Pond Reflections
The Pond Systems Newsletter

Information for the Pond Owner


INTRODUCTION TO
BIOLOGICAL FILTRATION

Part I : The Background Basics

What is biological filtration?

Why is it important?

What is a biological filter?

Where do the bacteria come from?

If the bacteria occur naturally, why is a bio filter necessary?

Why do ponds turn green?

Part II: Biological Filtration

The Nitrogen Cycle

Two types of biological filters

Out-of-pond pressurized filters

Set up and Maintenance

In-pond-filter

Out-of-pond pressurized filter

Pumps

What can go wrong?

Part I : The Background Basics


What is biological filtration?

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.

Why is it important?

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.

What is a biological filter?

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.

Where do the bacteria come from?

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.

If the bacteria occur naturally, why is a bio filter necessary?

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.

Why do ponds turn green?

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.


Part II: Biological Filtration


The Nitrogen Cycle

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.

Nitrate is eliminated by "denitrifying bacteria" that live in the bottom mud. These mud dwellers are anaerobic bacteria that die in the presence of oxygen, and, as they do their work converting nitrate to free nitrogen, also release hydrogen sulfide and methane gases - the swamp smell. Most clean, well aerated backyard ponds do not have an anaerobic environment or chamber, and nitrate thus accumulates in the pond. It is nowhere near as toxic to fish as are ammonia and nitrite, but it is a nitrogen fertilizer and will encourage an algae bloom. Fortunately for beleaguered pond owners, there are now available proprietary formulations of bacteria that eliminate nitrate in the presence of oxygen. These bacteria can be added to the pond and thus allow the nitrogen cycle to be completed so that algae will not colonize the pond. Otherwise, plants, periodic partial water changes or algae must be relied upon to remove the nitrate.

Two types of biological filters

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.

An in-pond filter has a submersible pump attached, and all the equipment is in the pond. The advantages are ease of installation and cost economy. The disadvantage is that you have to reach into the pond to clean the filter. Some in-pond filters have only a sponge for the bacteria to live on and do not work very well because the bacteria are killed off every time the sponge is cleaned. Others, such as the Bio+Plus, have separate mechanical and biological chambers, and are much more efficient.

Out-of-pond nonpressurized systems are the oldest form of bio filter - a simple gravel bed. Usually the water is pumped into the filter below the gravel, with a gravity flow back to the pond, although with proper engineering the reverse can also be set up. Gravel bed filters are custom built at the pond site and are very difficult to clean unless expertly engineered.

Out-of-pond pressurized filters

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.

What makes a pressurized filter so easy to maintain is the multiport valve that controls the flow of water through the filter. There are backwash and rinse positions that allow pond water to be used to clean the filter. The dirty water is discharged to waste, and the whole process only takes a minute or so. If the sand grain size is too small, however, backwashing will not be effective, and water will channelize through the filter, greatly reducing efficiency.

Set up and Maintenance

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.

In-pond-filter

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.

Out-of-pond pressurized filter

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.

PVC pipe, usually 1 1/2" or 2" depending on pipe length and pump size, is trenched from the pond to the pump, which is normally located next to the filter but need not be. However, since the pump must be shut off everytime the position of the multiport valve is changed, the on/off switch should be near the filter. There two discharge pipes from the filter; return-to-pond, and waste. The return-to-pond pipe may be T'd and valved so that water may return via the waterfall or through an underwater jet.

Pumps

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.)

Sometimes a two-speed pump makes a lot of sense. It runs on low all the time to save energy costs and on high for backwashing or when a major waterflow is desired. The new electromagnetic submersible pumps draw about one-seventh the energy of that of a similarly sized conventional pump, but they are limited in size.

What can go wrong?

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.

The bacteria typically die off every winter and must be reintroduced in the spring before algae colonizes the pond. Both nitrifying and denitrifying bacteria should be added on a regular basis.

The sand in a pressurized filter should be checked after backwashing to ensure that it is clean. If not, a larger grain may be required. In-pond filters should be thoroughly cleaned every winter


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