Green Way To Clean Water


Alina Tatashin
University of New Hampshire at Manchester
E-mail: aab43@unh.edu
Class: Organic Chemistry, Fall 2010
Professor: Dr.Kenick
Yuriy Tatashin, who is a Liquid Chemist Engineer, provided research and guidance assistance with this project.
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Take Water Sample

About
Reverse Osmosis (R.O.) Membrane System is used to produce extremely purified water. This process of filtering water does not produce by itself any by-product such as toxic chemicals. In addition, the membranes can be reused many times through the regeneration process. However, cleaning R.O. membranes can be improved and be more eco-friendly. Current method requires a large amount of complex chemicals, including acids, caustic, oil by-products and thus leaves large environmental footprint. I believe that it is possible to find a better solution for regenerating the membranes and that is making clean water by utilizing practice and principals of green chemistry.

Goals

  1. The project's goal is to make the process of purification of water “greener” that is to find a way that has no harm to the environment, or at least reduces it. The project deals with organic chemistry because R.O. membrane regeneration process uses a number of organic chemicals.
  2. The project will involve creative thinking to come up with alternative methods for membrane regeneration. I will try to reduce the amount of toxic and non-green chemicals to regenerate the membranes to the required level.

R.O. System
Reverse Osmosis Membrane system is used to produce extremely purified water and the main advantage compare to the alternatives is that the process by itself is green: no toxic chemicals are used in the water purification process. First, lets study what the osmosis and the reverse osmosis mean.
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Osmosis

Osmosis is the movement of water molecules from an area of low solute concentration, through a selectively-permeable membrane, to an area of high solute concentration until it reaches equilibrium.

Reverse Osmosis


Reverse osmosis is the method of moving solvent from the area of high concentration through a semi permeable membrane to a region of low solute concentration. However to do so, pressure must be applied in order to overcome osmotic pressure. This method is widely used for water purification in large-scale manufacturing plants, and also in many households. Today, this method is very popular in many types of industry: medicine (purification of proteins),
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food (production of concentrated milk and juices), electronic manufacturing (chips, printed circuit boards), and drinking water (desalination of sea water, and household water purification). Depending on the usage pattern of the membranes and the size of the dissolved particles which have to be separated from the solvent, one chooses appropriate size of the pores of the R.O. membranes. The smallest pores are used for the highest quality of purification. For example, in industries such as electronic or medical, when all cations even as small as Na, K, and small anions as F, Cl must be removed.

R.O. Membrane


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A semipermeable membrane coil. (Public domain)

A Membrane is made from polymer organic compound Polysulfone. Production of Polysulfone membranes is very complicated process and therefore there are only a few companies in the world specializing in production of this material. Layers in the membrane consist of very small pores. There are several types of membranes based on the size of the pores:
  1. Around 0.1 nanometer: Osmosis Membranes. After water passes through a reverse osmosis filter, it is essentially a pure water.
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    My father verifies membrane ID
  2. Around 1 nanometer: Nano-filtration Membranes. They remove most organic molecules, nearly all viruses, most of the natural organic matter and a range of salts, and divalent ions. Nano-filtration is often used to soften hard water.
  3. Around 10 nanometer: Ultra-filtration Membranes: remove microorganisms and some viruses.
  4. Around 100 nanometer: Micro-filtration Membranes. They are used to remove microorganisms, but viruses remain in the water.

Regeneration Dilemma
Some of the chemicals used for R.O. cleaning have different additives, such as detergents. In turn, production of detergents requires non-reusable resources of the planet. In some cases it is hard to clean membranes without such additives, but in other cases this is possible. When membranes were not used to remove organic compounds the use of detergents can be avoided. This can help owners of private sewer systems, as these detergents kill useful bacteria. Membranes become dirty because of different salts such as CaSO4, MgSO4, and Fe salts and other inorganic chemicals with low solubility like compounds of silica. They also can get dirty from organic compounds such as oil, fatty acids, bacteria and wastes of their activity.
Experiment
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Test pH
For the experiment we took four membranes of the same type which have been used for six month at the medical device manufacturer. As all the membranes were used in one R.O. system designed to clean details, we could conclude they had almost the same level and type of clogging. It was also known the quality of water the manufacturer uses and the type of contamination during R.O. process.
Analyzing the customer's feed water we can conclude that most contaminants found in the membranes will be precipitated salts of Calcium and Magnesiums, and most likely Carbonates and sulfates. This is deduced because customer's water total hardness (Ca++; Mg++) is about 127mg/L as CaCO3. We want to check if it is possible to clean R.O. membranes with a greener solution. To do this, we cleaned two membranes with the solutions containing detergents and the other two membranes we cleaned without detergents but only with NaOH and HCl. R.O. cleaning
With detergents
  1. Cleaning with Ultrusil 103: 35-40 grams/litter and maintaining pH below 3.
  2. High pH solution is used to remove organic contamination, bacteria slime, and compounds of silica (Si). In the experiment we used RoClean P111: 35-40 grams/litter adjusted by pH to be above 10.
Without detergents
  1. Low pH solution used for the first step of cleaning to remove precipitated salts. During this step acid, such as HCl is used. In our experiment we used 30% HCl to make a 3% solution.
  2. High pH solution used for the second step of cleaning to remove organic contamination with bacteria slime. In our experiments we used 50% NaOH to make 2% solution.

The water in the cleaning process had 30C - 35C degree. After each step the membranes were cleaned by using Deionized water.
Parameters of Feed WaterThe following water was used to get the test results:
  • Feed water pressure: 120psi
  • Temperature of water: 13
  • Conductivity:290 microsiemens/cm

Before Regeneration

Membrane #
Conductivity microsiemens/cm
Flow of product water, gpm
Rejection of contaminants
SDI10107
22.26
0.4
93%
SDI10108
22.10
0.5
93%
SDI10109
22.42
0.5
93%
SDI10110
22.42
0.5
93%

After Regeneration

Type of cleaning
Membrane #
Conductivity
microsiemens/cm
Flow of product water
gpm
Rejection%
With detergents
SDI10107
2.86
1.1
99%
SDI10108
3.18
1.1
99%
Without detergents
SDI10109
2.86
1.1
99%
SDI10110
3.02
1.2
99%

Discussion of the Results
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Conductivity Meter

All four membranes have the same flow rate of product water in the range between 1.1 and 1.2 gallons per minute after the cleaning procedure. It means that both of our cleaning methods opened the same type of clogged porous in the both sets of membranes. Since manufacturer’s specification for this type of membranes has parameters more than 1 gallon per minute, the regeneration was successful. Rejection of all four membranes is 99%. It means our cleaning in both methods did not destroyed the porous and they can reject all ions as manufacturer specifies. Both methods of cleaning with detergents and without detergents showed the same results.
Conclusion


We can use for R.O. membranes cleaning greener solutions because the final results are the same, when customers uses membranes to clean water with known inorganic containments.
Extra Information
Pretreatment is a required procedure designed to prolong the life of the membrane. Before water reaches R.O. system, it goes through the two or more different media filters (depends on the quality of feed water):
  1. Granulated Activated Carbon is used before most of R.O. systems. Since membranes are made from Polysulfone and the cellulose tri-acetate, chlorine may easily damage it. Granulated Activated Carbon removes chlorine from the water.
  2. Often when feed water is very hard. Hardness of water is described by the amount of minerals present in the water. Softening is a method of removing Ca and Mg by ion exchange.
  3. Colloidal and particulate fouling prevention
For neutralization of waste water after cleaning we mixed wastes and tested pH. When it was in the range of 6-8.5 we started the discharge.