Sludge Volume from Dry Solids

Sludge is almost entirely water, and the whole economics of biosolids handling comes from that fact. A tonne of dry solids at 4% solids and specific gravity 1.02 occupies 1000/(1.02 × 1000 × 0.04) = 24.5 m³, about 6500 gallons. Thicken the same tonne to 6% and it fits in 16.3 m³; dewater it to 20% cake and it is 4.9 m³: one fifth the trucks, one fifth the hauling bill, for solids that never changed at all.

The relationship is a reciprocal, which is why the payback on thickening is so lopsided: going from 1% to 2% halves the volume, while going from 20% to 21% barely moves it. Typical figures are 0.5–1.0% for waste activated sludge straight from the clarifier, 4–6% for primary sludge, 2–6% for gravity or dissolved-air-flotation thickened sludge, 15–25% for belt-press or centrifuge cake, and 30–40% off a plate-and-frame press. Two field cautions: percent solids means dried at 103–105 °C to constant weight, not "what the meter said"; and the specific gravity here is that of the wet sludge, which for anything under 10% solids is within 2% of water, so the term matters for cake and hardly at all for thin sludge.