Filtratie bedrijfsvoering_eng.pdf

8/10/2019 Filtratie bedrijfsvoering_eng.pdf

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TECHNICAL COOPERATION

PDAM PONTIANAK - OASEN GOUDA

FILTER MANAGEMENT

Section of Production:

INSTRUKTUR OASEN: THE NETHERLANDSIr. Peter Mense. OASEN :Mr.Rob van Klaveren. Managing Director : Ir. A.B.I.M. Vos de Wael

Director Operations : Ing.H.ArdeschProject Manager : Ir.A.HaasnootTeam members of Oasen

Gouda, 21 mei 2007REPUBLIC OF INDONESIA

Mr. H.Nuijten. PDAM PONTIANAK:Managing Director : Ir. SyahrilTechnical Direcor : Mr.Masriyatno

Team members of training centre PDAM Pontianak


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Filter management

In order to have the filtration process run optimally proper management is necessary. This tutorial deals

with the most important management aspects.


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It is important to log the various necessary maintenance activities. This canbe done in an automated system, but the easiest and most practicalmethod is to create an overview similar to figure 1, and keep it up to date.Figure 1 displays the most important maintenance activities. Depending onthe construction of the filter and the characteristics of the filtration processadditional activities can be necessary. Add these activities to the planning.

Filter management

Figure 1


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Daily inspections Level of the water layer above the filter bedThe water layer above the filter bed is influenced by two factors:

the load on the filter (higher load means a higher water level) the contamination of the filter bed (the dirtier the filter bed, the higher thewater level)

The filter must be cleaned, by backwashing, when the water level almost reaches theoverflow sill. Backwashing will clean the filter bed and restore the capacity.

Optically inspecting the top layer of the filter bedLarge pieces of debris, like plastic or leaves can sometimes get stuck in the filter. Thisdebris can negatively influence the filtration process because a part of the surface isblocked. It is very important that these obstructions are removed during the dailyinspections. This can easily be done with a net of some sort.

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Daily inspections

Inspecting the water distribution over the filters, and adjusting it if necessaryThe distribution of water over the filters must be as even as possible. Unevendistribution will cause one filter to receive a heavier load then the other, causing it toclog up faster, and increasing the backwash frequency of that particular filter.During peak flow the preferential filter will receive too much water, causing it tooverflow. This in its turn will prevent the plant from reaching her the maximumcapacity. It is therefore very important to regularly check the distribution of waterover the filters. Note that different flows will require a different distribution over thefilters. This means that an optimum must be determined between the distribution andthe different flows.

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Backwashing

Daily inspections Backwashing the filterThe filtration process causes the filter bed to become contaminated, which, in its turn,causes an increase of the filter bed resistance. A filter is cleaned by reversing the waterflow, and hereby backwashing the filter with clean water. This backwashing can bedone with water alone, but air can be added as well. Usually water and air will not beused simultaneously to prevent the filter bed from washing away. When relativelycoarse filter material is used, it can be possible to backwash using water and air

simultaneously.The most common backwashing procedure consists of one session, lasting a couple ofminutes, using only water, followed by a second session, only using air, and finishedwith a third session, again consisting of only water.


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Backwashing

Daily inspections Backwashing the filters at a previously determined indicator (time or water layer level)There are two possible indicators to determine whether a filter must be cleaned:

1. Based on a previously determined level of the top water layer (e.g. 20 cm. under theoverflow sill);

2. Based on a determined period of time (e.g. once every 24 or 48 hours).

An immediate backwash is necessary when the filter overflows.It is critical that the backwashing happens regularly, according tot the chosen indicator. If this is not

the case the filter will clog in such a manner that it cannot be cleaned by backwashing. When thishappens the filter material must be externally cleaned or even completely replaced.

De backwash velocity and interval depends on the level of contamination, the filter material, and theconstruction of the filter. The following values can be used as an indication.

The backwash velocity for a sand filter ranges from 25 to 40 m/h; The backwash interval when using water and air ranges from 2 to 4 minutes; A complete backwash will, on average, require 3 to 5 times the total water volume of the filter.The proper backwash velocity can be determined experimentally. When the velocity is relatively high

it is advised to periodically check that no filter material is washed away during backwashing. Thiscan be done by positioning a net with a small mesh size or a white bucket in the overflow drain todetermine whether or not filter material has been washed out.


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DefinitionsTo properly understand the various aspects several definitions are critical

Surface: This indicates the size of a two-dimensional area. Surface is sometimes called size,especially for ground surfaces. The dimension of surface is square meter, of m 2 .

Volume: This is a measure for the space an object occupies (three-dimensional). An the volume ofan angular object equals length x width x height. The dimension is cubic meters, or m 3 .

Time: in seconds, minutes or hours

Velocity: The velocity of a certain movement indicates the distance traveled by an object over a

certain period of time. E.g. is the distance increases with 3 kilometers every hour, then the velocityequals 3 km/h (three kilometers per hour). The dimension of velocity is meters per second (m/s) or kilometers per hour (km/h).

L L

B B

Lengte:L =

Volume:Q = L x B x H

L

H

Oppervlakte: A = L x B

Filter management

Length

L=

Surface

A = L x B

Volume

Q = L x B x H


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Flow (discharge) measurements (volume/time)In reality it is often necessary to have an accurate estimation of the discharged amounts of water.To obtain this estimate a container with a known volume is filled with water. The amount of timenecessary to fill the container is logged. Later on the flow can be calculated by dividing the knownvolume by the time necessary to fill the container. On a small scale this can be done by using a

bucket. On a larger scale a filter basin can be used. The principal in both situations is the same: aknown volume is filled while measuring the time needed to fill it. The flow is then calculated bydividing the volume by the time it took to fill it.

Example: An empty basin of 1 m 3 is filled with water in 72 seconds. What is the measured flow, inm 3 /h?

Solution: 1 m 3 /72 seconds = 0,00139 m 3 /sec = (x 3600 sec) 50 m 3 /h

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Another example:Flow measurement using a real filter:Carefully measure and document the (internal) length and width of the filter basin. Lower the water layer until the filtermaterial becomes visible. Now measure the distance between the filter bed and the overflow sill. Multiply thesemeasurements (length x width x height) to obtain the available volume above the filter bed. Take out your stopwatch.Close the outgoing valve of the filter and open the incoming valve.

Start your stopwatch as soon as the surface of the filter bed is submerged. Stop the stopwatch when the filter starts to

overflow. Document the measured time.

Dividing the calculated volume (m 3 ) by the measured time (s) will yield the incoming flow of the filter (m 3 /s or, whenmultiplied by 3600 s, m 3 /h).

Example:Length (L) = 7 m, Width (B) = 4 m, Height (H) = 0,75 m, Time (T) = 720 sFlow = Q = (L x B x H) / T = (7 x 4 x 0,75) / 720

Q = 0,0292 m3

/sQ = 105 m 3 /h

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Filtration velocity (flow (discharge) / surface)The filtration velocity is the velocity with which the water percolates through the filter bed. Thisvelocity can be used to indicate the load on the filter. It can also be used to calculate if the filter hassome capacity left.The filtration velocity is the flow (discharge) that is distributed over a filter, divided by the totalsurface area of that filter.

Example:Flow = 72 m3/h, surface area = (L x B) = 8 x 2,5 = 20 m2Filtration velocity = (flow / surface area) = 72 m3/h / 20 m2 = 3,6 m/h = 0,001 m/s = 1 mm/s

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Calculating backwash flow

Monthly inspections Inspecting the capacity of the backwash pump

Determining the discharge of the backwash pumpA backwash pump is not always equipped with a flow meter. It is, however, important to regularlyinspect the capacity of the pump. Due to wear and tear, or internal defects, it is possible that thepump can discharge less water over time, which, in its turn, can cause the filters to becomecontaminated. The flow of the backwash pump can be measured with a simple measurement.First, carefully measure and document the internal length and width of the filter basin. Decrease the

water lever until the filter bed becomes visible. Measure the distance between the filter bed and theoverflow sill. Multiply the length, width and height to obtain the available volume above the filter bed.Take out your stopwatch.Start the backwash pump and start the stopwatch as soon as the filter bed becomes submerges. Stopthe stopwatch as soon as the water reaches the overflow sill. Document the time. Divide the volume(m 3) by the time (s). This yields the capacity of the backwash pump (m 3 /s or, when multiplied by3600 sec, m 3 /h)

Example:Length (L) = 7 m, Width (B) = 4 m, Height (H) = 0,75 m, Time (T) = 120 s

Flow = (L x B x H) / T = (7 x 4 x 0,75) / 120 = 0,175 m 3 /s = 630 m 3 /h


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Backwash velocity (flow (discharge)/surface)It is important to be able to calculate the backwash velocity of a filter. If the velocityis too low, the filter will not be cleaned properly, and contamination will occur faster.It the velocity is too high, the filter material can be washed out. The calculation

method is similar to the method used to calculate the filtration velocity. The onlyexception is that, in the case of backwashing, the water is delivered by the backwashpump instead of the filtration process.Calculating the backwash velocity is done by dividing the flow of the backwash pumpby the surface area of the filter bed.

Example:Flow= 630 m 3 /h, surface = (L x B) = (7x4)= 28 m 2Backwash velocity = (flow/surface)= 630 m 3 /h / 28 m 2 = 22,5 m/h = 6,25 mm/s

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Monthly inspections Inspecting the functioning of the blowerThe blower is usually not equipped with a flow meter. By experience, however, it can quite accurately bedetermined whether or not the blower discharges a sufficient air flow. The indicator for this is theintensity of the bubbling in the water during the backwashing with air.A second method to inspect the blower is checking the pressure gauge on the backwash air pipe. If thisgauge is not present it must be installed. The proper pressure the blower should produce can be foundin the technical specifications. This value should be displayed near the pressure gauge (e.g. by writing itdown using a felt-pen or a permanent marker), to enable the operator to easily observe the pressureand thereby check if the blower is functioning properly.For extra safety the piping system should be equipped with a safety valve in the air pipe. This safetyvalve enables the air to escape when the blower is turned on and all other valves are closed. Using thissafety valve is not dangerous and will not damage the installation when it is used occasionally.

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Monthly inspections Measuring filter bed heightThe height of the filter bed must be measured monthly. The data obtained from thesemonthly measurements must be logged so that it can be compared to the data fromother filters. An overview of this long term data will give an indication if the height ofthe filter bed is decreasing. This is tremendously important because a decreasing filterbed height means that sand is escaping from the filter. This can either be caused by adefective backwash nozzle, or by a too high backwash velocity.

Filter management

Bottom 2007 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec250 cm Filter 1 150 148 149 150 148250 cm Filter 2 147 149 147 148 150

250 cm Filter 3 150 149 150 151 151250 cm Filter 4 152 150 152 154 156250 cm Filter 5 148 149 147 148 150

A B B B B B B B B B B B B

B

A

H=A-B


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Monthly inspections Inspecting the functioning of the valvesIn order to backwash a filter properly it is vital that neither air nor water can escape to other filters. Itis therefore important to inspect all the valves monthly. This can be done in the following way:

Backwash valves (water) :

Close one of the outgoing valves, fill the filter until it starts overflowing, and then close the incomingvalve. Start the backwash pump while all the valves are closed. Wait for three minutes, and when thefilter does not start overflowing it can be assumed that the backwash valve of that specific filtercloses properly. Be alert that the test does not exceed 5 minutes, because the pump axle, whichneeds constant cooling, does not get cooled when there is no flowing water. Exceeding the 5 minutemark may result in damaging the pump.

Backwash valves (air):Verify that the piping system is equipped with a safety valve. Only proceed with this test when thissafety valve is present and functioning properly. Start the blower while all the valves are closed.When all the valves close properly the air will escape through the safety valve. When a backwashvalve is leaking bubbles in the filter will appear.

Filtrate valves:Fill a filter until it starts overflowing. Leave it alone for approximately 10 minutes with all the valves

closed. When the level drops one or more valves are leaking, and further research is necessary todetermine which one. If the first two tests showed no leaking valves it will most likely be the filtratevalve.

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Filtratie bedrijfsvoering_eng.pdf

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