User:AGauke/Sandbox

=Belt Press Filters=

Process Characteristics
Belt press filters are designed for solids capacity, by weight or volume, rather than wastewater flow. Solids concentration must be determined based on the concentration of primary solids in the wastewater and further solids that may precipitate during treatment. Solids concentration for a process will vary, thus the design must have the capacity to deal with varying solids concentration.

The feed to a belt press filter depends on the type of solids, desired product and filter design, however for many sludge types the feed dry solids concentration is typically in the range of 1-10%. The resulting dewatered sludge (or cake) dry solids concentration typically falls in the range of 12-50%. Dilute feed solids concentration results in a cake of higher moisture content whilst a higher feed solids concentration yields an improved solids filtration rate and drier end product.

The input to a belt press filter is generally measured as the rate of dry solids loading (mass of dry solids per time per belt width). Again, the input solids loading is dependent on the sludge type and filter media, thus there is great variation in the dry solids loading rates of operating belt press filters. Typically, lower range solids loading rates fall in the range of 40-230 kg/hr/m belt width and high range solids loading rates fall in the range of 300-910 kg/hr/m belt width. Whilst loading is important for measuring production rate, it is also important to consider the thickness of the cake that forms in the gravity drainage section. Cake thickness affects the permeability of the filtration media and the filtration rate. Testing for the particular sludge type must be conducted to determine the optimum cake thickness. In some cases where filtrate recovery is important, it may be necessary to introduce a cake washing step.

The primary objective of a belt press filter is to dewater process sludge and much of this dewatering occurs in the gravity drainage zone. The gravity drainage zone can achieve a 5-10% increase in solids concentration. 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. Sludge conditioning is the addition of chemicals to promote flocculation of particles to form a thickened sludge and to promote dewatering. Dewatering can be promoted by the addition of surfactant and flocculation is achieved via the addition of high molecular weight polymer. Flocculation is improved with optimum polymer dosage, polymer dilution and mixing. The pH of the feed slurry must also be monitored and controlled as low pH decreases flocculation. 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. The effects of sludge conditioning are most apparent in the gravity drainage zone which can be easily replicated on a laboratory scale where the optimum conditioning strategy can be determined. For a belt press filter to be industrially viable it must be economically efficient and thus maximum throughput is desired. Without sufficient conditioning, the gravity drainage is generally the limiting process step, but with optimum dilution the limiting process step can be shifted to the compression zone.

In the compression zone of a belt press filter, the filter cake is compressed between the two belts and passed over rollers to exert pressure on the cake. There is an optimum number of rollers above which a drier product is not necessarily the result. Drier product is obtained from reduced belt speed rather than increased pressing time.

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

The efficiency of a belt press filter is often assessed based on the dry solids content of the product cake, solids recovery and lateral migration of sludge on the belt. Solids recovery is the percentage of dry solids recovered from the feed sludge. Solids recovery is dependent on the filter media which must be selected for good permeability to promote dewatering but with pore diameter sufficiently small so that solids recovery is not greatly decreased. It is important that the belt press filter has an effective belt washing section so that blinding does not decrease the permeability of the belt. Solids recovery is directly related to filtrate quality and thus the filter media and process arrangement must satisfy the desired cake and filtrate qualities. Dry solids content is a measure of the degree of dewatering. The degree of dewatering is increased when the belt speed is decreased. Lowering the belt speed reduces the capacity of the process. The following correlation relates input mass flow rate to belt speed :

$$Q_0=m_0 s_b L_{sludge0} $$

Where Q0 = mass flow rate (kg/s), m0 = mass loading (kg/m2), sb = belt speed (m/s) and Lsludge0 = initial width of sludge across the belt (m). Thus to maintain industrial scale economic throughput at lower belt speed, the mass loading and width of sludge across the belt must be increased. It has been found that increasing the solids loading slightly decreases the dry solids concentration of the cake while significantly increasing the potential for sludge to overflow the belt. Lateral migration of sludge on the belt is a measure of how the sludge spreads across the width of the belt. Increased lateral sludge migration means that sludge is escaping the edge of the belt and overflowing into the filtrate. Therefore, increased lateral sludge migration negatively impacts filtrate quality and dry solids recovery.

Generally, the minimum design discharge cake thickness is 3-5 mm. This ensures that the cake is thick enough to discharge and is easier to remove from the belt.

Reference E not listed

Heuristics of the design
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. Belt press filters have the lowest flocculent consumption.

Typically, a belt filter press receives a slurry ranging from 1-4% feed solids and produces a final product of 12-35% cake solids.

Increasing the feed solid concentration increases the solid filtration rate, minimises cake moisture content and produces a more homogenous cake. The addition of flocculants in a pre-treatment step has the same advantages. The optimum dosage level can be found by monitoring the viscosity of the slurry. If the feed slurry is so dilute that rapid settling occurs the filtration rate is decreased and the filter cake contains higher moisture content as a result of stratification.

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 with a wide variety of materials and weaves 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 clothes produce a clearer filtrate and do not block as rapidly while 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 available and the zipper-type have a longer life span as they provide less discontinuity. Seamless belts have the longest life span and are more expensive and it should be ensured that the belt press is compatible with a seamless belt.

Increasing the temperature of the feed slurry increases 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 passing dry steam through the deliquored cake to raise the temperature of the remaining moisture or other drying methods.

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.The thickness of the cake does not have to be critically controlled when cake moisture is a controlling factor. In general thicker cakes are dryer than thin cakes as the moisture transferred when the vacuum is realised is relatively constant.

Necessary post-treatment systems
Completely clear filtrate cannot be obtained using belt press filters except in rare circumstances. Thus further treatment may be required on the discharge before reuses or discharge. If the filter is downstream of a clarifier or thickener the filtrate (and wash water) can be recycled back into the clarifier to reduce the required filtrate clarity and allows for the use of more durable cloths. If recycling or reuse is not an option the filtrate may be discharged. Further treatment before discharge (filtration or chemical treatment) may be required.

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. The polymer content makes it more suited to this than a cake conditioned with ferric chloride and lime.

Recent developments
Developments in belt press filters include: Cloth Developments: Double weave incorporates different yarn types to combine the specific advantages of each. A double weave woven wire belt is also possible increasing the life span and durability of the belt over a conventional wire belt.

Belt press filter using three belts: The addition of a third belt allows for independent speed and types of belt in the pressure and gravity zone. This allows the filter system to accommodate higher hydraulic loadings occurring with dilute feed sludge (feed solid concentration below 1.5 %). The three belt system is more efficient with both a higher production rate and cake solid concentration at the expense of mechanical complexity.

V-Fold Belt: The V-fold belt is similar to the belt filter press with the main difference being that only a single belt, folded along the centreline is used. The technology has not been widely proven. A final dry weight solids content of 9-13% can usually be achieved; this is smaller than competing technologies. 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).V fold belts have a small footprint, low energy and wash water consumption and low capital and operating costs. They are self-tracking and can processsludge of varying composition, reducing operator involvement.