Stabilizing Recycled Water Using CaCl2 and NaOH

The permeate from the Reverse Osmosis (RO) system is corrosive and low in pH because it removes most of the dissolved solids and other substances such as calcium and bicarbonate ions that give calcium hardness and alkalinity. Dehydrated lime (Ca(OH)2), or calcium hydroxide is the most commonly used chemical to stabilize the permeate since it is the cheapest and gives both alkalinity and calcium hardness. Lime saturator and slurry injection system are commonly used technologies to prepare the powdered lime before injection. Both systems are operator intensive and do not perform well with large variations in flow and frequent shut downs. In addition, they require frequent maintenance, due to lime characteristic. Scales are formed throughout the system and a lot of effort is needed to remove them.Terminal Island Water Reclamation Plant (TIWRP) uses a lime slurry injection system to stabilize its RO permeate. The dehydrated lime is mixed with RO permeate in a mixing tank, then the mixture (lime slurry) is injected into the permeate discharge line to the Chlorine Contact Tank (CCT). However, the system has caused many operational and maintenance problems that have resulted in preventing the Plant from delivering recycled water consistently to the Dominguez Gap Barrier (DGB), in which the recycled water is injected to prevent seawater intrusion as well as to replenish groundwater.When the liquid chemicals, 25% of Sodium Hydroxide (NaOH) and 35% of Calcium Chloride (CaCl2), were tested to comply with a Langelier Saturation Index (LSI) limit, not only could the limit be met but also the problems associated with use of lime were resolved. Sodium hydroxide was used to increase pH and alkalinity of the recycled water and calcium chloride was used to increase calcium concentration in the recycled water. These chemicals made it easy to control pH and turbidity of recycled water in the CCT.