TREATMENT OF SEAFOOD-PROCESSING WASTEWATERS IN MESOPHILIC AND THERMOPHILIC ANAEROBIC FILTERS

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, SO4(2-): 0.6-2.7 g L-1). The presence of high sodium ion concentration 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 SO4(2-), transformed into H2S during the anaerobic degradation process, may caus 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.