Understanding filtration › 2. The equipment

CHAPTER 2.3

Belt filters

Here, the cloth does not stay put: it is an endless belt that moves forward, carries the product, is dewatered, releases the filter cake and comes back round after washing. Two main families share this principle, the vacuum belt filter and the belt press. Their belts wear for reasons all their own.

Nicolas Ract · 7 min read · October 2026

The vacuum belt filter

The principle

A grooved, perforated rubber carrier belt supports the filter cloth and slides over a vacuum box. The slurry is deposited onto the cloth; the vacuum draws the liquid through the cloth, then through the grooves and holes of the belt, into the vacuum box. The filter cake forms, can be washed and then dried by drawing air through it, and drops off at the end of the belt [2].

Its main strength is cake washing, in several successive zones: co-current (two to three displacement washes are common, according to one manufacturer) or counter-current when wash liquid is limited or the filtrate must not be diluted [2]. The applications cited are mainly in mining and chemicals: concentrates and tailings, washed ores, hydrometallurgy, calcium sulphate from phosphoric acid production [2].

What wears

• The wear belts that seal between the carrier belt and the vacuum box: one manufacturer gives a typical service life of around 12 months, depending on the application and maintenance [2].
• The carrier belt, whose splice is cold-bonded below 70 °C and hot-vulcanised above, according to the same manufacturer [2].
• The filter cloth, which rubs on the belt and rollers, is washed on every revolution and blinds with fines. Field experience: on these filters, the cloth rarely tears in the middle; it wears at the edges and at the joint, or ends up blinded.

The belt press

The principle

Widely used for sewage sludge and some industrial sludges, the belt press passes the previously flocculated product between two endless cloths. The fact sheet from the U.S. Environmental Protection Agency (EPA) describes four zones [1]:

• conditioning: a small tank or mixer where the polymer flocculates the sludge;
• gravity drainage: the sludge drains on a flat or slightly inclined belt; its solids concentration increases by 5 to 10 %;
• the wedge zone (low pressure): the two belts come together and form a cake firm enough to withstand what follows;
• the pressure zone: the belts pass around rollers of decreasing diameter, which squeeze and shear the cake.

According to the same source, the cakes obtained range from about 12 to 50 % dry solids depending on the type of sludge [1]. The cake is generally less dry than with a filter press (see chapter 2.1), but the machine runs continuously.

What the EPA says about belt wear

• an average service life of about 2,700 operating hours, within a very wide range of 400 to 12,000 hours [1];
• belt replacement as the main maintenance item, hence the value of a machine designed for quick belt changes [1];
• the need to screen or grind the sludge to prevent sharp objects from damaging the belt [1].

Filter belts and their joints

A filter belt is a fabric, usually synthetic and monofilament, so that it releases the cake well and withstands washing (see chapter 3.2). To form a loop, its two ends must be connected by a joint.

Field experience: the most common are joints made with metal or plastic clipper hooks connected by a pin, spiral joints, and endless-woven or welded belts. Clipper hooks are fitted quickly on site, but they form a thicker, stiffer zone that marks the rollers and scrapers, and where the belt often ends up failing. An endless belt removes this weak point but requires the machine to be dismantled for fitting. The choice is made with the machine builder and the belt manufacturer.

Poor tracking fatigues the joint: it can cause stress and failure of the joint, creasing, fraying of the edges and product leaking at the sides [4][3].

Tracking

A moving belt tends to drift to one side. A tracking system, electric or pneumatic, keeps it centred: sensors detect the edge, and a steering roller corrects its path [4]. The causes of drift cited by a belt manufacturer: insufficient tension, uneven loading, misaligned or worn rollers, uneven drainage across the width, thermal expansion [4].

For a belt press, one manufacturer gives a simple guideline: the seam line must stay straight. An offset of 4 to 6 inches (about 10 to 15 cm) across the width is considered acceptable; at 12 inches (about 30 cm) or more, action is needed. The most frequent cause is uneven distribution of the sludge across the width, and the effect of an adjustment only becomes visible after one to two hours [3].

Belt washing

After cake discharge, the belt passes a spray bar of nozzles that washes it before it receives product again. The EPA recommends washing the belts at least daily, at the end of each shift, with high-pressure water; it stresses that this washing can consume a lot of water and time, and that an automatic system using recycled water reduces this cost [1].

• Field experience: blocked nozzles leave longitudinal blinded strips, visible on the cloth when held against the light; drainage becomes uneven across the width, and the belt starts to drift.
• Field experience: a spray bar set too close or excessive pressure wears the cloth, as on a filter press (see chapter 4.4).

Typical wear patterns

As with filter press cloths, record at each replacement the operating hours, the cause and the location of the defect: this is the basis of the history log described in chapter 4.5.