Inferences made from mineral elements characterisation of anaerobic digestion substrates: preliminary results from avocado oil processing byproducts

The mineral element levels in avocado oil processing byproducts were evaluated to make inferences about these byproducts' biodegradability behaviour. The mineral assays were determined using ICP-OES methods. Literature was also reviewed to understand how different mineral elements detected in substrates could impact biogas production under anaerobic digestion (AD) conditions. It was noted that there is no consensus among scholars on absolute mineral requirement levels for optimal digester performance since digesters use different substrates operating under varying conditions in most cases. When mineral elements are below certain values, the digester operates sub-optimally. When certain mineral levels are exceeded in digesters, the minerals become toxic to the AD microorganisms and destabilise the biogas digester. Only Zn levels across all avocado oil processing byproducts and K levels in cold-pressed decanter wastewater (CDW) exceeded the toxic level limits pegged at 1 mg/L and 3000 mg/L respectively. These may therefore need corrective action to avoid potential inhibition of the AD process. Mineral assays that seemed adequate and required no supplementation or detoxification were K, P, Fe, Cu, Se and Cr across the studied byproducts. All the other elements (excluding the Hg, As and Cd which are toxic) may require supplementation for optimal digester performances when using avocado oil processing byproducts as biogas digester substrates. However, these inferences may be affected by issues of mineral bioavailability which is a substrate-dependent phenomenon. Mercury cadmium and arsenic are naturally toxic to microorganisms and must be removed or detoxified if detected in substrates. The current study indicates the significance of mineral elements' characterisation in explaining and potentially guiding the optimisation of substrate biodegradation to biogas.