Why is post treatment necessary in Commercial RO Plants?
Finished water from RO facilities, if not adequately handled, can cause corrosion in downstream fixtures and pipes, including the distribution system. Chemical addition to alter water quality is part of the post-treatment process. When water quality allows for less, or in some circumstances, corrosion control chemical use, a limited amount of raw water is blended with permeate before disinfection and distribution. If corrosion occurs, it can create issues ranging from aesthetics (e.g., "red" water generated by iron) to serious public health concerns (e.g., elevated lead concentrations).
Depending on the application of product water at the point-of-use, post-treatment is necessary, for example, RO permeate is re-mineralized prior to distribution for potable use. For applications that demand high-purity water (pharmaceuticals) or ultrapure water, RO product water is polished to further de-mineralize it.
Corrosion control treatment methods
The following are the major post-treatment options for membrane permeate water:
• pH balancing
• Alkalinity correction
• Calcium supplementation
• Anti-corrosion agents
• Mixing and matching
pH Adjustment
The production of insoluble compounds on exposed pipe walls is induced by adjusting the pH. The operational mechanisms for this corrosion control approach are passivation. Chemicals such as lime, soda ash, sodium hydroxide, potassium hydroxide, and carbon dioxide are used to modify the pH level. Source waters with low to moderate hardness and alkalinity (between 80 and 150 mg/L as CaCO3) are best for pH correction.
This treatment method is frequently employed instead of calcium carbonate precipitation. Increased generation of disinfection by-products at pH levels over 7.8, decreased chloramines disinfection efficacy at pH values below 7.8, and a larger risk for calcium carbonate scaling in the distribution system pipe at pH above 7.9 are some of the problems with pH modification.
Alkalinity Adjustment
Alkalinity adjustment is widely employed to cause the production of insoluble compounds on the distribution system's pipe walls. Carbonate passivation is performed by incorporating pipe components into a metal hydroxide/carbonate protective coating.
Passivation is the operational mechanism for this corrosion control technique. This corrosion management approach works well with low-alkalinity source waters and is typically employed instead of calcium carbonate precipitation. The concentration of dissolved inorganic carbonate (DIC) in the source water is changed through alkalinity adjustment. Lime, soda ash, sodium bicarbonate, sodium hydroxide, potassium hydroxide, and carbon dioxide can all be used to modify alkalinity. For alkalinity correction, sodium bicarbonate is preferred.
Although sodium hydroxide adds minimal alkalinity to the water, it can produce significant pH changes.
Calcium Adjustment
The modification of the calcium carbonate system's equilibrium for the source water is the mechanism for this corrosion control technique. The goal of this treatment process is to create a protective layer of calcium carbonate on the pipe walls. Calcium addition or removal is not required for calcium carbonate precipitation, which is achieved by adjusting the pH and alkalinity of the source water. The key to this treatment method is to provide the right circumstances for calcium carbonate saturation to occur. The pH/alkalinity is adjusted to provide the appropriate conditions for the calcium and carbonate ions to surpass their solubility limits in water.
Odour Control
Some groundwaters may include significant levels of H2S, which will not be eliminated by membrane processes since it is a gas. To eliminate H2S and other generated gases such as CO2, a post treatment consisting of a single or two stage odour control system may be necessary (if excessive pre-treatment acids are added).
Corrosion Inhibitors
Inhibitors have found wide spread use as a method of corrosion control. The most prominent forms of inhibitors used are polyphosphates, zinc phosphates, and silicates. The inhibitors control corrosion by several mechanisms, including sequestering of the corrosion by-products, specifically lead and copper, scale inhibition, development of a coating film on the pipe walls and buffering the water at the desired pH. Operating data indicate that the choice of inhibitor depends upon pH, alkalinity, calcium and total hardness, chloride, sulphide, iron concentrations, and dissolved oxygen levels of the source water.
Adding of Blenders
Blending Adding or blending pre-treated source water into the (permeate) product water can help in stabilizing the product water thereby reducing the impact of the before mentioned issues. However, blending introduces the need for disinfection of the pre-treated water prior to or after blending. Unfortunately, blending will not stabilize the product water completely hence permeate will still need to have some level of calcium and alkalinity (alkalinity being the more important parameter) present. This can be accomplished by employing either lime or limestone treatment.
If the source of the water to be blended with the product water from the reverse osmosis system is from a ground source from a limestone or chalk geological formation, the amount of lime treatment will be substantially reduced. Blending of variable and differing water supplies where desalted water serves as one of the supplies is becoming more frequent. Concern has also been expressed about the impact of extremes of major ion composition or ratios for human health. There is limited evidence to describe the health risk associated with long-term consumption of such water, although mineral content may be augmented by stabilization processes typically used by utilities practicing desalination (WHO).
Conclusion
Over the last 50 years, reverse osmosis technology has become significantly more efficient. Innovative design, enhanced membrane materials, more efficient pumps, and improved pre-treatment processes, as well as antiscalant chemicals, are all contributing to this.
To get the most out of reverse osmosis (RO) for your commercial or industrial application, look for RO systems from reputable companies like Netsol Water Solutions. The firm is unquestionably an expert in the field of water and wastewater treatment. The firm provides a variety of water treatment options, including whole-house RO systems, commercial RO systems, industrial RO systems, Water Softeners and much more.
