FILTRATION DESIGN
and
INSTALLATION
TECHNICAL COOPERATION
PDAM PONTIANAK
-
OASEN GOUDA
Section of Production:
Design INSTRUKTUR OASEN: Ir. Peter Mense. Mr.Rob van Klaveren.
and
Installation THE NETHERLANDS OASEN : Managing Director : Ir. A.B.I.M. Vos de Wael Director Operations : Ing.H.Ardesch Project Manager : Ir.A.Haasnoot Team members of Oasen
Gouda, 21 mei 2007 Mr. H.Nuijten.
REPUBLIC OF INDONESIA PDAM PONTIANAK: Managing Director : Ir. Syahril Technical Direcor : Mr.Masriyatno Team members of training centre PDAM Pontianak
Filtration: design and installation Contents • • • • • • • • •
History Construction Filter bottom Backwash nozzles Filter material Water distribution Water level regulation Preferential flow Capacity calculation
History
A century ago filtration of river water was hardly necessary. The rivers where generally clean and unpolluted. Water was extracted from the river by hand or by simple pumps and was used by people without any form of purification. Due to the increasing population and industry the river water became more 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 enormous surface 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 sand filtration, however, demanded more management because cleaning the filters is done by backwashing which requires water and air pumps. In addition, the construction of the filter housing must be altered in order to allow the filter bed to be backwashed with water and air. This presentation focuses mainly on the construction of rapid sand filters.
Construction
A filter is mainly comprised of a filter housing containing filter sand. Depending on the available budget 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.
Filter bottom In conventional filtration water is distributed on top of the filter. Gravity causes the water to seep through the filter bed. On or underneath the bottom a drainage system is 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 situations this backwashing procedure is done with a combination of water and air. The filter bottom therefore has different functions.
Function filter bottom
filtrate
backwash water
backwash air
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)
Filter bottom
The first slow sand filters were equipped with a drainage pipe on the bottom of the filter housing (see figure above). This design was also used for the first rapid sand filters, and uses drainage pipes with small holes in the bottom. These drainage pipes are covered with support layers which should prevent small sand particles from being drained away. These support layers should also cause an even distribution of the backwash water. For the distribution of backwash air a separate pipe system must be placed on the bottom.
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
Filter bottom
The figure above shows a filter with modern backwash bottom. This double bottom is equipped with synthetic 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 be backwashed using water and air simultaneously.
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 backwash velocity • 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
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 than de 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 attached to the nozzle. This pipe causes an air cushion, which in its turn causes resistance, to form underneath 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 necessary backwash air. As this is a difficult calculation and sometimes different amounts of air are used during backwashes, a second hole can be made in the pipe. When a larger amount of air is used it can also escape 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 filter bottom is very accurately horizontal, in order to get an even distribution of air over the filter. This alternative is therefore not preferred.
Backwash nozzles
Utilizing the wrong type of backwash nozzle, or an improper installation procedure can cause unwanted situations: •During the installation it incidentally occurs that some of the backwash nozzles cannot be fitted due to obstructing parts underneath the filter bottom. In these case it is necessary to completely seal the nozzles 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 all nozzles are the same type and dimensions. When this is not the case the following consequences can occur: ¾A
nozzle fitted with a too short pipe: The air cushion underneath the filter bottom will escape through this shorter pipe, causing a local fountain of air which will inevitably wash away the filter material surrounding the nozzle. The other parts of the filter bed will hardly receive any air during the backwash procedure. ¾A
nozzle without holes in the pipe: the missing holes will prevent air from escaping through the nozzle, which in its turn will result in the fact that the filter bed directly above the nozzle will not receive any air during the backwash procedure. It will also cause an uneven en therefore less optimal distribution of air over the rest of the filter bed. ¾Nozzles
without pipes: These nozzles are only suitable when the backwash is done without the use of air. Never install this type of nozzle in filters that are backwashed with air, because the absence of pipes will cause an uneven distribution of air over the filter and the filter will most
Backwash nozzles After a filter has been emptied completely, for an external cleaning, it can be set up again from the beginning. 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 diameters the 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 have been 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 the distribution of air and water over the filter. If a nozzle is not fixed properly and becomes dislocated, filter material can escape through the hole and into the lower compartment. This will prevent the filter from being 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 even distribution 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 and remove the nozzle. Most likely the nozzle has been damaged or the pipe is too short or the holes in the pipe have the wrong diameter or are at the wrong height. Replace the nozzle by a proper one. 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 either the pipes are to short or the holes in the pipe have the wrong diameter. Check the thickness of the filter bottom and if the holes in the pipes are located at a sufficiently distance from the underside of the filter bottom. •If the air is distributed evenly over the filter bed the filter material can be deposited.
Filter material
0,4-0,63 mm
1,0-1,6 mm
1,7-2,5 mm
A filter is designed for a specific filter capacity. The following table shows the starting points when designing and installing both slow and rapid sand filters. A very important aspect is the use of proper filter material. Sand is most commonly used, and the sieving procedure is therefore very important. This procedure 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
Filter material
After the nozzles have been installed and the air distribution has been checked, the filter sand can be deposited. 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 measurement and 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 nozzles from 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 to ensure an even distribution of the filter sand and to flush out any potential pollution. Depending on the sand fraction it might not be advisable to backwash using water and air simultaneously, because sand might be washed out; •After filling the filter with sand the distance between the top of the bed and the overflow sill must be measured. By subtracting this measurement from the first one (distance between the filter bottom and the overflow sill) the thickness of the filter bed is calculated. Note these measurements and store them carefully, for they are important for the proper maintenance of the filter.
Water distribution
Figure 1
Figure 2
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 least resistance. Therefore the two centre filters receive more water then the outer ones. This will cause 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 valves leading to the centre filters are slightly closed, causing a similar resistance as the resistance to 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.
Water level regulation
The water level regulation regulates the water layer on top of the filter. This water layer 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 and therefore require more maintenance. The most common method however, requires hardly any maintenance, and regulates the water level using an overflow pipe. This regulator will cause the water level to increase as the filter becomes clogged. The disadvantage of this system is that the water level must increase in order to force more 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 any maintenance at all, and the water level on top of the filter is a good indicator of the status of the filter bed and whether it requires backwashing.
Preferential flow
Figure 1
Preferential flow When 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 the filter bed. This cavity causes the filter bed to become thinner locally, allowing water to pass more easily through the bed. This occurrence is called a preferential flow, which causes an uneven distribution of water over the filter surface, and has a negative influence on the water quality, because the distance through the filter becomes less.
Capacity calculation
A filter is designed for a specific capacity. In order to calculate the capacity of an existing filter the main functionality (slow or rapid filtration) and the surface of the filter must be known. As shown in table 1 the velocity through the filter bed can range from 1 to 5 mm/second. The capacity can therefore be calculated by multiplying this velocity by the surface area of the filter. Example: A rapid sand filter with a surface area of 20 m 2 has a maximum capacity of: 20 m2 * 0,005 m/sec = 0,1 m3 /sec = 360 m3 /h This 360 m3 /h is the absolute maximum of the filter. In order to ensure a proper water quality a lesser velocity is recommended. The minimum capacity if the filter is: 20 m2 * 0,001 m/sec = 0,02 m3 /sec = 72 m3 /h
