• Process Optimisations
    for AD and RNG Projects
    At Celignis Biomass Lab


There a large number of factors to consider when running an anaerobic digestion (AD) facility, including:
  • The feedstock(s) used and their composition.
  • Whether multiple feedstocks (co-feeds) are used and, if so, in which proportions.
  • The Organic Loading Rate (OLR), defined as the amount of organic matter that is fed to the reactor per unit time and per unit reactor volume.
  • The Hydraulic Retention Time (HRT) and Solid Retention Time (SRT), defined as the average time periods over which the liquids and solids are retained in the digester.
  • The viscosity of the feed and digestates and the strategies that can be employed to reduce the negative effects of high viscosity.
  • Whether, and to what degree, nutrients and trace metals are supplemented to aid with digestion.

If a biogas plant is operating below expectations, it may be due to one of the above factors being inappropriate for the process conditions of the facility, leading to negative effects on biogas and biomethane yields. Underperformance of the biogas plant and downtimes due to accumulation of inhibitors means that revenues for the facility may end up lower than they could otherwise be and it may also mean that process residues (digestate) arise in greater quantities as a result of less efficient digestion of the feedstock(s).

Furthermore, if these performance issues are not addressed, matters may continue to deteriate over time, increasing the risk of potentially catastrophic events, such as a digester crash.

It is therefore of key importance that the process is optimised, and that limits for key process indicators are established, in order to maximise revenues from the facility and ensure that plant performance is stable and predictable over time. Such optimisations can be designed and experimentally validated at the lab-scale prior to implementing them at the AD facility.

Some of the process optimisations that Celignis personnel can undertake for AD projects are detailed below.

Co-Digestions of Feedstocks

The type of feedstock used is of great importance as it will influence the optimal AD process conditions as well as the quality of the biogas and digestate. AD facilities that rely on a small subset of agricultural residues and energy crops and on-site AD facilities that deal with the outputs of a particular industrial process tend to have relatively consistent supplies of reasonably homogeneous feedstocks. However, stand-alone AD plants that use feedstocks from a variety of sources tend to have inconsistent levels of supply as well as heterogeneous feedstock compositions that can cover wide ranges of levels in: complex carbohydrates, proteins, lipids, sugars, and acids.

Co-digestion with nutrient and pH complimenting feedstocks can be one of the several ways to improve the process efficiency. However, the fine-tuning of the co-feed ratio of different feedstocks for co-digestion, organic loading rate, and hydraulic retention time optimisation will be dependent on the particular feedstocks being considered.

To further complicate matters, each feedstock acts differently when it is used individually and in combination with other feedstocks. This is due to the interaction effects with other feedstocks and balance/imbalance in nutrients, buffering capacity, C/N ratio, among other factors. For example, the waste from fruit juice industry by itself is not a good feedstock for biogas production as it can cause acid-crash in the digester. However, when it is co-digested with protein-rich waste the stability and long-term performance of the digester improves significantly.

At Celignis we follow a scientifically-based Design of Experiments (DoE) protocol for undertaking optimisation trials for projects where a number of feedstocks are being considered. The criteria and boundaries for this DoE are formulated in close collaboration with our clients, considering factors such as the availability of each feedstock and whether certain feedsocks are preferred over others (due to higher gate fees, for example).

The experiments that we then undertake are typically based on lab-scale digestions (either batch tests, i.e. the biomethane potential (BMP), or continuous digeistions) over which feedstocks are included/excluded or added at varying proportions. The results of these experiments are then interpreted by our AD team leding to recommendations on which feedstocks are suitable and unsuitable and at which levels the suitable feedstocks can contribute to the total feedstock mix.

For example, in a scenario where we consider a number of feedstocks for a municipal-waste AD facility we can produce a Pareto chart showing the effect of each individual feedstock on the mix. In this chart the median line (“0”) indicates the baseline and the bars towards the positive axis indicate positive contribution and the bars towards the negative axis indicates negative contribution regarding BMP. The values indicate % variable effect.

In another scenario we can optimise the relative amounts of each feedstock contributing to the total mix, again based on experiments undertaken in our laboratories. Given the number of potential variables at play (feedstocks, individual feedstock proportions etc.) there can be an extremely large number of permutations available. However, we carefully design our experiments so that, initially, we scope across the different options in a targeted and efficient manner. This initial scope then provides us with the boundary regions of most interest, within which we can undertake a number of additional, more focused, experiments in order to find the optimum scenario. Such an approach reduces the total number of experiments undertaken and is more time- and cost-effective.

The figure presented here shows the percentage of methane in the biogas and how it varies according to the relative proportions by which fruit waste is co-fed with two different feedstocks. It can be seen that the optimum ratio varies substantially according to which co-feed feedstock is used.

Such data can also feed into our Techno-economic Analyses (TEAs) of potential AD projects, whereby other factors (e.g. gate fees, predictability of feedstock supply) can factor into the metrics and inform what feedstock mix is ultimately proposed.

Improved Biogas Composition

AD efficiencies can also be reduced when organic matter is converted to carbon dioxide rather than methane. This phenomenon can be avoided by increasing the concentration of methanogens in the AD reactor either by augmenting with a high-methane producing inoculum source or by providing feedstock and process conditions that can enrich methanogens concentrations.

At Celignis we can undertake such experiments targetting improvements in biogas composition.


Bioaugmentation is a recently developed strategy for enhancing anaerobic digestion efficiencies. It involves the use of microbes or enzymes, grown outside of the AD system, that are chosen according to their specificity for a particular stage of the AD process, being added to the digester in order to improve feedstock digestion. Historically, this approach has focused on the bioaugmentation of microbes (e.g. cellulolytic bacteria) or enzymes to assist with the hydrolysis stage, however it is also possible to work on augmentations that improve the other stages (acidogenesis and methanogenesis) of the AD process.

At Celignis we can undertake experiments to assess the specific microbial activities of samples from your digesters, leading to data on the specific activities for each stage (hydrolysis, acidogenesis, and methanogenesis). Based on these results we can work on a bespoke bioaugmentation strategy that will target an enhancement in activity for those stages that are underpeforming.

This strategy will then be tested, at various levels of bioaugmentation, in a series of appropriately-designed lab-scale experiments (undertaken in either batch (i.e. BMP tests) or continuous digestions).

The results of these experiments then allow us to provide recommendations on appropriate bioaugmentation strategies, considering other important factors (such as the cost of the extra microbes or enzymes).

Click here to read more on our specific microbial activity tests.

Nutrient Supplementations

Nutrients are essential for maintaining stable microbial populations and for efficient enzymatic activities throughout the four stages (hydrolysis, acidogenesis, acetogenesis, and methanogenesis) of anaerobic digestion. Certain trace elements, such as iron and selenium are also useful in combating the toxic effects from H2S, volatile fatty acids (VFAs), and ammonia. Trace elements can also help in increasing the organic loading by increasing the overall kinetics of anaerobic digestion.

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.

We can also test the effects of nutrient supplementation in lab-scale experiments whereby the strategy is compared against the control in our biomethane potential tests. Additionally, nutrient supplementation approaches can be tested in continous digestions, lasting three months or longer, undertaken in our laboratories using reactors up to 12 litres in volume.

Click here to read more about our services for testing nutrient supplementations in AD systems.

Optimised Organic Loading Rates (OLRs) and Inoculum Adaptation

A greater Organic Loading Rate (OLR) will allow for more substrate to be processed in a given period of time and reactor area. However, it is important that an increase in the OLR should be performed in a systematic way to avoid accumulation of VFAs, undigested solids, ammonia, and hydrogen sulphide.

Accumulation of any of these components will lead to a signficant drop in the digester performance and may even lead to digester crash.

Our continuous digestion experiments are particularly useful to see what OLR can be reached, and at what level of performance, for a given feedstock or set of feedstocks. The general approach we follow is that we start with a relatively low OLR which is then gradually increased over a period of weeks with the biogas outputs and process indicators carefully monitored.

Where an increase in the OLR leads to accumulation of the above-mentioned compounds or a decrease in biogas yields we will consider strategies (e.g. nutrient supplementation, bioaugmentation) to obtain the maximum yields at maximum achievable OLR. In cases where these strategies are not successful we then return to the prior ORL level, where the efficiency of digestion was not impacted, and assign this as the practical maximum OLR for continuous operations. These lab-scale trials allow for OLR experimentation without running the risk of negatively affecting digester operations.

Additionally, continuous digestions allow us to see whether inoculum can adapt to novel feedstocks, or initially unfavourable process conditions, by implementing a structured adapataiton and start-up phase.

Additional Information on Process Optimisations

Feel free to get in touch with us if you have any questions about our services for process optimisations 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.

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.

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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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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.

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 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.