Sorption and release of organics by primary, anaerobic, and aerobic activated sludge mixed with raw municipal wastewater

Abstract

New activated sludge processes that utilize sorption as a major mechanism for organics removal are being developed to maximize energy recovery from wastewater organics, or as enhanced primary treatment technologies. To model and optimize sorption-based activated sludge processes, further knowledge about sorption of organics onto sludge is needed. This study compared primary-, anaerobic-, and aerobic activated sludge as sorbents, determined sorption capacity and kinetics, and investigated some characteristics of the organics being sorbed. Batch sorption assays were carried out without aeration at a mixing velocity of 200 rpm. Only aerobic activated sludge showed net sorption of organics. Sorption of dissolved organics occurred by a near-instantaneous sorption event followed by a slower process that obeyed 1st order kinetics. Sorption of particulates also followed 1st order kinetics but there was no instantaneous sorption event; instead there was a release of particles upon mixing. The 5-min sorption capacity of activated sludge was 6.5±10.8 mg total organic carbon (TOC) per g volatile suspend solids (VSS) for particulate organics and 5.0±4.7 mgTOC/gVSS for dissolved organics. The observed instantaneous sorption appeared to be mainly due to organics larger than 20 kDa in size being sorbed, although molecules with a size of about 200 Da with strong UV absorbance at 215-230 nm were also rapidly removed.

Fig 1. Absorbance at 650 nm and 254 nm and concentrations of TOCp and TOCd in the mixtures of wastewater with effluent (Control), anaerobic digester sludge (ADS) and primary sludge (PS) of different concentrations.

The original TOC concentrations and absorbance values before sedimentation are also shown. Averages of duplicate measurements are shown with the error bars representing the individual measurements. An asterisk (*) above a column indicate that the removal efficiency was significantly different (p<0.05, n = 2) from the control.

Fig 2. Absorbance at 650 nm and 254 nm and concentrations of TOCp and TOCd in the mixtures of wastewater with effluent or activated sludge suspension.

Original refers to the concentrations before sedimentation. Control, low addition, and high addition refers to the concentration in the supernatant after 30 min sedimentation with zero (control), 0.30–0.39 gTSS/L (low addition), or 1.04–1.37 gTSS/L (high addition) activated sludge. Averages of duplicate measurements are shown with the error bars representing the individual measurements. ABS650 was not measured on sampling day 6.

Fig 3. Absorbance at 650 nm and 254 nm and concentrations of TOCp and TOCd in the mixtures of wastewater with effluent (Control), 0.30–0.33 gTSS/L of activated sludge (AS low), 1.04–1.16 gTSS/L of activated sludge (AS high), or activated that had been starved for either 1, 3 or 6 days.

The low concentration of the starved sludge was 0.28–0.32 gTSS/L and the high concentration was 0.97–1.13 gTSS/L. The original TOC concentration and absorbance values before sedimentation are also shown. Averages of duplicate measurements are shown with the error bars representing the individual measurements. Arrow pairs indicate that the removal efficiencies for two treatments were compared. An asterisk (*) above the arrow pair indicate statistically significant difference in removal (p<0.05, n = 2).

Fig 4. Removal of TSS and TOCd in kinetic tests carried out on two different sampling occasions.

The legends show addition of activated sludge in gVSS per litre. Control samples (Ctrl) did not contain added activated sludge. Symbols show measured data whereas lines were fitted using Equation 2. Each measurement point is the average of two replicate tests with the error bars representing the individual measurements.

Fig 5. Concentration of TOCd in sorption tests with azide-inhibited (A) and live activated sludge (B).

The solid lines show fits using 1^st-order kinetics for the measurements points between 1 min and 60 min. Two repeated tests were carried out with each type of sludge.

Fig 6. HPSEC of samples from sorption tests with aerobic activated sludge and 1 min mixing.

Black lines show controls without addition of sludge. Red lines show samples with addition of 0.97 g/L VSS. The vertical lines show the retention time of polyethylene glycol standards of known molecular weight.