Running head: Cost Efficient Methods of Desalinating Brackish and Wastewater 1
Cost Efficient Methods of Desalinating Brackish and Wastewater
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Title:Cost Efficient Methods of Desalinating Brackish and Wastewater
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Table of Contents
3Introduction
3Desalination Methods
4Reverse Osmosis
5Ion Exchange
6Capacity Deionization
7Thermal Distillation and Crystallization
8Conclusion
10References
Introduction
Given the current world population of 7.4 billion which is growing rapidly, access to safe water is a serious challenge. Mismanagement of water resources and excess withdrawal of surface and ground water has led to scarcity of water resources in some parts of the global. Whereas media outlets are fully of scary stories relating to the impending water crisis, 71% of the earth’s surface is covered with water bodies. However, less than 1% of it is liquid fresh water as 97% is saline while the rest is ice(Oelkers, Hering, & Zhu, 2011). Water problems are a leading cause of diseases in developing nations; where water related illness such as cholera, dysentery, typhoid and bilharzia consume large volumes of the health budgets. Such challenges can be managed by adoption of better water resources management and adoption of desalination technologies.
Water is made renewable through a natural process where water drained into the ocean evaporates and forms clouds leading to rainfall. The rainwater is held in fresh water bodies where human beings and animals can make use of it. Nonetheless, the disruption of the necessary ecological conditions through deforestation and pollution has compromised the ability of water to renew through the natural process (Oelkers, Hering, & Zhu, 2011). In attempts to reverse the trend, several nations have adopted different desalination methods depending on their needs and budgets(Shannon, Bohn, Elimelech, Georgiadis, Mariñas, & Mayes, 2008).
Desalination Methods
There are two broad ways of classifying brackish and wastewater desalination methods, namely: membrane methods and thermal processes (phase change methods). This paper discusses four methods namely: capacitive deionization, reverse osmosis, ion exchange, and thermal distillation and crystallization.
Reverse Osmosis
Reverse osmosis (RO) is a popular membrane process of treating water for industrial and domestic consumption. The method is known to yield water of high level of purity. RO entails passing water which is under high pressure through a semi-permeable membrane forcing it against osmotic gradient to produce high-purity water(Gregory, Vidic, & Dzombak, 2011). The pressure on the pump to be used depends on the osmotic pressure of the solution which is determined by the amounts of dissolved salts. Reverse Osmosisa concentrate as a by-product that needs to be disposed. The semi-permeable membrane allows water molecules to pass will screening other contaminants. The process is able to eliminate organic molecules, salts of single valence, bacteria and other suspended particles(Xu & Drewes, 2006).Highly functional systems eliminate 95-99% of the dissolved contaminants depending on their size and charge(Alklaibi & Lior, 2004). Some of the modern systems have been able to reduce the disposable concentrate to below 20% of the total flowback water.
The use of high pressure in the process means the method should consume high amount of energy. As such, it becomes uneconomical to treat water with total dissolved solids (TDS) exceeding 40,000mg/L using this method(Gregory, Vidic, & Dzombak, 2011). According to Karagiannis and Soldatos (2007) the cost of desalinating seawater through reverse Osmosis ranges between 0.50$ and $0.70$ per cubic meter while those of brackish water desalination lie between 0.07$ and 0.08$. RO is the most popular method of desalinating brackish water due to the economic aspects even though it has seen increased use in seawater desalination in the recent years. However, the size of the plant plays a significant role in choosing the method to be used since the volume affects costs. Below is a table showing the costs against the volume of water produced for the two types.
Table 1: Cost versus the capacity of water produced
Note. Reprinted from “Water desalination cost literature: review and assessment”.Copyright 2007 by Agricultural University of Athens.
Ion Exchange
Ion exchange (IX) is a reversible chemical process mostly used on dilute solutions such as boiler feed-water. In IX method, wastewater is passed through a bed containing ion exchange materials where the unwanted ions are replaced with other better ions. For example, sodium ions can replace magnesium ions. The process starts with water entering the top of a specially constructed tank via pipe which is distributed over the ion exchange material’s bed. The now treated water is then collected at the bottom of the tank by a collector. In newer systems, the flow is reversed to have service water and regenerant flowing in opposite directions. These systems achieve high operating capacity and reduced leakages. In the softening process, water is passed through a resin granules-filled softener where sodium ions form the granules replace magnesium ions. The resultant water has no water hardness and has to be hardened to prevent problems associated with softness. The process is ideal for water whose non-carbonate hardness of below 350mg/L(Konikow & Kendy, 2005).
The exchange material is like a car battery and has to be regenerated; usually done in three steps. First, the usual flow is reversed to wash away any suspensions on the bed. Then regeneration chemicals are introduced into the system to get rid of any suspended matter. Finally, the remaining materials are rinsed out. Ion exchange delivers the lowest boron concentration compared to other methods like RO(Fritzmann, Löwenberg, T, & Melin, 2007). The method however has two distinct challenges: brine disposal issues and health problems created by the exchanged sodium ions. The level of salt concentration in the feedwater determines the costs in IX method. As a result, ion exchange is generally a more expensive way to treat brackish water as compared to reverse osmosis. As a result, the method is mostly applied in instances that require high selectivity(Lipnizki, Adams, Okazaki, & Sharpe, 2012).
Capacity Deionization
The process was conceived in the 1960s but took clear direction in the 19702. It relies on the charge separation concept achieved when porous electrodes are subjected to fast charge and discharge conditions enabling them to store and free ions(Hou, Huang, & Hu, 2013). The salts are extracted form water by use of electrical charge applied between a cathode and an anode as demonstrated in figure 1 below(Zhao, 2013).
The method does not require pressure pumps and chemicals like other processes and the electrode potential required is low. As such, the method is very efficient as it consumes low energy and has great capacity. Additionally, the electrodes are ease to regenerate while there is no by-product that requires disposal(Shannon, Bohn, Elimelech, Georgiadis, Mariñas, & Mayes, 2008). In more modern systems, membranes are added to the capacitive deionization devices to improve efficiency.
Figure 1: Capacitive Deionization process
Note. Reprinted from “Theory and Operation of Capacitive Deionization Systems”.Copyright 2013 by Wageningen University.
Since the method has high efficiency, is simple to operate and leaves no residue, its operating costs are low. In fact, the method is increasingly being viewed as an alternative to reverse osmosis(Suss, Porada, Sun, Biesheuvel, Yoon, & Presser, 2015).
Thermal Distillation and Crystallization
When water contains very high levels of TDS, use of membrane methods is often not viable and thus the best desalination method becomes thermal distillation and crystallization. The method employ on evaporation of the desired water leaving the unwanted dissolved molecules in the residue(Shannon, Bohn, Elimelech, Georgiadis, Mariñas, & Mayes, 2008). Vapor compression, multi-effect distillation and multi-stage flash are the common approaches to thermal treatment.
In this technology, pure water is obtained by passing the brackish water through a heat exchanger that condenses gas. The method is highly effective as 99.5% of the dissolved salts are eliminated cutting the disposal costs by up to 75%(Gregory, Vidic, & Dzombak, 2011). The process eliminates salts from water of containing TDS of above 125,000mg/L. the downside of this method is the high energy required for the distillation and condensation plus the slow flow rates which are limited at 300m3/d. the low flow rates necessitate construction of big plants to achieve high capacity. In modern systems, mechanical vapor-compression technologies are employed to concentrate the flowback highly slashing the operation costs. These methods are the most efficient for treatments for
