How to Select a Pond Filter
Also see "How to choose
your pond pump"
- Introduction to pond filters
- Type of filters
- Filter Selection
- Small & Shallow Ponds (up to 950 litres approx.)
- Plain Water Filtration - No Waterfall or Stream
- Filters for Water Falls & Streams
- Using an In-Pond Filter
- Using an out of pond filter
- Pressurised Filter systems
- Gravity Discharge Filters
- Formulas for determining pump & filter size
- Actual Pond Volume
- Effective Pond Volume - Taking into account pond depth, sunshine
& climate
- "Total Head" - maximum pumping height - Taking into
account hose length and diameter
- Allowance for Fish Stocking Level
- Elbows - The effect they have on flow
- Power consumption
- Hose Flow rates
Introduction to pond filters
Maintaining a healthy pond environment with crystal clear water
usually requires a filtration system. Filtration systems perform
one or several of the following functions:
- Mechanical Filtration - traps debris in a
material of some type for later removal during cleaning
- Biological Filtration - provides an environment
where beneficial bacteria efficiently feed on impurities in the
water breaking down fish wastes and other organic matter
- Ultra-Violet Clarification - causes single-cell
algae (waterborne algae) to clump together under the action of
ultra-violet light, greatly increasing the efficiency of filtration.
Most filters for mid-size and large ponds employ a combination
of mechanical filtration and biological filtration, with ultra-violet
clarification specified for particular pond types.
Depending on brand and model, mechanical filters can consist of
layers of foam pads, trays of gravel, foam cartridges, or brushes.
Some models combine several of these filtration media into one filter.
In addition, many filters have a chamber containing plastic bio-media.
These are specially designed plastic pieces 25 to 50 mm in diameter.
Ten cubic centimetres of these pieces can contain as much as 1 square
metre of surface area. This large surface area encourages the growth
of beneficial bacteria colonies - the secret of effective bio-filtration.
Once the colony is established, it converts ammonia from fish waste,
dead organic matter and other impurities into nitrites and later
into harmless nitrates which help promote the growth of plants.
Without help, it can take up to seven weeks for a bacteria colony
to take up residence and grow large enough to be effective in a
typical bio-filter. This process can be accelerated through the
introduction of 'seed' bacteria, available either in dry, granular
or liquid form.
The rate of water flow through the bio-filter will have a major
effect on its effectiveness. All filters have a maximum flow rate
which must not be exceeded. Slower flow rates may result in the
volume of the pond not passing through the filter as often as required
for efficient filtration. Excessive flow will not allow the bacteria
sufficient time to clean the water and may dislodge the bacteria
from the bio-media.
Care must also be take when cleaning a bio-filter. Mechanical elements
(filter foam) should be gently rinsed in a separate container of
pond water. If the filter contains more than one layer of filter
foam, clean each in rotation. The bio-media should be disturbed
as little as possible so that beneficial bacteria are not disturbed.
Types of Filters
There are a number of different filter designs.
In-Pond Filters:
These small filters, referred to as pre-filters, fit on the intake
of the pump or around the pump itself and allow the pump's discharge
to supply a fountain head or waterfall if desired. They are usually
only used in smaller ponds and tend to be more difficult to maintain
as they must be removed from the pond for cleaning. An in-pond pre-filter
is not to be confused with a removable pump pre-filter. Removable
pump pre-filters are usually small foam filters designed to protect
the pump from large particles that may damage its impellor. Pump
pre-filters whilst not designed to filter the pond water do have
some minimal effect in reducing the amount of suspended solids in
the water .
Out-of-pond Filters:
There are two main types of out-of-pond (above-ground) filters,
'pressurised' and 'gravity-discharge'. These type filters have the
advantage of being readily accessible for routine maintenance as
they are not immersed in the pond.
Pressure filters
Pressure filters such as the Oase FiltoClear, Cyclone or Hozelock
Bioforce range allow you to filter the water and discharge it under
pressure to the top of a waterfall. The amount of lift that can
be achieved after the filter discharge is limited and should generally
be restricted to less than a metre. These types of filter are fairly
easy to camouflage. They can be hidden behind a shrub or buried
adjacent to the pond, up to the level of the lid. Click
here for a list of pressure filters.
Gravity-discharge filters
Gravity-discharge filters must be placed so that the water discharges
freely by gravity only. To operate a waterfall or stream with a
gravity-discharge filter, the filter must be situated at the highest
elevation with its discharge pipe feeding the waterfall/stream by
gravity. Of course, water must be pumped up to the filter with a
suitable pump. These filters are for above ground use only and are
usually set at the edge of the pond. Click here for
a list of gravity discharge filters.
Filter Selection
Please calculate the Actual Pond Volume and use
this figure to calculate the effective volume of
your pond, adding in an allowance for fish stocking levels if necessary.
All filters have a maximum pond volume that they are rated
for. It is essential to choose a filter that is rated for the
effective volume of your pond (with an allowance for fish stocking
levels if applicable).
Small & Shallow Ponds (up to 1000 litres approx.)
Due to the special requirements and unique circumstances found
in small and shallow ponds, including most pre-formed units, we
recommend to use a pond filter that is rated for double
the effective pond volume. For exampe, if the effective
volume of your pond is 600 Litres, look for a filter that is rated
for ponds up to 1,200 Litres.
Plain Water Filtration - No Waterfall or Stream
This is filtration at its simplest. All you require is a pond filter/pump
combination that is rated for the effective volume (plus an allowance
for fish stocking level, if applicable) of your pond. Start by selecting
a pond filter that has a maximum pond volume rating one level greater
than the effective volume of your pond. Go to our main
pond filters index and select from the range of filters listed
there. Pick any filter that is sized correctly and is appropriate
for your particular setup be it in-pond or above-ground.
Installation
In-pond Filters.
If an in-pond filter is the most suitable, simply place it in the
pond and plug it in. Most of these filters are supplied as a package
with a pump included. Many come with small fountain heads or can
be used to operate a small waterfall or spitter.
Above-ground Filters.
If an above-ground filter is the most suitable, the type you choose
will depend upon the effective volume of the pond as well as the
type of installation that you have planned. Above-ground filters
do not include a pump and this must be purchased separately.
Hint: Remember to place the pump inlet as far away from the filter
discharge as possible so that water circulation within the pond
is maximized.
Each filter has a maximum flow rate. This is the maximum flow of
water that the filter can accept from the pump. If this flow rate
is exceeded the filter will not work efficiently and might overflow
and/or suffer damage to the filter unit itself.
Total head height
To select a suitable pump the total head from the pond surface
to the filter intake must first be calculated.
Having calculated the total head , go to our main index page and
select an appropriate
pump based on the required flow rate at the calculated total
head. Select a pump that will deliver, but not exceed, the maximum
flow rate required by the filter. The final selection would be based
on consideration of such criteria as initial cost, power consumption,
and manufacturer's warranty. The yearly operating cost comparison
figures on the website make it easy to determine which pump will
be most economical to operate over the long term. Often, a less
expensive pump with high power consumption will quickly run up power
costs which more than offset the cost difference for a more expensive
pump with lower power consumption.
Tubing Size
The required tubing size connecting the pump outlet to the filter
inlet is determined by the maximum flow rate of the selected pump
and the intake fitting size of the pond filter. Most pond filters
feature step-down hose tails to accommodate several different tubing
diameters, for example from 20mm to 40mm. Filter intake fitting
sizes are shown for each filter we list, and recommended tubing
sizes are shown for each pump that we sell. Please click on the
pump model in our online
pond store for more details. Take care in choosing the tubing
diameter that is most appropriate for the filter and the pump. If
you can not find a listing for your pump, you can use the hose flow
rate chart to determine the correct hose size for the required flow
rate (please check to ensure that it will fit on the filter intake).
A hose adapter or a combination of adapters may be required to attach
the hose to the pump and/or the filter.
Filters for Waterfalls & Streams
Choosing a filter to use in conjunction with a waterfall or stream
can be a little complicated. The first requirement is a pond filter/pump
combination that is rated for the effective volume (plus an allowance
for fish stocking levels, if applicable) of your pond. Select
a filter that is rated for the effective volume of your pond
and suits your particular setup, be it in-pond or above-ground,
from the range listed.
The flow requirements for your waterfall must now determined.
The flow rate required will depend on the visual effect desired
and the width of the waterfall that you have constructed. Please
go to the How to Select a Pump/Waterfalls & Streams
section to calculate your flow requirements. You will need this
figure for use in later calculations.
Using an In-Pond Filter
Please take a moment to review the introduction above where pump
sizing requirements and filter selection are covered. All of the
information below is based upon the material previously covered.
If you have a small pond and waterfall, an in-pond filter may be
suitable provided that the desired flow rate down the waterfall
can be achieved. Many in-pond filters are supplied packaged with
a suitable pump. Look up the flow rate for the pump/filter package
and compare it with the flow requirements that you calculated for
your setup. If you have more flow than required, many of these filters
also come with small fountain spray heads through which some of
the flow is diverted. If the waterfall is to be very small, it may
be possible to have both a fountain display and a waterfall. If
the flow is barely adequate, replacing the fountain head with a
short piece of tubing which can be pinched off/cosed and will improve
flow from the waterfall. If the flow rate over the waterfall is
not adequate, a separate pump will be needed to supplement the flow
of water.
Using an out-of-pond filter
Above ground filters offer advantages in terms of maintenance,
design flexibility and filtering capacity but are not usually supplied
packaged with a pump. In sizing the pump for an out-of-pond filter
a decision must be made to either run both filter and waterfall
with a common pump or to use a separate pumps for each item. If
separate pumps are to be used simply size a pump for each application
separately. Operating both waterfall and filter from one pump is
often not possible as the requirements for each item are incompatible.
There are two basic types of out of pond filters (covered in the
next section), 'pressure' and 'gravity discharge' each of which
has its own particular characteristics and applications.
Pressurized Filter systems
Please take a moment to review the introduction to this section
where sizing requirements and filter selection are covered. All
of the information contained hereafter is based upon this.
Pressure filters can be used in two different types of installations.
- The simple setup where the filter is at pond level and the
water is pumped through the filter and flows straight back into
the pond, or
- When a waterfall or stream comes into play, requiring water
to be pumped up hill.
The second situation presents many challenges in choosing the correct
pump and filter because of the pressure and friction loss which
occurs in pumping water to the top of a waterfall or stream. Whilst
the water emerges from pressure filters still under pressure just
50cm of vertical lift and/or 2m of tubing run after the filter discharge
can decrease the flow rate to unacceptable levels. For this reason
the amount of vertical lift and the length of horizontal hose run
after the filter discharge should be kept as small as possible.
The best indication of what you can expect is in the performance
rating (flow rate versus head height) for the pump that you have
chosen to operate the pressurized filter.
Hint: When water passes through a pressure filter there is a flow
loss of about 15%, so this factor must be built in to your calculations.
Although it may be more difficult to install your filter at the
top of the waterfall, it is preferable to do so to minimise the
pressure on the filter, particularly in systems operating under
significant head.
Whichever setup you plan to use, calculate the total head at the
intake of the filter and be sure to note the maximum flow rate for
the filter you have chosen. Then go to our main pumps page and select
a pump that will deliver, but not exceed, the maximum flow rate
for which the filter is rated at the total head of your system.
The final selection is up to you. Criteria would be initial cost,
power consumption, and manufacturer's warranty. The yearly operating
cost comparison figures on the website make it easy to determine
which pump will be most economical to operate over the long term.
Sometimes, a less expensive pump with a high power consumption will
cost more to operate over the long term.
The final consideration would be the flow rate from the discharge
hose of the filter. Does this correspond to the flow rate requirements
for your waterfall that you calculated previously? If the volume
coming out of the filter is not sufficient there are several possible
remedies. You can use a larger filter (sized for a larger pond volume),
install a separate pump to operate and/or supplement the flow of
water coming down the waterfall, or use a larger pump and split
the volume of water between the filter and the waterfall.
The size of tubing required to connect pump to filter is dictated
by the maximum flow rate of the pump selected and the intake fitting
size of the filter. Most pond filters feature step-down hose tails
which will accommodate several different tubing diameters, for example
from 3/4" to 1-1/2" (simply remove the unwanted section
of the hose tail with a saw). Filter intake fitting sizes are shown
for each filter we list. Carefully select the tubing diameter that
is most appropriate for the filter and the pump. If you can not
find a listing for your pump, you can use the hose flow rate chart
to determine the correct tubing for the flow rate required, however
please remember to ensure that it will fit on the filter intake.
A hose adapter or a combination of adapters may be required to attach
the tubing between the pump and the filter.
Gravity Discharge Filters Such as Oase Biotec
Using a gravity discharge filter for an installation with waterfall
and/or stream has some limitations. Since these filters rely on
gravity to operate they must be situated at the top of the stream
or within the waterfall so that the water flows freely through the
filter. If you are able to achieve this, your waterfall will be
limited to the amount of water that is flowing through the filter.
As mentioned earlier, the pump should be placed as far away from
the waterfall as possible to maximize water circulation within the
pond. Calculate the total head to the position where the filter
intake will be located. Each type and size of filter has a maximum
flow rate listed for it. This is the flow rate that must be supplied
to the filter at the total head previously calculated.
Now go to our main pumps page and select an appropriate pump that
will deliver, but not exceed the maximum flow rate the chosen filter
will require at the total head of your system. The final selection
is up to you but you may wish to consider such criteria as initial
cost, power consumption, and manufacturer's warranty. The yearly
operating cost comparison figures on the website make it easy to
determine which pump will be most economical to operate over the
long term. Sometimes, a less expensive pump with a high power consumption
will cost more in the long term due to its operating costs.
Hint: For filtration purposes, rather than using a pump with an
integrated foam pre-filter, we recommend you use a pump that
comes with a screen housing. The foam pre-filters clog up quite
easily and require a lot of maintenance. All Oase
pumps and Hozelock pumps are supplied with
only a screen (cage), other pumps such as Pond master have a removable
pre filter.
The correct size tubing to connect the pump to the filter is dictated
by the maximum flow rate of the selected pump and the intake fitting
size of the filter. Most pond filters feature step-down hose tails
which accommodate several different tubing diameters, for example
from 3/4" to 1-1/2". Filter intake sizes are shown for
each filter. The amount of vertical lift and the length of hose
run after the filter discharge should be kept as small as possible.
Select the tubing diameter that is appropriate for the filter and
the pump. If you can not find a listing for your pump, you can use
the tubing flow rate chart to determine the correct
tubing for the required flow rate (please ensure that the tube selected
will fit on the filter intake). A hose adapter or a combination
of adapters may be required to attach the tubing to the pump or
the filter.
The volume of water flowing through the filter will control the
flow rate that is available for your waterfall. You will however,
lose a little volume to the filtration process and the flow will
decrease as the filter becomes clogged. Taking this into consideration,
does the flow rate that you will be getting out of the filter correspond
to the flow rate requirements for your waterfall that you calculated
previously?
At the beginning of this section we said that things can get a
little more complicated when combining a pond filter with a waterfall.
Why? Because the amount of water discharged from the filter may
not correspond with the amount of water necessary to produce a good
flow of water over your waterfall. It is possible that you will
require more water coming down the waterfall than the filter is
capable of supplying. If this is the case you have two options,
you can either install a second pump to supplement the flow to the
waterfall, or use a pump that provides a greater flow rate than
required for the filter. In the latter case the water coming from
the pump must be split into two streams, one stream would be piped
directly to the top of the waterfall, and the second stream would
be piped to the filter and then make its way down the waterfall.
Most likely ball valves or some method of regulating the flow into
one of the two lines will be necessary to ensure that the maximum
flow rate for the filter is not exceeded.
Power consumption
The average cost of power in Australia is about $1.20 per watt
per year, so a 10 watt pump/light/filter running 24 hrs a day will
cost about $12.00 a year (23 cents per week)
How to calculate annual cost: Watts divided by 1000 x the price
of electricity in $ per kilowatt hour x 24 hours x 365 days (assuming
continuous operation)
Useful Conversions & formulas
You may find the following helpful:
Volume
One Imperial Gallon |
= |
1.20 US Gallons (approx) |
One Imperial Gallon |
= |
4.54 litres (approx) |
One US Gallon |
= |
0.834 Imperial Gallons (approx) |
One US Gallon |
= |
3.78 litres (approx) |
Power
To calculate DC power consumption: |
= |
Volts x Amps = Watts |
To calculate AC power consumption |
= |
Volts x Amps x Power factor = Watts |
To calculate Amps |
= |
Volts divided by Watts |
Traditional Horse Power to watts conversion (But be careful*)
1/4 hp |
= |
187 Watts |
1/2 hp |
= |
377 Watts |
3/4 hp |
= |
559 Watts |
1 hp |
= |
746 Watts |
1 1/2 hp |
= |
1116 Watts |
2 hp |
= |
1496 Watts |
* These figures are very old and unreliable as they come from
a time when motors were not as efficient as today. People from the
old school of thought quote Horse Power but it has no real value
with today's pumps. It's all to do with efficiency so for example,
a pump made in China that can produce 1,000 litres per hour may
be a 50 watt pump while a German pump of much higher quality may
only need half as many watts to pump 1,000 litres per hour.
|