• Digestate Analysis
    for AD and RNG Projects
    At Celignis Biomass Lab


Digestate is the residual matter left after the anaerobic digestion (AD) process. Nutrients are preserved in the digestate during AD meaning that it can be rich in macronutrients such as nitrogen, phosphorous, and potassium, as well as in micronutrients. Hence, it can have value in being applied to the soil as a fertiliser. Additionally, the fibre in the solid residues of digestate can help to build up the humus content in the soil which is advantageous for soils of a low carbon content.

Digestate can also improve a number of physical parameters of the soil, such as available water capacity, workability and drainage. In selling the digestate to local farmers as a fertiliser, AD/RNG facility operators can increase their revenues from the process, whilst farmers can at the same time make significant savings in using nutrient rich digestate as opposed to more-costly mineral fertilisers.

The Anaerobic Digestion and Bioresources Association (ADBA) stated that "one tonne of artificial fertiliser replaced with one tonne of biofertilser saves 1 tonne of oil, 108 tonnes of water and 1 tonne of carbon dioxide emissions".

Digestate also has other potential applications other than as a soil amendment. The fibre from the digestate can be valorised in a wide variety of means, including uses as animal bedding, in fuel pellets, as a construction material, and as a feedstock for the production of high-value biobased products.

The digestate output from anaerobic digestion contains high volumes of water and can be treated by a variety of methods including: drying, ammonia stripping and drying, biological treatment, and reverse osmosis. The liquid and solids fractions of the digestate can be separated in a number of ways, but typically mechanical separation is used. The separated solids can be either composted or dried and pelletised, although composting is not necessary if the final application of the digestate is for land-spreading.

Before using digestate for any applications, it should be tested for fibre content, fibre composition, chemical oxygen demand (COD), biological oxygen demand (BOD), NH4-N, nitrogen-phosphorous-potassium (NPK), micronutrients, trace elements, heavy metals, microbial load and pathogens.

Important Analyses for Digesates

Digestate Chemical and Biological Analysis

Celignis Analysis Package P94 (Digestate Chemical and Biological Analysis) includes a number of important analyses that will help to assess the suitability of the digestate for land applications. The nitrogen, phosphorous and potassium (NPK) ratio of the digestate is an important parameter for its applicability as soil enrichment or fertiliser and the free ammonia content in the digestate is an indicator for the possible ammonia emissions from direct land application.

Major & Minor Elements

Analysing for the major and minor elements in the digestate, along with the concentrations of heavy metals, will allow for decisions to be made with regards to whether it is a suitable material for land application and does not exceed regulatory limits. Additionally, free ammonia data together with the major elements (calcium, magnesium, phosphate) will help in designing ammonia recovery strategies (as ammonium phosphates, ammonium magnesium phosphates, ammonium sulphates etc.) using existing technologies.

Digestate Impurity Content

The digestates originating from industry process streams and mixed solid waste streams, such as municipal solid waste, should be tested for impurity content (plastic, metals, glass, gravel).

Volatile Fatty Acids

We have developed an optimised chromatography protocol for determining not only the total content of volatile fatty acids (VFAs) but also quantifying the different species present.

Residual Biogas Potential

The efficiency of the AD process can be tested by performing simple mass balances based on the composition of the feedstock and its volatile solids content coupled with the same set of analyses on the digestate. An efficient biogas plant will maximise the digestion of organic matter and its conversion to methane, resulting in minimal amounts of organic material in the digestate.

However, in many cases AD facilities do not fully digest the feedstock, typically as a result of high organic loading rates and low hydraulic retention times. Hence, it is often warranted that the digestate is tested for its residual biogas potential (RBP) in order to determine how efficiently the feedstock has been digested.

The RBP is a lab-based test, based on the BMP test, that takes digestate as the starting material and determines how much biogas can be produced from its digestion in controlled conditions. Any biogas produced in the RBP tests can be considered to be lost potential yields from the previous digestion of the sample in the AD system.

We can undertake residual biogas potential (RBP) analysis on the digestate from AD facilities in order to determine the biogas yields from this residue. This test can be undertaken for between 14 and 28 days and the Deluxe version of these analysis packages also includes all analytes listed in the Digestate Analysis package.

The Celignis Analysis Package(s) that determine the RBP are listed below:

Request a QuoteRBP

Other Digestate Analyses

The positive (soil enrichment) and negative (phytotoxicity) effects of the digestates by land application are studied by: (1) Conducting seed germination test; and, (2) Plant growth trials.

In the plant growth trials, in addition to plant growth (root and shoot length), the effect of digestate on leaf area, chlorophyll content of the leaf, stomatal densities and nutrient use efficiency are also assessed upon request.

We will also soon launch ecotoxicity tests that will estimate the toxicity or beneficial effects of digestate across different microscopic organisms (bacteria, microalgae, planktonic crustacean), aquatic and terrestrial plant species.

Additional Information on Digestate Analyses

Feel free to get in touch with us if you have any questions about our analytical services or if you are looking for opportunities to get the most value from your digestate. 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.

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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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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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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.