Forward Osmosis Tech For Wastewater Reuse
By Dr Xiaodong Wang 17 April, 2020
Dr Wang from Qingdao University of Technology introduces a competitive & energy efficient solution for desalination & wastewater use
Drinking water resources have become a severe factor affecting social-economic development. Conventionally, drinking water is produced mainly from safe water sources, i.e. surface water, groundwater water. However, surface water pollution and declining urban water levels have reduced the availability of water resources.
Unconventional drinking water sources such as seawater and wastewater has been considered as alternatives to natural freshwater. For example, seawater desalination is being used by several countries to satisfy freshwater demand. Despite advances in reverse osmosis membrane technology, seawater desalination via reverse osmosis is facing three main challenges: (1) energy efficiency, (2) net water recovery, and (3) generation of pre-treatment sludge. Domestic wastewater contains many molecules with varying diffusivities. Wastewater reused for drinking needs sound proof pathogen elimination and better water recovery.
Water recovery can be increased 45 % using a hybrid FO/RO system
Enter forward osmosis tech. Forward osmosis (FO) has been increasingly studied in the past 10 years, which is most often used as a pre-treatment prior of reverse osmosis (RO) desalination. Studies have shown that water recovery can be increased up to 45 % by using a hybrid FO/RO system (Liyanaarachchi et al., 2016). Since the driving force of forward osmosis process is the osmotic pressure difference, the high salinity of seawater leads to limited flux.
Forward osmosis is a biological term describing plants drawing water from soils: water molecules pass through a semi-permeable membrane, at the same time ions and particles are retained by the membrane.
Forward osmosis process in water treatment is a natural process for highly concentrated water to draw water molecules from less saline water. Sodium chloride has been employed as the draw solution due to its low cost and solubility. The concentration is usually between 0.3-6M, but it is often used at the concentration of 0.5M to simulate seawater as the draw solution. The advantages and disadvantages of FO membranes are listed below:
- Good water quality, removal of pathogens and micropollutants as well as particles
- Energy efficiency
- Adapted to industrial wastewater
- Reduced membrane fouling and simple cleaning compared with RO membrane
- Low water fluxes
- Reverse solute leakage
- Concentration polarisation of saline solution on both side of the membrane
A new commercial FO membrane is available
A new commercial FO membrane is available in the market and is set to help solve challenges currently faced by seawater desalination.
A general framework for unconventional water resources
Seawater is abundant in coastal areas and this is where an integrated freshwater production solution can be implemented – from which water can be produced from a combination of treated wastewater and seawater. This new energy-efficient freshwater production approach is illustrated below.
In coastal areas an integrated freshwater production solution can be implemented…
…from which water can be produced from a combination of treated wastewater & seawater
The effluent from wastewater treatment plant which has passed through secondary or tertiary treatment goes to forward osmosis process as the feed flow, while sea water is used as the draw flow. The water transport will happen naturally from the feed slow to the draw flow, while particles, pathogens and other pollutants will be retained in the feed side. The seawater would be diluted by the water molecules during this natural process. The diluted seawater can be further used for reverse osmosis desalination. The pressure and energy consumption will be reduced because seawater ion concentration has been reduced during the FO process.
The hybrid FO/RO system for reusing wastewater and seawater has proved technically and economically feasible in studies. The maximum FO flux can reach as high as 13.2LMH, and total salt rejection rate can reach 99%. The concentrated water from RO can be reused as the draw solution for FO process.
The energy consumption of the hybrid process is much lower compared with conventional reverse osmosis process…
…the overall cost is 0.35 – 0.70 USD/m3
With proper pre-treatment of the feed water, such as using ultrafiltration, nanofiltration or ion exchange, the energy consumption of the hybrid process is approximately 0.90 kWh/m3, which is much lower compared with conventional RO process (3.02 kWh/m3). The overall cost of applying the hybrid system for freshwater production is 0.35 – 0.70 USD/m3. Therefore, this hybrid FO/RO system can be a competitive solution for both seawater desalination and wastewater reuse with high recovery, lower cost and higher water quality.
Requirements & limitations for FO to reach full scale application
Internal concentration polarisation (ICP), membrane fouling and low flux are the three main issues limiting the large scale application of forward osmosis as well as hybrid FO/RO. ICP is exclusive for RO membrane happening within the porous of the supporting layer of membranes. ICP may further lead to membrane fouling and reduce the flux. Study on ICP prevention is necessary. Some hybrid system utilise ion exchange as an intermediate solution to reduce ICP.
Further treatment of the WWTP effluent will reduce membrane fouling. Ultra-filtration and ion exchange are necessary in some cases to reduce particle and TDS.
A higher osmotic pressure is essential to make the water flux. The hybrid FO/RO process can be used in coastal areas but seawater erosion of the wastewater sewer system will affect the application of FO significantly. In some of the coastal cities in China, the Cl- concentration can reach 1500 – 2000 mg/L due to the erosion of seawater to the sewer system. As a result, the osmotic potential is reduced which therefore lead to low flux of the RO process.
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