Mechanical pond filtration

Whenever koi pond filtration is discussed, attention tends to be focused on the biological aspects, with the emphasis being placed on maturation rates, testing for ammonia and nitrite and avoiding new pond syndrome at all costs. But effective pond filtration results from the combination of a number of complementary processes that interact with pond water in a given order; the first process being mechanical (or primary) filtration.

Mechanical filtration is not an area of filtration that we spend a great deal of time discussing, perhaps because once it is fitted, it gets to work immediately and does not require any further nurturing from ourselves. Furthermore unlike the biological water parameters, there is not an ‘everyday’ water test that we can use to test how well a mechanical filter is performing, settling for the arbitrary measure of our own eyesight. (It could be argued that in the short term, unlike the biological water parameters such as ammonia and nitrite, water turbidity or clarity is only of aesthetic importance.) But I strongly believe that mechanical filtration is by far the most important element in a pond filter’s design, and because of its poor cousin relationship with biofiltration, mechanical filtration is often inadequate.

Not only will inadequate mechanical filtration lead to frustratingly poor water quality, but also to a limited biofiltration performance as your filter matures through the years.

Why is mechanical filtration so important?

Efficient and effective mechanical filtration is essential because of the way we choose to keep koi. Not only do we expect to see as our koi swimming in clear and colourless water, we also plan our filters to function best under these conditions. Compare our situation with that in a typical mud pond in Niigata, and you start to see why we, the koi keeper, are the driving force behind mechanical filtration. A mud pond is arguably the best environment for koi, yet it has no clarity to talk about, and at best can be described as a muddy hole filled with dirty water. Koi thrive in these conditions, and the suspended clay particles act as the microscopic media in a natural fluidised bed onto which the environment-sustaining bacteria are attached. So we don’t filter mechanically for the benefit of our koi, but so we can see them. But the long-term stability of our pond environment is also at stake if we do not filter mechanically as our primary filtration must protect the media within our biological chambers from debris that overtime would settle and smother the media and the bacteria within it. This could lead to large-scale die-offs of bacteria, jeopardising the stability of the pond’s water quality parameters. So working backwards, if we skimp on mechanical filtration, not only will the appearance of our pond be poor, but we risk an unpredictable and acute deterioration in the biology of our pond.

Different types of mechanical filtration.

The objective of mechanical filtration is to remove particular debris from the water that would either cause the water to become turbid or settle in the bio-media. Sources of particulate matter will include food, fish waste, flocculated algae and external debris that is blown into the pond (Sand, leaves, grit etc). The particulate matter must be removed and collected efficiently to allow it’s easy removal, without disturbing other parts other filter media. When discussing the efficiency of a mechanical filter we discover a paradox that we need to overcome (usually by compromise. This paradox arises when designing a mechanical filter to remove debris efficiently and down to as small a particle size as possible. It would be easy to do so, but the filter’s ability to remove even the finest particulate matter must be balanced by how frequently the koi keeper is happy to clean or maintain that filter. In fact, the majority of pond owners, when choosing a filter, strike a balance between filter efficiency and performance and its maintenance needs (i.e. how often you have to roll your sleeves up to work on the filter). If maintenance was not an issue, then we would choose ultra-fine pore filtration systems that intercept the finest of particles, but would need cleaning or backwashing on a daily basis. (And I thought pond keeping was a leisure activity!) And this is the paradox.

What do we mean by an efficient filter? Is it one that removes all debris in a single pass and needs cleaning daily or one that does not require regular cleaning but does a reasonable job over time? Inevitably, this results in a compromise between the two, although ‘The Answer’ has tried to overcome this paradox by combining an ultra-fine screen with a self-cleaning backwashing mechanism.

There are several different methods of removing debris mechanically, all of which have been embodied in different filter designs. Some of these designs have varied in their fashionability over the years as new ideas of how mechanical filtration should be executed have developed.

1. Modifying flow characteristics.

a. Settlement. all debris held within a water column is subject to gravity and will sink if it’s density is greater than that of water and it’s particle size is of sufficient size to overcome drag. The more energetic a water body, the less likely the particulate matter will settle, as the water movement will act against the force of gravity. It is widely accepted that organic particles less than 25 microns will not settle under their own weight through the force of gravity due to the small differential between the overall mass of the particle and the density of the water (this is why microscopic algae that causes green water cannot be removed by settlement). These particles must be removed by other means. Settlement (also known as sedimentation) occurs most effectively when the water column is at its slowest, and a settlement chamber is designed to slow water down. In fact, by default, every pond’s most effective settlement chamber is the pond itself – a feature we can either utilise (by the strategic placement of bottom drains) or find to our cost when we discover that a thick covering of black silt has settled on our pond bottom. We need to encourage the particulate matter to travel from our pond into a settlement chamber where it is both encouraged to settle and is easily removed by pulling the drain.

Settlement chambers are best fed by gravity, rather than a pump as a pump increases the velocity of pond water and also macerates the particulate debris making settlement less likely. The design of a settlement chamber can be improved to enhance settlement; design changes focus on slowing down the water:

a. wide pipework to and between chambers

b. Large area of settlement chamber to slow down the water

c. Baffle or weir boards cause water to slow and change direction

d. The addition of brushes or other mechanical media to slowdown and deflect water flow.

e. Strategically placed drain to purge the chamber and remove all settled organic debris.

Box Out

Settlement in the natural world. If you have ever studied the bottom of a stream or river, you may have noticed that it is at it’s rockiest when the flow rate is at it’s fastest, depositing silt and other fine debris when the river widens and the flow rate drops. This is the same principle that is used in a settlement chamber.

b. Vortex. A vortex chamber modifies the water’s flow characteristics by creating a gentle centrifugal force on the particulate laden water. The water is introduced into the circular vortex chamber via a wide gravity-fed inlet that is positioned to cause a tangential flow within the chamber. The water is also forced upward leaving the chamber via a central exit in the middle of the chamber where the water will be cleanest. The friction between the water and the sides of the chamber, together with the centrifugal force and changing direction all act to cause the debris to drop from the water. Like the settlement chamber, a vortex should have an easy access drain that enables the rapid purging of the chamber to remove the accumulation of the oxygen-demanding organic debris.

A series of vortex chambers works well, where one leads into the other, processing the water further to remove any particulate matter that may not have been removed in the primary vortex. It is also quite common to add media to intermediary vortex chambers just before the dedicated bio-chambers as these polish the water further by intercepting the finer particles. This method starts to use entrapment, which is a method of mechanical filtration in itself.

2. Entrapment.

Where settlement works by removing energy from the water flow, entrapment works in the opposite way, by forcing your water to flow through media that then intercepts and strains the water ‘colander-style’. This is of course the method of mechanical filtration employed by pump-fed ‘black box’ filters, where a graduated set of foams (running from coarse to fine) works in unison to remove debris from the pond water. These in turn also mature to have a biological action, being referred to as biomechanical media.

The biggest problem to overcome with entrapment media is the selection of the pore size. Too fine and it collects everything but blocks very quickly, too course and it may mean it’s low maintenance, but only because it does not intercept much debris. Consequently, maintenance and cleaning is a big factor with entrapment media. Initially, entrapment media pore sizes can be have been quite coarse, but their efficiency will increase over time as the media matures; yes, that’s right mechanical media matures! A living slimy, mucilaginous biofilm coats the microscopic surfaces of mechanical media, filling the tiny interstices within the media effectively making the pore size even smaller. Add to this the accumulation of additional microscopic particles of debris and the pore size becomes so fine that even the finest of particles start to adhere and are trapped in this living mechanical media. It will, nevertheless reach a stage where it requires cleaning or backwashing to remove the accumulated organic debris, unfortunately setting back its efficiency.

Entrapment devices with built-in cleaning.

1. Sand pressure filters. These filters run a very thin line between being very effective and to labour intensive of. That is, they require regular backwashing. Such a filter consists of a glass-fibre pressure vessel inside which a bed of silica sand sits on top of a fanned network of porous pipework through which the mechanically filtered water is pumped. On account of the pressures required, a sand pressure filter is usually driven by its own dedicated external pump which also doubles as a back washing pump once the flow is diverted through a very ingenious multi-valve system. Sand pressure filters were very fashionable in the late 1980s to the mid-1990s but have been superseded by higher performance self-cleaning units such as…..

2. The Answer. This takes entrapment a stage further by being able to marry the maintenance limitations up of an ultra-fine micron mesh screen with a continuous self-cleaning system which prevents the need to manually clean the screen. This allows clean water to pass through the unit, leaving even the finest of particular matter to drift around in the first mechanical chamber where it is retained until purged.

3. Bubble bead. The beads (from which these filters get their name) are the ‘business end’ of the filter and are made from a buoyant polyethylene plastic, each of which is 1/8″ in diameter. They achieve both mechanical (solid entrapment) and biological (bacterial breakdown of waste) functions in a similar way to an undergravel filter that has been turned on its head. Where gravel in an undergravel filter serves both mechanical and biological functions on the base of an aquarium, the buoyant beads do exactly the same but in the top of the filter. That is where the similarity ends, as typically, a bead filter is cleaned more frequently (on a weekly basis) where as an undergravel filter could be cleaned once every 3 months!

The beads supply a large surface area (but are not porous, unlike other biological media) attracting a film of bacterial growth within the steady stream of pond water. As the beads accumulate and ‘bed down’ at the apex of the filter, they also form an effective mechanical filter, screening the water of particulate matter down to 20 microns. Finer particles (less than 20 microns) can also become trapped in the sticky bacterial film that coats each bead.

Eventually, the floating bed of the multi-functional beads will become blocked with the debris that they have been designed to intercept. Evidence of this will be a drop in the throughput of water through the filter (particularly visible if the bubblebead filter outlet feeds a waterfall).

Cleaning involves a backwashing process. In these ‘hourglass’ units, the inlet is closed and an adjacent backwash valve at the bottom is opened. Air is also introduced into the lower chamber just beneath the constriction to relieve the vacuum within the vessel, allowing the water and bead bed to drop under their own weight , down through the constriction, breaking up and agitating the bed of accumulated debris and beads as it goes. The discharged nutrient-rich water is diverted onto the garden or down a drain while the perforated pipework within the chamber retains all of the beads in the filter. The discharge valve is closed and the pumped water reintroduced from the pond to re-float the beads.

Some models of bubblebead filters are fitted with pressure sensitive valving which means that when the pump is turned off, the filter drains and backwashes automatically through the discharge valve. To take full advantage of this, a simple timer can be installed to cut the power to the pump for 15 minutes each day, thereby backwashing your filter automatically each day. You will need to install an auto top-up system to maintain the water level in the pond, thus making backwashing a completely automatic process.

In summary, for every pond, mechanical filtration starts in the pond itself, with the first objective being to prevent the particulate matter from settling in the pond (the largest settlement chamber). This is achieved by using either a strategically placed submersible pump or bottom drain(s), incorporated into a pond that has been designed without dead spots. The choice of mechanical filtration is then purely personal, but one that complements the layout of your pond filter and the budget and space allowed. Mechanical filtration is usually the most inadequate in a koi filter system, but is essential for not only providing crystal-clear viewing water, but also the long term stability of your biomedia. I am convinced that if there was a water test to measure how effective our mechanical filtration performed, we would all dedicate more time in planning and space in constructing a hyper-efficient mechanical filter.

Kill blanketweed and string algae.