Marine recirculating aquaculture systems need to move water, capture solids, support nitrification, add oxygen, and strip carbon dioxide. Pairing a low-head PolyGeyser bead filter with an airlift pump can combine those jobs into a compact, low-water-loss treatment train.
Educational summary adapted from a 2005 article by Ronald F. Malone and Sripavani Gudipati on PolyGeyser/airlift filtration for recirculating aquaculture systems.
Key takeaways
- RAS treatment has five core jobs. A recirculating system must circulate water, capture solids, biologically convert ammonia, add oxygen, and remove carbon dioxide.
- PolyGeyser filters combine solids capture and biofiltration. These floating bead filters can operate as bioclarifiers, meaning one unit can help clarify water and support nitrifying bacteria.
- Airlifts complete the low-head treatment train. A properly designed airlift can circulate water while also adding oxygen and stripping carbon dioxide.
- Low head loss is the key design constraint. Airlifts can move large amounts of water, but they cannot lift water very high. Tank elevations, piping, media selection, and filter backwashing must be designed around that limitation.
- This approach is especially attractive for marine hatchery, broodstock, and fingerling systems. It minimizes water loss and can reduce mechanical complexity, but heavily loaded growout systems may still need supplemental aeration or degassing.
Why low-head filtration matters in marine RAS
Marine recirculating aquaculture systems are often used where water quality, biosecurity, and water conservation matter. Hatcheries and broodstock facilities may need to hold animals for long periods, reduce pathogen exposure, and limit the discharge of nutrient-rich saltwater. A simpler, lower-water-loss filtration strategy can make those systems easier to operate and easier to isolate.
The article focuses on a practical pairing: a self-washing floating bead filter called a PolyGeyser and an airlift pump. The PolyGeyser handles solids capture and biofiltration. The airlift moves water while providing gas exchange. Together, the two devices can address the five basic operations of a recirculating system in a streamlined layout.
The five jobs every recirculating system must handle
A recirculating aquaculture system is not just a tank and a pump. It is a water treatment loop. In the PolyGeyser/airlift approach, two pieces of equipment divide the five essential jobs:
| RAS function | What it protects against | Primary device in this approach |
|---|---|---|
| Water circulation | Stagnation, poor mixing, uneven oxygen and waste distribution | Airlift pump |
| Solids capture | Uneaten feed, feces, turbidity, organic loading | PolyGeyser floating bead filter |
| Biofiltration | Ammonia and nitrite accumulation | PolyGeyser bead bed and biofilm |
| Oxygen addition | Low dissolved oxygen stress | Airlift pump |
| Carbon dioxide removal | CO2 buildup, pH stress, reduced gas transfer | Airlift pump |
What a PolyGeyser filter does
A PolyGeyser is part of the floating bead filter family. In a bead filter, plastic media floats under a screen to form a granular bed. As water passes through the bed, larger particles are strained or settle behind beads, medium particles are intercepted, and small particles can attach to the biofilm on bead surfaces.
The same bead surfaces also support bacteria. Heterotrophic bacteria grow on organic solids, while nitrifying bacteria convert ammonia to nitrite and nitrite to nitrate. When a bead filter is sized and managed for both clarification and nitrification, it is often described as a bioclarifier.
How the self-washing cycle works
The PolyGeyser design described in the article has four main internal zones: the bead bed, drop chute, charge chamber, and sludge basin. A slow feed of air fills the inverted charge chamber. When the chamber fills, a trigger releases the captured air through the bead bed. The burst of air rapidly mixes the beads, releases trapped solids, and pushes dirty wash water into the chamber and drop zone.
The backwash event takes only about 10 to 15 seconds, so influent flow does not need to be interrupted. Between backwash events, wash water settles and solids collect in the sludge basin. Water loss is minimal because the filter recycles its own backwash water; water is mainly removed when sludge is intentionally withdrawn.
Why marine systems benefit from this design
Marine systems can be hard on equipment, especially metal parts. The article highlights that PolyGeyser backwashing does not require electronic controls, moving mechanical parts, or metal components inside the washing mechanism. That simplicity can be valuable in saltwater hatchery and nursery settings.
Why gentle, frequent backwashing can improve performance
Backwashing is not just a cleaning step. It shapes the biofilm that performs nitrification. If backwashing is too aggressive or too frequent, nitrifying bacteria can be stripped away faster than they regrow. If washing is too infrequent, the biofilm and trapped solids can become too thick, reducing flow and limiting ammonia transfer into the nitrifying layer.
The source article summarizes earlier studies showing that frequent but gentle backwashing can improve nitrification. One cited dataset for enhanced nitrification media showed that, as washing increased from 2 to 12 backwashes per day, measured head loss fell from 19.7 inches to 8.0 inches and nitrite-N fell from 0.48 mg/L to 0.09 mg/L. In that test, more frequent washing reduced clogging while preserving enough protected biofilm for nitrification.
Enhanced nitrification media supports this strategy by increasing bed porosity and providing protected pockets where nitrifiers can survive backwashing. The article notes that this media can nearly double nitrification capacity when managed properly and can also reduce head loss enough to make airlift operation practical.
What an airlift pump contributes
An airlift pump works because an air-water mixture is less dense than water alone. When air is injected near the bottom of a submerged pipe, the lighter mixture rises and spills out above the water surface. In a RAS, that motion can move water without a conventional water pump.
The advantage is that one air delivery system can help perform three jobs at once: circulation, oxygen addition, and carbon dioxide stripping. Airlifts also have few moving parts, can be simple to maintain, and can be backed up by emergency air delivery during outages.
| Airlift advantage | Why it matters |
|---|---|
| Lower energy potential | When the system is designed for very low lift, air can circulate and aerate water efficiently. |
| Reduced equipment complexity | The water pump and some secondary aeration or stripping equipment may be reduced or eliminated in suitable systems. |
| Reliability | Fewer moving parts can reduce routine upkeep and mechanical failure points. |
| Biosecurity and isolation | Multiple isolated systems can be served by centralized air delivery while keeping water loops separate. |
| Backup planning | Emergency air delivery can support circulation and gas exchange during power interruptions. |
The tradeoff: airlifts are low-lift devices
Airlifts can move substantial water, but they cannot push against high pressure. They require an integrated low-head design. Retrofitting an airlift into a high-head system is often difficult because pipe friction, filter head loss, and elevation differences can quickly exceed what the airlift can overcome.
The design principle: keep head loss low
The article explains that conventional pump-driven bead filter systems historically operated at much higher pressures. Airlifted systems must be designed differently. For a PolyGeyser/airlift pairing, the filter, screen placement, drain lines, pipe velocities, and air injection depth all need to be selected so the airlift only has to overcome a small lift.
In the examples discussed, airlifted PolyGeyser systems often operated with cumulative head loss from tank to filter effluent in the range of about 6 to 15 inches. That range is compatible with air injection depths around 48 to 60 inches in the configurations described by the authors.
6-15 in.
Typical cumulative head loss range reported for airlifted PolyGeyser applications.
48-60 in.
Common air injection depth range discussed for compatible airlift configurations.
10-15 sec.
Approximate duration of a PolyGeyser backwash event described in the article.
How the combined treatment train works
In the described layout, water leaves the culture tank and enters the PolyGeyser above the screen plate. It is distributed below the floating beads, then moves upward through the bead bed where solids are captured and ammonia is biologically converted.
The filtered water then enters a drop pipe that creates a pressure column feeding the airlift tube. Air is injected below the water surface. The rising air-water mixture returns water to the tank, while also adding oxygen and stripping carbon dioxide. In effect, the water completes a loop that clarifies, biofilters, circulates, aerates, and degasses in one integrated circuit.
The article emphasizes that the physical placement of the filter relative to tank water level is critical. In many configurations, the effluent screen or pipe is placed 12 to 18 inches below the tank water level so there is enough pressure to move water through the screens, drains, and bead bed under worst-case conditions.
Design rules from the article
The authors present simplified interim design rules for PolyGeyser/airlift combinations. These are useful as educational planning concepts, but final designs should be checked against current manufacturer guidance, site-specific hydraulics, species requirements, and professional engineering review.
| Parameter | Rule of thumb from the article | Plain-language meaning |
|---|---|---|
| Flow, Q | Defined by system needs | Start with the turnover rate, animal load, and water-quality target. |
| Lift, L | About 3-15 inches | Keep the vertical lift small enough for an airlift to handle. |
| Airlift pipe sizing | About 450 gpm/ft2, or 1 ft/sec apparent water velocity | Select pipe diameter so water velocity in the lift tube stays conservative. |
| Approach and drain piping | About 900-1350 gpm/ft2, or 2-3 ft/sec | Avoid excessive friction losses before water reaches the lift. |
| Injection depth, S | About 4 x L | Place air injection deep enough below the dynamic water level to create lift. |
| Air delivery | 0.275 x Q, with Q in gpm and airflow in cfm | Estimate blower flow using a conservative gas-to-liquid ratio. |
| Air delivery pressure | S + L, in inches of water | Size the blower for both injection depth and system losses. |
Educational note: These values summarize the article’s interim design guidance. They are not a substitute for detailed hydraulic design or updated manufacturer specifications.
Example from the article
For a 40 gpm target flow using a four-inch airlift with about four feet of air injection depth and roughly 10-12 inches of lift, the authors estimate an airflow requirement of about 11 cfm and an injection pressure of about 60 inches of water.
Where this approach fits best
Airlifted PolyGeysers are presented as a strong fit for marine systems where water loss, equipment simplicity, and biosecurity are priorities. They are especially relevant for broodstock and fingerling systems, where water quality and reliability can be more important than maximum production intensity.
| Application | Fit for airlifted PolyGeyser approach | Design caution |
|---|---|---|
| Marine broodstock | Strong fit because water quality, biosecurity, and stable long-term operation are valuable. | Monitor TAN, nitrite, dissolved oxygen, CO2, pH, and alkalinity closely. |
| Fingerling and nursery systems | Strong fit for compact treatment with minimal water discharge. | Design conservatively for sensitive life stages and startup acclimation. |
| Growout systems | Possible in suitable low-head layouts. | Heavy loading may require supplemental aeration and degassing before biofiltration capacity is reached. |
| Retrofits | Case-by-case. | Existing high-head piping, elevations, and filters may limit feasibility. |
Design checklist for educational planning
Use this checklist to frame early discussions before detailed engineering:
- Define the biological load: estimate feed rate, animal biomass, species sensitivity, and target TAN and nitrite levels.
- Set the water flow target: determine turnover and recirculation flow before sizing the airlift and filter.
- Protect low-head operation: minimize pipe friction, avoid unnecessary elevation changes, and keep screens and media clean.
- Place the filter correctly: confirm that tank water level, filter screen elevation, and air injection depth work together hydraulically.
- Plan backwashing and sludge removal: frequent automatic backwashing can maintain low head loss, but settled sludge still needs removal.
- Size air delivery for both flow and pressure: blower selection must satisfy cfm and pressure requirements at the operating point.
- Monitor gas transfer limits: confirm that oxygen addition and CO2 stripping are sufficient, especially in high-density systems.
- Build in redundancy: protect animals with alarms, backup air, emergency oxygen, and a clear response plan.
Glossary
Airlift pump: A pump that uses injected air to create a rising air-water mixture and move water with little or no mechanical pumping.
Bead filter: A filter that uses floating plastic media to capture solids and support biofilm growth.
Bioclarifier: A treatment unit that performs both solids clarification and biological filtration.
Enhanced nitrification media: Bead media designed to improve porosity and protect nitrifying bacteria during washing.
Head loss: The loss of hydraulic energy caused by elevation, friction, screens, media, and other restrictions.
Lift: The vertical distance or pressure the airlift must overcome to move water back into the tank or next process.
Nitrification: The microbial conversion of ammonia to nitrite and then nitrate.
Submergence: The depth of air injection below the water surface or dynamic water level in an airlift system.
TAN: Total ammonia nitrogen, a measurement that includes unionized ammonia and ionized ammonium.
Frequently asked questions
What is the main advantage of pairing a PolyGeyser with an airlift?
The combination lets two devices address five major RAS treatment functions: the PolyGeyser captures solids and supports nitrification, while the airlift circulates water, adds oxygen, and removes carbon dioxide.
Does an airlifted PolyGeyser eliminate the need for a water pump?
In a properly designed low-head system, the airlift can provide the main recirculation flow. However, the system must be designed around airlift limitations from the beginning.
Why is frequent backwashing helpful?
Frequent, gentle backwashing can keep the bead bed open, reduce head loss, and maintain a thinner biofilm that allows ammonia to reach nitrifying bacteria. The goal is to clean the bed without stripping away too much nitrifying biofilm.
Is this approach suitable for every growout system?
Not necessarily. Heavy growout loads may exceed the airlift’s gas transfer capacity before the PolyGeyser’s biofiltration capacity is reached. Supplemental aeration or degassing may be needed.
Can this be added to an existing high-pressure filtration system?
Sometimes, but it is not a simple drop-in retrofit. Airlifts require low head loss and careful elevation control, so existing tank drains, pipe runs, filters, and water levels must be evaluated.