Abstract
Nutrient depletion in Tanzanian sisal production has led to yield decreases over time. We use nutrient mass balances embedded within a life cycle assessment to quantify the extent of nutrient depletion for different production systems, and then used circular economy principles to identify potential cosubstrates from within the Tanzanian economy to anaerobically digest with sisal wastes. The biogas produced was then used to generate bioelectricity and the digestate residual can be used as a fertilizer to address the nutrient depletion. Life cycle assessment was used in a gate-to-gate assessment of the anaerobic digestion options with different cosubstrates. If no current beneficial use of the cosubstrate was assumed, then beef manure and marine fish processing waste were the best cosubstrates. If agricultural wastes were assumed to have a current beneficial use as fertilizer, then marine fish processing waste and human urine were the best cosubstrates. The largest reduction in environmental impacts resulted from bioelectricity replacing electricity from fossil fuels in the national electricity grid and improved onsite waste management practices. There is significant potential to revitalize Tanzanian sisal production by applying circular economy principles to sisal waste management to address soil nutrient depletion and co-produce bioenergy.
Highlights
Sisal (Agave sisalana) was imported from Mexico’s Yucatan Peninsula into Tanzania during the late 1800s [1]
The assessment indicated that most wastes from plants were not suitable, as they had a C:N ratio above 25, but that wastes from livestock production, sugar cane trash, cowpea residue, and grass clippings were suitable, as they had C:N ratios of less than 25
This is known as a comparative gate-to-gate life cycle assessment (LCA), and results in certain midpoint impact categories (MICs) appearing as emission sinks, rather than emissions sources, which are more usual for LCA studies
Summary
Sisal (Agave sisalana) was imported from Mexico’s Yucatan Peninsula into Tanzania during the late 1800s [1]. Tanzania was the world’s second-largest producer of sisal in. 2019, producing 15% of the world’s 220,363 tonnes of sisal fiber [2] behind Brazil, which produced 39% of the world’s production. The fiber fraction in each sisal leaf ranges from 2.7% to 7.3% [3] and the average Tanzanian value is. 4%, indicating that each tonne of sisal fiber generates 24 tonnes of solid waste material (dry weight) [4]. At most sisal processing sites in Tanzania, this waste composts in retention areas in an uncontrolled fashion, leading to both anaerobic and aerobic decomposition. In the 1970s, researchers found that successive cycles of sisal cultivation without the use of fertilizers or recycled, composted sisal waste material depleted nutrient levels in the soil [5,6]. Subsequent research has consistently confirmed this effect and the adverse
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