How waste water and sewage is treated

The wastewater from our homes, schools and businesses is returned to the natural water cycle. Depending on where we live it may be into the sea, a river or into the ground water. All the dirty water from our homes is collected together and leaves our homes through sewer pipes to be treated.

Step 1

Wastewater flows from our homes into the sewers. For most of our homes the sewage pipes lead to a sewage works, although many houses in rural areas are not connected to a sewage system. In this case the house will have its own septic tank, which must be emptied when full or will have a bio-sewage treatment tank. The waste water collected from sewers on the other hand will go to a treatment plant which can vary in size

Step 2

The sewage is screened when it arrives at the sewage plant to remove items such as rubbish, which should not have been flushed down the toilet in the first place. Metal grids are used to trap this material. This adds to the expense of treatment and the material has to be removed in order to be disposed of in landfill

Step 3

The sewage passes into a settlement tank where solid material settles out and can be removed as sludge. The water and sludge will then go separately for further treatment.

Step 4

The sludge passes to tanks where it is broken down by bacterial action. In many plants air is mixed into the sludge to speed up the process. It can also be done in an enclosed plant with the bacteria producing methane gas rather than carbon dioxide. The methane gas, which is the same as natural gas, can be used as a source of energy. The solid remains are a rich fertiliser and can be spread on land after tests are carried out to ensure it is safe. When spread on agricultural land livestock must be kept off the land for three months as a safe guard and as with farm waste, it cannot be spread during very wet weather conditions.

Step 5

The water also undergoes treatment with bacterial action. However, a different process is used. The water passes through tanks full of stones. These stones harbour bacteria that feed on any remaining organic material in the water converting it into water and carbon dioxide

Step 6

Pure water is produced which can be released back out into the environment, usually a river or the sea. In many major cities the water is recycled and may go back into the supply system or may be diverted for non-domestic water usage. Before the water is released it is tested to ensure it is sufficiently pure.

Sometimes different approaches may be used in treating sewage from smaller towns and villages. A short rotation Coppice willow wastewater treatment system was provided for the town of Donard in 2007, preventing discharge to the local river. The willow trees clean the effluent water through using the nutrients in it as food. The first crop of willow will be harvested in 2009 producing 188 tonnes of woodchip per annum, used locally as an alternative to oil -saving 179 tonnes of CO2 emissions annually. Lower electricity consumption will result in operating cost savings of approximately € 1,200 a year.