Environmental Aspects of Concentrate Disposal

The amount of matter available on our planet is limited and final and has been so from the time of its formation (barring solar radiation and a few meteorites). Nature operates by the continual recycling of materials, including that of water.

The water cycle is a slow one, taking thousands and even millions of years. In principle, each drop of clear water which we consume has passed through the whole cycle. The natural cycle involves three major energy consuming processes: evaporation with the aid of solar energy, evaporation with the aid of thermal energy and biological processes whereby membranes and biological pumps utilise chemical energy.

In this modern world where over six billion people are crowded onto the surface of the earth, the right to live a quality life is being recognised as a basic human right, it is our duty to look for alternative recycling processes that would complement and replace the natural ones. The estimated deficit in the Israeli water budget as estimated in the year 2001 is 200-500 million m3 per year. A desalination plant such as the one which is presently being built in Ashkelon is intended to supply 100 million m3 water per year and would replenish a quarter of the estimated deficit. Additional desalination plants are being planned for Hadera, the Haifa Bay area and the south of Israel.

Seawater desalination processes contribute positively to the environment and to humanity but at the same time they are accompanied by some negative impacts on the environment which may be minimised by proper planning. Most of these impacts are local by nature.

Desalination may impact on five domains: the use of the land, the marine environment, the intensified use of energy, the groundwater, and noise pollution. The impact on land use is caused by the use of the coastal land for the purpose of building factories, thus converting the coastal area into an industrial zone instead of an area of tourism and recreation. The impact on the marine environment takes place mainly in the vicinity of the concentrated brine discharge pipe.

Even though the concentrated brine contains natural marine ingredients, its high specific gravity causes it to sink to the sea floor without prior mixing. In addition, chemicals that are added to the water in the pre-treatment stages of the desalination process may harm the marine life in the vicinity of the pipe’s outlet. The actual placement of the discharge pipe may also damage sensitive marine communities. A desalination plant may also affect the environment indirectly, such as via the intensified use of energy by the plant.

This increased use of energy results in an increased production of electricity by the respective power station, which in turn results in increased air pollution, pollution by coal dust, thermal pollution, etc. The impact on groundwater mainly occurs if pipelines carrying seawater or brine are laid above an aquifer. It also occurs in the case of feed drilling. In such instances the aquifer may be damaged either by infiltration of saline water or by disturbances of the water table. A desalination plant may also generate noise pollution as the process of reverse osmosis requires high pressure pumps which generate noise.

Therefore the plant must be located at a suitable distance from population centres. Technological means may be employed in order to minimise noise intensities7.

The main impact on the environment is due to the discharge of the concentrated brine to the sea, and its magnitude depends on environmental and hydro-geological factors characteristic of the sea: bathymetry, waves, currents, depth of the water column etc. These factors would determine the extent of the mixing of the brines and therefore the geographical range of the impact.

Hopner and Windelberg10 divide the global marine habitats into 15 categories according to their sensitivities to the effects of desalination plants. According to the hierarchy that they suggest, the sites most suitable for the construction of desalination plants are regions of high energy oceanic coasts (no. 1). The most sensitive regions (no. 15) are Mangal, mangrove flats. Because of the diversity of species characteristic to them, Coral reefs are rated at 13.

Composition of discharge brines

In all processes of desalination, discharged brines, the concentration of which is higher than that of the natural seawater, are returned to the sea. The concentrations of the brines are usually found to be double or close to double that of natural seawater. In addition to the high concentration of salts, this discharge water contains various chemicals used in the pre-treatment stage of the desalination process, including various de-fouling materials. In the case of evaporation plants, thermal pollution is also produced.

The types and the amounts of the chemicals used depend on the chosen technology and the required quality of the product water. Chemicals that are likely to be found in the brines include anti-scaling materials, surfactants, and acids used for the lowering of pH. The salts returned to the sea are identical to those present in the feed water, but they are now present at a higher concentration. In plants of reverse osmosis, the discharge concentration is 30-70 %, or 1.3-1.7 times that of the original seawater. This is a higher concentration than the one found for MSF plants where the return ratio is 1.1-1.5. 13

The chemicals used in the pre-treatment of seawater are mainly: NaOCl or free chlorine, used for chlorination, preventing biological growth (anti-fouling); FeCl3 or AlCl3, used for the flocculation and removal of suspended matter from the water; H2SO4 or HCl , used for pH adjustment; SHMP (NaPO3)6 and similar materials which prevent scale formation on the pipes and on the membranes; NaHSO3, used in order to neutralise any remains of chlorine in the feed water.

All these materials (in concentrations and amounts which are similar to those used in desalination plants) are approved for use by the American E.P.A. and most of them are used in drinking water systems. Furthermore, most of these materials naturally dissolve in seawater and may contribute ions identical to the ions already present in seawater.

Cleaning of the membranes is conducted three or four times a year, and the chemicals used are mainly weak acids and detergents (citric acid, sodium polyphosphate and EDTA which is used in order to remove carbonate deposits).

The rinse water is kept in a titration container and after being treated (titration, neutralisation of the cleaning materials), it is disposed off either by transporting it in closed containers to an authorised salt disposal site, or by the continuous flow of small quantities together with the discharged brine back to the sea. The high dilution ratio (about fifty to one million) ensures very low concentrations of rinsing materials in the brine returned to the sea.

It should be noted though that the highly concentrated brines may increase the water turbidity especially in view of the fact that part of the added reagents, mostly those that contain iron, are dark black or red. This type of pollution is referred to as ‘optical pollution’. In areas where living creatures are present under the dark and turbid plume, the reduction in light may hinder the process of photosynthesis. The environmental sensitivity to optical pollution varies from one habitat to the next. This pollution carries more significance in clear water seas and less so in the turbid water of the easter Mediterranean sea.

Dispersion of the concentrated salts

The major environmental problem associated with a desalination plant is how to get rid of the surplus of concentrated brines. In most cases, these brines must not remain on land because of the danger they pose to the underlying groundwater and because of other potential and severe environmental impacts.

A natural disposal site for these brines is the sea, but an appropriate technology is required in order to insure the proper dispersion of the concentrated solutions and thus minimise their adverse effects on the marine environment. Several alternative techniques are available for this purpose, and the choice between them would depend on the particular conditions in the area, taking into consideration the environmental, engineering and economical aspects.

The alternative techniques are: Discharging the brines by a long pipe far into the sea; Discharging the brines via the outlet of the power station’s cooling water; Directing the brines to a salt production plant and (though not recommended) Direct discharge of the brines at the coastline.

Discharging the brines by a long pipe far into the sea

As the brines would be routinely returned to the sea, they would form a plume of highly saline water, corresponding to their amount and to the conditions of the sea (depth, bathymetry, currents, etc.). This plume would sink to the sea floor and its effects would extend over a range of hundreds of metres.

As this presents a continuous and cumulative source of pollution, it would result in a continuous damage to the biota within the plume’s vicinity. It is therefore desirable to place the point of brine discharge far away from the coast and from rocky areas which are rich in organisms, as well as far away from areas where a large number of people are involved in activities such as recreation, touring, fishing etc.

A possible additional measure would be the Installation of diffusers on the discharge pipe, in order to boost the process of natural (turbulent) dilution. The diffusers enable an increase in the pressure of the entering solutions and also increase the volume of seawater in contact with the brine, therefore improving the mixing. The success of the diffusers operation depends on their number and on their spacing. It is possible to improve the dispersion efficiency by using special diffusers, such as “Red Valve” diffusers. These boost the brine pressure at the outlet of the discharge pipe and thereby improve the dilution. Another option is the use of diffusers directed at an angle of 30°-90° to the sea floor, so that the concentrated brine is pushed in the direction of the surface of the sea 2 .

Prosperous The main effects on the marine biota would be in the vicinity of the discharge pipe and would be related to the increase in the concentration of salt. This would mostly affect benthic organisms dug in the sandy bottom as well as planctonic organisms. The salinity is expressed in weight of salts per litre (‰) and in most seas and oceans its value varies between 32‰ to 38‰, which is the range to which most marine creatures have adapted. The eastern part of the Mediterranean is more saline than its western part. In the Red Sea salinities may reach a value of 41‰.

Marine organisms exist in an osmotic balance with their environment and an increase in the concentration of salts in this environment may result in the dehydration of cells, decrease in the turgor pressure and death (mainly of the larvae and young individuals). The biomass in Israel’s Mediterranean coasts is composed of species that have originated from Pacific and Atlantic species. The Atlantic species, found in the Eastern Mediterranean, are at the limit of their tolerance to the water’s salinity, while species that have originated in the Pacific can cope more easily with an increase in salinity.

The sensitivity of the invertebrates, mainly that of crabs, varies but in general it is found that long abdomen invertebrates are more sensitive to an increase in salinity than short abdomen ones. The larvae of crabs and of other invertebrates which float in the water are more sensitive than the adults to changes in salinity 3,9,12 .

Data from systematic monitoring of the dispersion of concentrated brines in marine outlets is scarce, and the only information we have available is from Cyprus and the Canary Islands. Two desalination plants operate in Cyprus: the plant in Dhkelia, which has operated for four years and the new plant in Larnaca, which has operated for a few months only 5,10.

An impressive study carried out in the Canary Islands. The work included both a survey and the monitoring of the dispersion of concentrated brines past the outlet of the discharge pipe, and the influence on the marine flora14. The research was carried out at the plant of Maspalomas II. The plant produces about 17 thousands m3 a day. The discharge pipe is 300 metres long, its diameter is 60 cm and the water depth is 7.5 metres.

It should be noted that the topographic structure of the sea floor in the area is characterised by a shallow shelf that extends out a few metres, followed by a steep fall off. The sea in the region of the island is often rough, and the tide rises about two metres. The measurements were carried out by divers under calm sea conditions. Even though dilution was satisfactory at the surface of the sea, sinking of concentrated and dense solutions to the bottom was still observed.

In measurements that were conducted later in the region of the plume, a concentration of more than 60‰ was detected at a distance of 100 m from the outlet, and as a result other regions within the plume are to be monitored. The plume took an elongated form, resembling a salty underwater ‘river’ flowing in the direction of the fall line. Impact on the local marine flora in the vicinity of the outlet was observed.

Discharging the brines via the outlet of the power station’s cooling water

This option suggests using the hot water discharged from the power station for the dilution of the concentrated brines. The main environmental advantage is the high dilution ratio achieved. An additional advantage lies in the relatively low specific weight of the hot water, which would partially offset the high specific weight of the brines and would therefore reduce their tendency to sink to the bottom.

In essence it is possible to combine a power plant and a desalination plant, for instance by the shared utilisation of the marine environment: via the water feed system, the concentrate flow system, or the evaporation technology as well as in efficient modes of utilisation of electric energy and steam.

In a paper that presents options for lowering the price of desalinated water, estimates that the combination of desalination plant and a coastal power station with sea water cooling would result in a 8%-10% reduction to the total cost of the seawater supply systems and the concentrate discharge. Altman 1 presents a concept for a plant which would combine electric power production and seawater desalination. The main advantages of such a process lie in the combined use of the seawater feed systems and in the more efficient use of fuel energy.

The combined seawater feed systems may include a shared pumping system and a shared preliminary filtration unit. The advantages are obvious: reduced impact on the marine environment at a reduced price. An excellent example of a combined process is to be provided by the Ashkelon plant, wherein the concentrated discharge from the desalination plant and the coolant seawater from the power station will be discharged jointly into the sea.

This will insure sufficient brine dilution so that the concentration of the discharged solution would be up to only 10% higher than seawater. This slight increase in concentration would be offset by a slight increase in temperature so that the specific gravity would remain the same as that of seawater, and the sinking of brines to the sea floor would be avoided 15 .

In the models used to simulate the Ashkelon desalination plant it was assumed that the plant would produce 100 million m3 of water per year and would return to the sea about 18,000 m3 of brine at a concentration of 73.5 TDS. This brine would then be diluted by 158,000 m3 of hot water coming out of the power station. Under these conditions the dilution at the point of discharge is 10%, and its effect would disappear at a distance of a few hundreds of metres from the point of discharge. Some limited deposition of brines at the sea floor would therefore be expected.

Directing the concentrated brines to a salt production plant

This option, whereby the salts pumped from the sea are utilised for salt production rather than returned to the sea, presents many environmental and economical advantages. Its only drawback is the small number of salt producing plants found in the vicinity of desalination plants. If this option is to be used, there would be an advantage to the additional processing of the brines through the membranes, thereby increasing their salinity at the point of discharge.

This option is partially employed in Eilat. The “Mekorot” plant in Eilat is based nowadays on reverse osmosis and produces almost 12 millions m3 of desalinated water each year. Part of the feed water is brackish water from drilled wells (9 millions m3 in concentrations of 3500-6000 mg chlorides per litre) and the rest of the feed is seawater. The volume of brines generated from the brackish water is reduced by 70%.

while the volume reduction of brines generated from seawater is 50%. It follows that the brines exit the plant at concentrations that are 2.0-2.5 times higher than the concentration of seawater.

The brines are then transferred from the plant to the Salt Company ponds and any surplus, is transferred to the Eilat bird watching centre. At the grounds of the centre the brines are combined with brines from other sources (the fish growing farms, seaweed growing plant), and are then transferred in an open canal to the sea.

As the canal passes through an area which is a highly saline marsh and as the flow is by a strong current, it seems that there is no penetration of brine water into the groundwater. The canal’s outlet is located in the northern beach area and to the best of our knowledge the rate at which the brine disperses in the sea has not been monitored.

Direct discharge of the brines at the coastline

The option of discharging concentrated salt solutions directly at the coastline is not recommended by the authors of this article, although under certain conditions (small plants, insensitive shore) it should be given some consideration because of economical factors. As the brine water is continuously returned to the sea it will form a plume of high salinity seawater. The effect will be noticeable at distances of hundreds of metres from the outlet.

Even if the brines would be mostly diluted at a short distance from the outlet, during the many days in which the sea is calm, the secondary dilution would be negligible. On those days the damage to the coastal habitats would be high. This method is not recommended for seas with high sensitivity, or for large desalination plants, nor for areas with population of high environmental awareness.

Summary

A proper environmental treatment of brines is paramount to the operation of a desalination plant. Proper groundwork is necessary if society is to be allowed to benefit from such a plant. The technology of each plant should be chosen in accordance with the local marine and environmental conditions. All emissions from the plant as well as their impact on the environment should be continuously monitored. The monitoring results would serve in re-assessing the plant modes of operation. In order to enrich our knowledge, special effort should be placed on the investigation of the environmental impact of presently operating plants, in preparation for building new ones.

References

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• Perez Talavera, J.L. Quesada Ruiz. J.J (2001) Identification of the Mixing Processes in Brine Discharges Carried Out in Arranco del Toro Beach, South of Gran Canaria (Canary Islands) Desalination 139:277-286.
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Published: 10th Jan 2007 in AWE International