| Natural filtration represents a sustainable technology to enhance the quality of the source water. Slow sand filtration (SSF) and riverbank filtration (RBF) are examples of natural filtration that have been used. SSF is a low-cost water treatment technology often used by small municipalities and households. It can be used to improve the quality of the source water in the aftermath of a natural disaster as well as in underdeveloped countries and rural areas. RBF, on the other hand, can be used as a primary treatment or a pre-treatment of water for municipalities and regional authorities. Similar removal processes, such as filtration, biodegradation, and adsorption, occur during both natural filtration technologies. However, during RBF the additional processes of chemical precipitation, redox reactions and mixing can occur. SSF and RBF showed similar removal efficiency in terms of turbidity (20--90% vs. 70--90%), total coliforms and E. coli (1--3 logs in both filtration technologies).;Two SSF units, disposed in parallel or in series, coupled with two post treatment point-of-use (POU) devices (an ultraviolet water treatment, UV unit, or an activated carbon impregnated with silver nanoparticles, AC unit) were used to examine the ability to treat source water with high levels of turbidity (> 20 NTU), high bacterial content (E+6 to E+7 MPN/100 mL of total coliforms and E. coli), and pharmaceutically active compounds (PhACs, at concentration of 50 microg/L). Six PhACs, caffeine, carbamazepine, 17-beta estradiol (E2), estrone (E1), gemfibrozil, and phenazone were selected using a multi-step approach based on: i) occurrence in the environment, ii) toxicity and pharmaceutical class, iii) environmental fate, iv) behavior under different redox conditions, and v) availability of analytical standards and adequate instrumentation.;Turbidity removal ranging from 40% to 80% and removal of total coliforms and E. coli greater than 95% was consistently achieved by SSF regardless of the configuration (series vs. parallel) of the units, the hydraulic loading rate, the starting turbidity of the source water and the presence or absence of PhACs. However, the presence of a post treatment unit, such as an UV unit, is desirable to further enhance the quality of the source water, especially in the presence of bacterial loads greater than 10 6 MPN/100 mL. SSF can also be used to treat water with a high concentration of PhACs such as a wastewater treatment plant spill or a pharmaceutical industry spill. However, the nature and concentrations of the PhACs, the duration of the spill, and the age of the SSF may significantly impact the overall performance of the filtration unit in terms of bacterial removal. Among the selected PhACs, complete removal of caffeine, and partial removal of E2 and E1 (11--92%) were achieved by both SSF units. Adsorption and biodegradation are the main removal mechanisms for the selected PhACs. None to limited (< 10%) removal of carbamazepine, gemfibrozil, and phenazone occurred.;Columns and two side-by-side sandbox were used to investigate the role of oxygen (aerobic vs. anoxic), temperature (summer vs. winter), and level of organic matter (TOC = 3, 10, 20 mg/L) on the removal of selected PhACs during simulated RBF. The same PhACs investigated during SSF were also used for the simulated RBF. RBF can be effectively used to remove most of the PhACs present in surface waters. However, the geochemistry of the RBF site is expected to play a key role in their removal. Depending on the compound, removal of PhACs may predominantly occur due to biodegradation, but environmental variables such as oxygen and temperature may enhance or limit biodegradation. Limited and slower removal of selected PhACs may occur during the winter temperature conditions. Limited removal of carbamazepine (< 10%) and gemfibrozil (< 30%) occurred regardless of the different environmental conditions. Among the different PhACs, removal of phenazone occurred only under aerobic conditions, while removal of caffeine was highly impacted by the level of organics as well as by the temperature. The occurrence of air beneath the riverbed can enhance the development of locally present aerobic conditions, that leads to an enhanced removal of redox sensitive PhACs. |
