Enhanced synergistic degradation efficiency using hybrid hydrodynamic cavitation for treatment of tannery waste effluent

Abstract

The treatment of recalcitrant bio refractory pollutants especially common in wastewater emanating from the tannery industries is a major challenge to the industries due to the absence of an efficient treatment technology, high operational costs as well as in view of the stringent rules imposed by the various environmental and governmental regulatory bodies. This study reports the degradation of organic pollutants present in tannery waste effluent (TWE) using hybrid hydrodynamic cavitation (HC) such as HC + ozone (O3), HC + hydrogen peroxide (H2O2) and HC + Fenton's reagent. HC treatment alone caused a reduction of 14.46% chemical oxygen demand (COD), 12.60% total organic carbon (TOC), 10.01% total dissolved solids (TDS) and 34.82% total suspended solids (TSS) of the TWE samples at the optimum inlet pressure of 500 kPa within 120 min. It also caused an increase in biodegradability index (BI) value from 0.33 to 0.43 indicating increased biodegradability, whereas dilution of the TWE samples did not enhance the HC process efficiency. HC combined with O3 was effective as COD and TOC reduction increased to 26.81% and 17.96% respectively at the optimum loading of 7 g/h of O3. HC combined with H2O2 also significantly enhanced the degradation efficiency to a maximum of 34.35% COD and 19.71% TOC reduction due to the enhanced generation of hydroxyl radicals. However, HC combined with Fenton's process was found to be the most efficient hybrid process for the treatment of TWE at a maximum reduction of 50.20% COD and 32.41% TOC respectively at FeSO4.7H2O/H2O2 ratio of 1:3 (w/w). The requirement of H2O2 per g of COD reduction for HC + Fenton reduced to 1.95 g/g compared to the requirement of 3.02 g/g for HC + H2O2 process. Moreover, HC + Fenton's treatment increased the BI value from 0.28 to 0.46, which is an increase by 64% compared to an increase by 30% using HC alone. These hybrid techniques enhanced the BI of treated TWE and therefore may be used as a pre-treatment tool by integrating into existing biological treatment units in the conventional effluent treatment plants, in order to cause enhanced degradation of highly bio-recalcitrant organic pollutants. Other enhanced benefits include higher rate of mineralization of organic matter with lower cost of treatment. Hybrid HC + Fenton was the most energy efficient approach than HC process with a 75% reduction in the energy requirement and 56% reduction in treatment cost per mg of COD reduction of TWE.