Belt filter

[1] The process of filtration is primarily obtained by passing a pair of filtering cloths and belts through a system of rollers.

The belt filter is mainly used for dewatering[2] of sludge and slurry and juice extraction from apples, pears and other fruits, as well as grapes for winemaking, etc.

Belt filters are used both municipally and industrially in a range of areas including urban sewage and wastewater treatment, metallurgy and mining, steel plants, coal plants, breweries, dyeing, tanneries, as well as chemical and paper factories.

[2] Belt filters are considered simple and reliable, with good availability, low staffing, easy maintenance and a long life.

[2][4][7] The belt filter is most advantageous when installed such that it is open and viewable at floor level for easier adjustment and monitoring.

Although all problems can be controlled by enclosing the filter, the enclosure reduces essential visibility and easy access to the machine for maintenance and repairs, leading to the expensive automation of the process.

[2] Belt filter designs are crafted using manufacturer design and performance data, operating installations, pilot testing, surveys of similar plants and testing of the wastewater solids[2] to obtain a desired dewatered solid percentage from the sludge or slurry to be processed.

[2][5] Preconditioned slurry, which is flocculated and/or coagulated depending on the feed and process, is thickened in the gravity drainage zone.

[7] Belt filters are very versatile and are made to suit the sludges, slurries or mashed fruit to be processed.

For a feed or treatment process which produces unpleasant odours, volatile emissions, pathogens and hazardous gases like hydrogen sulphide[2][7] the belt filter can include fume hoods or even be completely enclosed in a gas tight housing.

[7] Due to the reduced visibility and increased corrosion associated with enclosure, the belt filter process can also be automated.

[2] Belt press filters are designed for solids capacity, by weight or volume, rather than wastewater flow.

[2] The resulting dewatered sludge (or cake) dry solids concentration typically falls in the range of 12-50%.

[2] The degree of dewatering in the gravity drainage zone is greatly dependent on the type of solids, the filter media and the sludge conditioning.

The dewatering achieved in the gravity drainage zone is adversely affected if the sludge is poorly spread across the belt or the residence time is insufficient.

[10] It is important to find the optimum value for each conditioning parameter as too much polymer or mixing can have a negative impact on flocculation and greatly increase operating expenses.

[10] For a belt press filter to be industrially viable it must be economically efficient and thus maximum throughput is desired.

[11] The overall performance of a belt press filter is improved where variations in parameters such as sludge type, feed solids concentration and conditioning are minimised.

Therefore, increased lateral sludge migration negatively impacts filtrate quality and dry solids recovery.

In general centrifuges and other competing technologies do not show a significant cost advantage compared to the belt press filter, for the same cake dryness.

If increasing the feed solid concentration is not practical the addition of flocculants in a pre-treatment step has a similar result.

The minimum feed concentration that results in a homogeneous cake is determined by observing a sample of the slurry.

If rapid settling occurs the filter cake formed will not be homogeneous and the filtration rate is decreased.

[5] The minimum cake discharge thickness for horizontal belt press filters is in the region of 5 mm.

[5] The filter cake usually has a high enough solid concentration to allow for all types of disposal methods without further treatment including recycling back into the process, landfill/composting and incineration.

Cloth developments include the double weave which incorporates different yarn types to combine the specific advantages of each.

The three belt system is more efficient with both a higher production rate and cake solid concentration at the expense of mechanical complexity.

Currently this technology is suited to small-scale applications (up to approximately 3000 L of slurry per hour as the maximum belt size is 0.75 m).