORP controller suffers deficiencies in the data recording software, which confounded efforts to correlate ORP set point with other measured parameters.[khv1]These factors result in operators tending to the ORP control unit to adjust the control setpoint on a month-to-month basis, to meet the constraints of their EPA permit. Overall, however, the control unit is far more responsive to effluent quality fluctuations than human operators can be in taking hourly residual measurements. The equipment combination has reduced chemical consumption while maintaining similar performance for compliance with specified permit limits for coliform and chlorine residual in the effluent discharged to the receiving waters of Cook Inlet. And the resulting chemical and labor savings yield a payback period of about four years.As of this writing, AWWU has relocated the induction mixer to inject axially into the flow. And having confirmed software code problems in August 2003, the vendor delivered a new datarecording program (beta version 2.27) very recently. Under the new configuration and with the new software, operators will develop a new data record to guide the continuing improvements to this process.