Industry Solutions

RAS Sludge Thickening After Drum Filtration: Settling, DAF or Direct Dewatering?

Drum-filter backwash is not finished sludge. Compare gravity thickening, DAF and direct dewatering for RAS solids before choosing the next treatment step.

A rotary drum filter removes suspended solids from recirculating aquaculture water, but filtration is only the first step in solids management.

The backwash stream leaving the drum is much smaller than the main recirculation flow and contains the captured solids. However, it is still a liquid sludge stream—not a final dewatered cake.

Depending on the RAS, the backwash may contain:

fish feces;

uneaten feed;

fine suspended solids;

biofilm fragments;

organic floc;

dissolved nutrients;

wash water;

and, in some systems, additional solids from side-stream treatment.

The next engineering question is therefore:

What should happen to the drum-filter backwash after the solids have been captured?

For some systems, simple gravity thickening may be enough.

For others, flotation, mechanical thickening or dewatering may be more suitable.

The correct choice depends on solids concentration, settling behavior, sludge stability, operating scale and the final disposal route.

Drum Filtration and Sludge Thickening Are Different Duties

A drum filter is primarily a solids-capture device.

Its job is to separate suspended particles from the high-flow RAS water stream while allowing most of the water to return to the system.

Thickening has a different objective:

reduce sludge volume by increasing the solids concentration before storage, transport, dewatering or disposal.

Dewatering goes further.

Its objective is to remove enough water to produce a much more concentrated, often handleable solids phase.

These three functions should not be confused:

capture → thicken → dewater

Not every RAS needs all three stages, but larger systems should evaluate each one separately.

Why Drum-Filter Backwash Is Often Still Too Dilute

The solids captured on a drum screen are removed with wash water.

That creates a backwash stream whose concentration depends on:

influent solids load;

screen micron;

filtration area;

backwash frequency;

backwash duration;

wash-water flow;

feed rate;

fish biomass;

solids characteristics;

hydraulic loading.

A filter that backwashes frequently may produce a larger volume of more dilute sludge.

A filter operating with longer intervals between cleaning cycles may produce a smaller but more concentrated backwash stream, although excessive solids retention on the screen is not desirable.

There is therefore no universal backwash-solids concentration that can be assumed for every RAS.

The actual stream should be measured.

Start With a Mass Balance

Before selecting thickening equipment, collect two basic values:

backwash volume per day;

solids concentration in the backwash.

From these, estimate the dry-solids mass.

A simplified relationship is:

Dry solids load = sludge volume × solids concentration

Use consistent units.

This calculation helps answer whether the plant is dealing with:

a large hydraulic volume with a low solids mass;

a small concentrated sludge stream;

or a genuinely high dry-solids load that justifies mechanical dewatering.

Without this mass balance, equipment can easily be oversized or selected for the wrong duty.

1. Measure the Backwash Stream Directly

Do not estimate sludge concentration from the drum-filter model alone.

Measure or sample:

average backwash volume;

peak backwash flow;

TSS or total solids;

settleable solids;

sludge pH;

temperature;

salinity where relevant;

visible particle characteristics;

sludge age before treatment.

Where operating conditions vary, sample during more than one production period.

Useful comparisons include:

normal feeding;

peak feeding;

tank cleaning;

harvesting or grading;

high-biomass periods.

The backwash stream can change significantly even when recirculation flow remains stable.

2. Check Whether the Solids Settle

Settling behavior is one of the most useful first tests.

Place a representative sludge sample in a transparent cylinder or settling column and observe:

how quickly solids separate;

whether a clear supernatant forms;

whether the sludge compacts;

whether solids remain suspended;

whether part of the sludge floats;

whether the sludge becomes difficult to settle after storage.

If the solids settle readily, gravity thickening may be the simplest next step.

If the solids remain fine and suspended, another process may be needed.

Option 1 — Gravity Thickening

Gravity thickening is often the first option to evaluate because RAS solids can be fragile and are best removed from the water with minimal additional breakdown.

Possible equipment includes:

sludge cones;

settling tanks;

radial-flow settlers;

conical-bottom tanks;

dedicated gravity thickeners.

The basic process is simple:

1. backwash enters the thickener; 2. solids settle; 3. clarified supernatant is removed; 4. concentrated sludge is withdrawn from the bottom.

When Gravity Thickening Can Work Well

It may be suitable when:

solids settle reliably;

footprint is available;

sludge flow is moderate;

operation can be kept relatively quiescent;

the final target is only pre-concentration;

downstream dewatering will occur later.

What Can Limit Gravity Thickening

Performance may become difficult when:

particles are extremely fine;

sludge starts to float;

gas formation lifts settled solids;

sludge is stored too long;

hydraulic surges disturb the settled blanket;

salinity or wastewater characteristics affect settling.

For RAS applications, sludge should generally be removed and managed before excessive biological degradation breaks particles into finer material or releases nutrients back into the water.

Option 2 — DAF for RAS Sludge Thickening

Dissolved air flotation can be considered when solids do not settle well or when the project needs flotation rather than gravity sedimentation.

DAF creates fine bubbles that attach to suspended material and carry it to the surface.

For RAS sludge, it may be worth investigating when:

solids are light;

part of the sludge tends to float;

fine suspended material remains after gravity testing;

footprint is limited;

chemical conditioning can improve separation.

However, DAF is not automatically the next step after every drum filter.

It adds:

recycle/pressurization equipment;

energy demand;

controls;

surface skimming;

possible coagulation/flocculation;

chemical consumption;

another sludge stream to manage.

For this reason, gravity thickening is often worth testing first when the solids settle reasonably well.

Option 3 — Direct Mechanical Dewatering

Some projects want to move directly from drum-filter backwash to mechanical dewatering.

Possible equipment includes:

screw press;

belt filter press;

chamber filter press;

other project-specific dewatering equipment.

This can be practical when the sludge concentration and dry-solids load are high enough for the selected machine.

The key question is:

Is the drum-filter backwash concentrated enough to feed the dewatering unit efficiently?

Very dilute sludge can force a dewatering machine to process a large amount of water for relatively little solids.

In that case, a separate thickening stage may:

reduce hydraulic load;

reduce operating hours;

reduce equipment size;

stabilize polymer dosing;

improve overall operation.

Mechanical dewatering should therefore be selected from dry-solids load and feed concentration—not only from liters per hour.

Gravity Thickening vs DAF vs Direct Dewatering

Project Condition	Gravity Thickening	DAF	Direct Dewatering
Solids settle readily	Strong candidate	Often unnecessary	Possible after concentration check
Solids are light/poorly settling	May be weak	Worth evaluating	Depends on feed concentration
Backwash is very dilute	Useful as pre-thickening	Possible	Often inefficient without thickening
Limited footprint	May be constrained	Often compact	Equipment dependent
Low operating complexity desired	Strong advantage	More complex	Moderate to high
Chemical use should be minimized	Favorable	May require chemicals	Polymer often required
Final goal is only volume reduction	Strong candidate	Possible	May be more than needed
Final goal is transportable cake	Pre-step only	Pre-step only	Required
High dry-solids load	Possible	Possible	Stronger case
Small RAS / low solids mass	Often adequate	Often excessive	May not be justified

This table is a screening tool, not a final design.

3. Check the Final Disposal Route

The required final solids condition depends on what happens after treatment.

Possible routes include:

discharge to municipal wastewater treatment;

land application where permitted;

composting;

landfill;

off-site waste contractor;

further drying;

nutrient recovery;

digestion or other stabilization.

If the sludge can legally and economically be transported as a liquid, thickening may be enough.

If hauling cost is high, a dewatered cake may be more attractive.

The disposal route should therefore be defined before selecting the final solids-treatment stage.

4. Protect the Sludge From Excessive Breakdown

Fresh RAS solids are different from sludge that has been stored for a long period.

With extended storage, biological activity can:

break particles into smaller material;

increase dissolved organic matter;

release nutrients;

generate gas;

create odor;

change settling behavior.

That can make later thickening more difficult.

A good solids-management system removes sludge from the RAS quickly and moves it through the thickening/storage system without unnecessary retention.

Tank mixing should also be matched to the objective.

A storage tank may need enough mixing to prevent uncontrolled deposits, but excessive shear can break fragile particles into finer solids.

5. Check Supernatant and Filtrate Return

Whenever water is separated from sludge, decide where that water goes.

Possible streams include:

thickener supernatant;

DAF clarified water;

screw-press filtrate;

belt-press filtrate;

filter-press filtrate.

Do not automatically return these streams to the RAS.

They may contain:

fine suspended solids;

dissolved nutrients;

organic matter;

residual polymer;

treatment chemicals.

The return point should be evaluated based on water quality and the overall system design.

In some projects, these streams are better sent to wastewater treatment rather than back to fish production.

6. Consider Freshwater and Seawater Conditions

Salinity affects more than corrosion.

It can also influence:

sludge behavior;

chemical conditioning;

equipment materials;

polymer selection;

supernatant return strategy.

For seawater RAS, material compatibility should be reviewed for:

tanks;

pumps;

screens;

piping;

fasteners;

dewatering equipment.

Do not select sludge equipment from a freshwater installation without checking compatibility.

A Practical RAS Sludge-Thickening Data Checklist

Data Group	Information to Collect	Why It Matters
RAS Production	Species, biomass, feed rate	Solids-generation context
Drum Filter	Flow, screen micron, backwash cycle	Source of sludge
Backwash Volume	Average and peak daily volume	Hydraulic duty
Solids Concentration	TSS/TS in backwash	Thickening and dewatering basis
Dry-Solids Load	kg DS/day or kg DS/h	Equipment capacity basis
Settling Behavior	Settling column / visual test	Gravity-thickening potential
Sludge Stability	Storage time, odor, flotation	Process timing
Salinity	Freshwater or seawater	Materials and chemistry
Final Target	Thickened liquid or cake	Process selection
Disposal Route	Sewer, land, landfill, contractor	Required final condition
Utilities	Power, water, polymer space	Equipment feasibility
Return Stream	Supernatant/filtrate destination	RAS protection

A Better RAS Sludge-Management Workflow

Step 1 — Capture Solids Quickly

Use drum filtration or another mechanical separation process to remove particles from recirculating water before they break down further.

Step 2 — Measure the Backwash

Determine:

volume;

solids concentration;

dry-solids mass;

variability.

Step 3 — Test Settling

Establish whether simple gravity thickening is technically realistic.

Step 4 — Define the Final Target

Decide whether the project needs:

pre-concentrated liquid sludge;

storage reduction;

pumpable thickened sludge;

mechanically dewatered cake.

Step 5 — Compare Thickening Options

Evaluate gravity settling, DAF or another concentration process based on the actual sludge.

Step 6 — Evaluate Mechanical Dewatering

If cake is required, select equipment from:

feed concentration;

dry-solids load;

sludge characteristics;

polymer requirement;

operating schedule;

final cake target.

Step 7 — Manage Separated Water

Define where supernatant and filtrate will be treated or discharged.

Common RAS Sludge-Management Mistakes

Treating Drum Filtration as Final Dewatering

A drum filter captures solids but normally produces a liquid backwash stream.

Further concentration may still be required.

Specifying Only Sludge Flow

Sludge flow does not show the actual solids mass.

Always include solids concentration and dry-solids load.

Assuming a Fixed Concentration Target Can Always Be Reached

Achievable thickening depends on sludge behavior and the selected process.

Confirm it by testing.

Storing Fresh Sludge Too Long

Extended storage can change particle size, settling and nutrient release.

Sending Dilute Sludge Directly to a Large Dewatering Machine

Pre-thickening may reduce hydraulic load and improve the overall dewatering system.

Returning All Filtrate to the RAS

Separated water should be checked before it is returned to the fish-production loop.

Example: Why the Same Drum Filter Can Produce Different Sludge Duties

Consider two RAS facilities using similar drum-filter technology.

System A

moderate feed;

stable biomass;

moderate screen opening;

short backwash events;

solids settle well.

A gravity thickener may provide a simple pre-concentration step before periodic sludge removal.

System B

higher feed;

finer screen;

more frequent backwash;

fine suspended solids;

limited disposal volume.

The project may need:

larger thickening capacity;

flotation testing;

or mechanical dewatering after pre-concentration.

The filter model alone does not determine the downstream sludge system.

What Data Should You Send for a Preliminary Review?

Provide:

fish species;

current and future biomass;

daily feed;

drum-filter flow;

screen micron;

backwash frequency;

estimated or measured backwash volume;

TSS or total-solids concentration;

freshwater or seawater;

settling observations;

current sludge-storage method;

target sludge concentration or final cake requirement;

available footprint;

disposal route;

available power, water and polymer facilities.

If concentration data are not available, a representative backwash sample and basic settling test can still help define the next engineering step.

Technical Basis

RAS engineering guidance consistently treats microscreen filtration as a solids-capture step that creates a separate backwash waste stream requiring further management. Public aquaculture guidance also emphasizes removing captured solids quickly to reduce breakdown, nutrient release and re-suspension.

Published RAS sludge-management literature shows that sludge concentration can vary widely depending on the solids-separation process. This is why downstream thickening should be selected from measured sludge characteristics rather than from a universal concentration assumption.

The practical process sequence is:

capture the solids → characterize the backwash → thicken if required → dewater only when the final disposal route justifies it.

Need Help Planning RAS Sludge Thickening?

Send HydroNovus the drum-filter flow, screen micron, backwash volume, measured solids concentration, fish biomass, daily feed, freshwater or seawater conditions, settling behavior and final sludge-disposal requirement.

HydroNovus can provide a preliminary RAS sludge-thickening and dewatering review covering:

backwash characterization;

gravity-thickening potential;

DAF suitability;

dry-solids load;

mechanical dewatering options;

sludge storage;

supernatant/filtrate handling;

and the additional data needed before equipment selection.

Final thickening performance, dewatering capacity, chemical dose and disposal design should be confirmed using project-specific sludge testing and engineering data.