Method for determination of methane potentials of solid organic waste

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Abstract

A laboratory procedure is described for measuring methane potentials of organic solid waste. Triplicate reactors with 10 grams of volatile solids were incubated at 55 °C with 400 ml of inoculum from a thermophilic biogas plant and the methane production was followed over a 50-day period by regular measurements of methane on a gas chromatograph. The procedure involves blanks as well as cellulose controls. Methane potentials have been measured for source-separated organic household waste and for individual waste materials. The procedure has been evaluated regarding practicality, workload, detection limit, repeatability and reproducibility as well as quality control procedures. For the source-separated organic household waste a methane potential of 495 ml CH4/g VS was found. For fat and oil a lag-phase of several days was seen. The protein sample was clearly inhibited and the maximal methane potential was therefore not achieved. For paper bags, starch and glucose 63, 84 and 94% of the theoretical methane potential was achieved respectively. A detection limit of 72.5 ml CH4/g VS was calculated from the results. This is acceptable, since the methane potential of the tested waste materials was in the range of 200–500 ml CH4/g VS. The determination of methane potentials is a biological method subject to relatively large variation due to the use of non-standardized inoculum and waste heterogeneity. Therefore, procedures for addressing repeatability and reproducibility are suggested.

Introduction

Several batch methods exist for measuring methane potentials of waste. The basic approach is to incubate a small amount of the waste with an anaerobic inoculum and measure the methane generation, usually by simultaneous measurements of gas volume and gas composition. However, the technical approaches in terms of pretreatment of the sample, inoculum, gas measurement technique and incubation vary significantly among the published methods (Adani et al., 2001, Eleazer et al., 1997, Harries et al., 2001, Heerenklage & Stegmann, 2001, Owen et al., 1979, Owens & Chinoweth, 1993). Some of these differences originate from the purpose of measuring the methane potential and from the type of waste samples measured. Most of the methods are geared to either very homogenous samples, such as sewage sludge, or to samples with low methane potentials, such as waste from old landfills or biologically pretreated waste.

We have adapted and modified existing procedures, in particular inspired by Angelidaki and Ahring (1997), to an easy-to-operate method of determining methane potentials of solid waste samples generating high amounts of methane. Here, the procedure and the experiences obtained from measuring methane potentials of more than 100 waste samples during a 2-year period are described.

The method was used to characterize organic waste, separated from household waste, regarding methane potential as relevant in the context of treatment by anaerobic digestion. The main goal was to determine the methane potential in terms of STP (STP: standard temperature and pressure) ml CH4 per gram of organic waste expressed as volatile solids (VS). The determination should be reliably with a reasonable incubation period and with a minimum workload. The methane generation as a function of time may also be of interest for identification of inhibition or adaptation. These priorities have fostered a procedure including extensive homogenization of the solid waste sample, a large inoculum, incubation at 55 °C for 50 days, and direct measurement by a gas chromatograph (GC) of CH4-mass produced.

Section snippets

Equipment and supplies

The following equipment and supplies were used:

  • Two-liter glass bottles with a thick rubber septum (chlorobuthyle rubber, Apodan Nordic, Copenhagen, Denmark) were used as reactors. The exact volume of each bottle was determined by weighing the water contained in the bottle.

  • An incubator at 55 °C for the incubation.

  • Inoculum from a thermophilic biogas plant.

  • A 1 ml glass syringe with pressure lock (VICI, Precision Sampling Inc., Baton Rouge, LA, USA) to allow sampling of a fixed volume at actual

Results and discussion

The DTU-methane-potential-measurement has been used routinely on source-separated organic household waste and occasionally on specific organic waste fractions, for example paper, fat and protein. All the results below are given at STP conditions.

Conclusions

A laboratory procedure is described for measuring methane potentials of organic solid waste showing high methane potentials, for example in the context of treating the waste by anaerobic digestion. The lower detection limit of the presented method based on experiences from about 100 sample is of the order of 70 ml CH4/g VS, which suggests that modifications (primarily ratio of inoculum to sample) must be made if waste samples with low methane potentials are to be measured. The biological

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