Feature34Filtration+Separation January/February 2009

In this, the first of what is intended to be a set of articles reviewing recent developments of the technologies of filtration and its closely related separative processes, the main topic is sedimentation under the force of gravity (centrifugal sedimentation, the province of the high speed disc separator and the decanter, will be covered in a later article). This is coupled with separation by a screen or sieve, which is largely gravity driven, and separation in a deep bed (or sand) filter, where, again, the driving force is often that of gravity.

The full range of equipment types to be covered therefore is as follows, although not all will have noteworthy developments:

• gravity clarifiers and thickeners,

• lamellar separators,

• classifiers and elutriators,

• flotation cells,

• screens, both stationary and moving, and

• deep bed granular (sand) filters,

with which are also included the surface biological reactors, or trickle filters, used for treating water and wastewaters with similar purpose to other items in this list.

The article will look at developments in the listed equipment types, but also at novel items that aim to replace them.

Gravity sedimentation

The separation of solid particles from a suspending liquid by sedimentation under the force of gravity is characterised by the need for large land areas on which to stand the settling tanks. It is potentially a very efficient process, with actual efficiency determined by the settlement area devoted to the task. It

is also a relatively low consumer of energy, even though it often has to deal with high volumetric flow rates, because the flow rates in terms of flow per unit cross sectional area of flow channel are quite low. If, therefore, there is land available, at a reasonable price, gravity sedimentation is an attractive means for clarifying dilute suspensions. For this reason, it is extremely widely used in fresh water clarification, in municipal and industrial waste water treatment, and in mineral processing and similar industrial applications.

However, there is not always sufficient land available, or there are higher priority uses for

the land that is currently occupied by a large clarifier, so that considerable development has gone into improving the clarification process in order to achieve the same separation efficiencies in a smaller ground area.

Clarifiers and thickeners

Clarifiers, which are intended to produce a clear overflow, and thickeners, which aim to produce an underflow considerably more concentrated than the feed suspension (often with a clear overflow as well), have been improved in performance by the use of coagulants and flocculants mixed with

Filtration and separation technology:

What’s new in sedimentation? I

n the first of a set of articles covering new developments in a number of broad classes of filtration and other separation equipment types, Ken Sutherland looks at such developments in the fields of gravity sedimentation, flotation and screening.

Figure 1: A high aspect ration feedwell. Image courtesy of Outotec Australia.

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Feature 35Filtration+Separation January/February 2009

the feed suspension so as to increase the average size of the suspended particles, and so hasten settling. The appearance of synthetic flocculants several decades ago gave a marked increase in separation efficiency, and modern developments have concentrated upon

achieving rapid and complete contacting of the flocculant with the suspended solids, and, of course, in having available progressively more efficient flocculants.

An example of the improved flocculant addition and mixing system is the “next generation” feedwell introduced by Outotec, of Australia, particularly for thickeners (see Figure 1). Feedwell designs had changed little over the past 40 years or so, but the demands placed upon them have become more severe as flow rates have increased, and energy considerations have become more important. The patented Outotec design has benefited from computational fluid dynamic modelling, and features an inner floor of vanes, separating the feedwell into two zones, an upper zone for mixing and a lower one for aggregation.

Although synthetic polymers have been of huge benefit to gravity sedimentation processes, they are not the only flocculant of current interest, as is shown by Veolia Water’s Actiflo™ compact clarification process, which uses microsand particles as the seed for floc formation. The denser sand particles give the suspended solids much improved settling characteristics, resulting in high overflow rates and short retention times. The resultant small settling tank footprint can be as little as one twentieth that of conventional plants. The

microsand particles can be recovered from the settled sludge and recycled within the plant.

What must be counted as a major development in the field of clarification is the appearance on the treatment process marketplace of several large companies, who have all spotted the value of the water and wastewater business. These include Axel Johnson/Parkson, GE, Severn Trent, Siemens and F L Smidth, in addition to Veolia, all of whom now offer clarification plant in the form of efficient packages.

The existence of a major process need in the clarification of such large quantities of liquid as is presented by the water and wastewater sectors attracts, not surprisingly, any number of new equipment developments aimed at taking some of this business from the conventional clarifier. These are mostly filters rather than sedimenters, but one of the most intriguing is the Fibra filter, which is an in-line device, consisting of a bundle of fibres each parallel to the liquid flow, which are squeezed together pneumatically to act as the filter, and then expanded and forward flushed to clean (see Figure 2). Although demonstrated so far only on relatively small industrial waste applications, there is no reason why sufficient modules should not be mounted in parallel to achieve any required throughput.

Figure 2: The Fibra filter, which is an in-line device, consisting of a bundle of fibres each parallel to the liquid flow. Image courtesy of Fibra.

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Feature36Filtration+Separation January/February 2009

Lamellar separators

One of the ways in which the footprint for gravity settlers has been decreased was to cut the “floor” of the settler into several pieces, and then to stack them one above the other. In order for any collected sludge to be able to escape, the sections have to be mounted at an angle (usually 45-60°) to the horizontal. The resultant tilted-plate (inclined-plate or lamella) clarifier has become a familiar feature in sedimentation systems, either as the main separator, or as a polishing separator working on the overflow.

The main operating feature of the tilted-plate separator is that a suspended particle has only a small distance to fall before it reaches the “floor” and so be removed from suspension. Its disadvantages are its inability to produce a sufficiently high solids concentration in the underflow, and the chances of scaling or deposition on the lamella surfaces.

The tilted-plate separator as the main settlement zone in a packaged water treatment plant can be seen in versions of the Veolia Actiflo™ system mentioned above, while Siltbuster’s range of mobile separators, employing tilted-plate packs, are a common sight wherever a short term need for clarification arises.

Flotation systems

The opposite of sedimentation in the sense that the suspended solids are caused to rise to the top of the separation vessel rather than fall to the bottom, flotation works by attaching an air bubble to each suspended particle or floc, giving it the buoyancy to rise. Originally generated by perforated sparge tubes or mechanical agitation at the base of the vessel, most flotation systems are now of the dissolved air type, in which air is dissolved in a flow of clean water at high pressure, and the resultant super-saturated water is injected into the suspension, the dissolved air coming out of solution as fine bubbles, of the order of 50 µm in diameter.

A major improvement in the DAF system has come from allying it with a lamella separator, which significantly reduces the footprint of the flotation unit. Such a system can be seen in the Siemens RC Series flotation plants, where the fastest rising particles go straight to the top of the flotation tank, while the slower rising ones are separated in the plate pack. Siemens’ flotation plants are sold as featuring DAF and induced air flotation, while GE has a new technology for waste water treatment that it terms “entrapped air flotation”.

Already well known for its Split-O-Mat® series of flotation plants, Envirochemie launched last year a new Flomar series, among which the HL range features a high performance separation package especially suited to the food and meat industries.

Screens

Screens are not sedimentation devices, but they are covered in this article because the driving force is often that of gravity, and they are a significant component of the treatment processes for water and waste water, in common with the clarifiers just discussed. Screens exist in a wide range of types, from the simple in-line strainer to the complex rotary drum screen or the multi-deck gyratory separator. They are used for separation of particles from a liquid flow, usually in a position upstream of a more delicate filtration requirement, and for separating two or more fractions from a mixture of differently-sized solid particles, either in the dry or wet state. For this classification duty, an essential feature of the screen is the accuracy of its apertures, and its ability to retain this accuracy in use. This results in large-scale use of metallic media, either perforated plates, or crimped wire mesh.

The most developed of the screen types is the vibratory (or gyratory) screen separator, which is now available in sizes up to 2 m in diameter, for cleaning of recycled water, and other scalping and classification duties. As evidenced by the pages of Filtration + Separation, new features are available from manufacturers, such as Kason, Russell Finex, and Sweco on an almost annual basis (see Figure 3).

Deep bed filters

Although also not a sedimentation device, the deep bed filter is included here because many do operate by means of gravity (i.e. hydraulic head), and the greater proportion of them is used in the water and wastewater treatment applications alongside (or instead of) gravity clarifiers and flotation plants. They work actually by depth filtration within the interstices of the sand particles (except for the slow sand “filters” that use a biological reaction in the topmost layer of the bed). The collected materials must be removed

periodically, which is done by flushing with water in upward flow, to expand the bed and release the trapped solids, after which the bed solids are allowed to settle back to their normal layer height.

The developments of the deep bed filter over the years have mainly been aimed at overcoming its innate limitations – the wish to filter downwards through coarse particles and then fine for the most effective filtration, whereas the expansion and resettlement of the bed during flushing leaves the sand with the finest particles on top and the coarsest on the bottom. The most effective solution to this problem has been the multi-media bed, which has layers of different materials essentially of differing densities laid in the bed with the least dense, which resettles most slowly, as the top layer, and the most dense at the bottom.

One of the most recent developments of the deep bed filter has been its use in the tertiary stages of water and waste treatment, as a polishing filter after the secondary active process. An example of this is SevernTrent’s TETRA™ Deep Bed tertiary filter, which is a downflow unit that can be readily switched on and off, thus optimising operating costs. This has been further developed as the TETRA Denite™ system, which combines solids capture with the biological removal of nitrogen (as nitrate).

A different kind of bed medium is now supplied by the Inversand Company, in the form of a manganese greensand that not only removes suspended particulate material, but also dissolved iron, manganese, arsenic and hydrogen sulphide. The latest version, Greensand Plus, has much greater resistance to attrition.

The most important development of the fixed deep bed filter has been its adaptation to a moving bed format, in which the bed of granules falls downwards from a separation zone, to be picked up in a flow of transporting fluid, which carries it upwards to a cleaning zone, from which it falls back to the filter to complete the cycle. This requires just one grade of bed particle, but takes up much more vertical space than the fixed bed. Parkson’s DynaSand™ is a good example of such a system, now used for enhanced nutrient (i.e. phosphorus) removal.

Contact:Ken Sutherland Tel: +44 (0)1737 218868Email: ken.suth@ntlworld.comKen Sutherland has managed Northdoe Limited, his process engineering and marketing consultancy, for over 30 years. Northdoe is essentially concerned with filtration and related separation processes. He has written numerous articles for Filtration & Separation and for Filtration Industry Analyst, and also four books on separation processes, most recently an A to Z of Filtration and the fifth edition of the Filters & Filtration Handbook, both for Elsevier.

Figure 3: The most developed of the screen types is the vibratory (or gyratory) screen separator. Image courtesy of Russell Finex.