Hi Guys,
Bacteria.....
The most people see them as single creatures, but is not true.
Bacteria live like people in a city...we call this city a biofilm.
So aerobic and anaerobic live together...see it as layer.
Nitrification vs denitrification is also....
Anoxic vs. Anaerobic
In discussing the technology behind this nitrate reduction, it is important to understand the difference between anoxic and anaerobic condition
Anoxic conditions exist when the water does not contain elemental oxygen (usually in the form of dissolved oxygen), but there is oxygen in the form of oxygen-bearing inorganic compounds like nitrates (NO3-) or sulfates (SO4-). This oxygen can be used in the oxidation of organics in the tank. As in most oxidation-reduction reactions, when one species is oxidized another has to be reduced. It’s kind of a zero-sum game.
In order to establish an anoxic or anaerobic environment, there are two factors that have to be addressed: oxygen transfer rate and oxygen utilization rate (OUR). This is true whether you are looking to achieve the anoxic condition in a liquid medium or a biofilm. Oxygen can be transferred passively through the air/water interface, or actively through the use of aerators, waterfalls, fountains and the like.
The oxygen consumption rate will be related to the organic load, which is determined by a few factors:
- How many fish will this tank hold and how many can the bio-exchange really support?
- How much work will the new owner have to do to maintain the tank? How easily is this system cleaned?
- Will the mechanical filter remove sufficient waste to allow the bio-reaction to occur naturally?
The organic load in the tank creates oxygen demand, and the bacteria convert it to carbon dioxide, water and (additional) bacterial cells. In most cases, in a filter it is most desirable to have anoxic rather than true anaerobic activity. The reason for this is that many anaerobic metabolites are malodorous and, in some extreme cases, can be toxic. A tank can even become “too anoxic,” as measured by something called oxidation-reduction potential (ORP), when sulfates are chemically reduced to hydrogen sulfide (the source of that notorious rotten egg smell).
The Solution
Fortunately, it is relatively easy to get into the “sweet spot” where water is anoxic enough to reduce nitrates, but not so anoxic that hydrogen sulfide generation occurs. You also avoid true anaerobic metabolism, where a lot of foul-smelling, possibly toxic byproducts like butyric acid can be produced. The answer: an anaerobic biofilter.
For most anaerobic biofilters for tanks, the biomass grows on a fixed film on the media in the filter. While this biofilm may seem just like a slimy layer on the media, even a thin biofilm contains millions of bacteria per square inch, many layers deep.
The biofilm is an often misunderstood and vaguely accepted terminology. In a biofilm (see diagram 1) you will generally have a thin film of water that clings to the outside of the biofilm due to the adhesive and cohesive properties of water. It is these same properties of water that provide for capillary action in plants and help water get to the top of the tallest of trees. Soluble organics and some inorganics, like ammonia, diffuse into the biofilm through this layer of water.
In the first layer of the biofilm, there is an aerobic zone. The depth of the aerobic zone is determined by the strength of the water in terms of oxygen demand and loading per unit area of the media, and the dissolved oxygen content in the water. As the oxygen is consumed as it diffuses through the biofilm, and organic and inorganic compounds are still present, an anoxic section develops.
In this section, any oxygen-bearing compounds will be utilized as what are called “electron acceptors,” providing a means to oxidize the organic compounds to carbon dioxide, while reducing the oxygen-bearing inorganic compounds to nitrogen gas (from nitrates) and hydrogen sulfide (from sulfates). Fortunately, the utilization of nitrate is preferred over the utilization of sulfate and most water will have more nitrate than sulfate.
The process dynamics of diffusion and biological activity in a biofilm.
The key to getting the correct dynamics in the anaerobic or anoxic filter is proper sizing of the filter with respect to loading. If a filter is oversized and the loading is too light per unit area of surface area, there will not be enough oxygen demand to create an anoxic zone. However, if the loading per unit of surface area is too great, the film may grow too thick and create a true anaerobic zone, which can lead to undesirable results. So, an oversized or undersized filter may give less than the desired results.
One of the best filtermedia which can hold an biofilm is so called KNS Kaldness K1....google for it.
It is a moving bed filtermedia which holds excellent a biofilm and you can easily control the thickness of the biofilm thus the concentration of nitrate just by adding filtermedia or take it away.
Hope this gives some ideas ..
