Arsenic in groundwater can constitute a persistent nuisance for water treatment facilities when it exceeds the admissible limit of 10 μg/L. Recently, a stricter limit has been set as a new challenging target by many companies in the Netherlands, which is below 1 μg/L. However, most of the groundwater treatment plants have been conventionally designed solely for the removal of the most common undesirable groundwater constituents, namely iron, manganese and ammonium. The current research aimed at the investigation of the operational conditions facilitating As removal in biological rapid filters simultaneously with the required Fe removal. This improved As retention should be correlated with an extended length where its adsorption takes place, thus with the deeper Fe penetration inside the bed. Therefore, this was attempted in this research. The process water used throughout the experimental tests contained As(III) and Fe(II) in order to simulate a typical anoxic groundwater quality. For the most part, a triple-layer filter bed was used, consisted of anthracite, sand and garnet. The different settings under examination involved a range of pH values (7.8, 7.1 and 6.4), two filtration velocities (2.5 m/h and 5 m/h) as well as the recirculation of the filtrate back to the feed stream. Finally, the multimedia bed was compared with a single-layer sand filter.
The results of the conducted pilot column filter experiments revealed that high pH values were accompanied with high oxidation rates and thus with the creation of Fe flocks, already in the supernatant water. Due to this, at pH 7.8 and 7.1 lower Fe concentrations were detected in the effluent, denoting a higher Fe retention as compared to pH 6.4. Interestingly enough, a relatively deep Fe penetration was observed for every pH value tested. Regarding As removal, it was evidently favored by high pH values owing to the oxidation-floc formation removal mechanism of Fe (homogeneous reaction), which prevailed under those conditions. On the other hand, at pH 6.4 the adsorption-oxidation mechanism was predominant (heterogeneous reaction), which obstructed the AsOB activity. As far as the tested filtration velocities is concerned, they didn’t seem to significantly impact both Fe spread over the bed as well as As removal at the higher pH levels. Nonetheless, this was not the case for pH 6.4, in which the slow flow rate enabled the generation of more and larger Fe flakes (due to sufficient residence time in the supernatant water), which were then retained in the top part of the bed. The high flow rate on the other hand allowed Fe to reach deeper in the filter. Surprisingly, As removal seemed to be improved at 2.5 m/h, despite the Fe flocks accumulation in the upper layers. Possibly, the short experimental times not allowing equilibrium to be reached could comprise a reasonable explanation of this unexpected result. Furthermore, the filtrate recirculation stream didn’t seem to positively influence As removal. The induced dilution effect resulted in a relatively large dispersion of Fe inside the filter bed, however the essentially halved incoming Fe concentration was not sufficient to adsorb the oxidized As(V). Finally, the comparison between the multimedia bed with the single-layer filter reveals a considerably wider Fe dispersion over the bed height in the former case, which in its turn promotes a more efficient As removal. The overall conclusion of the current study is that triple-layer bed filters facilitate a more gradual Fe removal and its deeper penetration in the bed as compared to single-layer filters. This fact stimulates As removal and additionally allows for longer filter run times. Moreover, heterogeneous Fe removal seems to obstruct As oxidation by AsOB and therefore homogeneous reaction can be considered as more favorable in terms of As removal. This specific removal mechanism becomes predominant at pH levels above 7, when sufficient oxygen is available. Lastly, the operational setting of filtrate recirculation back to the filter inlet, displays a negative impact regarding As removal.

Activated carbon (AC) is commonly used in drinking water treatment plants (DWTPs) to remove organic micro-pollutants (OMPs), and it is effective in adsorbing a wide range of OMPs. However, its adsorption efficiency can be affected by natural organic matter (NOM). NOM is a complex matrix and widely exists in water bodies. During the drinking water treatment process, the large size fraction of NOM may block the pore of the granular activated carbon (GAC), and the small size fraction of NOM can compete with OMPs and occupy the sites. The goal of this study is to increase the OMP adsorption efficiency of AC filtration in the drinking water treatment process. The objective was to investigate the feasibility of the competitive NOM removal by zeolite adsorption prior to powdered activated carbon (PAC). Meanwhile, the filtration performance and advantages of nanofiltration (NF) with 1000 Da membranes were found out. Zeolite - AC and NF- Zeolite - AC combined treatments were exterminated. In this study, the properties of zeolites were reviewed and the zeolite with FAU framework was selected as the hydrophobic adsorbent. Batch experiments with FAU and PAC were conducted to examine the adsorption performance of 10 common OMPs in different water samples. Comparative batch tests on UV effluent and NF permeate were carried out with two stages, zeolite pre-treatment and PAC treatment. Ultrapure water and demineralized water were involved as the reference to prove the influence of NOM from adsorption isotherms. Dissolved organic carbon (DOC) measurements were provided by the DWTP. NOM is confirmed to hinder the OMP adsorption on AC according to DOC measurements and adsorption isotherms. NOM smaller than 1000 Da is proved to cause the competition with OMPs. FAU-type zeolite pretreatment was failed to remove the competitive NOM removal.