Aquaculture Wastewater Treatment for RAS Systems
Land based fish farming (aquaculture) is a growing industry as wild-caught fish are becoming depleted because of modern sophisticated methods. There are two types of tank growing operations. One is a flow through system where surface water continuously flows into and then out of a tank. With this method, there are concerns about biosecurity. The other method that is not as widely used, but is growing in popularity, is Recirculating Aquaculture Systems (RAS) where, like a fish tank in your home, the water is continuously filtered and treated to remove impurities. Although most water is reused, RAS systems generate aquaculture wastewater by periodically discharging purge water to control:
- Nitrate accumulation
- Dissolved solids buildup
- Fine organic accumulation
This purge stream is the primary wastewater of concern and what would need aquaculture wastewater treatment prior to discharge.
RAS Aquaculture Market – Small but Rapidly Growing
There are 6,500 commercial RAS systems worldwide. 850 new RAS systems were commissioned between 2023 and 2024. They are more likely to be located in areas with water scarcity, strict discharge limits, or limited coastal access. They are also used more often for hatcheries and nurseries for early life stage after which, the fish are moved to ponds or net pens. RAS systems are most economically viable when operated at high stocking densities.
Typical Purge Water Composition
| Parameter | Typical Range |
| TSS | 100 – 2,000 mg/L |
| VSS | 60–80% of TSS |
| Total Nitrogen (TN) | 20 – 150 mg/L |
| Ammonia‑N | <1–5 mg/L (usually low) |
| Nitrite‑N | <0.5–2 mg/L |
| Nitrate‑N | 20 – 120 mg/L |
| Total Phosphorus (TP) | 2 – 30 mg/L |
| Orthophosphate‑P | 1 – 15 mg/L |
| Particulate P | Bound to fine solids |
| COD | 100 – 1,500 mg/L |
| BOD₅ | 30 – 300 mg/L |
| TDS (freshwater RAS) | 300 – 2,000 mg/L |
Conventional Treatment Methods
a) Methods used for treatment of the purge water will vary depending on the discharge limits. For small or lightly regulated RAS systems this may be the only treatment: Gravity separation of heavier solids in basins or clarifier tanks or screen filters.
| Removed | Not Removed |
| Coarse settleable solids | Fine solids (<50 µm) |
| Large fecal particles | Dissolved nitrogen |
| Particulate phosphorus | Dissolved phosphorus |
b) Where simple solids removal is not sufficient, more advanced methods are used including flocculation or DAF systems.
| Removed | Not Removed |
| Suspended solids | Nitrate |
| Some COD | Dissolved nitrogen |
| Particulate phosphorus | Dissolved phosphorus |
c) When limits for Nitrogen apply, constructed wetlands (where land and climate allow), aerated lagoons, or biological denitrification reactors are used.
| Removed | Not Removed |
| BOD | Suspended solids |
| Nitrogen | All phosphorus |
| Some phosphorus |
Key Limitations of Conventional Methods (Why RAS Operators Struggle)
From a practical standpoint, conventional systems suffer from:
- Poor removal of fine colloidal solids
- No intrinsic nitrate removal without complex biology
- High sludge production
- Large footprint (for wetlands, basins)
- Chemical dependence
- Difficulty meeting tightening discharge limits
This is exactly why many RAS operators start with conventional systems—but later look for higher‑intensity separation technologies for aquaculture wastewater treatment.
Conventional Treatment vs VSEP® Membrane Filtration
VSEP® (Vibratory Shear Enhanced Processing) is a unique vibratory membrane filtration system. The fundamental distinction is how separation is achieved:
- Conventional methods rely on gravity, chemistry, or biology to transform or aggregate
- Membrane methods rely on a physical barrier to separatecontaminants by size or molecular weight.
This difference drives everything else: footprint, sludge generation, performance consistency, and regulatory confidence. The following observations are based on operating data from commercial RAS facilities and industrial membrane installations worldwide. This page was developed by senior process engineers with over 30 years of experience in membrane filtration, industrial wastewater treatment, and high‑shear separation systems.
Comparison at a Glance
| Aspect | Conventional Methods | VSEP Membrane Method |
| Solids removal | Moderate (coarse) | Excellent (fine & colloidal) |
| Phosphorus removal | Good with chemicals | Very high |
| Nitrogen removal | Complex (denitrification) | Diverted into Reject |
| Flow profile | High‑volume tolerant | Best – low‑flow, hi‑strength |
| Footprint | Large to very large | Small |
| Process stability | Variable | Highly consistent |
| Operator skill | Low–moderate | Moderate |
| CAPEX | Lower upfront | Higher upfront |
| OPEX | Chemicals, sludge hauling | Energy, membranes |
How VSEP® Should be Used in Practice
Conventional treatment is best used for:
- High‑flow
- Dilute wastewater
- Less strict discharge limits (especially phosphorus)
VSEP membrane treatment is best used for:
- Low‑flow
- Strong wastewater
- Very strict discharge limits
- Water reuse is valuable
- Land and labor are constrained
Conventional treatment is typically designed for discharge. VSEP treatment enables water recycling where reuse is desired for aquaculture wastewater treatment.
Hybrid Conventional + VSEP Membrane
In many RAS facilities, the optimal solution is a hybrid system:
- Existing conventional treatment handles bulk solids
- VSEP treats the majority of the flow
- Conventional systems treat the small VSEP concentrate stream
This approach minimizes capital risk while dramatically improving performance. Conventional systems become increasingly complex as standards tighten, and membranes are being evaluated as enabling technologies rather than luxury upgrades.
Bottom Line for Decision‑Makers
- Conventional methods are proven, familiar, and often sufficient for basic compliance.
- They struggle with fine solids, footprint, sludge, and nitrate complexity.
- Membranes excel at consistency, compactness, and fine solids/phosphorus removal.
For RAS operators under tightening discharge limits, membranes are increasingly viewed as the shortest path to predictable compliance.
VSEP RO Systems are Applied to Strong Wastewater with Strict Discharge Limits
The Vibratory VSEP membrane system is specifically designed for difficult effluents with tight discharge limits. It is not the right fit for dilute effluents or when simple conventional treatment can comply with the limits, but it is a good fit for more challenging wastewater.
Why VSEP Performs When Conventional Spiral RO Struggles
When spiral membrane systems are fighting unstable operation, the root cause is almost always the same: Spiral RO systems struggle with organics, suspended solids, and scaling species unless extensive pretreatment is used. Conventional Spiral RO use crossflow velocity to reduce fouling. But shear at the membrane surface decays to near zero due to the narrow feed spacer and this results in rapid fouling and scaling at the membrane face. This often creates a need for prefiltration, chemical dosing, softening, or antiscalants.
VSEP Shear at the Membrane Surface is Why it Works
VSEP exists to eliminate those limitations and requires no pretreatment other than coarse solids removal prior to aquaculture wastewater treatment. The VSEP membrane module oscillates 50 times per second, generating extremely high shear directly at the membrane surface. This allows for:
- Little or no fouling
- Solids remain in suspension
- Allows VSEP to run on waters that spiral RO simply cannot tolerate.
VSEP maintains flux, stability, and uptime even on high‑TSS, high‑organic, or viscous feeds.
Case Study – Shrimp Farm
In 2016, New Logic provided a demonstration VSEP machine to a shrimp aquaculture operation in Minnesota. In this RAS operation, the difficulty was in removing very small colloids that would not settle or be removed by conventional methods. The VSEP machine using a tight ultrafiltration (UF) membrane (5 kDa) operated at 98% recovery leaving only 2% of the volume as concentrated solids. The UF filtrate contained zero suspended solids. The process required treatment of 2000 gallons per day (gpd) and so a small VSEP machine was used to recycle water with no turbidity or TSS.
VSEP UF Demonstration Equipment for Shrimp Wastewater
Grab Samples Taken Showing the UF Filtrate in the Middle
Pilot Testing and Performance Projections
New Logic offers pilot testing—including full‑scale demonstrations—to evaluate wastewater treatment performance. Pilot testing is needed to design a permanent full-scale system
Pilot testing allows:
- Verification of treated water quality
- Measurement of flux, recovery, and fouling behavior
- Accurate operating cost projections
- Optimization for reuse or ZLD configurations
Get Started
If you are evaluating ways to improve sustainability, recycle water, minimize wastewater risk for aquaculture wastewater treatment —VSEP can help.
- Request a Performance Projection: info@vsep.com
- Learn about Pilot Testing Options: https://www.vsep.com/technology/pilot-testing/
- Learn More About the Technology: https://www.vsep.com/technology/
About VSEP and New Logic Research
- More than 450 industrial VSEP systems installed globally
- Technology covered by U.S. and international patents
- Used in food, beverage, wastewater, and agricultural applications
- Engineering team with chemical, metallurgical, and mechanical engineering backgrounds
This page was prepared by New Logic Research’s senior process engineers with backgrounds in membrane process engineering. Our team has over 30 years of direct experience designing, building, and piloting high‑shear membrane systems for industrial applications.
Regulated Industries Served:
- Mining
- Food and Beverage
- Drinking Water
- Refineries
- Oil and Gas
Frequently Asked Questions (FAQ)
What is purge water in a Recirculating Aquaculture System (RAS)?
Purge water is the small fraction of water periodically discharged from a RAS to control the buildup of nitrate, dissolved solids, and fine organic materials that cannot be fully removed within the recirculation loop. Although purge flows are low, contaminant concentrations are typically high and require aquaculture wastewater treatment prior to discharge or reuse.
Why can’t conventional solids removal fully treat RAS purge water?
RAS purge water is dominated by fine colloidal solids and dissolved nutrients, which do not settle well and are only partially removed by screening, clarification, or DAF. As discharge limits tighten, conventional systems often require multiple treatment steps, chemical addition, and large footprints to achieve compliance.
When is membrane treatment necessary for RAS wastewater?
Membrane treatment is typically evaluated when:
- Discharge limits for phosphorus or nitrogen are strict
- Fine suspended solids persist after conventional treatment
- Water reuse is desired to reduce intake and discharge volumes
- Land, labor, or sludge disposal options are limited
Membranes are especially effective for low‑flow, high‑strength purge streams, which are common in RAS facilities.
Does VSEP remove nitrogen from wastewater?
VSEP membrane systems do not biologically destroy nitrogen. Instead, they physically separate and concentrate dissolved and particulate constituents, including nitrogen species, into a smaller reject stream. This enables improved nitrogen management through reduced discharge volume, reuse strategies, or downstream biological treatment where required.
How is VSEP different from conventional spiral‑wound RO systems?
Conventional spiral RO systems rely on crossflow velocity to limit fouling, which decays near the membrane surface. VSEP uses high‑frequency vibration to generate high shear directly at the membrane face, allowing operation on wastewater containing higher levels of suspended solids, organics, and scaling species with minimal pretreatment.
Can VSEP be used with existing conventional treatment systems?
Yes. Many RAS facilities use VSEP as part of a hybrid treatment approach, where conventional processes remove bulk solids and VSEP treats the majority of the purge flow. The concentrated reject stream can then be managed using existing infrastructure, minimizing capital investment and operational disruption.
Is VSEP suitable for all aquaculture operations?
No. VSEP is not intended for high‑flow, dilute effluent where simple conventional aquaculture wastewater treatment already meets discharge requirements. It is best applied to challenging wastewater streams with tight discharge limits, where advanced separation or water reuse provides measurable economic and regulatory benefits.