Human Biology
Plants
Ecology
Genetics

Micro-organisms
Viruses
Bacteria
Fungi & Yeast
Microbial Populations
Pathogens
Staying Clean
Food and disease
Sewage treatment
Antibiotics
Fermentation

Introduction

Sewage is domestic and industrial waste, and water drained from rainfall. Although sewage is for the most part harmless (it is 90% water) it contains pathogens that can cause abdominal cramps, diarrhoea, vomiting and even paralysis.

Activated Sludge

This strangely named method is the most common type of sewage treatment.

diagram of the activated sludge sewage method

First the sewage is screened where larger solid particles are removed.

The non-solid remains fo to the first settling tank where the sludge falls (settles) and goes to the digester.

In the oxidation pond, compressed air is pumped in to vigourously agitate (stir) and aerate (give air) the liquid. Various aerobic bacteria digest and break down organic material, producing nitrates, sulphates, carbon dioxide and water.

Solids are settled out more in the 2nd settling tank. About 10% of this sludge is returned to the oxidation pond.

The digester is a large vessel with little air. The sludge is stirred and heated whilst anaerobic bacteria produce methane which is burnt to heat the digester and generate electricity for the sewage works.

Treated sludge sludge is dried and used as fertiliser on farms.