Topic 10.1.4: Waste Water Treatment

What is the path that water takes after it goes down the drain?

It passes through sewers and then finally to sewage treatment plants

What are some sources of agricultural waste water?

Agricultural waste from animal farms and nutrient run-off which is collected from fields

What is the purpose of processing domestic and agricultural sewage?

To remove organic matter, harmful microbes, particulates, and toxins from waste water before it is safely returned to freshwater sources

Why is it important to process domestic and agricultural sewage?

If this process did not take place, it could potentially pose health risks for the population

What are the stages of sewage treatment?

**Screening & Grit Removal**:

* The first stage of treatment removes large materials such as plastic bags and twigs and grit by **screening**

**Sedimentation**

* Water in a settlement tank is allowed to stand still in the tank while heavier solids sink to the bottom creating **sewage sludge**, whilst lighter matter which is also known as **effluent**, floats to the top

**Aerobic Digestion**

* The effluent is removed and treated by biological **aerobic digestion**
* This involves pumping air into the water to encourage the breakdown of organic matter and other microbes by aerobic bacteria

**Anaerobic Digestion**

* **Anaerobic digestion** is then used to break down the sewage sludge from the bottom of the settlement tank. It is firstly removed and placed in large tanks where bacteria break it down
* Anaerobic digestion releases **methane gas** as a by-product from the organic matter in the sludge. Methane gas is used as a source of energy and the leftover, digested waste as a **fertiliser**