Co-digestion multiplies profitability but introduces dynamic uncertainty. The first risk is not total ammoniacal nitrogen (TAN): it is the free NH3 fraction. It is governed by the pKa 9.25 equilibrium, modulated by pH and temperature. A digester that incorporates pig slurry, WWTP sludge or poultry by-products without auditing the mixture works with an inhibition variable out of control. This article explains how to calculate free NH3, the thresholds that separate adaptation from collapse, the critical trace nutrients, and how Smallops anticipates these interactions before they show up in biogas production.
In the biogas market, co-digestion is sold as a universal solution: more substrate, more biogas, more margin. The operational reality is more complex. When an anaerobic digester receives nitrogen-rich substrates — pig slurry, WWTP sludge, blood meal or poultry by-products — without a prior analysis of the mixture, the total ammoniacal nitrogen rises steadily. The problem is not the TAN itself, but the fraction that adopts the non-ionised form: free ammonia, or NH3. This molecule crosses cell membranes, decouples the proton gradient and halts methanogenesis. The result is a production drop that is wrongly attributed to other factors.
What free-ammonia inhibition is in anaerobic digestion
Total ammoniacal nitrogen (TAN) comprises two species in dynamic equilibrium: the ammonium ion (NH4⁺) and the non-ionised free ammonia (NH3). The proportion between the two depends on pH and temperature according to the acid-base equilibrium with a pKa of 9.25. The higher the pH and the higher the temperature, the larger the NH3 fraction. Free NH3 diffuses freely across cell membranes, interferes with the proton balance, inhibits key enzymes of anaerobic metabolism and decouples ATP synthesis. At 35°C and pH 7.5, with a TAN of 3,000 mg N/L, the free NH3 concentration is approximately 150 mg N/L, right at the incipient inhibition threshold for non-acclimated consortia.
How free NH3 is calculated from TAN, pH and temperature
The acid-base equilibrium formula is: NH3 (mg N/L) = TAN × [1 / (1 + 10^(pKa − pH))]. The pKa depends on temperature: pKa = 0.09018 + (2729.92 / T in Kelvin). At 35°C (308 K), pKa = 8.95; at 55°C (thermophilic, 328 K), pKa = 8.62. This means a thermophilic digester generates twice as much free NH3 as a mesophilic one with the same TAN. The Smallops team calculates free NH3 at every visit using the laboratory-measured TAN, the real-time pH and the digester temperature. This variable cannot be estimated: it must be calculated.
Inhibition thresholds: acclimated vs non-acclimated consortium
The technical literature distinguishes two types of methanogenic consortium according to their history of exposure to ammonia. A non-acclimated consortium shows incipient inhibition at 150 mg NH3-N/L and collapse above 700 mg NH3-N/L. A consortium acclimated through a gradual increase in nitrogen load over 60-120 days can tolerate up to 1,000 mg NH3-N/L with no significant production drop. Acclimation occurs naturally when the nitrogen load rises slowly. It occurs in a directed way when the technician plans the dose increase with weekly analytical monitoring. A digester to which pig slurry, poultry manure or other N-rich waste is added all at once — going from 0 to 30% of the mixture in a week — differs drastically from one in which the slurry is increased by 5% per week.
Trace nutrients: critical enzymatic cofactors in co-digestion
Co-digestion has a poorly documented effect on the availability of trace nutrients: it dilutes the concentration of inorganic cofactors essential for methanogenesis. Methanogenic microorganisms require minimum concentrations of iron (Fe), cobalt (Co), nickel (Ni), molybdenum (Mo), selenium (Se) and tungsten (W) to synthesise their key enzymes: coenzyme F430 (Ni), coenzyme F420 (Co), methyl-CoM reductase (Ni) and formate dehydrogenase (W, Mo). The effective concentration in the digester must be between 0.05 and 10 mg/L for each micronutrient. A mono-substrate digester (slurry) may have the correct profile. A multi-substrate co-digestion digester can end up with specific deficiencies if the mixture is not audited for mineral composition. The addition of substrates rich in inorganic particles — such as ash, sand or soil — can also precipitate the micronutrients, reducing them to non-bioavailable forms.
Dissolved H2 accumulation and syntrophic β-oxidation in co-digestion
When co-digestion incorporates fat-rich substrates — used vegetable oils, dairy-industry flotation fats, slaughterhouse fat — the syntrophic β-oxidation of long-chain fatty acids (LCFA) produces hydrogen as a by-product. If the partial pressure of H2 in the liquid exceeds 10^−5 atm (equivalent to 10^−4 atm in dissolved H2), β-oxidation becomes thermodynamically unfavourable and stops. The syntrophic acetogenic bacteria depend on the hydrogenotrophic methanogens to consume that H2. If free NH3 is inhibiting the methanogens, the H2 pressure rises, the acetogens stop oxidising, propionate accumulates and FOS/TAC rises. This cascade explains why, in co-digestion, ammonia inhibition and fatty acid accumulation appear together: they have a common cause.
Operational case: mixture audit at an agro-industrial plant with pig slurry
An agro-industrial plant with a 2,000 m³ digester operating with 70% pig slurry and 30% food-industry by-products detected a sustained drop in biogas production of 18% over 45 days. The plant’s on-site doctor attributed the drop to mechanical problems. The Smallops audit measured a TAN of 4,200 mg N/L, pH 7.9 and a temperature of 37°C.
Diagnosis: free NH3 calculation
With TAN = 4,200 mg N/L, pH 7.9 and T = 37°C (310 K), pKa = 0.09018 + (2729.92 / 310) = 8.90. NH3 = 4,200 × [1 / (1 + 10^(8.90 − 7.9))] = 4,200 × [1 / (1 + 10)] = 4,200 × 0.0909 = 382 mg NH3-N/L. This concentration exceeds the severe inhibition threshold (>250 mg N/L) for non-acclimated consortia. The propionate in the digestion liquor was at 1,850 mg/L (normal <500 mg/L), confirming inhibition of the syntrophic acetogens by H2 pressure.
Plan and intervention applied
The intervention followed three steps. First, reduction of the slurry fraction from 70% to 45% in the mixture over 3 weeks, incorporating more low-nitrogen substrates (vegetable waste with a C/N ratio of 30-35). Second, controlled acidification of the pH from 7.9 to 7.5 by adding acidic effluent from pre-treatment, reducing the NH3 fraction. Third, micronutrient supplementation with Fe (5 mg/L), Co (0.1 mg/L), Ni (0.1 mg/L) and Mo (0.05 mg/L) in the form of soluble salts.
Result at 90 days
Result at 90 days. TAN reduced to 2,800 mg N/L; free NH3 calculated at pH 7.5 = 140 mg N/L (below the incipient inhibition threshold); propionate in the liquor <400 mg/L; specific methane production recovered to 98% of the historical level; C/N ratio of the mixture stabilised at 22-25. The plant kept the corrected mixture and started a gradual acclimation programme to increase the slurry fraction back to 60% over 6 months.
Frequently asked questions
How is free-ammonia inhibition calculated in a digester?
By measuring total ammoniacal nitrogen (TAN) in mg N/L through laboratory analysis and applying the formula: NH3 = TAN × [1 / (1 + 10^(pKa − pH))], where pKa = 0.09018 + (2729.92 / T in Kelvin). It is not enough to measure the TAN: the free fraction must be calculated using the real pH and temperature of the digester.
What is the optimal C/N ratio in anaerobic digestion with co-digestion?
The optimal C/N ratio in anaerobic co-digestion is between 20 and 30. Below 20, there is an excess of nitrogen and a risk of ammonia inhibition. Above 30, nitrogen becomes limiting for the bacterial biomass. Nitrogen-rich substrates (slurry, sludge, hydrolysates) must be mixed with carbon-rich substrates (vegetable residues, cereal by-products, pre-treated straw) to balance the mixture.
How is a consortium acclimated to a high ammonia concentration?
Directed acclimation consists of increasing the fraction of nitrogenous substrate in the mixture by 5-8% per week, with weekly monitoring of TAN, calculated free NH3, FOS/TAC, propionate and specific methane production. The increase must be paused if FOS/TAC exceeds 0.4 or propionate exceeds 500 mg/L, and resumed when both parameters return to the normal range. The full process requires between 60 and 120 days.
What trace nutrients does methanogenesis need and at what concentration?
The key micronutrients for methanogenesis and their optimal ranges in the digestion liquor are: iron (Fe): 1-5 mg/L; cobalt (Co): 0.05-0.1 mg/L; nickel (Ni): 0.05-0.1 mg/L; molybdenum (Mo): 0.05-0.1 mg/L; selenium (Se): 0.01-0.05 mg/L; tungsten (W): 0.01-0.05 mg/L. In co-digestion with multiple substrates, elemental analysis of the mixture and the liquor should be carried out at least every 3 months.
How Smallops anticipates interactions in co-digestion
Co-digestion does not fail for lack of substrate. It fails for lack of monitoring of the interactions between substrates. The Smallops work in co-digestion plants follows a fixed sequence: first, an audit of the current mixture with calculation of free NH3, C/N ratio and micronutrient balance; second, design of the optimal mixture with safety margins against inhibition; third, a plan for the gradual increase of nitrogenous substrates with a defined monitoring frequency; fourth, quantitative stop criteria if the operational indicators deteriorate.
Every mixture change in co-digestion is a technical intervention, not a commercial decision. It requires prior calculation, continuous monitoring and quantitative validation criteria. A digester that does not measure free NH3 does not know how many metres from collapse it is operating.
References and regulations
Hansen, K.H. et al. (1998). Anaerobic digestion of swine manure: inhibition by ammonia. Water Research, 32(1), 5-12.
Rajagopal, R. et al. (2013). A critical review on inhibition of anaerobic digestion process by excess ammonia. Bioresource Technology, 143, 632-641.
Sprott, G.D. & Patel, G.B. (1986). Ammonia toxicity in pure cultures of methanogenic bacteria. Systematic and Applied Microbiology, 7, 358-363.
Ministerio de Agricultura, Pesca y Alimentación (MAPA). Guide to best available techniques in biogas facilities.