Mechanical vs Biological Filtration: What’s the Difference?

1. Start by Separating the Waste Streams

The easiest way to understand mechanical and biological filtration is to stop thinking about equipment names and instead ask what form the waste is in. Koi ponds continuously receive waste from fish metabolism, feeding, the environment and the pond's own microbial community. Some of that waste exists as visible or microscopic particles. Some is dissolved in the water and cannot be caught by a screen or filter mat.

Particulate waste

Particulate waste includes fresh faeces, uneaten feed, leaves, pollen, filamentous algae fragments, insect debris, dead biofilm and mineral or organic particles. Large particles may settle rapidly. Finer particles can remain suspended for hours or days and make the water appear hazy. Extremely fine particles may pass through coarse screens and become part of the pond's fine suspended-solids load.

From a filtration perspective, particulate waste is valuable because it can still be physically exported. The sooner it is removed, the less time it has to be shredded by pumps, colonised by bacteria, dissolved into the water, or converted into additional oxygen demand and nitrogen load.

Dissolved nitrogen waste

Fish release nitrogenous waste largely as ammonia, and decomposition of organic material can generate additional ammonia within the system. Once ammonia is dissolved in the water, a drum screen, sieve, brush chamber or settlement tank cannot simply catch it. The system therefore needs a biological process that converts ammonia first to nitrite and then onward to nitrate through nitrification.

This difference between particles and dissolved compounds explains why clear water and safe water are not the same thing. Mechanical clarity is visible; biological performance often is not. The only reliable way to assess ammonia and nitrite control is with water testing and a clear understanding of the biofilter's loading and operating conditions.

2. Mechanical Filtration: Capture, Separate and Export Solids

Mechanical filtration is any process that physically separates particles from the circulating water. In a koi pond, that separation may occur by gravity, screening, straining, depth capture, flotation or a combination of mechanisms. The equipment can look very different, but the goal is the same: move waste out of the water column and place it somewhere it can be discharged easily.

Mechanical filtration is only complete when the waste is removed

A filter that catches waste but stores it in the water is only halfway through the job. Brushes, foam, mats and static media can trap impressive amounts of debris, but the trapped material remains biologically active. Bacteria continue to decompose it. Oxygen is consumed. Fine particles and dissolved organic compounds can be released downstream. The filter may gradually become a waste reservoir rather than a waste-removal device.

This is why maintenance accessibility matters as much as nominal capture efficiency. A slightly coarser mechanical filter that is flushed every day can outperform a theoretically finer filter that is so unpleasant to clean that the owner delays servicing it for a week.

The main mechanical separation mechanisms

Settlement

Settlement chambers and vortex-style units reduce water velocity so heavier particles can fall out of suspension. They are simple, use little energy by themselves and can work well with intact, settleable solids. Their limitation is space: fine particles settle slowly, so effective settlement generally requires generous chamber volume, suitable hydraulic loading and calm flow. Compact chambers operated too quickly often look impressive but allow a large proportion of the solids to remain suspended.

Screening

Sieve filters and rotary drum filters separate particles using screens. A sieve commonly allows water to pass through wedge-wire while larger particles remain on the screen surface. A rotary drum filter continuously or intermittently washes captured solids from a fine screen to waste. The major advantage of screening is that solids can be exported quickly instead of remaining immersed in the recirculating water for long periods.

Straining and depth capture

Brushes, foam, filter mats, static plastic media, bead beds, sand-like media and similar devices capture particles as water moves through a tortuous path. These systems can achieve excellent polishing, but pressure loss and clogging increase as solids accumulate. Their real performance therefore depends on cleaning frequency, backwash effectiveness and whether the media can be serviced without disturbing the rest of the system.

Particle size is only one part of performance

Manufacturers often describe mechanical filters by screen aperture or nominal micron rating, but pond performance cannot be predicted by one number alone. Actual capture depends on particle shape, deformability, flow velocity, screen loading, bypass leakage, water level, filter cleanliness and whether large particles are broken into fines before reaching the separator.

A pump located before the main mechanical filter can chop soft faeces and debris into smaller particles. This does not automatically make pump-fed systems unsuitable, but it changes the solids-removal challenge. In gravity-fed systems, bottom drains and skimmers can deliver relatively intact waste to a sieve or drum before the pump, which is one reason this arrangement is popular in high-performance koi ponds.

What mechanical filtration cannot do

Mechanical filters cannot directly solve an ammonia or nitrite problem simply by making the water clearer. They may reduce future biological load by removing organic material before it decomposes, but dissolved ammonia already present in the water passes through ordinary screens, brushes and settlement chambers. Mechanical filtration also does not automatically remove nitrate, dissolved salts or every dissolved organic compound.

That limitation is important when troubleshooting. If the pond is clear but ammonia is rising, buying a finer screen is unlikely to address the primary failure. The question should shift to biological capacity, oxygen, flow, pH, alkalinity, recent feeding, stocking changes and whether the biofilter has been damaged or bypassed.

3. Biological Filtration: Convert Ammonia and Nitrite

Biological filtration provides a controlled habitat for microorganisms that process dissolved waste. In a conventional koi biofilter, the most important function is nitrification: the aerobic oxidation of ammonia to nitrite and then nitrite to nitrate. This is why the biological stage is often described as the pond's ammonia-processing engine.

The nitrogen pathway in practical terms

Protein in koi feed contains nitrogen. Fish retain part of that nitrogen for growth, while a substantial fraction eventually returns to the water through metabolism and waste. Ammonia in water exists in equilibrium between un-ionised ammonia and the ionised ammonium form; the balance is strongly affected by pH and temperature. Because ammonia toxicity changes with water chemistry, a single total-ammonia number should never be interpreted without context.

In an established aerobic biofilter, ammonia-oxidising microorganisms convert ammonia toward nitrite, and nitrite-oxidising microorganisms convert nitrite toward nitrate. Hobby literature often simplifies these communities to two named bacterial genera. In reality, nitrifying biofilms can contain diverse ammonia oxidisers, nitrite oxidisers and other microorganisms, and the exact community can vary with the system. For pond management, the useful concept is that nitrification is a living biological process that needs stable environmental conditions.

A biofilter does not make nitrogen disappear

This is one of the most important distinctions in the entire filtration system. A conventional nitrifying biofilter changes the chemical form of nitrogen; it does not remove all nitrogen from the pond. Ammonia becomes nitrite and then nitrate. Nitrate is generally less acutely toxic than ammonia or nitrite, but it can accumulate in closed systems and is normally controlled by water exchange, plant uptake, low-density dilution, or dedicated denitrification processes in more specialised systems.

What the biofilm needs

Nitrification is aerobic, so the biofilter needs a reliable oxygen supply and good contact between the water and the colonised media. It also needs appropriate pH and alkalinity because nitrification consumes alkalinity and tends to acidify the system over time. Temperature, ammonia concentration, organic load and hydraulic loading all affect nitrification performance. A filter with enormous theoretical surface area can still perform poorly if oxygen is low, water short-circuits around the media, or solids smother the active surfaces.

The Southern Regional Aquaculture Center and aquaculture engineering literature consistently treat solids removal and biofiltration as separate but connected unit processes. Research on nitrifying trickling filters likewise identifies dissolved oxygen, pH, bicarbonate/alkalinity, organic matter, temperature and hydraulic loading as important performance variables. These principles translate directly to koi ponds even though stocking patterns and equipment brands differ.

Common biological filter formats

Koi ponds use several reactor styles to provide colonised surface area. Moving-bed filters keep buoyant plastic media in motion with strong aeration. Shower and trickle filters expose thin films of water to media and air, giving excellent gas exchange when correctly designed. Fixed media such as Japanese matting, structured plastic, ceramic media and other porous substrates can also support large biofilms. Bead filters and some packed beds can perform both mechanical and biological functions, although the two jobs can compete if solids accumulation reduces oxygen and flow through the bed.

4. Mechanical vs Biological Filtration: Side-by-Side Comparison

5. Why Mechanical Filtration Should Usually Come First

In most koi systems, the main mechanical stage is placed before the main biological stage. This is not simply tradition. It protects the biofilter from a workload it is poorly suited to handle and preserves more of the biofilter's surface area and oxygen for nitrification.

Solids create oxygen demand

Organic particles are rapidly colonised by heterotrophic microorganisms that use carbon-rich material as food. Their respiration consumes oxygen. If large amounts of faeces and uneaten feed enter the biofilter, oxygen is spent decomposing solids at the same time that nitrifiers are trying to oxidise ammonia and nitrite. In a heavily loaded system, that competition can reduce the biological filter's effective capacity.

Solids can clog and channel media

Fixed media and packed beds are particularly vulnerable to solids accumulation. Sludge fills void spaces, increases resistance to flow and encourages water to take easier paths through the reactor. Once channeling develops, part of the media receives too much flow while other areas become poorly supplied with oxygen and dissolved waste. The filter may still contain the same amount of media, but far less of it is being used effectively.

Fine solids can migrate surprisingly far

Aquaculture literature notes that fine suspended solids can make up a large share of the suspended-solids load in recirculating systems and can contribute to oxygen demand and fish-gill irritation. This is why a system that removes only large faecal pieces can still benefit from secondary polishing or improved primary separation when stocking and feeding are high.

Early removal prevents waste from changing form

Fresh faeces are easier to remove than the dissolved and microscopic material produced after they break apart. Every hour that solids remain trapped in warm, oxygenated pond water gives decomposition more time to convert a removable particle into dissolved nutrients, bacterial biomass and fine sludge. Mechanical filtration therefore reduces not only visible dirt but also the load placed on downstream biological and polishing stages.

Design principle: The best place for solid waste is not inside the biological filter. The best place is outside the recirculating loop, in a waste drain or collection point that can be discharged easily.

6. Combined Filters: When One Device Performs Both Jobs

Not every pond has a completely separate mechanical chamber and biofilter. Some equipment intentionally combines the two functions. Bead filters, static media chambers, foam blocks, matting chambers and certain compact pressurised filters can capture solids while also supporting bacterial biofilm. Combined filtration can be practical, especially where space is limited, but the design has to respect the conflicting requirements of the two processes.

The basic compromise

Mechanical filtration benefits from trapping particles. Biological filtration benefits from clean, oxygen-rich, evenly distributed water contacting the media. As a combined filter accumulates solids, it often becomes better at trapping finer particles but worse hydraulically: head loss rises, oxygen can fall inside the media bed, and water distribution becomes less uniform. Backwashing restores hydraulic capacity but also disturbs some biofilm.

Research on bubble-washed bead filters illustrates this trade-off. Such filters can provide both solids capture and nitrification, but backwash frequency and adequate flow become critical as loading rises. More solids and higher feed loading require more aggressive maintenance to prevent the bed from becoming oxygen-limited or hydraulically restricted.

When combined filtration makes sense

• Small or medium ponds where space is restricted and stocking is moderate.
• Retrofits where the owner needs improved clarity and additional biofiltration without rebuilding the entire filter house.
• Systems where the combined unit can be backwashed thoroughly and frequently without creating excessive labour.
• Situations where a separate prefilter removes coarse debris before the combined media bed, reducing the amount of waste it must store.

When separation is usually superior

• High feeding rates, dealer systems, grow-out ponds or heavily stocked collections.
• Systems where stable biological capacity must be preserved during frequent mechanical cleaning.
• Installations using fine automated solids removal such as an RDF followed by a dedicated moving bed or shower.
• Ponds where rapid waste export and low manual maintenance are priorities.

7. The Two Stages Must Be Sized Differently

A major design mistake is to choose both mechanical and biological filtration from pond volume alone. Pond volume is important for circulation and residence time, but it is not the most direct measure of waste production. Two 20,000-litre ponds can require very different filters if one contains six lightly fed koi and the other contains forty large koi being fed heavily in warm weather.

Sizing the mechanical stage

Mechanical filtration must be able to accept the system's actual water flow while removing the expected solids load without excessive head loss or constant blockage. Important variables include the number and size of koi, feeding rate, leaf and environmental debris, screen aperture or media grade, pump location, plumbing arrangement and how frequently the waste-removal mechanism operates.

For an RDF, a useful question is not merely the manufacturer's maximum flow, but how often the drum is expected to wash at the intended solids loading and water level. For a sieve, consider whether the screen can accept peak flow without bypass. For static media or mats, consider how quickly pressure loss and clogging develop and whether the cleaning process is realistic for the owner.

Sizing the biological stage

Biological capacity should be related to the amount of feed and nitrogen entering the system, not merely the number of litres in the pond. Feed rate is a practical proxy because protein metabolism drives much of the ammonia load. Professional recirculating aquaculture design commonly rates biofilters against feed loading or ammonia-nitrogen load under specified temperature, oxygen and water-chemistry conditions.

Media surface-area claims must also be interpreted cautiously. Manufacturers may quote total surface area, protected surface area or theoretical microscopic area, but only the area that receives oxygenated water and remains biologically accessible contributes reliably. Reactor design, media fill fraction, aeration, flow pattern and organic fouling can matter as much as the headline surface-area number.

Build reserve into both stages

Koi collections rarely remain static. Fish grow, owners buy more koi, feeding increases, and warm weather raises metabolic demand. Designing every component at its theoretical limit leaves little protection against a clogged screen, reduced pump flow, a heat wave, a sudden batch of new fish or an unusually heavy feeding period. Sensible reserve capacity is therefore more valuable than chasing the smallest possible filter footprint.

8. Flow Rate Affects Mechanical and Biological Filtration Differently

Flow connects the pond, mechanical stage and biofilter, but 'more flow' is not automatically better in every chamber. Each process has a hydraulic range in which it works effectively.

Mechanical filtration and flow

If water moves too quickly through a settlement chamber, particles that should settle remain suspended. If a screen filter is driven beyond its practical hydraulic capacity, water levels can become unstable or bypass may occur. If flow through dense mats is too high, head loss increases. Mechanical equipment therefore needs to be sized for real operating flow, not a pump's zero-head catalogue rating.

Biological filtration and flow

A biofilter needs enough flow to deliver ammonia and oxygen to the biofilm and to prevent large stagnant zones, but contact must also be distributed effectively. In moving beds, media motion and aeration are part of the reactor's mixing system. In showers, the water should spread across the media rather than form a few concentrated streams. In fixed beds, poor distribution can leave large areas underused.

The best target is therefore not simply maximum turnover. It is adequate pond circulation combined with the hydraulic conditions required by each treatment stage. Measuring actual flow after pipe friction, fittings and static lift is far more useful than sizing from pump labels alone.

9. Maintenance: The Cleaning Rules Are Almost Opposite

Mechanical and biological filters should not be maintained in exactly the same way. Mechanical media is cleaned because captured waste is undesirable. Biological media is protected because the attached biofilm is the treatment process itself.

Mechanical filtration: clean early and thoroughly

Mechanical stages should be flushed before solids remain long enough to decay extensively or restrict flow. Automatic drum filters achieve this by washing the screen repeatedly. Sieves require routine rinsing. Settlement chambers need regular purging. Brushes, foam and static media need cleaning before accumulated waste becomes anaerobic, compacted or difficult to remove.

Biological filtration: preserve stability

A mature biological filter may occasionally need maintenance to remove excess sludge, dead biofilm or blocked passages, but the objective is not to sterilise every surface. Clean enough to restore flow and oxygenation while preserving a large active biofilm population. Avoid allowing a biofilter to sit stagnant for long periods without circulation or aeration, especially in warm water, because oxygen can be depleted quickly inside loaded media.

Chlorine and other disinfectants can damage the microorganisms that the biofilter depends on. Where fixed biological media must be rinsed, pond water or suitably dechlorinated water is generally preferable to exposing the entire mature biofilm to untreated chlorinated water. Exact cleaning methods depend on the media and reactor design, so manufacturer guidance and measured water quality should take priority over rigid routines.

After medication or a system interruption

Some fish treatments, disinfectants, prolonged power failures, severe oxygen depletion and major filter cleaning can reduce biological performance. After any event that may have disturbed the biofilter, test ammonia and nitrite more frequently and be prepared to reduce feeding until the system demonstrates stable processing again.

10. How to Diagnose Which Stage Is Failing

Troubleshooting becomes much faster when symptoms are matched to the job each filtration stage is supposed to perform. Water clarity, ammonia and nitrite should be evaluated separately rather than treated as one overall 'filter quality' score.

11. Real-World System Layouts

Example A: Modern gravity-fed koi pond

A common high-performance arrangement is bottom drains and skimmers feeding a gravity-fed RDF or sieve, followed by a dedicated moving-bed or shower biofilter, then pumps returning water to the pond. The mechanical stage receives relatively intact solids before the pump. The biofilter receives cleaner water, which helps preserve oxygen, media movement and hydraulic capacity.

This layout clearly separates the jobs and makes troubleshooting easier. If suspended solids rise, inspect the screen stage. If ammonia rises, inspect the biofilter and its operating conditions. The drawback is higher capital cost, more plumbing and the need for careful water-level design in gravity-fed systems.

Example B: Pump-fed retrofit

In an existing pond where bottom-drain plumbing cannot be rebuilt, a submersible or dry pump may send water to a sieve, pressurised filter or compact mechanical stage, followed by biological media and a return waterfall or shower. This can work well, but the pump may break soft solids into smaller particles before they are captured. Fine polishing and more frequent cleaning can therefore become more important.

Example C: Bead filter plus shower

A bead filter can provide substantial mechanical polishing and some biological activity, while a downstream shower provides a dedicated, oxygen-rich nitrifying stage and gas exchange. In this arrangement, the bead filter should be backwashed frequently enough that it does not become an organic-sludge store. The shower then receives cleaner water and can focus more effectively on biological treatment.

Example D: Dealer or high-stock holding system

Dealer tanks experience abrupt changes in biomass, repeated feeding, handling debris and sometimes frequent medication or quarantine procedures. For these systems, separating mechanical and biological treatment is particularly useful. Automatic solids removal, large aerated biofilters, easy isolation valves, bypass capability and redundant air/pumps can reduce the risk of a single maintenance event disrupting the entire life-support system.

12. Why UV Is Neither Mechanical nor Biological Filtration

UV clarifiers are often listed as part of the 'filter system', which can create confusion. A UV unit exposes passing water to ultraviolet radiation. In koi ponds it is commonly used to control free-floating algae and can also affect some microorganisms that pass through the unit, depending on dose and operating conditions. It does not physically remove faeces, leaves or suspended debris, and it does not provide the nitrifying biofilm needed to process the pond's ammonia load.

When UV damages or aggregates suspended algae, the resulting material still has to be removed or processed elsewhere. A pond can therefore have a powerful UV unit and still need excellent mechanical and biological filtration. UV should be viewed as an auxiliary treatment stage, not a replacement for either core process.

13. Common Myths About Mechanical and Biological Filtration

Myth 1: Clear water means the filter is healthy

False. Clear water mainly tells you that suspended particles and algae are controlled. Ammonia, nitrite, nitrate, dissolved organics, low oxygen or unstable pH can all exist in visually clear water.

Myth 2: More bio-media always means more filtration

Not necessarily. Additional media only helps when it is exposed to adequate oxygenated flow and when the reactor is not overloaded with solids. Packing media too tightly can reduce circulation and create unused zones.

Myth 3: Mechanical filters remove ammonia

Ordinary mechanical filters do not directly strain dissolved ammonia from pond water. Good solids removal can reduce future ammonia production from decomposing waste, but it does not replace a functioning biofilter.

Myth 4: Biological media should never be cleaned

Biological media should not be sterilised casually, but it may need controlled cleaning when sludge, dead biofilm or blocked passages impair flow and oxygenation. A dirty, stagnant biofilter is not automatically a healthy biofilter.

Myth 5: A combined filter is always inferior

Combined filters can work very well when loading is appropriate and maintenance is consistent. The limitation is that solids capture and biological stability impose different demands on the same media. Dedicated stages simply make those demands easier to optimise independently.

Myth 6: Pond volume is enough to size filtration

Volume helps determine circulation, but fish biomass and peak feeding rate are stronger indicators of waste production. Serious koi systems should be designed around both hydraulic demand and biological load.

14. Designing the Interface Between Mechanical and Biological Stages

Good filtration is not merely about buying two correct devices; the connection between them matters. Water leaving the mechanical stage should reach the biofilter with minimal bypass, sufficient oxygen and suitable flow distribution. Waste drains should be independent of clean-water returns. Valves should allow either stage to be isolated for service without forcing dirty water around the system.

Avoid hidden bypass paths

Water always follows the path of least resistance. A poorly sealed screen, overflowing chamber, internal gap around matting, or incorrectly set water level can allow a significant fraction of flow to avoid treatment. Because the pond may still circulate normally, bypass can go unnoticed until water quality or clarity deteriorates.

Keep waste drains genuinely separate

When an RDF, sieve or settlement chamber discharges waste, that dirty water should leave the recirculating loop. Waste that is flushed into a sump and then pumped back toward the pond has not been removed. The plumbing should make the direction of waste obvious and irreversible during normal operation.

Allow for maintenance without shutting down life support

Valves, unions, bypasses and drain points are not accessories. They determine whether a filter can be cleaned quickly and whether biological circulation can continue while a mechanical component is serviced. In valuable collections, redundancy is worth planning from the beginning because pumps, air systems and screens eventually need maintenance.

15. Practical Design Checklist

Before finalising a koi pond filtration layout, work through the following questions separately for the mechanical and biological stages.

Mechanical stage

• What is the real operating flow through the unit after plumbing head loss?
• Can the device accept that flow without bypass or excessive water-level change?
• What types of solids dominate: large faeces, leaves, fine suspended material or algae?
• Does the pump break solids up before they reach the separator?
• How frequently will the unit need cleaning or automatic washing at peak feeding?
• Can captured waste be discharged completely from the recirculating loop?
• Is the cleaning process easy enough that it will actually be done on schedule?
• Will a secondary polishing stage be needed for fines?

Biological stage

• What is the expected peak daily feed rather than just the pond volume?
• How much biological reserve exists for future fish growth or higher summer feeding?
• Does the media receive strong, even oxygenated flow?
• Is aeration sufficient for both koi and nitrification during warm weather?
• Are pH and alkalinity monitored so nitrification is not limited by water chemistry?
• Can sludge or dead biofilm be removed without destroying the entire mature biofilm?
• What happens to circulation and aeration during a power failure or pump service?
• Can the biofilter be isolated from medications or disinfectants if required?

Whole-system checks

• Do bottom drains, skimmers and returns move waste toward the filter instead of allowing dead zones?
• Is the mechanical stage upstream of the main biofilter wherever practical?
• Are actual ammonia and nitrite results used to judge biological performance?
• Is visible clarity evaluated separately from chemical water quality?
• Are waste drains, isolation valves and service access built into the layout?
• Is there room to expand filtration as the koi collection grows?

16. The Most Useful Mental Model: Remove First, Convert Second

A high-performance koi pond follows a simple logic. Waste that can be physically removed should be removed as early as practical. Waste that cannot be screened out, especially dissolved ammonia, should then be biologically converted under stable, oxygen-rich conditions. The cleaner the water entering the biological stage, the more of that stage's capacity can be devoted to nitrification rather than sludge handling and heterotrophic decomposition.

This 'remove first, convert second' model also makes future upgrades easier. If water clarity is poor but ammonia and nitrite are controlled, upgrade solids collection or polishing. If water is clear but nitrogen compounds are rising, increase or repair biological capacity and correct its operating environment. If both are poor, investigate circulation, loading and the entire sequence rather than adding random media.

17. Special Considerations for High-Stock and Dealer Systems

In a lightly stocked ornamental pond, the distinction between mechanical and biological capacity may be forgiving. In dealer ponds, quarantine systems and grow-out facilities, it becomes critical. Feeding can be intense, fish biomass may change overnight, and solids production can increase sharply after new fish arrive or feeding resumes. A filtration system that was stable yesterday can be overloaded today.

For these systems, mechanical filtration should be able to export solids continuously or very frequently, and biological capacity should be based on realistic peak feeding rather than average occupancy. Multiple tanks sharing one biofilter also introduce biosecurity considerations: excellent filtration does not prevent pathogens from moving through shared water. Isolation capability and operational discipline are therefore part of system design, not separate concerns.

Redundancy is also more important. A failed air pump can affect koi and nitrifying bacteria at the same time. A blocked mechanical stage can reduce total flow and starve the biofilter. Backup aeration, spare pumps, high-water or low-water alarms and emergency overflow paths can protect valuable stock when a single component fails.

Conclusion

Mechanical and biological filtration are not alternative ways to perform the same job. They are different treatment processes that handle different forms of waste. Mechanical filtration captures and exports particles. Biological filtration supports living microbial communities that oxidise dissolved ammonia and nitrite. One primarily removes matter; the other primarily changes its chemical form.

The strongest koi systems respect that distinction. They collect solids efficiently, remove them before prolonged decomposition, deliver cleaner oxygenated water to the biofilter, and size biological capacity around actual fish and feeding load. They also make maintenance simple enough that the system remains effective months and years after installation.

Once these two stages are understood separately, filter selection becomes much easier. Instead of asking whether an RDF, bead filter, moving bed or shower is 'the best filter', the better question is: which job does this component perform, how well does it perform that job under my load, and what other stage is needed to complete the system?

The next article in this series, FL-003, will examine biological filtration in greater depth and explain how biofilm, nitrification, media, oxygen, pH, alkalinity, temperature and loading determine real-world performance.