Mechanical Filters

Many newcomers to aquaponics are confused by the need for both types of filtration in a system. If the biofilter and bacteria are going to break down the waste from the fish, why would you need to trap and remove so much of it? This is because the bacteria need relatively low solid waste levels to perform optimally, and solid waste also has a serious clogging effect throughout the system when it’s allowed to accumulate.

The most basic mechanical filters for aquaponics systems are simple water tanks. These tanks have one or more layers of tightly stretched, woven netting for the water to flow through. Solids are trapped and sink to the bottom of the tank or stay trapped in the filter until manual cleaning. This can require two to four cleanings every single day in a large commercial system. Aside from netting, passive filters can also be built with sand, gravel or other fine particles that will allow water to flow through while keeping the solid waste behind.

More advanced, active filter units clean themselves for a reduced maintenance schedule. They also typically trap far more solid waste and capture smaller particles of dissolved waste as well. Some of the most popular, active filter units suitable for commercial aquaponics include:

• Rotary drums; that splash the water around in a rotating screen drum to separate out solids while adding oxygen. Self-cleaning, but still need maintenance at least once a week.
• Radial flow and swirl tanks; that use a naturally generated cyclone effect by offsetting the inlet and outlet pipes to send waste to the bottom of the container. Both work best on larger solids and do little for dissolved solids.

You’ll likely need both static mesh filters and at least one active filtration unit for most large-scale aquaponics systems. Check the water filtering capacity of the commercial filtration units to determine how many you’ll need. This will be based on the system’s total gallons of water, fish stocking rate and desired water quality parameters.

Biofilter Ideas and Designs

In both media bed and deep water culture/raft growing systems, the plant troughs are generally considered the only biofilters needed. This is because both media beds and the net pots used in the floating rafts are filled with porous growing mediums that can host millions of nitrifying bacteria. For a nutrient film technique (NFT) or highly stocked raft system, you may need a separate biofilter, or two, to host enough bacteria to power the nitrogen cycle. Fifty-five or 100-gallon tanks filled with loosely wadded plastic netting work surprisingly well as bacterial hosts and biofilters. The mesh can be shaken and gently rinsed on a regular schedule to wash away any accumulated solid waste without losing the bacteria growing on the surface. The netting is widely available and easy to replace whenever needed. Containers full of sand, gravel and growing mediums like expanded rock can also function as biofilters as long as the material is porous enough to host the bacteria.