www.BioTopics.co.uk
Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

SEWAGE TREATMENT


Humans and animals have always produced waste materials which may cause problems in the environment, but the scale of these problems has increased recently, along with our awareness of environmental issues.

Sources of waste

Direct and indirect human origin
faeces and urine from domestic lavatories

soaps, detergents and food residues from domestic sinks

detergents, water treatment, etc from washing machines

detergents, water treatment, food from dishwashers

Agricultural
animal faeces and urine from livestock

silage and other food effluents

chemical fertilisers (nitrate) from arable farms

pesticides

Most of these substances can be removed or made much less harmful by biological treatments at sewage works or similar processing plants designed to do the same "remediation" processes.

Industrial
chemical residues including (organic) solvents

oil, fuel, hydraulic fluid

acids, alkalies, heavy metals

Scrap metal, etc

You should be able to add more :


Drainage

runoff from roofs, roads, etc :
large volumes of water with grit, salt etc

disinfectants

These categories pose different problems, some of which act in opposition to biological treatments.

Consequences of wastes in the environment


Many waste substances look unsightly or smell, but the main amenity that is affected by pollution by wastes is water. Industrialists used to consider water courses to be ideal to carry away wastes from their factories, perhaps due to the reliability of river flow and sea tides, although population growth has placed greater pressure on these resources.

Eutrophication is one of the major consequences of the waste substances mentioned before finding their way into water.

The main substances of biological origin cause increased growth of bacteria. fungi and algae, as well as fast-growing plants which can tolerate rank conditions. Therefore, nutrient-rich discharges have the effect of changing the character of the plant and animal life in the region of the outflow.

Initially, saprobiotic activity occurs:
bacteria and fungi use any carbon- and nitrogen-containing substances which can be oxidised to provide energy for their growth. They bring about a depletion of the dissolved oxygen content, increasing the BOD, and may release acids.

The BOD (Biological oxygen demand) is a measure of the amount of oxygen needed to complete the oxidation of the organic load and restore the water to its usual oxygen content,

Consequently, fish are killed, along with the community of invertebrates on which they feed, and this has effects on birds and mammals (including Man). After a while, only anaerobic bacteria can exist, and these produce smelly hydrogen sulphide gas, and deposit other black sulphide compounds.

What do water authorities do if they know that such a pollutant has been released into a river?
>pump oxygen gas /air into water,
or pump water into fine jets to oxygenate it
and catch and transfer fish to unpolluted water

The growth of bacteria and algae may have the effect of preventing light from passing though the water, so that submerged plants (often the best "oxygenators") cannot photosynthesise , and die, adding to the pollution load. Underwater visibility is reduced, causing problems for animal life.

Only a limited amount of plant life can tolerate such conditions, although those that do may grow profusely and block water courses. Algae are often unsightly and smelly, blue-greens (not really algae - more close to bacteria) may produce poisons.

Nitrate and phosphate are slightly different. These are simple "salts" - inorganic fertilisers - and they may cause uncontrolled growth of green plants. Nitrate often leaches out of agricultural land, and phosphate may originate from water treatments used with detergents.

What does nitrate turn into in anaerobic conditions? (clue: refer to the nitrogen cycle)
> nitrogen gas


Principles of action of a sewage works


Edexcel
The first stages in a sewage works involve physical processing:

- screening to filter out larger floating debris
- settling to remove grit
- primary settling to separate main solid material from liquid

Solids become removed as sludge which passes into tanks for further treatment, and the liquid is either treated in an aeration tank ("activated sludge"!) or by the trickling "filter" process. Each of these treatment processes involves biological activity.

sewage treatment processes
Name of stage Type of
micro-organism
Aerobic or
anaerobic?
Time
taken
Temperature Resulting
compounds
Anaerobic
digestion
bacteria
(Methanobacterium)
anaerobic 2-3 weeks 30-40C methane,CO2, Hydrogen
Activated sludge bacteria, protoctists,fungi aerobic 8 hrs ambient? CO2, soluble salts
Trickling filter bacteria, fungi, protoctists aerobic days/hours? ambient? CO2, soluble salts
The (liquid) product from these processes is the "purified" effluent. It may be chemically treated to prevent further growth of micro-organisms and to reduce odours. In some cases, mineral salts may be chemically precipitated.
In what ways may the (solid) product from the anaerobic digestion stage be disposed of, apart from agricultural uses?
sold for (domestic) fertiliser / dumped at sea / incinerated?
What benefits might a farmer get from spreading this "digested" sludge on his land?
It contains nitrates, phosphates,sulphates etc to enhance agricultural productivity - These are fertilisers so they increase plant growth, e.g. get more grass for animal feed, or greater production of crops - using less artificial fertilisers (expensive)
Apart from the digested sludge itself, what other product results from the anaerobic digestion process?
(biogas) methane,carbon dioxide, hydrogen,[hydrogen sulphide?]
To what use can this be put?
burned to power plant / provide electricity / power to national grid

What will happen to the (liquid) product from the activated sludge and trickling filter processes?
(fairly harmless so) allowed out into rivers / water courses / sea

What will the following substances be converted into in the activated sludge and trickling filter processes, and where do the resulting substances go ?
carbon compounds hydrogen sulphide (H2S) ammonium ions (NH4+)
carbon dioxide CO2
escapes into air
sulphates (SO42-)
(mineral ions) dissolved in liquid
nitrates (NO3-)
(mineral ions) dissolved in liquid

What is the general (chemical) name for these conversions?
oxidation
It is quite normal to pour disinfectants based on chlorine compounds ("bleach") or phenolic compounds down the toilet or drain.
What effects do you think this might have at the sewage works, if overdone?
It would kill and prevent micro-organisms (in "trickling filter" beds/ activated sludge) from purifying sewage, so effluent would still contain noxious substances, as well as poisonous disinfectant compounds.


In practice, it is not normal for the operation of a sewage works to be affected by disinfectants such as bleach used in household toilets and drains. Try to explain this.
Disinfectant will be diluted by other waste so bacteria will not be killed, and sewage will still be converted into safe products.


Is the name "trickling filter" a good one? Explain your answer.
It is not a true filter - and does not sieve out solid particles.
The liquid contains dissolved and extremely small particles which are removed by living organisms.

Why is it important for the liquid to trickle, rather than be pumped at high volume?
Micro-organisms need the correct balance of oxygenation and food supply, and could be washed away/ not get enough air.

What is the function of the clinker (or gravel or plastic pieces) in the trickling filter bed?
As a support medium, providing a large surface area for attachment of the micro-organisms which convert the wastes into simpler substances

What happens to solids from secondary settling of the output of the activated sludge or trickling filter process?
These are recycled back into the tank/bed they came from, to be acted on again.





Return to the previous unit? Return to the contents page? Another look? ../icon1/back.gif Next Unit?


www.BioTopics.co.uk    Home     Contents     Contact via form     Contact via email     Howlers     Books     WWWlinks     Terms of use     Privacy