Anaerobic Digester Loading Calculator

Anaerobic digesters live or die by how hard you load them. Enter your sludge feed and digester volume to check volatile-solids loading and solids retention time against standard design ranges.

Sludge feed rate

Flow unit

Sludge total solids (% by weight)Thickened primary/WAS blend is typically 3–6%.

Volatile fraction (% of TS)Typically 65–80% for combined primary + WAS.

Sludge specific gravityDefault 1.02 for thickened sludge.

Digester working volume

Volume unit

Digester Loading

Item	Value

How the calculation works

Formulas
Sludge fed (lb/day) = flow (gal/day) × 8.34 × specific gravity × TS% ÷ 100
VS fed (lb/day) = sludge fed (lb/day) × VS% ÷ 100
VS loading (lb VS/ft³·day) = VS fed (lb/day) ÷ digester volume (ft³)
SRT (days) = digester volume (gal) ÷ sludge feed (gal/day)
1 ft³ = 7.48052 gal

Digester type	Typical VS loading (lb/ft³·day)	Typical SRT (days)
Conventional (low-rate)	0.04 – 0.10	30 – 60
High-rate, mesophilic	0.10 – 0.35	15 – 30

Notes & limitations

Loading ranges are from Metcalf & Eddy wastewater engineering references (conventional 0.04–0.10 lb VS/ft³·day; high-rate up to ~0.35). Ten State Standards require a minimum ~15-day SRT at 95 °F for mesophilic high-rate digestion. Confirm against your state design standards.

SRT here assumes a completely mixed digester with no recycle — the simple volume-over-feed definition. Staged or temperature-phased systems need a per-stage analysis.
VS loading above ~0.35 lb/ft³·day risks volatile-acid accumulation, foaming, and digester upset; watch the volatile-acids-to-alkalinity ratio in the lab.
SRT below 15 days at mesophilic temperatures risks washing out methanogens. Thermophilic digesters run shorter SRTs but need tighter temperature control.
This tool checks loading only — it does not predict VS destruction, gas yield, or pathogen reduction.

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