The methanogenic toxicity and anaerobic degradability of potato starch wastewater phenolic amino acids

Potatoes, which are important agricultural feedstocks for the starch industry, contain tyrosine and phenol oxidase. Since l-dopa can be formed from tyrosine by phenol oxidase, both tyrosine and l-dopa are presumably present in potato starch wastewaters. The purpose of this study was to evaluate the methanogenic toxicity and anaerobic degradability of these two phenolic amino acids. Tyrosine was found to be negligibly toxic to methane bacteria, while l-dopa caused from 40% to 50% inhibitions of the methanogenic activity at a concentration of 327 mg liter^-1. The toxicity of l-dopa occurred only if anaerobic sludge was exposed to l-dopa in the presence of Volatile Fatty Acids (VFA). The l-dopa toxicity could be minimized by maintaining low VFA concentrations in the media and by adapting the sludge to VFA prior to l-dopa exposure. Both tyrosine and l-dopa were anaerobically degraded to C̀H₄; however, only tyrosine was degradable after prolonged operation of continuously VFA-fed, granular sludge, packed columns. Phenol and p-cresol were identified as phenolic intermediates of anaerobic tyrosine degradation. Both p-cresol and m-cresol were identified as phenolic intermediates of anaerobic l-dopa degradation.