Hydrodynamic cavitation using orifice plate configurations and arrangements for tertiary treatment of palm oil mill effluent

Abstract

Hydrodynamic Cavitation (HC) is one of Advanced Oxidation Processes (AOPs), which generates and utilises hydroxyl radicals (HO·) as its oxidising agent. It has been studied for different applications to treat pharmaceuticals waste, seawater and microalgae, where much effort has been conducted to enhance its performance such as using pH, aeration and hydrogen peroxide (H2O2). However, the production of HO· using multiple-plate combination has not yet been studied. The use of pH, aeration and H2O2 has proven to give significant improvement for HO· formation, but these have not being studied previously using multiple-plate combination. The use of HC as a tertiary treatment for POME has not being reported before. Therefore, in this study, the enhancement of the HC has been investigated using double and triple orifice plate configurations and arrangements. The best system was then tested on biologically treated palm oil mill effluent (BT-POME). As the colour of POME is difficult to remove the performance of ponding treatment system was evaluated to understand the causes of colour in POME. The experiments were conducted in a labscale HC system, treating 10 L samples for reaction time ranging from 30 to 180 min. The effect of pH (2-7), aeration (2-10 L/min) and H2O2 dosing (50-200 mg/L) were explored. The performance of the HC system was based on iodine liberation, and removal of colour and chemical oxygen demand (COD). The byproducts of BTPOME degradation was identified. Additionally, the performance of an existing ponding system treating POME was assessed and the relationship between colour and few selected parameters were studied. Within the range of the experimental conditions used in this study, the HC orifice plate configurations and arrangements were found to have significant effects on HO·generation. The iodine liberation for both double and triple plate were higher than that of a single plate. The HO·generation was also affected by the arrangement and the distance between the plates; arrangement plate of P3P2 with 10 cm distance gave the highest iodine liberation (1296 mg/L). The performance of HC was enhanced under the effect of pH, H2O2 and aeration as compared to HC alone. For the conventional ponding treatment system, the anaerobic pond played the most significant role in treating POME with removal up to 97%. Among the pollutants analysed, colour has strong relationship with phenolics, tannin, lignin and carotene, indicating the roles of these compounds in causing colour of POME. The degradation of BT-POME by the HC system was not encouraging as only up to 14.7% of colour was removed, with lower removal of COD. The addition of H2O2 and aeration have significant effect in removing COD, while pH and addition of H2O2 have significant effect on colour removal. The degradation of BT-POME, particularly phenolics and tannin/lignin was found to form catechol and ρ-benzoquinone as by-products. The study showed another approach in improving HC system performance but further work is required before the system can be applied in treating BT-POME effectively.