Anaerobic digestion of wastewater derived from the pressing of orange peel generated in orange juice production

A study of the anaerobic digestion of wastewater from the pressing of orange peel generated in orange juice production was carried out in a laboratory-scale completely stirred tank reactor at mesophilic temperature (37 degrees C). Prior to anaerobic treatment the raw wastewater was subjected to physicochemical treatment using aluminum sulfate as a flocculant and to pH reduction using a solution of sulfuric acid. The reactor was batch fed at COD loads of 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 g of COD. The process was very stable for all of the loads studied, with mean pH and alkalinity values of 7.5 and 3220 mg of CaCO3/L, respectively. The anaerobic digestion of this substrate was found to follow a first-order kinetic model, from which the specific rate constants for methane production, K(G), were determined. The K(G) values decreased considerably from 0.0672 to 0.0078 L/(g h) when the COD load increased from 1.5 to 5.0 g of COD, indicating an inhibition phenomenon in the system studied. The proposed model predicted the behavior of the reactor very accurately, showing deviations of <5% between the experimental and theoretical values of methane production. The methane yield coefficient was found to be 295 mL of CH4 STP/g of COD removed, whereas the mean biodegradability of the substrate (TOC) was 88.2%. A first-order kinetic model for substrate (TOC) consumption allowed determination of the specific rate constants for substrate uptake, K(C), which also decreased with increasing loading, confirming the above-mentioned inhibition process. Finally, the evolution of the individual volatile fatty acid concentrations (acetic, C2; propionic, C3; butyric, C4; isobutyric, iC4; valeric, C5; isovaleric, iC5; and caproic, C6) with digestion time for all loads used was also studied. The main acids generated were acetic and propionic for all loads studied, facilitating the conversion into methane.