With its discharge site located in Chesapeake Bay, the 1.9-mgd wastewater treatment plant in Havre de Grace, Md., must comply with a fecal coliform limit to 14 mpn (most probable number), the state standard for direct bay discharges, and the chlorine content must be less than 0.1 ppm. Until May 1997, operators used a flow-pacing system and routinely fed 15 lb. more sulfur dioxide than chlorine to ensure adequate dechlorination.
Costly chlorine violations led Havre de Grace in May 1997 to install an automated, demand-based chlor/dechlor controller, a Strantrol(r) 900 High Resolution Redox(tm) (HRR) controller manufactured by USFilter Stranco Products, Bradley, IL,. This step, taken without state or federal compulsion, was viewed positively by the Maryland Department of the Environment, which recently had lowered a Havre de Grace fine from a possible $12,500 to $5,500.
The automated chlor/dechlor system has significantly reduced chemical costs by reducing overfeed of sulfur dioxide, which allows operators to add less caustic for pH control.
Three Channels
The automated controller at Havre de Grace provides three channels for monitoring and controlling wastewater chemistry based on data received from sensors. The first channel, used for chlorination control, maintains the disinfection strength necessary to hold fecal counts at 2.0 mpn or less, well within the 14-mpn limit required for the Chesapeake Bay direct discharge site.
Three Channels
The second channel monitors and controls dechlorination. Because of the brief lagtime between the sensor and the point where the water is injected with sulfur dioxide, the controllers response to chlorine levels is accurate and almost instantaneous.
Three Channels
The third channel is not currently in use at Havre de Grace, but is being considered for pH control or monitoring the rotating biological contactors (RBCs) to develop a nitrification curve. A future plant conversion to biological nutrient removal (BNR), a possibility currently under review, would allow operators to control BNR with the third channel sensor. The sensor would monitor nitrifier activity in the nitrification basin, providing a basis for improving process control through adjustments to aeration, pH, and sludge return.
Cost, Compliance Benefits
Since the installation of the automated controller, the plants sulfur dioxide usage has dropped 50 percent to an average of 30 lb. per day. Instead of feeding up to 100 gpd of caustic, the facility now feeds as little as six gpd to maintain pH, lowering caustic supply costs by about $5,400 per year.
Cost, Compliance Benefits
Based on chemical savings alone, the plant projects a 30-month payback on the cost of the automated chlor/dechlor controller system.
Cost, Compliance Benefits
Fewer chemical deliveries and less chemical usage also translates into time saved by employees. Less chemical handling, especially of highly corrosive caustic, makes operations safer and more efficient. Making use of the third channel on the controller to gain insight into RBC performance (or future BNR performance) also will increase the plants efficiency by taking much of the guesswork out of process control.
Cost, Compliance Benefits
The most important benefit demand-based chlor/dechlor control brings to the Havre de Grace WWTP is the ability to comply with tough regulations and avoid costly fines. The Maryland Department of Environment reacted positively to the installation of automated control at Havre de Grace, indicating that the plant is resolving potential problems before they become the source of violations.
Cost, Compliance Benefits
The efforts of the staff to make the plant run as efficiently as possible have paid off. The Maryland Rural Water Association selected the Havre de Grace WWTP as the "Wastewater System of the Year" for 1997.
