Filtration and Nozzles

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FILTRATION

DESIGN and INSTALLATION


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TECHNICAL COOPERATION

PDAM PONTIANAK - OASEN GOUDA

Design and Installation

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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Contents

History

Construction

Filter bottom

Backwash nozzles

Filter material Water distribution

Water level regulation

Preferential flow Capacity calculation

Filtration: design and installation


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History

A century ago filtration of river water was hardly necessary. The rivers where generally clean andunpolluted. Water was extracted from the river by hand or by simple pumps and was used by peoplewithout any form of purification. Due to the increasing population and industry the river water becamemore polluted and water purification became necessary. Originally slow sand filters were used. These

filters consisted of large housings, containing sand, with a drainage pipe in the bottom. The enormoussurface required by this slow sand filtration triggered a desire for a more efficient purification technique.This new technique was called rapid sand filtration, which required a lot less filter surface. Rapid sandfiltration, however, demanded more management because cleaning the filters is done by backwashingwhich requires water and air pumps. In addition, the construction of the filter housing must be altered inorder to allow the filter bed to be backwashed with water and air.

This presentation focuses mainly on the construction of rapid sand filters.


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Construction

A filter is mainly comprised of a filter housing containing filter sand. Depending on the availablebudget and the required lifespan the filter housing can either be made of concrete or steel.

Even though the construction differs, the basic idea and functionality is identical.


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Filter bottom

In conventional filtration water is distributed on top of the filter. Gravity causes thewater to seep through the filter bed. On or underneath the bottom a drainage systemis constructed, which allows water to exit the filter, and keeps the sand in place.

Rapid filtration requires the filter to be periodically backwashed. In many situationsthis backwashing procedure is done with a combination of water and air.

The filter bottom therefore has different functions.

Function filter bottom

filtrate backwashair

backwashwater

Functions of the filter bottom:

draining away filtered water preventing the filter material from draining away allowing distribution of backwash water (not relevant for slow sand filtration) allowing distribution of backwash air (not relevant for slow sand filtration)


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The first slow sand filters were equipped with a drainage pipe on the bottom of the filter housing (seefigure above). This design was also used for the first rapid sand filters, and uses drainage pipes withsmall holes in the bottom. These drainage pipes are covered with support layers which should preventsmall sand particles from being drained away. These support layers should also cause an evendistribution of the backwash water. For the distribution of backwash air a separate pipe system must

be placed on the bottom.

Filter bottom


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Filter bottom

Characteristics of a filter bottom with perforated pipes: Ancient principle, seldom used Separate backwash air system necessary For an even distribution of backwash water the diameter of the holes in the

pipes must be carefully calculated

A support layer underneath the filter bed is necessary


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Filter bottom

The figure above shows a filter with modern backwash bottom. This double bottom is equipped withsynthetic backwash nozzles. These nozzles keep the filter bed in place and allow water to drain away.Backwashing the filter can be easily achieved. By using nozzles with perforated pipes the filter can bebackwashed using water and air simultaneously.


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Filter bottom

Characteristics of a filter bottom with backwash nozzles:

Double bottom construction

Lower compartment often accessible to check for leakage of filter material (broken nozzle) Backwash water and air can be distributed through the filter bottom simultaneously

The amount of nozzles varies from 50 to 90 per m2 and depends on the desired maximum backwashvelocity

The width of the slits in the nozzle must be smaller than the smallest diameter of the filter material,standard width varies from 0,5 to 1,0 mm

Support layer generally not necessary


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Backwash nozzles

Backwash nozzles differ in type and size. The main parts are the nozzle and the pipe.The width of the slits in the nozzle is chosen based on the filter material and must be smaller thande smallest particle of the filter material.In order to create an even distribution of air over the entire filter bottom during backwashing,resistance is necessary over the filter bottom. This resistance is created by the pipe that is attachedto the nozzle. This pipe causes an air cushion, which in its turn causes resistance, to formunderneath the bottom. This cushion can be varied in thickness by changing the position of the hole

in pipe. The diameter of the hole can be accurately calculated to fit the amount of necessarybackwash air. As this is a difficult calculation and sometimes different amounts of air are used duringbackwashes, a second hole can be made in the pipe. When a larger amount of air is used it can alsoescape through this second hole. The distribution of air over the filter will remain even.A pipe with a groove can be used as an alternative. For this, however, it is necessary that the filterbottom is very accurately horizontal, in order to get an even distribution of air over the filter. Thisalternative is therefore not preferred.


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Utilizing the wrong type of backwash nozzle, or an improper installation procedure can cause unwantedsituations:

During the installation it incidentally occurs that some of the backwash nozzles cannot be fitted due toobstructing parts underneath the filter bottom. In these case it is necessary to completely seal thenozzles to prevent a disturbance of the air distribution.

In some cases it also occurs that a different type of nozzle is included. It is very important that allnozzles are the same type and dimensions. When this is not the case the following consequences canoccur:

A nozzle fitted with a too short pipe: The air cushion underneath the filter bottom will escapethrough this shorter pipe, causing a local fountain of air which will inevitably wash away the filtermaterial surrounding the nozzle. The other parts of the filter bed will hardly receive any air duringthe backwash procedure.

A nozzle without holes in the pipe: the missing holes will prevent air from escaping through thenozzle, which in its turn will result in the fact that the filter bed directly above the nozzle will notreceive any air during the backwash procedure. It will also cause an uneven en therefore lessoptimal distribution of air over the rest of the filter bed.

Nozzles without pipes: These nozzles are only suitable when the backwash is done without theuse of air. Never install this type of nozzle in filters that are backwashed with air, because theabsence of pipes will cause an uneven distribution of air over the filter and the filter will mostdefinitely become clogged.

Backwash nozzles


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Backwash nozzles

After a filter has been emptied completely, for an external cleaning, it can be set up again from thebeginning.The first step is to check all the nozzles:Are all of them the proper type, and are they all the same;Are there enough nozzles available;Are all of them fitted with perforated pipes, are the holes the right diameter and are all the diametersthe same, are all the holes located on the same height

After the nozzles are checked they can all be installed:Take care during the installation of the nozzles that already installed nozzles are not damaged;The nozzles must each be fixed manually;After all the nozzles have been installed the operator must check every single one of them if they havebeen fixed properly;Loosely fitted nozzles must be always be tightened. If this cannot be done then the nozzle must be

completely sealed, so that neither air nor water can pass through. This will have the littlest effect on thedistribution of air and water over the filter. If a nozzle is not fixed properly and becomes dislocated, filtermaterial can escape through the hole and into the lower compartment. This will prevent the filter frombeing backwashed properly and it must therefore be emptied entirely to refit the nozzle;Fill the filter with approximately 20 cm of water;Backwash the filter using only air. If the nozzles are installed properly there should be an evendistribution of air over the filter;The following situations can occur:

An air fountain occurs. Step into the filter and locate the faulty nozzle, turn off the blower andremove the nozzle. Most likely the nozzle has been damaged or the pipe is too short or the holesin the pipe have the wrong diameter or are at the wrong height. Replace the nozzle by a properone. If this cannot be done, seal the hole entirely;A wave occurs in the filter. The wrong type of nozzle has been used. It is most likely that eitherthe pipes are to short or the holes in the pipe have the wrong diameter. Check the thickness of thefilter bottom and if the holes in the pipes are located at a sufficiently distance from the undersideof the filter bottom.

If the air is distributed evenly over the filter bed the filter material can be deposited.


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Filter material

A filter is designed for a specific filter capacity.The following table shows the starting points when designing and installing both slow andrapid sand filters. A very important aspect is the use of proper filter material. Sand ismost commonly used, and the sieving procedure is therefore very important. Thisprocedure will be elaborated on in a different tutorial.

Slow sand filtration Rapid sand filtration

Grain 0,1-0,3 mm 0,5-3 mm

Filtration velocity 0,03-0,3 mm/s 1-5 mm/s

Duration Months Days

Cleaning Skimming Backwashing

Table 1

0,4-0,63 mm 1,0-1,6 mm 1,7-2,5 mm


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Filter material

After the nozzles have been installed and the air distribution has been checked, the filter sand can bedeposited.The first step is checking the new filter sand:Is it the proper fraction (check this using the instructions on filter material and sieve curves);Is the material clean, not polluted;

Is there enough material available.

After the filter sand has been checked the filter can be filled:Measure the distance between the filter bottom and the overflow sill carefully. Note this measurementand be sure to remember it, because you will need it later on;Fill the filter with a meter and a half of water. This amount of water is necessary to protect the nozzlesfrom getting damaged by the falling sand;

The filter sand can now be deposited;During the depositing, try to distribute the sand as evenly as possible over the filter;During the filling of the filter it must be regularly backwashed using water and air. This is necessary toensure an even distribution of the filter sand and to flush out any potential pollution. Depending on thesand fraction it might not be advisable to backwash using water and air simultaneously, because sandmight be washed out;After filling the filter with sand the distance between the top of the bed and the overflow sill must bemeasured. By subtracting this measurement from the first one (distance between the filter bottom and theoverflow sill) the thickness of the filter bed is calculated. Note these measurements and store themcarefully, for they are important for the proper maintenance of the filter.


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Water distribution

It is preferable to distribute the load evenly among the filters.

The four filters in figure 1 are unevenly loaded. The water chooses the path of leastresistance. Therefore the two centre filters receive more water then the outer ones. This willcause the centre filters to become overloaded, which will reduce their effectiveness and will

have a negative effect on the quality of their output.

Figure 2 shows the same four filters, the only difference being the fact that the two valvesleading to the centre filters are slightly closed, causing a similar resistance as the resistanceto the outer filters. This, in its turn, will cause an even distribution of water among the filters,minimizing the chances of a filter becoming overloaded, and maximizing the water quality.

Figure 1 Figure 2


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Water level regulation

The water level regulation regulates the water layer on top of the filter. This waterlayer ensures an even distribution of water over the filter surface. This, in its turn,results in an even distribution through the filter bed, optimizing the filtration process.

Regulating the water level can be done in different ways. There are some regulators,which regulate the water level using valves. These are usually more fragile andtherefore require more maintenance. The most common method however, requireshardly any maintenance, and regulates the water level using an overflow pipe. Thisregulator will cause the water level to increase as the filter becomes clogged. Thedisadvantage of this system is that the water level must increase in order to forcemore water to percolate through the filter. This, in its turn, requires a higher filter

construction. The advantages of this system are the fact that it requires hardly anymaintenance at all, and the water level on top of the filter is a good indicator of thestatus of the filter bed and whether it requires backwashing.


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Preferential flow

Preferential flowWhen a filter is not equipped with a water level regulator the filter sand can

easily be dispersed at the point of entrance.Figure 1 shows a situation where the incoming water forms a cavity in thefilter bed. This cavity causes the filter bed to become thinner locally,allowing water to pass more easily through the bed. This occurrence iscalled a preferential flow, which causes an uneven distribution of waterover the filter surface, and has a negative influence on the water quality,

because the distance through the filter becomes less.

Figure 1


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Capacity calculation

A filter is designed for a specific capacity. In order to calculate the capacity of an existing filter themain functionality (slow or rapid filtration) and the surface of the filter must be known. As shown intable 1 the velocity through the filter bed can range from 1 to 5 mm/second. The capacity cantherefore be calculated by multiplying this velocity by the surface area of the filter.

Example: A rapid sand filter with a surface area of 20 m2 has a maximum capacity of:20 m2 * 0,005 m/sec = 0,1 m3/sec = 360 m3/hThis 360 m3/h is the absolute maximum of the filter. In order to ensure a proper water quality alesser velocity is recommended.The minimum capacity if the filter is:20 m2 * 0,001 m/sec = 0,02 m3/sec = 72 m3/h

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Filtration and Nozzles