Treatment of softdrink industry wastewater using an integrated anaerobic/aerobic membrane bioreactor

Abstract

Most softdrink industries in developing countries are moving towards wastewater reuse or recycling. Water and wastewater reutilization, costs of treatment and disposal guidelines, remain the most critical factors for the development of sustainable water use for softdrink industries. Wastewater reuse or recycle has potential in the softdrink industry, depending on the wastewater characteristics’ concentration and volume. During this study, an integrated laboratory scale anaerobic/aerobic sidestream membrane bioreactor (MBR) system was used for treating softdrink industry wastewater (SDIW). The aim was to evaluate the system’s performance, and identify potential opportunities to recycle the water, and therefore reduce freshwater intake and minimise wastewater production. The objectives were to: evaluate: 1) treatment efficiencies for the individual stages; 2) biogas production in the anaerobic stage; and 3) the overall performance of the integrated system under different operating conditions. The SDIW used in this study was classified as medium to high strength wastewater with a total chemical oxygen demand (CODt) ranging between 2 242 and 11 717 mg/L and a biological oxygen demand (BOD) of up to 1 150 mg/L. The major pollutants in the SDIW were caustic soda; dissolved sugars, namely fructose (1 071 mg/L) and sucrose (6 900 mg/L); with the pH ranging between 6.1 and 12. The SDIW was characterized by total suspended solids (TSS) of 66 mg/L, as well as fats, oils and greases (FOG) of 40 mg/L. The maximum turbidity and colour was 65.3 NTU and 42 mg Pt/L, respectively. All the physiochemical properties and inorganic parameters were within the within the City of Cape Town’s (CCT’s) industrial wastewater quality discharge standards by-law (South Africa, 2006). Excluding the total dissolved solids (TDS) and electrical conductivity (EC) with maximum values were 1 050 mg/L and 1 483 μS/cm, respectively. Anaerobic pre-treatment of this SDIW was studied using a laboratory-scale expanded granular sludge bed (EGSB) reactor maintained at mesophilic temperature of between 35 to 37˚C. An organic loading rate (OLR), upflow velocity (Vup) and hydraulic retention time (HRT) of 10.9 kg COD/m3d, 0.85 m/h and ~11.8 h, respectively, resulting in COD treatment efficiencies of up to 93% CODt. An increase in nitrate (NO3-) in the EGSB product stream was an indication of an anaerobic ammonium (NH4+) oxidation (ANAMMOX) process. Anaerobic digestion (AD) of SDIW in the EGSB resulted in biogas production with methane (CH4), carbon dioxide (CO2), nitrogen (N2), and oxygen (O2), concentrations of up to 70%, 11%, 14.8%, and 4.1%, respectively. At the OLR and Vup of 10.9 kg COD/m3d and 0.85 m/h, respectively, the EGSB produced 16.7 L/d of biogas. The EGSB anaerobic pre-treatment

resulted in stable treatment efficiencies for the removal of organic constituents, as well as biogas production without adding an external carbon source. The MBR post-treatment satisfactorily operated at a feed flowrate of up to 33.7 L/d, OLR of 2.3 and 3.1 kg COD/m3d for the anoxic and aerobic zones, respectively, and an HRT of approximately 0.41 h for both zones. The average CODt removal achieved was 86%. The dissolved oxygen (DO) concentration of 2.1 mg/L in the anoxic zone combined with an aeration rate and DO concentration of 11.8 L/min and 5.7 mg/L, in the aerobic zone resulted in NH4+; NO3-; and orthophosphate (PO43-), removal rates up to 90%; 55% and 39%, respectively. However, the MBR post-treatment did not decrease the orthophosphate concentration to within the SANS 241:2011 drinking water standards. The integrated EGSB-MBR treatment for SDIW was able to achieve an overall CODt removal efficiency of up to 94%. Although the MBR performance was successful the EC, TDS, PO43-, and colour concentrations in the ultrafiltration (UF) permeate did not meet the CCT’s industrial wastewater standards by-law (2006) as well as the SANS’ drinking water standards 241:2011 and required further treatment for reuse.