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8 FILTRATION

The purpose of filtration is to remove suspended particles from water by passing the water through a medium such as sand. As the water passes through the filter, flock and impurities get stuck in the sand and the clean water goes through. The filtered water collects in the clearwell, where it is disinfected and then sent to the customers.

Rapid sand filter (high-rate granular media filtration)

Filtration is usually the final step in the solids removal process which began with coagulation and advanced through flocculation and sedimentation. In the filter, up to 99.5% of the suspended solids in the water can be removed, including minerals, floc, and microorganisms.

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Surface water with low turbidity and low colour can treat by DIRECT FILTRATION - there is no sedimentation prior filtration. When chemicals used rapid mixing is needed and flocculation stage is either eliminated or reduced mixing time less than 30 min.

The water quality standards needed for direct filtration varied, but example quality standard of colour (mgPt/l) and turbidity (NTU) must be under 25. Often pilot testing is valuable in determining the efficiency of direct filtration to conventional treatment. - filtration rate 3-10 m/h - cheaper than conventional treatment and chemical costs lower

Direct filtration can be used also example when remove Fe from groundwater (oxidation by air or by chemicals before). In direct filtration it is not sensible to dose metal based coagulant prior filtration as this would itself impose a heavy solids loading on the filters, leading short filter runs. Thus the coagulant normally used is a polymer dosed at low rate, like. Filter construction http://www.youtube.com/watch?v=Uf8Rn_W6sys

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Polymers used two main types. Moderate molecular weight cationic polymers (DADMA) are added ahead of flocculation to strengthen the floc while relatively high molecular weight nonionic polymers (polyacrylamides) are added just before filtration to aid in floc removal.

Polymer aids can be troublesome in some respects. The powdered form of the polymer is very slippery, so spills should be cleaned up quickly. In addition, extended use of polymer aids may gum up the filters. As a result, polymer aids are often used like coagulant aids - in extreme situations to improve the water quality for a short time.

Another type of filtration, known as in-line filtration (kontakti-suodatin), involves operating the filters without flocculation or sedimentation. A coagulant chemical is added to the water just before filtration and coagulation occurs in the filter. In-line filtration is often used with pressure filters, but is not as efficient with variable turbidity and bacteria levels as conventional filtration is.

Requirements

Filtration is now required for most water treatment systems. In US there is requirement that filters must reduce turbidity to less than 0.5 NTU in 95% of each month's measurements and the finished water turbidity must never exceed 5 NTU in any sample.

As you will recall, TURBIDITY alone does not have health implications. So, why the strict regulations? Although turbidity is not harmful on its own, turbid water is difficult to disinfect for a variety of reasons. Microorganisms growing on the suspended particles may be hard to kill using disinfection while the particles themselves may chemically react with chlorine, making it difficult to maintain a chlorine residual in the distribution system. Turbidity can also cause deposits in the distribution system that create tastes, odors, and bacterial growths.

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However, turbid drinking water has other troublesome implications as well. Sand filtration removes some cyst-forming microorganisms, such as Giardia which cannot be killed by traditional chlorination. Cysts are resistant covers which protect the microorganism while it goes into an inactive state.

In US regulations require that at least 99.9% of Giardia cysts and 99.99% of viruses be removed from drinking water. Since it is difficult to test directly for these microorganisms, turbidity in water can be used as an indicator for their presence. By requiring a low turbidity in the finished water, treatment plants are ensuring that few or no Giardia are present in finished drinking water.

In a few locations, surface waters are used for domestic purposes without filtration. In these situations, the water is obtained from a watershed which includes only undeveloped areas. The watershed is patrolled and carefully managed to prevent contamination.

MECHANISMS OF FILTRATION

There is many mechanisms in filtration and many kind of classificate them, here one way to classificate those most important for water purification.

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Straining

Passing the water through a filter in which the pores are smaller than the particles to be removed. The picture below shows an example of straining in a filter, the floc cannot fit through the gaps between the sand particles, so the floc are captured. The water is able to flow through the sand, leaving the floc particles behind.

In many cases, the pores between sand particles in the filter are much larger than the particles captured by the filter. It has been suggested that small particles become wedged between sand grains as filtration occurs, making the pore spaces smaller and allowing the filter to strain out yet smaller particles. However, a clean filter will produce clean water before any of this pore size-reduction has occurred. Therefore, it is now believed that straining is not an important part of most filtration processes.

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Adsorption

The second, and in many cases the most important mechanism of filtration, is adsorption. Adsorption is the gathering of gas, liquid, or dissolved solids onto the surface of another material, as shown below:

Coagulation takes advantage of the mechanism of adsorption when small floc particles are pulled together by van der Waal's forces. In filtration, adsorption involves particles becoming attracted to and "sticking" to the sand particles. Adsorption can remove even very small particles from water.

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Biological Action

The third mechanism of filtration is biological action, which involves any sort of breakdown of the particles in water by biological processes. This may involve decomposition of organic particles by algae, plankton, diatoms, and bacteria or it may involve microorganisms eating each other. Although biological action is an important part of filtration in slow sand filters, in most other filters the water passes through the filter too quickly for much biological action to occur.

A electron photomicrograph of the complex biological matrix found in the schumtzdecke, or biolayer, in a slow sand filter

Plastic carrier – large surface to grow for micro-organisms

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Absorption

The final mechanism of filtration is absorption, the soaking up of one substance into the body of another substance. Absorption should be a very familiar concept - sponges absorb water, as do towels.

In a filter, absorption involves liquids being soaked up into the sand grains, as shown below:

After the initial wetting of the sand, absorption is not very important in the filtration process  

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Some mechanisms whereby particles in a flow of water thorough a filter may come into contact with a particle of filter media

Collision mechanisms in a granular filter

Filter types

There is many ways of classification filter: - open filters, pressure filters - up flow – down flow - physical, chemical, biological filters In here is discussed more of - rapid filters (open & pressure) (physical filter) (pikasuodatin) - slow sand filters ( flow rate so small that biological processes have time to happen) (hidassuodatin)

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RAPID FILTERS / Reduction efficiency In practice those sand filters have grain size ca 1 mm and overflow rate some 6 m/h, typical particle removal efficiency is shown next picture. Very small particles are removed predominantly through diffusion and large particles by straining. Particles around 1µm are removed largely by interception and sedimentation, and these processes are less effective.

Typical particle removal efficiency of granular filter Those typical improvements what can be attained by rapid filtration to water quality are ca: Parameter Before After filtration Turbidity (clay), mgPO2/l Colour (Fe, humus) mgPt/l KMnO4-number, mg/l Fe (Fe(OH)3), mg/l Al, (Al(OH)3), mg/l Microbe,giardia cyst,removal%

20 50 30 2 3

0,5 5 5 0,1 0,2 15-99 % reduction

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Number of filters / size

For cost point of view, one filter is the most ideal. It is good to have at least 2 filters parallel, so when 1 filter is in backwashing mode water income filtering can directed to other filters. In this short time some overloading of other filters is acceptable. Many times, however, four filters are the minimum number that should be used to allow filter washing and occasional need for filter to be out of use for maintenance.

Filter can be quite big, 420 m2 have been reported. But practical maximum size a filter is 90 m2provided the plant is not extremely large. And many times it is better to do more smaller unit - example water amout for backwashing is wery bin in big filters.

Construction

Open rapid filter: 1- water in, 2- filtrated water 3- backwash air 4- backwash water 5- backwash water out 6. sand 7- filter floor 8- purified water tank Backwash http://www.youtube.com/watch?v=vsTuj9W-9h0 http://www.youtube.com/watch?v=itfrOB2ech0 Water W http://www.youtube.com/watch?v=whwEBxqa3yU

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Open rapid gravity filter using a plenum floor

Rapid sand filters in water plant

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Sandfilters & pipings for bacwashing under filters

Used sand and overflow rate of open rapid filter

Grain size 0,8-1,2 mm - thickness of sand 1,0 – 1,2 m - overflow rate 5-6 m/h - water surface min. 1,0 m up from sand surface - max hydraulic loss in filtration 1,5 – 2,0 m Grain size 0,5-1,0 mm and 0,4-0,7 mm - thickness of sand 0,7 – 1,0 m - overflow rate 5-6 m/h - water surface min. 1,0 – 1,25 m up from sand surface - max hydraulic loss in filtration 1,5 – 2,0 m

Nowadays some bigger overflow rates has used, 6-8 m/s. The space reserve for sand extending in backwashing is at least 50 % (outlet channels). Outlet channels normal 0,7-1,0 m over the sand, weir-overflow rate 10-20 l/s/m.

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In single media filters the upper part of filter takes most of the particles. this phenomena is boosted a by backwashing, when larger grains settler faster .

Higher loading rates are possible with dual media filters /up around 12 m/h, or for coarse filters for manganese removal up around 15 m/h

Many media filter,

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Very common is dual media filter, sand & anthracite (effective size = d10, uniformity coefficient = d60/d10, number is sieve pass %)

Backwashing, basic types

Filters can run several days without washing. Quite typical is backwashing once per day. Can control automatic by time, pressure loss or water turbidity. Water used for bacwashing 2-4 % of treated water.

1 water backwashing - sand extend 30 - 50 %, water flow 10-15 l/m2/s (36-50 m/h) - 10-20 min 2 water + air backwashing - 5-10 min air 15- 20 l/m2/s (50-75 m/h) ja water 2-4- l/m2/s ( 7-15 m/h - 5-10 min water 10-15 l/m2/s (36-50 m/h) Air makes the abrasion between sand particles better

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Various types of uderdrain systems, a) with gravel support b) without gravel support (more popular).

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Sometimes it is used surface agitators with up flow water.

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Development of sub atmospheric pressure within gravity filter

Filter sand clogging from the top first. If the head loss in the sand at any point exceed the static head of the water on the filter, sub atmospheric pressure will be induced which may cause dissolved air to be given off from the water and result in air binding of the filter. In poorly designed or operated filters, this tends to happen at around the time that the filter needs washing. Level control on the filtered water outlet such that negative pressure cannot develop in the bed is easy way of avoid that.

Development of negative pressure in rapid gravity filter. Lines 1 to 2 represents the change in pressure through the filter as the media becomes blinded. Line 5 results in the development of negative pressure within the media.

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Performance during working period

Rapid gravity filter can produce relatively high turbidity water (up 0,5 – 1 NTU) after backwashing. This should drop rapidly to around 0,2 NTU or less around 30 min. the turbidity of water grows then along using time when the filtering capacity of media is approached.

Using cycle of gravity sand filter. Those peaks of loss of head are typically in the time when other filters are backwashed and the flow grows temporary.

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PRESSURE FILTERS

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Good in small water plants – possible to have only one pumping.

 

In communal use normal upright cylinder , height 2-4 m. - thickness of sand 0,8 – 2,5 m - overflow rate 7-10 m/h (7 m/h common) - sand extending in backwashing ca 50 % - pressure same than in water network - backwashing water 8-15 l/m2/s - backwashing air (if used) 15 l/m2/s

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INVERTED FILTERS (käänteissuodatin)

Up flow filter where in bottom is coarser filter media. Overflow rate without grate 6-8 m/h and whit grate 15-30 m/h. (arinallinen (grate) käänteissuodatin)

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CONTINUOUS FILTRATION

The DynaSand® filter is based on the counter flow principle. The water to be treated (red arrow) is admitted through the inlet distributor (1) in the lower section of the unit and is cleaned as it flows upwards through the sand bed, prior to discharge (blue arrow) through the filtrate outlet (2) at the top. The sand containing the entrapped solids is conveyed

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from the tapered bottom section of the unit (3), by means of an airlift pump (4), to the sand washer (5) at the top. Cleaning of the sand commences in the pump itself where impurities are separated from the sand grains by the turbulent mixing action. The contaminated sand spills from the pump outlet into the washer labyrinth (6) in which it is washed by a small counter current flow of clean water. The separated solids (brown arrow) are discharged through the wash water outlet (7), while the grains of clean sand (which are heavier) are returned to the sand bed (8). As a result, the bed is in slow, constant downward motion through the unit. Compressed air for the sand pump is provided by via the control panel. Thus, water purification and sand washing both take place continuously enabling the filter to remain in service without interruption. Can operates rates up 15 m/h, depending of use.

• Drinking water treatment.

• Industrial effluent treatment and water supply.

• Recovery and reuse of water.

• Treatment of wastewater before discharge.

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Process water treatment to a box board manufacturing plant. Nearest place where used in water plant: Meri Lapin Vesi.

SLOW SAND FILTER (hidassuodatin)

 

  Slow sand filter boosted by GAC  

 

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 Slow sand filter in Vaasa waterworks 

 

‐ biological filter ‐ used only treatment for good raw water or secondary treatment for taste enhance ‐ water 0,7 – 1,0 m over the sand ‐ over flow rate 5‐40 cm/h (normal 10‐20 cm/h) ‐ maturation can take 3‐6 weeks – time biology need for start ‐ when loss of head 40‐60 cm, skim (peel) 2‐3 cm top sand, when sand layer near 70 cm – sand washing/ replace ‐ inside or outside  ‐  outside – skimming not in winter (only 2 times/year)  

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 Typical slow sand plant layout 

 

 

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 Access ramp and slow sand filter 

 

Under drain installation 

 

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On the ground build slow sand filter 

 

 

Sieve graph of good slow sand filter sand. It would be good if the sand used in slow sand filter would be quite near sieve graph over, where. ‐ d10      0,35 mm ‐ d60 / d10    2,5 < 0,2 mm   < 1 % > 2 mm    < 1 % ‐ Fe < 1 %  &  no humus or impurities 

 

 

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DIATOMACEOUSA EARTH FILTERS 

are compact, high efficiency filters which are suitable for armies in the field, swimming pools and meeting short term emergencies and in industry. 

They are small and depending on the deposition of filter powders of diatomaceous earth on the porous filter candles for their filtering action. They cannot deal with highly turbid eaters and because extremely high head loses in the filter their running costs are high.