Tanzania Sisal Industry: Auditing and Characterization of Sisal Post-Harvest Wastes as a Bio-Resource for Bio-Refining

Headnote

Abstract: A waste audit case study was conducted for sisal post-harvest waste (SPHW) generated at Fatemi sisal estates in Tanzania to evaluate its potential for use as substrate for mushroom cultivation and biogas production by integrating quantitative with qualitative methods and laboratory analysis. Annual generation of fresh sisal boles waste (SBW) and remnant leaf stubs (RLS) per hectare was estimated at 22 and 44.5 tons, respectively. The chemical compositions of SBW and RLS as well as the leachate extracted from the fractions were established. Regardless of the fraction analyzed, contents of volatile solids, total carbon, total organic matter were in the ranges of 93-98%, 49-55% and 91-98% on dry weight basis, respectively. Total sugar contents of the solid and liquid fractions ranged between 8-27% and 30-35 mg/l, respectively. The chemical oxygen demand (COD) content of the SBW and RLS fractions were between 20 and 26 gO^sub 2^/l, respectively. The high values of organics contained in the SPHW fractions is indicative of its potential as feedstock for a biorefinery that produces food (mushroom), feed, biofuels (biogas, bioethanol), biochemicals and other bioproducts. On the basis of the established characteristics, an innovative approach for the utilization of the waste is proposed.

Keywords: Sisal waste, biorefinery, utilization, management.

1. Introduction

Sisal is a semiarid and marginal land crop of the tropics whose leaves are used for extraction into natural hard fibers by either wet or dry decortications (Kimaro et al., 1994). Tanzania is the forth world's largest sisal producer currently at 24,600 tons of sisal products per annum earning the country foreign exchange through exports. However, sisal fibre production is a high waste process currently using only 2% of the plant and the remaining 98% biomass being various fractions of wastes. Sisal processing wastes include; sisal leaf decortications wastes (SLDW), short fibres (flume tow), wastewater and sisal dust. The industry also produces post harvest waste (SPHW) that comprises of sisal stems (comprising of boles and leaf remnant stubs) and poles (Muthangya et al., 2009). SLDW is commonly disposed offraw into the environment and SPHW is disposed offbefore replanting and mostly by burning.

Although currently a menace to the environment, the 98% could be regarded as a bioresource not a waste (TSA, 1995;Bisanda and Enock, 2003). Sisal waste is now known to be more valuable than the 2% of fibre if fully exploited to produce high value commodities such as edible mushrooms, biofuels, biofertilizer, foam liquid (cement foaming substance which enlarge sand and cement mixture), organic acids (lactic acid, citric acid, acetone, butanol), composite, low calorie dietary fibers, functional foods, sweeteners, thickeners in ice creams, sandwich spreads, chocolate products, breads and pastries etc, fine and green chemicals (Kimaro et al., 1994; TSA, 1995;Bisanda and Enock, 2003; Muruke et al., 2006; Mshandete and Cuff, 2008; Elisante and Msemwa, 2010;Mshandete, 2011). Adding value to waste will make the industry more competitive through diversification of its markets. Despite this big potential, it is only one factory in Tanzania that is utilizing SLDW at industrial scale for biogas to electricity generation of 150-300 KWh/day from a 1700 m3 biogas plant since 2007. As for the rest, sisal waste remains grossly underutilized.

In order to be able to utilize sisal waste for commercial production, information on their availability, quantity and composition is crucial. Whereas elaborate information on generation rates and characteristics of sisal wastewater and decortications waste is provided in the literature (Mshandete et al., 2004; Mshandete et al., 2006; Muthangya et al., 2009) while that on SPHW is scanty (Mshandete, 2011). The aim of this study was to establish the quantity and quality of SPHW available for cultivation of oyster mushrooms and using the spent SPHW for biogas production.

2. Materials and Methods

2.1 Waste Quantification

Quantification of SPHW involved a field survey to Fatemi sisal estate, which is owned by Mohammed Enterprises Tanzania Ltd (MeTL) and is located in Morogoro, Tanzania for site reconnaissance. Information was gathered on acreage of the estate, sisal density of sisal plants per hectare, rates of generation of the waste and its current management methods through guided interviews of the estate management staff.

Twelve samples of sisal boles with the attached leaf stubs were collected randomly from one-hectare farm area and transported to the laboratory. They were each weighed before the leaf stubs were removed from the core-sisal boles by using a machete and categorized by weight to small, medium, large and extra-large. The fresh weights of the core-boles and leaf stubs fractions were recorded. Both fractions were then chopped into 3-5 cm pieces and dried in the sun and their weights recorded once more. Fresh and dry samples of the two fractions were taken for physico-chemical analysis according to Gumisiriza et al. (2009).

2.2 SPHW Leachate (juice) Extraction

The chopped pieces of the core-bole and leaf stubs were crushed and minced using a laboratory mincer. One hundred grams of the processed core-bole and leaf stubs were weighed using an Adventurer TM balance (Ohaus Corp, Pine Brook, NJ, USA) each separately into 1000 ml of tap water. Extraction of leachate (juice) from core-bole and leaf stubs was done by hot water extraction method according to Gaafar et al. (2010). The mixtures of core-bole and leaf stubs and tap water were brought to a boil at 100°C in two separate pans using a laboratory table hot plate (E.G.O. (Elektro-Gerätebau GmbH), Germany) and boiling was continued for 20 minutes. The boiled slurry was leftto cool to room temperature. To extract the leachate (juice) the slurry was squeezed manually through two layers of cheesecloth. The filtrate obtained was subsequently filtered manually by squeezing through a 2 mm mesh size plastic sieve. The extracted liquid was referred to as leachate (juice). The solid leftafter leachate (juice) extraction was sun dried for five days and was referred to as biomass residue after leachate (juice) extraction.

2.2 Analytical Methods

Determination of total solids (TS), volatile solids (VS), total nitrogen (TN), ash content, chemical oxygen demand (COD), biological oxygen demand (BOD5) was done according to standard methods (APHA, 1995). On the other hand, total carbon was estimated according to Allen (1989) and Nelson and Sommer (1996). The organic matter content was determined by the dry combustion method described by Jiménez and García (1992) and Lyimo et al. (2002). Total fibers were determined by the permanganate method as Neutral detergent fiber (NDF) and Acid detergent fiber (ADF) according to Goering and Van Soest (1970). Total sugars of SPHW fractions were determined by phenol-sulphuric acid method according to Dubois et al. (1956). Total dissolved solids (TDS), conductivity and pH were measured using a portable pH microprocessor probe and meter (HANNA-Italy). Minerals content were determined as described by AOAC (1984) and Allen (1989). Crude fiber was determined using Near Infrared Spectrophotometer (NIRSystems, Inc, USA). Volatile fatty acids and alkalinity were determined by titration method described by Buchauer (1998) and Lahav and Morgan (2004).

3. Results

3.1 SPHW Production and Disposal at Fatemi Sisal Estate

The sisal plant currently grown at Fatemi sisal estate is Agave hybrid 11648. The estate's area under cultivation is 2,000 hectares each with a population of 5,000 plants grown in double rows. The sisal post harvest wastes comprises of sisal poles, core-sisal boles and stubs of leaves that remain on the boles after every cutting. The sisal bole is the intermediate part between roots and stem covered with a hard lignified cortical layer or bark of 2-3 mm thick, onto which leaves are attached. SPHW fractions account for 50% of the sisal plant and are produced at the end of the plants' productive life (6-12 years). The average weights of the various sizes of sisal boles with the attached leaf stubs are shown in Table 1. The weights of the core-boles and leaf stubs ranged between 19 and 129 kg. The largest and smallest core-sisal boles without leaf stubs weighed between 44 and 4 kg and their sisal leaf stubs weighed between 85 and 15 kg, respectively. Generally, the weight of the leaf stubs contributed much more to the total weight of the waste than the core-boles (Figure 1).

Poling of sisal plants is not uniform and differs even on the same plot due to many factors including type of soil and available soil nutrients, dry and rainy season, temperature, humidity and other microclimate factors. Experience revealed that after 3-4 years of economic life, about 40-50% of the plants are uprooted and 80-90 % are uprooted within 5-12 years in order to replant the farm. Therefore it is assumed that each year 10 % of sisal plants are uprooted before replanting new ones. Based on X-large category (44 kg sisal bole and 85 kg sisal leaves remains), about 22 tons and 42.5 tons of sisal boles and leaf stubs, respectively can be projected per hectare annually from Fatemi sisal estate. When sisal boles and sisal leaves remains were chopped and sun dried for seven days, they became 35% and 38%, respectively of their original fresh biomass. These translate into an annual production of about 15,000 tons of dry sisal boles and 32,000 tons of sisal leaf stubs. Currently, the boles are crushed and leftto rot on the farm and the poles are sold/and or given for free to be used by the local community around the factory.

3.2 Characterization of SPHW

The chemical compositions of the various fractions of the core-sisal boles and the leaf stubs are shown in (Tables 2 and 3). Generally, the solid fractions were rich in biodegradable substances in terms of volatile solids (VS) in the range of 93-98% and total sugars ranging between 8 and 27 mg/l. The fresh SBW fraction had the highest VS content and its dried fraction had the least. On the part of total sugar content, the fresh RLS fraction had the highest content whereas the biomass residue after leachate extraction from the same had the least content. Furthermore, the solid fractions were characterized by high crude fiber content in the range of 25-81% and high C:N ratios ranging between 24 and 285. The biomass residue after leachate extraction from SBW fraction had the highest crude fiber and C:N ratio values. Similar to the solid fractions, the leachate extracts were found to be rich in biodegradable substances in terms of VS at about 93 and 97% and in total sugars at about 30 and 35 mg/l whereby the leachate from the RLS fraction had the highest content of VS and total sugars.

4. Discussion

4.1 Quantification of Sisal Post-Harvest Waste

Availability of a substrate in abundance and at low or not cost is the basis for choosing to utilize it for industrial production and in this context SPHW qualifies. In this case study, the annual generation of SBW and RLS fractions was estimated at 15,000 and 32,000 tons of dry weight, respectively for the 2000 ha Fatemi sisal estate (Table 1 and Figure 1). Applying this generation estimation rate to a total of 52,000 hectares under cultivation in Tanzania (Semotony, 2012), total SPHW generation by the sisal industry amounts to 1,222,000 tons annually. To our knowledge, this study is for the first time reporting elaborate information on generation of SPHW in Tanzania, and the region. Brief information on relative weights of sisal boles, which tallies with the findings of this study was reported by Kimaro et al. (1994).

4.2 Composition of Sisal Post-Harvest Waste Fractions

Suitability of any biomass as feedstock for processing to bioproducts very much depends on its chemical characteristics (Table 2 and 3). The various solid fractions of SBW and RLS were characterized by high organic matter contents of up to 97% in terms of VS and about 55% in terms of total organic carbon. These values are similar to those obtained previously for sisal leaf decortications pulp by Mshandete et al. (2004) and Muthangya et al. (2009). Apart from high organic carbon content, the fractions had high total sugar contents in the range of 8-27%. These results are in agreement with the high (16.3%) inulin sugar content reported by Elisante and Msemwa (2010) for sisal boles. Furthermore, all the fractions showed low hence safe levels of light metals including Na, Mg and Ca, which are required for microbial growth but cause inhibitory effects when found in high proportions as previously documented (Chen et al., 2008). Judging from their solid and organic matter content, the total sugars in particular, the waste fractions generally contain easily biodegradable substances hence potential feedstocks for bioconversion to a wide range of value added products such as organic acids, alcohols, pharmaceutical chemicals and biogas through applying a biorefinery approach (Francesco, 2010).

Despite the high organic matter profile, the fractions had high crude fiber content ranging between 25 and 81%. This characteristic makes the fractions to be categorized as heavily biodegradable (hydrolysable) materials requiring pre-treatment for subsequent bioprocessing including when anaerobic digestion is an option. Besides, their high C:N ratio in the range of 24-201 necessitates codigestion with nitrogen rich materials for an improved anaerobic digestion process (Mata-Alvarez et al., 2012). This effect was previously demonstrated by Mshandete et al. (2004) by co-digesting sisal decortications pulp with fish processing waste.

This study further characterized the extracts (leachates) of the waste fractions to explore the possibility of utilizing them instead of the solid materials. The chemical profiles of the leachates from the fresh SBW and RLS were to a greater extent found to be similar to those of their original fractions (Tables 2 and 3). Use of the leachate instead of the solid fractions may be an innovative approach for easy handling and pumping in bioreactor systems. The leachate extraction cost may be off-set by the benefits considering the sample percolation methods available (Gaafar et al., 2010). If the option is anaerobic digestion for biogas production, biomass percolation has been shown to increase methane production and ensure waste stabilization (Busch et al., 2009). The biomass residues after leachate extraction was characterized by very high crude fiber values in the range of 63-81% and their total sugar content was about 50% of that obtained for the original fractions and the leachates. Nevertheless, the rest of the parameters were similar to the rest of the fractions, which still makes them potentially suitable substrates for bioconversion to a number of products including mushrooms

4.3 Proposed Integrated Utilization of Sisal Post-Harvest Waste

As mentioned previously, sisal fiber production is conventionally a high waste industry with low profitability at high capitalization hence unable to be sustainable and compete in the global market (UNIDO, 2002). In order to ensure ecologically sustainable industrial development, reversal of the current situation from utilizing only 2% of the sisal plant to 98% was proposed under the Cleaner Production Programme (UNIDO, 2005). This would be achieved through diversifying processing and products. From the sisal leaf decortications waste, production of wax, alcohols and other chemicals or biogas and biofertilizer was proposed. For the post-harvest waste (boles and roots), they proposed production of alcohol, inulin and pharmaceutical chemicals. On the basis of the results of this study, we propose an innovative two-in-one approach within the UNIDO framework (Figure 2). In the first option, the entire sisal bole is used for cultivation of mushrooms and the mushroom spent substrate is co-digested with cow-dung manure anaerobically for biogas production. The effluent of the process known as biogas manure is valuable biofertilizer. In the second option, the two fractions may be utilized together or separately depending on the desired products. Here, the chopped and crushed fractions are used for leachate extraction and the leachate may be used in a biorefinery for production of a wide range of products such as inulin, organic acids, ethanol and biogas. The biomass residue after leachate extraction may be utilized for mushroom cultivation as well as for animal feed production using white rot fungi. Again the mushroom spent substrate may be a feedstock for anaerobic digestion process. Unlike the UNIDO proposal for the post-harvest waste, this approach has more options for more products giving the industry more flexibility in choices, which is an additional competitive advantage.

5. Conclusions

Although highly fibrous and poor in nitrogen, sisal post-harvest waste produced by cultivation of Agave hybrid 11648 has high contents of organic matter and rich in total sugars. These characteristics make it potentially suitable for production of a wide range of high value commodities. The waste generated by Fatemi sisal estate is currently available for free and in abundance awaiting exploitation.

Tables and Figures:

Acknowledgements

We wish to acknowledge the Swedish International Development Cooperation Agency (Sida) for the financial support. We thank the BIO-INNOVATE programme for including environmental biotechnology as one of the research themes in the programme. The participation of Fatemi Sisal Estate Management under the leadership of Mohammed Enterprise Tanzania Limited (MeTL) is highly appreciated.

References

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