Advantages of Trickle Filters
  • ZackZack October 2012
    Posts: 232
    Trickle Filters

    Trickle filters, or trickle towers, are filters that are designed for biological filtration of pond water in 'dry' mode. 'Dry' mode filtration simply means that the filter media are not submerged under water. In the case of the trickle filter, the pond water is instead allowed to drip in small 'trickles' through the filter media, which are usually hundreds (or even thousands) of bio-balls stacked onto each other to form a column or tower. The primary purpose of trickle towers is to reduce the nitrate levels of the pond water and turn green water into gin-clear water.
    Trickle towers reduce the nitrate level of pond water by exposing the water to a large surface area of good aerobic bacteria that consume nitrates. The large surface area is achieved by choosing filter media that has a high surface area-to-volume ratio, such as bio-balls (plastic balls that have pegs). For trickle filters to work, however, their filter media need to be fully colonized by the good aerobic bacteria. This colonization occurs naturally with time, usually in 3-4 weeks after the pond is filled with water. Colonization may be accelerated by mixing some 'old' filter media in with the new ones, which in effect 'seeds' the new filter with a small colony of bacteria.
    The term 'aerobic' means 'oxygen-breathing', so aerobic bacteria need oxygen in order to thrive and expand in large colonies. Trickle filters allow better exposure of the filter media to oxygen than wet filters, since these media are not submerged under water. This is why trickle filters are good biological filters - because they provide an oxygen-rich environment needed by the bacteria that perform the actual biological filtration.
    The trickle filter is a simple contraption. It basically consists of just four parts: 1) a spray bar or drip plate; 2) a media container or holder; 3) a filter exit or pond return; and most importantly, 4) filter media that's suitable for trickle filtration. The trickle filtration process is just as simple: 1) pond water is pumped into the spray bar or drip plate; 2) the pond water is 'sprinkled' by the spray bar or drip plate onto the filter media; 3) the aerobic bacteria in the filter media consumes the nitrates in the water; and 4) the nitrate-free water is returned to the pond by the filter exit.
    A spray bar or drip plate is just that - a bar (such as a pipe) or a plate with many small holes underneath, just like an ordinary garden sprinkler. The spray bar or drip plate is placed over the filter holder, so that water that trickles from the many tiny holes of the spray bar or drip plate will land on the filter media inside the holder. The distribution of the holes should be in such a way that the water droplets are evenly dispensed over the top surface of the filter media. Also, the holes shouldn't be too small, because undersized holes get clogged up quickly, no matter how efficient the preceding mechanical filter is.
    The filter holder can be anything that will hold a stack of bio-balls (or any other filter media suitable for trickle filters) inside it. The stack of bio-balls is ideally over 18 inches in height, although the higher the stack the better. Height matters because it determines how much surface area the water droplets will travel through from the time they hit the top bio-balls to the time they've cascaded down to the bottom ones before returning to the pond. The more surface area the droplets get in contact with the more nitrates will be removed from them.
    Since bio-ball stacking height is important in maximizing the 'contact time' of the water droplets to the bacteria on the bio-ball surfaces, filter holders are usually in the form of upright towers, hence the name 'trickle towers.' Of course, any container that looks like a tower (much taller than it is wide) can be used as a trickle filter media holder. It is common to see trickle filter made out of used drums.
    At the bottom of the filter holder is the filter exit or pond return, which is usually just a spout that returns the water to the pond by gravity. Since the filter media must not be submerged in a trickle filter, it goes without saying that the entire filter holder must be above the pond water line. So should the spout be if you want to use it in agitating the pond surface with the returning water to enhance aeration.

    Thus, another benefit of using a trickle filter is excellent aeration, not only because of the exposure of the water to air as it travels down the filter, but also because of the pond surface agitation that it can provide. By the way, some hobbyists have their filter exits below the water surface, using subsurface piping to draw air to the pond bottom, also to promote aeration.

    Bio-balls are a popular choice as trickle filter media because they have a high void fraction and a high surface area-to-volume ratio (something like 160 sq. ft for every cu. ft. of 1" bio-balls). A high void fraction simply means that when the bio-balls are stacked, there are large air gaps in between them that allow excellent oxygenation of their surface areas. The high surface area of the bio-balls allows them to contain larger colonies of good aerobic bacteria, as pointed out earlier. Thus, any filter media that have a high void fraction and a high surface area can be good alternatives to bio-balls in trickle towers.
    A trickle filter must be treated as a biological filter, and not as a mechanical filter. Debris will not be able to pass through the tiny holes of the spray bar or drip plate, which will easily get clogged up if the water they get has some suspended particulates in it. Thus, an efficient mechanical filter must always come before a trickle tower, so that the water fed to the latter will already be free of debris.

    A well-designed trickle filter is said to be tens to hundreds of times better than wet filters in terms of biological filtration efficiency. Koi hobbyists who have attached trickle towers to their conventional filters attest to the effectiveness of this accessory in achieving crystal-clear pond water, even without uv sterilizers. This is why more and more koi enthusiasts are turning to trickle filters for help nowadays.
    My kois are my lover

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