The Potential of Cassava Biomass as a Feedstock for Sustainable ...

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Journal of Energy and Power Engineering 8 (2014) 836-843

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PUBLISHING

The Potential of Cassava Biomass as a Feedstock for Sustainable Biogas Production in South Africa Vincent Okudoh, Cristina Trois and TilahunWorkneh School of Engineering, University of KwaZulu-Natal, Durban 4041, South Africa Received: November 11, 2013 / Accepted: December 25, 2013 / Published: May 31, 2014. Abstract: Cassava is currently being investigated for biogas production in South Africa as it offers multiple benefits such as high yields of starch and total dry matter. The chemical constituents of the cassava biomass were determined using standard methods. Using a locally fabricated laboratory batch fermenter, anaerobic digestion was carried out in a 25 L capacity digester maintained at 36 ± 0.5 oC. Pre-treatment of the cassava biomass with spoilage fungi, Aspergillus niger and Penicillium species yielded large amounts of fermentable sugars for digestion. Cassava slurry was made and mixed with zebra droppings (2:1 v/v) and loaded into the digester of 20 L working volume. Analysis results showed an increase in most nutrients after pretreatment except for starch which decreased from 76% to 60% as a result of its hydrolysis to fermentable sugars by the spoilage fungi. Theoretical biogas yields were between 0.71 nm3 and 0.75 nm3 per kg VS (volatile solids) destroyed while the total biogas yields of between 250 nm3 and 300 nm3 per kg VS fed into the digester was obtained after 20 days residence time. Cassava is not yet a staple food in some BRICs countries like South Africa and the peels and other by-products of its processing are equally suitable for energy production. The use of cassava will be an alternative feedstock strategy for several rural biogas projects running with cow dungs inside South Africa. In addition, opportunities exist for decentralized, cheaper and socially advantageous bioenergy production from cassava considering that fuel and electricity needs are not satisfied in many rural areas. Finally, the incorporation of cassava anaerobic digestion facility at different scales will deliver additional benefits like the incorporation of nutrients and residual carbon into the land as fertilizer. Key words: Cassava, biomass, biogas, bioenergy, sustainable energy.

1. Introduction Energy security for the future and better use of natural resources are the key issues facing many countries of the world today. Inequality exists in the distribution of fossil fuels and about 70% of countries and in particular African countries rely on imported fuels to meet their ever increasing energy demands. A huge chunk of their national budgets meant for national development are diverted to imports leading to poverty, poor infrastructural development and spread of lethal diseases. Fossil fuels are non-renewable and its combustion produces GHG (greenhouse gas) emissions that contribute to global warming. Renewable and more environmentally friendly sources Corresponding author: Vincent Okudoh, Ph.D., research fields: renewable energy, biomass, biogas, anaerobic digestion and novel antibiotics discovery. E-mail: [email protected].

of energy are needed to address these problems. The conversion of biomass to energy is a good alternative with additional benefits such as job creation, rural economy development and improvement in environmental quality [1]. In Africa, there are lots of degraded and unutilized land that can be used for biomass and bioenergy production. e.g., South Africa currently utilizes only 20% of its 120 Mha (Million hectares) of total land mass for biomass production [2]. An estimated 317 EJ (Exajoules) per year of bioenergy can be produced in sub-Saharan Africa by the year 2050 [3]. The production of bioenergy and in particular biogas will provide a much cleaner energy source than fossil fuels with lesser GHG emissions. Biogas is a product of microbial anaerobic digestion and comprises of mainly methane and carbon dioxide. Other gases present in trace amounts include H2S, NH3,

The Potential of Cassava Biomass as a Feedstock for Sustainable Biogas Production in South Africa

H2 and N2 [4]. Biogas can be used for heating, as a fuel or natural gas equivalent and converted to electricity. The quality and yield of the biogas produced depend on the type and composition of the feedstock used [5, 6]. Cassava (Manihot esculenta Crantz) biomass offers huge potential as feedstock for bioenergy and in particular biogas production with multiple benefits. It thrives in drought conditions and requires low input of agro-chemicals [7]. Its carbohydrate yield per hectare (4.742 kg/carb), dry matter (38.6%) and starch contents (80.6% dry weight) are higher than most crops [8-10]. Cassava water footprint is the smallest (21 m3/GJ) compared to all other crops [11]. Most biogas studies are biased towards industrial wastewaters and only recently has energy crops such as fodder beets and cassava been focused. In South Africa, there is no record of biogas studies from cassava biomass. Therefore, the objective of this study is to convert cassava biomass into biogas through anaerobic digestion for the first time in South Africa. And identify the key factors affecting the biogas yield in the attempt to optimize methane yield and productivity. In the present work, we explored two cassava varieties, the bitter TBS (tuber from South Africa) and the corresponding TBM (tuber from Malawi), in terms of their chemical composition. We also checked the composition of cassava peel for its biogas production suitability determined by the amount of available carbohydrates for microbial conversion to energy. In the first report, we present the results of the chemical analysis together with biogas methane potential of TBS which contained less proportions of fiber than TBM and cassava peel.

2. Materials and Methods Fresh cassava samples were collected from a farm in Makhathini area, Northern KwaZulu-Natal, South Africa and from commercial farmers in Malawi. The samples were first washed with water, air dried and then chopped into pieces (1 cm3). Some of the samples were peeled mechanically with a sharp knife before

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chopping. All the samples were sun dried for two days before milling in a Scientific RSA Hammer mill SER No. 400 equipped with 2 mm sieve mesh to obtain cassava flour. TS (total solids) and MC (moisture content) were quantified by heating fresh cassava biomass at 105 oC to constant weight using Sartorius moisture analyzer model MA35. The following parameters were also determined: ash content by furnace method; NDF (neutral detergent fiber), ADF (acid detergent fiber), and ADL (acid detergent lignin) by Van Soest method; crude protein and carbon by Dumas combustion method; starch and sugars by enzyme method. Elementary composition was also done on ICP (inductively coupled plasma). All the cassava biomass analysis was performed according standard methods [12]. A flowchart indicating all the steps used for biogas production is shown in Fig. 1. Some milled cassava samples were pretreated by inoculating with a mixture of spoilage fungi Aspergillus niger and Penicillium sp., and left at ambient temperature for 30 days [13]. Only the samples of pre-treated TBS cassava biomass were used for the present batch experiment. All samples were stored in a refrigerator set at 4 oC before use. Dried cassava biomass was made up as slurry (250 g/L) with water in 5 L plastic container. An inoculum: substrate ratio of 2:1 was used for the digestion [14] after purging the system of oxygen with ultrapure N2 gas. Fresh Zebra (Equus quaggaburchelli) droppings collected from Albert Falls Game Reserve were used as inoculum to startup the experiment [15]. Zebra droppings (0.5 kg) were mixed with water (5 L) in a plastic container and incubated at 36 ± 0.5 oC for 7 days [16] before loading in a digester (Fig. 2). The digester of 25 L (40.5 cm height × 14 cm radius) capacity was locally constructed at the Agricultural Engineering Centre, University of KwaZulu-Natal, South Africa and has a working volume of 20 L. An electrically heated blanket surrounds the cylindrical digester body equipped with a tap at the bottom where samples for analysis can be collected. The top plate has

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Fig. 1 Flow chart showing steps used in cassava biogas production.

Fig. 2 Diagram of digester used for Cassava biogas production.

two outlets for gas collection and inspection and can be opened for cleaning. It also has a mechanical stirrer for mixing that runs top-down through the top plate of the digester. A single-stage batch digester maintained at 36 ± 0.5 o C (Fig. 3) was used for AD (anaerobic digestion). NaHCO3 (10 g/L) was also added to the digester when necessary to maintain the pH at neutral [17].The batch experiments were carried out according to recommendations by Angelidaki et al. [14]. The digester was loaded with fresh substrate (100 g/L)

every four days and the contents were manually mixed three times daily with a mechanical stirrer for 30 days. Another digester containing only the inoculum was used as control. The biogas yield from the inoculum control test was subtracted from the total biogas produced [18]. Since cassava biomass is deficient in vitamins and minerals, the digester medium was supplemented by a mineral and trace element solution [19, 20]. The volume of the biogas produced was measured by downward displacement of water and collected by means of a water-column to which a gas storage bag


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Fig. 3 Picture of digester design and arrangement.

(tyre tube) was attached (Fig. 2). The water in the gas collector was adjusted to pH 2 with H2SO4 to avoid CO2 dissolution [21]. All gas volumes reported are corrected to STP (0 oC, 110.3 KPa) as described by Walker et al. [22] cited by Suhartini et al. [23].

3. Results and Discussion The results of cassava biomass characterization showed that the two cassava varieties TBS and TBM showed little difference in composition except in fibre and minerals such as iron, zinc and manganese. The TBM variety contained higher amounts of fibre (6.43%-9.08%) than TBS (3.99%-6.41%). Higher proportions of fibre are indeed known to yield much less biogas because they are poorly degraded by most microbes [24, 25]. Both varieties are rich in carbohydrates with sugar content of approximately 78%. The pretreated TBS variety used for AD showed an increase in the amount of most nutrients tested except for starch where there was a marked reduction from 76% to 60%. This is mainly due to starch hydrolysis to sugars by the spoilage fungi to a form that can be readily utilized for energy production. The trace heavy metals (Fe, Zn and Mn) content was quite high for the cassava peel which also showed higher amounts of protein and minerals than the cassava tubers. Trace

metals especially Fe are very essential for methane fermentation [26, 27]. The cassava peel also showed lower starch content (61%) but higher amount of fermentable sugars (79%) than the cassava tubers (76%-78%). A summary of the physical and chemical characteristics of the cassava biomass is shown in Table 1. Theoretical calculations on the composition of biogas produced and the concentration of methane were determined using the Buswell equation (Eq. (1)) [28] based on the chemical composition of the cassava biomass. It should be noted that in practice all the ammonia (NH3) dissolves in the slurry, hence only CO2 and CH4 may be calculated using the equation. CcHhOoNnSs+ ((4c – h – 2o + 3n + 2s)/4) H2O → (4c – h + 2o + 3n + 2s)/8) CO2+ ((4c + h – 2o -3n -2s)/8) CH4 + nNH3 + sH2S (1) where, C = carbon (mol), H = hydrogen (mol), O = oxygen (mol), N = nitrogen (mol), S = sulfur (mol), n = No. of moles nitrogen, s = No. of moles sulfur. Preliminary results of the biogas potential from cassava biomass showed that the theoretical yields are between 0.71 nm3 and 0.75 nm3 per kg of VS (volatile solids) destroyed and a methane content of 50% (Table 2). Total biogas yields of between 250 nm3 and 300 nm3 per kg VS fed into the digester was obtained after 20

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Table 1

Average physical and chemical composition of cassava biomass#.

Content Unit TBS TBM TBM peel TBS pretreated Ash % 2.78 3.60 3.72 3.37 Moisture^ % 58.05 58.75 76.85 55.82 TS (total solids) % 41.95 41.25 23.15 44.18 VS (volatile solids) % TS 93.37 91.27 83.93 92.37 Fat % 0.22 0.18 0.31 2.23 Adf % 3.99 6.43 12.73 8.84 Ndf % 6.41 9.08 14.31 13.14 Adl % 2.80 3.54 6.60 5.43 Protein % 3.18 3.30 5.47 4.13 Nitrogen* % 0.51 0.53 0.88 0.66 Sugars % 77.45 77.36 78.47 73.70 Starch % 76.28 78.32 61.42 60.17 Ca % 0.11 0.15 0.24 0.17 Mg % 0.13 0.09 0.12 0.14 K % 1.01 1.29 0.77 1.07 Na % 0.05 0.02 0.03 0.04 K/Ca + Mg % 1.64 2.27 0.89 1.38 P % 0.12 0.16 0.09 0.15 C % 39.02 39.09 39.67 41.25 Zn mg/kg or ppm 13 17 24 15 Cu mg/kg or ppm 3 1 3 3 Mn mg/kg or ppm 11 4 37 11 Fe mg/kg or ppm 62 35 205 95 *Nitrogen % calculated by dividing protein by 6.25, moisture % of fresh cassava biomass, moisture % of dried sample ±7.78% oven dried; # Data based on average of three replicates and on 100% DM basis, TBS-Tuber SA, TBM-Tuber Malawi. Table 2 Biogas methane potential of cassava biomass. Parameters HRT OLR Biogas yield (calculated) Total biogas yield CH4 (calculated) pH Temperature

Units d Kg VS m-3 d-1 nm3 per kg VS nm3 per kg VS % o C

0-5 4 0.25 0.71 0 50 5.8-7.2 35 ± 0.5

6-10 4 0.25 0.75 250 50 4.6-4.7 36 ± 0.5

Days 11-15 4 0.25 ND 300 50 7.0-7.5 36 ± 0.5

16-20 4 0.25 ND 280 50 6.8-7.0 36 ± 0.5

HRT―hydraulic retention time, OLR―organic loading rate.

days of residence time. This result is similar to one obtained by Anunputtikul and Rodtong [29] working with 20 L digester working volume. High biogas yields are usually related to the high carbohydrate content of cassava. This substrate provided enough TS for microbial growth during biogas production [21]. The biogas produced was effectively collected over acidic

water of pH 2. The electric blanket set at level 2 was very effective in maintaining the temperature at 35 ± 0.5 oC. By the end of the 4 days HRT, the effluent pH became lower (4.69) than that of the inlet (7.5) indicating the formation of organic acids by the acidogenic bacteria. The pH was then adjusted to


neutral with NaHCO3 solution to allow for proper functioning of the methanogenic bacteria. The optimal pH for methanogens is between 6.8 and 7.5 [14]. The introduction of zebra droppings as an inoculum was suitable for the conversion of cassava biomass to biogas. It provided microorganisms capable of hydrolyzing carbohydrates during the fermentation. Also the mineral supplements enhanced the performance of the microbes. The volatile acidity/alkalinity ratio in this test is supposedly higher than 1:1 at acidogenesis step due to the presence of more acidogenic bacteria as values above 0.8 indicates trouble in the bioreactor [30]. The reason may be that cassava biomass contains large amounts of carbohydrates which allows rapid onset of acidogenesis stage. This leads to accumulation of volatile fatty acids [31]. The best solution for cassava biomass digestion is to divide the acidogenic and methanogenic phases for a much better process efficiency [32, 33].

4. Conclusions The chemical composition of cassava biomass as determined showed high biogas production potential based on its large amounts of carbohydrates, dry matter (TS), VS and low fiber contents. However, its carbon to nitrogen ratio (C: N = 39:1) is higher than normal and may have to be co-digested with animal manure such as zebra droppings to bring the normal ratio of about 20:1. Higher amounts of trace metals from cassava biomass means a much stable digester operation but will require supplementation with some minerals to boost microbial growth. The performance of the locally designed digester was suprisingly good with the electric blanket maintaining a stable inside temperature of 36 ± 0.5o C throughout the duration of the experiment. The results obtained are quite promising and efforts are being put in place to optimize the biogas production. The benefits of using cassava biomass for future crop-based biogas plants can be many folds. (1) It reduces the need to use lands available for food production and artificial

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fertilizers as they can be cultivated in degraded lands; (2) The biogas produced from the crop’s biomass can provide decentralised, cheaper and socially advantategeous energy especially for rural areas of South Africa where fuel and electricity needs are currently inadequate. Therefore, it is important to investigate the biogas production from cassava biomass in detail and find the best ways to optimise the biogas process. For identifying the biogas potential of cassava biomass, the following aspects are of special interest in the future: (1) The composition of the biogas and its methane content; (2) Determination of the SMA (specific methanogenic activity); (3) The identification of the cellulolytic microorganisms involved using 16 S rRNA. The project will address all these aspects in order to find the most suitable way to optimize the biogas process. It is postulated that cassava is a very good feedstock for biogas production in South Africa.

Acknowledgments Our sincere thanks go to the following UKZN (University of KwaZulu-Natal) staff: Prof Stefan Schmidt for advice; Allan Hill for digester design and Sthembile Ndlela for materials supply. We also thank Funmi Faloye for constructive criticism and Shireen Naicker

from

KwaZulu-Natal

Department

of

Agriculture for assistance in biomass analysis. This work was sponsored by UKZN College of Agriculture, Engineering and Science research fund.

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