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In a mechanical and biological sewage plants solids generally occur as screened material, sand trap material and fats, as well as primary, excess and precipitated sludge.

Screen ed material from coarse and fine screens 5-15 l/(P x a)
Sand trap material from sand trap 5-12 l/(P x a)
Fats from fat separator Generally part of sand trap

The amount of solids to be dis­posed of after biological stab­il­isation is much more affected by the type of catchment area and amount of industrial waste water occurring than the type of sludge stabilisation. If a large sewer system is connected to a sewage plant and the population density is low, an increased amount of sludge is often found with respect to the population equivalent; it is generally 20-60 g DS/(P x d) or 7-25 kg DS/(P x a). In addition to precipitated sludge, 10-30 kg DS/(P x a) must be disposed of in plants with P precipitation.

Primary sludge PS Sludge material from the primary sedimentation basin ; 20-60 g/(P x d); grey-yellow; putrefies quickly; contains pathogenic germs and worm eggs; depends on catchment area (industrial waste water) and sedimentation time
Excess sludge ES The bacterial mass formed in the activation basin which must be removed from the system; depends greatly on the waste water purification method; 15-60 g/(P x d); in smaller and medium-sized plants the excess sludge is often fed back to the inflow for primary sedimentation (--> sediments together with PS)
Precipitated sludge Sludge which arises by addition of chemicals; with simultaneous precipitation in the ES; with 1.5-2 g of P to be removed by precipitation/(P x d), 10-15 g DS/(P x d) of precipitated sludge is produced
Filtration sludge From downstream filtration stages; the sludge material arising when the filters are flushed is generally fed back into the sewage plant inflow; little effect on the sludge production of a sewage plant

PS and ES are almost always stabilised together.

The properties and constituents of the sludges depend essentially on the producers of the waste water. The only way to protect the environment from pollutants is to retain them at the point of production – at the industrial and commercial sites.

The treatment of sedimentation sludge is primarily focussed on reducing the water content (increasing DS by concentration, dehydration, drying) and reducing putrescibility (stabilisation). Depending on the options for accommodating the treated sedimentation sludge, other requirements may apply, for example requirements for hygienisation, reducing the content of org. substance or preventing leaching of heavy metals. The choice of method and process design of the sedimentation sludge treatment plant depend on the options for storing the prepared sludge – it is to be matched to the sedimentation sludge recycling/disposal stage.

It should generally be ensured that anaerobic conditions for the sludge are avoided during sludge treatment, in order to avoid unintentional re-dissolution of P.

The sludge treatment methods can be divided into three large method groups.

sludge stabilisation conversion into a non-putrefiable form
anaerobic , aerobic or chemical
concentration, dehydration, drying reducing water content and volume
decontamination killing pathogens (important especially in the event of agricultural use)

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