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Trickle Filters – Nitrification or Degassing

What a trickle filter is and how to design one for aquaculture or even aquaponics is a question on many peoples minds entering the commercial and even the home aquatic field.  They are often overlooked.  In this article we take a closer look at trickle filters and a sneak peek at the one we use for mineralization in UAE commercial aquaponic system. What is a trickle filter? Very simply a trickle filter is any random or structured packed media that has water “trickling” over the top of the media.  Simple really.   They can be made out of just about anything that will hold the media out of water but the media generally must have a low surface are to prevent clogging.  For example you may be able to make a trickle filter out of gravel stacked in trays on top of each other, but too much gravel and the water will channel and not flow evenly over time. One of the many benefits of a trickle filter is the capacity to passively or purposely bring the gasses in the water back to atmospheric equilibrium.  Effectively reducing harmful build up of potentially deadly gasses, especially in high density recirculating aquaculture systems using pure oxygen, even if connected to a hydroponic system such as the “Earthan Cycle”.  Science + Math = Growth. What about Nitrification? Trickle filters make for great bio filters however the drawback and really the only one is their size compared to other options for biofiltration.  When you consider most trickle filter media is up to 200 m2/m3 compared to a moving bed bio reactor with a kaldnes type media of 850 m2/m3, to achieve the same ammonia removal rate the trickle filter needs to be up to 5 times bigger. As you can see in the above bio media comparison photograph, there is a difference between different types of bio media, even the kaldnes copies have large variations in specific surface area.  The top half of the picture shows a media with 100m2/m3 surface area and its general design tends to block in a trickle filter as the structure is a little too tight and as the bio film grows on it, it does not slough off as the higher surface area media in the bottom half which has some capacity for self-cleaning and has a surface area of 200m2/m3. When we talk about ammonia removal rates the difference between moving beds and trickle filters gets much wider.  A moving bed will have an estimated ammonia removal rate of around 0.35 g/m2 where as the trickle filters are as low as 0.1 g/m2 making the trickle filter a further 3.5 larger to achieve the same result. Choosing the right media We have seen plenty of random packed media used in trickle filters, similar to waste water treatment facilities where a gravel or other random media is stacked in a column and water is trickled over it.  You have seen me do it with kaldnes type media but more for polishing temporarily.  These are great for very low densities and small home type systems.  In a commercial context, the clogging of the media severely reduces the already low nitrification rate and over time can be catastrophic for the commercial grower.  It will creep up on you. In most cases commercial growers will use modular blocks, like the one you saw in the first picture (bioblocks).  They will either be a cross flow (like above) or a vertical flow.  Which one is used is dependent on your biological demand or organic loading on the system.  If it is high, such as waste water, the vertical ones are generally used to prevent clogging.  If in a low BOD such as aquaculture, then either the cross flow or the vertical flow can be used. I personally like the modular blocks, while they can be expensive, they save having specialized support and a filter can be built onsite very quickly.  One of my other preferences is the combine both a moving bed to get most of the surface area and a trickle filter, usually over the top of a bio filter at a 70:30 ratio.  I fine that combination delivers on both the degassing and nitrification needs of a commercial farm. Mineralization in integrated systems When including the full waste reclamation from a fish farm and integrating it into a hydroponic sub-system, a trickle filter works perfectly and very efficiently for the mineralization of the organic wastes from the fish farm.  You see above the trickle filter in the UAE aquaponic system I built.  In that case, the media we wanted to use was not available so we improvised (like a lot of things over there).  This media is a substrate support for under lawns to prevent them from compacting.  It was a pain to build sheet by sheet but it works perfectly. Designing a trickle filter for an aquaculture facility is very basic, however designing them for mineralization is somewhat more complex.  This is because you have to account for all the loading and each nutrients biological oxygen demand which can be frustrating to assess with so many varieties of fish feed and levels of waste from the many different fish species and feeding practices.  But it is doable and very effective. The effectiveness of the trickle filter in an integrated system is quite astounding.  Part of this success is due to the trickle filters capacity to keep gasses in control, reduce denitrification and loss of nutrient and break up the organic solids to much smaller particles for very quick breakdown. For example, we would run this filter for three days without adding to the hydroponic system, then introduce the mineralized goodness.  The difference in growth in the hydroponic system in the three days following was not believable.  Much to the confusion of the guys I was working with.  They had thought I had added some magic to the system during that time. The key here is keeping the “brew” as aerobic as possible and often aeration being 5% efficient is not adequate and will not keep up with the loading in mineralization, especially when you have to turn it off to let the solids settle before decanting the goodness off.  The other side is the foam machine.  Those of you that have run large aerated tanks with organic solids will have gone through a stage of massive foam production.  The is the fractionating of the proteins in the solids combining with the nutrient salts.  A trickle filter will act as a foam suppressor and prevent that mess from happening. What about smaller systems? You can use a trickle filter by itself as your primary biological filter in smaller systems and they are often a filter of choice.  However they often need large areas because the suitable bio-media has a very small surface area and they can suffer reduced nitrification capacity if the media is not-self cleaning or is not cleaned regularly. Trickle filters are very simple in their design and management and perform a vital role in degassing in fish densities over 30kg per 1000 liters (>30kg/m3).  Densities below that seldom suffer from a CO2 build up as the water movement and aeration is enough to manage that job. This can do both degassing and gassing depending on the amount of a particular gas is in the water in comparison to the balance of gas in the air.  For example, if you oxygen levels are low, the trickle filter will introduce more oxygen, if your carbon dioxide is high it will remove it. Size is probably a limiting factor in small home aquaculture systems, such as using the larger surface area of 200m2/m3and feeding 1kg per day, you will need approx. 1 cubic meter in comparison to a moving bed biofilter using kaldnes type media which will need around 0.1 cubic meters (100 liters). The great benefit to trickle filters is the reduced operating cost because you are getting aeration from the water trickling rather than supplying 125 liters per minute of air per 1000 liters of moving bed.  While you have to pump the water up to the top of the trickle filter, with good planing you can gravity feed this where your sump is in the ground and trickle filter is on top. In future posts we will talk more about the design of these wonderful filters along with some video of my one here at home.  I am currently building a very large one which I will share with our members when we finish.. that will be a fascinating watch no doubt.  Stay tuned!…

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