Impacts of Chemical-Assisted Thermal Pretreatments on Methane Production from Fruit and Vegetable Harvesting Wastes: Process Optimization

Abstract

The increasing population creates excess pressure on the plantation and production of fruits and vegetables across the world. Consumption demand during the whole year has made production compulsory in the covered production system (greenhouse). Production, harvesting, processing, transporting, and distribution chains of fruit and vegetables have resulted in a huge amount of wastes as an alternative source to produce biofuels. In this study, optimization of two pretreatment processes (NaOH and HCl assisted thermal) was investigated to enhance methane production from fruit and vegetable harvesting wastes (FVHW) that originate from greenhouses. NaOH concentration (0–6.5%), HCl concentration (0–5%), reaction temperature (60–100 °C), solid content (1–5%), time of reaction (1–5 h), and mixing speed (0–500 rpm) were chosen in a wide range of levels to optimize the process in a broad design boundary and to evaluate the positive and negative impacts of independent variables along with their ranges. Increasing NaOH and HCl concentrations resulted in higher COD solubilization but decreased the concentration of soluble sugars that can be converted directly into methane. Thus, the increasing concentrations of NaOH and HCl in the pretreatments have resulted in low methane production. The most important independent variables impacting COD and sugar solubilization were found to be chemical concentration (as NaOH and HCl), solid content and reaction temperature for the optimization of pretreatment processes. The high amount of methane productions in the range of 222–365 mL CH4 gVS−1 was obtained by the simple thermal application without using chemical agents as NaOH or HCl. Maximum enhancement of methane production was 47–68% compared to raw FVHW when 5% solid content, 1-hour reaction time and 60–100 °C reaction temperature were applied in pretreatments.

Pretreatment, Anaerobic Codigestion, or Both? Which Is More Suitable for the Enhancement of Methane Production from Agricultural Waste?

“…Similarly, a decrease was observed in the CMP with pretreatment using 2% H 2 SO 4 , while pretreatment with 2% NaOH increased the CMP by 94% compared to raw wheat straw [ 13 ]. Although it was proven that introducing high acid and alkaline concentrations for pretreatment results in the production of inhibitory substances [ 2 ], it was not expected for the CMP to be inhibited at very diluted acid concentration (0.1% H 2 SO 4 ) used in this study. As with pretreatment with H 2 SO 4 , inhibition in biogas production was observed when thermochemical pretreatment with 1.5% HCl was applied at 121 °C for 60 min to wheat straw and sugarcane bagasse [ 1 ].…”

“…It is well known that agricultural wastes produced in high quantities are a significant feedstock containing lignocellulosic residues for second-generation biofuel production [ 1 ]. On a local scale, agricultural wastes (AWs) consisting of roots, stalks, leaves and fruits originating from the production of tomato, pepper, cucumber, eggplant and courgette are the main causes of environmental problems in Antalya, Turkey [ 2 ]. Conventional disposal methods for these wastes, such as landfilling, uncontrolled burning and unconfined storage in the territory, result in significant environmental issues in urban areas [ 3 ].…”

“…Chemical pretreatment is also widely applied for the enhancement of anaerobic digestion by disintegrating the lignocellulosic structure. The main chemicals used in chemical pretreatment are acidic agents such as sulfuric acid and hydrochloric acid [ 2 , 3 ], alkaline agents such as sodium hydroxide and calcium hydroxide [ 2 , 12 ] and oxidants such as hydrogen peroxide [ 1 ]. While acidic pretreatment facilitates the hydrolysis of hemicellulose and the interruption of lignin [ 8 ], alkaline pretreatment provides the depolymerization of lignin and an increase in the porosity and surface area through solvation and saponification reactions [ 9 , 13 ].…”

“…While acidic pretreatment facilitates the hydrolysis of hemicellulose and the interruption of lignin [ 8 ], alkaline pretreatment provides the depolymerization of lignin and an increase in the porosity and surface area through solvation and saponification reactions [ 9 , 13 ]. The incorporation of thermal pretreatment into chemical pretreatment has been a subject of several studies due to its benefits such as decreased chemical consumption and increased biogas production [ 1 , 2 , 3 , 4 , 5 , 6 , 14 , 15 , 16 ]. Although thermochemical pretreatment with alkaline agents has been recommended for agricultural wastes with low lignin content, the most efficient chemical agent should be determined for each waste/feedstock with a different lignocellulosic structure [ 13 ].…”

Utilization of Whey for Red Pigment Production by Monascus Purpureus in Submerged Fermentation

“…The major criteria of sustainable industrial production is the recovery and reuse of these wastes as a resource within the cycle of circular economy [13]. Hence, researchers in recent years have concentrated on studies about the use of food industry wastes in the production of high value biotechnological products.…”