Filtration Module Gravel Filters Sec 5 Dec 00 Module Gravel Filters... · Page 3 of 33 Gravel Filters - Agricultural ... the only plastic filter widely used in the marketplace today

Page 1 of 33 Gravel Filters - Agricultural

Netafim School of Irrigation - Copyright

FILTRATION MODULE

SECTION 5

GRAVEL FILTRATION (AGRICULTURAL)



Table of Contents

Table of Contents 2 1 Introduction 3 2 Objectives 4 3 Background on the development of Gravel filters 5 4 Description and components of Gravel filters 5

4.1 Components of Gravel Filters 6 5 The process of filtration and backflushing 14

5.1 Gravel Filters – the Principle 14 5.2 The Filtration Process 15 5.3 The Backflush Process 16 5.4 When to back flush 17

6 Automatic batteries 17 7 Selection and Design 19

7.1 Pre-filtration 19 7.2 General Selection 20

8 Installation and Operation 24 9 Maintenance 26

9.1 Gravel Filters 26 9.2 Backup Filters 27

10 Applications 28 11 Summary and Conclusion 30 12 Acknowledgements 30 13 Review Questions 31

13.1 Beginner 31 13.2 Intermediate 32 13.3 Advanced 33



1 Introduction Water sources for micro-irrigation include municipal water, reservoirs, dams,

lakes, rivers, channels and bores. Solids contaminants include inorganic matter

such as sand, silt, clay, aquatic growth such as algae, fish and snails, vegetation

such as stubble trash, weeds and leaves and other such contaminants including

plastic bags, paper and bottles.

As global water quality is deteriorating and water quantity is becoming restrictive

as well as mounting economic pressure, agricultural producers need to be

utilizing their resources as efficiently as possible. Gravel filtration is one tool used

to increase their operating efficiency by improving and maintaining the efficiency

of their irrigation systems.

The use of irrigation in agriculture requires water to be filtered. Filtration for

agricultural uses serves the purpose of protecting irrigation systems from

clogging and/or abrasion. Therefore, its purpose is to stop only those particles

which may cause clogging or abrasion from entering the irrigation system.

It is necessary to consider the various water sources and the changes that occur

within the sources over time in order to select the correct filtration system that

suits the irrigation system, operator and budget.

This section being Gravel Filters (Agricultural) will cover all aspects of

The development of gravel filters

Description and components of gravel filters

The process of filtration and backflush

Automatic gravel filter batteries

Selection and design

Installation and operation

Maintenance

Applications

These areas will be covered in depth with the use of examples where possible

and be followed by a series of review questions.



2 Objectives On completion of this module, Gravel Filters (Agricultural), participants should be

able to: -

List all components of a gravel filter and describe each component’s role in

both single tank systems and batteries

Understand automatic batteries in terms of operation, application and function

Understand the filtration and backflush process for a single tank manual

system and an automatic battery

Design and select gravel filters for a given application

Understand the parameters and gravel filter selection criteria

Understand how to operate single tank or automatic batteries

Understand how to install single tank or automatic batteries

Understand how to maintain single tank or automatic batteries

Improve knowledge of the applications of gravel filtration systems



3 Background on the development of Gravel filters Media filtration has been occurring since the beginning of time. Ever since rain

and flooding water seeped through the earth into the crystal clear aquifers media

filtration was occurring.

In terms of agriculture, in the late 1960’s, early 1970’s, gravel tanks were being

used. Odis filtration began developing filters in Israel, carbon steel with a

protective coating. They were then similar in style as some of those used today.

Automation of these filter tanks soon followed and in the mid 1970’s the first

stainless steel filters were used. In 1993 Arkal started to produce the Arkal

Gravel Filter (AGF) - the only plastic filter widely used in the marketplace today.

Since those early days a tremendous amount of improvements have occurred.

The design of the “mushrooms”, the shape of the diffuser plates, the backflush

valve changed from two valves into one, the controllers have changed and even

simple changes such as easy to remove inspection lids.

Research and development has been ongoing which has led to the exciting

development of today’s gravel filter systems.

4 Description and components of Gravel filters Filters in general can be broken into two main types when looking at units that

are widely used in agriculture, surface filtration and volume filtration. Firstly

surface filtration. Screen filters are single surface, two dimensional filters. They

have only one retention point for solids and all apertures of the screen are

usually of uniform size. These two facets greatly limit the success of this type of

filtration therefore limiting screen filters to roles such as back up filters to gravel

systems or infield backup filters. Secondly, volume filtration (depth filtration) that

consists of gravel filters and disc filters. Both types of filters have multi

dimensional layers of filtration enabling far better efficiency and safety in terms of

collecting organic or inorganic material.



4.1 Components of Gravel Filters There are many components that compose the hardware of a gravel filtration

system. Whether the filtration system is a single tank system or a multi-tank

battery, the same components apply but naturally are increased in number in

accordance to the number of tanks. Refer Fig 1 below

Media filter body – The filter body or tank is a receptacle that contains the

gravel/sand media. It is a vertical tank consisting of one or more external

inspection or access ports, a top inlet and a bottom outlet. Three legs usually

support it.

Internally the filter body contains a diffuser plate, located just under the inlet. This

diffuser plate is designed to spread the incoming water over the filter media.

Located above the outlet, also internal to the filter body is the under-drain. The

under-drain has three purposes.

To allow clean water to exit the filter tank

To prevent the filter media from exiting the filter tank along with the clean

water

Fig 1

Command filter

Air valve

Pressure relief valve

Filter body

Outlet manifold

View tube

Vacuum breaker

Restriction valve

Inlet manifold

Backwash manifiold

Backwash valve

Pressure gauge



It distributes the backflush water evenly throughout the filter bed during the

backflush process.

This under-drain, in the case of ODIS consists of a welded, reinforced steel plate

that divides the tank into an upper and lower chamber. This plate is covered with

“mushroom” diffusers that are plastic, slotted conical cups that collect the filtered

water and also evenly disperse the backflush water uniformly in the flushing

process. ARKAL AGF gravel filters use a series of upper and lower diffuser arms

radiating out from the centre drum. These diffuser arms have the “mushrooms”

evenly spaced along each one. (Refer Fig 2 below) These arms or flutes are

specifically designed so that the orifice under the mushrooms is larger the further

they are away from the central hub. This is to allow even backflushing across the

media bed. The main problem with other types of filter tanks is that they are

unable to backflush evenly across the bottom of the bed or are unable to

backflush close to or against the tank walls. These factors lead to residual dirt

and foreign material being kept in the tank that builds up over time and grips onto

the gravel. This causes a blockage that forces the water through a smaller area

under higher velocity. This process forms tunnels or channels through the gravel

media causing poorer filtration. This also makes backflushing less effective.

Slots

Mushroom or diffuser

Fig 2



Filter tanks can be either coated carbon steel, reinforced polythene or stainless

steel type 304 and 316. ODIS filter tanks are carbon steel that have a 100 micron

protective coating of extra durable polyester applied electrostatically and oven

cured on a zinc phosphate layer. ARKAL AGF filter tanks have an outer layer

made from reinforced polyester with an inner layer of black polythene. Type 304

stainless steel is sometimes used; it is corrosion resistant but not rust proof. Rust

is a definite concern for SS304 if water salinity is above 750 ppm; some pitting

has been reported at even lower salt levels.

Both ODIS and ARKAL filter bodies have upper and lower inspection ports.

These are easy to remove covers that allow for visual inspection, addition or

removal of gravel.

ODIS gravel tanks are available in body diameters of 12”, 16”, 20”, 24”, 36” and

48”. They have a maximum working pressure of 10 bar.

ARKAL AGF tanks are only available in 48” and are rated to 6 bar. Higher ratings

are available upon request.

The gravel media is the critical factor in term of filtration efficiency. Netafim

recommends (in order of preference).

Crushed basalt #1

Crushed granite #11

Crushed silica #12, #16 or #20

The particles of sand should be crushed ensuring the grains have sharp edges

and are not round.

For surface and ditch water that contain a high level of algae and organic

materials 70 –75% of the media particles should be between the sizes of 1.23 –

2.25 mm. The other 30% can be outside those tolerances, but no smaller than

0.8 mm or no larger that 2.6mm.

For water with high levels of iron, the size of the media should be smaller; 70 –75

% should be between the sizes of 0.75 – 1.75 mm. The other 30% can be

outside those tolerances, but no smaller that 0.6 mm and not bigger than 2.0

mm.



The life of the gravel is directly proportional to the hours of filtration and the level

of abrasives in the water. Usually 4 - 5 years can be expected.

Media Mesh Micron (µ) Min Max Average

Crushed silica #20 170 –230 88 – 60 0.3 0.5 0.4

Crushed silica #16 155 –200 95 - 70 0.5 0.8 0.65

Crushed silica #12 80 - 130 170 – 120 0.8 1.3 1.05

Crushed basalt #1 140 -180 105 - 80 0.4 1.1 0.75

Table 1 Filtration Media (ex Arkal)

ODIS and ARKAL AGF filter tanks have level indicator marks located externally

for easy recognition of the correct media level.

Gravel filter manifolds consist of the inlet manifold (brings unfiltered water into the filter body), outlet manifold (takes filtered water to the mainline) and the backflush manifold (takes flush water and debris away from the filter system). Ideally the backflush manifold has a visible, above water level exit point, so a visual appraisal of backflushing can be done. A transparent viewing tube can also be used in this instance. The exit point of the backflush manifold should not exit at any point close to the footvalve. Ideally the backflush pipe should terminate downstream or quite some distance from the footvalve. Care must be taken not to run the backflush pipe too long in terms of distance or have a pipe with a diameter that is too small. The pipe diameter should be able to cater for the backflush flow with less than 5 metres /100 metres loss. The higher the loss the shorter the backflush pipe should be. 50 metres are about the maximum length possible. It is possible to see and set backflush times from this information. These manifolds can be manufactured with galvanized steel, stainless steel, and

epoxy coating or made entirely of polythene. The corrosive nature of the water

chemistry, as well as the application and budget will determine the most practical

manifold material for each application. Manifold diameters are sized in

accordance to maximum flow rate so as to give a minimum friction loss across

Equivalent Silica Size (mm)



the filtration system. This friction loss as well as friction loss inside the tank give

us the baseline pressure loss i.e. the pressure differential across the filter when it

is operating as clean. This baseline pressure loss is usually less than 5 psi. (0.3

bar.)

Most manifolds are modular and connect to valves and filter bodies via victaulic

couplings.

Each filter body is equipped with a Backflush Control Valve. The backflush valve

connects the top of the filter body to the inlet manifold and the backflush

manifold. Its role is to change the direction of the water flow thus changing a

filtration mode to a backflushing mode. The backflush valve consists of two built

in units, the normally open filter port and the normally closed drain port. Each

port has its own seat but share a common stem and diaphragm. The valve

operates as a mutually integrated unit, that is when one port is open the other

port is closed. This permits the valve to operate both modes. In filtration mode

the valve allows unfiltered water to flow from the inlet manifold into the filter tank.

When in backflush mode the inlet port closes and the drain port opens thus

reversing the flow of water and allowing filtered water and debris to flow from the

filter tank into the backflush manifold via the backflush valve.

This process can be initiated manually or automatically. Manual operation can be

initiated by switching the three-way selector, “Sagiv” to the open position. This

Sagiv can be located on the outside of the backflush valve.(Refer Figure 3 below)

A manual bleed screw can also be used to initiate backflushing. Either of these

operations will allow water to bleed of the vent command chamber of the valve

thus causing the stem and diaphragm to shift into the other mode of operation.

Figure 3

Manual/Semi automatic control of backwash valve



Automatically this same process occurs but is initiated electrically. An electric

solenoid or hydraulic relay initiates operation. Water pressure or a pneumatic

source then actuates the valve. (Refer Figure 4 above)

Most automatic filter stations have solenoids; AC or DC installed on the backflush

valve. A separate Backflush Controller can start a backflush cycle based on

pressure differential or time or both. The Backflush Controller is mounted on or

near the filtration system and can be of AC or DC operation. It is a stand-alone

unit that has the role of automating backflush sequences. The AC power source

is converted from 240 volt to 24 volt for the controller. The DC controller relies on

a battery that can be trickle charged by solar panel or from an alternator. The

backflush controller would be connected to a pressure differential switch that has

hydraulic control tubing connecting it to the inlet and outlet manifolds – this then

constantly monitors the Pressure Differential (PD).

The backflush controller, PD switches and solenoids are all protected by a

Hydraulic Command Filter. This filter consists of a 20mm up to 40mm disc filter

with a 120-mesh element that prevents contaminants from entering the system.

As flow through this filter are very low it is not necessary for this filter to be

automated.

Figure 4

Automatic control of backwash valve



The parts shown on Figure 5 above are

1 Filter Tank

3 Outlet manifold

4 Back flush valve

12 Hydraulic Command filter

14 Pressure Gauge – Outlet

15 Backflush controller

16 Pressure differential switch

17 Victaulic coupling

A check filter or check filters should be supplied and installed on every gravel

filter system. The check filter element should be 40 or 80 mesh depending upon

the application. The sole purpose of the check filter is to prevent any filtration

Figure 5

Hydraulic Command system



media from entering the irrigation system should a “mushroom” be displaced or

damaged. It is purely a precautionary device.

Two choices of check filter design are available. Single check filter, located on

end of outlet manifold or multiple check filters, one disc filter per filter tank. Both

have advantages and disadvantages. Firstly

Single check filters. A single check filter is usually a screen filter with a

manual ball valve for flushing. This valve is very simple to install and easy to

flush to see if gravel is being caught. However, if gravel is being caught it is

difficult to see which tank is losing these small amounts of gravel media. The

nature of screen filters is also that if the screen builds up with a massive load

of gravel or iron etc it can in some circumstances collapse thus contaminating

the mainline with the material. Secondly,

Multiple check filters. In this case one ARKAL check filter is located for every

tank. They are slightly harder to check as they have to be undone but they do

reveal which tank is yielding gravel media. Blocked ARKAL check filters will

unlikely collapse under heavy load but instead reduce in flow until a no flow

situation can occur. This can place extra load on the other filters.

A Backflush Restricting Valve needs to be installed in the backflush manifold beyond the last filter tank. The flow in the backflush manifold must be restricted in order to prevent gravel media from being flushed out of the gravel tank with the backflush water. In the backflush mode the turbulent action of the water fluidises and suspends the gravel in a boiling type of fashion. Proper adjustment of this valve will allow debris to be flushed out whilst retaining the gravel media inside the tank. Only a small trace of media will be lost through flushing and this is considered optimum. Check to see if media is being washed out. One method is to place your cupped hand or a container under the bottom of the stream as it exits the system. If gravel is present in large amounts the valve needs to be restricted. New gravel does contain many fines. Fill the filter with water to overflowing with a hose to float off the majority of the fines. Then back flush the filter 4 - 6 times to remove fines before adjusting the Back flush Restricting Valve.

A filter isolation valve, pressure gauge and a dual acting air valve are all items

that should also be included as part of the filter system. The isolation valve can

be used to keep pressure on the filters at start up or simple to shut the system



down. Pressure gauges are useful tools to check the efficiency of flushing. That it

is if after flushing the pressure gauge indicates the baseline pressure you will

know that back flushing is very efficient and there is no residual blockage. Air

valves are necessary for preventing air locks, preventing water hammer and

allowing the system to breath at start up and shut down.

Optional accessories may include Pressure Sustaining Valves, Pressure Relief

Valves, a Check Valve and a Rinse Valve.

Pressure Sustaining Valves can be used to ensure that there is always adequate

back pressure on the filters so as to get a proper back flush. The valve operates

by hydraulically throttling itself if the downstream pressure decreases past a set

point. A minimum pressure of 14 metres (20 psi) needs to be maintained. This

type of valve is a must on flat ground and where the filter has irrigation blocks

below it.

A Pressure Relief Valve upstream of the filters will protect the filters from over

pressure if for example infield valves close and the pump is still operating. At a

certain pressure this valve will hydraulically operate, opening to atmosphere and

venting to reduce pipe pressure.

A Check Valve should be installed downstream of the filters to prevent any back

flow of water once the pump has shut down. Naturally this can be problematic

where the field is a lot higher than the filter station. It also allows maintenance on

the pump and or filter station without the need for mainline drainage.

Finally, a Rinse Valve can be used to initially clean media or simply act as a

source of water at the pump and filter station.

5 The process of filtration and backflushing

5.1 Gravel Filters – the Principle Gravel filters operate by allowing unfiltered water to pass through a bed of

aggregate that captures the suspended material as the water passes through -

all inside a pressurized tank. This media captures the debris that would normally



jeopardize the integrity of the emitters in the field. The bed of aggregate

recommended is crushed basalt, has many varying aperture sizes that makes the

water travel through many passages on its way from the inlet to the outlet of the

filter. Due to the large volume and contact area between the water and the

media/gravel particles, various physical forces occur, thus retaining the

contaminant particles. Efficiency of large particle retention is very high as long as

the gravel is kept clean.

5.2 The Filtration Process During the filtration process, unfiltered water enters the gravel filter system via

the inlet manifold, cleaned and discharged via the outlet manifold. During

filtration the inlet ports are open and the backflush ports are closed, water flows

from top to bottom across the gravel layer. That is water flows over the diffuser

plate through the media and into the out the filter bottom through the

“mushrooms”. Refer Figure 6 below

Figure 6

The filtration process



5.3 The Backflush Process During the backflush process the inlet port closes and the backflush port opens

reversing the flow of water in the filter. Clean pressurised water from the other

filters now flows into the bottom of the filter, lifting the gravel media and freeing

the accumulated dirt and debris that will be then flushed out through the

backflush valve and into the backflush manifold. If the backflush restriction valve

is adjusted properly, the debris will be flushed out of the filter while the gravel

media, being of a higher Bulk Density than the debris will remain fluidised in the

filter tank.

Only one filter at a time should backflush. The water that is being used to

backflush one filter would have already passed through another filter or filters so

that the backflush process is carried out with clean, filtered water.

Refer to Figure 7 below

Figure 7

The backwash process



5.4 When to back flush Filter flushing (back flushing or back washing) can be carried out either manually

or automatically. Whatever the method, flushing should be timed in accordance

with one of the following practices.

Pressure differential – Backflushing will start whenever the pressure

differential across the filter drops to a predetermined level. A pressure

differential sensor can sense this level and flushing can be automatically

started. This predetermined level of pressure loss is 6 - 8 psi (3 - 6 metres)

over the baseline pressure loss. The baseline pressure loss is the measured

pressure loss when the filters are clean.

Time – It is recommended to backflush every 2 - 4 hours regardless of

pressure loss.

6 Automatic batteries A Battery or Filter Array simply consists of multiple filter tanks. This can mean 2

or 50 tank systems. More often than not batteries are automated so as to save

labour and maximise efficiency.

At least two tanks should be used. In a two-tank system, half of the water flows

through each tank. In a three tank system one third of the water flows through

each tank. Only one tank is back flushed at one time and it uses clean water from

the other tank(s) for the back flush. Tanks are back flushed in sequence at a time

set interval or when there is a large enough pressure differential. With two tank

systems especially, problems can arise where there is not enough flow or back

pressure for adequate flushing or interruption to flows into the field occur when

the filter goes into back flush. What occurs is all of the flow going through one

tank, to give clean water to back flush the other, puts the filtering tank into a high

flow situation. This can cause huge friction losses and channeling of the tank

media. Channeling is the term used when excessive velocities in a filter tank

cause the water to force “worm holes” through the media. These worm holes or

tracks cause filtration efficiencies to be reduced as actual contact filtering surface



area is reduced. It is therefore advisable to design a filtration system with at least

two tanks, preferable three or more. Ideally, back flushing a battery of filters,

because only one filter is back flushed at one time, can occur without interfering

with irrigation. In a back flush sequence the Dwell Time is the time period

between the flushing of each filter. This prevents two filters from flushing at one

time through overlap – it allows the valves time to revert to their normal operating

position. A usual setting is 15 to 60 seconds.

Three reasons why Filter Batteries are very flexible in terms of design.

A filter battery can be designed to any flow rate and water condition; it is not

limited by size.

Enlargement- when the project is done in stages, the filter station can be

enlarged in stages. In the first stage one installs the number of filters required

for the initial flow rate and simply leave vacant, plugged gaps for successive

stages.

Flexibility – when the flow rate and the water quality are not known exactly,

the filter battery can be designed to the maximum expected; manifolds,

concrete base, controller, etc. The actual array can be larger or smaller with

the ease of adding or subtracting tanks as required.

Automatic Filter Batteries can take three main shapes; L, T and W. All have the

same filtering capacity but with different manifolding.

“L” shaped batteries are essentially a straight-line system. Unfiltered water enters

one end of the line and exits at the opposing end as filtered water.

“T” shaped batteries have the inlet and the outlet at the centre of the battery with

the filter tanks being left and right of the inlet and outlet. Manifolds are usually

smaller than those of “L” batteries.

“W” shaped batteries tend to resemble to “L” shapes batteries side by side.

Unfiltered water enters one end and exits as filtered water at the far end in two

outlets. These batteries are often used for larger numbers of filter tanks in one

battery in limited spaces.



7 Selection and Design Selection and Design should be an objective, rather than a subjective exercise. Experience, however, does help.

7.1 Pre-filtration In order for the filtration system to work with the highest degree of success it is

highly recommended to install, whenever required, some pre-filtration or

screening facility.

Trash screens, grills made of simple bars or a punched screen act as a good,

solid mechanical barrier for holding off large pieces of debris from the pump

intake. It should have sufficient area so that the velocity of the water through it

is less than 0.3 metres/sec. If there are fish in the water entrance velocities

should be less that 0.15 metres/sec.

In most cases the specific location of the pump foot valve can greatly affect

the performance of the filtration system. In large dams the wind direction

should be considered. The foot valve should be located if possible up wind, to

prevent build up of algae and other vegetation around the valve. The foot

valve depth is also important. If it is too close to the bottom silt and clay will

be dragged into the system. This situation can become particularly acute

towards the end of the season when water levels are falling in the reservoir.

The optimum pumping depth should be between 1.2 and 1.9 metres below

the surface. Within this layer, algae are less likely to develop and the

concentration of the bio mass is minimal. Foot valves should also be fitted

with a screen. This will help keep large contaminants out that and allow more

efficient filter operation as well as allowing the foot valve to hold its prime.

Hydro cyclones (Sand separator) – a sand concentration of 50mg/L indicates

the need for this type of device. The hydro cyclone separates sand and other

solid matter with very little head loss and 90% or better efficiency. A hydro

cyclone can greatly compliment as gravel filtration system in terms of

removing a large portion of the sediment load before it reaches the filter

tanks.



7.2 General Selection Selection and Design of the filtration system are critical to the efficiency,

longevity and manageability of the entire irrigation system. The selection of the

gravel filtration system is based on:

Irrigation system (sprinklers, micro-sprinklers or drip)

Required filtration (mesh size or microns)

Type and quantity of impurities in the water

Required flow rate (capacity)

Maximum and minimum water pressure Future needs (modification, expansion)

Irrigation System

Prevention of emitter plugging is the main reason for gravel filtration systems.

Micro irrigation systems have relatively small outlets ranging from about 0.35 mm

– 0.85 mm in terms of drip and 0.5mm – 2 mm in terms of micro-sprayers.

Some heuristics exist as to the relationship between the emitter minimum

dimension and the filtration level desired. Charles Burt et al (1994) suggests that

all particles greater that 1/10th the diameter of the emission holes should be

removed for drip and 1/7th the orifice diameter of the micro-sprayer nozzle should

be sufficient.

The reason for this 1/10th or 1/7th rule of thumb is to minimize plugging through

“bridging”. Bridging is a term used when many small particles accumulate

together in a passageway and eventually “bridge” the hole.



Required filtration (mesh size or microns)

When talking disc or screen filters the terms mesh, mm and micron are used.

Their relationship is as follows. Refer Table 2 below

Mesh Micron mm

40 400 0.4

80 200 0.2

120 130 0.13

140 115 0.12

200 55 0.055

600 20 0.020

Recommended mesh sizes for micro-sprayers are as follows.

Litres/Hour Mesh

30 – 60 80 –100

70 – 120 60 – 80

160 – 240 40 – 60

Generally Netafim drippers require 120-mesh filtration.

In terms of gravel filtration the crushed basalt #1 has a particle size 1.2 - 1.8 mm.

Expected filtration at 50 to 75 m3/hr/m2 equates to 70 –100 microns.

Type and Quantity of impurities in the water Design should always take into consideration the type and quality of the water

source. By selecting the correct filter for the correct size and type of particles that

should be removed high retention efficiency and easy cleaning are ensured. This

can mean a basic water analysis in most cases. Two main areas are focused on

in terms of type and quantity of impurities in the water. These are Solid Mineral

materials and Large Micro-organisms. The Solid Mineral Material is primarily soil,

that is found in water sources that flood after rains, springs and rivers, in shallow

ponds and sand contained in bores. Soils can be lifted into the system via the

pump footvalve and vary in terms of type and quantity from site to site, season to

Table 2



season and even day to day. Large Micro-organisms include zooplankton and

algae that grow in sources that have ample air and light. Throughout the season

the quantity of micro-organisms will vary from minor to extreme cases.

Required flow rate (capacity)

The required flow rate has a direct effect on the number of filter tanks selected,

and their size. Each filter tank has a given filtration area (and volume for that

matter). A flow rate is then applied against this filtration area so as to determine

acceptable flow velocities and losses. The results of this process will naturally

differentiate as water quality varies from good, average and bad. The following

Table 3, GRAVEL FILTER SELECTION CRITERIA, illustrates the results. In

terms of multiple tank systems for larger flows it is simply a matter of

extrapolating the figures to arrive at the correct system size.

Maximum and minimum water pressure

Gravel filters need a minimum pressure for filtration and back flushing. This

minimum pressure, mostly for backflushing is required so that the valves will

operate. This minimum pressure for both ODIS and ARKAL AGF is 15 metres

(22 psi) of pressure during backflush. As mentioned it may be necessary to have

installed a pressure-sustaining valve to achieve these minimum pressures during

flushing.

Maximum pressures are

ODIS 100 metres (145 psi or 10 bar) and

ARKAL AGF 60 metres (85 psi or 6 bar).

Future needs (modification, expansion)

When selecting a filter battery try to have some understanding of any future

needs, expansions and modifications that may be planned in successive stages.

In terms of the filters’ hydraulic duty, if there is going to be an increase in flow

rate it is possible to plan for this change and allow for modulation of the system.



For example if the initial duty is to be 140 m3/hr in average water and increase

yearly by 140 m3/hr for three years thus giving a total future flow of 560 m3/hr

the system design would change. That is for 140 m3/hr we could use 4 x 36” or 2

x 48” However the total flow would require 8 x 48” or 14 x 36”. This total flow

would make 14 x 36” filters undesirable but be very suitable for 8 x 48”. The filter

station could be increased by 2 x 48’ tanks per year. This information is required

as a 2 x 48” system has a 150-mm manifold whereas a 8 x 48” system has a

300-mm manifold. The advantage in the above example is that with enough

information a 2 x 48” system could be supplied with a 300-mm manifold, with

blank inlets and outlets allowing for future tanks to be added as necessary. This

planning ahead saves a great deal of time, money and resources.

GRAVEL FILTER SELECTION CRITERIA

MAKE TANK DIAMETER RECOMMENDED FLOW RATE m³/hr

Inches mm Min Average* Max Backflushing

Odis 12 300 3.5 5 6 7

Odis 16 400 6 8 11 10

Odis 20 500 9 12 18 15

Odis 20 500 10 12 18 15

Odis 24 600 14 20 28 25

Odis 30 750 21 30 42 38

Odis 36 900 32 42 62 54

Odis 48 1200 62 72 120 95

Arkal 48 1200 48 65 80 60

Notes

*Good to bad quality (average grade agricultural water)

For dirtier water reduce the flow rate and/or consult Netafim

Pay attention to required backflush flow rates. It may be necessary to install a

pressure-sustaining valve downstream of the filter battery. Correct tank

selection needed to suit flow.

Table 3



Odis standard tanks are rated to 10 bar, Arkal standard tanks rated to 6 bar

Above flows are on a per tank basis. E.g. 24m³/hr would require 2 x 20” Odis

tanks.

Example selection

a) Duty 45 m³/hr surface water maximum pressure 75 metres --- Use 3 x 24”

tanks

b) Duty 60 m³/hr very poor water maximum pressure 40 metres --- Use 3 x

30” or 2 x 36” (Selection 1 is the better option)

c) Duty 220 m³/hr surface water maximum pressure 50 metres --- Use 3 x 48”

Odis or 4 x 48 AGF Arkal

d) Duty 350 m³/hr sand free bore water (high corrosion) maximum pressure 56

metres – Selection 5 x 48” AGF Arkal (4 x 48” Odis or 6 x 36” Odis would be

OK but due to the corrosive nature of the water the AGF tank would be

preferred as would the use of poly manifolds)

8 Installation and Operation A few points to assist Filters should installed on a concrete slab at least 100mm thick.

The slab should have a slight grade to allow for water run off. But not too

great that the manifold will not bolt together easily.

Grade should not direct water to electrical panel.

The tanks should be positioned so the access cover is easily accessible for

maintenance and inspection i.e. not positioned against a wall.

Distances between tanks

12” to 16” 600mm

20” to 24” 800mm

30” to 36” 1000mm

48” and AGF 48” 1320mm

Victaulic Clamp Assembly



Remove bolts from coupling, place gasket onto one of the grooved

components to be assembled. Mate the two grooved components and slide

the gasket over both so that it is centered between the two grooved fittings

place some gasket soap on the seal. Place the clamps around the gasket and

tighten bolts evenly.

Assemble all components before full tightening of bolts.

Backflush Valve Assembly

The valves are assembled on the top of the tank using victaulic clamps. The

arrow on the valve should point down towards the filter.

Adding filtration Media

Before any media is added to any of the tanks first inspect the tanks with the

aid of a torch to ensure that there are no foreign objects in the tanks (i.e.:

information brochures! ). And ensure that all the mushrooms are installed. In

the case of AGF ensure that all the plastic retaining pins that secure the

diffusion arms are in place.

The media fill level is indicated on the outside of each tank. When filling care

should be taken that the media is evenly spread particularly with 36” and 48”

and 48” AGF. A broom can be use to ensure that the media is evenly

distributed but care must be taken to ensure that the plastic mushrooms are

not damaged.

Number of 30kg Bags per tank

12” 2 Bags

16” 3 Bags

20” 4 Bags

24” 7 Bags

30” 9 Bags

36” 13 Bags

48” 23 Bags

AGF 48” 23 Bags



After the system has been run for a few hours it should be depressurized and the

gravel should be inspected to ensure that the media level is correct and that the

media is evenly distributed in the tank.

System Start up

Prior to pressurization of system

Program Back flush configuration into Back flush controller.

Check that all Back flush solenoids are working electrically.

Ensure that all inspection covers have been tightened up and are

secure.

All victaulic clamps have been tightened.

Command water filter is not isolated.

Backflush restriction valve is fully open.

Isolation valve to main line is open.

Pressurization of system

Start pumps and bring system up to operating pressure. Once

pressure has been achieved put the 1st filter into manual Back flush

and set restriction valve on the Back flush line to a point where no

gravel is being removed from the tanks.

Continue the manual Back flush until each of the tank’s Back flush

water is clean.

Note: Only Back flush one tank at a time!

9 Maintenance Maintenance is an essential yet often overlooked operation – routine maintenance is simple, inexpensive and cost effective.

9.1 Gravel Filters

Gravel (sand, media) filters are a very simple and reliable filtration system.

These filters generally require minimum servicing. At the end of the irrigation

season, remove the covers from the filters. Check the level of media in the tanks

and top up if required, manufacturers clearly mark the level of media required in



each tank (do not over fill the tanks). The media should be loose and level in the

tank. If the media appears hard, lumpy and uneven, it is an indication of

problems with the filter. The hard media surface should be treated, by mixing the

media with a broom handle, to free it up. Several manual flushes, for several

minutes may be required after the mixing, to flush the dirt from the filters. In some

cases if there are large quantities of algae, the media may have to be treated

with chlorine, to oxidize the algae and bacteria build-up from the media.

CHLORINE TREATMENT OF GRAVEL (MEDIA) FILTERS Backwash the filters first. Isolate the filtration system from the irrigation system by closing the isolation

valve downstream of the filtration system.

Fill up the filters with water (using external hose), to above media level. Add

four (4) litres of liquid chlorine to each of the filter tanks, and allow to treat for

12 - 24 hours.

With the chlorine still in the tanks, mix the media thoroughly, to free any

solids, which may remain in the tanks.

Note: care must be taken while mixing the media in the tanks, not to damage the

internal filtration elements (mushroom) at the bottom of the tanks.

Manually turn the filters (one at the time), to (ON) Back flush position.

Continue to flush the filter for several minutes, before moving to the next filter.

Continue to flush all filters in the system.

Open the isolation valve downstream of the filter, start irrigation system, and

check the pressure loss across the filter. A clean filter generally, should not

lose more then 5 psi (this may vary from one design to another).

Backup filters should be thoroughly cleaned and serviced at the end of each

season. However, in some cases, systems, which are operating with poor

water quality, may require more frequent cleaning.

9.2 Backup Filters

Disc Filters First wash the discs with fresh water – if scum and sediments persist then



The discs should be removed from the element (spine) and soaked in an acid

or chlorine solution for up to12-24 hours. HCl (Hydrochloric acid) mixed to a

diluted solution (1 to 2 litres of acid mixed with 10 litres of water), will remove

any buildup of organic and mineral salts from the discs grooves. (Refer to disc

Filter Module).

The discs should then be sprayed with high-pressure water before

installation, to remove any particles and the remains of acid. Damaged or

buckled discs should be removed and replaced with new discs (always use

the same color discs to replace damaged ones).

Clean check and lubricate all “O” rings and moving parts.This acts as both a

lubricant and preservative. Replace any damaged “O” rings with new ones.

Note: Use only recommended “O” rings (“EPDM” normally), and silicon based

lubricant to lubricate the “O” rings. Eg Molycote 111. Do not use Vaseline® -

petroleum based products will corrode the “O” rings.

Screen Filters – Screens should be removed and manually cleaned.

High -pressure water and a soft brush can be used, to remove particles from

the screen surface. The screen surface should be checked for damage and

tears.

Gaskets and “O” rings should be checked and replaced, if necessary.

All “O”rings should be lubricated (silicon based lubricant) prior to installation,

as per above.

The filter should be manually flushed at the end of the service, to ensure

correct operation.

10 Applications The main applications of Gravel Filter systems in agriculture are

Use as a primary filter for water from open water reservoirs and recycled

water.

Most effective form of filtration for water heavily contaminated with algae and

organic matter.



Used as a primary filter for water from bore holes.

Used in-conjunction with aeration for iron removal (covered in another

module)

Gravel media tanks have traditionally been the most popular filter for dirty water

applications. They are renowned for removing organic matter and are extremely

capable when it comes to high degrees of organic and inorganic material. The

depth of the gravel provides 3 dimensional depth filtration enabling it to store

much more debris than other filtration types prior to back flush.

Effective back flushing requires large back flushing flows to lift, fluidise the gravel

media and cause mass separation of the gravel media and the trapped

contaminants. Relative to disc and screen filters gravel systems require a large

percentage of backwash water – where the consumer is not sensitive to price, a

large footprint and



11 Summary and Conclusion The subject of gravel filtration in agriculture is broad topic. What we have

attempted to do is to gain a broader understanding of gravel filtration in

agricultural applications. We have discussed the development of gravel filtration

systems and described the components of the gravel filters. We have identified

the components and described their role and how they operate. From an

understanding of the components of the gravel filters we have covered in depth

the processes of filtration and back flushing. We have also identified when and

how back flushing should be done. Upon the understanding of the above-

mentioned processes, we have elaborated on automatic batteries. This has

allowed us to develop skills in selection and design for a given duty. Some

examples have clarified these skills. As with all mechanical systems installation,

operation and maintenance need to be carried out. This has been described in

some depth reaffirming our knowledge of gravel filtration. We have then briefly

reviewed gravel filtration applications and finalised the module with a series of

review questions.

12 Acknowledgements Thanks to Mac Ross and Andrew Crawford of Netafim for compiling the bulk

of this Section Acknowledgment to Arkal and Odis Filtration for information on their

equipment



13 Review Questions The questions are divided in to 3 sections – Beginner, Intermediate and Advanced. See below

13.1 Beginner 1) What level do you fill a gravel tank with media?

2) Can filter inlets and outlets be reversed? Why? Why not?

3) What does looking at the backflush water tell you?

4) Name two ways to look at backflush water whilst backflushing?

5) What causes media to exit the backflush manifold?

6) How do you prevent media exiting the backwash manifold?

7) Is inspecting the gravel necessary? What does it tell you?

8) How does gravel media trap algae and dirt?

9) Where would DC control be used compared to AC?

10) What is the minimum back flush time per tank?

11) How often should filters back flush?

12) In automatic systems what determines when filters backflush?

13) What are the maximum pressures Odis and Arkal recommend?

14) How often should gravel filter systems be serviced?

15) What is the role of the check filter? What mesh size is recommended?

16) What options are there for check filters of gravel filters?

17) Why is it necessary to use an air valve on the filter manifold?

18) How do mushrooms work?



13.2 Intermediate 1) What is the procedure if gravel media is found in the check filter?

2) How do you check if all filters are back flushing properly?

3) How long does media last before it needs replacing?

4) When is the use of a pressure sustaining valve necessary?

5) If there has been a power failure will the filters backflush? Why? Why not?

6) What is dwell time? How does it affect the flushing process?

7) What is the relationship between manifolding and filter battery efficiency?

8) What is the baseline pressure?

9) What does baseline pressure have to do with PD setting?

10) Describe the filtration process?

11) Describe the backflush process?

12) What is a diffuser plate? Why is it necessary?

13) Suggest two under drain designs? What are their advantages and

disadvantages?

14) How can the position of the foot-valve affect efficiency? Where should it be

located?

15) How should a check filter be maintained? How often?

16) What happens if you backwash both filters in a 2 filter battery at the same

time?



13.3 Advanced

1) How can gravel filters be used to filter iron? Describe the process?

2) How do you set the back flush control valve? What does it do?

3) What is the maximum distance the back flush pipe can be run from the filters?

What size?

4) What is channeling? How does it occur? How can it be avoided?

5) What is bridging? How does it occur? How can it be prevented?

6) What is better; a 2 tank system or a 3 tank system if the flow rate is the

same? Why?

7) What is the importance in the number of back flushes that occur on PD

compared to time? How would you find this information?

8) What are the problems associated with over pressure and under pressure?

9) What are the problems associated with over flow and under flow?

10) Why is a water sample beneficial for filter system sizing?

11) What are the advantages and disadvantages of injecting fertilizer before and

after the filters?

12) What are the advantages and disadvantages of single check filters and

multiple check filters?

13) Can gravel filters be used to remove sand from water?

14) If so will gravel media be removed while flushing the sand out of the tank?

15) Can you mix different types of media in the same tank?

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