The figure below depicts a schematic view of a hybrid desalination plant. The combined use of reverse osmosis and thermal systems in hybrid plants, makes such approach a suitable in many circumstances.
The proportion of using RO desalination plants and thermal desalination plants (MED) depends on several factors, such as the plant’s construction site and conditions, the trend of changes in electricity and water consumption, the network utilization coefficient (dispatching) of the power generation unit, the presence or absence of a water transmission network, and so on. Before deciding on the best desalination method, the site and location must be thoroughly examined.
Although, in recent years, the electricity required for membrane desalination plants in Iran has been supplied from the national grid – at a much lower price than the cost of power generation (including fuel costs, electricity conversion costs, and electricity transmission costs) – it seems necessary to consider large-scale dedicated power generation for water desalination to ensure uninterrupted power distribution across the country. Electricity and water cogeneration methods, as well as desalination approaches using a thermal hybrid method, can be considered in this case.
One of the challenges that thermal desalination systems confront in tropical and temperate countries is the increase and decrease in power usage during the summer and winter seasons, respectively. This is despite the fact that water demand is practically constant. As a result of lowering the producing unit's power requirement, the production of thermal desalination plants (MED) decreases. Such issues might also arise during system inspections, which are performed on a regular basis. As a result, constructing a reverse osmosis (RO) desalination plant alongside thermal desalination plants (MED) can improve water and electricity production flexibility.
Figure: Schematic view of a hybrid desalination plant
Another benefit of hybrid plants is that they lower the water production costs. The reverse osmosis (RO) method produces fresh water with a TDS level of 300 – 500 ppm, which is ideal for drinking; however, the water generated by the MED method has a significantly lower TDS (TDS<10ppm). Therefore, blending MED water with RO water minimizes the requirement for additive injection (additives that convert MED water to drinking water) and, as a result, lowers water production expenses. In this regard, we can consider employing a single pass in the RO method in hybrid plants to generate water with a greater TDS level.
Applying reverse osmosis leads to the increase in water TDS over time. In hybrid plants, due to the high purity of water produced by the thermal method (MED), membranes are replaced after longer periods of time, resulting in lower operating costs.
When compared to the single MED method, hybrid water desalination has a great environmental advantage: the water and salt concentration (Brine) produced by MED have a relatively high temperature that requires cooling to protect the environment. Because RO water has a much lower temperature, combining MED and RO water can significantly contribute to lowering the temperature of the end product.
Furthermore, the brine produced via RO has a high level of concentration. Returning it to the sea in large volumes poses a major risk to ocean life and marine ecosystems; but combining the low concentration brine produced through MED with RO brine will help moderate the water pumped out into the sea.