RO & Bio-

Among various desalination methods, Reverse Osmosis Method (RO Method) had a rather short yet rapid developing history. Now, it shares nearly half of the global desalinated water production capacity. As a new desalination tool, the project should be evaluated against traditional methods. In this page, you will see our partly comparison against RO method based on our results. Furthermore, how two methods might cooperate with each other to achieve an better effect.

Desalination method has been used for hundreds of years to produce fresh water, but large-scale municipal drinking water distillation plants began to operate during the 1950s (Gleick, 2006). Over years, this industry become more and more important due to the increasing water demand of human activities. Until 2013, global desalination water production capacity has increased to 85.9 million m^3/d. (Shumo, 2015)

Various desalination method

	Membrane method	Thermal method
Salt separation	Electrodialysis (ED) , Ion exchange
Water separation	RO membrane (RO)	Multistage flash (MSF), Multiple effect distillation (MED), Vapor compression (VC)

Table 6.3.1 Predominant desalination methods and their classification

As table 7.3.1 shows, the methods varies. For any ones concerns, we gives explanation to every methods listed in this table.

We will look close up to one of the method, RO method and its mechanism in next section.

Visit to Hangzhou Water Treatment Technology Development Center

On Aug. 8, 2015, we visit the Hangzhou Water Treatment Technology Development Center. It is the supporting unit of National Engineering and Technological Research Center for Liquid Separation Membrane and a base for the national membrane sector. Their vice director, Hongqiang Li, and several people from the research team received us.

During our visit, the center gave us a detailed presentation of RO method including its current situation, mechanism, whole process, technological and other difficulties. When we explained our project, they showed great interest on this new approaching way. The question they asked most is how efficient this method can be. Unfortunately, that time we were half-way of transformation, so we can’t answer very clearly. Yet, they still inspired us on combing two methods instead of replacing one.

After this visit, we follow the big picture that center gave us and filled in with details below.

The mechanism of reserves osmosis membrane

To understand RO method, the first step is to learn how it works. The mechanism of RO method can be explain through two parts: membranes and pressure.

First, RO membranes are special membranes. Its layered, web-like structure only allows water to pass through by a tortuous pathway. Unlike nanofiltration (NF), ultrafiltration (UF) or micro-filtration (MF), RO membrane reject the smallest contaminants, monovalent ions. This properties allows this kind of membrane to process seawater desalination and even other filtration process requiring removal of monovalent ions.

The other important elements of RO method is osmotic pressure. Figure 6.3.3 shows the simplest figure explanation how it works. In a normal status, water molecules always move from low salinity area to high salinity area. This force we called it osmotic pressure, yet, desalination process is working against osmotic pressure. Therefore, we have to give an artificial pressure called reverse osmotic pressure, which is where RO comes from. This extra pressure requirements is also the main cost of desalination process.

Difference between Our project and RO method

In a word, our project favors the starting point, and RO method favors the ending point. Using the human practice experience, we made a simple table 7.2.2 to compare their characteristic. As you can see, some actual of our project is remain unknown. That’s because it will take time to expand a lab project into industry, and we can’t jump to conclusion. This part will be further take into project results & modeling to discuss.

	Seawater RO	Our project (bio-desalination)
Main Energy form	Mechanical (electrical) energy	Light (solar) energy
Energy cost(kWh/m^3)	2.5-7	N/A (estimated to be lower)
Pretreatment requirement	High	Low (The more nutrient it contains the better.)
Operating temperature (℃)	<45	<45
Product water quality (Remove racial of total dissolved solids)	95%-99%(single RO pass)	estimating 18%-20%(based on lab result)
Typical water recovery	35-50%	N/A (depend on separation efficiency)

Table 6.3.2 The comparison chart of RO membrane and Our project.

The possibility of combing two technology

Overall, our project is expected to working independently with its own pretreatment and after-process, but we think it will be great to find a collaboration way with current method. Either way, our goal is to find the most efficient and feasible way for seawater desalination.

In last section, we mentioned that our project favors the starting point, and RO method favors the ending point. However, it is not enough if the power is only simple as 1+1=2. Here’s some reasons why we think that the combination contains a lot of potential.

Process order: The idea of combing two methods comes from that bio-desalination water separation process can work under similar circumstance of some pretreatment of seawater in RO desalination plant at first place. As figure 6.3.4 below, the pretreatment of seawater is complicated and throughout. The idea is to ensure that constituents, in the source water do not reduce the performance of the RO desalination facility (NEP, 2008)

After discussing with the Hangzhou Water Treatment Center, we think that bio-desalination should be add in the first step of seawater treatment. It is because the source water has the most impurity and will provide a more favorable environment for cyanobacteria. The other reason is either using its own separation method or altering a few steps of RO membrane pretreatment. The pretreatment of RO will ensure that there’s no cyanobacteria left behind during RO membrane process.

Energy cost: In some way, our project can be understand as one of the pretreatment of RO membrane method. It will lower the salinity. The lower salinity is the lower pressure RO method need, and the more water it will recover. The lower pressure is the lower energy it costs. This will also help expand RO membrane’s life and cleaning cycle.

However, the key is how our project will lower the salinity and if the costs is lower than original costs, especially energy cost. This part will be further discus in our modeling.

Reference

Gleick, P.H., 2006. The World’s Water 2006–2007, The Biennial Report on Freshwater Resources. Island Press, Chicago.

Greenlee, L. F., Lawler, D. F., Freeman, B. D., Marrot, B., & Moulin, P. (2009). Reverse osmosis desalination: water sources, technology, and today's challenges. Water research, 43(9), 2317-2348.

GWI. 2006b. 19th IDA Worldwide Desalting Plant Inventory. Oxford, UK: Media Analytics Ltd.

Miller, J.E., 2003. Review of Water Resources and Desalination Technologies. Available from: Sandia National Laboratories http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/2003/030800.pdf (accessed 25.05.08.).

NEP, U. (2008). Vital Water Graphics an Overview of the State of the World’s Fresh and Marine Waters.

Sauvet-Goichon, B., 2007. Ashkelon desalination plant –a successful challenge. Desalination 203, 75–81.

Shumo, L., Aifu, L., & Liang, Y. (2015). The Engineering Economy Analysis of Large-Scale Seawater Desalination Water Supply Project in China. China Science and Technology Innovation Herald, 16, 039.

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Team:SJTU-BioX-Shanghai/Human Practice/Desalination

RO & Bio-

Contents

Various desalination method