Koi Rescue
Phoenix Arizona

Designing a Koi Pond Biofilter / Bioreactor -- Bilfilm requirements

Koi fish need good filtration to keep them alive in a typical backyard pond, and serious filtration to keep them growing like they would in the wild. So, a large part of the koi hobby is fiddling with filtration systems so we can cram more koi into our ponds, and still keep them alive, happy and growing. But the problem is, there are many, many different commercial and DIY koi pond filters, all with conflicting theories on which works better, and why they work.

I have spent a lot of time reading about the different types & theories, and have found many contradictions. I also happen to have a unique perspective, I visit many backyard ponds while rescuing koi as part of Phoenix Koi Rescue. The majority of these ponds were created by people that have not put much research into commonly published koi pond and biofilter design guidelines, so the ponds end up at being a random guess at what the koi needs. This sort of makes each one a koi environment experiment. Being curious, I ask many questions when I arrive at the ponds and am always thinking about what went right, and what went wrong with the environment. Time after time, there are a couple of key factors that kept showing up as I'd visit the ponds with healthy growing koi.

High turn over rate -- Pumps with actual flow rates that cycle the pond at least 2 or 3 times an hour.
High aeration -- Waterfalls, fountains, trickle towers etc.
Regular water changes -- At least 10% per week, 20% or more is better.
Running Current -- Moving water so the koi have something to swim in
Table Scraps -- Yep, thats right, table scraps. People are feeding their koi fresh foods right off their plate, and it seems to work.

--- So what is the most efficient biofilter to use? ----
This is something I have wondered also, because after you get past the guessing stage and want to be a real koi enthusiast and employ an adequate filtration system, there are a number of factors to understand, and not many definitive published opinions on the subjects. Well, at least not much hard scientific data published in the typical koi enthusiast websites. Furthermore the efficiency of your filtration system will effect how much you spend on your pond, and how much electricity, water, and time you use. I hunted around and think I found some answers from the serious fish farming world, in the book Aquaculture Engineering, published in 2007 which cites the sources on it's opinions. With these opinions, filtration systems can be evaluated better, or you can design your own system.

Is bio media more efficient as submerged, or above the water as in a trickle filter / bakki shower?
Trickle style medium is not as efficient as submerged media. (31-33) Thats a shocker, there is currently a lot of hype about trickle and bakki filters being higher efficiency than submerged media. Something I have noticed though, of the shower filter systems running which are highly publicized, they also seem a bit like flow through systems where massive amounts of water change happens, so possibly what the trickle filters are mostly doing is aerating the water.

Does the biomedia (with bio film on it) need to be in dark, in the light, or does it not matter?
The filters need to be in the dark, or atleast shielded from the light, because the light reduces the nitrification process. (11,30)

Temperature range for best biofilm growth
Bacterial acitivity occurs between 5 degrees C and 30 C, with the most activity at 30 c. With temperatures above 30 c, the bacteria will start to die off. (13-15)

Best pH for nitrification
The best pH range for nitrification is between 8 and 9, which is high for a typical koi pond. The nitrification rate decreases by 90% when the pH falls below 7 down to 6. (12)

Oxygen concentration for optimum biofilm development and nitrosomonas activity
Experiments have shown that oxygen levels trhoughout the entire nitrification process need to be at or above 4mg per liter. (10) So adding a packed column aerator just ahead of the bioreactor is a good idea.

Start up time
In ideal conditions, it takes between 20 and 40 days to startup a biofilter. It depends on the factors, but that is a good average time. (28,35)

Best to leave your biomedia alone, or to vigorously clean it all the time?
Plastic media has been proven to show good results at hosting biofilm. Thin new biofilm has the highest nitrification rate, higher than older thick biofilm. (36) The reason the older film doesn't work as good, is because it is tougher for the oxygen and nutrients to get deep into the layers of the old biofilm. Moving bed / fuidized bed reactor with Kaldness K1 media caters just to this finding, in that the small plastic biomedia swirls around and "boils" in the reactor to knock off the old biofilm and allow new film to develop.

This is another controversial finding that contradicts a common belief -- for years the "skippy biofilter" built with tight weave pot scrubbers or chore boy pads has been preached as of the most effecient of DIY filter systems, with the operating instructions to never clean it.

-- Biofilter Media --
Kaldnes K1 media has 259 sqft of area per cubic foot of media, and has proven in aquaculture applications to be able to process 2 ounces of 40% protein food, per day, per 1 gallon of K1 media in a moving bed filter. Most koi filter manufacturers rate it at a lower effeciency, they rate it at 0.61 ounces of food, per day, per gallon of K1 media. For biofilter chamber volume, to get the K1 to boil properly, you can only fill it 40% of the way with the K1 media. To get new K1 media to boil, it has to have biofilm growth on it to decrease it's bouyancy to neutral. As it comes new from the factory, it is bouyant will just float at the top of your biofilter vessel.
Example Using 55 gallon Barrel
A 24" koi typicaly weighs around 134 ounces.
If you feed it 1% of it's body weight per day, then you are feeding 1.34 ounces of food per day, per koi.
1.34 / 0.61 per gallon of K1 = 2.19 gallons of K1 needed per 24" koi.
You can fill a 55 barrel 40% of the way with K1, so that is 22 gallons of K1
22 gallons of K1 / 2.19 gallons per koi = 10 of 24" koi per 55 gallon barrel filter

Bioballs (biopin) are typically around 160 sqft per cubic foot of media, so only have 61% of the square area per cubic foot of K1. I haven't been able to find a rating of feed per day / per gallon of bioballs, so the best I can do is guess by scaling down the rating of the K1 media -- which 61% of the K1 rating would be 0.37 ounces of 40% protein food per gallon of bioball media. But the problem is that bioballs typically are used as a static media filter, and thus would need more cleaning to knock off the old biofilm. From my experience, you can fill a container about 80% of the way and still be able to jostle the bioballs for cleaning.
Example Using 55 gallon Barrel
A 24" koi typicaly weighs around 134 ounces.
If you feed it 1% of it's body weight per day, then you are feeding 1.34 ounces of food per day, per koi.
1.34 / 0.37 per gallon of bioballs = 3.62 gallons of bioballs per 24" koi
You can fill a 55 barrel 80% of the way with bioballs, so that is 44 gallons of bioballs
44 gallons of bioballs / 3.62 gallons per koi = 12.1 koi per 55 gallon barrel filter

Aquaculture Engineering (2007) by Odd-Ivar Lekang
10. Haug, McCarty, (1971) Nitrification with submerged filters, Water Pollution Contro Federation 44:2086-2102
11. Wheaton, Hochheimer, Kaiser (1991) Fixed film nitrification filters for aquaculture. Aquaculture and Water Quality.
12. Odegaard (1992) Wastewater treatment. Tapir Forlag
13. Wortman, Wheaton, (1991) Temperature effects on biodrum nitification. Aquacultural Engineering, 10: 183-205
14. Fdez-Polanco, Villaverder, Garcia (1994) Temperature effect on nitrifying bacteria activity in biofilters - Activation and free ammonia inhib ition. water Sciend and Technology 30:121-130
15. Zhu, Chen, (2002) The impact of temperature on nitrification rate in fixed film biofilters. Aquacultureal Engineering 26: 221-237
28. Bovendeur (1989) Fixed-biofilm reactors applied to waste water treatments and aquacultural water recirculating systems
30. Allenman, Preston, (1991) Behavior and physiology of nitrifying bacteria. Commercial aquaculture using water recirculating
31. Thorn, Mattsson, Sorensson 1996 Biofilm defelopment in a nitrifying trickling filter. Water and science Technology, 34: 83-89
32. Wik, Mattsson, Hansson, Niklasson 1996 Nitrification in a tertiary trickling filter at high hydraulic loads - Pilot plant operation and mathematics modeling. Water Science and Technology, 32:185-192
33. Parker, Jacobs, Bower, Stowe, Farmer 1997 Maximizing trickling filter nitrification rates through biofilm control: research review and full scale application. Water Science and Technology, 36:255-262
35. Lekang, Kleppe (2000) Effeciency of nitrification in trickling filters using different filter media. Aquacultural Engineering 21: 181-201
36. Rusten, Hem, Odegaard (1995) Nitrification of municipal waste water in moving-bed biofilm reactors. Water Environmental Research 67: 75-86