Chapter 18 Chemistry of Water in the Environment

Transcription

1 Chapter 18 Chemistry of Water in the Environment World Ocean Freshwater Chlorination of Drinking Water Other Methods of Disinfection Ground Water pollution Tap Water vs. Bottled Water You will need to read the Course Pack to compliment the Textbook

2 18.5 The World Ocean Approximately 70% of the earth s surface is covered by oceans. Seawater contains about 3.5% dissolved salts by mass. The ocean contains many important compounds and minerals. However the cost of extracting them is prohibitive. Only sodium chloride, bromine and magnesium salts are obtained commercially.

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5 Desalination Because of the high h salt content, t seawater is unfit to be used as drinking water. Typically the dissolved salt content of drinking water must not exceed 0.05% by mass. Distillation on a large scale is complicated due to salt precipitation Desalination can be achieved using Reverse Osmosis (RO) Each Permeator contained in a RO desalination plant contains several million tiny hollow fibres.

6 Water naturally flows through a semi-permeable membrane from regions of higher water concentration to regions of lower water concentration. The largest desalination facility is in the UAE (Jebel Ali) Desalination is being viewed as viable method for producing drinking water by affluent cities in coastal regions. Desalination plants are common in Australia. However water stressed regions are generally in the interior or at higher elevations and it is more economical to transport fresh water to these areas. Desalination has adverse effects on the environment, disposal of waste brine, and marine life mortality.

7 18.6 Fresh water In 2004, total t Canadian household h use of water was estimated t at 8.3 billion litres per day, enough to fill rail tank cars The average daily freshwater domestic use per capita was 329 litres, of which 35% is for bathing, 25% for laundry and cleaning, 30% for toilet flushing, and 10% for cooking and drinking An adult needs about 2L per day for drinking! 60% of Canada s total electric power is generated from hydro sources 8% of Canada s area 8% of Canada s area is covered by Lakes

8 Dissolved Oxygen and Water Quality At 1atm and 20 o C, Water contains about 9ppm Dissolved Oxygen Biodegradable Organic material causes a depletion of dissolved oxygen as aerobic bacteria consume oxygen as they oxidize this oxygen-demanding waste. Sources of oxygen-demanding waste include: Eutrophication Dissolved oxygen can reduce to the point where anaerobic bacteria begin to produce CH 4, NH 3, H 2 S and PH 3, causing noticeable odours. Plant nutrients such as N and P stimulate excessive plant and algae growth cutting out light and increasing decaying organic material. The most significant causes of eutrophication include domestic sewage run-off The most significant causes of eutrophication include domestic sewage, run off from agricultural land, farms and golf courses.

9 Treatment of Water Supplies Treatment of Municipal water supplies involves (usually) five steps: 1. Course Filtration 2. Sedimentation Water is allowed to stand in large settling tanks so finely divided particles can settle out. To aid the process the water is made slightly basic by adding CaO. 3. Sand Filtration

10 4. Aeration. 5. Chemical Disinfection Typically Chlorine is used to destroy potentially harmful bacteria as it does not need to be generated in situ like Ozone. Check out the quality of your drinking water at the CRD website.

11 Disinfection by Chlorination The active chlorinating agent is HOCl, hypochlorous acid. It is generated by dissolving Chlorine in water Cl 2 (g) + H 2 O(l) HOCl(aq) + HCl NaOCl(s) + H 2 O(l) HOCl(aq) + OH - (aq) (for small scale) The ph of water must be closely controlled as a ph above 9 rapidly converts HOCl to the less effective OCl - ion HOCl + OH - OCl - + H 2 O Advantages of Chlorination include:

12 Residual Chlorine in water is converted to chloramines (NH 2 Cl, NHCl 2 and NCl 3 ). This combined chlorine is longer lived and provides residual disinfection. An acidic ph favours the less effective di- and tri chloramines and also corrodes piping. A ph just above 7 maintains the more effective monochloramine. The ph of CRD tap water is 7.15

13 Disadvantages of Chlorination include: Production of Chlorinated Organic substances Chlorinated Phenols are toxic, and have offensive odours and tastes. Formation of Trihalomethanes (CHX 3 ), such as chloroform. Continued exposure to just 30ppb of CHCl 3 can pose health hazards, including cancers and DNA mutations. Water often needs to be filtered with activated Carbon prior to chlorination. Use of Chlorine dioxide and Ozone for disinfection is becoming increasingly common since these produce negligible levels of CHCl 3 and chlorinated phenols Disinfection of water by chlorination has had major benefits in protecting human health and eradicating water borne disease such as typhoid and cholera. The advantages vastly outweigh the disadvantages. However, many of these benefits are still not available to people in developing countries.

14 Disinfection by other methods Chlorine Dioxide ClO 2, an exception to the octet rule! Is not a chlorinating agent, it is a powerful oxidizing agent and produces much less toxic organic by products. Unfortunately it can not be stored so must be generated on site Ozone Many North American and European cities use Ozone disinfection. It is more effective at killing water borne viruses, but again can not be stored, and can form toxic organic compounds such as formaldehyde. Ultra-Violet Irradiation Mercury lamps can be used to disrupt the DNA of micro-organisms to prevent replication and inactivate the cell. UV irradiation systems are portable and can be used to disinfect small population bases.

15 Softening of Hard Water Areas rich in Limestone often suffer from Hard water. Hard Water contains high concentrations of dissolved Calcium and Magnesium ions. Relatively harmless to health, when heated the water containing calcium ions and bicarbonate ions releases some CO 2 and precipitates CaCO 3 and other mineral deposits. The soda lime process uses Borax or sodium carbonate to precipitate out the Ca 2+ and Mg 2+ at the treatment plants Ion exchangers can be used in the home for softening hard water, sodium ions displace the magnesium and calcium ions.

16 Groundwater pollution by Organic Compounds Groundwater is a source of drinking water, shoddy waste disposal practices have resulted in contamination which is difficult and expensive to clean up. Landfills and Industrial wastes are a source of contamination. This leachate can include: Chlorinated organics Benzene, toluene, xylenes (BTX) from gasoline Depending on the density and solubility of the pollutant, they can persist for decades. Clean- up Pump and treat: Costly, huge volumes of water, re-polluted Bioremediation: Cleaning by natural processes

17 Nitrate contamination Groundwater can also be subject to pollution from the nitrate ion. This typically can originate from The run-off reaches streams and rivers Elevated nitrate levels are thought to increase the risk of certain cancers and can cause the rare but fatal infant disease methemoglobinemia