It was time to upgrade the water treatment plant, again.
For the third time since commissioning the Kuiper Water Treatment Plant, the City of Aurora, CO, and its engineers, CH2M Hill, began planning another upgrade project. This 70 mgd direct filtration plant was originally built in the late 1960s and had already been expanded twice between 1972 and 1976. Now the plant was to be upgraded again to meet anticipated future regulations and to replace aging equipment.
Included in the renovation was more mixing for the flocculation process. Mixing improvements came in the form of a new rapid mixer and new mixers in the completely remodeled flocculation basins. The basins also received new fabric covers.
Reclassification of the filters was an important part of the upgrade. The treatment plant is a direct filtration plant and the filters are the most important part of the process. Reclassifying the filters increased the plant capacity to 80 mgd. During the upgrade, the filters received a complete rebuild including new underdrains, new valves, filter troughs and media.
A new chemical building also was built, replacing a small, inadequate chemical storage area. The new structure provides space for existing chemicals as well as new chemicals to be used in the process. These chemicals include soda ash and chlorine dioxide.
The upgraded, newly-named Griswold Water Treatment Plant has operated successfully since its completion.
UV-blocking cover
During the upgrade planning process an opportunity was identified to change primary disinfection from chlorine to chlorine dioxide because of reduced DBP formation potential. To take advantage of the switch, the upgraded plant’s twin flocculation basins required covers to block sunlight since chlorine dioxide is very UV reactive. The covers would also keep wind-blown debris from entering the basins.
The basins each have three separate passes with flocculation mixers, and staff wanted to be able to remove the covers to access each pass individually.
Following a value engineering process, retractable, structurally supported covers manufactured by Geomembrane Technologies Inc. (GTI) were recommended due to the company’s ability to deliver a cover system with customized features, including individual access, at a competitive price.
GTI’s structurally supported covers consist of a sheet of high strength, UV protected, coated fabric tensioned across a series of low profile aluminum arches which span the basin openings. The fabric cover can be detached on three sides and retracted to expose the tank contents for inspection or maintenance. Rain or snow-melt water automatically runs off the cover to the tank perimeter. The fabric cover is NSF-61 approved for potable water applications.
Plant management reported that during the upgrade the cover installation was fast and efficient. Most importantly, the covers block sunlight, as required.
“Before the upgrade, operators saw a change in water quality each day when the sun rose. With the covers blocking sunlight, the water quality remains constant and the chlorine dioxide residual is maintained through the basin,” said Kirk Watson, Griswold Water Treatment Plant Supervisor.
Plant maintenance staff like the fact that the covers are essentially maintenance free and that they can roll open as much - or as little - of the cover as required. Being able to roll the fabric covers open, rather than disassembling a fixed cover, saves time.
The retractability feature became even more appreciated when a safety issue had to be addressed by plant staff. The plant initially developed a maintenance procedure for annual basin cleaning that required workers to enter the basins through hatches with hoses and cleaning equipment. Workers would clean residuals from the basin walls and floor and wash them into a center drain. Since the basin floors are sloped and tend to be very slippery, this maintenance function was considered a hazardous task.
After a worker slipped and fell while performing a cleaning, the maintenance procedure was revised to take advantage of the retractable covers. Now workers merely roll the covers open and wash the basins from the deck, without entering the basins or dragging hoses and equipment through the basin passes.
“Annual basin cleaning is safer and easier with our retractable covers,” Watson said.
About the Authors:
Brent Howe, P.Eng., is a Product Line Manager with Geomembrane Technologies Inc. (Tel: 506-452-7304). Kirk Watson is the Treatment Plant Supervisor of the Griswold Water Treatment Plant in Aurora, CO (Tel: 303-739-7980).
