Abstract
The aim of this paper is to describe a study of the anaerobic digestion of industrial citrus solid waste (ISCW) in both batch and semi-continuous modes for the production of bioenergy without the elimination of D-limonene. The study was conducted at the pilot plant level in an anaerobic reactor with a working volume of 220 L under mesophilic conditions of 35 ± 2 °C. Cattle manure (CM) was used as the inoculum. Three batches were studied. The first batch had a CM/ISCW ratio of 90/10, and Batches 2 and 3 had CM/ISCW ratios of 80/20 and 70/30, respectively. In the semi-continuous mode an OLR of approximately 8 g total chemical oxygen demand (COD)/Ld (4.43 gVS/Ld) was used. The results showed that 49%, 44%, and 60% of volatile solids were removed in the batch mode, and 35% was removed in the semi-continuous mode. In the batch mode, 0.322, 0.382, and 0.316 LCH4 were obtained at STP/gVSremoved. A total of 24.4 L/d (34% methane) was measured in the semi-continuous mode. Bioenergy potentials of 3.97, 5.66, and 8.79 kWh were obtained for the respective batches, and 0.09 kWh was calculated in the semi-continuous mode. The citrus industry could produce 37 GWh per season. A ton of processed oranges has a bioenergy potential of 162 kWh, which is equivalent to 49 kWh of available electricity ($3.90).
Highlights
Citrus belongs to the group of products with the highest world consumption, with oranges being the most consumed
The results presented are explained according to the research methodology, i.e., Stage 1: inoculation, start-up, and stabilization of the anaerobic reactor with
A study was conducted of the anaerobic digestion of citrus solid waste in both the batch mode and the semi-continuous mode without the elimination of D-limonene
Summary
Citrus belongs to the group of products with the highest world consumption, with oranges being the most consumed. 47.5 million tons [2]. In this context, Brazil (15.1%), China (7.3%), the European Union (5.8%), the United States (4.9%), and Mexico (4.4%) are the countries that produce the most oranges [2]. 28.7 million tons (60%) are consumed as fresh oranges and. 18.8 million tons (40%) are used in the citrus industry for the production of concentrated orange juice, essential oils, marmalades, jellies, potpourris, candied peel, jams, flavoring agents for beverages, health drinks, and essences that are used as food-grade products [3,4,5]. From the processing of oranges, 40–60% of the total tonnage of oranges is discarded as solid waste [6,7], so, worldwide, the citrus industry generates from 7.5 to 11.3 million tons of industrial solid citrus waste (ISCW) per year. ISCW is characterized by a high water content (80%); acid pH (i.e., pH values in the range of 3–5)
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