Abstract:
Anaerobic digestion is able to stabilise wastes, generate bioenergy and a product that is high in nutrients. AD is increasingly being used by many industries. It is important to accurately and precisely measure the potential biogas production from a variety of feedstocks and digestates. These measurements are valuable for performing plant technical and economic appraisals and for assessing stability of digestates. Three methods of measuring gas production from batch biodegradability systems were assessed. Results suggested that the commercially available Oxitop®-C system can measure total biogas but not methane production. The automated methane potential test system (AMPTS) provided accurate results for methane potential, but was inaccurate to measure total gas. The method that used Tedlar gas bags for gas collection was found unreliable with potentially large errors.
Key words
Oxitop; AMPTS; gas bag; biogas potential; methane potential.
Introduction
Anaerobic digestion (AD) of organic wastes is currently being encouraged by current European regulations and in many cases by incentives attributed to the renewable energy produced. AD systems in addition to energy generation are also able to recover valuable nutrients for beneficial use in agriculture. AD systems are increasingly being used by local authorities, water companies, agro-industries and farmers to digest wastes and energy crops. In Europe, in 2007, approximately 170 AD plants process over 6 million tonnes of substrate per annum with a large proportion of municipal solid wastes as feedtstocks (Monson et al., 2007). Digestion capacity is continually growing across Europe. In addition, many more digest agriculture residues and energy crops e.g. over 4500 AD plants in Germany alone.
The ability to determine accurately anaerobic biodegradability of substrates for digestion and of the digestate produced has become essential for designers and operators of AD systems as this can provide important information for technical, financial as well as performance evaluations. Accurate information about potential methane yields from various feedstocks is even more fundamental, specifically in AD plants where bioenergy constitutes the only revenue stream.
Anaerobic biodegradability is defined as the fraction of compound or compounds that can be converted into biogas under anaerobic conditions (Guwy, 2004). The biodegradability can be determined gasometrically by the volume of biogas or methane produced or by the amount of substrate depletion or the formation of intermediates or end products. Alternative to gasometric tests a number of methods have been developed to determine biodegradability of various materials (Lesteur et al., 2010). These methods include aerobic respirometry, composition analysis i.e. elemental (C, H, O, N, S) and component (carbohydrates, proteins, lipids, fibres, lignin), pyrolysis combined with GC, UV and Infra Red (mid and near) spectroscopy methods. At this stage, many of these alternative methods are either unable to provide what gasometric tests can in terms of kinetic information. In some cases there is an overestimation of the methane potential (in elemental analysis due to the non accounted inorganic components) or require a significant calibration phase e.g. NIR (Lesteur et al., 2011). So despite the time and costs involved in gasometric biodegradability based testing, the method is widely used and continues to generate significant interest in both the academic and industrial AD worlds.
There are two techniques for determining gas production. The first uses a manometric method which involves keeping the volume of gas constant and the pressure increase is measured, the second uses a volumetric method, which keeps the pressure constant and the volume of gas produced is measured.
Significant interest has in recent years turned to measuring digestability using batch Biochemical Methane Potential (BMP) tests and standardisation efforts of the methodology are on-going (e.g. Angelidaki et al., 2009). The BMP test has been stated to provide methane potential and digesters can be designed and operated from the digestion kinetics provided by the test (Buffiere et al., 2006). Jensen et al. (2010) concluded also that BMP tests were able to provide results for degradation extent and hydrolysis rate coefficient. These tests can also help to identify microbial inhibition, overloading, and adaptation (Hansen et al., 2004). Gas measuring methods have also been used to assess the effect of pre-treatments of a variety of feedstocks prior to digestion (Devlin et al., 2011, Ma et al., 2010).
Determining the biodegradability of digestates is also an important tool both in terms of assessing performance of AD plants and for determining the stability of these materials. Standards and specifications for digestate have been elaborated in a large number of EU-27 member states (JRC-IPTS, 2011). A high degree of stability of digestates is an important characteristic of organic products for use in land application as a stabilised digestate avoids issues such as further biodegradation leading to odour and other emissions. A digestate produced to a particular degree of stability also allows a better control of nutrients input and use. Therefore, in some countries digestate stability is also being required by regulators and as part of ‘quasi’ or product status accreditation schemes. In Germany, the Bundesgütesgemeinschaft Kompost (BGK) is the carrier of the quality label for digestate products and is recognised by RAL, the German Institute for Quality Assurance and Certification. RAL dictates total organic acids ≤4000 mg/l as an appropriate measure for the degree of fermentation for the digestates. It is nevertheless important to state that not all the organic material found in digestates is in the form of organic acids and therefore other ways of quantifying stability could be more appropriate.
In England, Wales and Northern Ireland, digestates can obtain End of Waste status by meeting the requirements of the Anaerobic Digestate Quality Protocol (WRAP/EA, 2009) and the PAS 110:2010 (BSI, 2010). In terms of stability the PAS 110 requires a screening of volatile fatty acids, which will need to be below 0.43 g COD/g VS, and a residual biogas potential of 0.25 l /g VS. The methodology proposed for performing residual biogas potential (Walker et al., 2010) used volumetric gas measuring methods either by using liquid displacement gas collection systems or impermeable gas storage bags e.g. Tedlar® gas bags. The latter was considered to provide results with a higher assurance that no gas loss had occurred but not so much kinetic data would be available as potentially the volume of gas produced would only be measured at the end of the test i.e. after 28 days.
It is important that the methodology used for measuring anaerobic digestability provides accurate results as over or underestimations can be detrimental both in terms of providing erroneous bioenergy production values and assessing the degree of stability from digestates. This paper does not discuss the most appropriate types of inoculums, inoculum:substrate ratios or the need to add nutrients or buffering although these parameters have a significant impact on testing regimes.
This paper aimed to assess three different methodologies for measuring the volume of biogas and biomethane produced in three methodologies for assessing the biomethane or biogas potential in batch reactors using cellulose as a standard test substrate.
