Determination of biogas potential of banana harvestingwaste and environmental life cycle assessment of utilizingstem waste for banana production in greenhouses in Türkiye

Abstract

In Türkiye, 548,323 tons of banana fruit were produced in 2019. Banana fruit mainly grows in the Mediterranean Region due to the favorable temperature conditions. The year-long average temperature, humidity, and specifications of the soil are the parameters that make this region preferable for banana production. However, the environmental conditions in the Mediterranean Region have disadvantages when the region is compared with the countries that are the motherland of bananas such as India, Uganda, Ecuador, and Brazil. The ideal growing temperature for bananas is 27° C. In the case of a production environment at this temperature, an average of 100 kg of fruit can be collected from a plant for each harvest, although it varies according to the type of banana plant. It is not possible to capture this temperature regime during the year in Türkiye. For this reason, 77% of the current banana production is made in greenhouse areas. Greenhouse areas are production areas designed to keep the indoor temperature as high as possible, especially in winter. However, with existing methods, producers cannot increase the indoor temperature above 15 ° C in winter. This situation leads to the 40-50 kg range of production yield in each harvest per tree in Türkiye. Although this amount, which was half the current yield in the previous periods, is accepted; when the increasing banana consumption is considered, producers have tried various methods to increase this yield even more. However, most of them have failed. One method that has been tried frequently has been to use a heat source to increase the temperature of the covered areas. Some producers have installed wood, coal, and even natural gas stoves in greenhouses, also known as greenhouses, and tried to heat the interior in this way. The average size of the greenhouses densely located in Mersin and Alanya regions is 3 decares; In greenhouses covering these and larger areas, heating by setting up a stove has not been an effective method since it could not heat the entire area homogeneously. The most effective method for increasing the yield of banana production in Türkiye is to spray the groundwater into the greenhouse to improve the indoor temperature to a higher level than the ambient temperature. In Alanya, where the average air temperature is 11.8 °C in winter, groundwater temperatures are around 15 °C. Therefore, the usage of groundwater by spraying is an effective method of heating the greenhouses during the winter period. While some producers feed the groundwater directly into the greenhouses, some producers use the boiler systems they have installed to heat the groundwater and give it into the greenhouse. Fossil and nonrenewable energy sources are widely used in the heating process. This study, aims to evaluate the wastes generated during banana production and harvest and to measure the environmental effects of this method to handle the banana production process with a more circular approach, which will be an alternative to fossil fuels. The banana harvest takes place approximately 9 months after the plant sprouts. Depending on the type of banana plant, the plant's height could reach 8-10 meters during that time. The main planted banana plant species in Türkiye are Azman, Dwarf Cavendish, and Grande Naina can grow up to 6 to 8 meters. When the fruit harvest is completed, the strongest sprout is left from the newborn roots of the banana plant, and the whole plant is cut and left as waste. During the production of every 1 kg of banana fruit, 4 kg of harvest waste is generated. Banana harvest waste consists of leaves, stems, roots, root stems, flowers, and raw fruits. These completely organic wastes contain a high yield of carbon, nitrogen, phosphorus, and potassium. Currently, some producers leave these wastes in greenhouses and wait for the rich nutrient content to return to the soil. Although leaving these wastes to decompose in an uncontrolled way has a positive effect on nutrient recovery, generally, the application of a non-homogeneously distributed dose to a certain area has a negative effect. In addition, a disease that occurs in harvested plants can pass into the soil as a result of decomposition and damage healthy or newly budded plants. The banana harvest waste is quite suitable for energy and nutrient recovery by anaerobic digestion method when all the mentioned effects and properties are evaluated. In the anaerobic digestion method, banana harvest wastes are digested in a controlled reactor in the absence of oxygen at constant temperature and pH. During the digestion process, various bacteria consume organic matter and pathogens in the harvest waste and produce biogas, the content of it mostly methane (CH4) and carbon dioxide (CO2) gases. The produced biogas has a flammable feature due to the CH4 gas content and thus can be used as a renewable fuel. This study was carried out in two stages. In the first stage, the biogas potential of banana harvesting waste that occurred in Türkiye was measured in a pilot-scale anaerobic biogas reactor built in Lüleburgaz, Kırklareli. Thus, the potential of banana waste as a renewable source was measured for the greenhouse production areas which use groundwater as the heating source. However, due to different reasons (logistics, technical problems, etc.), the reactor could only be operated for 30 days, and the system could not reach a steady state during this time. However, the results and experiences obtained, albeit for a short time, are given to shed light on future studies. It is not recommended that these results be used for design purposes as the steady state is not reached. The reactor design was completed based on the results of the characterization of banana harvesting waste samples collected from southern Türkiye. The reactor volume is calculated as 10 m3. Anaerobic digestion was carried out in mesophilic conditions for 30 days. The results collected in the 30-day experiment were verified by comparison with literature data. Then, parameters in the design of the pilot scale reactor and biogas production quantities were used as inputs in the life cycle assessment. In the second stage of the thesis, the life cycle environmental impacts of banana production and then its supply to the end user in Türkiye were investigated. The low groundwater temperatures in Türkiye inhibit the yield of banana trees in Türkiye and literature suggests that it is possible to double the yield of a single tree by increasing the irrigation water temperature to 27 °C. Hence, three different scenarios were studied. The first scenario, also known as the business as usual case was considered; in the second scenario heating the irrigation water by using natural gas was studied, and in the third scenario heating the irrigation water by using biogas produced on-site via the anaerobic digestion of banana stem waste was analyzed. The functional unit was chosen as 2 tons of bananas produced throughout the lifetime of the biogas production system. CCaLC2TM was used as software, and CML2001 methodology was used. A cradle-to-grave approach was employed. The production processes were modeled based on real-life data acquired from a real greenhouse in Türkiye. Six impacts (global warming potential, acidification potential, eutrophication potential, photochemical oxidant creation potential, ozone layer depletion potential, and human toxicity potential) were calculated. Results show that four of the six impacts decreased when biogas was used, suggesting that this practice has the potential to reduce the environmental footprint of banana production. The results were found to be in good agreement with the values reported in the literature. It was concluded that to reduce the environmental footprint of banana production, utilizing stem waste instead of the conventional practice of burning is essential, and special emphasis should be given to treating or utilizing the bioreactor digestate to further reduce the environmental footprint.