Filtration - Graded bed catches bugs more efficiently01 October 2004

By very carefully selecting the layers of material in a graded bed filter, it is said to be possible to catch 99.7% of bacteria and other micro-organisms down to 0.2 µm in diameter. A newly constructed filter, which is being piloted for Scottish Water, can process 216,000 litres of water per day, despite being only 18 in (457 mm) across. The filters are also very easy to backwash with a minimum of water.

A new company, Filter Clear, has been set up to exploit this technology. Target markets are water utilities, both drinking water and effluent treatment; makers of mobile water treatment units; and process equipment for the food and beverage industries.

Graded sand bed filters are very widely used in the treatment of both drinking water and effluent. Sand filtration is referred to in the Susruta Samhita, written in India some time between 2000 and 1000 BC. (Incidentally, this also refers to the desirability of boiling local drinking water, still wise policy today.) Modern industrial-scale sand bed filtration has been around since 1804, when John Gibb constructed a system of concentric sand and gravel filters in Paisley, near Glasgow, to clean water for his bleachery and the town. The main drawbacks with this simple approach are that the filters tend to be very large, because their throughput is limited by clogging in the upper surface layers, and a lot of unwanted microorganisms get through. There has therefore been a movement towards deep-bed filters where the water or effluent encounters the coarsest material first, and is therefore well cleaned by the time it meets the layers of finer material.

Frederick Spruce is a former senior research scientist, university teacher and researcher at UMIST, and long time professional member of IPlantE. He has spent much of his life improving filters, and has now developed and obtained worldwide patents on the Spruce Deep Bed Filter, currently undergoing trials with users and potential commercial manufacturers.

He says that "the nub of the matter is the different shape factors" of the media particles in the four filtering layers, along with their different densities and different particle sizes. The top (first) layer has the largest particle size and lowest density, while the lowest (fourth) layer has the smallest particle size and highest density. The particle shapes are roughly spherical, with a shape factor in the upper three layers of at least 0.6, while that in the fourth layer should be at least 0.55.

The particles are all porous material, and in order that the layers re-form when the filter is backwashed, particle sizes and densities are such that the settling rate increases by about 20% from the top layer downwards to the bottom layer.

Once the backwash cycle has been completed, the filter bed can resettle to its original configuration with a limited amount of intermixing of particles at the interfaces between the layers.

In order to support the fine media at the bottom of the bed, it is necessary to have an efficient underdrain system. The under-drain has to both distribute the water uniformly during backwashing and collect the filtered water without loss of media.

The volume of water required for adequate backwashing is said to be only 0.5-1% cent of the volume of filtrate obtained during a filtration cycle.

Tests and experiments

To simulate the process, filtration tests consisted of filtering a 60 ppm w/v suspension of insoluble silica particles with a size distribution of 4% less than 0.5 µm and about 55% less than 10 µm. The test particles were suspended in potable water with a pH in the range 6.5-7.4.

Throughout the experimental runs, filtrate quality remained between 0.00 and 0.10 NTU (nephelometric turbidity units), which represents 'ultra-clear filtrate', containing only particles less than 0.5 µm. (Clear-looking river water tends to be about 10 NTU.) Filter runs of 40-50 hours were achieved between backwashing cycles with a build-up of filter head loss of 3.5-4.5 psig. When the filter pressure drop reached about 7 psig, particle breakthrough began, followed by a progressive deterioration in filtrate quality.

A further 20-hour continuous test with a suspension of matter with similar properties to cryptosporidium (a microbe common in water supplies) showed that the filter had retained 99.954- 99.974% of particle counts down to 0.2 µm without the assistance of either a coagulant or coagulant aid. Turbidity throughout the run was 0.00-0.10 NTU.

Scottish Water is currently testing a full-sized unit as a tertiary filter on a small effluent treatment plant, with the aim of polishing the final effluent to meet the requirements of new, more demanding standards.

Applications

Filter units have been made ranging from 90mm in diameter with a throughput capacity of 500 l/hr, up to 2m diameter beds and a capacity of 180,000 l/hr. Filtration rates are in the range 10-15 gallon/min ft2 at 10°C and 20-25 gallon/min ft2 at 30°C.

Units with diameters of 100mm or 150mm can be skid-mounted in a LandRover or similar vehicle and used as a completely mobile unit for military or civilian purposes. (The traditional Royal Engineers Brigade Water Unit, by contrast, usually takes some hours to set up.) Larger units can be transported in a lorry or shipping container for emergency relief operations.

The basic technology is, however, far from limited to reducing the cost of supplying clean drinking water, since it is potentially applicable to any kind of liquid from which suspended matter has to be removed for any purpose. Here are some examples:

- Two-metre units are seen as a suitable size for municipal swimming pools, reducing the chlorine requirement.
- As a pre-treatment for reverse osmosis desalination, the removal of sub-micron sized particles is much higher than that of currently used systems.
- This technology is also considered to be ideal for the removal of impurities, bacteria and microorganisms from brining systems in the cheesemaking process.
- One mobile unit is currently being constructed for food industry supplier Prime Engineering of Wrexham.
- Good filtration also forms an essential part of modern brewing practice, otherwise the product is cloudy and has a malty taste unacceptable to most modern palates.
- Research and development is currently underway into the filtering of ship dip to allow water to be safely returned to the land, and the separation of oil and water from effluent to be passed into the sea from ships and oil rigs.
- Because the new filter requires low volumes for backwashing, it is also well suited to applications requiring reclamation of valuable solids, such as electroplating processes.
- Membrane filters are very effective for the complete removal of micron and sub-micron-sized particles, but have a tendency to clog when used to remove larger particles. If placed upstream of membrane filters, the new filter will minimise this problem and maximise intervals between backwash cleaning.

The Spruce filter is also capable of producing the required specification of feed water for the 'electrokinetic water battery'. This is a fairly new idea where water is pumped through channels in glass or some other insulator a few microns across to generate a voltage between the input and output ends. It is possible to do this because many insulators have an excess of electrons on their surfaces. These attract positively charged ions in the water, creating a thin, positively charged layer. When the water moves through a channel, the ions tend to pile up at the downstream end of the channel, while the electrons, trapped in the insulator, cannot follow them. The result is a small positive potential between the downstream and upstream ends of the channel. If these ends are connected by a conductor, a small current flows, which converts the kinetic energy of the water flow into electricity, without moving parts.

The effect has previously been used the other way round to pump water or other fluids in small-scale lab-on-a-chip devices, but researchers have lately been seriously looking at it as a means of generating electric power. Using a commercial porous glass filter 20 mm in diameter with a pore size of 10-16 µm, researchers at the University of Alberta in Canada produced a current of 1.5µA using tap water and a 30 cm pressure head.

Although this does not sound a lot, the process is considered to have considerable potential. The team at Alberta has calculated that it could greatly increase efficiency by using water with a higher salinity and by optimising other factors, such as external load resistance. "In more recent experiments, we have achieved a 1% conversion efficiency," explains team leader Daniel Y Kwok of the university's department of mechanical engineering. "We are not trying hard yet to maximise efficiency. We are still in the proof-of-principle phase." Kwok says that the battery's main advantages are its complete lack of environmentally polluting materials and moving parts.

Experimentation has also shown that the multilayer bed can also be used as an effective biological fluidised bed reactor.

SOE

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