What are the problems caused by the discharge of sewage?

It is estimated that each person uses an average of 180 litres of water per day, including uses such as toilet flushing, bathing, washing of clothes and kitchen activity. All this contributes to domestic sewage, 99.9% of which is water itself, while only 0.1% is organic matter. Organic waste contains oxygen-hungry bacteria. In the right conditions, these bacteria multiply rapidly, using up all the available oxygen dissolved in the water. BOD (Biological Oxygen Demand) is a measure of this de-oxygenation process; it is defined more fully later.

Locally sewage discharges are concentrated at a few points along the Great Stour. Small sewage treatment works (STWs) are found at Chilham, Chartham and Westbere. Canterbury STW at Vauxhall Bridge is much larger. It is "consented" to discharge up to 18,000 cubic metres per day (equivalent to 0.18 cumecs, which is approximately 10% of average river discharge, 1.68 cumecs).

What does "consent" mean?

Anyone wanting to dispose of domestic, industrial or agricultural effluents into a river must obtain legal permission from the Environment Agency in the form of a "discharge consent". This is given as an actual volume of waste, expressed as a discharge (cubic metres per day), a loading (kg per day), or as a concentration of specific chemicals (mg/litre). The amount of consent is not constant, depending on the quality of water from upstream, the flow rate, the amount of downstream pollution that can be tolerated, and the weather (e.g. the consent can be reduced in droughts). If the consent level is exceeded, the polluter can be fined up to £20,000. The Environment Agency monitor the effluent frequently and regularly to check that it is up to standard.

The Canterbury STW has a consent of 18,000 cubic metres per day under dry weather conditions. It has an ammonia consent limit of 5mg/litre (an important threshold for survival of fish), and a phosphate consent limit of 1mg/litre.