Six agricultural waste and their utilisation in creating useful sustainable, environmentally-friendly products.
By Oyewole Okewole
Agricultural processes produce waste at all levels throughout the value chain. They generally become an environmental nuisance when indiscriminately and unsafely disposed of. However, these agricultural waste products, through knowledge, research, and technology, can be potentially harnessed to produce wealth. Furthermore, these products are increasingly being converted into useful, sustainable products around the globe.
Refer here to the first part of this series that discussed five ways plant waste can be converted into value-added products – such as feedstock – or as part of another product.Â
Through this article, we will expatiate some of the conversion and utilisation of waste generated during the processing of some agricultural commodities. Agro-industry is characterised by quite robust classes and degrees of waste generated in solid, liquid, and gaseous forms. Some of the ways of processing the agricultural waste from the world’s staple crops will be addressed. Their social and economic impact and many other benefits that can be derived from their conversion and utilisation will also be discussed.
Rice Husks
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Rice husk is the most prolific agricultural residue in rice-producing countries around the world. In particular, approximately 25% of harvested rice paddy will give rice husks, and about 8% will produce rice husk ash, all by-products from milling rice. These by-products are efficient as energy sources for rice processing operations. In particular, processes like the drying and parboiling process can utilise rice husks as an energy input. They can also be further utilised for various applications in several industries such as construction, animal feed, food processing, electricity generation, equipment fabrication industry, and many others.
Some of these operations include rice husk usage as a strengthening agent in building materials, which is largely due to its absorbent and insulating properties. This makes it a major material for the production of rice husk brick. It is also a very good source of nutrients and fibre for cattle feeding. It is part of the materials used in partition board manufacturing.
Rice husk can be used as a substrate for the production of enzymes with other ingredients like bagasse and corn stalks using the solid state fermentation method. It is also a renewable energy source and is essentially utilised as fuel wood for direct combustion as rice husk pellets, briquettes, and biogas. In addition, it is used for the gasification process and alkali activation in the digital fabrication of materials used in 3D printers and Computer Numerical Control (CNC) machines. Other applications include its use for compost production and soil improvement. It is used as a soilless growing medium for crops like cucumber and capsicum spp. (pepper) in soilless farming, and also mixed with nitrogen-rich materials, being rich in carbon to make a well-balanced soil amendment.
The briquetting technology for example, is an approach to solving a processing and environmental problem. Rice husk briquettes are an affordable alternative fuel to firewood, coal, and other solid combustible fuels, which are produced by converting low bulk density biomass into high diversity solid fuels. The briquettes are a non-conventional source of energy, renewable in nature, eco-friendly, non-polluting and economical. To produce the rice husk briquettes, a briquetting machine is required. This machinery makes energy fuel briquettes from biomass waste by densification process.
Rice Husks Briquetting Machine.
Rice Bran
Rice bran is another by-product of rice processing that is often produced after the polishing process. According to the Rice Knowledge Bank, 100kg of paddy rice will generate approximately 5-10kg of bran. Rice bran is rich in crude fibre, protein, fat, ash, and sometimes fractions of rice hull. It has a high nutritive value and is a very good source of Vitamin B and E. It contains low quantities of antioxidants and is therefore used as low cholesterol-sourced food for humans. It contains 10-23% bran oil which makes bran an excellent binder for animal feeds. Rice bran can simply undergo some form of densification by adding a suitable binder (for example 1 – 3% of molasses, fat, or colloidal clays) and then pressing the composition under high pressure in pelletising machines or extruders to form cylindrically shaped pellets. These pellets are administered to animals as feed. Rice bran oil through solvent extraction is a high-quality vegetable oil for cooking.
It has also been discovered that rice bran has various medicinal uses. It is researched as the source of different phytoactive that indicates various pharmaceutical characteristics. It was reviewed that constituents of rice bran show anti-cancerous, anti-inflammatory, anti-diabetic, and anti-hypertensive activities.
[Read also: Why Mechanisation is Focal in Transforming Africa’s Agricultural Landscape (Part One)]
Empty Fruit Bunches (EFB)
Empty Fruit Bunches is one of the solid waste from crude palm oil processing. They represent about 20-22% of the fresh fruit bunches. They are generated from the stripping process of fresh fruit bunches to extract the fruits from the fruit bunches. They are a lignocellulosic material for making pulp and paper, photographic papers, and cigarette wrappers. They are also processed into briquettes for their good burning characteristics, and also as activated charcoal and biochar. EFB is also extensively utilised for packaging papers. The fibre in this case is combined with other fibre substances to improve the packaging strength. The soil-enriching characteristics have made the usage as manure with high nutritional sustainability, especially for plantations like oil palm and pineapple.
Furthermore, research indicates EFB use through the fibre cracking technology for livestock feeding with the combination of urea treatment, high temperature and pressure treatment to enhance the nutritional value. It is also converted to pyrolysis oil used in gas-fired power plants for electricity generation. More value-added products like bioethanol, fibre, and manure/fertiliser can be produced by utilising EFB. Pretreatment technologies of EFB have made it amenable to enhanced combustion. The technology process includes shredding and torrefaction.
Palm Oil Mill Effluents (POME)
One of the major environmental problems with palm oil processing plants is the POME. For every 1,000kg of crude palm oil produced, about 2.5-3.0m3 of POME is produced. POME is the liquid effluent from palm oil processing. It is a colloidal suspension that contains 95-96% water, 0.6-0.7% oil, 4-5% total solids or sludge including 2-4% suspended solids. The utilisation of POME has been researched to produce value-added materials and energy input which ensures a more sustainable operation during palm oil production and processing.
POME can be used for the production of biogas through anaerobic processes. The organic substance is generally biodegradable; therefore treatment through a biodegradable process can be achieved, which is based on anaerobic, aerobic and facultative processes. The anaerobic process has great potential for rapid disintegration of organic matter to generate biogas that can be used in electricity generation and ultimately save fossil energy. Economically, viable techniques of extracting methane and other nutritious content of POME have been used in industries like agriculture, medical, and energy sectors. The nutrients are excellent substitutes for inorganic fertilisers and can be recycled into manure. An equipment known as the Complete Stirred Tank Reactor (CSTR) uses microorganisms to digest the organic substance in POME in an airless environment, resulting in the production of methane, used in generating electricity.
[Read also: Why Mechanisation is Focal in Transforming Africa’s Agricultural Landscape (Part Two)]
Cassava Peels
Cassava peels are one of the primary waste generated in cassava processing. The peels can represent 5 to 15% of the root usually obtained from manual or mechanically peeling processes. They contain a high amount of cyanogenic compounds with relatively high protein content. They require to be processed to preserve their nutritional value. Some of these processes are engaged to reduce the cyanogenic glucoside while transforming them into useful products. They include ensiling, sun-drying, and size reduction.
Cassava silage is obtained by chipping the peels into equal lengths for uniform and compaction and subsequently wilting for about 48 hours to reduce moisture content which is usually at about 70-75% to approximately 40%. The peels can be sun-dried and then fed to livestock and as a composite of other feed additives in cattle and piggery for example.
Other utilisations of cassava peels include their usage in the production of bioethanol through the fermentation process, as a biomass for the production of activated carbon for wastewater treatment, utilised as material for the production of biogas.
Cassava Effluents or Slurry
Cassava effluents are liquid wastes generated from the processing of cassava roots. It has a high organic content with high concentrations of chemical oxygen demand (COD), biochemical oxygen demand (BOD), total solids (TS), as well as low pH acids. Co-digestion with a nitrogen-rich substrate, such as manure, could decrease the carbon-to-nitrogen (C: N) ratio and provide the required buffering capacity for stabilising the pH in order to increase methane (CH4) production — the biogas used for other applications.
Cassava effluents have shown positive potential as an input for the production of biogas, biohydrogen, bioethanol, biofertiliser, and electricity generation. The activities of microorganisms enhance their characteristics for the production of these products. Previous research conducted indicates activities integrating cassava peels and effluents co-digested with livestock wastes for the production of biogas.
Cassava Effluents as a feedstock for Biogas Production
There are other agricultural waste products generated from the processing operations of many other plant commodities. More of the utilisation of these wastes will be considered in our next article.
Oyewole Okewole. Agricultural Project Development Specialist.
Cover photo: Sabine van Erp from Pixabay
