Tomato-processing wastewaters generated from food-canning industry are typically considered difficult to biodegrade since they contain seed, skin and high particulate and colloidal fractions. A bench-scale and a pilot-scale anaerobic/aerobic system were evaluated and compared for the treatment of high-strength tomato-processing wastewater. The pilot-scale and bench-scale systems had comparable performance based on the removal efficiencies of SBOD, SCOD and TSS of 96-97%, 99% and 91-98%, respectively. Increase of anaerobic hydraulic retention time (HRT) from 0.25 day to 0.5 day resulted in significantly better settling characteristics of the mixed liquor confirmed by the decrease of diluted sludge volume index (DSVI) from 124-189 mL/g to 24-131 mL/g. Increase of temperature from 25°C to 32°C affected biomass activity and sludge settling characteristics, The biomass activity, represented by specific oxygen uptake rate (SOUR), increased from 0.15-0.23 mg O2/mg VSS-day at 25°C to 0.67-1.24 mg O2/mg VSS-day at 32°C. DSVI of mixed liquor increased from 24-131 mL/g at 25°C to 115-173 mL/g at 32°C.
Anaerobic technologies are widely used for the treatment of food-processing wastewaters. Anaerobic digestion of slaughterhouse wastewater containing pork fat particles revealed that approximately 35% of substrate neutral fat was hydrolyzed during pretreatment (Masse, et al. 2003). However, anaerobic technologies are not capable of treating wastewater to meet final discharge requirements (Moody and Raman, 2001) and anaerobic/aerobic treatment is employed to meet discharge requirements.
Hydraulic retention time (HRT) is an important variable in any biological treatment system particularly anaerobic treatment processes. Increase of HRT resulted in a decrease of propionate: acetate ratio in the acidogenic and digestion stages in a continuous anaerobic digestion system (Wang et al. 1997).
Temperature has mixed effects on the net volatile fatty acids (VFA) production. The net production went up by 15%when the temperature was increased from 22°C to 30°C while decreased by 23% at 35°C, indicating that 30°C might be the optimum for biomass growth (Banerjee et al., 1998).
The objective of this paper is to study the treatability of slowly biodegradable and nutrient-deficient high strength tomato-processing wastewater in pilot-scale and bench-scale anaerobic/aerobic systems. Due to the limited duration of the 52-day canning season for pilot study, the bench-scale system was built up to confirm the results of pilot-scale system as well as study the impact of HRT and temperature.
Figures 1a and 1b show the detailed setup of the bench-scale and pilot-scale anaerobic/aerobic biological wastewater treatment systems. The bench-scale system was composed of a 75 L influent storage tank with a heavy-duty high-torque mixer (Arrow 2000, Arrow Engineering Co. Inc., Hillside, NJ), a 5 L polyvinyl chloride (PVC) anaerobic tank with a mixer mounted on the top, a 10 L PVC aeration tank with a mixer on the top and copper coil air diffuser immersed in the mixed liquor, and an 8 L PVC clarifier. Two (2) immersion heaters with dial temperature controller (Cole-Parmer short rod 5.5” L, 500 W and short rod 5” L, 1100W, Labcor Technical Sales Inc., Montreal, QC) were inserted into the anaerobic and aerobic tank to increase the liquid temperature to 32 ± 2°C. Internal recirculation ratio R1 (from aeration to anaerobic) of 3Q and R2 (return activated sludge (RAS), from clarifier to anaerobic) of 2Q were employed while influent flow rate (Q) was set to 10 L/d in Periods II to IV. Three (3) adjustable peristaltic pumps (MasterFlex L/S, Labcor Technical Sales Inc., Montreal, QC) with L/S 14 and L/S 16 tubings were used to control the influent flow rate and the internal recirculation R1 and R2. The dissolve oxygen (DO) in the aerobic tank was controlled at 2-3 mg/L and pH of wastewater samples in the storage tank was adjusted to ~7 by adding sodium carbonate powder before the treatment. Tomato-processing wastewaters were collected from the field during the canning season and stored in a cold room maintained at 4°C. The similar pilot-scale system was composed of a 2 m3 anaerobic tank with a mixer, a 4 m3 aeration tank with air diffuser, an 1.2 m3 secondary clarifier, and a membrane ultrafiltration system. Detailed system setup and operating conditions of the pilot-scale system were discussed elsewhere (Xu and Nakhla, 2005). The operating conditions of pilot-scale and bench-scale systems are shown in Table 1.