Resource 28

Chemicals in sewage

Sewage is made up of household waste from toilets, kitchens, bathrooms and laundries, as well as wastes from industry. It is 99 per cent water. The major chemical impurities include suspended solids, nitrogen and phosphorus compounds, micro-organisms and organic matter.

The table below details five important components of sewage that are used to measure the success of sewage treatment.

Tasks
  1. Work out the percentage reduction for each of the five components listed.
  2. Use the virtual tour to find out how the levels of biochemical oxygen demand (BOD - a measure of the strength of the waste), suspended solids, ammonia and total organic nitrogen are reduced at the Western Treatment Plant.
Component Average readings of incoming sewage (mg/L) Median readings for released treated effluent (mg/L)
Total phosphorus 10 9
BOD 480 25
Suspended solids 400 41
Ammonia 30 11
Total organic nitrogen 30 24

Nitrogen in sewage

Nitrogen enters the sewage in urine or from industry (tanneries) and cleaning products (mainly as amines).

Why should we be concerned about nitrogen? In waterways nitrogen is a nutrient (food) that can result in a population explosion of algae and other plants. This also increases the number of micro-organisms, which are involved in breaking down dead matter.

The overall result is a decrease in the amount of dissolved oxygen present in the water due to the breaking down of plants, algae, bacteria and other micro-organisms. Other organisms, such as fish, die due to this lack of oxygen. There is then an overall change to the balance of the ecosystem.

The Western Treatment Plant contributes about 50 per cent of the nitrogen in Port Phillip Bay. The other 50 per cent enters Port Phillip Bay via the stormwater system. Most of the nitrogen in Melbourne's waterways comes from fertilisers.

Task
  1. Melbourne Water is hoping to reduce the nitrogen levels to around 5 mg/L by 2005. How could this be done?

Reducing the amount of effluent

Melbourne Water also wants to reduce the amount of treated effluent discharged into Port Phillip Bay by 10 per cent. The reduction can be achieved by either reducing the total volume of sewage entering the Western Treatment Plant or by recycling the treated effluent.

Tasks
  1. Make a list of possible ways that the amount of sewage entering the Western Treatment Plant can be reduced.
  2. Make a list of the ways recycled water can be used in our suburbs.