Exploring biogas potential data of cattle manure and olive cake to gain insight into farm and commercial scale production

This article presents raw data of volumetric biogas and its methane composition obtained from anaerobic digestion experiments conducted under lab scale condition. A commercial biogas industry in Trondheim (Norway) developed interest in using olive cake from a Danish farm (Combineering A/S, Birkerød, Denmark) as a substrate for its existing biogas plant. Moreover, local cattle farm owners wanted to evaluate the possibility of investing on a biogas plant using cattle manure generated on their own farmlands. Accordingly, an evaluation of biogas production potential of these substrates was performed and the obtained data in brief are presented.


Subject
Bioenergy Specific subject area Biomass for anaerobic digestion.

Type of data
Excel spread sheets, tables and images.

How data were acquired
An electronic balance (Entris 4202-1S, Sartorius, Epsom, UK) for weighing the samples, an in-situ water displacement apparatus for biogas quantitative analysis, a gas chromatograph (SRI instruments, Torrance, USA) for biogas compositional analysis, an oven for drying biomass to determine total solids, a muffle furnace (Nabertherm, Lilienthal, Germany) for biomass combustion in order to evaluate volatile solids, pH litmus papers for measuring pH,and Microsoft excel in a desktop computer for data record and analysis.

Data format
Raw and analyzed.

Parameters for data collection
Biogas volume and composition, total solids, volatile solids and pH.

Description of data collection
The total and volatile solid contents were calculated by using relevant equations in excel spreadsheet based on the sample data at the following processing steps: weighing, drying, weighing, combusting, and weighing again. The produced biogas in the reactor headspace was collected on a water displacement column resulting in reducing the water height equalled to the amount of measured biogas. The manually recorded biogas volume data were transferred to Microsoft excel spread sheets for analysis and to represent in terms of various other units, i.e., specific yield and daily yield. Furthermore, the data for methane and carbon dioxide contents in biogas were obtained from gas chromatography analysis and treated afterwards in excel spreadsheets. Value of the data • The presented data are of extreme importance to the local cattle farm owners in performing preliminary assessment to consider for an investment on a biogas plant. Furthermore, the data facilitate in determining the suitability of olive cake as a supplementary material to a commercial anaerobic digestion plant in Norway. • Both local community, farm owners, researchers and business stakeholders will be benefitted by accessing the data, as the demonstrated data will allow to make a quick assessment whether or not anaerobic digestion is a convenient option for treating investigated feedstocks to generate renewable biogas. • The current data give insights into biogas production potential of locally available cattle manure and olive cake. In order to enable more comprehensive assessment relevant to commercialization of a biogas plant, these data will lay a strong foundation for the calculation of many basic parameters of R&D interest using which more complex analyses such as economic and advanced experimental activities dealing with multiple and sophisticated parameter measurements can be developed. • The presented data can be easily interpreted, exchanged and extracted to strip out basic biogas parameters, which allow comparison with similar data generated through a similar or different methodologies in variable contexts, and thus making them as a valuable R&D reference.

Data description
The data presented in this paper include biogas potential, methane composition, total solids (dry matter) and volatile solids analyses of cattle manure (CM) and olive cake ( Fig. 1 a.), which were obtained from a local cattle farm in Norway (Ørland, Trondheim) and a Danish industry (Combineering A/S, Birkerød, Denmark) respectively. Table 1 outlines the description of the materials and important parameters associated with the data. Table 2 shows the input parameters design considered to set-up experiment. For each substrate to inoculum ratio (S:I), duplicate experiment was conducted and the resulting average data (data for statistical variation are given in supplementary file) are displayed in Table 2 . Table 3 shows the data for the total solids and volatile solids of inoculum, cattle manure and olive cake. The measurement of total solids and volatile solids were conducted in duplicate, and the resulted statistical variation together with the mean values are given. The standard [1 , 2] followed for these measurements are also reported ( Table 3 ).
Tables 4 and 5 illustrate the evolution of biogas yield (mL) for CM and olive cake respectively. In these tables, the demonstrated values corresponding to the retention days are expressed with respect to various parameters, i.e., cumulative yield (mL), daily yield (mL/d), specific yield (mL/gTS) and S:I, which are defined in Table 1 . Table 1 Terminologies used in data analysis.

Inoculum
The organic material containing bacteria used for setting up anaerobic digestion environment.

Substrates
The organic materials used as raw-materials for anaerobic digestion.

Total solids
The total solid component of the substrates and inoculum left after drying.

Volatile solids
The total organic component of the dried substrates and inoculum lost after combustion. Ash The inorganic component of the total solids left after combustion. S:I ratio The weight ratio between substrate and inoculum.

Anaerobic digestion
The biological degradation of organic substrates at temperature regime suitable to anaerobic microbes' metabolic reactions. Biogas The gas produced after series of complex biochemical reactions during anaerobic degradation of substrates.

Analytical methods
Total solids and volatile solids were measured analytically, where sample was first weighed, dried in an oven (B9025, Termax, Hagan, Norway) for 24 h at 105 °C, and subsequently combusted in a muffle furnace (LT 5/12, Nabertherm, Lilienthal, Germany) for about 5 h at 550 °C [1 , 2] . The measured numerical data from each of these steps were then incorporated to relevant equations given elsewhere in Ref. [5] for calculating TS and VS.
Another parameter of interest is pH, which for the reactor bottles was measured using pH litmus strips (Arcol AS, Lørenskog, Norway) for two to three times during the course of the experiment (no data given).
Additionally, the most important parameter, the quantity of biogas produced was analyzed routinely by employing a water displacement appartus [5] ( Fig. 1 c.), and the recorded data were calibrated to STP (standard temperature and pressure) prior to inclusion in Tables 4 & 5 . For biogas volume measurement, the collected biogas on reactor headspace was extracted using a syringe-needle tube connected to an aluminum bag (1 L Tedlar bag, Sigma Aldrich, Darmstadt, Germany) from which the biogas was passed through the inverted glass cylinder, resulting in a volume difference equalling to the amount of biogas channel through the water column. The volumetric biogas yield data were also converted to specific and daily yields and reported along with the cumulative yield in Tables 4 & 5 . Parallel to quantitative analysis, samples were also collected in glass vials (10 mL, Apodan A/S, Hørsholm, Denmark) for qualitative analysis using an in-situ gas chromatograph (8610C, SRI instruments, Torrance, USA). The chromatography data in terms of CH 4 and CO 2 content are expressed in Table 6 . The cumulative biogas yield at the end of the experiment represents the biogas potential of the respective substrate, and the percent methane content illustrates the quality of the obtained potential.
The deep insight into both of these parameters are an essential prerequisite to future research in assessing the possibility of a commercial or farm scale biogas plant design or even to increase the biogas productivity of the existing plants.

Declaration of Competing Interest
The author declares that he has no known competing financial interests or personal relationships which have, or could be perceived to have, influenced the work reported in this article.