Sludge Treatment & Disposal

Sources and characteristics of sludge (primary, secondary, digested); moisture content and volume relationship, specific gravity of sludge; sludge treatment train — thickening (gravity, flotation, centrifuge), stabilisation by anaerobic digestion (stages, factors, standard- and high-rate digesters, digester capacity, biogas) and aerobic digestion, conditioning, dewatering (sludge drying beds, filter presses, belt presses, centrifuges); Imhoff tanks; disposal and reuse — land application, composting, landfill, incineration — with solved numericals.

Sludge Treatment & Disposal

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📑 Contents (10 sections)

Last reviewed 16 Sept 2026 · 7 min read

Sources and characteristics of sludge

Sludge is the mixture of settled solids and water removed from treatment units.

Sludge	Source	Nature
Primary (raw) sludge	Primary sedimentation tanks	Grey, offensive odour, readily putrescible; solids roughly 3–7%
Secondary sludge — waste activated sludge	Secondary clarifiers after ASP	Brownish, flocculent, earthy odour when fresh; low solids (about 0.5–1.5%) — hard to thicken
Trickling filter humus	Secondary clarifiers after trickling filters	Brownish, relatively stable; higher solids than activated sludge
Chemical sludge	Chemical precipitation (P removal, CEPT)	Depends on chemicals
Digested sludge	Digesters	Dark brown to black, tar-like odour, far less putrescible; drains well

Sludge handling is often the costliest part of a treatment plant.

FormulaMoisture content and volume

Volume of sludge is inversely proportional to its solids content:

\frac{V _{2}}{V _{1}} = \frac{100 - p _{1}}{100 - p _{2}}

$p_{1}$ , $p_{2}$ = moisture contents (%) — e.g. reducing moisture from 98% to 96% halves the volume.

Volume of sludge:

V = \frac{W _{s}}{ρ _{w} S _{s l} P _{s}}

$W_{s}$ = mass of dry solids; $S_{s l}$ = specific gravity of sludge; $P_{s}$ = solids fraction (decimal).

Specific gravity of dry solids (volatile fraction $f_{v}$ with $S_{v}$ ≈ 1.0; fixed fraction $f_{f}$ with $S_{f}$ ≈ 2.5):

\frac{1}{S _{s}} = \frac{f _{v}}{S _{v}} + \frac{f _{f}}{S _{f}}

Specific gravity of wet sludge: $\frac{1}{S _{s l}} = \frac{P _{s}}{S _{s}} + \frac{1 - P _{s}}{1.0}$

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