Gas to Liquid Chemical Feed System Conversion at Seven Wastewater Treatment Plants

The Hampton Roads Sanitation District (HRSD) owns and operates 9 regional wastewater treatment plants in southeastern Virginia. HRSD's service area is divided into the North Shore and the South Shore by the Hampton Roads harbor. Seven of the 9 treatment plants are included in this project.A round trip to all 7 plants takes the better part of a day, and covers approximately 150 miles.HRSD recently made the decision to convert these 7 treatment plants from chlorine and sulfur dioxide gas chemical feed systems to sodium hypochlorite and sodium bisulfite liquid chemical feed systems. Significant issues involved in this decision to convert from gaseous to liquid chemical feed systems included: potential health impacts to HRSD staff and to the public; treatment plant process impacts; and costs to HRSD customers.Once the decision was made to convert to liquid chemical systems, HRSD decided to expedite the project schedule. All chlorine and sulfur dioxide gas had to be removed from the 7 treatment plant sites prior to the June 21, 1999 Clean Air Act (CAA) deadline for filing Risk Management Plans (RMPs). The remaining two plants previously were made compliant with the CAA by retrofitting the facilities with hazardous gas leak containment and scrubbing facilities. RMPs were filed for these 2 plants, which are located in more industrial settings than the other 7 plants.To meet this aggressive project schedule, an intensive, fast tracked implementation plan was devised for design and construction of the required facilities. The implementation plan included pre-purchase contracts for 90 chemical metering pumps, 32 fiberglass reinforced plastic (FRP) storage tanks, 8 chlorine mixers, an advanced sitework construction contract, and two final construction contracts. Design commenced in September 1998.Many problems have been addressed and resolved by HRSD over the past 15 months, including chemical leaks, temporary and permanent containment, chemical degradation, off gassing, mixing, storage, corrosion, and safety. HRSD's wastewater plant operators and engineers have been resourceful as they have worked with the contractor to avoid permit violations during installation of first temporary and then permanent chemical feed facilities.The conversion from chlorine and sulfur dioxide gas feed systems to sodium hypochlorite and sodium bisulfite liquid feed systems has been successful. Treatment plant operators no longer have to deal with large numbers of dangerous, highly toxic, compressed gas cylinders,“24/7/365”.This paper discusses problems, solutions, implementation issues, and performance of the new liquid chemical feed systems at the 7 regional plants. Operational issues considered during facility design include sodium hypochlorite leakage, sodium hypochlorite degradation and off-gassing, dilution water usage, disinfectant mixing, temporary chemical feed and storage, chemical crystallization, and disinfection and dechlorination control loops.This paper should be of significant interest and an excellent resource for others considering conversion from gas to liquid feed systems. This paper also discusses construction and startup issues encountered during the conversion from gaseous to liquid disinfection and dechlorination chemicals at the seven regional treatment plants.