An Introduction to Drum Filters for Koi Ponds

Many koi hobbyists will at some point search for filters for a koi pond, whether for a new build, upgrading existing pond system, or curiosity. One filter that will feature in searches and discussion will be the Rotary Drum Filter, usually shortened to Drum Filter. The drum filter is often cited as the pinnacle of koi pond filters, and for good reason. However if you don’t understand the design features and operation, you might be unsure whether a drum filter is right for you, or even what type you require. 

 In this post we will explore the basics of drum filters

• How a drum filter works
• Benefits of drum filters
• How to choose a drum filter

What is a Rotary Drum Filter?

A Rotary Drum Filter is a mechanical filter for removing solid waste from the koi pond environment. A fine mesh captures organic waste, and when this blocks, a sensor will trigger an automatic cleaning cycle, rotating the drum while jets spray the debris from the mesh screen. Drum filters can often be combined in one unit with a biological filter. 

How a Drum Filter Works

A drum filter is installed externally to a pond like most other filters. Usually a drum filter is fed water from the pond using a bottom drain though they can also be pump fed. Drum filters often have multiple inlets so it is possible to use them with a bottom drain and a skimmer, or multiple inlets. 

Due the high capacity, drum filters allow the flexibility of multiple inlets. This allows them to accept bottom drains and skimmers, and often more than one of each type

 

In the case of a gravity fed pond with a bottom drain, the water enters the drum filter in the ‘dirty’ side, and passes through a dividing panel, in to the drum. The drum is wrapped in a fine stainless steel mesh, usually 60-80 microns, which is the size of the openings in the mesh.

 

 

A close up of the mesh screen, this is taken with a macro lens at 1:1

 

How big is a micron? The micrometer, with the SI symbol μm, is one millionth of a metre, or one thousandth of a millimetre! The human eye can detect objects around 40 microns in size. 

Size	Object
1.5 - 10 microns	Single cell algae
50 -100 microns	Trichodina, a common parasite found in koi ponds
100 micron	The average thickness of a human hair
500 microns	The average length of a grain of sand

 

This mesh will block quickly in a koi pond. The cleaning cycles would be too frequent for a koi keeper, so the process must be automated. In a gravity fed system, the circulation pump will be returning the water to the pond at a constant rate - when the screen blocks, the water level in the clean side of the drum will fall, causing the sensor to trigger a cleaning cycle. When the cleaning cycle is triggered, a motor will turn the drum, and at the same time, a flushing pump sprays water from the outside of the mesh to blast away the debris, in to the waste chute. This periodic cleaning will take place approx 15-45 minutes depending on the size of the pond, filter, pumps, stocking, and some other variables.

 

 

The float switch is responsible for triggering the cleaning cycle. When the water level rises (in a pump fed system) or when the water level falls (in a gravity fed system). This particular float is in a pump fed system

 

Benefits of a Drum Filter 

1. Automatic Cleaning - Most koi keepers will be attracted to drum filters due to the automatic cleaning. Automatic cleaning of pond filters creates a better environment for the koi, and leaves the hobbyist with more time to enjoy the better parts of koi keeping. If cleaning filters is a chore, then the drum filter will remove 90% of pond maintenance. Drum filters are not maintenance free though!
2. High Flow Rates - For any koi keepers wanting to improve their growth rates, having a high flow rate through the filters can increase the potential for growth. By moving more pollutants to the filter system, it creates better water quality, increasing the appetite of the koi. The high flow rate also benefits clarity. If the koi produce solid waste faster than you can transport it to the filters, then you will see the fine particles floating in the pond. 
3. Multiple Inlets - Due to the high flow rates and automatic cleaning, most drum filters come with multiple inlets. This is especially good on larger ponds when you need more bottom drains to provide good clearance of the pond bottom, and more turn over in the pond. On smaller ponds, multiple inlets give you the opportunity to effectively filter a bottom drain feed and a skimmer without multiple filter units. Bottom drains and skimmers should never be joined before the filter as it reduces flow, and can lead to build up of waste in the low flowing pipe.
4. Compact - A drum filter is very small for its capacity. As an example, a drum filter capable of handling 35,000 litres per hour is not much larger than a black box filter barely capable of 5000 litres per hour, while outperforming most filters twice its size.
5. Improved Clarity - The high flow rate allows more water and debris to be delivered to the filter, and the ability to use multiple inlets means you’re able to draw water from various parts of the pond. Combining a bottom drain and a skimmer, you can collect the waste from the top and the bottom of the pond.

 

 

This drum filter has a tiny foot print, it has two inlets, and has a maximum quoted flow rate of 30,000 litres. It's half the size of some other branded filters that will only handle a quarter of the capacity.

 

 

This drum combi filter was installed in a tiny filter bay - no other filter would fit this space, offer the performance and flexibility to be fed from a bottom drain and a skimmer, and reduce maintenance to minimal levels. The pond in the background is approx 3000 gallons

 

How to choose a Drum Filter

There are two main styles of drum filter available on the market. Some units focus solely on mechanical filtration, though you can buy a Combi Drum or Drum Combi, which combines the mechanical drum filter with a biological chamber, usually utilising a moving bed bio. 

A koi pond does need some form of mechanical filtration to remove the solid debris and a biological filter to remove the ammonia and nitrite produced by the koi. A drum only unit can increase flexibility, having separate drum filter and bio chamber can allow for a different layout to fit unusual or busy spaces. Having separate components can allow a larger bio chamber, or provide different options for plumbing in extra pond equipment such as bakki showers. A Combi Drum filter can simplify pond plumbing, requiring only an inlet and outlet pipe. The Combi Drum filters often have a smaller bio, which is worth assessing if the bio capacity will suit your fish load.

After measuring your filter house and your fish collection, the next consideration would be budget. 

There are many brands of drum filters on the market, some are made in the same factory. When looking at brands, it’s important to consider 

• Initial purchase price
• Cost of spares
• Availability  

Drum filters are a mechanical-electrical item, exposed to water. At some point, it will need a component replacing. If you pay the highest price for the most premium brand, when that filter needs repairing, the cost of spares are usually at the top end of the price range too. Speaking of reliability, failure is often a reason many people argue against drum filters. Almost all drum filters on the market have a bypass to ensure the pond water can continue to be recirculated - maintaining the biological, ultra violet, and optional pond heating performance.

Almost all drum filters come with a built in UV-C or have the option to add one. Adding the UV inside the drum has two benefits. The close proximity of the lamp to the screen prevents biofilm accumulation on the screen, which would lead to a drop in flow rate and increased cleaning cycles. The second benefit is the replacement of a traditional inline UV, which adds restriction to the flow rate of your pumps. A typical submersible UV like those used in drum filters is often very efficient - 40w submersible UV's generally have a rated capacity of 7000 gallons. 

Some drum filters have basic controllers, others have a digital display for operating parameters, and some have WiFi/Cloud capability. In our opinion, CCTV offers more benefits to WiFi/Cloud options. If you’re a frequent traveller or work away often, well placed CCTV cameras can give you piece of mind that the pond and filter is working. 

Conclusion

If you were to visit our premises, you will see we have gone all in with the drum filter. Our drum filters save us lots of time, improves water quality saving money on treatments and making sure our koi are in good condition, and offers excellent clarity to better view our koi.  

If you want automatic cleaning, more time to enjoy your koi, a compact filter system, and high quality water, then a drum filter is for you. If you need more help in choosing the right drum filter for your koi pond, we have 8 different drum filters on site to demonstrate to hobbyists. Our experience fitting drum filters will enable us to guide you through making the right choice, and correct installation for optimum performance.