Aquaponic Mechanical Filters Explained


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The mechanical filter is a fixed component which is located after the fish tank. It comes in many shapes and sizes and its sole purpose is to remove the heavier fish waste (poop) and uneaten food.

The purpose of a mechanical filter in an aquaponic system is to remove heavy solids from the water stream leaving the fish tank before it makes its way into the biofilter.This is done to prevent fouling of the other components and roots of the plants.

The amount of waste produced by the fishes is directly related to the amount of food they eat.

Additionally the amount of fishes in the system and the waste that they produce would give a good idea on how large of a mechanical filter that is required for solid removal.

It is important to keep healthy fishes but over feeding can lead to many problems if the mechanical filter is not up to the challenge of efficiently removing the waste before the water enters the other parts of the aquaponic system.

Having a properly designed mechanical filter would ensure that only the ammonia from the fishes waste gets to the biofilter. But what’s the theory behind a mechanical filter that makes it so special? 

In this article we are going to visit the different types of mechanical filters and their designs so you can pick which one is best suited for your system.

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How to Efficiently get Solids from the Fish Tank to the Mechanical Filter?

Solid removal from the fish tank is important in maintaining a good water quality for the fishes to live in. The solids in a fish tank can be removed using a number of methods including bottom drains, side overflows and solid lifting outflow.

The best method recommended to efficiently remove solids from a fish tank is by using a solids lifting outflow (SLO) design. 

The SLO design which is most commonly used is having piping installed such that it goes down to the bottom of the fish tank where the heavy materials tend to settle. At the bottom of the tank the pipe is expanded and held approximately one to two inches above the bottom of the fish tank. 

This allows the heavy fish waste and other materials to be lifted and taken out of the tank by the conventional water flow in the system. The outflow is piped out such that it maintains the water level in the fish tank at that point.

A method that I have found that worked pretty well also is by having a slotted ‘L’ shaped piping extend across the bottom of the tank. The slots a cut so that only the fish waste is allowed to leave the tank and not the fishes.

This design is incorporated with the direction in which the water pump discharge enters the fish tank. By having the discharge directed to one side of the tank it creates a slow moving swirl effect in the fish tank and the fish waste moves in a circular direction until all is removed via the slotted pipe which extends across the bottom of the tank.

What does a Mechanical Filter do?

The Mechanical filter removes solids from the water stream leaving the fish tank. 

These solids may come in the form of fish poop, food or even smaller dead fishes which may make its way into the mechanical filter. 

The food which is fed to the fishes may be high in protein and many other nutrients for the fishes to grow quickly. The need for fast fish growth is exaggerated by the business side of the aquaponics industry which would yield higher profits for high fish growth in short periods of time.

Approximately 15 to 40 percent of the food fed to the fishes ends up leaving the fish tank with the water stream.

With faster fish growth the need for solids removal becomes a top priority since the fishes would tend to produce more waste of their own as they get bigger and are fed more. 

Oftentimes some fish food also becomes part of the water stream leaving the fish tank and ends up in the mechanical filter.

A properly designed mechanical filter would facilitate the easy and efficient removal of solids through a ‘blow down’ line located at the bottom of the filter which targets the heavy waste that settles there.

Types of Mechanical filters and their designs –

Mechanical filters and their designs may come in different shapes and sizes. Some separates solids by utilizing the change in water direction to ‘knock out’ the heavy solids which then sinks to the bottom of the filter for easy removal.

These are categorized as Solid Separators.

Solid Separators are the goto type of mechanical filtration when it comes to aquaponics.

Other Mechanical filters remove solids by use of a filter medium such as sponge, netting or filter pad material.

These are categorized as Solid Filters.

Solid Filters are best suited for smaller systems such as aquariums with smaller fishes.

The latter is more labor intensive and requires some degree of monitoring which if not kept relatively clean can lead to problems with the filter.

Solid Separators –

  1. Swirl filters
  2. Radial flow filters
  3. Baffle filters/ clarifier

Swirl Filter –

A swirl filter is designed to have solids separated by the centripetal force created by the circular motion of the water. This filter is designed such that the water inlet enters the bottom of the drum which is then directed to the side.

The flow of the water entering the drum pushes against the sides creating a circular motion. This circular motion causes heavier solids to be removed and dropped out of the conventional flow of the water which rises to the top where it then exits, solids free, through the overflow.

The drum is usually cylindrical in shape with a conical base. The reason for the conical base is to trap heavy solids at one central point where it can be removed by opening a single valve.

Radial Flow Filter – 

This filter gets its name from the design and water flow. A look from the top would show the water flowing into the filter and spreads radially outwards into a baffle pipe.

In a radial flow filter the water enters from the side and is directed by the piping to the inside of the baffle pipe. As the water enters the baffle pipe and moves downwards it tends to change speed and slow down drastically.

The slowing of the water ‘knocks out’ the solids which then sinks to the bottom of the filter where it can be removed by opening a single valve. 

A flaw in this design is the collection of the solids at the bottom where it spreads out. Not all the solids are removed when the drain valve is opened. 

What I have found is by creating a cone shaped base somewhat like the swirl filter significantly helps with the removal of solids.

Baffle Filter/ Clarifier – 

This type of filter uses a series of baffles built into the filter which increases the retention time of the water within the filter. The baffles also change the direction of the water flow which drops out heavier solids to the bottom of the filter where they can be removed.

A baffle filter follows a simple box design with baffles installed to increase water retention. The filter should be large enough so that the solids in the water have enough time to fall to the bottom of the filter. 

A large barrel or square shaped container can be used to contain the water. 

Slowing the water flow and increasing retention time allows the solids to settle over time, at the bottom of the filter.

Solid Filters –

  1. Trickle filter
  2. Raft filters
  3. Micro screen rotating drum filters
  4. Sand Filter 
  5. Screen Filter

Trickle Filter –

A trickle filter works by allowing water to flow by gravity from the top of the filter to the bottom. On its way down the solids from the water becomes trapped within the filter medium leaving only clean water to flow out.

This type of filter works great in aquaponics but it requires more maintenance since the filter medium can get clogged quickly causing water to backup and overflow.

Raft Filters – 

This follows the same concept like the trickle filters, but is designed to stay horizontal allowing water to flow through the filter media on its own until it exits the outlet.

The filter media is designed such that the first filter medium traps the larger solids and the subsequent filter media sequentially traps the smaller solids as the water flows through.

This type of filter also works great but as with the trickle filter the filter media can get heavily fouled and cause the filter to overflow if not maintained properly.

Micro screen rotating drum filters –

Micro-screen drum filters are suitable mainly for tertiary treatment, especially for removal of suspended solids in the water.

Water containing solid particles flows through the inlet pipe into the center of the filter drum, solid particles are caught on the inside of the filter cloth and the filtered water flows through the cloth out

Screen filters – 

This type of filter uses screens to remove heavy solids. It allows the incoming water to flow down a screen where heavier solids are trapped and removed. 

I really don’t recommend this type of filter for an aquaponic system since the screen gets easily fouled and requires more maintenance than the solid separators.

Sand filters – 

This type of filter requires that the water be forced through a bed of sand using a water pump. 

Sand filtration increases electricity usage because of the pump and is not practical with organic waste since the sand gets easily clogged. This type of filtration is better suited for aquariums and small ponds.

Types of filter Media Used in Solid filters – 

  • Sponge Filter
  • Cartridge Media
  • Filter pads with different space sizes 

Table of Pros and cons using solid separators and solid filters

Filter TypesProsCons
Swirl FiltersMaintenance Free, No electricity, Availability, Has to be DIY
Radial Flow FiltersMaintenance Free, No electricity, Very EffectiveAvailability, Has to be DIY
Baffle FiltersMaintenance Free, No electricity,Water retention time depends of particles to be removed
Trickle FiltersEffective and SimpleMore maintenance with filter media
Raft FiltersEffective and simpleMore maintenance with filter media
Micro screen rotating drum filtersHandles large volumes of water, very effectiveExpensive
Screen FiltersNot for aquaponics
Sand FiltersSimple, Limited space, best for aquariumsRequires electricity, requires maintenance, Not for aquaponics
Mechanical Filters Pros and Cons

Do Sponge Filters Work?

The use of sponge filters in aquaponics is a very cheap and effective method to separate solids, but it comes with some additional work. 

Sponge filters however, require more maintenance and a lack of maintenance would lead to backup and over flow of nutrient rich water which has a ripple effect on the entire aquaponic system.

Can you Use a Carbon Filter in aquaponics?

Carbon filtering is a method of filtering that uses a bed of activated carbon to remove contaminants and impurities, using chemical adsorption.

This chemical adsorption may not be so beneficial in aquaponics since it will adsorb the ammonia which is used by nitrifying bacteria to make Nitrates for plants. This is beneficial for the fishes and is very effective when used in stand alone aquariums.

However in aquaponics, it will starve the plants of the very nutrients the fishes are providing for the plants to grow.

Can you Reuse the Solid Waste from the Mechanical Filters?

The solid waste when drained from the mechanical filters can be used as fertilizer for plants grown in soil.

When drained from the filter the water will also contain nitrates and other nutrients along with the solid waste. These nutrients are also beneficial to plants which are grown on the ground and is an organic way of fertilizing these plants. 

This is a good way of integrating both aquaponics and conventional farming methods.

Consequence of not having a mechanical Filter –

  1. The water becomes fouled and looks murky
  2. The biofilter will eventually become clogged 
  3. The solids will gather on the roots of the plants and would prevent nutrient absorption and proper plant growth.
  4. The Ammonia concentration in the water would increase creating a toxic environment for the fishes.

The Takeaway –

Mechanical filtration in aquaponics is critical in removing solids and maintaining good water quality for both plants and fishes.

Although there are many types of mechanical filtration available which does a fantastic job of removing solids, Solids Separators are the best method for solid removal in aquaponics which requires little maintenance. 

An additional benefit from using a solid separator is that the waste can be used as an organic fertilizer for plants grown in soil.

JayLea

JayLea has a passion for plants and has been gardening since 2015. He has valuable knowledge about gardening and houseplant care and can answer everyday practical questions that every plant owner has.

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