SLS Technology Pty Ltd has consulting experience with provision of filter aid options such as Perlite, DE (Diatomaceous Earth) and Cellulose.
Filter Aid Testwork
SLS Technology provides testwork based filter aid recommendations reviewing the 3 most significant filter aid parameters;
Particle Size Distribution
Competent water treatment design engineers apply Filter Aid equipment when a size separation filtration in the particle size range between 5 and 50 micron is required. When removal of sub 5 micron particles in the raw water is important the competent design engineer will select other separation technologies such as media bed or micro membrane filters.
Filtration - Filter Aid Processes and Machinery
There are a wide variety of filter aid processes each with numerous machinery options. By removing suspended solids Filter Aid machinery are generally referred to as clarifying filters.
Precoat filters apply a coating of Filter Aid material typically in the nominal 5mm thickness range before processing of the raw water commences. The water treatment engineer applies a Precoat Filter where a simple size screening separation is desired.
Precoat filters can be either pressure or vacuum filter machines. The coating of filter aid is held to the filter septum or cloth by the velocity of the water permeating through the filter aid coating. Thus, flow must never be stopped, unless a recirculating flow can be immediately established.
Admix or Body Feed applications are where the Filter Aid material is added continuously with the raw water to build a filtration bed with some depth. The water treatment engineer applies a Body Feed machine where the additional adsorption of microscopic particles is preferable.
Perlite Ore is mined from volcanic silica glass deposits. Perlite Ore has a unique property; when it is heated it undergoes a dehydration, delamination and deformation resulting in the formation of Perlite.
High quality Perlite Filter Aid requires a tight particle size distribution and ideally a uniformity of particle shape. Perlite filter aid material is manufactured by particle sizing to meet the product specification. The Perlite particle shape distribution results from the ore purity and the heating process. Whilst both the particle shape and size range determines the observed filtration performance.
In simplest terms the particle shape and size range affect the pressure differential and the resultant clarity. But particle size also affects the mass per unit volume and hence cost per dose, and parameters during the backwash.
Astute filter aid users are aware of the importance of backwash efficiency. Particle size distribution affects the filter cake release and desorption rate during the backwash.
Perlite manufactures using a premium grade Perlite Ore can refine their pure Perlite products into as many as 8 particle size range specifications. Manufacturers who use inferior Perlite Ore are forced to offer specifications with wider particle size range.
Perlite filter aid material provides the user with a density advantage between 20-50% over Diatomaceous and milled filter aids, which enables a proportional cost reduction as filter aid usage is volume dependent, not weight.
Usable with Standard Filtration Equipment
Perlite filter aid can be used with both pressure and vacuum filtration equipment by merely replacing the identical filter aid volume.
For expert optimisation of the Perlite grade, SLS can assist with plant or laboratory filtration studies to identify selection of the optimum filter aid.
Diatomaceous Earth is a naturally occurring ore made over millennia consisting of the salicaceous skeletons of many different forms of diatoms. DE filter aid is created by refinement of DE ore.
Diatomite is a unique mineral due to its low density, high porosity and relative inertness, making it an ideal filtering medium. However being an ore it is seldom pure and being derived from living creatures it is rarely consistent in size.
Thus, different manufacturers will generate different specifications of DE varying in dryness, particle shape and size distributions and purity. In additions, some manufacturers will calcine the DE to enhance its insolubility and mechanical robustness, whilst other will sell the material with minimum QA virtually straight mined from the ground.
In general DE easily crumbles into a fine powder and therefore dust mask PPE are recommended for handling the filter aid. The US EPA and WHO recognise that respiratory health risks are associated with all forms of DE.
The typical chemical composition of oven-dried diatomaceous earth is 80 to 90% silica, with 2 to 4% alumina (attributed mostly to clay minerals) and 0.5 to 2% iron oxide. The Aluminium and Iron content generates a significantly different surface chemistry to Perlite and this can provide a significant filtration difference.
Cellulose and Other Organic Media
Cellulose Filter Aid is available in many different forms, the most common being produced by a steam processing of low grade woods into fibres. Less common cellulose Filter Aids are derived from rice husk, potato starch, cotton, hemp and corn cob.
Cellulose Filter Aid fibres are characterized by a high aspect ratio, which enables it to precoat a septum or filter cloth very easily.
Cellulose is the preferred Filter Aid in pH extremes where silica, iron or aluminium dissolution from DE cannot be tolerated in the process and in processes which are extremely silica intolerant. Noting, Silica is a component of wood and other organic plants that thus not all Cellulose Filter Aids are Silica free.
It is not unusual to find Cellulose Filter Aid products as a combination with Perlite or DE to give a synergistic enhanced ease of use. A typical example is where the weight of DE provides a greater cake release performance in a plate and frame filtration machine.
Filter aids find applications in many different industries and can be sourced from many different manufacturers, each in a range of specifications and widely different quality. Ideally, a filter aid possesses these properties;
It does not impart taste, colour or odour to the liquid being filtered.
It is insoluble in all water chemistry and temperatures.
When at rest it builds a jagged interlocking structure forming a myriad of porous channels when against a support membrane, such as cloth textile or steel mesh.