Factors affecting the microbial digestion of an industrial seaweed-based residue

Commercial preparation of a seaweed extract from the brown alga Ascophyllum nodosum for use as fertiliser and soil improver produces a sludge residue which requires remediation. This residue is rich in nutrients and offers the potential for other value-added products. The residue composition was analysed, a microbial digestion process for the residue was developed, and several factors affecting the digestion process were studied. The residue showed an alkaline pH (8.61?±?0.39) and 16% (w/w) total solids, which comprised 40.6% mineral, 29.5% fibre, 24.3% lipid, 4.9% protein and 0.5% polyphenols. The optimised digestion system included a 3-day anaerobic phase to decrease pH (from 8.96?±?0.40 to 7.72?±?0.38), the addition of an inoculum, followed by a 10-day aerobic phase where the insoluble material was digested. Every 3 days, the solubilised material was decanted and replaced with water to delay metabolite inhibition. The rate of digestion (decrease in insoluble material of 28.6?±?14.2% over 13 days) was influenced by the initial insoluble (R ²?=?0.773) and soluble (R ²?=?0.672) matter, the pH at the beginning of the aerobic phase (R ²?=?0.528) and by the accumulation of solubilised digestion products. A compositional analysis of the insoluble material after digestion showed that the lipid content of the residue was 96% digested and that the proportion of protein increased by 82.4%. Inocula and metabolite inhibition were critical features of A. nodosum residue digestion. Similar organic residues require a carefully chosen inoculum and a minimum initial insoluble content (65–70%) and/or a maximum soluble content (25.30%).