How to Improve your Mechanical Filtration Process
Knowing what you are trying to achieve may sound like an obvious place to start but it is amazing how many people start off on the right path and then get side tracked.
Mechanical filtration is exactly what it says it is.
It is a filter that filters out solid particles mechanically. It cannot, by definition, remove anything that is not solid in nature.
This simple statement presents more problems than it appears to solve. Solids in a Koi pond are quite numerous when you start to think about it. You have the obvious Koi wastes, and the occasional leaf. Then sometimes you get drowned insects and food that the Koi have managed to miss eating. You occasionally get bits of bark, stones, dirt, wind borne human refuse such as plastic bags, bottle caps, car keys etc.
Some only happen as an exception to the rule, but invariably the exception is what poses the greatest problems and threats. Your mechanical filter should be able to deal with as diverse a range of solid material as possible.
If this range of solid material is examined more closely it can be broken down into two types from a Koi pond perspective. The first type is that of solids that won't rot, and the second is the type that will rot. Non rotting solids are unlikely to present a problem other than blocking up the biofilter, or on the odd occasion dissolving poisons slowly into the water as may happen with a branch from a poisonous plant or a sheet of rusting metal.
Rotting solids as we've seen create havens for heterotrophic bacteria, some of which are not welcome in any Koi pond as they can lead to outbreaks of disease and Koi losses.
Mechanical filtration can operate on basically two principles. A sieve can be used to filter out particles by simply setting a mesh size that will trap all particles larger than the smallest opening in the mesh. Alternatively the difference in density between solid particles and pond water can be exploited to try and attempt to 'settle' the solid particles out to be flushed to waste.
Most Koi ponds use either of these two methods. A sand filter is effectively a sieve - with the sand acting as the sieve catching a great percentage of the solid material entering the filter. So too are brushes - crude but with some efficiency.
Sieve type filters rely on more intensive maintenance for cleaning purposes. Sand filters appear to offer the lowest maintenance through the back washing ability of these filters, but bear in mind the cost of all the water being thrown away. Not to mention the energy costs of using one of these on your Koi pond - they have a relatively high pressure drop which means a more powerful more expensive to run pump is needed.
Settlement type filters rely on slow moving water in which solids can settle out into a chamber from where they can be simply flushed to waste. This is typically a lower maintenance, cheaper task that rinsing brushes, or cleaning sieves, or backwashing sand filters for 20 minutes at a time.
But always keep your mind on the objective - removing solids! It does not help to have the greatest vortex settlement filter system in the world, if half the solids simply pass right on through as the density difference between them and water is not sufficient to ensure adequate separation! It also does not help to have a fine mesh screen that clogs up within 5 minutes of being installed.
Forgetting about everything other than Koi wastes for the moment I want you to think about the nature of the beast. We're not talking about an easy to grab, strong heavy particle here. Typical Koi waste breaks up as soon as you so much as look at it wrong and as soon as it does that it degrades into zillions of smaller particles that will never be trapped by a sand filter or settle out in a vortex settlement filter. Imagine the problem when pond water is taken from a pond and fed into a pump BEFORE any mechanical filtration has even taken place. It happens! A biological sludge soup from hell is the result with the bio filter virtually becoming completely heterotrophic and ammonia levels that shoot through the roof.
We know that our filter systems are fed by our bottom drains. These drains have to have a 'pull' effect that ensure that as much waste as possible gets sucked into them. Yet, this very pull action helps break up the solids we're trying so hard to get rid of even before we begin to think of removing them.
An efficient vortex system (more than one vortex!!) has a reasonable probability of removing between 50 - to 70% of Koi solids by MASS, IF, and only IF it has been properly designed. A vortex at these efficiency levels would typically be huge, bulky and awkward to install. Flow rates through vortexes are absolutely critical to their performance and I have not yet seen any pond engineer apply so much as the basics to vortex filter design. In the very best scenario, 30% of the solid waste by mass is passing into the bio filter stage and as we all know, this is not what we want at all.
Enter the sieve filters. Sand filters as mechanical filters are disasters waiting to happen. Brushes, pipes, tubes, beads - note - not the biological stuff, the mechanical stuff - all suffer from the same problem that vortexes do - the 30% of particles that are too small to settle out in a vortex will simply just pass through these filters as if they weren't even there. There are no magical properties to brushes that 'attract' the solids to them, quite the opposite in fact.
But a screen or mesh filter solves the problem by physically straining the water of all particles larger than a specified size, this size being the smallest hole in the mesh. However, with the volumes of solid in a Koi pond either you will need a mesh screen of several hundred square meters (not practical) or else you will be hosing it down every hour or so. You are guaranteed a rate of 90% or 95% or 99% solids removal by mass however, if you specify a small enough screen size. It's up to you.
More than all this though, your bio filter will be allowed to operate under nearly ideal circumstances. No solid organics means no heterotrophic bacteria which means less ammonia contributions by these bacteria, less competition for space for the beneficial nitrifying bacteria and a reduced bacterial count in your pond. This means fewer secondary infections, fewer disease outbreaks and longer living, happier pets.