In order of increasing cost and decreasing product moisture the most common dewatering options are a thickener, deep bed thickening, belt presses and membrane filter presses. 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. The cost of flocculant is often a major operating cost of dewatering equipment. Belt press filters in general have the lowest flocculant consumption for any of the listed processes excluding membrane filter presses and centrifuges.
Increasing the feed solid concentration increases the solid filtration rate, minimises cake moisture content and produces a more homogeneous cake all of which are desirable outcomes. If increasing the feed solid concentration is not practical the addition of flocculants in a pre-treatment step has a similar result. The optimum dosage level of flocculant can be found by monitoring the viscosity of the slurry.
Homogeneous cakes are desirable as if the feed slurry is too dilute the filter cake will contain higher moisture content as a result of stratification. 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.
The minimum cake discharge thickness for horizontal belt press filters is in the region of 5 mm.
The choice of belt is critical to the function of the belt press filter and a wide variety of materials and weaves are available. The filter cloth for a belt press filter should be as open as possible while maintaining the desired filtrate clarity or, if precoat is used, to prevent the loss of precoat. Lighter cloths produce a clearer filtrate and do not block as rapidly however their durability and life span is significantly shorter than heavier cloths. Both seamless and seamed belts are available. Seamed belts wear faster at the seam and cause wear at the rollers and the doctor blade. Zipper-type and clipper-type seamed belts are also available with the zipper-type having a longer life span as they provide less discontinuity. Seamless belts have the longest life span but are more expensive. Also it should be ensured that the belt press is compatible with a seamless belt.
Increasing the temperature of the feed slurry decreases the viscosity of the liquid phase. This is beneficial as it increases the filtration rate and decreases the cake moisture. The same advantages can be obtained by other drying methods such as passing dry steam through the deliquored cake to raise the temperature of the remaining moisture, or other drying methods can be utilised.
Cake thickness may have to be controlled or restricted when cake washing is required or the final cake moisture is a critical parameter. When cake washing time is a dominating factor the maximum filtration rate will occur when the minimum cake thickness for discharge is achieved. The time required for washing is increased by the square of the ratio of cake thicknesses. For example, if the thickness of the cake is doubled the washing time will increase roughly by a factor of 4.