Treatment of Sewage: Introduction

Treatment of Sewage: Introduction

The most modern of Watercare's wastewater treatment plants- including the plants at Mangere and Rosedale - use primary (mechanical), secondary (biological), tertiary (filtration) and ultraviolet (radiation) methods to treat domestic and industrial wastewater (sewage) and storm water. The average volume of wastewater treated is 300,000 cubic metres per day. Wastewater treatment is designed to safeguard public health and to protect the environment. Wastewater (sewage) is 99 percent water and usually contains:

Organic material - solid organic wastes such as food scraps, toilet wastes, paper etc. (including leaves/wood etc from storm water infiltration). Food processing and textile industries contribute large quantities of organic materials, ie fruit/vegetable pulp, wool etc.

Grease and oils - household wastes contain cooking oil/ fat, soap and body oils from baths / showers. Industrial wastes can contain greasy organic compounds and inorganic (mineral) oils.

Inorganic material - wastewater contains sand, silt and gravel (grit). Most of this comes from stormwater infiltration.

Nutrients - our bodies need nutrients like phosphorus and nitrogen and these are naturally excreted in our wastes. Some industrial wastes also contain nutrients.

Metals - tiny amounts of metals, ie iron, copper and zinc, are naturally present in human wastes. Others such as lead, chromium and cadmium can be present from stormwater run-off and industry.

Chemicals - as a result of household cleaning (eg dish washing detergents and shampoos) or through process wastes from industry, many different chemicals are contained in wastewater, some of which are toxic.

Micro-organisms - bacteria, viruses and other micro- organisms that live in the human gut and are excreted in large numbers. Most of these organisms are harmless and some are even beneficial. Sick people, however, can excrete large numbers of pathogenic (disease-causing) micro- organisms, which end up in the wastewater flow.

The contents of the stream will vary depending on the season, day, time and the type of industries being served.