Ancient civilizations collected rainwater  for drinking. That practice continues even today on the Big Island of Hawaii, where rainwater is a major source of drinking water for more than 10,000 people on the island’s northern tip.

At the Waimea Water Treatment Plant, water from streams fed by rainfall is collected in reservoirs and fed by gravity to the plant, where it is treated and distributed to residents of the Waimea, South Kohala and Hamakua areas. Production capacity is 4.0 mgd; daily production averages 2.0 to 2.5 mgd. Two groundwater wells are available to supplement the streamflow.

There are plans to increase capacity and add microfiltration to provide an extra barrier against Cryptosporidium and guard against turbidity spikes during heavy rainstorms.

Following the flow

Surface water is collected from the Waikoloa and Kohakohau streams in the mountains 5 miles above the treatment plant. The water flows to the Waikoloa reservoirs — four open, concrete-lined basins, three with 50 million gallons capacity and the fourth with 8.5 million gallons. All lie about a 1/2 mile from the treatment plant.

“Intake pretty much follows the rainfall pattern,” says Kawika Uyehara, water engineer with the County of Hawaii Department of Water Supply. Normally all four reservoirs are in operation. Rainfall averages 30 to 50 inches a year; dry periods are common from September through November. If necessary, the Waimea plant can draw on two groundwater wells near the plant to supplement the flow.

From the reservoirs, water flows by gravity to the plant where it is rapid-mixed, then coagulated and flocculated and settled in tube settlers. Caustic soda and C-9 orthophosphate are added for corrosion control, and the water passes through deep-bed gravity sand filters and disinfection with chloramines to guard against DBPs while maintaining the necessary chlorine residual. The collected solids are pumped to a lagoon and then to drying beds. Clarified effluent is returned to the treatment plant.

Automated process

Two operators certified by the State of Hawaii Department of Health run the plant, assisted by a Motorola MOSCAD SCADA system that interfaces with Wonderware software (Schneider Electric, Telemetry & Remote SCADA Solutions). “The system is primarily used for data acquisition such as plant effluent flow, turbidity levels and chlorine residual levels,” says Uyehara. Hach Company supplied the flow monitoring instrumentation.

Turbidity control is a major goal. Uyehara reports that turbidity levels coming into the plant can range from 1.0 to 6.0 NTU; spikes occur after rainstorms. The turbidity in the product water averages 0.05 NTU.

A covered reservoir stores the water before it is gravity fed to the Waimea distribution system, which consists of about 100 miles of pipe, generally cast iron, ductile iron and galvanized steel. The system includes four booster pump stations and 20 finished water reservoirs, also covered.

The staff maintains a full-time acoustic listening leak detection program, monitoring the distribution system regularly and investigating, identifying and repairing leaks. Based on historical results, less than 10 percent of the flow is identified as non-revenue water.

Resource recovery

In a land of abundant sunshine and water, the treatment operation uses both to reduce power needs and cut greenhouse gases. As the water flows from the raw water reservoirs to the treatment plant, it drives a hydrogenerator, which converts its kinetic energy to electricity.

The workhorse of the system, designed by SOAR Technologies and dedicated in 2009, is a Pelton turbine. As the turbine spins, energy is released to the shaft of a generator, creating a maximum of 40 kW, enough power to operate the treatment plant. Any surplus can be fed to the local utility.

Since the amount of electricity generated depends on water flow and hydraulic head — the vertical distance between the water level in the reservoirs and the turbine — power generation is not constant and the hydrogenerator operates intermittently.

The sun’s rays are put to use in the plant’s solids drying beds, which lie beneath a facility with transparent walls and ceilings that trap and concentrate heat, allowing the material to dry with no need for gas heaters or centrifuges.

Constant threats

As idyllic as the Hawaiian Islands sometimes seem, the Waimea plant faces challenges from earthquakes and volcanoes. An earthquake in October 2006 damaged one of the concrete storage reservoirs; the utility spent about $1.9 million to repair cracks and holes and to place geotextile fabric and Hypalon polymer liner (DuPont Water Technologies) inside the basin. The perimeter of the reservoir was also excavated and rehabilitated to prevent buildup of water against the walls.
Uyehara says no special precautions are taken against the possibility of future quakes, except that all new structures are built to current seismic and wind velocity standards. Lava flow from active volcanoes can pose an additional threat, although the most recent eruption and lava flow on the far east portion of the island did not affect the Waimea system.

Meanwhile, plans for membrane filtration are proceeding. Bids have been let for the microfiltration system. The membrane facility will also expand plant capacity, as the deep-bed sand filters limit production of finished water.

The addition will address what Uyehara says are the biggest challenges facing the Waimea team. “Our efforts will be focused on continued compliance with the requirements of the Surface Water Treatment Rule,” he says.