The following article is an email exchange between Jack McNeary and a respondent on an internet bulletin board. Portions of the exchange that were unrelated to the biological filter topic were omitted.
Question
I am a little confused !!!! Some of the older books talk about the bio-filter being half the surface area of the pond I assume that was to have vegetative matter use the nitrates created Also 02 only goes so deep with out separate air stones or equivalent. Is that why the depth does not matter much? If the above is true than the surface area of the filter with bio-balls, beads, filter mat, plastic forks, brushes, Spring-Flow, etc. is not important as long as aerobic respiration is going on. If I am on the right track than how much surface area of the media (beads, brushes etc.) does one need to provide for a pond per 1000 gallons. I guess I am a little skeptical about the advertisement that says "this is good for up to 5000 gallons and only measures about 2 ft x 2 ft Any comments that will enlighten me I will appreciate.- jack mcneary
The issues Jack raised relative to the sizing of a filter refer to biological filtration. The relationships he described were true up through the late 1970's when all blo-filters were gravel beds. There are at least two reasons the relationships Jack describe are two dimensional (square feet of filter needed per square feet of pond). Jack hit on one of them; if a gravel bed biofilter is too deep, the available oxygen is used before water reaches the lower portion of the bed and nitrification can not occur, As a simplification, you can think of nitrification as an oxidation process where bacteria attach oxygen to the nitrogen in ammonia (NH3) to make nitrite (N02) and finally nitrate (N03). If there is no oxygen available, the process
Response
First, there are several types of filtration, the most common in Koi-keeping being biological filtration and solids filtration. Biological filtration refers to nitrification processes whereby dissolved ammonia is converted to nitrite and nitrite is converted to less toxic nitrate. Solids filtration is the physical removal of particulate matter, usually through gravitational sedimentation and/or mechanical screening. There are interactions between nitrification and solids removal as much of the solids are organic matter in the process of decaying or mineralizing into inorganic material such as ammonia. If you remove the organic solids, you lessen the amount of nitrification which is needed to convert ammonia.
The issues Jack raised relative to the sizing of a filter refer to biological filtration. The relationships he described were true up through the late 1970's when all blo-filters were gravel beds. There are at least two reasons the relationships Jack describe are two dimensional (square feet of filter needed per square feet of pond). Jack hit on one of them; if a gravel bed biofilter is too deep, the available oxygen is used before water reaches the lower portion of the bed and nitrification can not occur, As a simplification, you can think of nitrification as an oxidation process where bacteria attach oxygen to the nitrogen in ammonia (NH3) to make nitrite (N02) and finally nitrate (N03). If there is no oxygen available, the process cannot occur.
The other reason the old relationships are two dimensional is that gravel biofilter beds channelize. As solids and bacteria biomass builds up in the gravel bed, the interstitial pores between gravel particles begin to fill up, clog and hinder water movement around the rocks. To accommodate the same flow, the water velocity must then increase in the passageways which are still open. This process continues until there are very few passageways open and these passageways are kept open by the very high water velocity moving through them. The process is usually called channelization. In the old days. we would try to decrease channelization and the footprint of a gravel biofilter (square feet of fil-ter per unit volume of system) by making a tall fil- ter housing with several layers o f gravel separated by a retaining plates and open space. Some individuals like Bill learned to break up channelization in gravel filters without digging the for the most part, the situation changed with the advent of buoyant media, fluidized beds, etc. Not only do these new biofilter technologies make media easy to clean, thjey make it easy to break up channelization.
The proper way to size a biofilter is to use a given amount of media surface area (the combined area of the surface of each media particle) for each unit measure of ammonia being added to (or produced in) the system. There are known relationships between the amount of nitrifying bacteria (as expressed by the amount of suitable surface area for nitrifying bacteria to grow on) and the nitrogen oxidation rate at a given temperature. The filter media (be it rocks or beads or sand or whatever) is simply a surface for nitrifying bacteria to grow on. If oxygenated water is kept flowing evenly past all the exposed surfaces, and the temperature and other environmental parameters are adequate for nitrification. then you should be able to calculate how much surface is needed for your ammonia load. It is actually a little more involved than this because the ammonia concentration influences the ammonia conversion rate of bacteria, but all this comes out in the wash under steady state conditions.
You can determine how much surface area is available for growth of nitrifying bacteria by determining the surface area of an individual media particle (bead, rock, whatever) and determining the number of particles per unit volume of packed media material. For polyethylene beads, this is 400 square feet of surface area per cubic foot of packed media. For half-inch river gravel, is something like 150 square feet of surface area per cubic foot of packed media. [NOTE: We are talking about the combined surface areas of each media particle, not the footprint of square-foot area of the filter bed discussed by Jack in the original post.
You can determine the amount of ammonia you must convert if you are feeding the pond at a relatively constant rate. You can calculate the amount of ammonia which will be released (produced) under steady state conditions by knowing the rate at which feed is added, the protein content (almost all the nitrogen is contained in the protein) and the molecular weight of ammonia (remember NH3). There are a few other sources of nitrogen like the ammonia dissolved in rain water, and a few other nitrogen losses like ammonia volatilization to the atmosphere, but these can be ignored in this application.
Under ideal conditions and feeding typical Koi food at a constant feeding rate of one pound per day, you will need at least 450 square feet of available surface for nitrifying bacteria to convert ammonia as fast as it is produced. Do not use these figures as a rule-of-thumb. Conditions are never ideal for long and with this combination of feeding rate and surface area there will be enough back ground ammonia to maintain a pretty heavy phytoplankton population (green water).
If you want a rule-of-thumb, have at least 1000 square feet of surface area for nitrify-ing bacteria for each pound of feed being added daily. Thus, a bead filter (with 400 square feet per cubic foot) would need about 2.5 cubic feet of bead media per pound of feed added per day while a gravel filter (with 150 square feet per cubic foot) would need about 6.7 cubic feet of media. However, there are other surfaces available for colo-nization of nitrifying bacteria besides the media in the biofilter. These include part of all of the pond walls and bottom, plants, containers. even some sus-pended solids particles. Thus, a pond 26 ft x 26 ft x 3 ft deep should have enough exposed surfaces for nitrifying bacteria to assimilate one pound of feed per day without a biological filter.
The sizing specifications which say a filter is good for a pond of a particular volume can work only when talking in generalities based on some typical fish density and feeding rate. Likewise, saying a fil-ter is good for a certain feeding rate would have to assume the pond bottom and walls do not contribute to nitrification surface area. Given these complica-tions, we are not likely to see the "standard" change any time soon. Most people do not know how much feed they will/do apply now, much less how much they could be feeding in a couple of years when the pond and fish are mature. You guys are right, when a biofilter manufacturer says their unit is good for a pond of a certain volume, it is bunk. Now you know why.