Reverse osmosis (RO) membranes are the leading technology for desalination of sea water because of their strong separation capabilities and exhibiting a great potential for treatment of waters worldwide A typical RO system consists of four major subsystems: pretreatment system, high-pressure pump, membrane module, and post treatment system.
The membrane manufacturers offer high salt rejection membranes for RO plants, and the membranes do not retain the initial salt rejection throughout the membrane’s lifetime (up to 7 years with effective pretreatment). Temperature, salinity, target recovery, and cleaning methods can affect salt passage through normal membrane.
The main drawbacks of RO technology are the limited recovery and the environmental impact of rejected brines. Recovery and brine concentration are limited because increasing the brine concentration in RO would increase osmotic pressure and thus the energy consumption as well as scaling on the membrane surface. Recovery of the seawater RO plant is 35 to 45%.
The key limiting factor to widespread use of inland desalination is the exorbitant cost of concentrate disposal. Membrane fouling is a major obstacle in RO. Fouling increases resistance, which in turn reduces permeate flux. Fouling can be controlled by feed pretreatment and membrane cleaning. Sometimes conventional pretreatment is not effective. An excessively advanced pretreatment system significantly increases the installation cost. In RO Plant, for occurrence of reverse osmosis, a very high pressure is to be applied on the concentrated solution and is directly related to the feed pressure and flow rate. The high salt concentrations found in seawater require elevated hydrostatic pressures (up to 7000 kPa); the higher the salt concentration, the greater the pressure and pumping power needed to produce a desired permeate flux. High-pressure pump sets and approximately 70% energy required for these pump sets. As the recovery of a RO unit increases, the osmotic pressure increases on the feed side of the membrane, thus increasing the feed pressure required. However, as the recovery increases, the feed flow required decreases (for a specific product flux), and for lower recoveries (35–50%), the overall energy requirement decreases with increasing recovery. Thus, a minimum energy requirement exists, typically at a recovery between 50 and 55%, which varies with feed salinity.
In RO process, the rejected brine effluent will be having high pressure and having a considerable percentage of feed pressure. This available residual brine pressure can advantageously be utilized to boost the feed pressure of the raw water by suitable arrangement/device. This is called energy recovery system.. Hydro turbines and impulse turbines are the two types of devices for recovering the residual energy available from the high-pressure feed stream. Energy recovery devices can provide net energy transfer efficiency from the concentrate stream to the feed stream of more than 95%. The coupling of energy sources with RO desalination plants has been an increased interest to development. Wind and photovoltaic solar energy are commonly paired with RO desalination. Overall, the energy sources most often used are solar energy (70% of market) and RO which has the majority (62%) of the renewable energy desalination market. The energy recovery devices installed in the RO process can lead to 25 to 30% of energy saving. Energy recovery devices play vital role in cost-effective production of fresh water by RO desalination.