THE BASIC NITROGEN CYCLE IN BIOLOGICAL FILTRATION

 

The Basic Nitrogen Cycle

The waste products from the fish in their initial form are mainly expressed as ammonia which is poisonous in even small quantities and especially so under certain water conditions - high pH - which will be discussed later.

In a mature pond a class of bacteria in the biofilter removes the ammonia almost as soon as it is formed.

The ammonia is converted into other nitrogen chemicals called nitrites.

Another type of bacteria also present in the biofilter then converts the nitrite chemicals into nitrate chemicals. These chemicals are less polluting and toxic than either ammonia or nitrites - both of which are poisonous.

These chemical processes are occurring all the time and will not stop unless the biofilter system stops working or the source of nitrogen is removed - ie no fish in the pond, or the water becomes devoid of oxygen.

The health of the pond is determined by this relentless ongoing conversion of ammonia to nitrates. It must NOT stop for the sake of your fish.

If pond water is changed frequently then to some extent the poison levels are "controlled." The more fish in the pond then the more the waste products to get rid of. Koi produce 3 times more waste than goldfish of the same size - because they eat 3 times more.

The fish food used has a significant impact upon the amount of waste produced. Do not use foods with high ash contents - this indicates low quality raw materials have been used in the food manufacture which result in water pollution levels being higher than necessary.

In a natural stream or lake fish concentrations are generally low. Waste products are converted to relatively harmless nitrates by naturally occurring bacteria. Rocks, submerged trees, plant roots, etc all help to purify the waste products from the fish by providing holding points for the bacteria to live on. The more bacteria there are the better the purification.

The bacteria need oxygen to survive and they get this from the water - oxygen is absorbed by the water due to the action of waves, waterfalls and wind.

In a garden pond a bio-filter is installed to make up for the unnatural conditions prevailing. A biofilter is designed to allow large concentrations of bacteria to operate effectively in a small volume within a garden pond environment.

Correctly specified and installed biofilters create healthy environments in which fish can live for many years and grow to their full potential.

In small ponds the filter can be installed directly in the pond. For larger ponds the filter needs to be installed outside of the pond. The pump connected to a biofilter must run 24 hours every day otherwise the bacteria will die from lack of oxygen contained in the circulating water. It is a good idea to have a waterfall and a fountain to increase the oxygen content of the water.

It is difficult to add too much oxygen to a pond.

Once the bacteria have died it will take approximately 5 more weeks for them to regain close to their previous levels.

What is happening inside a biological pond filter?

The primary job of a biofilter is to convert ammonia (or ammonium) to nitrites and then to nitrates using specific bacteria types. In practical terms this requires that ammonia, which is dissolved in the circulating water, must make contact with bacteria at an interface and in conjunction with oxygen in such a way that a chemical reaction can take place.

Once the chemical reaction has taken place then the product of the reaction which is nitrite or nitrate must be able to be transferred back into the body of the circulating water and removed from the immediate presence of the bacteria to make way for the next bit of ammonia.

Therefore a well-designed biological filter MUST have...

• A large surface area on which bacteria can grow and to which a maximum amount of pond water can be exposed
• Biofilters work best at higher flow rates/lower residence times - ie turbulent flows are best.
• The presence of large amounts of oxygen at the interface also creates the concentration differences required for good mass transfer. This is why top class koi keepers have vortex filters and blow massive amounts of air around the biomedia that is generally Japanese matting.
• Biofilters work best at higher temperatures.

Sand Filters For Use In Pools Are Bad For Garden Ponds

Sand filter are used for the mechanical filtration of solids, not to be confused with fluidized sand filters, which are used for biological filtration. Although the size of particle that the sand filters remove is dependant on factors such as the size of the sand particles, the depth of the bed of sand and the flow rate through the bed, sand filters are usually regarded to filter water to a nominal 10 microns.

... The first is a simple box structure that operates with a low pressure across the filter. The water flow-rate to cross sectional area ratio of such filters is low, and the filters tend to rapidly block in the first few centimeters, with the rest of the filter staying clean.

Such filters are only of use in applications where the use of pressure filters is impossible or the water is generally clean and there are only a few particles that need removing. Such an example may be a ground water supply which is thought to be contaminated with pathogens through seepage into the spring / borehole. Large filters of this design are difficult to clean effectively, usually resulting in the bed being periodically dug out and replaced by fresh sand.

Pressurized sand filters are in common use in many aquaculture applications. They consist of an enclosed vessel that is typically half to two thirds full with sand. Water is pumped into the top of the filter under a pressure of approximately 1-2 bar and is forced through the sand to a water-collecting device at the bottom that allows the water through, but not the sand particles.

The flow is then reversed to back flush the filters. Pressurized sand filters are expensive to use for high flows due to the cost of pumping the water through them. They are however used extensively in hatcheries and also some recirculation systems, where they are either plumbed in for all the water or as a side stream, where only a percentage of the water flows through.

Their limitations in recirculation systems is that, in addition to the operational costs, they use a lot of water for back flushing (a typical sand filter in a recirculation system will require back flushing 4-6 times a day for 5 minutes each time. The water flow rate whilst back flushing is the same as the flow rate when filtering).

This is exacerbated by the fact that sand filters in re-circulated water will also act as biological filters, and a layer of heterotrophic and nitrification bacteria will build up on the sand, causing channeling and increase back washing frequency.

The back wash process is insufficient to eliminate all the bacteria that soon multiply and block the filter again. A way round this is to add ozone or other disinfectant chemicals to the water when back flushing (the advantage of ozone here is that any residual amounts after back flushing will quickly be neutralized by the organic compounds in the water). Now that self-cleaning mechanical screen filters are available with screens of less than 10 microns, the use of sand filters is becoming less common."