• Continuous Digestions
    For AD Feedstocks
    At Celignis Biomass Lab

Continuous Digestions Compared with Standard Biomethane Potential (BMP) Tests

At Celignis we have a range of analysis packages that can determine the biomethane potential of feedstocks. The biomethane potential (BMP) can be considered to be the experimental theoretical maximum amount of methane produced from a feedstock.

We determine the BMP following the standard VDI-4630 protocol that involves mixing the organic substrate with an anaerobic inoculum in a closed reactor that is incubated at a set temperature, with the contents mixed, for a set period of time (between 14 and 40 days). During this period the sample is digested and biogas is produced. The volume of biogas is monitored allowing for a cumulative plot of biogas production over time to be derived. We then analyse this biogas for its composition, in particular the methane content, in order to allow the BMP to be determined.

These conditions are designed to allow for the maximal output of biogas, on a volatile solids basis, to be obtained from the sample. For example, the BMP test uses a very low concentration of sample, versus the inoculum, and thus any limitations (e.g unoptimal C/N ratio) associated with the sample are masked in the test. Additionally, the BMP test is a batch test meaning that, after the reactors are loaded, no adjustments are made to the contents of the reactor over the digestion period. Clearly, these conditions are very different from those that are experienced in a commercially-operating digester where there are regular additions of feedstock and removal of digestate and where the organic loading rate is signficiantly higher.

As a result, while the BMP test is a very useful quick and relatively low-cost test to evaluate a range of RNG feedstocks and to determine their maximal biogas outputs, it is recommended that lab-scale continuous digestions are undertaken for the feedstocks of most interest in order to ascertain what real-world biogas and biomethane yields may be and whether there may be any physcial or operational constraints associated with their processing. Such physical constraints may include the formation of scum layers, foaming, and mixing inefficiencies due to the viscosity of the feedstock. Continuous digestions can also determine the maximum achievable organic loading rate (OLR) of the feedstock to be determined.

How we Undertake Continuous Digestion Trials at Celignis

We design continuous digestion trials for your feedstock in order to provide you with the most relevant data to understand the true performance of your feedstocks and their biomethane potentials in real operational conditions.

While our standard batch BMP tests involve 1-litre reactors, our continuous digestions use larger reactors (we can use either 5-litre or 12-litre systems, according to your preference) so that the real operational and physical performance of the samples can be evaluated.

Our continuous digestions operate on a daily feeding and daily sampling protocol. We target the maximum achievable organic loading rate (OLR), with this approach initially involving operating at lower OLRs which are then increased slowly, in a methodoical manner. The specific strategy employed will be customised based on the feedstock to be process but, for example, when undertaking coninuous digestions of herbaceous materials we typically start at an OLR of 1.5 g/L with step-wise increases at 0.5 g/L over a period of around 3 months. The maximum achievable OLR depends on the time required for stabilising at each step wise increase.

As part of the continuous digestion experiments we can undertake the following analyses in order to understand the stability of the reactor:
  • Biogas production.
  • Biogas composition.
  • Biomethane production.
  • pH of the mixture.
  • FOS/TAC (volatile fatty acids to alkalinity ratio).
  • Ammonia.
  • Minerals and metals accumulated in the digestate.
  • Soluble to insoluble COD ratio in the digester.
  • Volatile fatty acids (VFAs) speciation.
  • Viscosity.

Operational sweet-spots in terms of organic loading, and FOS/TAC target value will be defined. Any characteristics of the feedstock (such as foaming, scum layer formations, excessive viscosity increase, increase in soluble COD, drop in pH, increase in ammonia, etc.) will be recorded for the conditions tested in continuous operation and troubleshooting will be performed to bring the digester back to the original performance level.

For feedstocks with suboptimal nutrients composition, a nutrient mix can be designed and the maximum achievable OLR and biomethane potential with and without nutrients supplementation will be determined. This additional optional will involve the use of two continuous digesters for the comparisons to be made.

Celignis Packages for Continuous Digestions

For our continous digestions we work closely with our clients in order to formulate the most appropriate experimental design and process conditions. Our pricing works on a per-reactor per-month basis and also reflects the amount of additional analyses the client requests over the course of the digestion. We typically recommend that continuous digestions be undertaken for 3 months, however shorter and longer periods can also be used and it is also possible to extend the experiment beyond the initially envisaged period.

Our capacity for undertaking these tests is limited by the number of suitable reactors that we have available at any one time. Due to the popularity of these continuous digestions we are currently working on increasing our bioreactor capacity in order to keep up with demand. If you would like us to undertake continuous digestions of your feedstock(s) we would recommend that you get in touch with us in advance in order to book available bioreactor slots so that we can allot start dates for your digestions.

The Celignis Analysis Package(s) that we use for continuous digestions of feedstocks are listed below.

Case Study on Continuous Digestion at Celignis

Celignis carried out continuous digestion experiments, for a company that produces biogas from OFMSW (the organic fraction of municipal solid waste) and other waste streams, in order to determine the maximum achievable organic loading rate and optimum feedstock mixtures. These experiments also determined the minimum organic loading rate to maintain the health of the plant in the scenarios where feedstock availability was limited.

This continuous digestion data, combined with the specific microbial activity tests (Specific Hydrolytic Potential (SHP), Specific Acidogenic Potential (SAP), and Specific Methanogenic Potential (SMP)) on the digestate, provided the plant with the microbial activity in the operational digester and adaptation strategies for the new feedstock.

The full suite of tests and data analysis performed by Celignis allowed the biogas plant operator understand the limitations of the feedstock, feedstock underload/overload effects, optimum feedstock loading, and process indicator (volatile fatty acids (VFAs), alkalinity, biogas production, biogas composition) ranges at different organic loading rates and feedstock mixtures. This allowed adapting the strategies in the biogas plant for maintaining the plant health under feedstock supply and composition variations.

Click here for more information on our Biological Consultation services for the anaerobic digestion/RNG sector, and here for information on our Process Optimisation services.

Additional Information on Continuous Digestions

Feel free to get in touch with us if you have any questions about our continuous digestions or if if you looking to see how well previous BMP data that you have obtained will translate to conditions more relevant to real-world processing. 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.

Dan Hayes

Celignis CEO and Founder

PhD (Analytical Chemistry)

Dreamer and achiever. Took Celignis from a concept in a research project to being the bioeconomy's premier provider of analytical and bioprocessing expertise.

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.

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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.

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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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Process Parameters

Celignis can undertake a range of key analyses for KPIs and advanced process monitoring. These include volatile fatty acids (VFAs); Alkalinity ratio (FOS/TAC); and redox potential. It is particularly imporant that these are monitored when undergoing changes of feedstock type, organic loading rate and hydraulic retention times.

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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 resins—XAD 4, XAD 7, XAD 16, and an anion exchange resin—Seralite 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.