Media beds can act as the biofilter in an aquaponics system, but mechanical filtration to remove dissolved solids is likely unless very low stocking and feed ratios are used. DWC/raft culture systems generally don’t need media beds for tougher fish like tilapia because the growing medium in the rafts supports enough bacteria for nitrate conversion. However, systems designed to grow more delicate fish like trout, should involve a separate biofilter in addition to the mechanical filters to control nitrates that would otherwise return with the recirculating water. Nutrient film techniques (NFT) definitely need a dedicated biofilter bed or tower to control nitrate levels.

Mechanical filters rely on screens of varying sizes that trap particles of solid waste as the water is forced through the material. Most aquaponics systems rely on static bag or plate filters that don’t move as the water flows through. Advanced moving filters can trap more sediment before needing cleaning, but they’re also expensive and can’t be built by hand from lumber and screen. Properly sized static screens usually only need cleaning once to twice a daily, while moving filters like self-cleaning drum and belt screens, can often go a day or two between manual cleanings. Mechanical filters still contribute to the bio-filtration process by growing colonies of helpful bacteria on the screen surface; converting a good amount of ammonia to nitrites and nitrates as the water flows through the material.

Sand filters are popular for filtering ponds and pools, but neither produces the high levels of dissolved solids you’ll find in aquaponics. The sand in these filters tends to clog very quickly and slow down flow enough to reduce water quality for the fish and starve the plants downstream. Avoid sand filters, along with biofilters based around gravel and similar aggregates, because they will quickly fill up with the sediment of solid fish wastes.

Water arrives in the filter tanks and biofilter bed after flowing through clarifiers, which are discussed below. For raft-based systems, with a high stocking density and large plant growing area, mechanical filter tank volume should be around 1.5% of the total system. For example, the 22,000-gallon UVI system includes two 185-gallon filter tanks for a total of 370 gallons of filter tank volume. Because drum and belt filters are usually about twice as effective as static ones, you should only need about half as much tank volume for these units.

As with general sizing for an NFT system, sizing for the filters in these aquaponics designs is complex and requires calculators that take all factors into account. The same tool you use for sizing the fish and plant tanks for NFT should also provide filter unit size recommendations based on feed ratios and protein levels.

Sizing and designing biofilters for aquaponics systems is another complex task that requires the use of calculators that can accommodate the sensitivities of your chosen fish species and other factors. Proper design of raft and media beds will eliminate the need for a separate biofilter, dramatically simplifying the design and installation of the system. Some biofilters rely on specialty bacteria-hosting media like foam or small porous balls, while others take the form of constructed wetlands full of purifying plants. Simple biofilters are built by setting up one or two 55-gallon drums stuffed with plastic bird netting or another fine mesh material. The hundreds of layers of scrunched material traps the majority of solid waste that makes it through other filters and settling tanks, while the huge amount of surface volume results in strong bacterial growth. Even a single 55-gallon biofilter is often enough for large commercial raft or media bed systems, while multiple units are required for more sensitive NFT designs.