Skip to main content


Buy Article:

$17.50 plus tax (Refund Policy)

Or sign up for a free trial


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.

Document Type: Research Article


Publication date: 2004-01-01

More about this publication?
  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

    A subscription to the Proceedings of the Water Environment Federation includes access to most papers presented at the annual WEF Technical Exhibition and Conference (WEFTEC) and other conferences held since 2000. Subscription access begins 12 months after the event and is valid for 12 months from month of purchase. A subscription to the Proceedings of the Water Environment Federation is included in Water Environment Federation (WEF) membership.

    WEF Members: Sign in (right panel) with your IngentaConnect user name and password to receive complimentary access. Access begins 12 months after the conference or event
  • Subscribe to this Title
  • Membership Information
  • About WEF Proceedings
  • WEFTEC Conference Information
  • Learn about the many other WEF member benefits and join today
  • Ingenta Connect is not responsible for the content or availability of external websites
  • Access Key
  • Free content
  • Partial Free content
  • New content
  • Open access content
  • Partial Open access content
  • Subscribed content
  • Partial Subscribed content
  • Free trial content
Cookie Policy
Cookie Policy
Ingenta Connect website makes use of cookies so as to keep track of data that you have filled in. I am Happy with this Find out more