GTI custom-designed the structurally supported covers to include special features chosen by the plant team, and to fit around the various pipes and light standards existing around the basin openings. Click here to enlarge imageWith the switchover to ozone, the plan was to reduce or eliminate chlorine feed at the head of the plant, thus reducing the formation of disinfection byproducts (DBPs). However, any attempts in the past to reduce chlorine feed have resulted in algal growth on the basins walls, launders, tube settlers, etc. At times, algal growth was significant enough to have caused in-plant taste and odor production.
The water retailers served by the district were expecting substantial reductions in DBPs from ozonation. If pre-chlorination were to continue at the same rate for algae control, the benefits of ozonation would be less pronounced. District staff knew that DBP reduction with algae control could be accomplished in two ways: through the use of alternative oxidants to chlorine or by creating a physical barrier to block the sunlight that algae requires to grow. Both solutions were found to be used effectively at water treatment plants contacted as part of the district’s investigations. Ultimately, district staff chose to go with basin covers because the cost of the basin cover installation can be recovered in about five years through hypochlorite reduction. The idea also encompasses the district vision of “getting cleaner, greener and leaner.”
District staff evaluated three types of basin covers: 1) buoyant fabric covers floating directly on the water, 2) high-strength fabric covers tensioned across a basin opening and suspended above the water, and 3) covers consisting of a structure spanning the basin opening. Fabric, fiberglass and aluminum cover materials were considered. All of the cover styles would block sunlight and thus control algae. Similarly, all the cover styles were manufactured with materials that were NSF 61 approved for drinking water applications.
The Penitencia plant team expressed a strong preference for retractable, structurally supported covers because the basins are frequently taken in and out of service, and this cover style is the easiest for basin viewing by the operations staff and accessing by the maintenance staff.