• Biological Consultation Services
    for AD and RNG Projects
    At Celignis Biomass Lab

The Importance of Understanding the Biology of Anaerobic Digestion

Anaerobic digestion (AD) is a fully microbial driven process and requires efficient performance from each class of bacteria involved in the digestion of the feedstock and its conversion to methane.

A wide variety of factors can affect the microbial population and its activity, these include the operating conditions of the AD reactor and the nutrient composition of the feedstock.

There are several process process parameters of the digester (such as FOS/TAC, the concentrations and distributions of volatile fatty acids, ammonia, and hydrogen sulphide concentrations) which can be used to evaluate the health of the digester.

Additionally, regarding the feedstock(s) used, underperformances of digesters can be due to a number of factors, including: incompatible feedstock mixes, high organic loading rates, or inherent very high or low concentrations of certain elements (e.g. sulphur) in the feedstock.

Relating all these data, and their complex interplays, with regards to the biogas and biomethane yields from the anaerobic digestion plant requires a detailed understanding of how the biological balance of the digester is affected. These associations, along with additional dedicated tests that we can undertake at Celignis (such as Specific Microbial Activity (SMA) tests and the Anaerobic Toxicity Assay), allow for such an expert to understand which class of bacteria is getting affected. This can then lead to the formulation of strategies to improve the performance of that class of bacteria.

Such strategies can often involve making alterations to the organic loading rate (OLR), the relative proportions of the different feedstocks used in the digester, and the use of additives (major and minor elements). An expert AD biological consultant can also use process data to spot early-warning indicators that, unless addressed, could lead to a decrease in reactor performance over time. By identifying these markers early, the consultant can recommend remediation strategies that should prevent such a reduction in digester performance. Examples of such early-warning signs include the presence of certain iso-forms of volatile fatty acids (VFAs) above threshold values identified by the biology expert.

Biological Consultation Services at Celignis

Our primary Biological Consultant is Dr Lalitha Gottumukkala, Celignis's Chief Innovation Officer. Her PhD focused on the development of a biological process and she has been actively involved in the development and evaluation of AD technologies for over 10 years. At Celignis Lalitha has been providing AD/RNG biology consultations to clients in Ireland and across the globe. She is able to pour through operational data from biogas plants and identify the correlations between process parameters and digester performance. This then leads to an understanding of what factors were responsible for fluctuations in digester performance and to recommendations as to how peformance can be improved and made more stable. Lalitha is supported by other members of the Celignis AD team who are available to undertake additional tests on digester samples and the feedstocks used, in cases where Lalitha identifies that additional data may assist in recommendations for improving the biological conditions in the digester.

Examples of charts that Lalitha produces in her review of digester process data are presented here. The outputs of her investigations will include recommendations on various parameters including: feed mix ratios, retention times, inhibitors, trace elements and additives supplementation; and other means of improving efficiency.

For example, we can provide optimal values for the presence of major and minor elements in the digester as well as upper and lower threshold values. For elements considered to be of major importance to reactor performance, in cases where the concentrations are below the recommended minimum values we would advise supplementation of these elements. This allows us to formulate a bespoke cocktail of additives, designed according to the specific requirements of the digester. Such custom cocktails are more likely to address digester peformance issues than off-the-shelf nutrient mixes. Similarly, if certain elements are found to be at concentrations over the upper threshold values, we can then suggest strategies for reducing these levels.

How it Works

Our Biological Consultation services are formulated according to the specific needs of our clients. We work in close association with plant operators to collect all the required data. We can review historical data (for example, those obtained at the plant) as well as analytical data from third-parties (i.e. other labs). We can also directly analyse your digester and feedstock samples at our laboratories for routine process data (e.g. FOS/TAC, VFAs etc.) and Key Performance Indicators (KPIs). Additionally, we can also undertake analysis methods targeted towards deep investigations of the progress or inhibition of microbial activities.

We would recommend that a biological audit of a digester is undertaken once every quarter or whenever new feedstock is used. Also, if you have experienced a drop in digester performance, or even a crash in the digester, our biology consultation services can help to identify the root cause and appropriate strategies to not only address the current issue but to also avoid the event recurring.

Case Study of a Celignis Biological Consultancy Project

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 AD Biology Consultations

Feel free to get in touch with us if you have any questions about our biological consultation services or if 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.

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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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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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.