Tag Archives: Water

Cleaning Water, Step by Step

After listening to and reading India’s blog post about water pollution, I decided to further investigate the methods used to clean the water we drink and bathe with. Covered in her blog post, she mentioned pollutants within the water such as bacteria, chlorine, nitrates, and heavy metals. With all these particles and microbial life swimming in our tap water, it’s a miracle we aren’t ridden with diseases. I began to wonder, what do private firms and the government do to remove and minimize all these pollutants?

Huangpu River Pollution
Huangpu River Pollution

So I began my search on the companies who clean Shanghai’s water supply. Veolia Water is one of the major companies who purify and distribute water to households around Pudong. Veolia first extracts their water from underground aquifers and surface water bodies. These areas are protected to prevent pollution. All the water then passes through a purification process, which includes coarse and fine screening, flocculation and settling, filtration, ozonation and chlorination. (Veolia Water, 2010)

Screening is a process in which water is ran through different sized screens to stop rocks and other larger objects from entering the rest of the system. It then moves into a system called flocculation and settlement. Within water, there are usually small clay or dirt particles are suspended, giving water the yellowish, brownish look. These particles are often negatively charged, preventing them from clumping together. Hydrated ammonium alum (NH4Al(SO4)2Ÿ 12H2O) is  added to the water to neutralize the negative charges, allowing particles to combine and form larger particles called flocs. The water passes through a paddle chamber that assist flocculation of particles. The following chamber allows the larger particles to settle due to gravity, removing the majority of the clay. (Drinking Water Treatment – Flocculation, n.d.)

Flocculation of Clay Particles
Flocculation of Clay Particles

After flocculation, water is passed through a gravel and sand filter that removes the remaining clay particles. However, this does not remove metal ions, nitrates, or microbial life from the water. Ozonation, the process of bubbling ozone through water to purify it, is often performed to clean water but is more costly than adding chlorine as a disinfectant. Ozone, O3, is synthesized by the use of UV light or electrical discharges. Bubbling ozone through the water kills microbial life. It also reacts with metal ions such as Iron and Manganese, creating insoluble metal oxides, which can be filtered out. (Oram, n.d.) Unlike ozonation, chlorination is relatively inexpensive and will continue disinfecting after leaving the water purifying plant. Chlorine is added into the water, which kills microbial life; however, it doesn’t remove metallic ions. (Drinking Water Treatment – Disinfection, n.d.)

Once the water leaves the purifying plant, it is transported to households across Shanghai, but chlorine and nitrates still remain within the water. In addition, faulty and leaky pipes allow contamination of minerals and other compounds into the water system.  To combat this, some houses have granulated active carbon filters and water softeners. Water passes through grains of active carbon (organic material or coal treated with heat) to react and trap chlorine and some trihalomethanes (THMs, carcinogens), preventing them from being consumed or absorbed by the skin while bathing. (Water Treatment Using Carbon Filters, 2012)

Water softeners are used to reduce calcium and magnesium ions. Although these metals are not harmful to the body in small amounts, they cause pipes to calcify and clog up, decreasing water pressure. Calcium and magnesium ions are replaced with sodium ions found on the ion exchange resin sites found within the filter. (Skipton, 2008)

Despite all these processes, nitrates still persist within the water. Currently, the only way to remove nitrates would be through reverse osmosis or demineralization, both of which require lots of energy and are expensive to maintain. (Runyan, 2011) Often, some people use faucet filters to further purify the water. These are often carbon filters, which still don’t remove nitrates from the water supply.

With all these methods used to purify the water, it still comes down to the question, is tap water in Shanghai safe to drink? Or is any tap water safe to drink for that matter? Ultimately, it becomes the individual’s decision. How much does one trust the government and others to handle their water? Do the benefits outweigh the cost of purchasing distilled water? Judgment of these crucial matters always lies within the person.

References:

Drinking Water Treatment – Disinfection. (n.d.). Tech Alive Home Page. Retrieved March 25, 2013, from http://techalive.mtu.edu/meec/module03/Sources-SurfaceWater.htm

Drinking Water Treatment – Flocculation. (n.d.). Tech Alive Home Page. Retrieved March 25, 2013, from http://techalive.mtu.edu/meec/module03/DrinkingWaterProcess.htm

Oram, B. (n.d.). Ozone Water Treatment, Ozonation, Ozonator Dirty bad tasting water, contaminated colored water, unfiltered water, bad smelling water. Private Well Owner Drinking Water Pennsylvania Ground Water Research . Retrieved March 25, 2013, from http://www.water-research.net/ozone.htm

Runyan, C. (2011). Nitrate in Drinking Water. NMSU: College of Agricultural, Consumer and Environmental Sciences. Retrieved March 25, 2013, from http://aces.nmsu.edu/pubs/_m/m-114.html

Skipton, S. (2008, October 8). Drinking Water Treatment – Water Softening (Ion Exchange). NebGuide. Retrieved March 25, 2013, from http://ianrpubs.unl.edu/epublic/live/g1491/build/g1491.pdf

Veolia Water | Production and supply of drinking water. (2010). Veolia Water | The world leader in water services and water treatment. Retrieved March 25, 2013, from http://www.veoliawater.com/solutions/drinking-water/

Water Treatment Using Carbon Filters (GAC). (2012, August 1). Health State MN. Retrieved March 25, 2013, from http://www.health.state.mn.us/divs/eh/hazardous/topics/gac3.pdf

Hydrophobia

Wouldn’t life be much easier if you didn’t have to spend time drying yourself off after a shower or a dip in the pool?  Well, certain molecules have that luxury.  Such molecules are known as hydrophobic molecules.  Hydrophobic molecules are molecules that are repelled by water.  This happens because water is polarized and forms hydrogen bonds with itself and since hydrophobic molecules are non polar they are unable to form new bonds with water.  Instead, the two repel one another.

But where it really gets interesting is when a substance is super hydrophobic.  Super hydrophobicy is achieved with very rough surfaces that can suspend small liquid drops.  A rough surface is necessary because when water is put on that surface it can’t fit into the very narrow channels due to the rough surface and the substance’s hydrophobic nature.  In turn, this causes the water to roll off the substance.

In another instance, even when a rough hydrophobic substance is submerged in water it comes out clean due to its hydrophobic properties, but that’s not all.  In fact, the microscopic bumps in the surface trap air in between them, creating this thin film of air separating the substance from the water. 

Interestingly, this effect has been used for millennia as a self-cleaning tool in plants.  The super hydrophobic nature of plants reduces the adhesive force on water droplets allowing water molecules to expend more energy forming spheres due to water’s self attraction.  Now, dirt particles on the leaf on a plant naturally adhere to these spheres which then roll off the leaf due to gravity; and presto chango the leaf is clean.  Furthermore, the fact the water doesn’t stick to the leaf prevents bacteria and fungi from growing on the plant as there is no water for the fungi or bacteria to grow. 

More recently, this concept has been applied to other areas, like solar panels for one.  Solar panels lose efficiency when covered with just about anything including dust, but by having a super hydrophobic coating applied to the solar panel this problem is averted in the same that plants clean themselves.  Moreover, having a wet solar panel would be problematic as algae would likely begin to grow.  Luckily, by keeping the panels dry through properties of super hydrophobic molecules the potential for algae growing is eliminated.   

Evidently, nature’s properties and adaptations have been twisted and morphed into more modern inventions and it’s interesting to see how influential nature is and how effective.

Desalination Salvation

Last class we were discussing what happens when you dissolve table salt, NaCl, in water, H2O. The intramolecular ionic bonds in the NaCl lattice are very strong and usually require a temperature of around 500 degrees Celsius to break. The fact that the ionic substance dissolves in the polar covalent substance, indicates that the attraction of the charges on the hydrogen and oxygen atoms of the polar covalent water molecules is greater than the electrostatic forces between the positively charged sodium ion and the negatively charged hydrogen atom. This means that the sodium and chlorine ions are surrounded by the negatively charged oxygen atoms and the positively charged hydrogen atoms, respectively. We then discussed what happens if all the water is evaporated, determining that once the water molecules are gone, the sodium and chlorine ions are once again attracted to one another and bond into their ionic lattice. This got me thinking, if it’s as easy as boiling some salty water to get the clean stuff, than why don’t all countries take advantage of this abundant resource to solve one of the world’s most pressing problems
I have since learned that it is not quite as simple…

To start I decided to learn more about methods of desalination. There are two primary types of desalination processes. The first is Reverse Osmosis and consists primarily of four steps.

Reverse Osmosis

The second is Multi-Stage Flash Distillation. This uses sudden changes in pressure to flash a portion of the water into steam. The water is then forced through remaining pressurized chambers, each time being introduced to a lower pressure, and flashing into steam. The steam is condensed on tubes of heat exchangers. In heat exchange the colder salt water enters the process and flows alongside the saline waste or distilled water that is already heated. The kinetic energy is therefore recycled and very little energy is lost. The combined effect of the pressure changes and the heat exchange methods make for one of the most efficient desalination processes.
Heat Exchanger
Although simple on paper, from my idea boiling the water and collecting the steam, came two very different and complex processes of desalination. As of now, the oceans as practical sources of water are just that: ideas. Though many regions have built, and use desalination plants, most notably the Middle East with 49.9% of the world’s desalinated output, the sheer amount of expensive energy needed makes most operations a dream. As well, cities far inland or highly elevated such as Mexico City, are unable to use this process, as it costs much more to transport and desalinate the brackish or sea water than it takes to transport clean drinking water. This is a blow to the benefits of desalination, as more often than not, these are the countries that need water the most.

Further hindering global use of desalination are the environmental concerns. As with most large plants, greenhouse gas emissions are a huge environmental risk. However, unique to the desalination plant, is the intake equipment. This has been described as “a giant vacuum” placed in the ocean, and is used to pull water into the plant. Death of marine organisms trapped in the grate is virtually guaranteed. One grate runs the risk of destroying the entire surrounding ecosystem. Another risk is the discharge of concentrated brine solution, a large plant could add as much as 1.5 billion litres of brine a day to the ocean.

The expensive and complex process, along with numerous pressing environmental concerns forces detailed consideration. Until there are more solutions than problems in desalination there remains no quick fix for the water question.

For further information on desalination see: http://www.fwr.org/desal.pdf, http://www.oas.org/usde/publications/Unit/oea59e/ch20.htm, http://www.ryde.nsw.gov.au/environment/water/desalination.htm, http://en.wikipedia.org/wiki/Multi-stage_flash_distillation,http://www.fwr.org/desal.pdf, http://www.fwr.org/desal.pdf , http://www.chemicals-technology.com/contractor_images/ast/3_heat-exchanger2.jpg , and www.nrdc.org/…/04sum/images/saline_diagram.jpg .