Treatment of seafood‐processing wastewaters in mesophilic and thermophilic anaerobic filters

Abstract

Wastewaters from fish‐canning industries have a high concentration of organic polluting substances (10–50 g chemical oxygen demand L−1 [COD]) and, in some cases, a high content of sea salts (Cl−: 8–19 g L−1, Na+: 5–12 g L−1, SO2−4: 0.6–2.7 g L−1). The presence of high sodium ion concentrations in wastewaters with high organic content traditionally is considered as a very negative factor for their anaerobic treatment. In fact, both the presence of Na+ and SO2−4, transformed into H2S during the anaerobic degradation process, may cause toxicity and inhibition on the methanogenic process. This work deals with the operation and treatment efficiency of two lab‐scale mesophilic and thermophilic anaerobic filters (MAF and TAF, respectively). So that the adaptation of anaerobic sludge to high saline concentrations is attained, a prolonged start‐up period of about nine months was necessary. After this, a stable operation and similar treatment efficiencies were reached, even when organic loading rate (OLR) as high as 9 kg COD m−3 d−1 (TAF) or 24 kg COD m−3 d−1 (MAF) were applied at chloride concentration of 13 g L−1. At these conditions, the COD removal reached 73% (TAF) and 64% (MAF), and the COD methanized reached 69% (TAF) and 66% (MAF). The sulphate in the influent was removed practically completely, leading to a H2S concentration in the biogas between 3–4%. In spite of the lower specific activity of sludge from MAF (0.21 g COD g−1 volatile suspended solids [VSS] d−1) than from TAF (0.66), the MAF reached a higher OLR than TAF. This fact can be explained because of the higher retention of sludge into MAF (72 g VSS L−1) than TAF (10 g VSS L−1). Two practical conclusions may be derived from this work: the thermophilic operation needs the use of a packing material with a higher capacity to retain biomass and the mesophilic operation requires a more frequent detachment of biomass from the support in order to avoid clogging problems.