Pond Filtration: Settlement and bio-towers.

Dear Ben,

I am currently in the middle of building a koi pond of around 2,000gallons. I am planning on using traditional settlement and wondered what flow rate and size of settlement you would advise. Would you also include brushes? And how good are trickle towers for filtration? Any information on settlement and filtration would be gratefully received.

Thank you for your query about mechanical and biological filtration. It sounds as though you have started an interesting pond project that is looking at mixing old and new approaches to koi pond filtration. Settlement is a very traditional method of removing solids effectively, and is tried and trusted, having many benefits. Meanwhile, you are also planning on combining this with a method of biological filtration that has not traditionally been married to settlement – namely trickle filtration. Usually, through ease of construction, settlement chambers lead on to wet biofiltration where the media is housed in chambers similar in construction to the settlement chambers. Incorporating a biotower is a refreshing change with several performance benefits, but also a number of related issues that you should be aware of.

Size of your pond.

Your 2000 gallon pond will determine the flow rate required through your filter. The useful rule of thumb guide is to turn over your pond’s volume every 2 hours. So your filter will need to handle 1000 gallons of water every hour. This flow rate is set largely from the biofilter’s perspective, as the beneficial bacteria within the biomedia require a regular and timely supply of soluble wastes on which to thrive. They also require a flow rate that enables them to breakdown wastes at a rate that matches the rate at which it is produced by your fish. Your plan for a trickle tower will mean that you will have to take into account a greater ‘head’ when purchasing your pump than if you’d opted for a wet biofilter, so that the 1000 gph turnover is still achieved at the higher head.

Mechanical Filtration.

Mechanical filtration serves two purposes in a koi pond.

1. Clear water. Mechanical filtration is the first type of filtration in a koi pond (and is sometimes called primary filtration for that very reason). It removes suspended debris from the pond by trapping or settling particulate matter. However, this is carried out largely for aesthetic reasons, so we can see our cherished and spectacular koi. Koi themselves do not actually require clear water (and in fact thrive in the murky conditions of a mud pond).

2. Filter function. Even a small filter system will handle several thousands of gallons of water every day. Mechanical filtration plays a key role in preventing solid matter from gathering in the biological media (in your case, the biotower media). If silt or other sediment was allowed to accumulate within the fine structure of the biological media, then the biological function of the filter would deteriorate, leading to water quality problems. It is vital that biomedia be kept as clean and unblocked as possible to allow a high-density of bacteria to populate the media. As soon as settled debris is allowed to create dead spots within the biomedia, then traces a toxic ammonia and nitrite it will start to accumulate in the pond water, having a knock-on effect on koi health.

There are several different methods of removing suspended solids from your pond water.

1. Settlement and vortex. These methods use subtleties of water flow within the chamber to settle out suspended particulate matter. When the speed of flow drops, the water’s ability to keep solids in suspension also drops, resulting in the settlement of solids in the chamber.

2. Entrapment. This method is used in standard external black box biofilters, acting in a similar way to a sieve. In a multi-chambered system, methods of entrapment can vary.

How do settlement chambers work?

Settlement chambers work by modifying the flow characteristics of your pond’s circulating water. The effectiveness of a settlement chamber starts well before the debris enters the chamber, as great care must be taken to prepare the debris for settlement. Particles must be kept as large as possible while still in the pond (as these will settle much better) – something that is best achieved by a bottom drain. A settlement chamber that is gravity-fed by a passive bottom drain that gently transfers solid debris from the pond to the first chamber is the preferred option. Compare this to say a pump-fed set-up where the pump not only macerates the solid matter into finer particles (that will not want to settle) but also energises the water when in fact we require the water to be as slow-moving as possible. For this reason, if you’re planning to use settlement chambers as your means of mechanical filtration, you also need to be planning to have a bottom drain arrangement (which may involve more than 1 bottom drain, depending on the layout of your pond).

How does settlement work?

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 and must be flocculated into larger particles by a UV). 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 in a gentle fashion where it is both encouraged to settle and is easily removed by pulling the drain.

The design of your settlement chambers can be improved to enhance settlement; design changes focus on slowing down the water:

a. Wide pipework to and between chambers. Your design should include 4″ pipework as this prevents ‘jetting’ and encourages water to move slowly through the filter. When installing your pipework, think ‘slow, slow, slow’ and only install pipework that will work in sympathy with the whole settlement concept.

b. Large area of settlement chamber to slow down the water. Your settlement chamber cannot be too large (but it can be too small). I suggest a minimum of 2 settlement chambers should be incorporated (but the more the better). The smallest chamber I would consider would be 30″ wide, 3′ deep and 3′ long, but of course, larger would be better. This gives the water entering via a bottom drain and 4″ pipework opportunity to slow down further and drop it’s particulate load.

c. Baffle or weir boards, sometimes referred to as up-and-over boards. These force slow moving water between your chambers to change direction, slowing the flow further, encouraging yet more debris to drop out of suspension. These should be incorporated between each chamber.

d. Drains to waste. Situated in the floor of each chamber, these allow the easy removal of settled debris. The weir board and stand-pipe set-up allows each chamber to be independently purged, without having to empty all chambers at once. It also allows the rapid resumption of filtration once the settlement chamber(s) has been purged.

The addition of brushes?

The key benefit of settlement over entrapment as a means of mechanical filtration is that it doesn’t become blocked over time. Furthermore, with well-placed drains for emptying each chamber, maintenance can be both a relatively quick and easy operation. The addition of brushes can change this. True – brushes will intercept and trap finer suspended solids, but should not be necessary if the area devoted to settlement is sufficient. They can also prove a real headache for cleaning. But if you feel your area for settlement may be rather limited then the addition of brushes for added particle entrapment can enhance your filter’s mechanical function.

Trickle Towers:

Perhaps the single greatest limiting factor for biofiltration is the availability of oxygen. Biofiltration is an oxygen-demanding process with billions of aerobic filter bacteria requiring oxygen to perform their oxidative processes. There are essentially two types of biofiltration, depending on where the media is situated.

a. Below the water = wet filtration

b. Above the water = dry / trickle filtration

Trickle filtration by its very nature provides the bacteria on its media with a concentrated, readily-available supply of oxygen compared to wet filtration. This is because compared to the typical 1% oxygen being dissolved in water in submerged media, atmospheric air contains a huge 21% oxygen, making oxygen less likely to be limiting, making biofiltration consistently more effective in a trickle filter.

So as far as biofilter efficiency is concerned, trickle towers offer many benefits. However, they are for more obtrusive than their wet biofilter counterparts (which are designed and hidden along the lines of a settlement chamber). Trickle towers must sit above the surface of the pond, be pump-fed (from the final mechanical chamber) and return to the pond via gravity. The media for a trickle tower can also prove to be expensive (unless you can improvise with alternative every-day media) and to achieve effective coverage of the media, will have to design some sort of rotating spray-bar. They can also cause the pond water to cool quite significantly – a feature of so much pond water coming into contact with the air.

In summary, your anticipated combination of traditional settlement and highly efficient biotower should prove to be a very effective combination for your pond. I’m sure your koi will benefit from your planning – send us a picture when you’ve finished it!

Kill blanketweed and string algae.