Zero Liquid Discharge process for Cold Desalination of Seawater, RO, and Super Saline Industrial Waters with Salt Repellent Technique
(WATER FOR ALL: FREE LICENSE FOR MINIMAL USERS)
Summary
- Pollution, the greatest problem faced by Desalination and chemical industries is being solved by the newly invented salt repellent technique.
- A new concept of Repellents for desalting brines have been introduced which removes all the dissolved salts from brines at freezing temperatures.
- The same technique is extended to recover potable water from sea waters, high salt brines, as well as to get boron/ fluoride free water.
- The process is further extended to treat polluted industrial waters by applying an innovative cold zero liquid discharge (cold- ZLD) system to reduce pollution and at lesser cost.
- The process enables to recover 70 to 80% of desalted water that can be used in industries and Agro-cultivation
- Remaining concentrated brine be evaporated by solar heat, eliminating pollution to surrounding streams.
- No washing of ice is required, enables to recover greater yield of potable water and sea salts.
- Process has been patented and know-how is offered on license basis to potential clients
introduction
Water scarcity is a growing concern worldwide, due to increased population, climate change, and industrialization. The best method to meet the water shortage is the use of Desalination of seawater, as inexhaustive quantities are available throughout the year.
Traditional desalination methods, such as distillation and reverse osmosis, are being used widely, but they create more problems, by discharging more concentrated brine back to the sea and polluting the marine ecosystem.
Further, the existing technologies are not satisfactory for desalting super saline waters due to limited effectiveness. The revolutionary Repellent Technique proposed herein offers solution that enables to treatment of highly concentrated brines, resulting in the production of desalted water with minimal environmental pollution.
Freezing of seawater:
When Seawater is cooled, it forms ice, and during freezing, some amount of salt gets trapped inside the ice. The Total Dissolved Salts (TDS) in trapped ice is so high that it cannot be used for human consumption.
Many attempts have been made by different scholars to remove the salt from ice by repeated cycles of freezing, washing, and melting the ice. It was realized that these brine pockets, containing highly saline water cannot be removed by washing ice externally and all such attempts failed and proved to be uneconomical.
Why Repellents?
Recognizing that salts are trapped inside the ice crystals, it becomes essential to remove all salts away from the seawater during the ice formation stage itself. To address this, a small amount of repellents is added. These repellents, during the nucleation of ice crystals, repel salts away from the ice crystals, resulting in the production of salt-free ice.
Salt Repellent Technique - Mechanism Explained:
- Water dissolves all the salts present in seawater.
- In ethanol, the Repellent, none of the sea salts are soluble or sparingly soluble.
- Both water and ethanol are easily miscible and give a clear solution.
- Freezing water to -4°C results in obtaining salt-free ice and concentrated brine. The presence of ethanol with water repels all salts away from the ice-freezing zone.
Freeze DESALINATION OF SEA WATER USING REPELLENTS
The process involves adding the required quantity of Repellents, cooling the seawater to sub-zero temperature to get pure ice, centrifuging and melting the ice to get desalted water. The presence of these Repellents removes all inorganic salts present in water and facilitates to get salt-free ice.
To illustrate the process, a measured quantity of seawater was taken, deaerated and filtered to remove any suspended impurities present. Repellents were subsequently added to the filtered seawater, and the mixture was thoroughly mixed. The prepared mixture was then cooled to -4°C, the point at which ice crystals begin to form.
Due to the presence of repellents in the mixture, the water / ice crystals formed repels all salt ions away from the ice slurry. As a result, the ice that formed are free of salts, making it suitable for producing potable water. The ice crystals are separated by centrifuging the slurry and the ice obtained is warmed to get pure potable water.
The concentrated brine obtained from the centrifuge still contained substantial amount of water. So it was desalted two more times, as above to get more potable water. The resulting product had TDS (total dissolved solids) ranging between 200 to 400 mg/lit in different experiments. The yield of potable water ranged from 50% to 70%.
A flowsheet of the above operations is furnished below:
Desalination of RO, Nano, & Super Saline Waters
Sea water in many Gulf countries as well as the retentates of RO and nano plants contain high dosages of salts and cannot be used for drinking or in industries and are let out without sufficient treatment.
To reduce such pollution problems, experiments were carried out with artificial seawater having a TDS of about 70,000 ppm, mixed with repellents and cooled when ice crystals started to form. Due to the presence of repellents, all ionic salts are driven away from ice crystals and the resulting ice is free of salts, which on warming produces desalted water. Thus, High salt brine was desalted from 69,820 ppm to 240 ppm as shown below.
Table 1: Test Results of Desalination of High Salt Brines( (All Figs. In mg/L))
| Test Parameters | Artificial Sea Water | Desalted Water | |
|---|---|---|---|
|
1 |
Appearance |
Clear Liquid |
Clear Liquid |
|
2 |
Total Dissolved solids |
69,820 |
240 |
|
3 |
Sodium as Na
|
27,456 |
59.4 |
|
4 |
Chlorides as Cl |
37,639 |
103 |
|
5 |
Magnesium as Mg |
1,245 |
4.81 |
|
6 |
Sulphate as SO4
|
2,011 |
24 |
|
7 |
Calcium (as Ca)
|
321 |
7.93 |
|
8 |
Potassium as K
|
846 |
5.9 |
|
9 |
Alkalinity (as HCO3)
|
144 |
31 |
|
10 |
Boron (as B)
|
2.3 |
0.02 |
DESALINATION OF POLLUTED INDUSTRIAL WATERS WITH ZERO LIQUID DISCHARGE - (COLD PROCESS)
Industries such as chemical, desalination, petroleum, mining, thermal power plants, and others generate significant amounts of effluents, leading to water pollution. To address this issue, a method called zero liquid discharge (ZLD) is employed, which utilizes various treatment technologies like reverse osmosis, evaporation, crystallization, membrane distillation, and forward osmosis to minimize discharge volumes and salt concentrations. One commonly used ZLD method involves evaporating reverse osmosis concentrates in multi-effect evaporators, but it requires substantial energy inputs.
An alternative approach is freeze desalination, which operates at lower temperatures and can recover additional water and salts. This Cold ZLD process reduces environmental damage and costs lesser compared to the traditional method.
Zero Liquid Discharge - (Cold Process)
Evaporating RO concentrates in multi-effect evaporators is a popular method of ZLD, but it requires high energy consumption, in the range of 100-300 kWh per cubic meter of water evaporated.
As an alternative, freeze desalting with IS repellents can be used effectively to recover an additional quantity of water and salts, at a lower temperature, as the Cold ZLD process, thus reducing environmental damage and costs.
In the Cold ZLD process, the concentrated brine left out in RO plants is mixed with IS repellents, cooled to frozen temperatures and desalted in three to four stages to recover additional desalted water and residual salts. To prove the effectiveness of the Cold ZLD process, an experiment was conducted with polluted wastewater, as detailed below.
In a typical experiment, dark brown polluted water from a chemical unit of Gujarat with a TDS of 40,000 ppm was used, mixed with IS repellents, and cooled to -10°C to get an ice slurry. Upon centrifugation, clear colorless ice crystals were obtained, significantly reducing the TDS. The concentrated brine from the centrifuge was further desalted, with two additional stages to get desalted ice with lesser TDS values. The ice samples from different stages were consolidated and subjected to desalination once more, resulting in highly purified ice/ water with a TDS of 400 ppm. The super-concentrated brine was evaporated under the sun for two days to get dry salt. The process flowsheet is the same as in Fig 1 above.
Table 2: Test Results of Waste Water(Figs. In mg/L)
| Parameters | Waste Water | Desalted Water | ZLD Precipitate | |
|---|---|---|---|---|
|
1 |
Total Dissolved solids |
38,860 |
415 |
7,34,259 |
|
2 |
Sodium as Na |
10177 |
75.1 |
164300 |
|
3 |
Magnesium as Mg |
983 |
20.6 |
1592 |
|
4 |
Calcium (as Ca) |
324 |
3.24 |
7496 |
|
5 |
Potassium as K |
1630 |
11.1 |
34503 |
|
6 |
Chlorides as Cl |
23286 |
168 |
434853 |
|
7 |
Sulphate as SO4 |
4005 |
6.76 |
1577 |
|
8 |
Alkalinity (as HCO3) |
445 |
71.5 |
2281 |
|
9 |
COD |
1558 |
BQL |
13948 |
|
10 |
pH |
7.29 |
7.15 |
6.44 |
|
11 |
Density, kg/cu.m. |
1.02 |
0.98 |
0.99 |
|
12 |
Colour |
Yellowish |
2 |
- |
|
13 |
Turbidity |
410 |
BQL |
- |
|
14 |
Boron |
2.36 |
0.38 |
169 |
|
15 |
Moisture |
- |
- |
16.60% |
The results demonstrate a significant decrease in the TDS value from 38,860 ppm to 415 ppm, COD from 1,558 ppm to below quantifiable limit (BQL), and Boron from 2.36 ppm to 0.38 ppm. Additionally, the effluent’s yellowish colour was completely removed, making the water stream suitable for reuse. The analysis of the precipitate reveals the accumulation of all pollutants, which can be utilized for the recovery of valuable chemicals.
Thus, for the First time, an innovative Cold Zero Liquid Discharge (Cold ZLD) system has been developed for treating polluted industrial waters to reduce pollution, raw material recovery and substantial energy conservation. It facilitates the recovery of 70% of desalted water for reuse in industries and compliance with regulatory requirements.
This novel Cold ZLD technology requires no pre-treatment, regardless of % salinity of salts in the brine, less power consumption, simpler operations and less corrosive equipment are required.
Boron Removal
Although boron is essential for plant growth, high concentrations in the soil can damage sensitive crops and disturb in human reproductive systems and removal of boron from water has become a necessity. The Freeze desalination experiments conducted with IS repellents reduced the Boron level from 2.3 mg/L to a remarkably low level of 0.02 mg/L which is well below the requirements set by WHO. The results are given in Table 1 above.
Cost of desalination
While other desalination processes require energy to remove 97% of the water to obtain potable water, freeze desalination focuses on removing only 3% of salts from the sea water. The consumption of chemicals in pre-treatment and other operations will be much lower.
In Freeze Desalination, sea water is frozen to a slushy ice by direct contact with boiling butane, which is nearer to freezing point of sea water. The heat required for vaporization of butane (165 btu/lb) Is obtained by the cooling the water (144 Btu/lb) to ice. Consequently, only marginal heat is required to freeze sea water to ice.
The cost of desalting depends mainly on the salinity of feed water and the energy cost at the site. For a 10 million gallons/ day Freeze Desalination of RO water, the energy consumption is estimated at 5.5 to 8 kwh/ m3. Further pilot plant studies are required to establish the energy consumption. Considering other benefits, Freeze desalination may be considered as an alternative process for desalination of saline waters.
To offset the high operating costs, brine mining is foreseen which provides high-purity chloride, bromide and iodide salts, all of which are in great demand. Thus, setting up an integrated Desalination plant with a downstream brine mining unit will result in zero cost for water, less pollution and recovery of valuable chemicals.
Benefits of Repellent Technique
- No pre-treatment of seawater is required.
- Only minimal amounts of repellents are used to repel 3% of salts, in contrast to voluminous chemicals used on the entire 100% of the water for pH adjustment etc.
- Suitable for high salt content brines with no limitation on salt content of brines. Waters of textiles, leather industries, Mines, Petro wells can be used.
- Simpler machinery like freezers, centrifuges, and Heat exchangers are used and project cost will be much lower.
- Lesser corrosion due to low temperature of operation. Inexpensive materials of construction, such as Engineering Plastics can be used
- Smaller plants of reduced capacity can be set up.
- With Cold ZLD units, the unit can be located away from the sea, as well.
Process Know-How
Take Action Now for Pollution Elimination!
Freeze desalination has not been used so far due to the challenge of removing the last traces of salt from the ice formed. However, with the introduction of repellents, a significant breakthrough has been achieved in getting desalted ice from saline water. This development removes a major obstacle in the advancement of freeze desalination technology. Patents filed by us on our above findings have been approved recently by US, India and PCT authorities.
Due to prolonged lockdowns and travel restrictions, our plans to establish a pilot plant for producing potable water with TDS of 500 ppm or less were postponed. Contacting machinery suppliers became challenging under these circumstances.
In view of the above, we offer our process data under two groups
Water for All
Water is an indispensable resource for people worldwide, including small-scale industrial units. Recognizing its vital importance to the general public, we propose to grant a Free open license for the utilization of our patented technology, for water usage up to 10,000 Liters per day.
As mentioned, we provide process know-how and we recommend small consumers to contact project consultants for implementation and commissioning.
For Bulk consumers
For industrial units and bulk consumers exceeding the aforementioned usage limit, a nominal royalty fee will be applicable. This fee will support our ongoing R&D efforts and facilitate for further advancement in this technology. Further, we offer testing of your water and recommend suitable IS repellents on a cost basis.
In addition, for bulk users, we have prepared a detailed Feasibility Report detailing the Salt Repellent Technique, along with comprehensive test reports, to cover
1, Desalination of Sea Water
2.Desalination of RO, Super Saline Waters, and
3.Zero Liquid Discharge for Polluted Industrial Waters (cold process).
Interested firms can access this comprehensive report for detailed insights.
By providing access to the Process Technology on a licensing basis, interested parties will have the opportunity to scrutinize process intricacies, evaluate the compatibility of technology with their water quality, and obtain essential design data for setting up a pilot plant. This strategic shift will significantly mitigate costs associated with acquiring desalinated water.
We also extend an invitation to EPC firms to utilize our process know-how for implementation on behalf of their clients, catering to both small and high volume users.
For further details, please contact us : gopirs@seawater.in
We sincerely await your early response.