Desalination plant discharge

Inland desalination plants are much more hmited in the availability of a suitable discharge site. Inland water bodies are often high in quality and have potential for use as water sources, limiting their suitability. Disposal is only feasible if the quality of the concentrate is high enough to be compatible with the receiving body of water (Younos 2005). 

The modeling and optimisation of concentrate discharge to reduce environmental impacts has been the focus of numerous studies (Al-Barwani and Pumama 2007, 2008 Alameddine and El-Fadel 2007 Bleninger and Jirka 2008 Malcangio and Petrillo 2010 Pumama and Al-Barwani 2004 Pumama et al. 2003). Much of this research has been for a project by the Middle Eastern Desalination Research Center, Environmental Planning, Prediction and Management of Brine Discharges from Desalination Plants. 

Discharge to surface water is the most economical form of concentrate management for seawater desalination plants, regardless of the discharge volume. Due to the availability of ocean discharge for seawater desalination plants, the cost of disposal tends to be less costly than for inland desalination. Costs include pumps and pipes. 

Concentrate can be harmful to the environment due to either its higher than normal salinity, or due to pollutants that otherwise would not be present in the receiving body of water. These include chlorine and other biocides, heavy metals, antisealants, coagulants and cleaning chemicals. Of particular concern is the effect of pollutants on delicate ecosystems and endangered or threatened species. However, with appropriate measures in place, the discharge of concentrate to surface water can remain a viable method for seawater desalination plants. 

Most cleaning chemicals used for membrane desalination plants are harmful to the environment. Common cleaning chemicals are outhned in Table 2.5. These cleaning solutions should be treated prior to discharge with membrane concentrate, or sent to a location where appropriate treatment can occur. 

Metal in the discharge can come from the source water, or be a product of corrosion. Note that due to their operation at elevated temperatures, when compared with membrane desalination plants, thermal desalination plants are... 

Co-location of a power plant and a seawater reverse osmosis desalination plant allows for the cooling water from a neighbouring power plant to be blended with the waste from a desalination plant before discharge (Voutchkov 2004). In such a process, seawater is used as the cooling water for the condensers in a power plant. This water is then used as both the feed for the desahnation process, and for blending to dilute the concentrate from the desalination plant. 

Co-location is not be suitable for all desalination plants. This process only becomes feasible if the volume of cooling water discharged from the power plant is at least three to four times greater than the capacity of the desalination plant (Voutchkov 2004). Furthermore, corrosion from power plant heat exchangers may elevate the levels of metal in the feed to the desalination plant, which may then damage the reverse osmosis membrane units (Voutchkov 2004). 

Alameddine, I., El-Fadel, M. Brine discharge from desalination plants a modeling approach to an optimized outfall design. Desalination 214(1—3), 241—260 (2007)... 

Bleninger, T., Jirka, G. Modelling and environmentally sound management of brine discharges from desalination plants. Desalination 221(1—3), 585—597 (2008)... 

Pumama, A., Al-Barwani, H.H., Al-Lawatia, M. Modeling dispersion of brine waste discharges from a coastal desalination plant. Desalination 155, 41—47 (2003)... 

Although the integration of RO with other pressure-driven membrane processes has led to significant improvements in membrane-based desalination process economics, another fundamental problem is the environmental aspects of brine discharge from reverse-osmosis desalination plants. 

In some cases, an estimate of corrosion rate may be gained from chemical analysis of the corrosive fluid. For example, the level of copper ions in the discharge from a desalination plant will indicate the rate of metal loss from the system. 

The irony of the water shortage in McMurdo is that Antarctica contains 90% of the ice that exists on the Earth and 70% of the fresh water. However, mining of ice and snow on Ross Island for use in McMurdo Station is prohibitively expensive and may violate the Antarctic Treaty because of potential environmental issues. For these reasons, the demand for fresh water in the station continues to be met by desalination of seawater, although this process is also expensive and does impact the environment in McMurdo Sound because of the discharge of hypersaline brine by the desalination plant and of wastewater by the wastewater treatment plant. 

Brine concentrate stream from seawater desalination plants is typically discharged to the sea. Flowever, the brine can be processed to recover salts for the chemical industry, e.g.,... 

The primary disadvantage with FO is typically the input energy needed in step 2 to recover the water from the draw solution. The novel aspect of this project is to eliminate step 2 by to using seawater or thermal brine to draw water from the produced water and then directly discharge the diluted seawater/brine into the Arabian Gulf. This is particularly applicable in Qatar since both the desalination plant and the produced water from the gas fields are in close proximity to each other. 

Ramon et al [17] compiled a list of commercial and prototype osmotic membranes and evaluated the resulting theoretical power densities according to their characteristics. The analysis showed that prototype lab-cast thin-film composite membranes based on a selective polyamide active layer can achieve water permeability in the range of 5-7 m/s-Pa, which result in theoretical power density in the range of 5-6 W/m with seawater feed and over 15 W/m in plant fed with I.IM brine discharged from RO-based desalinization plants. Theoretical power densities achievable with commercial membranes are below 3 W/m with seawater feed. 

Voutchkov N (2005) Alternatives for ocean discharge of seawater desalination plant concentrate. In 20th Annual WateReuse Symposium, WateReuse Association, Denver, 18-21 Sept 2005... 

Figure XXIV-23 shows the routing of input, output and internal mass fluxes. Fresh seawater enters the site from the open ocean into the man-made harbour and then moves down the canals separating the peninsulas. These canals feed the desalination plants brine exits to the right into a brine collection canal running between the BOPs and reactors. This brine discharge canal circles around the reactors. This circuitous route is taken to avoid the need for multiple bridges under the heavy capacity railroad in Fig. XXIV-23. The brine is rejected from the desalination plant at 10°C above the seawater inlet temperature - a normal practice for desalination plants - and the extended travel time of the brine from BOP back to the ocean allows opportunity for further cooldown of the brine before return to the sea. Alternately, that heat and brine may be put to profitable use such as heating acres of greenhouses or perhaps shellfish beds.

Cavitation can lead to the shutdown of a desalination plant. If there is cavitation in an ejector condensate pump, it will fafl to reach the discharge pressure and the required amount of condensate would not be extracted. One of the requirements of pumping liquid is that the pressures in any point in the suction arm should never be reduced to the vapor pressure of the liquid as this causes boiling (at reduced pressure). Too low a pressure at the pump suction must always be avoided so that cavitation is not caused. Cavitation in pumps is noticed by a sudden increase in the noise level and its inability to reach discharge pressure [114]. 

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