Quality of effluents from sewage treatment works and from industries

Before any wastewater can be discharged into the river or the sea, permission to do so must be obtained from the organizations that control pollution. As mentioned in Chapter 1, in England and Wales this is the Environment Agency (EA), in Scotland, the Scottish Environment Protection Agency (SEPA) and in Northern Ireland, the Environment and Heritage Department of the Department of the Environment (the addresses of these Agencies are given at the back of this book). These organizations were set up by government to prevent pollution of the environment - not just water but also the air and the land. They also control radioactive wastes.

The Agencies control the amount of waste that is released by a licensing system. The discharger makes an application for 'consent to discharge' and the Agency usually gives its permission based on what are known as 'consent conditions'. Answers must be given to a number of questions. If the effluent is going into a river, the following information is required:

What is in the wastewater?

Where is it being discharged?

What is the flow rate of the river, particularly in summer when the river flow is low and the effluent is not diluted so much as in winter?

What is the flow rate of the effluent?

What is the river going to be used for downstream, for example, is it going to be extracted for water supply?

If the discharge is going into the sea, the Agency will ask:

Where is the discharge entering the sea?

What are the currents in the area for the different states of the tide?

Are there any beaches nearby that people use for bathing in the summer?

Is the sea used for shellfish growing or for fish farming?

Once the answers to these questions have been obtained, the Agency has to decide what consent conditions to apply to the effluent. It has to achieve a balance between, on the one hand, using the rivers and the sea to get rid of our waste (it has to go somewhere!) and, on the other hand, making sure that the effluent doesn't pollute the water. The assumption is made that the rivers and the sea can cope with a certain amount of waste, but it is a matter of getting the balance right.

As an example, let's assume a new electronics factory wants to discharge its effluent into a river that is popular with anglers because it has many trout in it. The factory makes an application to the appropriate environment agency for a consent to discharge and says that its waste-water contains copper. The Agency has to decide on the maximum amount of copper it will allow to be present in the discharge so that it will not harm the trout. It needs information on the effects that copper has on trout and so it looks at all the scientific papers and reports that are available on this subject. Small amounts of copper are essential to life, but large doses are toxic, especially to plants. A complicating factor is that the toxic effect varies according to the hardness of the water. In hard-water areas such as south-east England, the calcium in the water reduces the toxic effect of the copper. So if the effluent is discharging into a stream in this area the Agency can allow more copper to be discharged than if the factory is being set up in parts of Wales or Scotland where the water is much softer and lacking in calcium.

One of the pieces of information that is available for deciding the consent conditions is the toxicity of copper to fish. This is usually expressed as the 96-hour LC50, which is the concentration of the substance that will kill half the fish (the 'lethal concentration') in four days.

For copper the 96-hr LC50 to rainbow trout is:

Concentration (pg/l) Water hardness (mg CaCO3/l)

From these figures you can see that copper is much more poisonous in soft water than in hard water. This effect is summarized in Figure 3.

This information tells you only the toxic level of copper, but the Agency has to allow copper to enter the river and make sure the fish are not affected. Another piece of information is that if trout are in water (hardness of 45 mg/l) with a concentration of copper of 10 pg/l, they show increased coughing, so even this concentration affects them adversely.

From these and many other pieces of information, 'safe' concentra-

20-100 500-1000

14-45 200-300

10,000

10,000

Hardness

Figure 3. Dependence of copper toxicity on water hardness Source: G. Mance, Pollution Threat of Heavy Metals in Aquatic Environments,

Elsevier, London, 1987

Figure 3. Dependence of copper toxicity on water hardness Source: G. Mance, Pollution Threat of Heavy Metals in Aquatic Environments,

Elsevier, London, 1987

Table 2. Safe concentrations for copper (ug/l) in different

hardnesses of water

Water hardness

Copper concentration

0-50

1.0

50-100

6.0

100-250

10.0

over 250

28.0

tions can be set and those for the UK can be seen in Table 2. Now that the safe level is known, the Agency can decide what is the upper limit that can be allowed to be present in the discharge from the electronics factory. For example, if the wastewater is entering a stream with a water hardness of 75 mg/l and the effluent is diluted 20 times by the river water, then the maximum concentration allowed will be 120 pg/l (0.12 mg/l), but if the effluent is going to a river with a water hardness of 300 mg/l then the limit will be 560 pg/l (0.56 mg/l).

This was an example of how the regulatory authorities decide how much waste a river can absorb. The great majority of industries, though, dispose of their wastewater through the sewerage system and it is then treated at the sewage treatment works. However, as we learned in the section on the sewage treatment processes, the purification of sewage depends upon the activity of micro-organisms. In other words, it's a biological process and, just as the environment agencies try to preserve the life in the rivers by controlling what goes into them, so the sewage authorities restrict the amount of toxic waste that enters the sewerage system in order to maintain the bacterial life in the sewage works. The sewage works, however, can absorb waste more effectively than rivers, so the standards are not as strict. This is why it is often cheaper for an industry to put its waste into the sewer rather than treat it to the standard required for going into a river.

Although the example above was about the control of copper pollution, the principal concern of sewage authorities and the environment agencies is to ensure that organic matter is purified to a level that will not affect the life in receiving waters. Later in this chapter we will learn about the effect that biodegradable organic matter has on water chemistry. The amount of organic matter in sewage and sewage effluent is measured by a special test called the biochemical oxygen demand or BOD test. The BOD is a measure of the amount of oxygen used up by micro-organisms as they decompose the degradable organic matter that is present. The test involves measuring the dissolved oxygen in the sample initially and then after it has been incubated in the dark for five days at 20°C. You may wonder how these parameters were decided! The test was devised in 1912 by the Royal Commission on Sewage Disposal and is supposed to simulate the discharge of organic waste into a river in summer time; the five days is an estimate of how long the effluent might be in the river as it makes its slow journey to the sea.

The BOD of untreated sewage varies from place to place but is about 300 mg/l, whilst the 'consent condition' for most sewage effluents is that the BOD should be less than 20 mg/l. In other words, sewage treatment has to remove over 90 per cent of the organic matter. The majority of sewage works achieve this. Crude sewage, as it arrives at the works throughout the day and night, varies both in the flow rate and in the BOD content, and these variations reflect our day-to-day activities. The greatest change takes place at mid-morning when the sewage from all our wakening activities (washing, using the toilet, having breakfast) goes into the sewer. There is another minor peak flow rate in the early evening when we've returned home from school and work.

Assuming that the sewage treatment works is operating efficiently and meeting the standard set by the regulatory authority, we can predict the main constituents in the effluent and look at their concentrations both in the effluent and the receiving water.

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