Why High TDS Water Breaks Conventional RO Design Assumptions?
Reverse osmosis systems are common in desalination and industrial water treatment and, in most designs, many of them are designed on standard operating assumptions. The assumptions have been successful in mid salinity sources, but fail in most cases at high TDS water. Once the total dissolved solids start to rise, the water behavior within an RO plant becomes very different, and the assumptions underpinning the conventional RO design assumptions become apparent, and the resulting performance, cost increase, and shortened life of the membranes.
High TDS Water in RO Plant
High TDS water is water of high concentration of dissolved salts, minerals and ions sodium, chloride, calcium, magnesium, sulfates and silica. These soluble compounds raise osmotic pressure, decrease water recoveries, and enhance scaling and fouling. The traditional RO systems are usually constructed with generalized recovery rates, pressure distributions and assuming membrane flux, which fail to capture these issues.
The system is also overloaded when it is being operated with high TDS water that is supposed to be treated with a design that operates at lower salinity feed. This incompatibility is among the major causes of poor performance or early failure of RO plants in high TDS systems.
Conventional RO design assumptions that do not work
Linear Pressure Increase Assumption
The first is that the higher the operating pressure, the higher the permeate flow. This relationship disintegrates in high TDS water. With salinity, the osmotic pressure suddenly increases decreasing the driving force across the membrane. There is a limit beyond which increased pressure yields decreasing returns, more energy is used and more mechanical work is applied to membranes without any significant benefit in terms of water yield.
Fixed Recovery Expectations
Design assumptions in conventional RO will be based on standard values of recovery. Water with high TDS will restrict the recovery that can be obtained because scaling is more likely to occur and polarization of the concentration. In trying to keep high recovery rates in these conditions high salt concentrations are rapidly concentrated at the membrane surface which hastens fouling and scale formation.
Standardized Membrane Performance
The other assumption is that membranes would be consistent regardless of the quality of feed water. Water with high TDS causes membranes to be exposed to high ionic strength and increased salt penetration. This may decrease salt rejection, improve conductivity of the permeate and extend membrane life, particularly when membranes have not been chosen in respect of high-salinity-specific performance properties.
Operational Problems due to High-TS Water
TDS water that is high imposes extra strain on other parts of RO plants other than the membranes. Pumps will be forced to work at the greater pressure, which will cause greater energy requirements and wear. Interstage pressure drops increase and the flow imbalance across membrane elements are even more pronounced. Pretreatment systems that are not configured to cope with lower TSS levels might not be able to control scaling ions adequately resulting in frequent chemical cleaning and unexpected downtime.
There are also less predictable methods of chemical dosing. The use of antiscalants by itself might not be sufficient to prevent high ionic concentration precipitation particularly when silica, sulfate, or barium is high. This also reflects the inadequacy of the established conventional RO design assumptions in the high-TDS case.
Re-evaluating RO Design of High TSS Water
The design of RO plants on high TDS water needs a radically different approach. Rather than straining the traditional systems, the designers should emphasize on staged recovery, optimized flux rates, and selection of the membranes. Narrower per-stage recovery, split-pass configurations, or partial desalination methods tend to offer superior long term performance than single-pass configurations that are aggressive.
Hydraulic modeling should consider the effects of higher osmotic pressure and salt concentration through each of the membrane elements. Increased operating pressure and corrosive environments also make the selection of material to use in piping and pressure vessels critical. Strategies during pretreatment can be improved with softening, selective ion removal or advanced filtration to stabilize the quality of the feed water to reach the membranes.
Energy and Cost Implications
The use of designs that have erroneous assumptions to treat high TDS water increases energy usage and operating costs. Power consumption grows out of proportion with the demands due to pressure. It is also inflated by frequent replacement of membranes, a higher number of chemical consumption and cleaning cycles. Operators can prevent the expensive redesigns and retrofits in the future by recognizing the shortcomings of conventional RO design premises.
Conclusion
High TDS water basically defies the principles under which various RO plants are traditionally built. High osmotic pressure, low recovery potential, high scaling risk, and variable performance of membranes are all factors that lead to the breakdown of the standard RO assumptions. These limitations should be considered in the development of effective, energy-saving RO systems capable of dealing with high salinity feed water. The individual design approach that considers the special behavior of the high TSS water guarantees the long-term performance, reduced operation cost, and life-long operation of the systems.
Do you need an advice or assistance on selecting the best water and waste water treatment unit? We have solutions for all your problems!
Let us know your problem, our experts will make sure that it goes away.
For an assistance or related query,
Call on +91-9650608473
Or write us at enquiry@netsolwater.com
