Source water entering the plant is called raw water. Upon entry, a scale inhibitor is injected to prevent scaling. The addition of this chemical is commonly referred to as “pretreatment”. At this point, the raw water becomes the feed water. After pretreatment, the feed water enters a series of cartridge filters.
Upon passing through the cartridge filters, the water is pumped with high pressure pumps into the RO production units for primary treatment. When the feed water travels across the RO membrane elements, it is separated into usable (product) and non-usable (concentrate) water. Pretreatment keeps dissolved solids in liquid form during this separation. As required by permit, the concentrate is then discharged from the system by way of a pipeline to a deep injection well.
The amount of concentrate removed in the RO process is approximately 15% of the feed water entering the system. The concentrate water is not drinkable nor is it suitable for irrigation due to the high dissolved solids concentration.
After the RO units separate the water into product and concentrate, the product water flows towards the degasifiers. Product water coming out of the RO units is of such high purity that it has little or no hardness. Prior to entering the degasifiers, some raw water is blended with the product water to increase alkalinity and hardness to a moderate level. This produces a more stable finished water for corrosion control. At this point, the water is called blend product. Approximately 17% of the total blend product is blend water.
The blend product water now enters the degasifers where a final contaminant needing removal, hydrogen sulfide, is stripped from the water. Hydrogen sulfide produces the objectional sulfur or “rotten egg” odor often found in well water. Blend product water cascades down into the degasifiers. It is forcibly mixed with air from a blower. The air strips the hydrogen sulfide from the water, and the combined hydrogen sulfide and air leaves through a tower connected to the top of the degasifiers.
The water now falls into the clearwell where chlorine and caustic soda are added. Liquid chlorine (sodium hypochlorite) is added for disinfection and removal of any remaining hydrogen sulfide not removed by the degasifiers. Also, orthophosphate is added to the water for corrosion inhibitor. Caustic soda (sodium hydroxide) also is added to raise the pH of the water.
This pH adjustment is the final step in the process of stabilizing the water for corrosion control. From the clearwell, the water is pumped to storage tanks where it is called finished water.
At this point, it is available for pumping to the consumer.