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IUE 9 Common effluent treatment plants (CETP) are often used for collective
treatment of tannery waste water. To reach the plant, sewers are used,
but sometimes their characteristics are not adapted to the special
requirements of typical tannery effluent. The following recommendations
are proposed to prevent any damage or accident that might happen when
using inadequate or inappropriate equipment. All sulfur compounds can generate hydrogen sulfide mainly under anaerobic
conditions. Tanneries processing raw hides/skins mainly use sodium
sulfide to remove hair or wool. Tannery effluent may contain sodium
sulfide and therefore it is highly dangerous for anybody to enter
a sewer without proper monitoring and without respiratory safety equipment.
Even with careful sulfide oxidation in the tannery, there is a risk
of H2S being developed in the sewer under acidic conditions and this
gas has severeeffects on unprotected humans, depending on concentration
and exposure time (see IUE Recommendations for Odour Control in Tanneries).
It is recommended that mechanical systems are used to clean sewers. Alkaline and acid floats should be kept separate in the tannery. Sulfide and other sulfur compounds are also a danger for concrete
sewers, because they can be oxidised to sulfate, which solubilises
the calcium content of cement and concrete and thereby damages the
fabric of the sewer. It is recommended that tanneries should use plastic
sewers or plastic lined sewers. PVC, polyethylene or fibre reinforced
plastic sewers are suitable for transporting tannery effluent. Tannery effluent typically contains a large amount of suspended solids
(1 to 3 g/l) and when the flow circulation is too slow, deposits can
occur, so clogging is likely. It is recommended that smooth materials
should be used for the sewer and to maintain a minimum slope of 1
cm per metre in length. Manholes should be installed at each angle
of the sewer and the distance between two manholes should not exceed
50 metres. In order to reduce clogging in the sewer, it is recommended that solid
waste should be separated from effluent streams, by using a screening
unit with holes or spaces between bars not larger than 10 mm. Some
mechanical screening equipment with finer mesh (2 mm or less) can
be used to reduce the quantity of hairs and fibres sent to the effluent
treatment plant. Appropriate pretreatment is required if effluents
are transferred to a common effluent treatment plant. In order to prevent any increase in volume of tannery effluent, it is recommended that rainwater from the tannery's roof should be collected as carefully as possible, then sent in a separate sewer to surface water disposal. Alternatively, if the collected volume is great enough to be of value, it might be used in processing, e.g. soaking or even dyeing, since it will be soft water.
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