Introduction Cause of water pollutionEffects of water pollution on the

environment Water quality

Nowadays, our surrounding waters have been assaulted with a heavy burden of pollutants.

The most obvious damage has been to coastal zones, where fouled waters and littered beaches have resulted in a reduction of resorts suitable for swimming.

The longer we use our surrounding waters as a giant garbage dump the more difficult and cost the cleanup will be.

Livestock waste - from pigs and poultry

Oil spillage - from ships

Residues of pesticides - from farms

Detergents in sewage - from commercial and domestic wastewater

Industrial effluents - from dyeing and electroplating factories

Livestock waste When an excessive amount of organic matter is

discharge into water, the existing aquatic micro-organisms will grow and multiply rapidly.

The oxygen dissolved in water is rapidly depleted. Because of the lack of oxygen, anaerobic micro-organisms begin to break down partially the organic matter, releasing foul-smelling and toxic gases like CH4, NH3 and H2S.

As a result, the aquatic organisms will die.

Oil SpillageThe effects of oil spillage may include the following:

Oil clogs the feathers of sea birds and prevent them from flying or swimming, oil interferes with the insulation provided by the feathers, so the birds may die of cold or pneumonia.

As oil is immiscible with water and floats on water, it blocks the oxygen supply to the sea water.

As oil is toxic, it kills marine life.

Residues of pesticides Pesticides cause immediate toxic effects on aquatic life and

accumulate along the food chain until a toxic level is reached.

Detergents in sewage Detergents may speed up the eutrophication, because phosphates in some detergents are nutrients to algae. Detergents may cause foaming in rivers and lakes, because some

synthetic detergents with branched hydrophobic chains cannot be

biodegraded within a short time.

Industrial effluents The toxic materials present in industrial effluents vary with the

industry involved. They may contain toxic metal ions of Cd, Cr, Cu, Hg, Ni, Pb and Zn;

cyanides, polychlorinated hydrocarbons, organic solvents, acids and bases.

1. Dissolved Oxygen (DO)

Dissolved Oxygen (DO) is an indicator of the oxygen content in water, It is measured in:

a. mg of O2 per dm of water, or b. percentage saturation of dissolved oxygen

The maximum concentration of dissolved oxygen in water at 16°C is 10 mg dm . If a water sample contains 6mg dm of dissolved oxygen at 16 ,the % saturation of DO is then :

6/10 x 100 = 60 The sample is said to be 60% saturated with oxygen.

The amount of dissolved O2 in water is important for aquatic life. Fishes normally require 4-6 mg dm of DO for survival.

Depletion of oxygen may be caused by biological degeneration of water matter.

The larger the amount of organic matter in the waste water,the lower is it DO.

2. Biochemical Oxygen Demand (BOD)

Biochemical oxygen demand (BOD) is an indicator of the water pollution. It is the amount of oxygen required to break down the organic matter present in a water sample.

If water has a high BOD, it indicates that a lot of organic waste is present and a lot of oxygen is required to break down the organic matter in the waste.

If water has a low BOD, it indicates the presence of a small amount of organic matter and there is little organic pollution.

Natural clean water has a BOD is about 1 to 4 mg dm .

If BOD is greater than the amount of oxygen replenished by air, then some fishes and aquatic life may die; organic debris accumulates, and anaerobic micro-organisms begin to multiply, producing unpleasant odours.

BOD is determined in the laboratory by incubating a sample of water for five days in the dark at 20 °C and measuring the amount of oxygen consumed. The BOD determined in this way is also known as five-day BOD, BOD5.

BOD5 value is commonly used to indicate the relative oxygen requirements of treated effluents and polluted waters.

8%

25%

5%

24% 18%

20%

electroplatingtanneries and leather fi nishingbasic metal industries, machinery and transport equipmentelectronic, electrical and scientifi c productsasbestos removalother

Major producers of chemical waste in Hong Kong

Solid Waste in Hong KongSolid Waste in Hong Kong Disposal of Solid WasteDisposal of Solid Waste

A. IncinerationA. Incineration

B. LandfillingB. Landfilling Pollution Problems associated with the Pollution Problems associated with the

Disposal of PlasticsDisposal of Plastics Solving the Plastic Pollution ProblemSolving the Plastic Pollution Problem

A. Development of Degradable PlasticsA. Development of Degradable Plastics

B. Recycling of PlasticsB. Recycling of Plastics

In Hong Kong,an average of 25,000 tonnes of solid waste were disposed of daily due to human activities of domestic households, commercial and manufacturing business and construction site in the territory.

This quality amounts to about 4kg of waste per head of population.

Some of the waste are potentially recyclable, e.g. metals, plastic, paper and glass.

A. Incineration

About 1/4 to 1/5 of solid wastes in Hong Kong is disposed by incineration.

Advantages of incineration: reduces the volume of the waste by approximately 85% kills the disease-causing organisms at high temperature makes no risk of groundwater pollution

Disadvantages of incineration: it may produce toxic air pollutants and increase the incidence of acid rain and photochemical smog

B. Landfilling

About 4/5 to 3/4 of the solid waste is disposed by landfilling. The solid wastes are compressed and covered with soil on landfill sites.

Advantage of landfilling: only very small amounts of harmful air pollutants are released.

Disadvantage of landfilling: the potential environmental problems associated with

landfilling of waste result mainly from the generation of landfill gas and leachate.

The tremendous amount of plastic items used today The tremendous amount of plastic items used today creates waste disposal problems. Moreover, the use of creates waste disposal problems. Moreover, the use of plasticizers and CFCs in the manufacture of plastics plasticizers and CFCs in the manufacture of plastics produce ecological and environmental problems.produce ecological and environmental problems.

Landfilling and incineration are two main ways to Landfilling and incineration are two main ways to disposal of plastic wastes.disposal of plastic wastes.

Problems associated with the landfilling of plastics : Problems associated with the landfilling of plastics : As plastic are chemically tailored for long life, they As plastic are chemically tailored for long life, they do not generally undergo decomposition in the landfill do not generally undergo decomposition in the landfill site. Thus the plastic waste can last for a long period in site. Thus the plastic waste can last for a long period in the environment. The plastic wastes in landfills may the environment. The plastic wastes in landfills may delay the reuse of landfills for other purposedelay the reuse of landfills for other purpose.

Problems associated with the incineration of plastics: If plastics are burned away by incineration, in addition to CO2, toxic gases may be formed, e.g. burning PVC or other chlorine-containing polymer gives HCl. HCl may contribute to the formation of acid rain.

Plastics wastes in sea endanger aquatic lives, e.g. plastics bags or fragments of plastic foam may be mistakenly eaten by fishes, since plastics are not digestible, the fragments of plastic will clog their digestive tracts, the small fishes will eventually die. Sea animals have been suffocated to death by plastic bags.

Plasticizers from the degradation of plastics may pollute water as they are released from landfill as leachate, e.g. PCBs are once common plasticizers that are found to be very stable in the environment and pose health hazard. Other common plasticizers such as phthalates are also found in leachate, they are safer than PCBs but are still harmful to us.

A. Development of Degradable Plastics Biopolymers - They are polymers made by living organisms, e.g. poly(hydroxybutyrate), PHB, is a natural polyester made by certain bacteria. Micro-organisms found in soil and natural water sources are able to break down the polymer. The degradation of PHB in the environment is usually completed within 9 months. However, PHB is 15 times more expensive than poly(ethene).

Photodegradable plastics - Light-sensitive functional groups such as carbonyl group (C=O) can be incorporated into the polymer chains. The long polymer chains will be broken down under the action of sunlight into shorter fragments which can then be biodegraded by micro-organisms in soil.

Synthetic biodegradable plastic - made by starch or cellulose incorporated into the polymer during production.As micro- organisms digest the starch or cellulose, the plastic is broken down into tiny pieces.The very small pieces left over have a large surface area which greatly speeds up their biodegradation.

Degradable plastics have been used for marking six-pack beverage rings, trash bags and disposable diapers.

B. Recycling of plastic

Direct recycling - This apply only to thermoplastic. The plastic in the waste are separated, cleaned, pulverized, and remoulded into plastic items.

Coding system developed by the society of the Plastic Industry for sorting out

plastics:

PETE - Polyethylene

HDPE - High density polyethylene

V - Polyvinyl chloride (PVC)

LDPE - Low density polyethylene

PP - Polypropylene

PS - Polystyrene

OTHER- All other resins

Disadvantages of direct recycling: the regenerated plastics usually have properties due to repeated thermal and mechanical processing,and can only be used for articles which are not subjected to high stress. Recent development on recycled plastics focuses on converting the plastic products with short service lives, such as foam, wrap and containers, to products with longer service lives, such as construction materials and plastic pipes.

It is not very economical to separate the plastic items from the other wastes at present. Moreover, plastic products are very difficult to be classified.

Plastic products contain different dyes. It is very difficult to remove them from the recycled plastics , the recycled plastics are usually black in colour.

Recycling of energy - This applies to most plastic. The plastic wastes are burned in the incinerators. The calorific values contained in plastics are quite high. Advantages of recycling of energy:

The energy obtained from burning plastic waste can be used for heating or generation of electricity. It is not necessary to separate the plastics. Less dumping area is required.

Disadvantages of recycling of energy:

Burning plastic produces toxic gases,e.g. PVC will give HCI. Expensive scrubber systems have to be used to remove them.

Recycling of chemicals by pyrolysis - This applies to all plastics:

Similar to cracking, the plastic wastes can be pyrolyzed (decomposed at high temperature in the absence of air) at 600-900 °C to give useful small molecules similar to those obtained from the fractional distillation of crude oil. These small molecules are mainly hydrocarbons.

The small molecules are separated by fractional distillation.

Some of the small molecules (e.g.CH4 ) can be used directly as fuels while other larger molecules may be employed in plastics production or used for other purposes.

Advantages of pyrolysis:

The products are useful raw materials for the manufacture of plastics-reduces the use of petroleum.

It is not necessary to separate the plastics. Less dumping area is required/saves more land for

other uses.

Disadvantages of pyrolysis:

Burning plastics produces toxic gases, e.g.PVC will give HCI. Expensive scrubber systems have to be used to remove them . The capacity of handling plastic waste by a pyrolysis plant is also small compared to the total amount of plastic waste produced.At present, the running cost of disposal of plastic by pyrolysis is much more expensive than landfilling.

Uses of diff erent pyrolysis f ractions

19%

7%

7%12%

37%

7%

propene (manufacyure of polyproene)ethene (manufacture of polyethene)butadiene (manufacture of rubber)miscellaneous (chemical raw materials)benzene (manufacture of various plastics)methene (manufacture of town gas)