• Analysis of Process Parameters
    for AD and RNG Plants
    At Celignis Biomass Lab

Why Monitoring Process Parameters of an AD/RNG Plant is Important

In order to achieve maximal biogas yields from a feedstock, key chemical and physical parameters should be monitored. These can be divided in to process parameters and process indicators.

Process parameters include: quantity and composition of feedstock; total solids (TS); volatile solids (VS); BMP; NH4-N; organic loading rate (OLR); temperature; pH; mixing; and hydraulic retention time.

Key process indicators are: total biogas yield; biogas composition; volatile fatty acids (VFAs); Alkalinity ratio (FOS/TAC); and redox potential.

The biogas plant operator should routinely check process parameters and indicators. Temperature, pH, biogas composition, and mixing should be checked twice a day, with TS, VS, and organic loading rate monitored once a day.

Monitoring of process parameters becomes particularly important when the plant operator is planning and implementing changes in the process. Changes of feedstock type, organic loading rate and hydraulic retention time (HRT, a measure of the average length of time the liquid/soluble compound remains in AD reactor) and solid retention time (SRT, similar to HRT but for solid compounds) should be undertaken gradually in order to allowing microbes to adapt to the new conditions.

During this shift in conditions, any changes taking place in the biogas composition, pH, VFAs concentration, and alkalinity should be closely and frequently monitored in order to predict and pre-empt failures in the AD process.

The VFAs and alkalinity ratio for a stable digester should be tested twice a month. However, more frequent testing is warranted in cases where digester instability results from changes in feedstock type and supply.

Key Process Indicators


It should be noted that pH is a key process indicator, but cannot be considered an early process indicator due to its slow response to the digester imbalances. This is due to the buffering capacity of carbonates and ammonia in the digester. The pH value gives an approximate indication on the state of the digestion process; a value between 6.8 and 8 is considered suitable for AD and this depends on the feedstock type and the type of digester used. It should be closely monitored while changing operating parameters such as feed or temperature. A drastic shift towards acidic values indicates possible acid-crash and a shift towards alkali value indicates possible ammonia inhibition.


Unlike pH, FOS/TAC is an early process indicator. FOS/TAC ratio is also called intermediate alkalinity (IA)/Partial alkalinity (PA). The values for FOS, or IA, represent the volatile fatty acids (VFA) accumulation in the digestion system and the values for TAC, or partial alkalinity, represents the bicarbonate buffering capacity of the system. VFA accumulation is a key indicator for process imbalance and bicarbonate buffering capacity stabilises the system when there is accumulation of VFAs. Anaerobic digestion systems without sufficient buffering capacity have high chances of failure with slight changes in process parameters.

Total Volatile Fatty Acids (VFAs)

VFAs are formed after the hydrolysis phase during the acidogenesis and acetogenesis phases of anaerobic digestion (the phases prior to methanogenesis). Accumulation of VFAs indicates an imbalance in the process and inhibition of the methanogenesis phase. VFAs beyond a certain concentration are toxic to methanogens and causes process imbalances and acid-crash by altering the buffering capacity and pH of the system. VFAs accumulation is normally caused due to: high organic loading rates; imbalanced C/N ratios; and toxins inhibiting methanogens, among other factors.

Total VFAs can be analysed by either summing up the individual VFAs obtained by high performance liquid chromatography (HPLC) or gas chromatography (GC) or by a simple titration method developed by Kapp (1984). Though FOS value, which is also called IA obtained by two-point titration method, is considered as total VFAs, it is not considered to be an accurate measurement. The Kapp method uses three-point titration and is considered more accurate for total VFA estimation.

Individual Volatile Fatty Acids (VFAs)

Individual VFA concentrations and their ratios give a very good indication of process imbalance. Acetic acid is the major VFA, followed by propionic acid. The ratio of acetic acid to propionic acid is a key process indicator and any drastic change indicates changes in the biology of the system and process instability. Accumulation of higher chain fatty acids (butyric acid, valeric acid) and branched fatty acids (isobutyric acid and isovaleric acid) indicate severe process imbalance. Chromatography techniques such as high-performance liquid chromatography (HPLC) and gas chromatography (GC) are used for individual VFAs analysis.

NH4-N (Ammoniacal Nitrogen)

Ammonia is produced when nitrogenous compounds are digested during biogas production. Total ammoniacal nitrogen (TAN) is the sum of ammonium ion (NH4+) and free ammonia (NH3). The equilibrium between ammonium ion and free ammonia depends on pH and temperature. With an increase in pH and temperature the equilibrium shifts from NH4-N to NH3 which is toxic to microbes.

Limits for ammoniacal nitrogen are very broad as it is very much dependent on the biology of the anaerobic digester and adaptability of microbes. A sudden shift from a low nitrogen feedstock to a nitrogen-rich feedstock causes severe process imbalances due to the accumulation of ammonia. The shift should be gradual at low organic loadings in order to adapt the microbial population to increased concentrations of NH4-N and NH3.

The ratio of ammoniacal nitrogen to total nitrogen allows the operator to standardise the ratio of ammonia to total nitrogen input. This relationship can be established by continuous monitoring of the plant for TAN and correlating with the composition of the feedstock (C/N ratio).

TAN, NH4-N and NH3 are interrelated and measured by either distillation methods according to US-American standard “APHA 4500-NH3-Nitrogen” (APHA, 1998) or by the Nessler method.

Biogas Composition

Typical biogas consists of 50-60% methane and 40-50% carbon dioxide and trace amounts of hydrogen sulphide (H2S), ammonia (NH3), hydrogen (H2), oxygen (O2) and water vapour.

Accumulation of H2S and NH3 indicates process imbalance and toxicity. Immediate measures should be taken to reduce the concentration of these gases by supplementing the digester with additives or changing the feedstock or organic loading.

Residual Biogas Potential (RBP)

Residual Biogas Potential (RBP) is considered to be one of the main advanced process monitoring parameters to determine plant efficiency. It is also an important analysis to estimate the biological stability and emission potential of the digestate. The most appropriate sampling point is therefore the last digester or post-digestion storage tank from which biogas is recovered, as this represents the end of the active process. On the same basis, it is suggested that the RBP test should be applied to whole digestates rather than to separated fractions, as separation is not itself a stabilisation process.

The methodology followed for RBP is similar to Biomethane Potential (BMP) analysis. To calculate the emission potential, the test is conducted at lower temperature (digestate storage tank temperature). Percentage RBP is calculated based on the daily average amount of methane generated at the biogas plant from raw feedstock.

RBP % = (Average methane potential of the digestate from biogas plant/ Average methane production of the biogas plant per day) x 100

Click here to read more about the RBP test.

Specific Microbial Activities

Anaerobic digestion is a four-stage process and downshift of any of these stages will imbalance the process and cause failure of the biogas plant. This can happen due to many reasons including: feedstock toxicity/over-loading; nutrient imbalance; inhibitors accumulation (H2S, ammonia, VFAs); and insufficient retention time. Hence, it is important to monitor the efficiency of the individual stages in order to understand the biology and to maintain the stability of the biogas plant. Hydrolytic and acidogenic activity are determined by the maximum consumption rate of a defined model feedstock (based on the biogas plant feedstock). The methanogenic activity is based on the maximum rate of methane production from sodium acetate. These measurements give an indication of rate-limiting steps in the process, so that strategies to improve the overall process kinetics and efficiency can be designed and employed.

Routine monitoring of these specific activities will give an indication of shift in the reactor biology with time. It can also be used to study organic and hydraulic overload.

Click here to read more about Specific Microbial Activity tests.

Nutrients and Trace Metals

Nutrients are essential for maintaining stable microbial populations and for efficient enzymatic activities throughout the four stages of anaerobic digestion. Certain trace elements such as iron and selenium are also useful in combating the toxic effects from H2S, VFAs, and ammonia. Each element has its role in microbial metabolism during anaerobic digestion and limitation of even a single element can cause process imbalances and decreases in process efficiency. Trace elements limitations can be mostly seen by VFAs accumulation and reduced biogas production. Trace elements can also help in increasing the organic loading by increasing the overall kinetics of the anaerobic digestion.

Excessive use of trace elements leads to inhibition. So, the feedstock should be analysed for trace elements and only the required elements at the required concentrations should be supplemented.

Click here to read more about the importance of nutrients and trace metals in AD.

How Celignis Can Help with Monitoring AD Process Parameters

Celignis can undertake a range of key analyses for KPIs and advanced process monitoring. These can be undertaken separately or as part of packages. These packages can be seen by clicking the links in the section above.

We have also integrated AD process indicators analyses with the Celignis Database so that the evolution of these parameters can be plotted over time. These plots also allow the user, or Celignis personnel, to set ranges for the KPIs and there will be indications/warnings if values obtained in analysis are outside these ranges (i.e. through control plots). Additionally, the Celignis Database presents data for the changes in each analyte from the previous month’s data and identify whether there have been any drastic changes.

Related to these analyses are Celignis's biological advisory services. We would recommend that a survey is undertaken once every quarter or when new feedstock is used. We work in very close association with plant operators to collect all the required data and samples for analysis. The outputs of our surveys will include recommendations on various parameters including: feed mix ratios, retention times, inhibitors, trace elements and additives supplementation; and other means of improving efficiency. Examples of data plots we provide as part of such biological consultancies are presented here. Click here to read more about these services.

Case Study on Monitoriong Process Parameters

A Germany-based biogas company that operates dozens of AD/RNG plants in Europe and the UK approached Celignis to support them in optimising their plant operations to allow for more consistent outputs and reduced downtime. As a result, Celignis provided Biological Consultancy support which involved us analysing the plant process data in terms of: feedstock loading (organic loading rate); recirculation strategies; biogas composition and yield; volatile fatty acids (VFAs); and alkalinity.

This detailed analysis of the plant process data allowed us to provide operational limits and indicators in the plant beyond common indicators such as VFA and alkalinity and acetic acid to propionic acid ratios (isoforms of volatile fatty acids, presence of traces of hydrogen in the biogas, Hydrogen sulphide and ammonia) and provided green, yellow and red zones for each of the indicators.

In addition to this, Celignis also developed a tool for the company to allow self-design of major and minor elements (nutrients) for the biogas plants based on the feed chemical composition. The tool was designed to be suitable for mono and co-digestion and allows for change from one feedstock to other, and for addition of a new feedstock to the co-digestion mix, without there being a negative affecting on plant performance.

Additional Information on Monitoring Process Parameters

Feel free to get in touch with us if you have any questions about our services for monioring process parameters and process indicators or if you are looking to address issues that you are experiencing with your current digester performance. Relevant members of the Celignis anaerobic digestion team will be happy to assist. Those team members with the most experience with undertaking these tests and interpreting the resulting data are listed below.

Lalitha Gottumukkala

Founder and Lead of Celignis AD, CIO of Celignis


Has a deep understanding of all biological and chemical aspects of anaerobic digestion. Has developed Celignis into a renowned provider of AD services to a global network of clients.

Kwame Donkor

AD Services Manager

BSc, MSc, Phd (yr 4)

His PhD focused on optimising AD conditions for Irish feedstocks such as grass. Kwame is now leading the Celignis AD team in the provision of analysis and bioprocess services.

Sajna KV

Bioanalysis Developer


Our Biomass Detective! Designs, tests, optimizes and validates robust analytical methods for properties of relevance to the anaerobic digestion sector.

Other Celignis Tests and Services for Anaerobic Digestion

Global Recognition as AD/RNG Experts

Celignis provides valued services to over 1000 clients. We understand how the focus of AD projects can differ between countries and have advised a global network of clients on their RNG projects. We also have customs-exemptions for samples sent to us allowing us to quickly get to work no matter where our clients are based.

Further Info...

Biomethane Potential

The biomethane potential (BMP) can be considered to be the experimental theoretical maximum amount of methane produced from a feedstock. In our laboratory, we have six BMP systems, comprising 90 reactors, that allow us to digest your samples and determine the biogas yield over periods of between 14 and 40 days.

Further Info...

Continuous Digestions

To help you evaluate how well your anaerobic digestion feedstocks will behave in real-world conditions we can undertake continuous digestion experiments. These operate at scales up to 12 litres and typically run for 3 months. We target maximum achievable organic loading rate (OLR) and biomethane potential.

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Toxicity Assays

The waste streams used in AD that arise from process industries may contain toxic or bacterial inhibitory compounds (e.g. antibiotics, polyelectrolytes, detergents). Our anaerobic toxicity assays can determine the presence of such toxicities and suggest the feeding limits for feedstocks.

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Process Optimisations

There a many factors to consider when running an AD facility. We can design and experimentally-validate optimisations of these factors at the lab-scale prior to you implementing them at your AD facility. Such an approach allows for greater benefits and lower costs than optimising the process at the commercial scale.

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Feedstock Analysis

Our analysts have characterised tens of thousands of biomass samples. We have dedicated analyses packages for the compositional parameters of most relevance to AD/RNG. Additionally, based on our detailed analyses we can recommend appropriate feedstock mixing proportions in co-digestion facilities.

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Biological Consultations

We're experts in the biology of anaerobic digestion. We pour through operational data from biogas plants and identify correlations between process parameters and plant performance. This understanding on the specific biology of the digester allows for recommendations as to how peformance can be improved and made more stable.

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Specific Microbial Activity

AD is a microbial process involving a sequence of stages (hydrolysis, acidogenesis, methanogenesis) to convert a complex feedstock to methane. We analyse samples collected from digesters and undertake tests to investigate how well they proceed with each of these stages of digestion.

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Technoeconomic Analyses

Our TEA experts work with you to evaluate the economic prospects of your AD/RNG facility, considering various scale, technology, and feedstock options. We apply accurate costing models to determine CAPEX/OPEX of simulated and pilot scale processes which are then used to determine key economic indicators (e.g. IRR, NPV).

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Digestate Analysis

Digestate is the residue after the anaerobic digestion process. It can potentially have value as a soil fertiliser. We offer a range of detailed analysis packages for digestate, allowing you to fully assess this resource and to determine the best use for it. Our team can also assist in evaluating digestate valorisation options.

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Project Development

The criteria for the development of a successful AD project are numerous and vary according to region, technology, and feedstock. We have a deep understanding of these regional, technical, and biological differences and have advised a global network of clients on effectively developing their AD projects.

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Nutrient Supplementations

Nutrients are essential for maintaining stable microbial populations and for efficient anaerobic digestion. We can suggest optimal values for the presence of major and minor elements in the digester as well as upper and lower threshold values. This allows us to formulate a bespoke cocktail of additives according to the requirements of the digester.

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Publications on Anaerobic Digestion By The Celignis Team

Ravindran, R., Donkor, K., Gottumukkala, L., Menon, A., Guneratnam, A. J., McMahon, H., Koopmans, S., Sanders, J. P. M., Gaffey, J. (2022) Biogas, biomethane and digestate potential of by-products from green biorefinery systems, Clean Technologies 4(1): 35-50



Global warming and climate change are imminent threats to the future of humankind. A shift from the current reliance on fossil fuels to renewable energy is key to mitigating the impacts of climate change. Biological raw materials and residues can play a key role in this transition through technologies such as anaerobic digestion. However, biological raw materials must also meet other existing food, feed and material needs. Green biorefinery is an innovative concept in which green biomass, such as grass, is processed to obtain a variety of protein products, value-added co-products and renewable energy, helping to meet many needs from a single source. In this study, an analysis has been conducted to understand the renewable energy potential of green biorefinery by-products and residues, including grass whey, de-FOS whey and press cake. Using anaerobic digestion, the biogas and biomethane potential of these samples have been analyzed. An analysis of the fertiliser potential of the resulting digestate by-products has also been undertaken. All the feedstocks tested were found to be suitable for biogas production with grass whey, the most suitable candidate with a biogas and biomethane production yield of 895.8 and 544.6 L/kg VS, respectively, followed by de-FOS whey and press cake (597.4/520.3 L/kg VS and 510.7/300.3 L/kg VS, respectively). The results show considerable potential for utilizing biorefinery by-products as a source for renewable energy production, even after several value-added products have been co-produced.

Donkor, K. O., Gottumukkala, L. D., Lin, R., Murphy, J. D. (2022) A perspective on the combination of alkali pre-treatment with bioaugmentation to improve biogas production from lignocellulose biomass, Bioresource Technology 351



Anaerobic digestion (AD) is a bioprocess technology that integrates into circular economy systems, which produce renewable energy and biofertilizer whilst reducing greenhouse gas emissions. However, improvements in biogas production efficiency are needed in dealing with lignocellulosic biomass. The state-of-the-art of AD technology is discussed, with emphasis on feedstock digestibility and operational difficulty. Solutions to these challenges including for pre-treatment and bioaugmentation are reviewed. This article proposes an innovative integrated system combining alkali pre-treatment, temperature-phased AD and bioaugmentation techniques. The integrated system as modelled has a targeted potential to achieve a biodegradability index of 90% while increasing methane production by 47% compared to conventional AD. The methane productivity may also be improved by a target reduction in retention time from 30 to 20 days. This, if realized has the potential to lower energy production cost and the levelized cost of abatement to facilitate an increased resource of sustainable commercially viable biomethane.

Donkor, K. O., Gottumukkala, L. D., Diedericks, D., Gorgens, J. F. (2021) An advanced approach towards sustainable paper industries through simultaneous recovery of energy and trapped water from paper sludge, Journal of Environmental Chemical Engineering 9(4): 105471


This study considered the possibility of reducing the environmental footprint of paper and pulp industry by producing bioenergy from paper sludge by using process wastewater instead of fresh water, and reclaiming water trapped in paper sludge. Experimental studies are conducted with streams from three different pulp and paper mills (virgin pulp mill (VP), corrugated recycling mill (CR), tissue printed recycling mill (TPR)) for sequential bioethanol and biogas production with simultaneous reclamation of water from paper sludge (PS). Total energy yields of 9215, 6387, 5278 MJ/tonne dry PS for VP, CR and TPR, respectively, were obtained for ethanol-biogas production. Virgin pulp paper sludge gave the highest yield for ethanol and biogas in stand-alone processes (275.4 kg and 67.7 kg per ton dry PS respectively) and also highest energy conversion efficiency (55%) in sequential process compared with CR and TPR. Energy and environmental case study conducted on virgin pulp mill has proven the possibility of using paper sludge bioenergy to reduce energy demand by 10%, while reclaiming 82% of the water from the PS, reducing greenhouse gas emissions (GHG) by 3 times and producing solids suitable for land spreading.

Gottumukka L.D, Haigh K, Collard F.X, Van Rensburg E, Gorgens J (2016) Opportunities and prospects of biorefinery-based valorisation of pulp and paper sludge, Bioresource technology 215: 37-49


The paper and pulp industry is one of the major industries that generate large amount of solid waste with high moisture content. Numerous opportunities exist for valorisation of waste paper sludge, although this review focuses on primary sludge with high cellulose content. The most mature options for paper sludge valorisation are fermentation, anaerobic digestion and pyrolysis. In this review, biochemical and thermal processes are considered individually and also as integrated biorefinery. The objective of integrated biorefinery is to reduce or avoid paper sludge disposal by landfilling, water reclamation and value addition. Assessment of selected processes for biorefinery varies from a detailed analysis of a single process to high level optimisation and integration of the processes, which allow the initial assessment and comparison of technologies. This data can be used to provide key stakeholders with a roadmap of technologies that can generate economic benefits, and reduce carbon wastage and pollution load.

Gottumukkala L.D, Parameswaran B, Valappil S.K, Pandey A (2014) Growth and butanol production by Clostridium sporogenes BE01 in rice straw hydrolysate: kinetics of inhibition by organic acids and the strategies for their removal, Biomass Conversion and Biorefinery 4(3): 277-283


Growth inhibition kinetics of a novel non-acetone forming butanol producer, Clostridium sporogenes BE01, was studied under varying concentrations of acetic and formic acids in rice straw hydrolysate medium. Both the organic acids were considered as inhibitors as they could inhibit the growth of the bacterium, and the inhibition constants were determined to be 1.6 and 0.76 g/L, respectively, for acetic acid and formic acid. Amberlite resinsXAD 4, XAD 7, XAD 16, and an anion exchange resinSeralite 400 were tested for the efficient removal of these acidic inhibitors along with minimal adsorption of sugars and essential minerals present in the hydrolysate. Seralite 400 was an efficient adsorbent of acids, with minimal affinity towards minerals and sugars. Butanol production was evaluated to emphasize the effect of minerals loss and acids removal by the resins during detoxification.