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Trickle Filters

by Elmer Epistola

Posted: October 27, 2004



Remember that age-old piece of advice that says you can't have clear water without using a uv sterilizer?  Well, no matter how popular it still is to many hobbyists today, more and more people are becoming convinced that there are other ways to get that highly-coveted crystal-clear water in their ponds.  Their new secret weapon? 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.



Figure 1. Bio-balls are excellent filter media for trickle towers



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.



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See also: Filtration Basics



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