Development of a full-scale wastewater treatment plant for licorice root processing factories: A hybrid biodegradation-lime-alum-ozonation process

Liquorice is a perennial legume that grows principally in Asia, the Middle East and some parts of Europe. The sweet root extract is mainly used in the pharmaceutical, food and confectionary industries. It contains 400 compounds, including triterpene saponins and flavonoids, which are responsible for liquorice's bioactivities. The wastewater (WW) arising from the processing of liquorice can have negative environmental effects and must be treated before being discharged into the environment. Different WW treatment solutions are available. In the last years, increasing attention has been paid to the environmental sustainability of wastewater treatment plants (WWTPs). The present article discusses a hybrid biological (anaerobic-aerobic) and post-biological (lime-alum-ozone) WWTP, designed to treat 105 m3/day complex liquorice root extract WW for agricultural purposes. The influent chemical oxygen demand (COD) and biological oxygen demand (BOD5) were found to be 6000-8000 mg/L and 2420-3246 mg/L, respectively. With a biological hydraulic retention time of 8.2 days and no addition of extra nutrients, the WWTP reached a stable condition after 5 months. Over the course of 16 months, its highly efficient biological treatment reduced COD, BOD5, total suspended solids (TSS), phosphate, ammonium, nitrite, nitrate and turbidity by 86-98 %. However, the WW's colour proved resilient: only 68 % of the colour was removed by the biological treatment, necessitating a combination of biodegradation-lime-alum-ozonation processes in order to reach 98 % efficiency. Thus, this study reveals that liquorice root extract WW can successfully be treated and reused for crop irrigation.