Biological filtration is the use of beneficial bacteria to eliminate organicwaste compounds from a body of water. It is distinguished from mechanical filtration, which is a process whereby water is strained and suspended material is physically removed from the water. The bacteria that do the work in a biological filter are part of the "nitrogen cycle," a series of events that also occurs in nature.
By eliminating the organic waste compounds in the water, biological filtration detoxifies the water and makes it safe for fish. Additionally, by removing the organic waste compounds, algae is controlled because those compounds are the nutriment that algae require in order to grow.
A biological filter or "bio filter" is simply a home for the
beneficial bacteria that perform the nitrogen cycle. The filter provides
surface area that the bacteria can live on and a recirculating water pump
ensures that water is constantly flushing over the bacteria so they can
obtain their necessary nutriment and oxygen.
The bacteria occur naturally in a pond. They live on fish and other underwater surfaces such as plant stems and rocks. In order to get the bacteria established in a filter or new environment, aquarium and pond owners used to borrow some gravel that had bacteria on it from an existing system. This cumbersome procedure is no longer necessary as there are now available bottled viable bacteria cultures in either living form or freeze dried spores.
Most backyard ponds have a much higher concentration of fish than would occur in nature, and the fish are usually fed a high-protein food. These facts result in a higher concentration of organic waste, i.e., ammonia,than the naturally occurring bacteria can deal with. A bio filter houses more bacteria, hopefully enough bacteria, so that ammonia and the other nitrogen compounds are completely eliminated.
Ammonia is at the root of most green water (algae) problems. As discussed below, ammonia forms naturally in the pond and is toxic to fish. Ammonia may be eliminated by the beneficial bacteria in a properly operating bio filter, but if it is not, the pond could become toxic were it not for algae. Algae is nature's safety net for fish. If ammonia levels rise, algae will colonize the pond and make it safe for fish by taking up the ammonia as nutriment.
The nitrogen cycle consists of three basic steps; 1) ammonia to nitrite,
2) nitrite to nitrate, 3) nitrate to free nitrogen. Ammonia is created
from two sources- fish and other animal waste, and decaying organic debris
(leaf litter, pollen, etc.) that gets into the pond. To complicate matters,
ammonia is present in two forms - free (NH3) which is very toxic to fish,
and ionized(NH4-) which is still toxic but less so. The higher the pH,
the greater the ratio of the more toxic free form to the ionized form.
Nitrosomonas bacteria oxidize ammonia into nitrite by the addition of oxygen,
and nitrobacter bacteria oxidize nitrite into nitrate. These two types
of bacteria are referredto as "nitrifying bacteria," and live
on surfaces in the pond, such as plant stems, rocks and even on the fish
themselves. They require oxygen in order to live and to perform their function.
There are two basic types of bio filters; in-pond and out-of-pond. Out-of-pond
filters are further divided into two types - pressurized and nonpressurized.
In-pond filters are typically used in smaller ponds, say up to about 1500
gallons, and out-of-pond filters are typically used in larger ponds, although
this rule is far from absolute. The main function of any type of bio filter
(i.e. providing a home to bacteria ), is the same regardless of design.
The differences are in cleaning, space requirements, and add-on enhancements.
Out-of-pond pressurized filters are typically swimming pool sand filters
that are modified for pond use. (Diatomaceous earth and cartridge type
swimming pool filters are generally not used in ponds, as they get clogged
up too fast.) These filters have the advantage of being very easy to clean
and, because the water is under pressure, are very flexible for variations
installation requirements. A problem for the inexperienced installer is
that all the instructions that come with these types of filters are for
spas or swimming pools - not for ponds. Sizing of pump, canister, and medium
are different for ponds, and only the side-mounted multiport valves provide
a sufficient backwash. When they are sized and maintained properly, they
provide excellent bio filtration.
Since the primary purpose of a bio filter is to provide surface area
for beneficial bacteria to live on, the size of the filter depends upon
the amount of organic waste the bacteria have to deal with. The amount
of waste is a function of the fish population, debris that blows in from
nearby plants and trees and water plants such as water lilies that drop
a lot of debris of their own. There's not much reason to have an over-sized
bio filter,as the bacteria population will only be as large as the organic
waste warrants. The exception to this rule is a poorly designed or hard-to-maintain
filterthat is inefficient and requires a larger surface area because the
bacteria are struggling to survive.
The filter should be placed as far away from a waterfall or fountain
as is practical. If the pond is deep, it may be set on blocks for easy
access and cleaning. Typically, tubing runs from the discharge of the submersible
pump that is attached to the filter to a waterfall or fountain. The water
then circulates through the full range of the pond, and stagnant areas
are avoided. If there is no waterfall or fountain, a venturi T should be
added to the discharge tubing to ensure proper aeration.
Water is drawn from the pond, through the pump to the filter and then
back to the pond. Typically a nonsubmersible pump is used that has sufficient
power for proper backwashing. The water may be drawn from the pond through
a surface skimmer, which helps considerably in maintaining good water quality.
The rule of thumb is to pump the volume of the pond through the filter
once an hour, with a higher turnover rate for small ponds (say, under 500
gallons) and a lower rate okay for larger ponds (say, over 2500 gallons).
The pump must operate 24 hours a day in order to keep the bacteria alive.
It is much better to have a smaller pump that is cheaper to operate running
all the time than to have a larger pump that is shut off part of the day.
Energy cost can be determined by knowing the amperage of the pump, which
is always listed. Volts x amps = watts. At 12¢ per KWH, one amp @
110 volts running 24 hours a day costs about $10.00 per month. For 220
volt pumps, one amp costs $20.00 per month. (Please note that the amps
of a 220 volt pumps are one-half those of a similar 110 volt pump, so that
the energy costs are the same.)
If the bacteria colony is not thriving, ammonia will accumulate and
algae will colonize the pond. The most common problems with bio filters
(beyond poor design) are shutting off the pump for part of the day, improper
cleaning, chlorine or chloramine in the water supply and copper leaching
into the water from copper pipes. As previously noted, the pump must operate
24 hours a day. If the water source has chlorine or chloramine added, it
can not be used to clean the filter as it will kill the bacteria. Furthermore,
a chemical detoxifier must be added to the pond in an amount sufficient
to treat new water added for topping up. If copper is present, it too must
be detoxified with a proprietary water conditioner.
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