Fouling of ceramic capillary filters in vacuum filtration of humic acid

In this study the fouling phenomenon of ceramic capillary filters in the fi ltration of humic acid was investigated by experimental work and by modelli ng. The fouling of humic acid on ceramic capillary filters was evident, and in most of the cases, irreversible. In some filtrations the fouling did no t seem to be permanent, it looked more like a gel layer on the filter. The effect of pH was clear: the filtrate fluxes decreased to almost zero at pn rt and 8.5, but at pH 11 the flux reduction was only 50%. The structure of the humic acid molecule differs in alkaline and acidic conditions. Also, at pH 4 the charge of the filter medium is positive and the humic acid molecu les are negative. The presence of ferric ions decreased the filtrate flux o f humic acid faster than in the absence of them. The behaviour of humic aci d was compared to other chelating agents. At ph 4 the difference between ot her chelating agents and humic acid was remarkable: tbe fluxes when filteri ng the others remained good during the filtration, but for humic acid the f lux decreased very fast to close to zero. Elementary analysis showed that the inorganic elements did not adsorb on th e filter surface. The fouling of humic acid fitted best to the standard blo cking model, which means that the molecules fouled tbe filters by direct ad sorption on the surface and the walls of the pores.