Cibolo Creek Municipal Authority in Schertz, Texas, just north of San Antonio, had a problem with algae growth at its water reuse facility. To combat the problem, the authority decided to install a cover over its reuse pond.
Algae in reuse water can cause a number of problems. Large populations of algae raise BOD and TSS levels above permissible limits, clog irrigation spray nozzles and color the water an aesthetically unappealing green. Water clarity determines water quality in most water reuse ponds. Many engineers are designing their reuse ponds or tanks with a cover to shade and thereby prevent the growth of algae.
In August 1999, the Cibolo authority installed a 120,000 square foot LemTectrademark Modular Cover System on its new 10,000,000 gallon reuse pond. In addition to algae control, the cover system prevents the dissipation of chlorine by UV radiation and reduces the amount of chlorine usage.
The cover also can handle the pond's 15 foot water level variation without requiring water or gas collection. The connection between casings in the cover allow for some expansion and contraction of the cover surface area and, more importantly, provides flexibility so that the cover can lay against the slope. Ease of installation, removal and repair also were important considerations.
James Smith, Systems Manager, reports the extremely low BOD (2 mg/l), TSS (2 mg/l), NH3N (1 mg/l), fecal coliform (1 #/100ml) and turbidity (1 NTU) levels achieved by the wastewater treatment plant are being maintained in the pond after the cover was installed. Even though the facility has the capability to re-chlorinate after the covered zone, it has not been necessary.
"Reclaimed water is going to be a big business for every utility. The clarity of that water is important, therefore it is necessary to cover any basin," said David Dennis, General Manager. "Our experience is that the combination of a reuse pond and modular cover is a very viable and cost effective alternative to covered tanks."
The LemTec modular cover's design uses individual casings of closed-cell extruded polystyrene insulation encapsulated by two sheets of 10 year UV-resistant, high density polyethylene (HDPE) geomembrane. The cover system's overlap design allows gases and rainwater to pass between the casings, thereby eliminating the need for water collection and removal systems as well as gas dispersal equipment.
The insulation in each casing provides floatation as well, allowing each casing to be self-supporting. When connected to other casings, the cover can support the weight of several crew members for maintenance and access.
