Fallout from the Clean Air Act - Getting Rid of the Gas

The Hampton Roads Sanitation District (HRSD) converted 7 of their 9 regional wastewater plants (15 to 40 mgd – 176 mgd total) from chlorine and sulfur dioxide gas feed systems to sodium hypochlorite (15%) and sodium bisulfite (38%) liquid feed systems. Issues involved in this decision included: health impacts to HRSD staff and to the public; plant process impacts; and costs to the rate payers. To meet scheduling requirements, an intensive, fast tracked design and construction implementation plan was devised and implemented. The plan included prepurchase contracts for 90 chemical metering pumps, 32 FRP storage tanks, 8 chlorine mixers, an advanced site work construction contract, and two final construction contracts.The conversion from chlorine and sulfur dioxide gas feed systems to sodium hypochlorite and sodium bisulfite liquid feed systems has been successful. HRSD's engineers and wastewater plant operators were resourceful as they worked with the contractor to avoid permit violations during installation of first temporary and then permanent chemical feed facilities. Even though HRSD operators are dedicated, well-trained professionals who have safely used chlorine gas for over 50 years, they have expressed significant relief at no longer have to deal with large numbers of dangerous, highly toxic, compressed gas cylinders, “24/7/365”.Challenges addressed in this paper included: off-gassing/gas binding in hypo feed lines; joint leaks, affects of hypo/bisulfite spills on concrete containment structures; health and safety issues associated with liquid chemical feed systems; piping hypo/bisulfite feed lines across congested plant sites; providing access to buried chemical feed lines for future maintenance; affects of hypo on viton; teflon; and EPDM; proper venting of FRP chemical storage tanks; chlorine mixing efficiencies; NPW hardness/scaling; and dechlorination control loops.This paper includes construction, startup, and operational issues encountered during the project.