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Water is a compound of hydrogen and oxygen, but in Nature it is mostly recycled without being broken down into its elements.

The Water Cycle

Fill in the gaps in the water cycle below, adding arrows.

The water cycle

What (form of energy) powers the water cycle?

> Sun's HEAT energy

What is the life process by which ALL living organisms release water vapour into the air?

Don't forget to add it above.

> respiration

What is the (much less important) process by which mammals sometimes release water vapour into the air? Why do they do it?

> perspiration > to keep cool / regulate temperature

Why do geographers use the term evapotranspiration?

> combines evaporation & transpiration on an area of land -

difficult to tell the difference (the rest is runfoff or underground)

Apart from water, what else do rivers carry to the sea?

> soluble mineral (inorganic) salts > sediment / silt

What will be the long-term consequences of this?

> sea gets salty (and even saltier!) > silting up (mudflats,

deltas etc - and erosion higher up)

Domestic Water Supply

Water supply and sewage disposal are everyday topics in which Biology, Chemistry, and Geography overlap. It is far too easy to overlook the role of Biological processes, especially changes due to micro-organisms, in each of these Human undertakings. For example, so-called sand filters develop a layer of bacteria which remove contamination from the water passing through. "Biological filtration" is important in both water supply and the treatment of sewage.

List three sources from which water may be obtained.

> bore holes into aquifers > reservoirs > abstraction from rivers

Which of these should, in theory, provide the purest water?

> bore holes

What else might (naturally) be in this water?

> minerals

What is the purpose of adding chlorine to the water supply?

> disinfection - kills bacteria

What is the purpose of sedimentation tanks?

> remove solid particles

How does the process involving coagulation tanks differ?

> chemicals are added to cause coagulation

Why does water abstracted from rivers need these processes?

> it is more impure/likely to be contaminated

Why do you think we use water from rivers?

> habitations grow up near rivers - flowing water has uses!

How might the activities of Man affect the water from bore holes?

> landfill sites / chemical spillages may contaminate

How might the activities of Man affect the water from reservoirs?

> pollution from sailing/fishing/animals

How might the activities of Man affect the water from rivers?

> sewage works / lowland industries

Sewage Disposal

Large amounts of water pass through sewage works. In fact, some people think that too much water is wasted in carrying away a relatively small amount of Biological waste, mostly in the form of faeces and urine. Washing machines and dishwashers also use up large amounts of washing water, which ends up at the sewage works. In some cases, industrial wastes may also be present.

Some of the processes at the sewage works are based on physical separation, to take the solids out of the water. Others are biological transformations, involving growth of bacteria and protozoa. There is also an anaerobic phase in which different bacteria break down sludge, in the absence of air. This produces a fairly harmless solid substance which may be used as an agricultural fertiliser, as well as methane gas which may be used to generate electrical power in some sewage plants.

Which parts of the process involve aerobically-growing organisms?

> aeration tank (activated sludge)> "biological filter"

It is essential that these wastes are kept separate from the water supply, so there are regulatory bodies which oversee both water supply and sewage disposal.

What sort of things (or, rather, organisms) could theoretically be transferred from sewage to water supply (clue: they must be very small), and what might that cause?

> bacteria > diseases , e.g. diarrhoea

If the effluent from a sewage works discharging into a river is not sufficiently treated, it may also contain organic matter which will be acted on by bacteria in the river. These may then use up all the dissolved oxygen in the water.
What effects might this have on life in the river?

> kill some or most living organisms,

leaving only species tolerant of pollution (& low oxygen levels)

Sewage effluent may also contain inorganic (mineral) nutrients which are the same as required by all green plants. Contamination of water itself by sewage effluent is not the main problem, but it may also cause other effects such as EUTROPHICATION which results in the growth of different organisms.
What sort of simple plant organism might be in the river?

> algae

Even the growth of simple plants may be a problem, because they sometimes also take in and use up oxygen.

Under what conditions would this occur?

> darkness / cloudy / overcast

(insufficient light for photosuynthesis)

Water Pollution

Many substances produced by Man can have harmful effects, if they get into the water, in the general environment as well as the public water supply. The possible effects may not only be bad for us, but also for other organisms which live in the water. In fact careful examination (by Biologists) of populations of seemingly trivial living organisms ("indicator organisms") can often give important clues about types and sources of pollution, which might go unnoticed by purely chemical analysis.
Pollution is not a clear-cut issue; most pollutants are, or were, of use to Man. The main problems are the quantity or form of the substance, and the way it is released into the environment. Even waste hot water from coastal power stations can affect aquatic populations. There are many examples where the ineptitude and short-sightedness of industrialists have caused problems.
Until recently, some industries clearly considered that control measures to reduce the pollution they caused would be uneconomic, or that the effects could not be blamed on them. Others simply expected that pollution released into water courses or the air from factory and power station chimneys would be somehow diluted, or else they hoped that Biological processes in the general environment would take care of an ever increasing pollution load. In fact they overlooked the likelihood of downwind deposition of pollutants, and buildup in the food chain.
However, public and scientific opinion now favours actions which are more sympathetic to the environment in general.

The main effect by which pollution is defined is damage to the living components of the environment, so it is a sadly expanding area of interest (and employment!) for Biologists.

List some general categories of substances which may be present as pollutants in water:

> factory waste > farm wastes, e.g. slurry,silage,manure

> fertilisers > oil

> pesticides > paper mill waste, etc.

Air pollution

There are several forms of air pollution, some of which interact with the carbon, nitrogen , oxygen and water cycles, but they are often confused. Try to find out information about the following:

Causes of pollution
Pollution type Possible source Apparent ill-effects
Soot > smoke > prevents photosynthesis by plants
Acid rain > SO2 > kills trees
CFCs > aerosols > ozone depletion extra UV (skin) cancer
"Greenhouse effect" > CO2 > global warming
Lead pollution > exhaust fumes > IQ lowering
Low level Ozone > Vehicle engines, esp. in cities > asthma

This topic has connections with other BioTopics units on:-

Feeding relationships 
Food chains and webs
Examples of food chains
Measuring population sizes
Ecological pyramids
Conversion of biomass and energy
Predator prey interdependence
Natural Selection

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