Industry Solutions
RAS Solids Management: From Feed and Feces to Drum Filter Backwash and Sludge Thickening
Build RAS solids management as one mass-balance path from feed and feces to tank drains, drum-filter backwash, thickening, reject water and dewatering.
In RAS, solids management is not a single equipment-selection problem.
It is a mass-balance problem that starts with feed and ends with final sludge disposal.
A useful process map is:
feed -> fish -> feces/uneaten feed -> tank solids collection -> mechanical filter -> backwash sludge -> thickening -> dewatering -> final solids
The reject water from thickening or dewatering must also be accounted for.
Solids-Flow Schematic
Feed -> fish biomass -> feces / uneaten feed -> tank solids capture -> drum filter -> backwash sludge -> thickener -> thickened sludge -> dewatering -> cake
Thickener reject water -> dedicated wastewater treatment / controlled recycle after mass-balance review
Published RAS research identifies rapid tank solids capture and microscreen filtration as important because delayed removal allows particles to break down and increases downstream loading: https://www.sciencedirect.com/science/article/pii/S0144860905000294
1. Define the Solids Sources
Main sources include:
- feces;
- uneaten feed;
- biofilm slough;
- tank-cleaning solids;
- filter-captured particles.
Rapid removal matters because retained solids can break down and release dissolved organic matter and nutrients.
2. Tank Hydraulics Come Before the Drum Filter
A circular tank or dual-drain system can concentrate settleable solids before they reach the mechanical filter.
RAS research has shown that good tank solids capture and pre-separation can reduce the solids load and backwash demand placed on a drum filter.
3. Drum Filter Selection Uses More Than Flow
Microscreen drum-filter capacity depends on:
- recirculation flow;
- screen micron;
- submerged screen area;
- TSS concentration;
- particle-size distribution;
- backwash frequency.
Published RAS work explicitly identifies these variables as key filter-selection factors.
4. Treat Backwash as a Sludge Stream
RAS literature describes sludge as a dilute mixture of solids and water from:
- drum-filter backwash;
- tank cleaning;
- other solids-removal operations.
This stream may contain both particulate nutrients and dissolved N, P and organic matter.
Worked Example: Backwash Volume and Dry Solids
Assume a drum filter produces:
- backwash volume = 5 m3/day;
- measured backwash TSS = 8,000 mg/L.
Convert concentration:
8,000 mg/L = 8 kg/m3
Dry solids captured in the backwash are approximately:
5 m3/day x 8 kg/m3 = 40 kg dry solids/day
If a thickening step reduces the sludge-water volume from 5 m3/day to 1.5 m3/day while retaining most of the solids, the downstream dewatering machine now handles much less liquid.
The design basis for the press is still the dry-solids load - approximately 40 kg DS/day in this simplified example - not merely the 1.5 m3/day thickened volume.
This example shows why both wet volume and dry solids must be measured.
Worked Thickener Capture Balance
Continue the earlier example:
- drum-filter backwash = 5 m3/day
- backwash TSS = 8 kg/m3
- incoming dry solids = 40 kg DS/day
Assume a measured thickener solids-capture efficiency of 90% for this illustrative test.
Captured solids:
40 x 0.90 = 36 kg DS/day
Solids leaving with reject water:
40 - 36 = 4 kg DS/day
If the thickened sludge is measured at 30 kg solids/m3 (about 3% by mass for an approximate 1,000 kg/m3 density), thickened-sludge flow is:
36 kg/day / 30 kg/m3 = 1.2 m3/day
Approximate reject-water flow is then:
5.0 - 1.2 = 3.8 m3/day
The reject stream therefore carries about 4 kg DS/day plus any dissolved nitrogen, phosphorus and COD. That load must be included in the receiving process.
The 90% capture value is an illustrative test result, not a universal thickener performance claim.
Published RAS sludge literature notes that RAS sludge is often dilute and that thickening/dewatering is commonly applied: https://www.sciencedirect.com/science/article/pii/S0048969720339206
Thickening Selection Boundaries
Settles rapidly in a column test
Gravity thickening deserves first evaluation.
Poor settling but good chemical floc formation
Coagulation/flocculation-assisted thickening may be more suitable.
Very limited footprint and continuous operation required
Evaluate compact mechanical thickening.
Reject-water nutrient recovery is important
Select a method that allows the liquid and solids streams to be measured and treated separately.
Backwash flow is already low and dewatering equipment accepts the feed
A separate thickener may not be necessary.
5. Decide Whether Thickening Is Needed
RAS sludge commonly has low solids concentration.
High water content increases:
- hauling volume;
- storage volume;
- dewatering hydraulic duty.
Possible thickening approaches include:
- settling;
- geotextile;
- coagulation/flocculation;
- mechanical thickening;
- flotation where appropriate.
The best method depends on sludge behavior and disposal route.
6. Account for Reject Water
Reject water can contain:
- fine suspended solids;
- soluble nitrogen;
- phosphorus;
- soluble COD.
If returned to the RAS or wastewater plant, those loads recirculate.
A solids-handling design therefore needs a reject-water destination.
7. Link Feed to Sludge Production
A useful operating dashboard tracks:
- daily feed;
- biomass;
- drum-filter backwash volume;
- backwash TSS;
- thickened-sludge volume;
- dewatered cake mass;
- reject-water flow.
This makes the solids line inspectable instead of anecdotal.
Decision Path
Observation | Check First | Likely Action |
|---|---|---|
Drum filter backwashes continuously | flow, TSS, micron, submerged area | verify filter capacity/solids load |
Backwash volume very high | screen micron, setpoint, nozzle/backwash logic | optimize filter operation |
Sludge too dilute for press | TSS and settling/thickening behavior | add thickening step |
Reject water has high nutrient load | soluble N/P/COD | route for treatment |
Biofilter fouls with solids | tank capture and mechanical filtration | improve upstream solids removal |
Process Acceptance Criteria
A practical solids-management review should be able to answer:
- how many kg dry solids/day are generated;
- how many m3/day of backwash are produced;
- what solids concentration reaches thickening;
- what volume reaches dewatering;
- where reject water goes;
- what final solids-disposal route is used.
If those quantities are unknown, equipment sizing remains weak.
Technical Basis and References
- Summerfelt et al., Solids removal in a recirculating aquaculture system where the majority of flow bypasses the microscreen filter, showing the importance of rapid solids removal and upstream solids capture.
- Factors influencing optimal micro-screen drum filter selection for recirculating aquaculture systems, identifying screen micron, submerged area, TSS and particle-size distribution as important capacity variables.
- Solids management in freshwater-recirculating aquaculture systems, describing RAS sludge as dilute and noting that thickening/dewatering is commonly used to reduce disposal volume.
Need Help Building an RAS Solids Balance?
- feed rate;
- biomass;
- recirculation flow;
- drum-filter micron and area;
- backwash frequency;
- backwash volume;
- backwash TSS;
- thickened-sludge concentration;
- disposal target.
HydroNovus can provide a preliminary solids mass balance and equipment-data checklist.