The Great Stour Project - River Pollution

Heavy Industry

Chartham Paper Mill and St Mildred's Tannery are two large industrial factories in the Canterbury area. Canterbury is not as industrialised as Ashford (upstream) or Sandwich (downstream) where more serious pollution problems exist, but any industrial processes like paper-making or tanning pose potential hazards by the range of strong chemicals employed, and the intensity and scale of production. Water is an essential ingredient in the manufacturing processes as well as the means to discharge effluent.

Industrial effluent is strictly controlled, but accidental spillages can still occur from time to time, and can be very serious since heavy metals such as lead, mercury, cadmium and copper may be involved. They are very toxic to aquatic life. Water may also be used as a coolant in the manufacturing process. Warm water discharge is a problem, because oxygen is less soluble at higher temperatures. There will be less dissolved oxygen. Fuel oil breakages are another potential danger. Oil is very pervasive and, floating on the water surface, can coat plants and animals, reducing oxygenation and (in animals) destroying the insulating properties of fur and feathers. Oil will reduce photosynthesis in green plants; it can enter the food chain by ingestion, and will bioaccumulate in the higher animals in the food chain.

Heavy industry (potential pollution)

Manufacturing industry, such as paper making and tanning, involves a wide range of complex organic chemical substances. Worst are non-aqueous phase liquids (NAPLs) which include chlorinated hydrocarbons widely used as industrial solvents. Concentrations of these compounds is allowed in drinking water only in miniscule amounts i.e. parts per billion or micrograms/litre. A tiny amount of dense NAPLs has the potential to contaminate very large volumes of water to levels in excess of the acceptable limit. Leather-processing, for example, uses a dense NAPL (tetrachloroethylene solvent) in the manufacturing process.

Experience in British Columbia (Canada) has shown that paper mill discharges have complex effects on river water, including increased suspended solids, increased temperatures, high BOD, lower pH, oil and grease, and saline solutions containing chlorine, ammonia, defoamers, resin acids, furans and dioxins. Dioxins are a group of powerful organic chemicals associated with chlorine bleaching; even in minute quantities, dioxins are a significant health hazard, and can have serious effects on wildlife. In Britain, many paper mills have invested in expensive water treatment plants; industrial effluent, rich in fibres, clay and other chemicals, has a strong BOD character.

In tanneries, the treatment of animal hides is also a complex manufacturing process, involving the use of ammonium salts, enzymes, organic salts and degreasing agents. Tanning is achieved by the addition of chromium sulphate and sodium bicarbonate; dyes, resin binders and waxes are applied in the final stages. There are a number of potential pollution problems: sulphide waste, chromium discharge, suspended solids, pigments and dyes. These all have a high BOD quality.

Heavy industry (preventive measures)

Paper mills usually have their own water treatment plants with sedimentation tanks. Chlorine pollution can be reduced using alternative bleaching methods (for example peroxide bleaching), and by using already-bleached imported pulp. Re-cycling waste paper is sometimes carried out but is not necessarily a better solution; de-inking processes are expensive and difficult. Chartham Paper Mill has its own treatment plant with IPPC authorisation and agreed Environment Agency consent; trade effluent is monitored regularly. The EA report some problems of BOD and discoloration in the river at Chartham, but there is no ammonia problem. General water quality at Horton, downstream of the mill, is "good" in three indicators (BOD, dissolved oxygen and ammonia), and the River Ecosystem Classification is also "good" here, supporting all types of fish species, game and coarse.

The primary aim of pollution control in a tannery is to remove excess sulphides and chromium discharges. Recycling wash water, spent chromium and dye solutions, reducing suspended solids in settling tanks, and removing toxic sulphides by oxidation are all recognised ways of achieving this. St Mildred's Tannery is linked directly to Canterbury STW through the public sewage system, and no effluent problems have been reported. Concentrations of chromium released at Canterbury STW amount to 8.2 ug/litre (micrograms per litre or parts per billion), against a background concentration of 1.1 to 1.3 ug/litre. Heavy metals such as chromium are potentially dangerous, but in such tiny concentrations pose no significant pollution risk.

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