Issues with brines and alternatives
Brine is a water solution with a very high concentration of salt or sodium chloride.
The term also applies to solutions with other salts. The most common applications known to consumers are highway dicing or as refrigerant in the refrigeration industries.
However, the brine generated as waste in industrial processes is a difficult problem that industries are facing. Uncontrolled discharge of these effluents may cause a negative environmental impact.
Its treatment poses numerous difficulties and purification of saline wastewater is not very effective by means of the technology usually employed: physicochemical or biologic treatments, etc.
Saline effluents must be properly managed. There are different solutions depending on factors such as flow rate, geographic location and whether or not there are other pollutants apart from salts, etc.
Different alternatives, such as membrane separation technologies, do not provide the desired results in the case of brine treatment.
This is mainly due to the high amount of rejects generated and the inconvenience caused by the presence of organic contaminants in filtration membranes.
Zero Liquid Discharge systems provide multiple advantages: it is the most respectful option for the environment, does not produce any discharge, generates an effluent of high quality water (this high quality water can be reused in the production process), and the concentration of salts can be crystallized with the option to reuse.
As the variety of industries that generate saline effluents is broad, the most representative are discussed below:
Desalination of seawater
Desalination involves obtaining fresh water for human consumption or industrial or agricultural use from seawater or brine.
This practice has become widespread in those regions with water deficits, where supply cannot therefore be guaranteed, over the past few decades the intensive production of desalinated water at a moderate price is currently possible, thus meaning that this solution is applied in many cases to solve water supply problems.
According to UN Water, the inter-agency mechanism for all water-related aspects at the United Nations, in February 2014 there were more than 16,000 desalination plants worldwide, with a production capacity of 70 hm3/day.
Irrespective of the technology used to achieve desalination, a fresh water flow and a waste or rejection flow are generated in all cases. The latter contains a high concentration of salts, which depends on the raw water being desalinated and the yield of the separation, which itself depends on the technology used.
Those techniques that result in a high separation yield generate a rejection flow with a very high salt concentration, and vice versa.
The textile industry is characterized by high water use, and this water must be of high quality.
The water used, whether obtained from the distribution network or from other sources, is subjected to a purification, normally a softening, process.
Ion-exchange resins, the regeneration of which generates an effluent with a high salt concentration, have traditionally been used to eliminate water hardness.
Moreover, high salt concentrations are required in the medium used for textile fiber dyeing process to ensure that the pigment is fixed to the fabric. These dyeing waters have a high salt content even after treatment.
Municipal solid waste (MSW) landfill sites generate leachate effluents, which must be treated so that they can be discharged with no environmental impact.
Generally, after various processes, the treated effluent is subjected to a reverse osmosis process in order to obtain a pure water flow that can be reused or discharged and a smaller flow containing the concentrated contaminants.
This effluent has a high salt concentration as it contains all the salts originally present in the leachates.
Salting and brine-preservation techniques have traditionally been used to ensure that foods can be stored for long periods of time without being attacked by microorganisms.
The brines are usually prepared from cold water, sodium chloride, sodium nitrite and flavouring substances. In order for the brine to have a preserving effect, the salt concentration in the product must be between 15% and 20%.
As a result, the salting and food preservation industries in general produce effluents with a high salt concentration.
The preparation of pickles (olives, gherkins, carrots, onions, etc. marinated in brine and vinegar) is an activity that generates effluents with an organic burden as well as high salinity.
These effluents must be treated prior to discharge and it is advisable to recover the maximum amount of water possible for reuse in the process.
Effluents from water-treatment plants
A wide variety of industries need a source of high quality (ultra-pure) water for use in their production processes, especially the pharmaceutical, food and textile industries, etc.
Such industries typically use ion-exchange resins to soften water or membrane-based processes (nanofiltration or reverse osmosis) for more complete treatments.
The effluents generated by these processes concentrate all the salts and impurities removed from the crude water. A high-water consumption in the process generates high waste effluent flows characterized by a high concentration of dissolved salts.
Leather tanning industry
The leather tanning industry is characterized by its high contamination potential due to both the reagents used and the effluents generated in the different processes.
Generally, the processes used in the tanning of animal skins are salting (with NaCl), softening (using sodium sulfide, sodium polysulfide or sodium carbonate), unhairing (using sodium sulfide, sodium hydrosulfide, amines, calcium hydroxide and caustic soda), liming (in a bath of caustic soda), deliming (using hydrochloric acid, sulfuric acid, boric acid, ammonium chloride, ammonium acetate and cyclic esters), pickling (chromium salts and formaldehyde), tanning, lubrication, drying, conditioning and finishing (using dyes and aniline).
The chemicals used in the different processes are incorporated into the wastewater as they are used.
The technologies used in these processes are increasingly clean, water-saving and reuse effluents, thereby reducing final contamination of the water.
Finally, treatment of the waters removes most of the contamination. However, the dissolved salts present in the effluents are not removed, thus meaning that they remain unaltered upon exiting the treatment plant and these waters have salt concentrations of up to 10,000 mg/L.
As a result, these waters, with this salt content, cannot be discharged into either the sewage network or public watercourses.
Treatment of water for energy-generation plants
Energy-generation plants need water of the highest quality to operate. This water is transformed into high temperature steam, which moves the alternator.
The ultra-pure water used is generally obtained by subjecting water from the distribution network, or from other sources, to a treatment process.
This process generates a waste effluent that concentrates all the impurities removed from the water. Such effluents are characterized by their high salt concentration and must therefore be treated prior to discharge.
Gas and oil wells
The gas and oil industry are also capable of producing significant brine effluents. Many gas and oil wells are found close to seams of rock salt.
The technique used to extract oil comprises drilling wells into which fresh water, which dissolves the salt and returns to the surface as a brine, is injected.
Oil is recovered by displacing it towards the surface by injecting water or brine. The excess brine must be treated or discharged into the sea in the case of an underwater field.
Brine management is not a simple task in the majority of cases. Factors such as flow rate, geographic location, the presence of contaminants other than salts, etc. all affect the choice of treatment technique.
The only solution in many cases is the treatment of brines, although other management routes may be available depending on the characteristics of each case.
Deep injection (DWI)
The deep-well injection (DWI) technique involves injecting the waste liquid into the subsoil via a deep well. It can be used to manage both brines and other liquid wastes provided that there is no environmental impact on the subsoil.
This is the case under the following four conditions, which are necessary and sufficient:
- There is a permeable formation able to take the waste.
- There is an impermeable formation that keeps the waste trapped for a sufficient period of time until it -becomes harmless.
- The conditions of both operations do not change during the operation.
- The DWI operation does not compromise other, more important resources.
As such, this management technique is only viable when these four conditions are fulfilled and when the brine flow is sufficiently high to justify it on economic grounds.
The technique of confining brines in pond is an option used in dry regions where sufficient land is available. The surface area and minimum depth of the pond can be designed based on the brine flow.
One of the main drawbacks of this technique is environmental contamination of neighboring aquifers due to the possible leakage of leachates.
Recovery of valuable products
Another means of managing brines is to treat them to obtain sodium chloride, calcium sulfate, magnesium hydroxide and calcium chloride using different sequential evaporation processes.
This option can be used when the brine is of marine origin and production thereof is moderate.
Treatment of brines using Zero Liquid Discharge systems(ZLD)
This option tends to be the viable management alternative in the greatest number of different situations, can be adapted to any brine production scale and is undoubtedly the most environmentally friendly.
The aim of this systems is to covert the saline residue into a flow of high-quality water and salts in a solid, crystalline form.
This water can be reused in the process itself due to its high quality, or in any other application, and the crystallized salts can be managed for possible recovery.
Depending on the initial concentration of salts in the brine, treatment comprises initial concentration of the effluent by reverse osmosis. If the concentration of the brine is already high, the reverse osmosis step can be omitted.
The concentrated brine is then subjected to a vacuum-evaporation process in which it is concentrated yet further and which generates a water flow that can be mixed with that produced by reverse osmosis.
The management of the concentrate is the most compromised aspect of the treatment. The aim is to minimize it as much as possible in the most cost-effective way, as the final destination will be a waste.
If it is possible, the salts are obtained in a solid, dry and crystalline form by way of a crystallization process. These salts can be recovered for use in road gritting, resin regeneration, etc.
The reverse osmosis process can be replaced by an electrodialysis system, which would also allow the brine effluent to be concentrated and produces a flow of water with a very low salt concentration.
If a residual energy source is available, this can be taken advantage of in the vacuum-evaporation process, thereby giving excellent results at a highly competitive price.
Then, vacuum evaporators have proven to be, by far, the most successful technology for the treatment of this type of wastewater, according to ZLD system.
On the one hand, this system enables the highest degree of concentration possible, up to the point of salt drying up completely; and on the other, it generated purified effluents, which are compliant with usual discharge limits, because of their extremely low conductivity and organic contaminant contents.
In any case, vacuum evaporators already have a proven track record for effectively treating brines generated by the following industries:
- Food industry
- Hams and prepared meat products
- Meats, canned fish and shellfish, fish farms
- Pickles, olives and other pickled foods
- Lupines, pine nuts, almonds, and other canned vegetables
- Animal offal
- Chemical and pharmaceutical industry
- Leather tanning industry
- General industry: rejects from reverse osmosis