Sewage Treatment

Sewage Treatment

Step 1: Screening

This step takes out large items that find their way in the sewage system (ie - cans, plastic bags, diapers - even false teeth and wallets). These items are placed into a dumpster.

Step 2: Primary Settlement

This step is designed to take out the 'big' dirt. This is done by pumping the water into large, deep tanks called sedimentation tanks. Here, heavy particles of solid waste sink to the bottom to form a layer of sludge.

The sludge is taken away for treatment, after which farmers in some states can use it as fertilizer.

The remaining water has a lot of invisible dirt that needs to be taken out in the next stage.

Step 3: Biological Treatment

In this step, billions of bacteria are put to work to break the waste down into harmless substances.

This is done in two ways:

1. Holding the water in large tanks and pumping in air continuously, so helping the bacteria grow and thrive (activated sludge).
2. Passing the water over beds of special stones (or filter beds) which are home to waste-eating bacteria.

At some treatment plants, the methane gas released by the bacteria is collected and used to generate electricity.

Step 4: Final Settlement

In this step, the water flows into a final tank where any remaining particles sink to the bottom and are removed. The remaining water can then be sent to rivers or oceans.