Drinking Water & Dissolved Oxygen in Water

Drinking Water&Dissolved Oxygen in Water

Water • Highly polar • Forms Hydrogen bonds • Has the ability to dissolve many chemicals

Distribution of Water on Earth Only 2,7% of the water on earth is fresh, 80% of this is stored in ice, almost 20% is ground water and a tiny fraction is present in lakes and rivers. Most of the fresh water is recycled in the water cycle.

Polluted Water • Irrigation – Pesticides and Fertilizers • Industry – Burning of coal + car fumes cause acid rain • Washing – Detergents and cleaning substances • Other sources of pollution such as oil spills

Water Demand • Increase in Population = Increase in agricultural and industrial products = Increased water demand. • This means that water needs to be treated and saved. Nevertheless, not all water is needed to be of drinking quality: • Toilet Flushing • Air conditioning • Irrigation • Industrial purposes

Reason to Treat Water • Kill microorganisms that can create diseases such as Cholera, Hepatitis A and dysentery. • Remove suspended particles • Remove and destroy unpleasant smells and colors

Bactericidal Agents • Two major bactericidal agents, substances that kill microorganisms are: • Chlorine Cl2 • Cl2 + 2e → 2Cl- • 2. Ozone O3 • O3 + 2e + H2O → O2 + 2OH- • The bacteria are killed when the Carbon and Hydrogen, which they consist of, are oxidized to carbon dioxide and water.

Chlorination • Advantages: • Effective against disease causing bacteria (Ex: Typhoid fever) • Longer retention time = long term protection against diseases • Cheaper to produce • Easily liquefied and shipped • Disadvantages: • Not effective against viruses (such as Polio) • Forms toxic chlorinated organic compounds (CHCl3, CCl4) • Leaves a chemical taste behind

Ozone • Advantages: • Effective against both bacteria and viruses • Needed in lesser quantities • Oxidized products less toxic than chlorinated organic compounds • Leaves no chemical taste behind • Disadvantages: • Expensive to produce • Shorter retention time (needs to be consumed soon after purification) • Must be produced on site (because it is highly reactive)

Ideal solution Use Ozone for killing the bacteria and viruses and then add a small amount of chlorine to provide protection until it is consumed.

Desalination • Sea water (3,5% salt) is not suitable for drinking, but it can be by the following processes: • Distillation • Reverse Osmosis • Ion exchange

Distillation = Separating volatile water from the non volatile salt by heating the solution Distillation of sea water can be made more effective with multistage processes in which the heat is re-used so that a larger amount of seawater is desalinated.

Reverse Osmosis = The movement of water from a high concentration (pure water) to one of lower concentration (less pure water) through a semi-permeable membrane (allows water to go through but no large molecules or ions). The advantage of this method is that less energy is needed.

Ion Exchange Process: 1. The positive ion exchange column replaces the metal ions from the water by hydrogen ions. 2. The negative ion exchange column replaces the anion in the water by hydroxide ions. The positive hydrogen ions and the negative hydroxide ions join to form water. When the columns are saturated with metal ions and anions a reverse process is done to obtain the initial ion exchange columns (by use of sulfuric acid and sodium hydroxide).

What else can we do? • Saving water – Water saving showers, smaller sized toilets, re-use of sewage water etc. • Conservation – Dams (which also provide electrical power), water reservoirs. Nevertheless, the are expensive projects and can cause problems. • Re-use – Waste water and sludge from sewage can be used as fertilizers, if major pollutants are removed. Also, recycling the water is useful, such as in industry.

Dam Water reservoir

Dissolved Oxygen • Pollutants of water: • Plant nutrients – Growth of aquatic plants • Suspended (un-dissolved) particulates • Toxic substances – Benzene, Chromium, Mercury • Dissolved minerals – increase in salt • Acidity – due to acid rain • Thermal pollution – Oxygen decreases in water and it speeds up reactions

DO (= Dissolved Oxygen Content) For plants and animals to line under water, the water must contain a certain amount of oxygen. The DO of a body of water shows how much oxygen is in it. One must note that oxygen is non polar an hence it is difficult to dissolve in water. Ex: at 20C 9mg/dm-3 or 9ppm (parts per million) is considered a good level. Below 4,5ppm it is considered polluted.

Plant Nutrients • Nitrates: • Human and animal wastes • Fertilizers • Phosphates: • Detergents • Fertilizers • These nitrates cause: • Excess growth of aquatic plants (algal blooms) • Bad smell and tasty water

Effects of excessive plant growth • Eutrophication = pollution caused by the excessive growth of plants. • Process: • Dead plants sink and are decayed by the presence of aerobic bacteria. The decaying process uses Oxygen • Depletion of Oxygen in the lower levels of the water body • Fish and other life die of asphyxiation. (red tide)

The ‘Red Tide’ is a phenomenon were certain species of plankton grow rapidly, and so decrease the DO. This plankton produces toxins that kills fish (if they didn’t already die from asphyxiation)

BOD (Biological Oxygen Demand) The plant nutrients mentioned all use oxygen to decay (aerobic bacteria) which results to oxygen depletion. Other pollutants also consume Oxygen. The BOD measures the amount of Oxygen consumed by these organic wastes over a period of time. Process: The sample is diluted by oxygen saturated water. After some days the decrease of Oxygn is measured by an Oxygen electrode.

Another effect of Oxygen Depletion • If most Oxygen is used up in a water body then no life is possible, not even that of aerobic bacteria. • Anaerobic bacteria do live without Oxygen and convert: • C to CH4 • N to NH3 and amines • S to H2S • These substances create bad smells and color the water, hence they are a clear indication of polluted waters

Drinking Water & Dissolved Oxygen in Water