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Leachate Treatment

Sand Filters including Moving Bed Sand Filtration

Process Type:

A sand filter is a simple filtration treatment device which can operate under gravity but is frequently operated under pressure to increase the flow through the media.

The primary effect is to remove suspended solids by filtration and possibly also adsorption.

The application of sand filters to raw leachate is rarely appropriate and is more applicable as tertiary treatment. The optimum application is to reduce levels of solids from up to 200 or 300mg/l, down to below about 30 or 50 mg/l.

Backwash water should be reused by returning directly into the biological treatment reactor.

As any filter operates it will accumulate solids within it. The presence of these solids will progressively fill the voids reducing permeability, and the pressure or head loss increases with usually a corresponding flow reduction.

When the pressure head loss becomes unacceptable, as solids are progressively entrained within the sand media, the filter needs to be washed. Such equipment is most commonly backwashed, by reversing the flow of water to an upwards direction.

Backwash water (generally, treated effluent is used), sometimes using air to agitate the sand media, with addition of air and/or chemicals and the released particles are usually discharged into a balancing tank, or may be returned into the original biological treatment process.

Backwashing is automatically controlled either by adjustable timers, or by sensors which detect when the pressure differential exceeds a pre-set value. During the period of backwashing, no effluent can be treated by the bed, unless the bed is progressively removed from the filter, washed, and returned to the filter, as in "moving bed filters".

The volume of backwash water produced will be determined by the concentrations of suspended solids in the feed, to some extent by their nature, and by the concentration of suspended solids required in the effluent.

Many proprietary systems for fixed bed filtration and continuously operating "moving bed" filtration are available, but few have been used in the UK for either raw leachate pre-treatment, or for final polishing of biologically treated effluents.

The resultant final effluent from a sand filtration system can have low levels of residual solids, and the application is particularly useful for final polishing before discharge to a river.

The interception of solids can also be a useful technique for the removal of substances capable of bioaccumulation, which may be present in biological solids, or in some colloids.

Recently, tertiary treatment of biologically treated leachate has been carried out using a recovered media made from waste glass, with a particle size range of 0.5-1.0mm. The much smoother surface of the recovered glass has enabled the media to be cleaned more effectively using simple backflushing, and the removed solids have been returned to the treatment process.

This has been particularly useful for maintaining nitrifying bacteria in the treatment process, and for the elimination of
List 1 substances from the discharge.

An alternative type of sand filter is the moving bed, or continuous backwash filter, which has been developed into several forms, the most well-known being the proprietary DynaSand system, which is currently in use in tens of thousands of applications Worldwide, since its introduction 2 or 3 decades ago.

The moving bed sand filter operates continuously, avoiding the need for periodic shutdowns to allow the sand to be backwashed, as sand is cleaned continuously by means of an internal washing system.

An airlift pump and draft tube, in the centre of the unit, recirculates sand and filtered particles from the bottom of the filter to the top of the vessel, which is usually open, into a separation box at the top of the unit.

Here sand is separated from the removed suspended particles by turbulent action, the heavier grains of cleaned sand falling back into the top of the filter, and the lighter solid particles flowing over a weir to waste. As a result, the sand bed is in slow, constant downward motion through the unit, water purification and sand washing take place continuously, and no moving parts are involved in the system.

Chemical flocculants (e.g. FeCl3) may sometimes be added to water being treated, to improve the performance of the process.

Advantages:

Low energy use and running cost when compared with membrane filters.

Disadvantages:

Will not remove the always remove smallest particle sizes.

Where best used:

After a biological treatment stage.

Costs comments:

Initial cost of plant is significant.

Sustainability comments:

Minimal use of resources other than energy. FeCl3 is a plentiful resource.

Energy usage comments:

Average.

Chemical usage/by-product production:

FeCl3 produces a sludge for disposal.


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