Mechanical Filtration

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Mechanical filters.

There are many ideas as to the importance of the first stage of filtration, but it is the most important stage to get right for a number of reasons:

The idea of the mechanical filter is to remove particles from suspension and in turn reduce the load on the biological section of the filter.
  1. If large quantities of waste get through to the biological section of the filter it will clog the filter media and reduce the filters capacity to break down waste products.
  2. The more organic waste there is the more oxygen is required to break it down to harmless products. More oxygen demand means less for the fish in times of hardship, such as in hot weather.
  3. The quicker the large waste matter is removed the less work the biological filter has to do and the lower the nitrate level will be, therefore leading to less food for the algae, and less algae growth.
  4. If waste is kept to a minimum the size of the biological filter can be smaller as it will have less work to do.
There are numerous methods for mechanical filtration, the most commonly used are a screen or media to remove or trap particles. The other method is to use a vortex.

The basic principle of the settlement tank is to slow the water down so that the suspended particles fall to the base of the tank, from where they can be removed with ease.

In order to break up and slow the flow of water there has been a number of medias created to do this.

  1. Filter brushes are the most commonly used and is in fact the one used incorrectly in most installations. The brush should hang in the flow so that the water has to pass through the side, not from beneath or through the end.
    The water level should be below the top of the brush to stop the water by passing the settlement system, by flowing straight over the top of the brushes.
    The brush systems' biggest down fall is that it is very messy and time consuming to clean.

    filter brushes

  2. Bio-Block is another media that is now available to the pond world. A simple media created by the joining of net tubes into ‘blocks’ which can be used in the filter either whole or cut to fit. This media has many advantages over others, the most important is the easy maintenance and it is almost impossible to clog ( good if you go on holiday). With flow rates of 7500 to 10000 litres per hour per m2 of surface area.
    The longer it is established the better it works as the bacteria build up a filter web between the net elements and trap the finer particles.
    Easy to clean without removal makes this an excellent media for all ponds.

    bio block

  3. The vortex is one of the most under used systems available, but many that are sold are poorly designed. The flow rate is very important with a vortex.

    The water enters the vortex at a 90º tangent to the side this causes the water to spin, creating a vortex ( see picture below for basic design ). At the correct flow the waste works its way to the centre of the tank and falls into a collecting chamber. If the flow is to fast the vortex will pick the waste up ( like a tornado) and carry it on to the next stage of filtration, this is not the desired effect.

    The exit of the vortex can also effect the efficiency of the unit. There are two trains of thought for this, the first is a centrally mounted pipe that is set just below the water level and collects the water from the centre of the water column(Exit 'A'), and the second (Exit 'B')is an outer trough that collects the surface water as it over flows into it from around the edge.

    The ideal flow rate is between 12,000 to 17,500 litres per hour per square metre of surface area, this is for tanks over 1m deep. Shallow tanks less than 1m generally don't work very well unless the flow rate is slowed right down to suit, but this tends to make them some what redundant as brushes tend to work better.

    If a vortex tank was 0.9m diameter (90cm) the surface area would be 0.9 x 0.9=0.81m2 so the average flow required would be 0.81 x 15,000= 12,100 litres per hour. As with any system the flow rate will differ from one pond set up to another so some amount of trial and error will be needed to get the best results. Although slower flow rates generally work better.


  4. Baffle tanks. An older system that works very well, requires very little maintenance, but needs a large tank to work as the flow rates are very slow. The baffles are set at angles from the vertical plain of 32.5º to 45º depending on the flow rate, this needs to be done very accurately. As the water enters the tank it works its way up to the surface, which ever way it flows it will travel the same distance and therefore the same speed depositing the waste particles one the way.If you have the space this is a very good system, but is difficult to set up and get started.

    baffle tank

  5. Pressurised sand filters.
    Sand filters where designed to remove low volumes of small particles in relatively clean environments, so the garden pond is not an ideal situation for them. They are expensive to purchase and run as they require a large pump with a high pressure rating to clean them. They require regular of cleaning as they clog very quickly and in some situations can cause gas bubble disease ( not a disease but a condition similar to the buoyancy in effect.)

    The principle of the filter is to force the dirty water through a sand bed and as the water passes through the particles are left behind and the more the filter clogs the better it works, but the flow rate slows down as well. The multi port valve is a single valve that allows the control of the filter from one easy selection from the settings, it allows, normal filtration,, back washing for cleaning, rinse to stop waste entering the pond and recirculation. This allows work to be carried out to the filter without stopping the pump, although it is always recommended to switch the pump while working with water.

    The back wash cycle requires a large pump, typically a 1hp, to back wash a 60cm filter. There is 100kg of sand, in a 60cm filter, to lift during the back wash process as the clean water is forced to the bottom of the filter and then up through the sand and out to waste.

    Leave them to swimming pools where the work well.

  6. Drum Filters.
    The hi-tec way and the best method for particle removal is the drum filter. They are the choice of the fish farmer as they offer the best of all worlds. The drum filter has many advantages over all other systems, they are:

    1. Drum filters can filter down to 20 microns with ease. At this level they can remove algae that causes green water, free swimming parasites and almost all waste particles leaving clear water to pass to the biological filter.
    2. Once dirty they clean themselves. This is an enormous advantage in this busy world, time is important to people and the less time spent cleaning filters the better.
    3. The waste is washed away so nothing is left to do.
    4. Compact. The efficiency of the unit means they can be very small and therefore don't waste space.
    5. Due to the quick removal of waste the nitrate levels remain lower for longer and less water changes are needed.

    The main drawback to drum filters is the cost, but the benefits of size, can often be recouped in the rest of the system as the requirements are low, no tanks or complex pipe work has it is built in.

    Drum filters biggest benefits are the lack of maintenance and efficiency, nothing comes close to these filters, hence their popularity in fish farms all over the world.