Abstract

An anaerobic digester converts organic materials into biogas and digestate in the absence of oxygen. The organic materials studied in this experiment include fibres (types of paper or cardboard), food waste, and woodchips, which serve as a bulking agent. To analyze digester performance, it is necessary to calculate an accurate mass balance based on the chemical oxygen demand (COD) entering and exiting the system. Digester performance refers to maximum efficiency and biogas yield. The COD of the biogas is known, but that of the feed and the digestate is not. This paper describes a method for measuring the COD of the feed materials and the digestate by creating representative aqueous suspensions of each. The challenges are to ensure that the suspensions are representative of the feed or digestate, and that samples of the suspension extracted for COD analysis are consistent and reproducible. To obtain an accurate COD measurement of the feed and digestate samples, a specific procedure was developed: each material was processed in a blender with deionized water, creating a pulp from which samples were pipetted during continuous mixing of the suspension. The conducted trials provided COD content values ranging from 1.27-1.59 g of COD/ g of dry feed, depending on the fibre. Standard deviations of the COD content ranged from 2.8% to 12.7%, indicating that the procedure is reliable and the results precise. The measured COD content values allow an accurate mass balance of the digester to be determined, ultimately providing a better understanding of the system as the total digestible material entering the digester will be known. An accurate mass balance can improve the efficiency of the digester in order to produce optimal quantities of biogas. The biogas can be harnessed into energy from otherwise useless waste. Further study in this topic can explore the COD content of wider ranges of organic solids as well as further optimize the procedure in order to provide even more accurate results.

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