Most membrane processes in the UK, such as reverse osmosis (RO) and nanofiltration (NF), are designed for industrial applications such as process water, high-purity water and boiler feed water. With increasing demands for potable water, several utilities are investigating RO or NF treatment of brackish or seawater sources. Anglian pilot tested RO of seawater and Essex & Suffolk pilot-tested brackish well water near Dagenham as a potable water supply. Thames is also evaluating RO membranes for treating water from the Thames estuary.
All membrane plants produce a concentrate, or brine, stream formed by the total dissolved solids (TDS) removed from the feedwater. The character of this stream will depend on the source water quality and the product water recovery of the system. Brackish water recoveries typically range from 50-80% and seawater recoveries from 30-50%.
TDS are a conservative pollutant which removal processes merely concentrate into a smaller side stream. TDS are never destroyed. Changes in water quality standards are making brine disposal to water bodies other than the ocean much more difficult. Consequently, brine will become a very expensive residual requiring proper disposal.
Surveys on brine disposal conducted in the US indicate the most typical method of discharge is to WwTWs, where the impact can be severe. The high TDS concentration can hinder settling as a result of changes in wastewater density, inhibit the biological treatment process and cause aquatic toxicity, which may limit the options for disposal or reuse. In addition, the discharge permits may limit TDS or specific ions. High TDS brines can also aggravate corrosion of the wastewater collection system and plant process equipment.
The most common alternative, disposal to the sea, might not be an option due to location or regulatory limits. Therefore alternative methods for treatment and disposal of brines need to be examined. Traditional alternatives include evaporation ponds, deep-well injection and mechanical evaporation. Evaporation ponds have traditionally been used to reduce the volume of the brine concentrate to the point of crystallisation, so it can be handled as a solid. This practice, because of its large site requirements, is often practicable only in arid climates. Deep-well injection and mechanical evaporation are capital-intensive and have high operation and maintenance costs. Alternative brine treatment technologies include:
- freeze techniques – directional freezing of brine concentrating the non-frozen portion of the liquid, resulting in desalted ice fraction,
- mechanical evaporation – use of mechanical energy to accelerate the evaporation process and to reduce volume,
- beneficial reuse of the brine – using the brine stream as a sodium source for on-site sodium hypochlorite generation,
- biological degradation – for contaminant, nitrates for example, removal prior to discharge.
Freeze techniques use the fact the freezing point is reduced in proportion to TDS concentration, with an approximate 2°C change in freezing point per mole of ions in solution. The trick is that freezing does not occur simultaneously throughout the solution. During the freezing process, lower TDS areas in the solution freeze first, at higher temperatures, concentrating the TDS in the remaining liquid. Consequently, the freezing process can produce relatively pure water in the ice/slurry phase and concentrated brine in the un-frozen liquid phase.
Freeze concentration has broad use in the food and chemical process industries but has not typically been used for RO brine separation. One of the challenges associated with brine separation is the presence of minerals in the brine feed, especially calcium, that can precipitate and foul the freezing surfaces. Currently available freeze concentration equipment is not designed for the unique constraints associated with brine concentration.
In recent tests on brine, the freezing process was taken to completion, generating a clearly visible block of high TDS ice, which could then be sliced from the remaining block and disposed of. In this demonstration, the TDS was raised from 700mg/l in the feed to 3,500mg/l in the concentrate. The concentrated brine usually requires additional treatment, evaporation or mechanical separation, for further volume reduction prior to disposal.
Capital and O&M costs for full-scale municipal freeze concentration installations are not readily available. A recent estimate for brine flows of 4,000m³/d showed capital cost around £7.617M and an electricity consumption of 100,000kWh/d.
Mechanical vapour recompression (MVR) is a thermal desalinisation process that can significantly reduce brine volume. MVR uses both heat and mechanical energy, as compared to conventional evaporation processes that use only heat to separate water from other constituents. In MVR, an innovative heat transfer system recovers and recycles heat energy while evaporating water. At reduced pressure and temperature, less energy is needed to raise the water to its boiling point. To make the process affordable, the MVR process uses heat exchangers to recover the heat from the condensing water to vapourise water from the brine, and other heat exchangers to recover energy from the condensate.
As the heat energy is transferred from the water vapour to the brine, the water vapour cools, condenses and is removed from the system as clean water with TDS concentrations typically less than 10mg/l. The concentrated brine remaining is collected and removed for additional treatment or disposal. MVR processes can recover up to 98% of the influent brine as clean product, reducing the concentrated brine volume accordingly.
Evaporating water at a lower temperature has several major benefits which include a reduction in scale formation with a concurrent reduction in the rate or severity of heat exchanger fouling. The lower temperature creates less corrosive conditions to reduce construction materials cost. Scale formation and heat exchanger fouling are major causes of poor MVR performance.
In traditional evaporation systems, steam serves as the source of energy. MVR uses several methods to reduce steam consumption – specifically by boiling the brine under a vacuum, conserving thermal energy by transferring heat from the clean water vapour to the influent brine and by using mechanical energy to increase the temperature of the vapour. Steam is required only for system start-up with electricity as the energy source thereafter.
While the MVR process is more energy efficient than conventional evaporation, it is still capital intensive and has high energy costs. An MVR for a 20Ml/d RO plant was estimated to have annual operating costs of £1.269M and construction costs of £12.695M. These costs do not include additional treatment and disposal of the concentrated brine produced by the MVR.
Chlorination is extensively used for wastewater disinfection prior to discharge or reuse. The use of liquid sodium hypochlorite (NaOCl) is increasing, as gaseous chlorine is being replaced due to safety concerns associated with transport and storage. Liquid sodium hypochlorite is available in strengths of 10-15% chlorine, however, storage is limited to less than 2 weeks to minimise solution degradation. On-site hypochlorite generation eliminates the need for long storage times.
Hypochlorite generation is a well-established technology that is commercially available and a beneficial reuse of waste flows. The NaOCl is created when an electric current is applied to a brine solution. Currently available generation equipment uses a brine solution composed of water and food-grade salt. However, RO brine is being investigated to determine its suitability as a replacement for purchased sodium chloride. In addition to eliminating disposal requirements, using RO brine as the salt source for hypochlorite generation may significantly decrease operating costs. Initial comparisons of operating costs show a 50% decrease by eliminating salt costs. Pilot facilities have been operated in Florida demonstrating the viability of this process.
Since generated hypochlorite tends to have higher bromate concentrations than purchased hypochlorite, it can not be used to disinfect potable water in the US due to US Environmental Protection Agency Disinfectant/Disinfection By-Product Rule restrictions. Consequently, an RO facility must find alternate uses for the hypochlorite. Possible uses include transport to a WwTW plant for effluent disinfection or sale to nearby industrial users. In addition, hypochlorite generation is energy intensive, requiring approximately 2.5kWh/453g of equivalent chlorine per day. Therefore, increases in electricity rates can significantly affect the economics of hypochlorite generation.
Some brines will contain high concentrations of contaminants, perchlorate or nitrate for example, that limit the disposal options. Biological degradation of specific contaminants in a brine stream may be considered as one treatment step in the disposal process. Although this technology has been tested for perchlorate and nitrate removal from ion exchange brines, its application is still in its infancy
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