• Project Development Assistance
    for AD and RNG Projects
    At Celignis Biomass Lab

Anaerobic Digestion (AD), often referred to as Renewable Natual Gas (RNG) in North America, is a technology that is seen to be of increasing importance given its ability to provide sustaniable renewable energy while also valorising feedstocks that may otherwise be problematic waste resources. This has resulted in AD projects proliferating thoughout the world over recent years. In particular, where nations or regions have implemented schemes that favour AD technologies, for example the Californian Low Carbon Fuel Standard (LCFS), the growth of the AD/RNG sector has been especially marked.

However, the criteria for the development of a successful AD project are numerous and their relative importances vary according to: the region in which the project is being developed; the technology being implemented; and the feedstocks being processed, among other factors.

The Celignis AD team have a broad and deep understanding of these regional, technical, and biological differences and have advised a global network of clients on how their specific AD projects can be most effectively developed. Below we outline some examples of how we can assist at the various stages of AD project development.

Before Constructing your AD Facility

Evaluating Your Feedstock(s)

If your Project is focused on the digestion of a specific feedstock (or number of feedstocks) then we will focus our initial work on the detailed compositional analyses and digestion (batch and continuous) experiments on these feedstocks. We can suggest, based on these results, the most appropriate AD technology (e.g. UASB or CSTR) for your feedstock(s) and we can also look at addressing any issues that may arise from the use of these feedstocks, for example feedstock toxicities and nutrient imbalances.

Choosing Feedstock(s) for Your Technology

Alternatively, you may be open to using different feedstocks but fixed with regards to the technology and size of AD process that you will be developing. In that instance we can audit a range of different feedstocks that you have available within the catchment area and evaluate which of these would be most suitable for the selected process and would allow for optimal biomethane yields and/or project revenues. We can then undertake lab-scale experiments where the co-feeding of feedstocks, and the supplementation of nutrients and/or microbes, is optimised.

Technoeconomic Analyses (TEAs)

Our TEAs can take places at several stages, and to varying levels of detail, during the project development process. We can explore various options (e.g. differing reactor sizes, variable levels of subsidies, feedstock supply curves) to investigate their relative impacts upon key project financial metrics (e.g. IRR, Payback Period etc.).We can also undertake Life Cycle Analyses to explore the carbon intensity of the process and we can examine how the project can fit under existing rgulatory and support frameworks (e.g. LCFS, D3 RINS, RED II etc.).

Risk Evaluations

Based on our expertise in biological consultations, biomass feedstocks, and process design, we can undertake an external review of your plans for your future AD facility with a target of identifiying areas in which the project can be improved and elements of the project that may be exposed to higher levels of risk. We then work on identifying and recommending risk-mitigation strategies.

Developments at an Existing AD Facility

Changes in Feedstock

If you are looking to change the feedstock(s) being processed in your AD plant, or just to modify the relative proportion each feedstock contributes to the total mix, we would strongly recommend that these changes are first tested at the lab-scale. This will allow for the effects of these adjustments, on biogas/biomethane yields and other important process parameters, to be understood and for approriate modifications to be made to the planned activities so that plant performance will not be negatively affected. Undertaking such modifications directly at the plant would mean that you run the risk of reducing digester yields or, potentially, of causing a digester crash.

Optimising Process Conditions

If you are looking to improve the performance of your digester, either for increased biogas yields or for greater profitability, then we would also recommend that any planned changes to the process are first studied at the lab scale rather than being directly implemented in your commercial process. Examples of process optimisations that we can undertake include: increasing the Organic Loading Rate (OLR); targeted supplementation with nutrients and trace elements; and bioaugmentations.

Ensuring Stable Digester Performance

In an ideal world there would be no performance issues with digesters as they would always operate under stable conditions. This would avoid the revenue losses associated with reduced efficiencies and would also mean that there would be no risk of potentially catastrophic digester crashes. The regular collection of relevant process parameters can help significantly with regards to identifying warning-signs of imminent drops in reactor performance and so can allow for appropriate mitigation strategies to be employed. These strategies would result in the predicted drop in performance being avoided and would ensure that the digester remains stable.

In order for this approach to work most effectively, the process analytical data should be interpreted by someone that is an expert in the process and biology of AD systems. At Celignis we have such experts who provide valued biological consultancy services to clients in the AD/RNG sector using such process data. We can be sent samples and obtain the analytical data ourselves within our labs or we can use analytical data that you provide us. We can also do a historical audit of your process parameters and correlate these to performance metrics of your digesters. From this work we can identify warning-signs and causative factors for prior events, allowing for appropriate mitigation strategies to be developed for avoiding such issues in the future.

Making Most of Your Digestate

Digestate is the residual matter left after the AD process. It can have value in being applied to the soil as a fertiliser plus it can potentially be valorised in many other ways, including as animal bedding, in fuel pellets, as a construction material, and as a feedstock for the production of high-value biobased products.

However, the potential avenues for valorising the digestate are highly dependent on its physical and chemical parameters. We offer an array of different analysis packages for digestate that help our clients figure out the best use for this side-stream. We are also familiar with regulatory requirements with regards to the different end-uses of digestate.

Additionally, we can provide further support in terms of the technoeconomic analysis of the various options for using, selling or upgrading your digestate.

Click here to read more about our services for AD digestates.

Project Development Case Studies

AD Feedstock Mixture Optimisation Tool

Celignis was approached by a large beverage production company to determine the feasibility of utilising their waste streams for biogas production and to determine the additional feedstock requirement to meet the full plant energy demand. Celignis performed the required biological and chemical analysis of the facility's waste streams and developed a spreadsheet tool for feedstock mixtures design to allow the conversion of the sugar and acid rich waste stream to biogas and to meet the energy requirements of the company.

The tool considered seasonality of the locally-available feedstock that could be used as co-feed with the sugar rich waste streams. Also considered, while designing the feedstock mixtures, were Renewable Energy Directive (RED) II GHG emission targets and waste to energy crops ratio.

Greenhouse has (GHG) emission reductions and carbon dioxide that could be captured and the total revenue generation from biogas and CO2 were also estimated. The tool allowed the company to make informed decisions on the project and understand the biogas potential and feedstock requirements to meet the target power requirement.

Continuous Digestions for Improved OLR

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.

Stabilising Plant Performance

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.

Feed Limits for New AD Streams

A biogas plant started underperforming when a new feedstock was used as co-feed to the plant. As the plant received an important gate-fee for this new feedstock, they did not want to discontinue its use but to instead use it in a controlled and scientifically-driven manner. Celignis was asked to provide support for: determining the toxic effects of the feedstock; the causes of it; and to provide feeding limits.

Celignis undertook chemical analyses on the feedstock and then custom-designed and performed Anaerobic Toxicity Assays (ATA) for the waste stream based on the analytical data. These experiments allowed for the determination of threshold feedstock-loadings, as co-feed, in order to avoid the toxic/inhibitory effects of the feedstocks.

The biogas plant is now benefiting from Celignis's support since they can make informed decisions on using feedstocks coming various process industries and so can maintain healthy digestion while incorporating new waste streams into the feed-mix of the RNG plant.

Additional Information on Project Development

Feel free to get in touch with us if you have any questions about how we can help you develop or optimise your AD project. Relevant members of the Celignis anaerobic digestion team will be happy to assist. Those team members with the most experience in these activities 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.

Oscar Bedzo

Technoeconomic Analysis Lead


A dynamic, purpose-driven chemical engineer with expertise in bioprocess development, process design, simulation and techno-economic analysis over several years in the bioeconomy 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.

Further Info...

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.

Further Info...

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.

Further Info...

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.

Further Info...

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.

Further Info...

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.

Further Info...

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).

Further Info...

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.

Further Info...

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.

Further Info...

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.

Further Info...

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.