Whereas the adsorption of organic micropollutants (OMP) onto activated carbon (AC) is relatively well studied, little is known about potential OMP desorption effects, especially in real waters. In this study, the impact of different fractions of drinking water DOM on OMP desorption from AC was examined. By different pre-treatments of a raw drinking water, a high molecular weight (hmw) and a low molecular weight (lmw) DOM solution were prepared. These solutions were used as background matrix in AC adsorption/desorption batch tests, simulating a drop of the OMP inflow concentration to a fixed-bed adsorber. The tests were conducted in parallel with three AC of different pore structures (microporous, mesoporous/balanced, macroporous). The tests were evaluated with respect to the extent of OMP adsorption and its reversibility, which represents the potential extent of OMP desorption. In terms of OMP adsorption, the lmw-DOM fraction induced a higher competitive effect on OMP adsorption in comparison to the hmw-DOM fraction. In terms of their impact on OMP desorption extent, both fractions led to very similar results. In case of the macroporous AC, both DOM fractions induce an enhanced OMP desorption that can be attributed to displacement effects in both cases. For the microporous AC, an increased irreversibility of OMP adsorption was found in both cases, which shows that DOM adsorption prevents OMP desorption, independently of the size of the adsorbed DOM compounds. Whereas results from this study as well as from former studies indicate that this effect might be induced by permanent pore blockages by adsorbed DOM, further results show that there could be more complex DOM interactions that lead to the decreased desorption in case of microporous AC. Nonetheless, the very similar impact of the different DOM fractions on the reversibility of OMP adsorption indicates that the potential extent of desorption is similar in different waters (with different DOM composition) and primarily depending on the pore structure of the used AC.@en

Powdered activated carbon (PAC) for organic micro-pollutant (OMP) removal can be applied effectively on wastewater treatment plant (WWTP) effluents by using re-circulation schemes, accumulating the PAC in the system. This technique is complex because several factors are unknown: (i) the PAC concentration in the system, (ii) specific and average contact times of PAC particles, and (iii) PAC particle loadings with target compounds/competing water constituents. Thus, performance projections (e.g. in the lab) are very challenging. We sampled large-scale PAC plants with PAC sludge re-circulation on eight different WWTPs. The PAC plant-induced OMP removals were notably different, even when considering PAC concentrations in proportion to background organic sum parameters. The variability is likely caused by differing PAC products, varying water composition, differently effective plant/re-circulation operation, and variable biodegradation. Plant PAC samples and parts of the PAC plant influent samples were used in laboratory tests, applying multiples (0.5, 1, 2, 4) of the respective large-scale “fresh” PAC doses, and several fixed contact times (0.5, 1, 2, 4, 48 h). The aim was to empirically identify suitable combinations of lab PAC dose (as multiples of the plant PAC dose) and contact time, which represent the PAC plant performances in removing OMPs (for specific OMPs at single locations, and for averages of different OMPs at all locations). E.g., for five well adsorbing, little biodegradable OMPs, plant performances can be projected by using a lab PAC dose of twice the respective full-scale PAC dose and 4 h lab contact time (standard deviation of 13 %-points).@en

The adsorption of organic micropollutants (OMP) onto activated carbon (AC) in real waters is strongly affected by dissolved organic matter (DOM). This study examines the impact of DOM quantity and composition in terms of OMP desorption from different AC, by using four different water samples. In batch tests, an OMP concentration drop in the influent of an AC treatment system was simulated. These tests were conducted with six AC products with different internal pore structures. The tests were evaluated with respect to the extent of OMP desorption by interpreting corresponding OMP adsorption and desorption isotherms. For each tested AC and each evaluated OMP the isotherms in the different water samples were qualitatively very similar. Thus, despite different DOM composition very similar OMP desorption extents can be expected in different waters. Among the AC products a clear trend can be seen in all waters, namely that increasing pore size results in increasing desorption. The OMP desorption extent was quantified by a simple Freundlich equation-based approach, expressing the relative position of corresponding adsorption and desorption isotherms via the ratio KF, Des/KF, Ads. Plotting KF, Des/KF, Ads of any given substance for the different tested AC in one water over the average AC pore size shows a linear correlation. This confirms that the OMP desorption extent in real waters is strongly impacted by the AC pore structure. Furthermore, it indicates that the average AC pore size might be a good tool to assess the vulnerability of treatment systems towards desorption.@en

Adsorption onto ferric hydroxide is a known method to reach very low residual phosphate concentrations. Silicate is omnipresent in surface and industrial waters and reduces the adsorption capacity of ferric hydroxides. The present article focusses on the influences of silicate concentration and contact time on the adsorption of phosphate to a micro-sized iron hydroxide adsorbent (μGFH) and fits adsorption data to multi-component adsorption isotherms. In Berlin drinking water (DOC of approx. 4 mg L-1) at pH 7.0, loadings of 24 mg g-1 P (with 3 mg L-1 initial PO43--P) and 17 mg L-1 Si (with 9 mg L-1 initial Si) were reached. In deionized water, phosphate shows a high percentage of reversible bonds to μGFH while silicate adsorption is not reversible probably due to polymerization. Depending on the initial silicate concentration, phosphate loadings are reduced by 27, 33 and 47% (for equilibrium concentrations of 1.5 mg L-1) for 9, 14 and 22 mg L-1 Si respectively. Out of eight tested multi-component adsorption models, the Extended Freundlich Model Isotherm (EFMI) describes the simultaneous adsorption of phosphate and silicate best. Thus, providing the means to predict and control phosphate removal. Longer contact times of the adsorbent with silicate prior to addition of phosphate reduce phosphate adsorption significantly. Compared to 7 days of contact with silicate (c0 = 10 mg L-1) prior to phosphate (c0 = 3 mg L-1) addition, 28 and 56 days reduce the μGFH capacity for phosphate by 21 and 43%, respectively.@en