A digester is the one unit in a treatment plant that can pay for itself — if the energy
arithmetic closes. This tool sizes the tank from solids retention time, checks the volatile solids
loading against the process configuration, works out the gas and the methane in it, then subtracts the
heat needed to hold the tank at temperature. It also runs the two stability checks that decide whether
a digester runs or sours: the VFA/alkalinity ratio and free ammonia.
1Sludge duty
Thickened sludge is the feed. Take the feed concentration from the thickener, not from
a textbook — it sets everything that follows.
2Digester sizing & operation
Two decisions: how hot, and how hard you run it. Temperature sets the minimum
retention time and the ammonia chemistry; configuration sets the loading window you are allowed to
design inside.
3Gas yield & energy recovery
Destruction and yield decide the gas; the conversion efficiencies decide what the gas
is worth. Keep the two separate — a good yield in a poor engine is still a poor result.
Results — size, gas & energy
Digester volume
—m³
Biogas
—m³/d
Net power from CHP
—kW
Item
Value
Basis
4Volume & loading
Volume follows from retention time; loading is what then tells you whether that
volume is defensible. A tank that is big enough by the clock can still be overloaded by mass.
Item
Value
Basis
5Gas, heating & energy balance
Gas out, heat in. In a warm climate the tank losses are modest and the sludge heating
dominates — the reason tropical plants often find digestion easier to justify than temperate ones.
Item
Value
Basis
6Stability — VFA, alkalinity & ammonia
Digestion fails chemically long before it fails hydraulically. These three numbers are
what an operator actually watches.
Item
Value
Basis
7Digested sludge & mass balance
Destruction removes volatile solids, so the dry solids leaving the digester are fewer
than those entering it — and the concentration drops.
Item
In
Out
8Instrument schedule
A digester only needs four measurements to be safe and three more to be efficient.
Gas flow, gas pressure and temperature are the three that are easiest to justify.
Point
Instrument
Why here
Quote the digester instruments at factory price
Biogas flow, gas pressure, temperature, pH/ORP and level — all mappable to our catalogue and supplied
at the manufacturer's ex-works price (our margin is the export rebate, not a markup on the device).
Response within 24 hours · Inputs stay in your browser — nothing is uploaded.
Sources & parameter basis
[1]
METCALF & EDDY / AECOM — sludge digestion: thickened feed 3–8 % DS, VS/TS 0.6–0.85 for municipal sludges, nitrogen content 2–6 % of dry solids.
[2]
Sludge nitrogen: primary sludge ~2.5–3.5 % of DS, waste activated sludge ~5–8 %; of that, roughly 30–70 % is mineralised to ammonia during digestion.
[3]
Loading and retention: standard-rate single-stage mesophilic 0.6–1.6 kg VS/m³·d at 30–60 d; high-rate (completely mixed) 1.6–4.8 kg VS/m³·d at 10–20 d. Mesophilic minimum ~15 d, thermophilic minimum ~10 d.
[4]
Tank geometry taken as a cylinder with liquid depth equal to diameter; multiple tanks split the volume but add surface area, so they raise the heat loss. Overall U for insulated walls and roof ~0.5–0.9 W/m²·K; floor taken at 0.5 × U.
[5]
Gas: 0.75–1.12 m³ biogas per kg VS destroyed (0.9 typical); 60–70 % methane. Methane LHV 35.8 MJ/m³ = 9.94 kWh/m³. Boiler/thermal efficiency 75–85 %; CHP electrical efficiency 30–40 %.
[6]
Stability: VFA/alkalinity below 0.3–0.4 is comfortable, 0.4–0.5 is a warning, above 0.5 the digester is heading for a sour upset. Optimum pH 6.8–7.4; methanogens are badly inhibited below 6.5.
[7]
Ammonia: total ammonia nitrogen 1500–3000 mg/L is inhibitory; the toxic species is free NH₃, whose fraction is 1/(1+10^(pKₐ−pH)) with pKₐ = 0.09018 + 2729.92/T(K). Free ammonia above roughly 50–150 mg/L inhibits — and because pKₐ falls with temperature, thermophilic operation is far more exposed.
Full derivation and worked example: anaerobic-digestion.html.
Switch to thermophilic and raise the pH to 7.8 — free ammonia triples without a single extra kilogram
of nitrogen entering the tank. That is the chemistry that makes thermophilic digestion harder than it
looks on a gas-yield spreadsheet.
