GAC is commonly employed as an adsorption media for the removal of a wide range of organic contaminants, including TCP, from drinking water. This treatment approach is currently being used at many drinking water treatment plants throughout the State. The water treatment benefits of GAC derive from the adsorption properties of the GAC material and the media’s high internal surface area, as opposed to filtration media, which captures contaminants between particles. Adsorption with GAC is a relatively “green” process in that the spent media is taken back by the supplier, captured contaminants are destroyed, and the carbon can then be reused in another treatment application. The proposed treatment system could be capable of reducing raw water TCP concentrations as high as 150 parts per trillion (ppt), much higher than current levels in the wells, and reduce TCP down to non-detectable levels.
The Project proposes to construct a centralized GAC water treatment plant to remove the TCP from the water produced by the four potable water supply wells, which are all located near the Project treatment site (See Figure 3 5). The flow from the four wells supplying drinking water merges at the existing facility and combines before being sent to an existing storage tank and blending station for the reduction of nitrate levels. The new facility would intercept the flow from the wells, direct it through the GAC treatment process and return it to a new, water storage tank. The facility would require six 12-foot-diameter steel pressure vessels for the GAC media to treat the initial maximum flow rate of 2,350 gpm; however, the facility would be designed to accommodate the addition of another four vessels in the future, which could increase the overall treatment capacity to 3,150 gpm. The GAC media must be backwashed when it is first installed in the vessels and may need to be backwashed periodically once placed into service. The District intends to send this backwash water, which contains NSF-61 (drinking water contact) certified carbon fines and TCP levels comparable to the raw water to an equalization tank and then pump it into the District’s non-potable water distribution system. Because the water has high hardness (the simple definition of water hardness is the amount of dissolved calcium and magnesium in the water when heated, deposits of calcium carbonate can form) that may interfere with the GAC treatment, the District plans on reducing the pH of the water before it reaches the GAC using carbon dioxide and then raising the pH back up after treatment using sodium hydroxide. The existing well pumps would also need to be upgraded due to the additional pressure loss through the GAC system. In addition to the GAC treatment vessels, the facility would include a new treated-water tank, backwash equalization tank, non-potable water pumps, storm water detention basin, chemical feed systems, and other associated appurtenances.
The Project would be capable of treating any combination of the wells at the same time including flow rates of up to 2,350 gpm initially (and up to 3,150 gpm should additional two vessel pairs ever be added) and would be designed to support a flow rate as low as 500 gpm in order to accommodate reduced speed pump operation during low demand periods, which typically occur late at night. Automated motor operated valves integrated with the site supervisory control and data acquisition (SCADA) system would be included at each vessel pair to make removing vessel pairs from service an automated process. The average volume of treated water expected to be produced is approximately 72 million gallons per month.
The existing facility is approximately 0.5 acres, and the proposed new facilities would be approximately 2.5 acres. Specific Project components include:
• Three pairs of GAC vessels (six total), expandable to five pairs of vessels in the
future: 12-foot diameter; 18-foot tall; placed on a concrete foundation of 3,500
• Excavations for the foundation and infrastructure would occur up to
approximately 5 feet in depth;
• Backwash equalization tank: 126,000 gallons; 33 feet in diameter; 24 feet tall;
ring wall footing;
• Treated water storage tank: 85,000 gallons; 27 feet in diameter; 24 feet tall; ring
• Well pump replacements (four total): two 100 horsepower (hp) and two 125 hp;
• Electrical service upgrade – to allow higher horsepower well pumps and non-
potable pumps to operate;
• Fixed standby generator; which will include an approximately 10,000-gallon
diesel fuel tank for storage;
• Chemical feed systems: One 5,000-gallon sodium hydroxide storage tank and
feed system and one 14-ton carbon dioxide feed system;
• One small diameter pipeline and electrical conduit between this main site and the
existing Santa Rosa 8 well building to the south;
• Piping, fittings, valves, and associated infrastructure;
• Backwash (non-potable water) pumps: two 75 hp pumps;
• Chain link fence: 8-feet tall with three strands of barbed wire; approximately
1,000 linear feet; and a new access gate off of Hill Canyon Road; and
• Site surfacing of ag base under crushed rock; asphalt paved driveway with
concrete pads at the offloading area for delivery trucks.
• Total site improvements area: ˜ 108,000 square feet.