Micro-pollutant removal from wastewater treatment plant effluent by activated carbon

In the recent years, the removal of micro-pollutants from treated wastewater has been highly advocated throughout Europe and the rest of the world. The relevant regulations and the suitable techniques have been proposed accordingly, which promoted the innovation of the conventional wastewater treatment plants (WWTPs). Activated carbon adsorption and advanced oxidation are regarded as the most promising technologies to attenuate micro-pollutant concentration in the treated wastewater (i.e. the secondary effluent). In this thesis, activated carbon adsorption of micro-pollutants was systematically studied with respects to the adsorption competition mechanisms and the practical applications for advancing wastewater tertiary treatment. Different forms and types of activated carbons were adopted in the particular operational system, the associated adsorption competitions between target micro-pollutants and the background organic matter (BOM) were illustrated with the aid of various BOM separation methods. Moreover, for a better exploitation of the activated carbon adsorption capacity (i.e. when integrating into the existing WWTPs), the appropriate dosing location and the dosing approach have been identified, respectively. In literature, there were two well-established pathways for BOM interfering with the target micro-pollutant adsorption on activated carbon: site competition and pore blocking. To differentiate these two pathways and examine which BOM fraction contributes most to the competition; batch adsorption tests and rapid small-scale column tests (RSSCTs) were performed, and an strong-base anionic exchange resin (AER) was used to separate the BOM. Results revealed that the AER was not effective to remove the site competition organics which interfered mostly with the micro-pollutants in the batch isotherm tests. These site competition organics appeared to be hydrophobic, and have a low molecular weight character. Moreover, the powered activated carbon (PAC) with the highest amount of primary and secondary micropores was subjected to the least site competition, and was thus selected for the subsequent RSSCTs, where, site competition and pore blocking occurred concurrently. In contrast to the batch adsorption tests, the retarded micro-pollutant breakthrough after AER pre-treatment indicated the relevance of AER removed organics for interfering with micro-pollutant adsorption in the dynamic filtration. This can be attributed to a less pore blocking effect due to the reduced amounts of ‘humic substances’ and ‘building blocks’ in the BOM. During the operation of granular activated carbon (GAC) filter, the preloading of BOM is a normal phenomenon before GAC contacts with the target micro-pollutants, this leads to an occupation of the adsorption sites and/or makes adsorption sites in small pores practically unavailable to the target micro-pollutants. To understand the preloading effects from wastewater effluent organic matter (EfOM) on the subsequent micro-pollutant (e.g. pharmaceutically active compounds, PhACs) adsorption. EfOM were differentiated by a nanofiltration (NF) membrane into fractions with size similar to or higher than that of the investigated PhACs. These two size fractions were pre-adsorbed onto two GACs with different pore structures. Comparison of the PhAC adsorption isotherms between the fresh GACs and the preloaded GACs reflected a significant reduction in the PhAC adsorption capacity after EfOM preloading, while the EfOM fraction which was rejected by the NF had a negligible impact. This observation emphasized the importance of the NF-permeating, low molecular organics for a direct site competition with the PhACs. Regarding the GAC pore structure, the one with a broad pore size distribution including both micropores and mesopores was able to adsorb an higher amount of EfOM, thus causing a higher PhAC adsorption reduction on the preloaded GAC. Furthermore, the PhAC adsorption reduction were correlated to their molecular physicochemical properties. PhACs with positive charge were found to have a less adsorption reduction than the neutral and negatively charged PhACs, due to the electrostatic attraction with the preloaded GAC surface. Additionally, hydrophobic PhACs within each charge group were generally more resistant to the preloading effects. Concerning the change of the GAC characteristics by a continuous preloading of the influent organic matter, spent GACs treating surface water and groundwater over a full operational cycle were collected, respectively. Comparing to the groundwater-spent GAC, which had a loss of only the secondary micropores, the surface water-spent GAC exhibited pore volume reduction covering a wide size range, due to the higher organic carbon concentration and also a broad molecular weight distribution of the organic matter in surface water. Specially, the higher amount of low molecular weight organics in surface water resulted in a diminishing of the primary micropores. Micro-pollutant (atrazine) adsorption tests were conducted to evaluate the reuse potential of the two spent GACs (after pulverizing). It was found that in addition to the reduced adsorption sites, the heavily loaded organic matter on surface water-spent carbon decreased atrazine adsorption capacity and hindered the adsorption kinetics (in a demineralized water), likely due to the induced water adsorption and water cluster formation on the spent carbon surface. However, a less adsorption competition was observed for atrazine adsorption in an organic matrix (i.e. the wastewater secondary effluent) on the surface water-spent carbon, because of the repulsion of the like-charged organic matter. This implies the suitability of reusing the surface water-spent carbon into the treated wastewater for micro-pollutant removal. When applying activated carbon e.g. PAC into the wastewater treatment processes for micro-pollutant adsorption, a better dosing location needed to be selected. The primary effluent and secondary effluent of a WWTP were considered in this respect. Primary effluent generally has a higher micro-pollutant concentration than the secondary effluent, and was thus supposed to facilitate the micro-pollutant adsorption onto PAC, while secondary effluent has less BOM concentration and was expected to cause less adsorption competition. Results showed a negligible PhAC uptake in the primary effluent in contrast to a significant PhAC uptake in the secondary effluent by fresh PAC, and this was mainly due to the site competition induced by the low molecular and hydrophobic organics, which were largely present in the primary effluent as compared to the secondary effluent. Moreover, these competing organics were able to replace the pre-adsorbed micro-pollutants (e.g. the negatively charged PhACs) from the PAC which was used for secondary effluent treatment. As such, recycling the PAC used for tertiary treatment into the e.g. activated sludge tank might not help to improve the overall micro-pollutant adsorption. The (fresh) PAC was thus utilized for the secondary effluent treatment and was integrated into a dual media tertiary filter (instead of recirculating into the activated sludge tank). As a simple and economic approach, PAC was directly added and immobilized inside the tertiary filter bed. In comparison to the batch adsorption tests where PAC was suspended, the immobilized PAC inside the filter bed can offer a better micro-pollutant removal, due to a constant micro-pollutant concentration gradient under the dynamic filtration condition. Analogously, a constantly improved micro-pollutant removal was observed as the immobilized PAC amount increased in the filter during PAC continuous dosing. In addition to the approach how PAC was added, the profile how PAC was distributed inside the filter bed also played a role in the micro-pollutant breakthrough. By manipulating the filter effluent valve, different ‘pre-embedding’ velocities were achieved, which can propel the PAC to transport downwards to the filter end. A higher pre-embedding velocity served to create a more homogeneous PAC distribution inside the filter bed, consequently, prolonging the micro-pollutant (e.g. sulfamethoxazole) breakthrough.