Treatment of ink effluents from flexographic printing by lime precipitation and boron-doped diamond (BDD) electrochemical oxidation.

Effluent treatment from flexographic printing was investigated by precipitation/coagulation and electrochemical oxidation over boron-doped diamond electrodes. Precipitation with 3 g/L of lime led to complete removal of effluent turbidity (initial value of about 410 NTU) but only about 20% chemical oxygen demand (COD) decrease (initial value of about 1,900 mg/L). Higher lime dosages (up to 15 g/L) had no effect on separation. On the other hand, coagulation with alum in the range 0.05-1 mM failed to enhance the extent of solids separation achieved by gravity settling alone (i.e. about 60%). Preliminary electrochemical oxidation experiments in the presence of sulphuric acid as supporting electrolyte showed that treatment performance (in terms of COD removal and decrease in sample absorbance at 300 nm) increased with increasing applied current. The latter was more efficiently utilized at shorter treatment times and lower currents with efficiency reaching 30%. Following lime precipitation, the effluent was electrochemically oxidized at alkaline conditions for 360 min yielding 64% absorbance reduction and 50% COD removal (this corresponds to 60% overall COD removal for the combined process). The rather low electrochemical treatment performance may be attributed to limestone deposition and fouling of electrodes and other electrochemical reactor components as evidenced by the gradual drop in conductivity/current throughout the operation.