Fifteen years ago, filtration for koi ponds was all about chambers and media. We had moved on from gravel beds at the bottom of our ponds and the choice was between a series of large vortex chambers or a multibay with a series of chambers combined in one unit. Probably the most popular setups had a large vortex for mechanical filtration in front of a multibay with a series of aerated chambers containing Japanese matting.
The issue with all of these setups was firstly that they took up a lot of space and secondly, they had been designed for static media and few people understood the benefits of the new moving bed solutions using media such as Kaldnes K1.
Evolution Aqua (EA) set out to build a filter that would offer a chamber suited for moving media and in the process, came up with a brilliant solution putting one vortex inside another.
The concept of a vortex is relatively simple – a large circular vessel, ideally with a conical base has dirty water entering at the side somewhere around the middle. The flow of water makes the contents spin and due to the laminar flow, movement is slower at the edge/outside and faster in the middle. Thus any large particles entering tend to get caught in the slower moving water and drop to the bottom – leaving clean water at the top and centre, which exits via a central pipe and heads to the biological filtration (see diagram).
The simplest solution back then, would have been 2 vortices – the 1st providing mechanical filtration and the 2nd containing the moving bed, but had EA gone that route – they would have made some money reselling Kaldnes or other media but they would not have had the basis for a viable business.
So instead, they built a brand new filter with a very small footprint which provided mechanical filtration in one vortex and biological in the other using Kaldnes K1 in a moving bed. Also, they made the investment and had moulds made allowing them to use injection moulded HDPE rather than the standard glass fibre used by most others – allowing them to produce in greater volumes and probably at a much lower unit cost than their competitors.
However, there were a number of odd choices and design compromises – as is the case with any filter.
Logically, the outer vortex would have been used for mechanical filtration – the greater volume and circumference would better allow particles to drop out and then the clean water could flow into the central vortex containing the moving bed. I suspect there were a couple of issues with this approach:
At the time, it was commonly accepted that the volume of the pond should pass through the filter once every two hours. This may have made sense with Japanese matting or other static media but is debatable with a moving bed solution. Anyway, designing a filter for a typical 4,000 gallon pond meant that EA’s new filter had to handle around 2,000 gallons an hour. A vortex of the size EA were designing would not be effective at removing suspended solids at these flow rates, so something more would be required to remove them and doing that in the outer ring of a dual concentric vortex solution would be difficult.
Moving bed filtration was new and the science was not that well understood as far as koi ponds went, even though the technology had been in use in waste water management for a long time. EA probably knew they needed a module with around 100 litres of media in 5 or 6 times that volume of water
Both of these issues probably led to the illogical conclusion to put the mechanical filtration vortex inside the moving bed vortex. In their defence, EA also had plans to introduce a self-cleaning drum filter based on the type of solution used on an industrial scale in waste water management, and having the mechanical stage in the inner vortex worked well for this and also, freed up the outer vortex which was larger for the moving bed biological filtration.
I got my Nexus 200 around 12 years ago and initially, it sat on the pond side and operated ‘pump fed’ – a pump in the pond pumped water into the Nexus, a sponge sat in the middle of the central section and dirty water entered from the pump, solids were caught in the sponge and clean water passed through to the outer vortex where the Kaldnes K1 provided biological filtration before the water overflowed the weir and back into the pond.
Unless you like washing smelly sponges and possess plenty of strength which allows you to wring and squeeze those smelly sponges endlessly, you should never use sponges in your filters – though having said that, they do make remarkably effective mechanical filters, albeit labour intensive ones.
This setup also had the disadvantage that when the sponge clogs, the inner vortex overflows into the outer vortex – not a problem in itself and a useful safety feature, but in my setup there were a number of occasions when both chambers overflowed and I acquired a pond full of K1 – and I really don’t recommend a day spent fishing K1 out of a pond!
In later iterations of the Nexus, EA have reduced the risk of overflow by adding an overflow pipe from the inner chamber direct to the outlet.
My 2nd iteration replaced the sponge with EA’s drum solution – the Answer 325. This sat in the central section – a squat stainless steel cylinder wrapped in a sieve, I guess 200 or 300 microns, with a revolving arm inside using clean water to push any debris off of the sieve. Another great concept but unfortunately ineffective, especially if any blanketweed was present, which it always is.
EA’s next bright idea was the Eazy. Use static K1 to do the job of a sponge or an Answer, but clean it simply by running air through it and flush the dirty water to waste. My Eazy has been in place pump fed and more recently gravity fed for about a decade now – a vast improvement on the sponge and the Answer.
So, having designed a brilliant new type of filter much smaller than others with 2 concentric vortices, and Kaldnes K1 incorporated in both the static/mechanical filtration and also the moving bed/biological stage and having used materials which allow greater production volumes and economies, what are the compromises and design issues which lead me to say it is imperfect:
The inner vortex is too small to operate as a vortex and the rate of flow ensures that any particles are carried with the flow rather than dropping out of the revolving water column
The design of the Eazy exacerbates this, see separate article on this subject for detail
When waste is to be flushed from the inner vortex, the waste piping is only 1.5” so it can block if you get leaves in the pond or blanketweed
The join between the inner and outer vortex is large enough for Kaldnes to pass through but too small for it to do so freely, so this join can and does block or reduce the flow. It only takes one accident, say removing the standpipe from the centre of the inner vortex when it is empty and the outer is full, and masses of K1 is forced through the join under pressure and it does not take long (or much blanketweed) to turn this from an inconvenience to a major issue). These kiss points between the inner and outer chambers were apparently altered on later models to prevent the passage of Kaldnes but this means the openings are smaller and thus much more likely to clog with blanketweed – most people love a Nexus but I have yet to meet anyone who thinks that a Nexus has any ability to perform when blanketweed is present, they just don’t get on which is a problem, as every pond will have blanketweed at some point…
After sponges, the Answer and then the Eazy, EA have come up with a new Eazy which contains micro-K1, new smaller Kaldnes which purports to trap even more of the particles suspended in the dirty water. I have no experience of micro-K1 but talking to someone who has upgraded, he assures me that for the 1st time ever, he has a Nexus that can catch fines! He also admits that he also has to maintain the unit more often as it will block if left for more than 3 days. By comparison, all of the earlier variations were only partially successful at trapping fine particles, whilst I can see the bottom of my pond (2m deep) I have never been able to say it is crystal clear as the turbidity (amount of small suspended particles) is quite high
I have never been able to get the stated flow rates through my Nexus. I currently have an Oase Aquamax 8000 Dry which is rated at about 1,200 gal/hr but I have to run it severely throttled (with a nearly closed ball valve) on the return to the pond as even without an Eazy in place and with the centre vortex empty, it cannot handle the 8000 and unconstrained piping
The airline at the base of the outer chamber is mounted on posts which keep it 3-4 inches off the bottom of the unit. You can argue the efficacy of this but whenever I have had to drain my Nexus and root around in the K1, it is obvious I have dead spots under the airline due to the build-up of mulm and crap at the base of the chamber
So what do I conclude? The Nexus is still arguably the best overall single unit on the market. A bead filter may have an equally small footprint and be better able to handle fines and easier to automate and thus manage remotely, but the Nexus in most cases provides a more flexible and complete solution despite its imperfections.