Digester Chemistry & Dosing for RNG

Anaerobic digestion is a coordinated microbial relay in four steps. Break any step and gas production falls before the operator sees it in the flow meter — which is why chemistry is monitored, not assumed.

The four-stage microbial chain

Hydrolysis — complex polymers (carbohydrates, proteins, lipids) broken into sugars, amino acids and fatty acids. Rate-limiting for fibrous/lignocellulosic feedstocks.
Acidogenesis — fast fermenters convert those into volatile fatty acids (VFAs), alcohols, H₂ and CO₂. The fastest step — and the source of acidification risk.
Acetogenesis — VFAs converted to acetate, H₂ and CO₂. Hydrogen-sensitive: elevated H₂ partial pressure stalls it.
Methanogenesis — methanogens produce CH₄ from acetate (~70%) and from H₂/CO₂ (~30%). The slowest, most fragile, most pH- and toxin-sensitive step.

The parameters that decide stability

Parameter	Target window	Why it matters
pH	6.8–7.2 (methanogens)	Below 6.5, methanogenesis collapses; correction lags the drop
Alkalinity	2,000–5,000 mg/L CaCO₃	Buffers VFA spikes; the early-warning reservoir
VFA:alkalinity (FOS/TAC)	< 0.3 stable · 0.3–0.4 watch · >0.4 act	Most reliable single indicator of a souring digester
Temperature	Mesophilic 35–40 °C · Thermophilic 50–57 °C	±1–2 °C swings stress methanogens; thermophilic is faster but fragile
OLR	2–4 kg VS/m³·day	Overloading outpaces methanogen uptake → VFA build-up
C:N ratio	20:1 – 30:1	High N (manure/protein) risks ammonia toxicity; high C starves biology
Free ammonia (FAN)	< 80–150 mg/L NH₃	Rises with pH & temperature; classic manure-digester inhibitor

Trace-element (micronutrient) dosing

Methanogen enzymes are metalloenzymes. Digesters running on energy crops, food waste or low-diversity feedstocks routinely become trace-metal limited long before anyone suspects biology. Controlled dosing of iron, cobalt, nickel, selenium, molybdenum and tungsten restores enzymatic activity and is one of the fastest ways to lift a stalled methane yield. Typical maintenance targets sit in the low mg/L to µg/L range and must be matched to feedstock and lab data — overdosing wastes chemical and can be inhibitory.

Antifoam

Foaming — driven by surfactants, protein-rich feed, overloading or filamentous growth — carries biomass into gas lines, fouls foam traps and can trip the plant. Automated, event-triggered antifoam dosing (rather than continuous overdosing) controls it at the lowest chemical cost.

Common digester upsets — and the chemistry response

Upset	Signature	Chemistry response
Acidification / souring	VFA:TIC rising, pH falling, gas dropping	Reduce OLR, dose alkalinity (bicarbonate/lime), stabilize feed
Ammonia toxicity	High TAN, sluggish methanogens, high pH	Dilute/co-digest to correct C:N, manage temperature & pH
Trace-metal deficiency	VFA creeping up at stable load	Dose Co/Ni/Se/Mo/Fe blend to lab target
Foaming	Level spikes, foam in gas line	Event-triggered antifoam; address root load/surfactant cause

Where Milton Roy fits

High-accuracy metering pumps and packaged dosing skids for nutrient, alkalinity, pH-correction and antifoam chemicals.
Automated, flow-paced or feedback-driven control (pH/ORP) for hands-off stability.
Chemically-resistant liquid-ends and materials matched to ferric, caustic, acids and trace-metal blends.

Digester Chemistry Management for Renewable Natural Gas