Figure 1 : Experimental set-up for the ultrasonication-facilitated electro-oxidation treatment.

With the growth in the volume of ink-jet printer production, there is a continuous demand for a technically and economically optimal solution for the treatment of ink-jet ink effluent. Currently, ink effluents are treated by the chemical oxidation process (Fenton’s reaction for example), and a major problem associated with this is the excessive use of chemicals with accompanying waste sludge. In this research, the feasibility of using a two-step process - ultrasonication along with Fenton’s reaction - was investigated. While the application of sonolysis (which uses ultrasound to cleave chemical bonds) to the degradation of specific organic compounds has been widely studied, it has not been exploited for treating ink effluents.

A study of synthetic waste waters containing various commercially available inks was conducted. Analysis of this resulted in the identification of seven high-risk non-compliance parameters: COD (Chemical Oxygen Demand), BOD₅ (Biological Oxygen Demand at 5 days) TDS (Total Dissolved Solids), phenols, copper, iron and sulphate concentrations. Of these, COD reduction was found to be the most stringent treatment criterion based on the industry-accepted standard Fenton’s oxidation treatment. TDS and COD were also proposed as critical parameters for the initial assessment of the quality of untreated effluents. To establish rapid and robust indications of the TDS and COD of untreated ink effluents, a correlation for TDS as a function of conductivity and turbidity was obtained. Furthermore, a deterministic approach based on Beer’s law of absorbance additivity was developed for determining COD of ink effluent mixtures based on absorbances at 210, 436, 525 and 620 nm, which were validated against experimental data. Based on a thorough analysis of the compositions of ink effluents, a list of simple and rapid on-site water quality parameters was proposed for monitoring the quality of the treated wastewater: UV absorbance at 210 nm, conductivity, pH, turbidity and colour.

The feasibility of using ultrasonication along with Fenton’s reaction was investigated for treating cyan ink effluents. A two-step treatment process was developed - the first step was an ultrasound-assisted electro-oxidation, while the second was chemical oxidation with hydrogen peroxide. The use of electro-oxidation in the first step significantly reduced the amount of iron needed compared with the conventional Fenton's reaction, resulting in a 98% reduction in the amount of sludge. It was found that ultrasonication in the presence of iron (from electrolysis) converted the ink components into reaction intermediates, which were more amenable to peroxide oxidation in the subsequent step. These intermediates were quantified by UV absorption at wavelengths within the range of 275 to 400 nm.

During the same reaction time, the ratio of the treated effluent COD_e to the initial untreated ink COD₀ was 0.15 (from 10150 to 1490 mg/L) for the 2-step process compared with 0.23 (from 13247 to 3020 mg/L) using Fenton's reaction alone. The same COD removal could also be achieved using the two-step process in about half the time of the Fenton's reaction alone. In addition, a kinetic study was also performed to further understand the reaction mechanisms with regard to the reaction order and the effect of temperature in the 2-step treatment process.

A potential treatment process incorporating sonolysis was developed based on both unit operations and a process phenomenon approach. In order to develop such a process, various correlations were determined for the different vital process parameters needed as the model input for computer simulation. In particular, correlations for the power density for the reaction vessel within the conventional ultrasound baths and the kinetic data for various treatment steps were obtained.