Methane production from anaerobic co-digestion of orange peel waste and organic fraction of municipal solid waste in batch and semi-continuous reactors

A large amount of orange peel waste (OPW) is yearly generated due to the high production of oranges all over the world. A high percentage of this waste is dumped every year without any proper treatment, thus causing environmental issues as a consequence of dispersion of biological degradation by-products (i.e. biogas and leachate) into the environment. An effective, suitable and environmentally friendly treatment for OPW is therefore required. Biogas production under controlled conditions by an anaerobic digestion (AD) process is highly recommendable and it has been demonstrated to be potentially high efficient when OPW is used as substrate. However, the high content of essential oils, mainly composed of limonene, a well-known antioxidant, can cause the inhibition of the biological activity. A low limonene concentration inside the anaerobic digester can be effective to avoid this inhibition. Therefore, anaerobic co-digestion of OPW with organic fraction of municipal solid waste (OFMSW) was carried out in order to reduce limonene concentration, by dilution, in the organic mixture. Three different mass ratios of the two substrates (i.e. 1:3, 2:2, 3:1) were considered (each of them with different limonene concentrations) and used in two separate experiments: batch and semi continuous. The content of limonene inside the digester is directly related to the organic loading rate (OLR). The maximum OLR that resulted in stable anaerobic co-digestion process performance was 2 gVS(L−1·day−1) corresponding to 26.7 mgLimonene(Ldigester−1·day−1) of limonene dosage. The highest methane production obtained from such an OLR was around 0.70 ± 0.05 LCH4(Ldigester−1·day−1).