Treatment of Sewage Raw sewage consists of water containing organic wastes faecestoilet paper food scraps detergents

The water becomes mixed with: plastic bags rags, road grit and many other materials

The raw sewage containing the mixture of materials arrives at a sewage treatment works A typical sewage treatment process

Preliminary Treatment Sewage will: be screened have grit removed be left for several hours to let solid particles settle in settlement tanks as sludge

Sludge from the settlement tank goes to the sludge digester for further treatment Sludge digesters (each digester is 140 ft tall and holds 3 million gallons of sludge) End product – pellets for fertiliser Methane gas produced

The liquid passing out of a container is called an effluent Effluent Secondary Treatment oxygen must be provided for the decay micro-organisms to do their work products will be harmless to the environment organic compounds broken down by micro-organisms

Secondary Treatment (providing oxygen) – two types effluent pumped up central pipe through rotating arms sprinkled onto a bed of stones (air spaces between stones) stones coated with micro- organisms which receive oxygen Micro-organisms feed on organic material and break it down purified effluent leaves Biological Filtration Aeration effluent mixed with activated sludge (rich in bacteria) aerated by blowing compressed air through it stirred with paddles Micro-organisms receive oxygen Micro-organisms feed on organic material and break it down purified effluent leaves

‘Purified’ sewage passes to final settlement tank Remaining suspended matter settles out as activated sludge Effluent now harmless and can be safely released into nearest waterway Effluent to river Activated sludge used to produce methane gas

raw effluent treated water suitable for discharging to a watercourse

Methane gas fermentation from activated sludge Microbes feed on sewage sludge Anaerobic respiration (fermentation) Methane gas produced Methane can be used as a fuel to drive some of the other processes at the sewage works Methane from sewage is used in developing countries for cooking and heating Methane fermenters

Sewage Sludge Incineration Agriculture Sludge is disposed of in many ways: Dumped at sea Sludge pellets ready for market as fertilizer

Surfers against sewage publicises health risks from polluted water This is dried toilet paper on the beach in Troon, Scotland

Aerobic respiration – complete breakdown of sewage organic material +oxygenenergy+ carbon dioxide +water Anaerobic respiration – incomplete breakdown of sewage organic material a little energy +methane gas

Fermentation and micro-organisms Methane fermentation Micro-organisms release energy from fresh manure in the absence of oxygen (anaerobically) to produce methane gas Chemical oxygen demand (COD)

If the products of anaerobic respiration were released into the waterway They would be used by further micro-organisms in the river These micro-organisms would use up the river’s oxygen supply Semi-treated sewage, therefore, is said to have a ‘Chemical Oxygen Demand’ The aim of a sewage treatment plant is to reduce the COD of the effluent to the lowest possible level before releasing it into the waterway

Need for a range of micro-organisms mixture of many different complex organic materials Sewage fats carbohydrates proteins each species of micro- organism is only able to break down a few substances

many different species are needed to ensure complete breakdown of all sewage materials to carbon dioxide, water and simple inorganic substances In high concentration, however, some of them could later pollute the waterway by causing an algal bloom Bacteria ProtozoaFungi Most of the inorganic substances are harmless Algal bloom

Special bacteria may therefore be used to remove excess ammonium compounds, nitrates and phosphates Ammonium compounds Nitrates Phosphate Bacterium type 1 Bacterium type 2 Bacterium type 3 Harmless nitrogen gas released to air Phosphate stored in granules