Technology: Washing Plant
Washing, classifying (separation by size or weight) and
dewatering of processed aggregate are important activities at most
sand and
gravel quarries and at an increasing number of
hard rock quarries (Photo 21). These activities are carried out in a
washing plant, which takes the quarried material, uses wash water to remove
silt and clay, and recovers clean aggregate products (sand and gravel, and
crushed rock aggregate). The material removed consists of silt- and clay-sized material (finer than 63 microns) that is naturally present in sand and gravel, or is created during
extraction (e.g.
blasting) or during subsequent
processing (e.g. crushing). Silt and clay may be present as a coating on grains of sand and larger rock fragments or present as discrete particles. Washing improves the efficiency of other activities such as crushing and screening and the quality of the final product. It may also be used to recover fine sand as a commercial product or as material to be blended to create another product.
 Photo 21. Washing plant
A typical washing plant will aim to remove 96 - 98% of material below 63 microns and 100% of material below 150 microns. Removal of the silt and clay from the wash water is required before it can be re-used or discharged to the local water environment. Re-use is increasingly important as control of water
abstraction becomes more stringent.
Typically, washing plants consist of a selection of equipment drawn from the following list, used in series or parallel as appropriate to a particular operation's requirements:
- Scrubber barrels: clean stone, gravel and sand by attrition ;
-
Log washers: remove tough or plastic clays from sand and gravel and crushed materials;
-
Sand screws: separate water and silt from sand;
- Gravel washers: wash coarse sand or crushed stone and gravel and dewater the cleaned material;
-
Hydrocyclone classifiers: reclaim fine sand, using centrifugal force to separate coarser material from fines;
- Classifying tanks: recover coarser material (sand) from large volumes of water through settling;
-
Thickeners,
plate filter presses and
belt filter presses: recovery of clean water (for re-use or discharge) by removal of clay and silt (as a filter cake with a low moisture content) (Photos 24 & 25).
 Photo 25. Filter cake from plate press
Other common equipment includes dry and wet screens, which are described elsewhere. For further details on dry process screening
click here. Not all the items noted above are always necessary. A typical washing plant flowsheet might include:
- feed hopper and conveyor (with a magnet to remove tramp metal);
- gravel washer (e.g. for separation of coarse and fine aggregate);
- sand screw;
- hydrocyclone to remove minus 63 micron material;
- dewatering screens;
- thickener and filter press or a silt lagoon (Photos 42);
 Photo 42. Silt lagoon
 Photo 43. Sand tower and stockpile
Although not strictly defined as part of the washing plant silt lagoons (also known as settling lagoons) are an integral element of managing dirty water produced by the plant (and recovering clean water for recirculation back to the plant at some operations) and they are therefore also considered here.
A washing plant can be small relative to its throughput. Compact washing plants are becoming more common and can be installed in the floor of the
quarry, at quayside operations producing
aggregate from
marine sand and gravel and in urban locations where secondary aggregates are being produced. By choosing the appropriate type and number of unit processes, washing plant can be designed for small, medium and large-scale applications. Generally, as the throughput increases, so does
water use. It is important, however, to distinguish between water use and water consumption see Figure 1).
Water passing through the washing plant may be recycled back to the plant, reused elsewhere on the site or discharged. Each of these options will require treatment of the contaminated wash water to remove contaminating
silt and
clay (the degree of treatment may be different depending on the chosen option; acceptable standards for
reuse of water on-site may be lower than for discharge to local surface waters). Although there are significant opportunities for recycling of water, top up water is still required in all washing plants. Therefore, washing plant are water consumers as well as water users .
 Figure 1. Water use and water consumption. Copyright BGS NERC.
Until recently there has been little incentive to treat water as anything other than a freely available and cheap commodity and only limited signs of investment in approaches that promote the minimisation of water use and consumption. A rapidly evolving regulatory framework means that this approach must now change. Growing pressure on water use by quarrying operations will encourage water-saving and may encourage (and make economically viable) the application of dry processes (for further details click here)
to remove quarry fines as an alternative to the washing plant. In some UK locations an excessive use of water by industry is putting water-based ecosystems at risk and ongoing
climate change is likely to have a significant negative impact on water availability.
Scrubber barrels
Scrubber barrels are horizontal drums (occasionally with a slight slope) in which
feed material is cascaded with water, using attrition and impact to remove silt and clay present as discrete particles or as coatings (Photo 22) . Generally, the feed material and process water are introduced together at the lower end of the drum, which rotates and agitates the feed/ water mixture. Lifters (otherwise known as baffles) can be fitted within the scrubber to facilitate cascading and to help move the feed along the scrubber. The wash water flows over a weir at the upper end onto a screen, which separates the silt and clay from the sand and gravel.
 Photo 22. Washer barrel
Scrubber barrels are generally autogenous that is they rely on the tumbling action of the material and interaction between the particles to remove the silt and clay. Residence time is relatively short, of the order of two minutes or so. The configuration (diameter to length ratio) can be chosen according to the degree of washing/cascading action that is required for optimum liberation of fines. Throughput is determined by the size of unit, ranging from less than 100 up to 500 tonnes per hour. Larger throughputs can be delivered through the use of appropriate units in parallel. The barrels require 23 to 68 litres of wash water per minute depending on the feed rate351.
Key finding
A 500 tph scrubber barrel, 4 m in diameter, might use up to 800 m3 of water per hour, but the vast majority of such water can be captured, cleaned and recycled, reducing consumption to a smaller top up amount.
Scrubbers are generally used to begin the process of removing silt and clay from the raw feed, but they may not fully liberate the silt and clay from the feed material. Where scrubber barrels do not offer a suitable level of performance, log washers can be used (e.g. for the treatment of difficult contaminating clays that cannot otherwise be effectively removed).
Log washers
Log washers are comprised of a gently sloping trough (5 - 10º, with 8º being typical) full of water, within which shaft-driven paddles rotate (the shafts were originally made from logs and this is where the name originates).
Feed is fed in at the lower end of the unit, while water is fed in at the upper end. The abrasion-resistant paddles transport the feed upwards against the flow of water. The rotating paddles maximise attrition and allow the washers to remove clay, agglomerated rock, soft stone, friable waste and fine sand from the feed material. Clay, fine sand and other materials are removed via an overflowing weir at the lower end of the washer, while cleaned
aggregate is discharged at the upper end (often onto a rinsing screen).
Light duty washers are used to handle materials that require only moderate scrubbing. Heavy-duty washers (with a higher-rated motor, thicker shafts and heavier paddles) are used where plastic clays are present that resist moderate scrubbing.
Increasing the trough slope reduces the throughput and increases the retention time, increasing exposure to abrasion and cleaning action on the feed material. Conversely, lowering the slope increases the capacity, but reduces the abrading action and retention time. The log rotation speed can also be adjusted upwards to increase the scrubbing effect. However, this can be counter-productive if it substantially reduces the residence time.
Generally, log washers are most efficient when dealing with narrow feed size ranges (e.g. 40-20 mm or 20 mm 351), taking feed up to 40 mm in diameter, and therefore some pre-screening may be necessary to improve efficiency.
Sand screws
Sand screws (also known as
fine material washers) are based on the principle of the Archimedes screw with a drain trough to one side. As the washed material is
conveyed up an inclined box via a spiral screw shaft, water drains back into the
dewatering trough. From there the water drains back into the settling area and is discharged via an overflow weir. The ideal operating angle for a sand screw is 18º from the horizontal. The pitch of the screw is designed to convey the maximum amount of material at this operating angle. Sand screws can be of substantial size (e.g. 2 m diameter, 10 m length) and handle several hundred tonnes of material per hour. An alternative to a sand screw is a bucket wheel classifier, which performs the same function but operates by scooping the sand out of the classifying tank (Photo 44).
 Photo 44. Classifying tank
The sand screw performs three functions: removal of water (dewatering),
classification and washing. They remove excess water from washed sand so that the sand can be
conveyed and stockpiled. By adjusting flow (velocity) and depth of water in the settling pool, they can be used to effectively classify material and generate a range of products. Although these sand screws are generally most effective in generating fine sand in the size range 63 to 150 microns, by adjusting the setting rate, products with coarser or finer size gradings can be generated. The action of flowing water carries away silt, clays and other lightweight materials (such as wood and organic matter) generating a washed sand product. The tumbling of the sand also produces an abrading action of sand against sand, which helps to remove coatings of clay and other materials as the screw moves the sand up the slope.
When choosing the size of a sand screw, the type of material, its size analysis, the desired product specification, the required production rate and availability of water are all important factors. It is better to choose one that is too large (over-sized) as it is possible to slow the screw speed to deliver the required performance, while it is not possible to increase screw speed on an under-sized unit without detrimentally affecting performance (e.g. by loss of material into the overflow).
The position in the washing plant will vary according to particular application, but will in general be in the final stages (as the output is generally considered to be a product). Sand discharged from a sand screw is dry enough to be transported directly to storage.
Sand screws are very efficient in the removal of fines and the production of a fine sand product. They are particularly useful where some additional (secondary) scrubbing is required to remove coatings present on the desired material fraction. However, the density of slurry in the settling pool must be carefully controlled; an increase in density can result in more coarse particles being lost with the overflow discharge. Equally, an increase in screw rotation can cause agitation in the settling pool and the subsequent loss of particles above the lower cut-off size. Water discharged from sand screws may require treatment (for example in a silt lagoon or thickener / filtration unit) to remove the contained fine material prior to offsite discharge or reuse within the washing plant or elsewhere onsite.
Unlike classifying tanks, sand screws cannot produce more than one product (with a defined particle size range) at a time nor can they remove excess materials from the middle of the gradation range (being limited to the removal of excess fine or coarse material). If these applications are required, then a classifying tank may be more appropriate.
Gravel washers
Also known as coarse material washers,
gravel washers scrub, rinse and dewater coarse sand, crushed stone and gravel. A shaft with a combination of paddles and spiral segments conveys the material through the washer. The paddles serve two functions
scrubbing and fluidisation. By agitating the bed of material the rock-against-rock interaction helps scrub clay and coatings from the gravel. Fluidisation facilitates the removal of unwanted lightweight material from the gravel by allowing it to float free and be removed via an overflow. The spiral segments are primarily used to convey material through the unit and increase the handling capacity of the unit (paddles alone are relatively ineffective at moving material through the washer). The spirals also help wash the gravel by rolling the bed of material.
Gravel washers can be used for several applications. They can be used simply for washing or the removal of unwanted materials. They can also be used for scrubbing the raw feed prior to screening. The configuration of the unit will depend on the nature of the application. They are particularly effective for the removal of organic matter, soil, dispersed clays, and other unwanted lightweight materials in the size range 3 - 75 mm.
Hydrocyclone classifiers
These fine sand reclaiming units have no moving parts (Photo 23). They operate on the principle of centrifugal force, which separates coarser size fractions from finer size fractions. They are often used for the separation of silt and clay from sand, as well as the production of concrete and building sand products. For the removal of silt and clay they normally operate with a cut point in the 40 - 75 micron range; for the production of sand they operate with cut points in the 100 - 250 micron range. The cut point can be adjusted, for example, by changing the internal diameter of the outlets or by adjusting the flow/ pressure of the inlet.
 Photo 23. Washing plant cyclone
Key finding
Using a hydrocyclone gives an accurate cut point because it works on a combination of particle density and particle size. The result is less waste silt and a better quality sand product at the lower end of the particle size range.
Feed is pumped into the
hydrocyclone through a tangential entry, which imparts a swirling motion to the feed suspension. This generates a vortex (central air core) within the hydrocyclone. Coarser (and denser) particles move outwards to the inner wall of the unit and then move down through the apex valve discharging as an underflow product. The finer particles, and most of the water, are discharged through the vortex finder into the overflow product. Any water leaving in the underflow product will carry with it fine solids. Therefore to produce a clean sand product it is common practice to use a second stage hydrocyclone to treat the diluted underflow product from the first stage.
Classifying tanks
Classifying tanks are rectangular in shape, 23 m wide, and up to 10 m in length. They are used to recover sand from large volumes of water by the process of settling. Through the use of multiple discharge valves at the base of the tank, they are able to generate more than one sand product (based on particle size). These products are generated as thickened slurry that requires further
processing, such as
dewatering, before it is ready for
stockpiling and sale. The tanks also efficiently recover water. These tanks may be used prior to the sand screw in order to reduce excess water (and thereby reduce the size of the washer).
A sand/water slurry of pre-screened material less than approximately 10 mm enters the classifying tank through a feed box, which is designed to slow the velocity and direct the flow of slurry as it enters. The slurry stream progresses along the tank, with the particles settling in a position determined by their
density and mass. This means that the coarsest particles settle nearer to the feed box, and finer materials settle progressively further down the tank, separating the sand into its component sizes. Silt and other lightweight unwanted material overflow the weir with the process water.
A range of particle sizes will accumulate at any given point along the bottom of the classifying tank. The narrowness of the size range is determined by the length of the tank, the flow of water, and, less importantly, the overflow head of the wastewater.
Discharge valves are placed along the bottom of the tank (normally 6 to 11, depending on the length of the tank). Material released from these valves is continuously recombined in appropriate proportions of fine and coarse particles to produce the desired end product. The blended product then proceeds to further dewatering. A control system is used to ensure that the blending process is conducted both accurately and efficiently by diverting varying portions of the sand accumulating at each valve station to each of the sand products (according to a predefined recipe for that specific product). A properly controlled classifying tank is able to simultaneously produce two controlled (blended) products and one residual product. The general strategy is to generate a primary blended product that uses most of the sand available to it, a secondary blended product from the sand not used in generating the primary product, and a residual product not used in either of the blended products.
Classifying tanks can perform certain functions that a sand screw alone cannot: they can produce more than one product (with a defined particle size range) at a time and they can remove excess materials from the middle of the gradation range (whereas as sand screw can only remove excess fine or coarse material).
The primary limitation of classifying tanks is that as the feed gets coarser so do the products (and if the feed gets finer, so do the products). Therefore, the control system must be able to accommodate this; simpler control systems are acceptable where the feed is consistent. If the feed is of a variable nature, more sophisticated controls may be required to keep the product within its specification range irrespective of variation in the feed. The downside to this is that in ensuring the specification range is maintained, more material may be rejected, reducing the production rate.
Thickeners and plate or belt filter press
The new limits on water
abstraction, control of effluent discharge, combined with space restrictions and concerns about
lagoon stability have increased pressure for maximising re-circulation of water without the use of lagoons. As alternatives,
thickeners and
filter presses, are becoming more common. Thickeners have slowly rotating rakes that promote settling of
suspended solids and transport solid material downwards toward a central discharge point. The process is assisted by the use of automated dosing with selected chemical
flocculants. Clarified water overflows at the top of the tank and is recycled. The sludge from the thickener is either sent to a silt lagoon for further consolidation or directed to a filter press that can squeeze the sludge to remove remaining water and produce a dry
filter cake.
Key finding
Installing a thickener generally reduces the need for four or five silt lagoons down to one lagoon. Installing a filter presses may eliminate the need for a lagoon at all
Thickeners can substantially reduce the volume of slurry requiring subsequent handling, reducing it to less than one tenth of the volume of unconsolidated sludge in a silt lagoon. The solids content in thickener sludge is also significantly higher than in a silt lagoon, with 40% solids easily achieved. Thickeners are available to deal with a wide range of flows, from less than 10 up to several thousand cubic metres per hour (Photos 24, 45 & 46). Operating costs per cubic metre treated are generally low, but may exceed the direct and most obvious costs associated with silt lagoons. The capital cost of a thickener will also generally exceed the cost of developing a silt lagoon to deal with the same flow. However, there may be other good reasons beyond the obvious financial costs for opting for a thickener (such as land availability, concerns about
sterilisation of
aggregate resources, the need to rapidly recycle clean water, health and safety or environmental concerns associated with lagoons).
Key finding
Wharf operations (for processing marine sand and gravel) may adopt the use of thickeners and press filters to treat the silt and clay, saving space and recovering a large proportion of the water for recycling back to the washing plant.
Thickeners are compact relative to silt lagoons handling the equivalent flow and are useful where land availability is limited or where health and safety risks or maintenance issues for a silt lagoon are unacceptable. More importantly, they can make a significant contribution to rapid recycling of clean water, which is increasingly important as water availability becomes more limited. In these cases, thickeners and press filters may be viewed in a more competitive light despite their higher capital cost.
 Photo 24. Washing plant thickener
 Photo 45. Silt thickener
 Photo 46. Flocculant dosing system
Key finding
Ultimately lagoons may become a thing of the past due to increasing restrictions on water.
The flocculant-treated slurry is continuously introduced into a sludge bed in the main thickener tank, where settling and consolidation occur, to form a thick concentrated sludge . The clarified (clean) water is discharged over the top of the tank, from where it can be recycled or discharged from the site. Concentrated solids are removed from the thickener by pump and can be transferred to a settling area where the material can continue to dewater and dry. However, the use of
filter presses can completely eliminate the need for settling areas or silt lagoons and promote rapid recycling of a majority of water used in the washing plant via a closed circuit (minimising the need for top-up water).
Key finding
There are cost savings associated with switching from silt lagoons to a thickener or filter press system. The main one relates to waste handling, with 95% reductions possible (e.g. from £1000 for the removal of wet silt to £50 for the removal of the equivalent volume of filter cake).
Belt filter presses consist of a continuous filter screen belt constructed of a woven manmade material (Photos 47, 48 & 49)351. Wet slurry is distributed evenly on the filter material, which is then passed through a series of rollers that squeeze the water out, with the water passing through the woven material to be collected for recycling or discharge. At the final end drum the filter cake is removed from the filter belt by scraper and collected for use or disposal. The clean filter belt is then recharged with fresh slurry, and so the process continues.
 Photo 47. Belt filter press discharge
 Photo 48. Filter cake from belt press
 Photo 49. Water recovered by belt press
Plate filter presses are a batch process involving the formation of a hard and stable cake under pressure (Photos 25, 50 & 51). Fully automated press filter systems are available that have automatic cloth washing and assisted cake release to minimise downtime. Compared to the settled solids present in silt lagoons, cake produced by filter press is more easily handled for disposal or further processing due to the low moisture content (less than 10% in some cases) and higher density.
Photo 25. Filter cake from plate press
 Photo 50. Plate filter press
 Photo 51. Plate filter press close-up
Key finding
Choosing the correct filter press is important. Too much clay in the silt slurry can cause blinding of the cloths, making them impermeable. This in turn creates back-pressure that triggers the machine to release the filter cake too early, before it is sufficiently solid.
| Production + Process Technology |
Table 4 |
|
| Characteristics of belt and plate press filters |
| Characteristics |
Belt press |
Plate press |
| Process type |
Continuous |
Batch normally 2 to 3 cycles per hour |
| Capacity |
Capacity depends on belt width and speed |
Capacity for each cycle depends on the plate area multiplied by number of chambers. Cycle length depends on moisture content of feed |
| Feed requirements |
Does not require pressurised feed |
Feed pressurised to 7.1 kg / cm2 (7 bars) or 15.3 kg / cm2 (15 bars) |
Example sizes
|
Smaller: 1 m belt width at 3.5 tonnes per hour (tph) |
Smaller: 2 m2 plate area x 1 chamber |
| Larger: 3 m belt width at 22 tph |
Larger: 1500 m2 plate area x 60 chambers or more |
| Maintenance |
Contains many fittings, but can be fitted with automatic greasers; maintenance is generally not complicated |
Fewer moving parts, but requires plates and cloths to be in good condition to work at optimum level |
| Belt must be kept at the correct tension and tracking (can be automated) |
Requires less continuous monitoring |
| Belts are washed continuously by spray bars just before the new feed |
Plate cloths require cleaning at least once per week using pressure washer or automated |
| Generally cloths last longer |
Generally cloths wear out more quickly |
| Life depends on quality of construction and maintenance |
Life depends on quality of construction and maintenance |
| Product / output |
Generally wetter filter cake (3% to 5% more moisture) but depends on feed moisture and speed |
Generally drier filter cake but depends on feed moisture and length of cycle. |
| Adjustable feed rate, slower speed will result in drier cake but lower capacity |
Only cycles when full so reduced feed rate means less cycles per hour. |
| Capital and operating costs |
Example capital cost = £65,000 to £70 000. Generally, anything less than 10 tph is less expensive than plate press |
Example capital cost: £180,000 £200 000. Automated washing machine would add approximately £35,000. |
| Needs flocculant at 0.5 kg per tonne of solids at a cost of £2.40 to £4.00 per kg |
No flocculant required, but may need lime if the silt is clay-rich |
| Pump size: 15 kW for 10 tph unit |
Pump size = 37.5 kW for 10 tph unit, (because it works at pressure) |
| Pump costs are lower open ended feed, not at pressure, lower height |
Pump costs are higher greater pressure, higher off the ground. |
| Infrastructure costs lower machine just high enough to fit a conveyor underneath to collect the filter cake |
Infrastructure costs higher machine needs to be raised up and requires a more robust filter cake collector |
| Water recycling |
Filtered water recycled in closed circuit |
Filtered water recycled in closed circuit |
Silt lagoons
The separation of the fines and water mixture generated by the washing plant is essential before the water is recycled, reused or discharged off-site. One common option is the use of silt lagoons where the solids are allowed to settle under gravity or with the aid of chemical
flocculants.
Silt lagoon can include lined or unlined
excavated areas (below ground level), bermed areas (above ground level) and sometimes for small volumes of water manmade tanks (Photos 26, 42, 52 & 53). The preferred shape of silt lagoons is generally rectangular, with a length to width ratio of about 5 to 1 in order to prevent short-circuiting (the fast passage of solids through the lagoon without settling). A relatively narrow width also facilitates easier removal of accumulated sediment from the longer sides of the rectangle. The distance between inlet and outlet in each lagoon should be maximised, giving the suspended solids the maximum time to settle out of the water.
 Photo 26. Discharge into silt lagoon
Photo 42. Silt lagoon
Although the design can vary, they all work by slowing the flow of surface waters to facilitate the settling and consolidation of suspended solids. Their use may be constrained by land availability and site topography. Typically, they comprise one or more linked voids, developed in areas of prior working or in areas that have been set aside for water treatment. The volume of water and its suspended solid content determines the rate of filling (that is, the solid mass flow into the lagoon). If used, silt lagoons typically treat water from multiple sources (e.g. washing plant and contaminated run-off) and are designed based on the total volume of water likely to require treatment.
Key finding
Some sites use silt lagoons to treat water from the washing plant and once full they are restored and revegetated rather than being cleared of sediment and reused.
A common approach is to construct two or more ponds in series so that the water becomes progressively cleaner as it passes from lagoon to lagoon, with the coarsest material removed by the first pond, and the finer suspended solids by the subsequent pond(s). This also allows one or more ponds to continue operating while another is being cleaned out. If a lagoon is over a certain size or raised up above surrounding land then it will need to be registered with the Health and Safety Executive and its design and geotechnical stability assessed.
 Photo 52. Silt lagoon discharge point
 Photo 53. Silt lagoon at hard rock quarry
Key finding
Silt lagoons need to be of a sufficient volume to allow the silt particles to settle out and this becomes more difficult as the lagoon becomes full.
Water in silt lagoons may percolate through the base and be lost to the water table. If water is recycled from the silt lagoon, this water loss (and water lost through evaporation) will need to be replaced by top-up water.
Flocculants can be used to promote the aggregation of (ultra) fine particles, accelerate settling, facilitate
dewatering and reduce the lagoon size required to treat a certain volume of water. Ideally they should be added via automated dosing stations to minimise chemical costs, reduce the risk of overdosed chemical being transferred to the lagoon discharge and because excess use can have a detrimental effect on settling. Flocculants may be particularly important during storm events when discharge quality can deteriorate rapidly.
Key finding
Operators realise that silt lagoons are not a cost free option. They have to be fenced and maintained. There is a safety risk that needs to be managed and if they have to be emptied there is a cost associated with that.
The decant rate in silt lagoons needs to be low to minimise water currents and allow sufficient time for settling to occur. The lowest decant point should be set so that the non-decanting volume is about 30% of the total lagoon volume (i.e. only 70% of the pond volume is live storage). If they are to be reused, lagoons should be cleaned out when the sediment level reaches about 20% of design volume. The need for sediment removal should also be reviewed after every major storm event or sustained period of heavy rainfall.
Key finding
There may be delays in restoration of silt lagoons while the silt dries out. The length of time needed for this will vary depending on the level of the water tablesome former silt lagoons will never fully dry out.
Where possible
forebays should be used for silt lagoons. These are designed to slow water entering the main lagoon and should be around 10% of the total volume of the lagoon and 0.5 1.0 m deep. Water should enter the forebay at right angles to the weir that feeds into the main lagoon. The forebay helps slow water and promotes quiescent (stilling) conditions in the lagoon. A completely level and non-erodable (concrete) spreader should be installed between the forebay and lagoon to act as a weir and to further dissipate the water flow energy across the full width of the lagoon.
Baffles can be positioned to modify water flow and minimise the areas that are not effectively settling solids and the recirculation or re-suspension of solids. The correct placement of baffles can also assist with increasing the effective distance between inlet and outlet. Baffles that can be easily removed will facilitate periodic clearing of sediments.
When appropriate, floating discharge points should be installed. These take water from the top of the water column (where suspended solids are at a minimum). They also allow constant discharge rather than discharge only when the pond level reaches a fixed height discharge point.
The installation of automated suspended solids monitors and stop valves at the discharge point should be considered (to ensure that no out-of-specification water is able to leave the site, instead being held in the silt lagoon).
Where space is available, the presence of reeds or other aquatic vegetation in lagoons can enhance the removal of suspended solids by slowing the water and physical filtration by the plant roots. In general terms
wetlands can be described as areas flooded or saturated by surface water or groundwater often or long enough to support those types of vegetation and aquatic life that have specially adapted to saturated soil conditions. Constructed or engineered wetlands attempt to duplicate natural systems and can be designed using settling ponds and lagoons as their starting point. Natural generation of
reedbed and wetlands can occur in suitable settling ponds and lagoons, leading to low maintenance, self-sustaining systems. Artificial or accelerated promotion of reedbed and wetland species may be labour intensive until the systems are established, and may require specialist assistance in design and maintenance. It is important to note that not all sites will be suitable for reedbeds or wetlands. In the quarrying sector many of the additional benefits presented by reedbeds and wetlands (e.g. removal of dissolved inorganic and organic contaminants) may be largely irrelevant, reducing the financial case for specific construction. Reedbeds and wetlands will therefore rely on an existing need for settling lagoons in the majority of cases.
Due to the plastic nature of the settled sludge, the area of the lagoon may have restricted land uses after the aggregate operation has closed. For example if any construction with load-bearing structures is planned, complete removal of the sludge may be necessary. However, with proper planning lagoons can be designed for restoration to a wide range of habitats, as part of landscape improvements and for other beneficial end-uses.
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