Theoretical and Experimental Analysis of the Role of Sludge Age on the Removal of Adsorbed Micropollutants in Activated Sludge Processes

Abstract

This paper analyses how sludge age, the most important design parameter for activated sludge processes, affects effluent concentrations of those micropollutants which are removable only by adsorption on activated sludge flocs. A mathematical model has been developed for sequencing batch reactors (SBRs) treating readily biodegradable substrates and micropollutants, in order to calculate the effect of sludge age on liquid- and solid-phase concentrations of micropollutants, both during the startup and at steady state. It was shown that the increase in sludge age, at fixed values of the other parameters, causes an increase of the concentration of micropollutants both in the liquid and in the solid phase. At any given sludge age, the effect of process parameters (e.g., biomass kinetic and stoichiometric parameters, influent concentration of readily biodegradable substrate) on the removal of the micropollutant was discussed. Experimental data were collected from two parallel laboratory-scale SBRs operated under identical conditions and feed composition, the only difference being sludge age (4 vs 26 days). Removal of cadmium and lead in the two reactors was compared. Effluent cadmium concentration was significantly higher at higher sludge age (25% of the influent concentration vs 12% at the lower sludge age). Lead removal, on the ther hand, was not significantly affected by sludge age and other removal mechanisms, as precipitation, likely occurred. Other literature data on removal of micropollutants in activated sludge processes were critically analyzed in light of the findings obtained in this study.

Biotransformation of a highly chlorinated PCB mixture in an activated sludge collected from a Membrane Biological Reactor (MBR) subjected to anaerobic digestion

“…As a consequence, several recalcitrant hydrophobic pollutants occurring in industrial wastewaters are nowadays fed to WWTPs dedicated to urban wastewater treatment [3][4][5][6]. Since such contaminants are mostly removed through sorption into biosolids, the resulting sewage sludge is very often contaminated [7][8][9]. Thus, a Draft addressed to regulate the use of sludge as fertilizer on the basis of its contamination has been developed by the European Commission [10].…”

A Review on Biological Processes for Pharmaceuticals Wastes Abatement—A Growing Threat to Modern Society

“…Thus, a probable restriction to co-metabolic biodegradation is the availability of other substrates, particularly when the ratio of toxic compounds to other biodegradable substrates in the influent is high and when the SRT is reduced. 162 • Competition with abiotic removal mechanisms: Biological degradation is possible if the substrate is dissolved in the liquid phase. In biological WWTP, the competing removal mechanisms of adsorption and air stripping decrease the concentration of the toxic compounds in the liquid phase, thus reducing the substrate availability for biodegradation.…”

Potential and Limits of Biodegradation Processes for the Removal of Organic Xenobiotics from Wastewaters

“…The tendency of a substance to be removed by air stripping depends on the Henry's constant of the substance and on the air flow rate. On the other hand, the tendency of a substance to be removed by adsorption on activated sludge flocs depends on the partition coefficient of the substance between the liquid and the solid phase and on the solids retention time (sludge age) 18, 20. The partition coefficient between an organic substance and activated sludge microorganisms is usually considered proportional to the octanol‐water partition coefficient of the chemical 18, 19.…”

“…The effect of sludge age is due to the fact that only the produced biomass is available for adsorption, and the rate of biomass production depends on the applied sludge age. Therefore the higher the sludge age, the lower the removal of a given substance by adsorption 20.…”

“…However, if the substance is not biodegradable a reduction in adsorption will cause an increase in the substance concentration in the effluent liquid phase, with potentially undesired toxic effects on the receiving water bodies. Analyzing literature data from 57, Dionisi et al 20 observed that lindane removal in biological wastewater treatment plants decreased as the SRT increased, and this explained with the fact that the main removal mechanism for lindane was adsorption and not biodegradation.…”