Sludge - Collection & Dewatering

The goal is to capture solids early and concentrate them — remove as much water as possible — so transport, storage and further treatment stay manageable. Fresh particles are easier to remove than degraded sludge. See what aquaculture sludge is.

See also

• What is aquaculture sludge?
• HTC, digestion and nutrient recovery
• Drum filters
• Protein skimmers
• RAS
• Flow-through
• MBBR

Common collection systems

• Drum filters and microscreens (backwash becomes the sludge stream)
• Settling tanks and lamella settlers
• Hydrocyclones and settling cones
• Protein skimmers (fine and dissolved organics, especially in seawater)
• Biofilter purge systems
• Thickening tanks
• Geotextile dewatering bags
• Belt filters, decanters and filter presses

In RAS the sludge is often already concentrated at drum filters and cones. In flow-through it is more dilute and is often captured in a sedimentation basin before the receiving water. In ponds the material sits on the bottom; in sea cages it settles on the seabed under the farm.

Why sludge volume must come down

Reducing sludge volume lowers transport cost, storage need and disposal cost. Dewatering does not remove the nutrients — it removes water. Dry-matter content and odour depend on how quickly sludge is handled and whether it is aerated, covered or treated further.

Simple sludge calculator

Enter production and FCR. Choose whether there is a biostage (secondary sludge) and optional dewatering. Moisture follows the technology but can be edited. Primary DS is the midpoint of 200–400 g/kg feed from the mass-flow figure — edit if you have a feed analysis.

Simple sludge calculator (orientation)

Biological stage (secondary sludge)

No biostage MBBR Fixed bed Trickling filter Sand filter / biostage

Dewatering

None (raw backwash) Thickening / settling Geobag Belt filter Decanter Filter press

Production (tonnes/year)

FCR (kg feed / kg gain)

Primary sludge (g DS/kg feed)

Secondary sludge (g DS/kg feed)

Moisture after treatment (%)

Feed–

Primary sludge (DS)–

Secondary sludge (DS)–

Total DS–

Dry solids–

Wet sludge–

Volume (≈ 1 t/m³)–

Wet sludge per tonne fish–

If not dewatered (raw, 99% moisture)–

Method / defaults Feed = production × FCR. Primary DS default 300 g/kg feed — midpoint of the 200–400 g TSS/kg feed already used in the mass-flow figure (faeces + feed waste). Secondary DS default 25 g/kg feed when a biostage is selected (0 with none) — biofilm/residual BOD, editable. Moisture defaults: raw 99%, thickening 97%, geobag 80%, belt 84%, decanter 78%, press 72%. Wet mass = DS / (1 − moisture/100). Volume assumes 1,000 kg/m³. Orientation, not a permit or plant design calculation.

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Remove solids early and often

When solids remain in the loop, oxygen demand, CO₂ and ammonia rise and the biofilter fouls. Upstream solids removal is what protects MBBR, fixed-bed and trickling filters. The best biofilter is protected by the best solids-removal system.