In the year 2019, around 8 million tons of solid waste was generated in Kenya and the rate of generation is likely to double by the year 2030, driven by socio-economic development and rapid population growth [1]. Between 2009 and 2019, the country’s population increased by an average of 1 million people annually, while at the same time, Kenya achieved a middle-level income status, which resulted into an increasing demand for food and other resources [2]. Further, this implies that a significant amount of bio-waste is generated. To alleviate these challenges, efforts should focus on resource efficiency and circular economy. This would imply that resources are used efficiently throughout all stages of their lifecycle and throughout the supply chain, by emphasizing minimizing waste or transforming them into high quality secondary raw materials [3–5].
In Kenya, and similarly to many other (developing) countries, these bio-wastes are often burned or disposed of in open landfills, directly leading to environmental pollution and to climate change through particulate and greenhouse gas (GHG) emissions. Furthermore, due to prevalent tropical climate, the pile of bio-waste often becomes a habitat to pests and rodents and hence becomes a nuisance with corresponding health implications [6].
Since agriculture is the main economic activity in Kenya, contributing directly and indirectly to over 50% of the country’s gross domestic product (GDP) and providing livelihoods to over 80% of the entire population, the sector is expected to generate high quantity of bio-wastes [7]. Indeed, according to research conducted in Kenya, organic wastes from urban markets alone (excluding waste or inedible parts that remains at the farm gate and collection centers), forms over 4,000 kt representing over 50% of all wastes produced in the urban centers per year [8]. However, despite the high quantity of these types of waste and their related environmental and health hazards, the rate of valorization is just about 40 kt accounting for about 1% of the total urban organic waste alone per year, thus, bio-wastes is a key contributor to waste management problem in Kenya [8, 1, 6]. This can be ascribed to the dispersity of waste generation, but also, there seems to be little knowledge about the potential benefits of waste valorization. In cases where these wastes are utilized, they are turned into relatively low added value products such as mulch, compost, energy (by burning) or in best case scenario, as animals feed [8, 9].
At a policy level, Kenya adopted the “Transforming our world: the 2030 Agenda for Sustainable Development”, which aims to make the human environment safe, resilient and sustainable, ensuring sustainable production and consumption patterns and also reducing GHG emission by 30% by 2030 under the United Nation (UN) Framework Convention on Climate Change [1]. Strategic decisions now have to be made on how to achieve these targets. Science could support interventions towards an effective conversion of wastes from the food chain to usable, preferably high-valued products.
As a starting point for policy implementation in this context, mapping of the situation of these wastes should be performed, including current quantities, origin and destinations. One of the tools that can be used for this purpose is Material Flow Analysis (MFA). Indeed, MFA has been used for the analysis of organic waste flow in amongst others, Europe [10], UK [11], US [12], Asia [13] and proven to be useful in the support of organic waste management and recycling policies by allowing visualization and quantification of flows that are to be managed. Nevertheless, agricultural and food waste generation and destination in Kenya, and in broader sense, many developing countries still lacks mapping as there is no systematically collected data. There is, however, a bit of unpublished literature, sometimes in local languages, and some other local information which is generally not accessible to a broad audience [14].
Within this specific Kenyan context, the main objective of this research is to study the potential of bioresource material recovery from selected Kenyan agricultural-based waste. This paper identifies quantity, occurrence, current usage and disposal trends of the most important fruit, vegetable and nut waste streams in Kenya. The information is summarized in a comprehensive MFA, to track the mass flow of selected waste streams from production through processing, consumption and disposal. Finally, the potential bioresource materials recoverable from the waste streams and the logistics of recovery are evaluated.
1.1 Preface: selection of case study
Within this study, a broad inventory of fruit, vegetable and nut wastes quantities in Kenya was made that informed the selection of different case studies for the valorization potential of the waste streams. Apart from the waste quantities, the case studies were also selected based on their inherent differences, including, dry versus wet waste, soil grown vis-a-vis above ground (tree) based wastes. Table 1 gives an overview of top ten fruit, vegetable and nut wastes in Kenya. Based on this inventory, we selected banana, potato and coconut waste as case studies, for each of these groups. In Table 1, the potential quantity of waste (column 7) is derived from the production quantity (column 5) and the potential waste proportion (column 6) found in literature.
Table 1
Top ten fruits, vegetable, and nuts in in Kenya in the year 2018
Product class | Production Rank | Agricultural product | Nature of waste | Production in KenyaFW (kt)[15] | Approximate potential waste (%) | Derived potential quantities of waste (kt) FM |
Fruits | 1 | Bananas (plantain, dessert and other)a | Pseudostem, peel, stock, leaves, bud, rotten pulp | 1447 | 88e,f | 1273 |
2 | Mangoes, Mangosten, Guavasa | Pomace, peel, stone/seed, seed coat | 676 | 47f | 318 |
3 | Tomatoa | Pomace, cull tomato | 599 | 20g | 120 |
4 | Pinneaplesa | Pomace, peel, crown | 349 | 46f | 161 |
5 | Avocadoa | Pomace, peel, stone, seed coat | 234 | 26f | 61 |
6 | Water melona | Peel, seed/rind | 189 | 48f | 91 |
7 | Papayaa | Pomace, Peel, seed | 134 | 47f | 63 |
8 | Fruit citrus nesa | Pomace, peel, seed | 84 | 34f | 29 |
9 | Orangesa | Pomace, peel, seed | 72 | 24f | 17 |
10 | Fruit tropical fresh nesa | Peel, seed/stone, rind, skin, pomace | 39 | 27f | 11 |
Vegetable | 1 | Irish Potatoesb | Peel | 1870 | 16f | 299 |
2 | Cassavab | Peel | 946 | 28h | 265 |
3 | Sweet potatoesb | Peel | 871 | 22f | 192 |
4 | Beans, dryc | Hull, germ powder, husk, Broken grain | 837 | 18g | 151 |
5 | Cabbages and other brassicasa | Trim losses | 674 | 20f | 135 |
6 | Vegetables, fresh nesa | Trim losses | 670 | 20f | 134 |
7 | Spinacha | Trim losses | 169 | 20f | 34 |
8 | Pigeon peasc | Hull, germ powder, husk, Broken grain | 86 | 40g | 34 |
9 | Onions, dryb | Outer leaves, tunic, basal plate, roots | 35 | 11f | 4 |
| 10 | Beans greenc | Pods | 34 | n.d | - |
Nut | 1 | Coconutsc | Shell, husk | 105 | 47i | 49 |
2 | Macadamia (nut nes)c | Shell | 27 | 40-45j | 11 |
3 | Groundnutsd | Shell, karnel | 21 | 25j | 5 |
4 | Cashew nutsc | Shell | 14 | 55-65j | 8 |
5 | Bambara nut | Shell | 3.4 | | |
6 | Areca nutsc | Shell, husk | 0.1 | 60-80j | 0.7 |
7 | Nutmeg, mace and cardamomsc | Shell, seed | 0.06 | - | - |
| 8 | Chestnutc | Shell | n.d | 25k | - |
| 9 | Walnutc | kernel, green husk, shell | n.d | 67l | - |
| 10 | Hazel nutc | Shell | n.d | 67i | - |
a Wet, on ground waste (tree), b Wet, underground waste, c dry on ground waste, d dry underground waste, e [16], f [17], g [18] h [19], I [20], j [15], k [21], i [22], n.d: not determined |
Banana is a herbaceous plant in the genus Musa and family Musaceae and is cultivated mainly for its fruit [23]. Banana plants consist of a rhizome (corm), a succulent stem called pseudostem, leaves, and a stock (rachis or peduncle) which supports an oval inflorescence consisting of deep purple waxy bracts (male bud) enclosing the female and male flowers in the lower and upper rows respectively. The female flowers eventually develop into banana fruits (hand). The fruits are harvested after every four to six months of planting [24] and are normally covered by a thick skin (peel) which is peeled off to expose the edible part (pulp). Except for the pulps and sometimes the rhizomes, all parts of the banana plant are potential waste [23]. These potential by-products are about 88% of the whole banana plant [16] which translate to an estimated 0.22 kt of waste per hectare in countries such as India, where the average yield of banana is about 30 t per hectare [25].
In Kenya, banana is the most important fruit in terms of productivity and is available throughout the year. For example, in 2017, banana fruits accounted for about 14.5% of the total value of horticulture and 35.9% of all fruits produced [26]. Typically, there are two cultivars of bananas in Kenya namely the desert (Musa sapientum) - the ripe edible cultivar and the plantain (Musa paradisiaca) - the cooking cultivars [27]. The two cultivars generally have similar morphology and are generalized as bananas [27] and will be considered so in this study.
Irish potato (Solanum tuberosum L.), is a root vegetable in the family of Solanaceae and genus Solanum. Due to its high nutritional value and adaptability to different climates, Irish potato tuber is the second most important staple food and strategic security crop in Kenya after maize and accounts for about 50% by volume of all vegetables produced and approximately 23.5% by value of the horticultural produce [28, 29]. It is grown by around 800,000 farmers who retain around 10% of their produce as seed potato and sell the rest mainly at local markets [30, 31]. Annually, about 1.5 to 2 million tonnes of potatoes are produced mostly by a few large-scale farmers and many small-scale farmers [32, 29]. Irish potato plants consist of stems, branches, leaflets, roots, auxiliary buds, flowers and an enlarged portion of the stolon called tubers. Maturity is characterized by dehauliming, senescence or vine desiccation involving stem, branches and leaves yellowing or turning brownish and eventually drying and dropping off or being mechanically removed to allow tuber skin to harden and reduce damage to the tubers during harvesting [33, 29]. The main part that remains after desiccation is the tuber, of which during harvesting and subsequent processing, generates rotten/rejected tubers and peels as the main potential wastes comprising about 25% and 12% of the total potato production respectively [32, 34].
Apart from farm gate production wastes for both bananas and Irish potatoes, further wastes is generated during transportation, packaging, storage, sales and consumption due to, among other factors, adverse weather effects such as temperatures, poor road networks etc.) [35]. Furthermore, rejects are created due to attacks by pests and diseases, damage or inefficiencies during harvesting and processing, or as a result of over-supply and competition [27, 35, 36].
Cocos nucifera L. is a palm tree that belongs to the family Arecaceae and genus Cocos. The coconut palm tree produces a nut referred to as 'coconut' and is the highest productive nut in Kenya and accounted for over 58% of productivity of all nuts in 2018 [15]. In the same year approximately 10 million coconut trees with over 277 million nuts made up the Kenyan coconut industry of which 95% are in the costal belt [37, 38]. The Coconut tree consists of roots, trunk, fonds or leaves, flowers and the nuts which are commercially exploitable. The roots can be used as dye stuff and for medicinal purposes but are often left in the field as dry stocks to be used for firewood; trunks are used for; fuel, making furniture, building huts and other physical structures, canoe building, coffin making and performing important cultural practices such as crafting of grave posts (kigango) [39]. Fonds, such as fallen, dry or used coconut leaves (kanja) are used mainly as roof thatching material (makuti) for both the local houses and exotic tourist hotels and also for stuffing pillows and mattresses, making brooms, weaving mats (mkeka) and fish traps, while sap from inflorescence (fruit bud), stem or root can be tapped to produce toddy (mnazi) which is fermented to produce an alcoholic drink (palm wine) [40, 39, 38, 41]. The nuts are nutritious and are consumed at a tender stage (madafu) and at their mature stage in form of copra oils, copra cake, hence, the coconut trees are thus useful as cash crop due to their multi-purpose uses [42, 43]. The processing of coconut generates shell and husk as the main by-products and constitutes about 12% and 35 % of the whole fruit by weight respectively [20]. Also, coconut cake meal is produced as a by-product of copra processing but due to its nutritional value, it is being used in confectionery [44].
For these 3 case studies of banana, Irish potatoes and coconut, a Material Flow Analysis in Kenya context was performed.