SODIUM INHIBITION IN THE ANAEROBIC-DIGESTION PROCESS - ANTAGONISM ANDADAPTATION PHENOMENA

The effect of sodium on the methanization of volatile fatty acid (VFA) mixtures was evaluated for three different sludges. Sodium concentrat ions causing 50% inhibition ranged from 3 to 16 g l(-1) in the absence of nutrients or other salts, showing a higher tolerance to sodium in the sludges obtained from the digesters treating high saline wastewate rs. This fact is considered to be a consequence of sludge adaptation t o sodium. Furthermore, the adaptation of methanogenic bacteria to sodi um was also found during the batch assays: After 40 days of digestion, two different sludges (subjected to 6.9 and 21.5 g Na+ l(-1), respect ively) increased the relative methanogenic activity from 0% to about 4 5% of the blank activity. The antagonism phenomena, due to the presenc e of cations and anions contained in the assayed media, greatly influe nced the sodium effect on anaerobic sludges. Sodium concentrations cau sing 50% inhibition may be increased by 4 to 10 g l(-1) or more when s ea water is used instead of NaCl. Nutrients also influence the sodium toxicity. The effect of sodium in each step of the anaerobic digestion process appears to be different depending on the sludge. For an adapt ed sludge, sodium caused 50% inhibition of propionic, acetic, and n-bu tyric utilizers at concentrations of 10.5, 17, and 19 g l(-1), respect ively, an indication that propionic utilizers had less adaptation pote ntial. These concentrations ranged between 5 and 6 g l(-1) for an unad apted sludge. The methanization of a more complex substance, such as g lucose, was less extensively affected than the methanization of VFA. R esults from continuous assays agree with those obtained from batch ass ays.