Agro-industrial co-digestion in biogas is the main strategy for diversifying the digester diet and increasing productivity above a single substrate. But it is also the main source of operational instability. For example, the seasonal variation of VS in alperujo can reach ±25%, silage loses BMP with storage time, and fruit and vegetable waste concentrates peaks of simple sugars. Correct management requires a minimum quarterly characterisation in triplicate and dynamic adjustment of the organic loading rate (OLR) to the real diet, not to the nominal diet in the contract.
Agro-industrial co-digestion in biogas is the most profitable and, at the same time, the most unstable operational scenario in anaerobic digestion. Combining two or three substrates in a balanced proportion raises the specific methane yield by 15 to 30% compared with a single substrate, improves the C/N ratio, diversifies the supply of trace nutrients and reduces dependence on a single supplier. The downside is that the digester diet stops being stable: it changes with the agricultural season, with the supplier batch, with the weather of the period. This article describes why agro-industrial substrates change, how to characterise them to avoid operational OLR deviations, and how to adjust the digester regime to a variable diet without entering the alert zone.
Why agro-industrial co-digestion biogas is more volatile than acknowledged
In the food and agri-livestock industry, substrates reach the digester with a composition that rarely matches the composition declared in the supply contract. Three mechanisms explain this divergence: the inherent variability of the operation throughout the year, processing variability, and storage variability (silage and other agri-food by-products progressively lose fermentable organic matter; fresh waste degrades in the silo at rates that depend on the ambient temperature).
In practice, an agro-industrial co-digestion plant operating with substrate characterisation once a year has a typical deviation between nominal OLR (calculated with contract data) and real OLR (measured with triplicate VS of the current batch) of 15-25%. That deviation is the silent cause of most instability episodes attributed to “biology”. It is not biology: it is a significant variation in the feed.
Substrate characterisation: real VS vs declared VS
Volatile solids (VS) are the fraction of the substrate actually available for methanogenesis. Substrates with similar TS (total solids) can have very different VS: two batches of pig slurry at 7% TS may have a VS of 5.8% in one and 4.1% in the other. If the plant calculates its load using TS instead of VS, or uses the VS declared by the supplier instead of the laboratory-measured one, the real OLR deviates systematically from the nominal value, which has a direct impact on methane generation.
The Smallops operational rule is quarterly re-characterisation in triplicate as a minimum, and by batch for highly variable substrates (alperujo from different olive mills, seasonal fruit and vegetable waste, silage near the end of the campaign, slurry, etc.). Triplicates are essential because the spatial heterogeneity within the substrate itself generates analytical dispersion of 5-10% in VS; without triplicates, real dispersion cannot be separated from sampling error.
Alperujo: seasonality and lipid composition
Alperujo (the by-product of two-phase olive mills) is one of the most interesting and most volatile substrates in agro-industrial co-digestion. It has a high theoretical BMP (280-380 NmL CH4/g VS), but that figure hides significant variability for two reasons. First, the lipid fraction of alperujo varies between 5 and 15% of VS depending on the harvest and the centrifugation regime. Lipids have a very high BMP (700-1,000 NmL CH4/g VS) but are also the most inhibitory fraction: above 100-200 mg/L of long-chain fatty acids in the digester, syntrophic β-oxidation slows down and propionic acid begins to accumulate.
Second, the polyphenol fraction of alperujo, known for its natural antimicrobial character in the preservation of olive oil, also affects the methanogenic consortium. Total polyphenol concentrations above 5 g/L in the digestate indicate excessive alperujo input and are associated with a slowdown in kinetics without explicit decoupling (FOS/TAC does not rise, but CH4 yield does fall). Operational control requires seasonal polyphenol analysis when the mixture includes more than 15% alperujo on a VS basis.
Silage: effect of prior fermentation on BMP
Maize silage is the reference energy substrate in co-digestion because of its stable availability and predictable BMP (290-340 NmL CH4/g VS). The lactic fermentation that occurs during ensiling transforms soluble sugars into lactic acid, which improves preservation.
What is critical operationally is that the BMP of silage decreases with storage time at a rate of approximately 0.5-1% per month under well-sealed conditions, and much faster (3-5% per month) in silos with air ingress. A plant opening a silo in May with a BMP measured the previous November may be working with a real BMP between 5 and 15% lower than assumed. If the OLR is kept constant, the real load is proportionally lower and productivity falls with no apparent biological cause.
Fruit and vegetable waste: seasonal peak and sugar peaks
Fruit and vegetable waste (by-products of fruit and vegetable packing plants, wholesale market rejects, juice processing residue) has an attractive BMP (350-450 NmL CH4/g VS) and enters the mixture very easily because they are usually cheap or free substrates. But they have three operational features that make them dangerous in co-digestion without control.
- Simple-sugar concentration: fruit waste contains 40-70% fermentable sugars in VS. A sudden input of this substrate generates an acidogenesis peak that methanogenesis cannot absorb at the same rate, and FOS/TAC rises in a characteristic way with low propionic and high acetic acid.
- Strong seasonality: waste availability varies with the fruit campaign. In August-September there may be surpluses that tempt operators to increase the fraction in the mixture; in February-March the waste is reduced and the plant must adjust the diet. If the OLR is kept constant, the sugar load varies even though the plant believes it is operating stably.
- Low input pH: many fruit and vegetable wastes arrive with a pH of 3.5-4.5 due to spontaneous fermentation during transport. If the fraction exceeds 15% in the mixture without pH pre-correction, the digester loses buffering capacity and FOS/TAC becomes unbalanced faster than the operator anticipates.
How to adjust OLR dynamically in agro-industrial co-digestion
Dynamic adjustment of OLR to the real diet requires a simple but disciplined four-step system repeated at every significant change of mixture:
1 – Measure VS and BMP of the new batch or campaign
Measure TS and VS of the new batch or campaign. If the substrate is highly variable (alperujo, fruit and vegetable), repeat the characterisation for each significant new batch. The cost of three quarterly analyses is marginal compared with the cost of operating for a week with a poorly calculated OLR.
2 – Recalculate the real OLR in kg VS/m³·day
With the real measured VS, recalculate the effective organic loading rate of the digester. If the new OLR exceeds 110% of the nominal, reduce the flow or the percentage of the changing substrate until returning to the design OLR.
3 – Increase the frequency of sentinel variables
For 2-3 weeks after any significant substrate change, switch the frequency of FOS/TAC and individual VFAs from weekly to twice-weekly. This reinforced monitoring makes it possible to detect incipient decoupling before it turns into an alert.
4 – Adjust the mixture, not the geometry
If the changing diet destabilises the digester, the correct response is not to modify the HRT or the mixing regime, but to adjust the mixture (reduce the fraction of the problematic substrate, compensate with a more stable substrate, and also add buffer if the pH drops). The digester geometry is fixed; the mixture is the operational variable.
Summary table: BMP ranges and operational features
| Substrate | Typical BMP (NmL CH4/g VS) | Seasonal variability | Main operational risk |
|---|---|---|---|
| Alperujo (two-phase) | 280-380 | ±25% between campaigns | Lipids (LCFA) and polyphenols |
| Maize silage | 290-340 | 5-15% depending on storage | BMP decreasing over time |
| Fruit and vegetable waste | 350-450 | High seasonality | Sugar peaks and low pH |
| Pig slurry | 180-280 | ±15% depending on animal diet | High TAN, free NH3 |
| WWTP sludge | 150-350 | Stable (primary vs secondary) | Low real BMP, low energy |
Operational case: 1 MWe agro-industrial plant with a seasonal diet
1 MWe plant (mesophilic 37 °C) with a nominal diet: 50% pig slurry, 30% maize silage, 20% fruit and vegetable waste. Design OLR 3.8 kg VS/m³·day. Historical specific production: 0.36 Nm³ CH4/kg VS. Seasonal symptom: every August-September productivity falls to 0.28-0.30 Nm³ CH4/kg VS.
Diagnosis of the analytical characterisation
Triplicate re-characterisation of the three substrates in August: the fruit and vegetable waste (peach and nectarine campaign) has a VS of 12.5% versus the 9.5% declared in the contract; simple sugars represent 58% of VS.
Root cause: the real OLR during August-September is 4.5 kg VS/m³·day (18% above nominal), with a very high fraction of fermentable sugars.
It is not seasonal biology: it is acidogenesis accelerated by hidden overload.
Operational intervention
Reduction of the fruit and vegetable fraction from 20% to 13% during the peak campaign months (July-September), compensating with an additional 7% of maize silage. Reinforcement of the frequency of FOS/TAC and individual VFAs throughout the season. Neither HRT nor mixing is modified: only the mixture.
Result after a full campaign
Key results after one full campaign. August-September specific productivity: 0.35 Nm³ CH4/kg VS (vs 0.28-0.30 historical, +17%). FOS/TAC in the stable zone (<0.35) during 91% of the campaign period. Zero excursions to the crisis zone (>0.5).
Cost of the intervention: four additional quarterly characterisations in triplicate (~2,400 EUR/year). Economic recovery: the annual dip period is reduced from 3-4 weeks to zero, with an estimated saving of 28,000-35,000 EUR/year in production not lost. ROI: ≥12x.
Frequently asked questions about agro-industrial co-digestion biogas
Why does the BMP change between campaigns of the same substrate?
The BMP of an agro-industrial substrate changes between campaigns for three reasons: biological variability of the crop (alperujo composition depending on early or late harvest), processing variability, and storage variability (silage losing 0.5-1% per month). A reliable BMP according to VDI 4630 must be measured every campaign or with every significant change of supplier.
How is a seasonal substrate characterised correctly?
The minimum characterisation of a seasonal substrate includes TS (total solids), VS (volatile solids), pH, elemental composition (C, N, P) and a reliable BMP according to VDI 4630 with an ISR ratio ≥2 and acclimated inoculum. Recommended frequency: quarterly minimum for stable substrates, by batch for highly variable substrates. Triplicates are mandatory to separate real dispersion from sampling error.
What should you do when a new substrate arrives at the plant?
Before introducing a new substrate into co-digestion, run three steps: full characterisation in triplicate (TS, VS, BMP, pH, composition), gradual input at 5-10% of the mixture over 2-3 weeks with reinforced FOS/TAC and VFA frequency, and stability validation before raising the fraction to the final target. Skipping this sequence is the most frequent cause of instabilities attributed to “unpredictable biology”.
What is the typical BMP of alperujo?
The typical BMP of alperujo (two-phase olive mill by-product) is between 280 and 380 NmL CH4/g VS, with a seasonal variability that can reach ±25% between campaigns. The lipid fraction (5-15% of VS depending on harvest) raises the theoretical BMP but also the risk of inhibition from long-chain fatty acids. Total polyphenols > 5 g/L in the digestate indicate excessive input and are associated with a slowdown in kinetics without explicit decoupling.
Does your plant have a seasonal dip every year?
It is probably not biology, it is outdated analytical characterisation. Request an Operational Excellence Diagnosis and we will audit the analytical traceability of your co-digestion.
References and regulations
Mata-Alvarez, J. et al. (2014). A critical review on anaerobic co-digestion. Renewable and Sustainable Energy Reviews, 36, 412-427. doi.org/10.1016/j.rser.2014.04.039
Bouallagui, H. et al. (2009). Mesophilic biogas production from fruit and vegetable waste. Renewable Energy, 34 (1), 80-86. doi.org/10.1016/j.renene.2008.04.007
VDI 4630 (2016). Fermentation of organic materials. vdi.de/richtlinien/details/vdi-4630
Borja, R. et al. (2006). Anaerobic digestion of two-phase olive mill effluent. Process Biochemistry, 41 (6), 1268-1274. doi.org/10.1016/j.procbio.2005.12.010