Industry Solutions

How to Size a Rotary Drum Filter for RAS: Flow, Screen Micron, Solids Load and Backwash

Size an RAS drum filter from flow, peak load, solids, screen micron and backwash duty - not flow rate alone. Use this practical engineering checklist.

A rotary drum filter is one of the most important solids-removal units in a recirculating aquaculture system.
It is also easy to size incorrectly if selection is based only on nominal flow.
Two RAS projects may both circulate 100 m³/h and still require different drum-filter configurations because they differ in:
  • fish biomass;
  • feed rate;
  • solids production;
  • screen micron;
  • water level;
  • peak loading;
  • available backwash pressure;
  • redundancy;
  • and cleaning strategy.
The useful sizing question is therefore not:
“What drum filter handles 100 m³/h?”
It is:
“What hydraulic and solids load must this filter handle at the selected screen opening under real operating conditions?”
This article provides a practical data checklist for RAS operators, integrators and engineers evaluating a rotary drum filter.

Start With System Flow, but Do Not Stop There

The first number normally provided is recirculation flow.
That is necessary, but it is not enough.
Collect:
  • normal recirculation flow;
  • maximum recirculation flow;
  • short-term peak flow;
  • number of parallel lines;
  • bypass flow;
  • operating water level;
  • allowable upstream water-level rise;
  • allowable downstream water-level drop;
  • redundancy requirement.
The filter must pass the required hydraulic flow without creating an unacceptable head difference or frequent emergency bypass.
A filter selected too close to its nominal hydraulic limit may become unstable when the screen becomes partially loaded with solids.

1. Define the Actual RAS Flow Through the Filter

Not every pump flow necessarily passes through one drum filter.
Confirm the process arrangement.
Ask:
  • Is the filter on the full recirculation flow?
  • Is only a side stream filtered?
  • Are there multiple filters in parallel?
  • Is there a bypass?
  • Does the flow change with feeding or tank operation?
  • Will future biomass increase raise recirculation flow?
Use the actual design flow through the filter rather than total pump nameplate capacity if the process arrangement is different.

Average Flow vs Peak Flow

Average flow is useful for normal duty.
Peak flow matters for:
  • water-level stability;
  • hydraulic loading;
  • bypass risk;
  • screen differential;
  • emergency operation.
Where pumps are variable-speed, document the full operating range.

2. Estimate the Solids Load

Hydraulic flow determines how much water reaches the screen.
Solids load determines how quickly the screen becomes covered.
Useful information includes:
  • daily feed;
  • biomass;
  • feed conversion data where available;
  • expected fecal solids;
  • uneaten feed risk;
  • TSS entering the filter;
  • tank-cleaning events;
  • grading or harvesting events;
  • sludge recirculation or side-stream return;
  • production peaks.
In many RAS systems, feed rate is one of the most useful practical indicators of solids production.
A filter handling moderate flow with a high solids load may backwash more frequently than a filter handling the same flow with a lighter solids load.
Do not compare filters by flow rating alone.

3. Select Screen Micron From the Process Objective

Smaller screen openings can capture finer solids, but they also increase hydraulic resistance and backwash demand.
Larger openings generally pass water more easily but allow more fine solids downstream.
The selection should therefore start from the process objective.
Ask:
  • What solids need to be removed?
  • Is fine-solids control important for fish health or water clarity?
  • Is the filter protecting another unit?
  • What screen opening is practical for the expected solids load?
  • How much backwash capacity is available?
A finer screen is not automatically better.
If the selected micron is too fine for the solids load and hydraulic duty, the result may be:
  • frequent backwashing;
  • higher backwash-water use;
  • larger pressure demand;
  • rapid screen loading;
  • unstable upstream water level;
  • increased maintenance.
The correct screen opening is a balance between capture performance and operating stability.

4. Check Screen Open Area, Not Just Drum Diameter

Two drum filters with similar outside dimensions may not provide the same effective filtration area.
Review:
  • screen surface area;
  • submerged area;
  • effective open area;
  • panel layout;
  • drum diameter;
  • drum length;
  • screen support structure;
  • internal flow path.
The water only passes through the portion of the screen that is hydraulically active.
As solids accumulate, the usable open area temporarily decreases until the backwash cycle restores it.
That is why effective filtration area matters.

5. Understand Water-Level Control

Many automatic drum filters use water-level difference to trigger drum rotation and backwashing.
As solids accumulate on the screen:
1. hydraulic resistance increases; 2. the upstream water level rises or the downstream level falls; 3. the level sensor detects the difference; 4. the drum rotates; 5. the backwash nozzles clean the screen; 6. captured solids are discharged to the waste trough.
For reliable operation, confirm:
  • normal upstream water level;
  • normal downstream water level;
  • trigger level;
  • maximum allowable differential;
  • sensor arrangement;
  • emergency overflow/bypass level.
Poor hydraulic layout can cause frequent cycling even when the filter itself is not undersized.

6. Size the Backwash System With the Filter

A drum filter cannot operate correctly if the screen cannot be cleaned.
Backwash design should include:
  • required water pressure;
  • available water pressure;
  • backwash flow;
  • nozzle quantity;
  • nozzle condition;
  • nozzle orientation;
  • pump capacity;
  • backwash-water source;
  • solids discharge route.
If site pressure is insufficient, a dedicated backwash pump may be required.
If the screen opening is very fine or the solids are sticky, cleaning demand may increase.
Backwash should therefore be considered part of the filter system, not an accessory added after the drum is selected.

7. Check What Happens to the Backwash Waste

The drum filter removes solids from the recirculating water, but it creates a concentrated waste stream.
That stream must be handled.
Review:
  • backwash-water volume;
  • waste trough discharge;
  • sludge collection;
  • gravity drainage;
  • sludge thickening;
  • disposal or recovery route.
In larger RAS facilities, the solids-management line can become a significant part of the overall wastewater design.
A good drum filter improves the RAS loop only if the captured solids are removed from the system efficiently.

8. Consider Redundancy

A single filter may be acceptable for a small system, but larger commercial RAS projects often need to consider redundancy.
Questions include:
  • Can production continue if one filter is offline?
  • Is there a bypass?
  • Can two smaller filters operate in parallel?
  • Can one unit carry partial load during maintenance?
  • How long can the RAS operate without mechanical solids removal?
For high-value fish production, the cost of downtime may be more important than the cost difference between one and two filter units.
Redundancy should therefore be decided from production risk, not equipment price alone.

9. Check Materials and Water Chemistry

Material selection should match the water.
Important factors include:
  • freshwater or seawater;
  • salinity;
  • chloride exposure;
  • cleaning chemicals;
  • temperature;
  • structural requirements;
  • corrosion environment.
Common construction choices may include combinations of:
  • PP;
  • stainless steel;
  • corrosion-resistant screens;
  • suitable seals and fittings.
Do not assume that one material specification is appropriate for every RAS.
Marine systems require particular attention to corrosion resistance.

10. Check Solids Characteristics

Not all solids behave the same way on a screen.
Consider whether the solids are:
  • soft fecal particles;
  • fibrous;
  • oily;
  • sticky;
  • fragile;
  • large feed particles;
  • fine suspended solids.
Some solids form a filter cake quickly.
Others pass through or smear across the screen.
The screen opening, backwash system and operating cycle should match the actual waste characteristics.

A Practical RAS Drum Filter Sizing Checklist

Data Group
Information to Collect
Why It Matters
Flow
Normal, maximum and peak flow
Hydraulic capacity
Biomass
Current and future fish biomass
Solids-production basis
Feed
Daily and peak feeding rate
Practical solids-load indicator
TSS
Influent TSS where available
Screen loading
Screen Micron
Required capture size
Hydraulic resistance and solids capture
Water Levels
Upstream, downstream, allowable differential
Automatic control and head loss
Backwash
Pressure, flow, pump, water source
Screen cleaning
Solids
Fecal, feed, fibrous, oily, sticky
Screen and cleaning behavior
Materials
Freshwater/seawater, corrosion conditions
Material selection
Redundancy
Duty/standby, parallel operation
Production reliability
Waste Handling
Backwash drain, sludge collection
Solids-management integration

A Better Selection Workflow

Step 1 — Confirm Hydraulic Duty

Define:
  • normal flow;
  • peak flow;
  • future flow;
  • water levels;
  • redundancy.

Step 2 — Define Solids Duty

Estimate:
  • biomass;
  • feed;
  • TSS;
  • solids characteristics;
  • peak solids events.

Step 3 — Select the Screen Objective

Define what particle size needs to be retained and why.

Step 4 — Check Filtration Area

Confirm that the effective filtration area is appropriate for both hydraulic and solids loading.

Step 5 — Verify Backwash

Check pressure, flow, pump capacity and waste discharge.

Step 6 — Verify Materials

Match the construction to freshwater, seawater and chemical conditions.

Step 7 — Check the Solids Line

Confirm where the captured sludge goes after backwashing.

Step 8 — Review Redundancy

Decide how the RAS operates during maintenance or equipment failure.

Common Drum Filter Selection Mistakes

Selecting From Flow Alone

The same flow can carry very different solids loads.
Include feed, biomass and TSS where available.

Choosing the Finest Screen Available

Finer screening may improve solids capture but can also increase cleaning frequency and hydraulic resistance.
Choose micron size for the process objective.

Ignoring Peak Flow

A filter that works at average flow may become unstable during pump peaks or future expansion.

Ignoring Backwash Pressure

Insufficient pressure can turn a correctly sized drum into a poorly performing filter.

Ignoring the Waste Stream

Captured solids still need collection, thickening, dewatering or disposal.

Ignoring Future Biomass

RAS facilities often increase stocking or production after commissioning.
Where expansion is planned, include it in the filter basis.

Example: Same Flow, Different Filter Duty

Consider two RAS facilities, each circulating the same water flow.

System A

  • moderate biomass;
  • moderate feed;
  • larger screen opening;
  • stable solids load;
  • strong backwash pressure.

System B

  • higher biomass;
  • higher feed;
  • finer screen;
  • more variable solids loading;
  • limited backwash pressure.
Even with identical hydraulic flow, System B may require:
  • more filtration area;
  • stronger backwash;
  • more frequent cleaning;
  • additional redundancy;
  • or a different screen strategy.
This is why flow rating alone is not enough.

When Should You Consider a Finer Screen?

A finer screen may be worth evaluating when:
  • fine-solids control is important;
  • downstream units need better protection;
  • water-clarity targets are tighter;
  • solids loading is manageable;
  • adequate backwash is available;
  • additional filtration area can support the hydraulic duty.

When Can a Finer Screen Become a Problem?

A finer screen can become difficult to operate when:
  • solids load is high;
  • solids are sticky;
  • backwash pressure is weak;
  • filtration area is limited;
  • peak flow is high;
  • maintenance access is poor.
The right screen is therefore the one that meets the solids-removal objective while remaining hydraulically stable.

What Data Should You Send for Preliminary Drum Filter Selection?

Provide:
  • system recirculation flow;
  • maximum flow;
  • fish species;
  • current biomass;
  • future biomass;
  • daily feed;
  • peak feed;
  • influent TSS where available;
  • preferred screen micron;
  • freshwater or seawater;
  • upstream and downstream water levels;
  • available backwash pressure;
  • backwash-water source;
  • required redundancy;
  • existing tank or pipe dimensions;
  • available installation space;
  • photos or process drawings.
This information provides a much stronger basis than selecting only from a catalog flow rating.

Technical Basis

Mechanical microscreen filtration is widely used in recirculating aquaculture systems to remove suspended solids before they break down into finer particles and dissolved waste.
RAS engineering guidance commonly treats solids removal as a system function involving recirculation flow, solids production, screen size, hydraulic head and backwash performance rather than a single nominal flow rating.
That engineering principle leads to a simple selection rule:
size the drum filter for both water flow and solids duty at the selected screen opening.

Need Help Selecting an RAS Rotary Drum Filter?

Send HydroNovus the recirculation flow, peak flow, biomass, feed rate, TSS if available, preferred screen micron, water levels, backwash conditions, freshwater or seawater information, and the required redundancy.
HydroNovus can provide a preliminary rotary drum filter selection review covering:
  • hydraulic duty;
  • screen micron;
  • filtration area;
  • solids loading;
  • backwash requirements;
  • materials;
  • redundancy;
  • and sludge-discharge considerations.
Final filter sizing and performance should be confirmed using the actual RAS operating data and project-specific hydraulic conditions.