View Full Version : Part 5 How the Filtration System Functions

12-31-2015, 10:33 AM
Part 5 How the Filtration System Functions

An understanding of how the filtration system keeps the tank pristine clean without any maintenance for a month, begins with a description of the Nitrogen Cycle. A simplified diagram is shown below:

5-1 The Nitrogen Cycle in the Aquarium:


There are two processes occuring simultaneously in the aquarium: the first is decomposition and the second is nitrification. Decomposition begins with food and results in ammonia. The second is nitrification beginning with ammonia and resulting in nitrate.

Decomposition--Decomposition is a very common process in nature as evident by the foul smell of a dead animal or decaying vegetation. It results when organic compounds (food) are placed in the tank. This leads to bioload and biomass as defined in the following link:


When I wrote the post above, I took the liberty to redefine the terms bioload and biomass as follows:

Bioload--organic and decomposed organic material suspended or dissolved in the water column, usually controlled with water exchange.

Biomass--organic and decomposed organic material loose at the bottom of the tank (consisting of feces and uneaten food) or attached to the bottom, sides and everything within the tank. This is usually controlled by syphoning the tank bottom, wiping the sides and bottom clean, and removing decorative items for cleaning. Biomass accumulates on surfaces in the tank because organic compounds are transported in the water column and then become attached to the surfaces.

The undesired affects from decomposition include ammonia and other toxic compounds. Note that there are three ways that ammonia is produced: decomposition of uneaten food, decomposition of feces, and ammonia released by the fish (as a result of decomposition taking place in the digestive tract of the fish).

Nitrification--As illustrated in the diagram above, the nitrification process consists first of converting ammonia (NH3) to nitrite (NO2) and then converting nitrite (NO2) to nitrate (NO3). These are chemical processes produced by nitrosomonas and nitrobacter bacteria. The chemical equations for these processes are:

2NH3 + 3O2 -> 2NO2 + 2H3O oxidation of ammonia to form nitrite

2NO2 + O2 -> 2NO3 oxidation of nitrite to form nitrate

Note that in both of the chemical equations above, there must be a source of oxygen available in order for the chemical reactions to take place. The amount of dissolved oxygen (O2) in the water column is reduced by these reactions and thus must be replenished so that the process can continue. The amount of dissolved oxygen in water is known as the redox potential, also known as ORP. If the redox potential is not sufficient, then the chemical reactions will not take place and ammonia will increase in the water column.

Note also that the number of bacteria will grow in numbers until they are capable of processing all of the ammonia and nitrite as fast as it is produced. That is why ammonia and nitrite levels remain at zero in a tank with a properly established biological filter. Also, the amount of nitrate produced is proportional to the amount of available ammonia.

How Purigen Works--Purigen is a synthetic adsorbent that is unlike any other filtration product. It is not a mixture of ion exchangers, but a unique macro-porous synthetic polymer that removes soluble and insoluble organic compounds from water at a rate and capacity that far exceeds any other product on the market. It does not absorb ammonia, nitrite, or nitrate, but it does reduce them because a reduction of organic compounds in the water column result in less ammonia and thus a reduction in nitrite and nitrate.

In order for Purigen to be effective, the product must be fluidized, thus the need for a reactor. Simply placing a bag in the path of water flow doesn't work very well because the tiny beads become compacted and this does not allow for sufficient water contact.

How Purigen Eliminates Bioload and Biomass--When Purigen is fluidized properly it absorbs nearly all of the suspended and dissolved organic compounds in the water column, thus eliminating bioload. Biomass consists of two parts:

1) Decomposing organic compounds attached to the sides, bottom, and everything else in the tank. This form of biomass is eliminated because Purigen eliminates bioload, and without it, suspended organic compounds are not transported in the water column.

2) Feces and uneaten food on the tank bottom. Organic compounds in the food and feces acts as a binder that holds it together. When Purigen removes nearly all of the dissolved organic compounds in the water column, the organic compounds in the food and feces quickly dissolve and this causes them to shrink as they disintegrate. Water circulation then transports the suspended portion to the mechanical filter, and the dissolved portion to either the purigen reactor where it is absorbed or flushed out of the tank by water exchange.

How Purigen Improves Water Clarity--When food is introduced to the tank, some of it is suspended in the water column as particles. Particles greater than 25 micron are trapped in the mechanical filter while particles less than 25 microns remain in the water column when they pass thru. Light is scattered when it hits this suspended material and it makes the water appear cloudy. Purigen eliminates nearly all of the dissolved organic compounds in the water column, and this causes the organic material contained in the suspended particles to become dissolved, thus causing the particles to disintegrate into material too small to be seen. What results is a visible difference in water clarity that is even more apparent when sunlight shines on the tank. The water becomes so clean that the fish seem to be "floating in air."

How Algae is Eliminated--Algae lives in two stages, the first is algae spore, and the second is common algae. The spore stage requires ammonia, and the common algae stage requires either nitrate or phosphate. So the way to eliminate algae is to minimize ammonia, nitrate, and phosphate.

Before I developed my filtration system, I tried reducing the amount of light to control algae and this helped some, but the result was not satisfactory. At that time I had everything in place for the filtration system except the purigen reactor. When the reactor was placed on line, I was surprised that the algae was totally eliminated (even if the lights were on 24/7).

My tank has zero algae because the ammonia, nitrate, and phosphate are all zero as illustrated in the following picture.

5-2 Water Parameters:

How to Achieve Zero Ammonia, Nitrate and Phosphate--The use of purigen greatly reduces the amount of ammonia produced because it absorbs organic compounds from uneaten food and feces, thus leaving the only major source of ammonia coming from the digestive tract of the fish. Less ammonia results in less nitrate. Substantial water exchanges further reduce nitrate to the level demonstrated in the picture above.

Depending on the source water, nitrate and phosphate can possibility enter the tank when water changes occur. I am fortunate that my source water (private well) contains neither. If it did contain either of these, there are two possible remedies. The first is to remove them with the use of an RO system. The second is remove them with absorbents.

It is also important to be selective and careful as to how and what to feed the fish. Some foods have high levels of phosphate and these need to be avoided. I tested for phosphate levels in various foods by dissolving a known amount in water and then recording the resulting levels of phosphate. Also, it's important to feed the fish such that they consume as much of the food as possible because this increases the effectiveness of the filtration system.

Why Water Circulation is Important--Feces and uneaten food fall to the bottom of the tank where they will decompose and cause problems if they not removed. Organic compounds acts as a binder and holds them together. Organic compounds are soluble, and when they dissolve in water the uneaten food and feces disintegrate into suspended organic and inorganic compounds. A small portion of the food (about 3%) consists of heavy solids and it collects on the tank bottom where circulation is minimum. These heavy solids are not harmful to the fish and do not decompose. The rate at which disintegration takes place depends of the level of saturation of the organic compounds in the water column. Circulation assists this process as illustrated in the following example. Take a glass of water and place a teaspoon of rock salt in it and wait. It will take a long time to dissolve. Then take a glass of water, place a teaspoon of rock salt and stir. The rock salt will dissolve much quicker.

Referring to Figure 4-4, the water entering the tank is directed horizontal across the bottom of the tank (where the uneaten food and feces exists) towards the lift tubes which transport the water to the top of the tank. Generally it takes about an hour for the food and feces to shrink completely when new purigen is in the reactor. This increases to about two hours when the purigen has been used for about a month.

Why the Mechanical Filter Requires a Large Surface Area--When uneaten food and feces disintegrates, a large portion of this becomes trapped by the mechanical filter. This loads up the filter until it begins to get plugged and needs to be cleaned. How soon this occurs depends on the surface area of the filter and the volume and types of foods fed to the fish. The mechanical filter for my system is contained in a canister which has a pressure gauge that indicates when servicing is necessary. With ten adult discus in my tank the pressure gauge begins to indicate an increase about midway thru the fourth week.

Why Large Water Changes are Necessary--Since purigen is so effective at absorbing organic compounds and thus reducing decomposition, it might seem that large water changes could be decreased, but this is not the case. This is because there are three possible sources of ammonia: uneaten food, feces, and the digestive system of the fish. Purigen absorbs the organic compounds in the first two before decomposition can take place, thus eliminating them as a source of ammonia. However, the digestive process that takes place in the fish involves decomposition which produces ammonia and other toxic compounds that are released into the water column when the feces disintegrates. The ammonia is processed by the biological filter and this results in nitrate. Both the toxic compounds and the nitrate are reduced to acceptable levels with the large water change.

01-01-2016, 12:48 AM
I have a few questins related to this article. I am currently planning to upgrade to a 125 or 150 gallon display tank (or just dreaming?).

You use air to run the lift tubes. Would you consider power heads or the newer style circulation pumps suitable alternatives?

I assume that the relative lack of DOC in your tank also eliminates or greatly reduces "surface scum"? Or, is this handled by the water change overflow, or a combination?

What are the essential attributes of your setup that achieve the desired water "mixing" without being too much flow for the fish? If you turned the water over 5x/hour, what do you think would change? What if you reduced the lift tube count by 50%? I suppose I am looking for how I might tune/adapt this to other situations, or "rules of thumb."

I must run filtration under the tank. How loud is the water pump? Always a subjective question, but I currently enjoy the complete lack of detectable noise from an Eheim canister. The small air pump, an airstone bar in the tank and the resulting popping bubbles are much louder in the room, and occasionally my wife complains. I wonder how a 1000 gph pump would compare.

Have you ever explored the advertised denitrification properties of Seachem Matrix?

01-01-2016, 04:16 PM

You asked some good questions. I'll answer them one at a time:

You use air to run the lift tubes. Would you consider power heads or the newer style circulation pumps suitable alternatives?


The bottom on the tank is where you want to produce the circulation as shown in the picture above. This is where the feces and uneaten food tends to collect. Circulation causes them to dissolve into the water column so that they can be carried to the mechanical filter and then the purigen reactor, thus removing them from the tank. Since I don't have much experience with power heads or the newer style pumps, I really can't say if they are suited for the job.

I assume that the relative lack of DOC in your tank also eliminates or greatly reduces "surface scum"? Or, is this handled by the water change overflow, or a combination?

I would say that it is a combination. Note in the picture above that the lift tube outlets are pointed at the overflow tube (the black pipe). This pushes anything on the surface over towards the overflow tube. Everything on the surface is then flushed out during the water exchange. This is particularly effective for any food floating on the surface such as freeze dried black worms. In my tank the surface remains spotless clean.

What are the essential attributes of your setup that achieve the desired water "mixing" without being too much flow for the fish? If you turned the water over 5x/hour, what do you think would change? What if you reduced the lift tube count by 50%? I suppose I am looking for how I might tune/adapt this to other situations, or "rules of thumb."

Good water mixing is attained by how the new water enters the aquarium:


Note in the picture above that new water flows thru the pre heat tank and then is mixed with aquarium water at the input to the filter pump. New water is introduced at the rate of 1 g/m, while tank water draw flows at the rate of 16 g/m ( 1000 g/h / 60 m/h ). This mixture is then returned to the tank at the bottom thus resulting in good mixing.

The reason that the resulting flow is not too much for the fish is due to three factors:

1) Note in the top picture above that water entering the aquarium is forced to flow in 360 degrees.

2) Referring to pictures 2-1 and 2-2 in Part 2 that the water inlet to the aquarium is hidden inside the imitation log. This breaks up the flow.

3) Also note in pictures 2-1 and 2-2 that there are a number of decorative items that further breaks up the flow.

All three of these factors reduce water currents to an extent that the fish are totally unaffected by it. It does not require very much flow at the point where the feces/uneaten food exists. If you can observe it moving ever so slightly, then the flow is sufficient.

As far as reducing the number of lift tubes or the amount of circulation, try it and see if it works.

Sorry, I can't help you much with the noise factor. One thing that does help some is to make the bubbles smaller.


01-01-2016, 11:30 PM
Thank again Paul.