Editor’s pickMyths and misconceptions about SWRO desalination in India

29-11-2019
myths and misconceptions,desalination,India

Water stress in different regions of India have by and large led to the acceptance of Seawater Reverse Osmosis (SWRO) desalinated water as an alternate solution to alleviate drought. However, myths and misconceptions still exist, and this article seeks to address these misconceptions and provide clarification.

MYTH: Desalinated water from Municipal Desalination Plants do not have enough minerals and use of desalinated water depletes minerals from the body
SWRO plants are designed to be flexible, with modular and multistage arrangements to produce water of any required specification – potable water produced by the plants are of high quality, reliable and free from bacteria and viruses. Potable water from these SWRO plants are also designed to meet ISO 10500 standards, or the World Health Organisation (WHO)’s specification. Plants can also produce high-purity processed water for industries with Total Dissolved Solids (TDS) of <10mg/l.

The typical analysis of selected potable water parameters from a desalination plant are as shown below:

Hence, it is not true that potable water produced by the municipal SWRO plants do not have minerals.

MYTH: Desalination plants kill fish and other marine life in the region
In designing desalination plants, protection of marine life is given priority. The intake is designed to provide a large flow area so that water enters the intake velocity head at very low velocity, so fish, fish eggs and other forms of marine life are not drawn inside. The intake velocity head is located deep in the sea, so the protection of marine life against being drawn along with seawater is ensured. A typical intake velocity head arrangement in illustrated in figures 1 and 2.

Figure 1 : Intake velocity head (Bar Screen Type)  

Figure 2 : wedge wire type Intake screen

Desalination plants ensure that only saltwater is discharged without any impurities or chemicals, and chemical cleaning solutions are neutralised before being discharged into the sea. Many plants remove sludge from backwash water before the water is discharged. The outfall discharge is located far away from the shore, deep in the sea with the multiple nozzle diffuser design to recirculate seawater and dilute brine salinity so that it is closer to the salinity of seawater at a radius of 50 metres, and marine life is not affected. Environmental protection agencies also monitor the quality of the seawater, close to the outfall.

MYTH: Desalination Plants increase salinity of well water close to the shore
In terms of percentage of low-salinity water recovered from seawater, recovery falls within the range of 40-45%. The salt rejected from the 45% low-salinity water separated in the RO plant is returned to the sea as brine along with 55% seawater. However, the brine gets diluted in salinity, and it comes close to the salinity of seawater at a radius of 50 metres from the discharge point. Hence, the salinity of the seawater near shore remains unaltered.

MYTH: Desalinated water is very expensive
The cost of water produced by desalination has come down considerably in the last 30 years due to developments including membranes with higher productivity, innovation in energy recovery devices, economy of high-capacity units, and more efficient higher-capacity pumps.

New desalination technologies are bound to further reduce the cost of desalinated water – for example, the cost of electricity (which forms >75% of the O%M cost of an SWRO plant in India) can be brought down further by using green energy, as compared to power from a thermal power plant.

MYTH: Desalination consumes more energy
In the 1980s, SWRO consumed approximately 8.5 to 9 kWh/m3 of water produced, with a smaller plant capacity and plant recovery in the range of 30-35%. Today, innovations in energy recovery devices such as RO membranes with high flux and high-capacity plants with more efficient pumps have brought energy consumption down to 3-3.5kWh/m3.

Research is currently being concentrated towards efforts to develop new desalination technologies like forward osmosis and the membrane distillation process, and working with advanced membrane technologies such as Aquaporin & graphene to bring power consumption down.