Utilization of pineapple crown to enhance methane production of dairy cow manure

Babatope Tade Oje-Adetule, Sutaryo Sutaryo, Daffa Wahyu Pratama and Agung Purnomoadi

Department of Animal Science, Faculty of Animal and Agricultural Sciences, Diponegoro University, Semarang, Central Java, Indonesia 50275
soeta@lecturer.undip.ac.id

Abstract

Increase in both domestic and industrial wastes owes to the geometric increase in world’s population. This is as a result of corresponding increase in anthropogenic activities. Agricultural wastes have caused a huge menace to the environment due to poor disposal and one of the results is methane emission, a potential greenhouse gas. However, research has revealed the value of methane as a means to appeal to the global energy demand for renewable fuel. Monodigestion of Dairy Cow Manure (DCM) in anaerobic digester is usually plagued by low methane production, hence co-digestion has been suggested, although under controlled conditions. The aim of this study was therefore to co-digest DCM and Pineapple Crown (PNC) at different PNC proportion in the mixed substrate (PNC 0%; PNC 16.63%, PNC 28.71%, and PNC 38%). The co-substrates positively impacted (p<0.05) on methane production by 21.89; 38.91 and 49.27% for PNC 16.63%, PNC 28.71% and PNC 38% respectively, compared to that of the control reactor (PNC 0%) in terms of L/kg volatile solids (VS) added. The low total ammonia nitrogen concentration, total volatile fatty acids, stable methane production almost neutral pH values demonstrated by the digested slurries, suggest that PNC has the capacity to increase methane production of DCM at least the proportion of PNC up to 38% in terms of VS.

Keywords: co-digestion, dairy cow manure, energy, methane, pineapple crown

Introduction

The world’s population is really on the rise with a corresponding use of most available land for industrial and residential purposes, especially in urban centres. This has resulted in an increased generation of both domestic and industrial wastes, and will further increase. According to Gerber et al (2013), the proposed demand for meat and milk will increase by 73% and 53% respectively by 2050. Landfills have always been the destination of world’s wastes. But with the shrink in available land space and emission of methane; a potential greenhouse gas, there is an urgent search for alternative and sustainable waste management.

Livestock manure causes a huge environmental nuisance. This is influenced by the recent increase in livestock production, in a bid to support the increasing human population demands, most of which comes from cattle. Hence, the natural ecosystem requires an urgent reclamation to avoid its total breakdown as a result of the build-up of waste from livestock. However, according to Tufaner and Avsar (2016), manure contains substantial elemental substrates for biogas production. This benefit plus the fact that such substrate does not compete for arable lands and the increasing global energy demand for biofuels in lieu of fossil-based fuels, have increasingly earned it a viable spot in research.

Anaerobic digestion (AD) for manure management has numerous advantages and one of them is biogas production, which according to Sutaryo et al (2020), stands as an alternative means of producing gaseous fuel from biomass. The ongoing renaissance of biogas through AD mechanism in recent biochemical research is due to its tripartite contribution as renewable source of energy, digestate (fertilizer) and abatement of greenhouse gas emissions. This pathway allows the combination of two substrates with different characteristics for the production of renewable bio-fuel (bio-CH₄). This could help achieve the sustainable development goals responsible for women and youth empowerment with respect to market systems. However, the presence of recalcitrant lignin from forages, high level of moisture and ash concentration in dairy cow manure (DCM) (Li et al 2021), will disallow methanic economic balance of biomass. Angelidaki and Ellegard (2003) opined that methane production does not exceed 20 m³/t of animal manure, but to make biogas an adequate replacement for fossil fuels, it must rise beyond 20 m³/t. At such end, it could cater for the impending threat of declining fossil fuel reserves, fluctuating oil prices and climate change.

Furthermore, large amount of biowastes is generated from food fruit processing industries (Azevedo et al 2021). Pineapple (Ananas comosus) ranks third after banana and mango in world fruit production (Zhang et al 2014). It is cultivated on a total area of 1.02 million hectares, producing up to 24.8 million tons/year (Ming et al 2015). This is as obtained from Costa Rica, Brazil, Philippines, Thailand, Indonesia, India, Nigeria, China, Mexico and Columbia (Hossain 2016). This world production score of pineapple indicates an inevitable size of waste generation, and this according to Dahunsi (2019^a) is capacious of conveying pathogens of global health importance.

Pineapple waste, which include peel, pulp, core and crown are residues from canned juice industries and has been used in animal feed, but not so conspicuous. Pineapple crown (PNC) (Photo 1) forms the upper portion of pineapple usually discarded at harvest for logistic reasons or at point of sale. It makes up to 10-20% of the entire fruit weight and could be as long as 20 cm (Suoto et al 2010). Although they are productively utilized but its reuse is not commensurate with production, hence, while some end up in landfills, a whole lot are still facing disposal issues. Despite the technological advancement spelt out by science and the huge decry for sustainable production, pineapple crown is still mostly discarded, even though its commercial use could be explored for profitable uses.

Reports about the characteristics and content of PNC are still rare. But according to Braga et al (2015), it has a higher heating value (18.9 MJ/kg), compared to its counterpart biomass like straw (17.7 MJ/kg) and bagasse of sugar cane (16.3 MJ/kg). They also recorded a moderate bulk density (420.8 kg/m³). These two biomass indices define the potential energy ability of biomass per unit of volume. Also, due to its high biological and chemical oxygen demand, the fibres from PNC could replace conventional mechanical materials in composites. These fibres are renewable and expressly available. Hence, if appropriately deployed could resolve its waste challenge.

The ability of the agro-industry fruit residue to improve the inefficiency of sole AD of manure has not been highly researched. But reports have indicated the efficacy of food waste in terms of biogas yield and cost effectiveness, (Koch et al 2016). Reports (Chulalaksananukul et al 2012; Choonut et al 2014; Dahunsi 2019^b) have so far shown the potentialities inherent in pineapple waste use, but, up till now, PNC has not been used as a co-substrate with DCM for biogas production. Hence the aim of this study was to assess the influence of PNC as a co-substrate to enhance AD of DCM, in a CSTR at mesophilic conditions (36±1 ºC), and a hydraulic retention time (HRT) of 22 d. For these purposes, the co-substrate was fully characterised (chemically), and the methane content evaluated in AD trials. In addition, this study also evaluated the post digestion of digested slurry from the laboratory scale digesters

Materials and methods

The experimental setup for the study comprised a continuous experiment and post digestion tests. The continuous experiment evaluated the digester performance with varying proportions of PNC in the mixed substrate, while the post digestion test determined the residual methane potential of digested slurry obtained from the continuous experiment.

Continuous Experiment

Four laboratory-scale 7 L continuously stirred tank reactors (CSTR) (Photo 2) were used. The start-up phase commenced by put 5250 g of inoculum into each reactor. This was followed by feeding the digester with 238.6 g of DCM on the second d, after a corresponding amount of slurry has been evacuated from the base of the digester. This adaptation period was run for three weeks, then followed by data collection. The contents were fed through tube, submerged below substrate level to avoid air ingress. The arrangement was made airtight for the AD process. The reactors had a reaction active volume of 5.25 L and each was double-layered with stainless steel and incubated in a chamber at 37 °C. The reactors were operated at 22 d hydraulic retention time (HRT) for effective waste digestion in AD process. The treatments were proportion of PNC in the final substrate in term of VS (Table 1), with PNC 0% which was solely fed DCM served as control reactor. The continuous experiment was performed for 66 d that corresponded to 3 HRT.

Post digestion test

The post digestion test was conducted with 0.5 L infusion bottles, maintained at anaerobic state, using a rubber stopper and sealed with aluminum crimp. It was fed 200 g of digested slurry, harvested at day 40-45 from each digester of the continuous experiment. Each batch digester maintained at 37 ˚C with the aid of an incubator and airspace was flushed with nitrogen for two minutes. Each test was conducted in triplicates and allowed to run for 30 d.

Photo 1. Pineapple crown that used in this study	Photo 2. Bio-digester configuration

Starter and Substrate

PNC was collected from local pineapple market in Semarang. It was sun drying for 2-3 d followed by ground using hammer mill with 1 mm screen size to facilitated during feeding to the laboratory bio-digester scale. The chemical composition of PNC is presented in Table 2. DCM was taken from lactating dairy cows of the Research Farm of the Faculty of Animal and Agricultural Sciences of the Diponegoro University. Collection of dairy cow faeces was performed once a week and diluted in ratio 1/1.75 with tap water to reach final total solid (TS) of ca 6% with the aim to facilitated during feeding to the digester and later refrigerated. The substrate properties can be seen in Table 1.

Digested slurry at room temperature, from the active biogas digester at the Faculty of Animal and Agricultural Sciences, Diponegoro University, Indonesia, served as the starter for the continuous experiment. The digester treats DCM alone. The digested slurry from the digester was immediately transferred to the laboratory scale digesters. The pH value, TS, and volatile solid (VS) the inoculum was 7.28, 7.77%, and 5.30% respectively.

Analytical methods

Measurement of methane produced in the continuous experiment was conducted by routing the produced biogas through 0.5 L infusion bottles that contained 4% NaOH (Merck®, cat no: 1064981000) solution (Gelegenis et al 2007) using a 5 mm Teflon tube for removal of CO₂. The NaOH solution was changed on a weekly basis. The gas was collected daily with a 5 L Tedlar gasbag (Hedetech-Dupont, China), and measured daily using the water displacement method as described by Sutaryo et al (2020). Equally, methane produced from the post digestion experiment was conducted as mentioned above. However, instead of a 5 L Tedlar gas bag, a 1 L bag was used.

The substrate and digested slurry pH values was measured using a pH meter (Ohaus® ST300 pH meter). TS and VS was analysed according to APHA (1995). Total ammonia nitrogen (TAN) concentration was measured by distillation according to APHA (1995). Total volatile fatty acids concentration (VFA) was determined by titration method. Subsequently, data was statistically analysed using analysis of variance at the 5% confidence level according to Gomez and Gomez (2007). A Duncan multiple range test was applied when there was a significant effect of the treatment on the observed variables.

Results and discussion

To study the impact of PNC on the methanogenic effect of dairy cow manure, PNC was characterised in the present study. The chemical composition of PNC is as shown in Table 2. From the results obtained, the VS was 76.04%, which indicates high VS content, hence making it quite suitable as co-substrate with DCM for biogas production, since the DCM has TS content 7-9% (Angelidaki and Ellegard, 2003). Cellulose content was fairly high (37.74%) and other biodegradable elements which makes it an appropriate substrate for the AD process. From the Table 2, it was noticed that lignin content was quite low (1.19%). This shows that PNC is not recalcitrant like most other co-substrate, hence a further pre-treatment process might not be necessary. However, a 20 C/N ratio is fairly appropriate for optimum methane production. With respect to literature, C/N should be maintained within 20-30 in AD as an undesirable ratio will lead to accumulation of TAN, free ammonia or VFAs within the digester (Kainthola et al 2019). As a result of excessive ammonia, pH will increase within the digester thus hampering microbial microbial growth as most microbes thrive at near pH value.

Effect of co-substrates on methane yield

The trend of methane production of each treatment is presented in Fig. 1, while the correlation of PNC proportion in the mixed substrate with methane production in term of L/kg substrate, L/L active digester volume, and L/kg VS can be seen in Fig. 2, 3 and Fig. 4 respectively. The VS proportion of PNC in the final substrate was 0%, 16.63%, 28.71%, and 38% respectively and there was a corresponding increase in methane yield. Sidik et al (2013) gave a similar observation with the use of palm oil mill effluent as co-substrate to improve the methanogenic effect of animal manure. What this infers is that the introduction of additives enhanced methane production which owes to the fact that there was an increased nutrient concentration made possible by the additives in the digester. This according to Mshandete et al (2004), could improve the rate of microbial proliferation, hence a higher methane production.

Furthermore, PNC 38% gave the highest methane yield. This was followed by PNC 28.71%_, then PNC 16.63% and PNC 0%. Although, methane yields for PNC 28.71%, spiked at about d 14 (week 2), followed by a sharp decline and then a steady increase till week 9 when it began to experience a gradual decline. This sharp increase could be associated with a shorter lag phase growth, increased activity of methanogens and bioavailability of substrate. However, PNC 38% among the treatments showed the highest steady progression regarding methane production. This might be due to a positive synergestic impact between the amount of PNC and DCM. This suggests PNC as a veritable co-substrate for AD.

Fig. 1 presents the methane yield in terms of L/L active digester volume in a continuous experiment. The value of biogas as biofuel is defined by its content of methane. There was significant effect (p<0.05) regarding the co-digestion of DCM and PNC on methane production. Methane production in terms of L/kg VS added in the reactors (PNC 0%, PNC 16.63%, PNC 28.72%, PNC 38%) were 130.26, 158.78, 180.94 and 194.44 L/kg VS respectively (Table 3). The profile of methane produced varied in response to the substrate’s mixing ratio. Baek et al (2020), indicated that monodigestion of cattle manure was 109.2 L/kg VS and according to Dong et al (2019), reports indicated that methane production from DCM at 25 HRT in a plug flow reaction under mesophilic condition was in the range 150-220 L/kg VS. This therefore ensured the methane produced by PNC 0% biodigester in this experiment to be comparable with initial studies.

In the unit of L/kg substrate the co-digestion of PNC and DCM in PNC 16.63%, PNC 28.71% and PNC 38%, gave increase of methane produced by 33.43%, 75.04%, and 109.59% compared to PNC 0%. This is quite significant (p<0.05), and therefore suggests PNC as a potential co-substrate. However, based on the submission of Angelidaki and Ellegard (2003), it could be said that none of the treatments met the suggested economic utility value of organic waste, which is in excess of 20 m³ of CH₄ per m³ of biomass. In terms of L/L digester volume, it was 34.37%, 75% and 112%, while in terms of L/kg VS added, it increased against the control by 21.89%, 38.91% and 49.27% respectively. The percentage increase of methane produced as indicated by the study, in terms of L/kg substrate and L/L active digester volume were higher, compared to the L/kg VS added. This could be ascribed to the co-substrate impact of the substrates (DCM+PNC) as compared to DCM alone. With respect to this, the denominator value regarding PNC 16.63%, PNC 28.71% and PNC 38% for L/kg substrate added and L/L active volume had the same denominator as PNC 0%, but in L/kg VS added, the denominator in the calculation of methane produced was higher for PNC 16.63%, PNC 28.71% and PNC 38% than for PNC 0% (Sutaryo et al 2021).

Figure 1. Trends of methane production of the continuous experiment	Figure 2. Methane yield (L/kg substrate)

Figure 3. Methane yield (L/L digester)	Figure 4. Methane yield (L/kg VS)

Variables in the liquid phase

There was no observable significance (p>0.05) in all the recorded variables with respect to the co-digestion of DCM and PNC, except for TAN concentration (Table 3). The VFA concentrations (PNC 0%= 92.50; PNC 16.63%= 105; PNC 28.71%= 105 and PNC 38%= 98.75 mM) were stable throughout the experimental period with no negative impact on methane yield. Formation of VFAs in excess of the required level could halt the anaerobic process as a result of a drop in pH. According to Guo et al (2021), VFAs (28.8 g/L) could inhibit methanogenic activities.

Similarly low values were noticed for TAN concentrations of digested slurry of PNC 0%, PNC 16.63%, PNC 28.71% and PNC 38% which were 172, 201.29, 210.00 and 245.47 mg/L respectively. The TAN inhibitory threshold as opined by Yenigun and Demirel (2013) is in the range of 1700-1800 mg/L for an unadapted mesophilic-conditioned inoculum. The higher levels of TAN concentration of PNC 16.63%, PNC 28.71% and PNC 38% than that in PNC 0% in the digested slurries are correspondent with the higher level of crude protein of the substrates (Table 1). This claim was supported by Jiang et al (2019), that degradation of nitrogenous matter among AD substrates yield ammonia. Sutaryo et al (2014) reported that TAN concentration increased (2150-3620 mg/L) as a result of adding 0.35% urea. The pH values within the reactors for all digested slurries were all within tolerable range enough for methanogenesis to continue undisturbed. The ideal pH for an AD process, according to Mao et al (2015) should remain within the range of 6.8-7.4. As described earlier, the higher the level of degradation, the higher the ammonia deposition, which would correspondingly impact the reactor pH. This could describe the direct proportionality between pH as TAN increased. The VS reductions in the experiment were 27.62, 24.09, 25.15 and 26.50 % for PNC 0%, PNC 16.63%, PNC 28.71% and PNC 38% respectively.

Post digestion test

The methane production of PNC 38% in terms of L/kg digested slurry, gave the highest yield (6.38 L/kg) while the least was recorded for PNC 0% (3.82 L/kg). The higher VS concentration of PNC 28.71% and PNC 38% _­ could be ascribed to the significant difference of methane yield (p<0.05) recorded for both digested slurries as compared to PNC 0% and PNC 16.63%. This outrightly confirms the need for a longer HRT or post digestion. This is so as to cater for the full methanogenic ability of the explored substrates in the biogas plant.

Conclusion

This study investigated the possibility of improving the methanogenic potential of DCM by co-digesting it with PNC. This was suggested for a better nutrient balance and increased methanogenic activities to improve methane yield. A positive impact was recorded with methane yield increasing at different levels of PNC in the treatments, in terms of active digester volume (34.37, 75 and 112% respectively) and L/kg VS (21.89, 38.91 and 49.27% respectively) when compared with the control. The apparently low levels of TAN and VFA indicates a successful and stable operating digester. Further indications include pH within recommended values and a stable methane production of digested slurries. This therefore concludes PNC as a potential co-substrate item that could enhance the methanogenic behaviour of DCM at least the proportion of PNC up to 38% in terms of VS.

Acknowledgement

The authors would like to thank the Diponegoro University (grant number: 569/UN7.D2/PP/VII/2022) for financing this experiment.

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