In multiple studies, slow sand filtration has been useful to remove and eliminate propagules of plant pathogens: many species of phytophthora – P. cinnamomi , P. cryptogea , P. nicotianae , and species of Pythium , Fusarium , E. coli , and nematodes including Radopholus similis. These zoosporic fungi, viruses, and nematodes cause crop damage including severe root rot and if not treated in a recirculating nursery and greenhouse system, widespread epidemic may impact all the crops within the facilities. While SSF has been shown to remove plant pathogens, its efficacy to remove other compounds has yet to be explored in depth. Contaminants of emerging concern , also named “emerging contaminants”, are a group of contaminants that consist of pharmaceutically active compounds, endocrine disrupting compounds, personal care products, plasticizers, pesticides and herbicides, and flame retardants that are found in trace amounts in the environments, primarily discharged from wastewater treatment plants that insufficiently treat these contaminants through secondary and tertiary treatment. Non-point sources, such as overland flow during rainfall or land drainage in agricultural areas deliver veterinary medicines and pesticide runoff to surface water or groundwater. The existence of emerging contaminants is unknown in the environment due to the lack of monitoring and due to their low concentrations in surface waters. Many of these compounds vary due to their application and consumption from region to region and will depend on the efficiency of removal by wastewater treatment plants. Technologies already proven effective in removing contaminants are activated carbon ,maceta 25l ozonation and advanced oxidation processes , and membrane filtration. However, these processes are costly and require large amounts of resources to operate.
In turn, bio-filtration systems are simple to operate, relatively low in cost and maintenance, removes both turbidity and propagules of pathogens, and overall improves the quality of the water. Thus, bio-filtration like SSF may offer a low cost alternative for the treatment of contaminants in wastewater.The most commonly researched bio-filtration systems to treat emerging contaminants are managed aquifer recharge processes. Managed aquifer recharge processes are robust and cost-effective systems and include a variety of applications such as aquifer storage and recovery, infiltration ponds, percolation tanks, soil aquifer treatment, and sand dams. MAR systems like bank filtration and artificial recharge are adopted by wastewater treatment plants to reduce the cost of using more costly advanced treatment systems like nanofiltration and reverse osmosis. MAR systems are typically adopted if surface water quality is inadequate or if the amount of raw water like in groundwater is not sufficient ; MAR systems can replenish these natural systems. However, in comparison to SSF, biodegradation in MAR systems are very important mechanisms, much more than sorption as sorption sites can become exhausted or desportion can occur. This practice may not be as easy to maintain like slow sand filters, as sand can be back washed and the supernatant of the disturbed schmutzdecke can be drained. Also, it is suggested that MAR such as bank filtration may not be optimal for smaller operating facilities that have less land space. There have been many studies that focus on biofilm reactors such as bank filtration and underground dams that can remove organic micropollutants but these are not usually designed, only grown in natural conditions. Therefore these systems are very limited and may need to overcome obstacles of releasing WWTP effluent water directly to natural waters. However, given substantial research on MAR systems, pharmaceuticals and endocrine disrupting compounds, which are usually main contaminants of interest due to their potentially adverse effects on human health and aquatic life even at low concentrations , have shown positive results for their removal.
Most of these studies were laboratory scaled and field studies. Some contaminants researched in MAR systems are endocrine disrupting compounds , antiseptics , pharmaceuticals , and disinfection by-products. Baumgarten et al. investigated the removal of poorly degradable antibiotic sulfamethoxazole in laboratory columns for its removal in bank filtration. Results showed that 60% of sulfamethoxazole was removed within 14 days of column passage in aerobic conditions while no removal occurred under anoxic conditions in a 2 year long system operation. The degradation of poorly degradable compounds in aerobic conditions may give bank filtration its benefits in the top layers of sand, but shows complications with anoxic conditions, which can be representative of some MAR and SSF. Adaptation of the system may require long operation time as would be realistic in an actual bank filtration site. Thus, more biofilter systems with MAR and SSF should be studied for their processes that provide both oxic and anoxic conditions. Typically, varying redox conditions are effective for removing redox-sensitive organic micropollutants during MAR. Maeng et al. summarized the literature in a review regarding removal efficiencies of CECs using bank filtration and aquifer recharge column and full scale studies. To briefly summarize that work, there are CECs that are promising for removal by filtration and others which are more recalcitrant such as carbamazepine are ineffective. Antibiotics in a study by Heberer et al. investigated 19 targeted antibiotics at a lake bank filtration site in Berlin, Germany for 2.5 years. They detected 7 out of 19 target antibiotics. All antibiotics were completely removed after 2-4 months of travel time except for sulfamethoxazole, which in previous studies was discovered to be redox-dependent that degrades more effectively in anoxic conditions. Depending on the residence time, removal of antibiotics can increase. It can be concluded MAR is an effective treatment step for removing antibiotics, giving way for other filtration studies to filter veterinary antibiotics that may potentially be found in runoff from dairy farms. For non-steroidal anti-inflammatory drugs and analgesics, many have been removed at rates greater than 50% during bank filtration and aquifer recharge systems.
Field and laboratory scale studies have shown significant removals of diclofenac, ibuprofen, naproxen, and phenazone during soil passage. Diclofenac, ibuprofen, and naproxen have moderately high octanol-water partition coefficients , suggesting sorption would be the main mechanism of removal. Phenazone, however,frambuesas en macetas is more redox-dependent and can be removed under oxic conditions than anoxic conditions. Therefore, it is necessary to monitor the pH during soil passage as these NSAIDs may remain as ionic species in the aquatic environment, with more potential to be sorbed. Anticonvulsant pharmaceuticals have been shown numerous times their persistency in degradation in multiple treatment methods. Carbamazepine is one of the notorious poorly degradable compounds and has low removal in wastewater treatment plants. Drewes et al. showed no change in carbamazepine and primidone concentrations in soil aquifer treatment for estimated travel times up to six years. The extended research on this contaminant concluded bank filtration and aquifer recharge are not effective for anticonvulsant removal. Antidepressants removal has still yet to be studied. A study by Snyder et al. investigated three antidepressants and their fate during a pilot scale bank filtration. Fluoxetine was removed significantly at 99% and meprobamate was only 66%. There could be more research developed in understanding the fate of more antidepressants since they are commonly used drugs in the pharmaceutical industry. Lipid regulators are similar to NSAIDs, where they remain in ionic species. Thus, pH plays an important role in the removal mechanism. One of these lipid regulators, clofibric acid, is a common metabolite of clofibrate, and is detected frequently in the aquatic environment. Interestingly, research suggests clofibric acid concentrations increased at bank filtration sites in Germany due to the high consumption of liquid regulators during the 1990s. They discovered clofibric acid present in deeper layers of the aquifer. Lipid regulators and NSAIDs can be included in a joint research to test pH conditions for their removal. Steroid hormones are also a very particular group of CECs because they can produce potentially adverse effect on human health and aquatic life even at very low concentrations. However, laboratory scale and field studies using bank filtration in Berlin, Germany showed positive results. 17β-estradiol and 17α-ethinylestradiol were not detected in surface water from Berlin and estrone was removed greater than 80%. Snyder et al. used batch experiments and field studies with bank filtration and demonstrated estrone, 17β-estradiol, and 17α-ethinylestradiol were removed by biodegradation and sorption.Given conditions of certain MAR, steroid hormones and potentially a wide range of endocrine disrupting compounds can be reliably treated. For pesticides/herbicides, most studies have been focused on atrazine.Atrazine is resistant in the environment and penetrates through the surface and subsurface due to its high mobility, persistence, low vapor pressure, and massive application since it has been in use since 1959. Ho et al. and Zhang et al. showed triazine herbicides like atrazine were poorly removed by biofilters, but some studies have shown that atrazine can be readily biodegradable in aquatic environments, with reported removal rates ranging from weeks to years.
Based on the previous studies, it is apparent that emerging contaminants can be removed and have shown adequate removals with a variety of conditions; aerobic and anaerobic conditions, biological substrate feeds, bacterial community development, filter mediums, variety of physicochemical properties of contaminants, and type of leachate water filtered using a SSF system. However, despite the variance in studies, most research agrees that the general contaminant removal increases over time as filter matures. Now that available freshwater resources are continuously limited and increases in world population have raised the pressure on natural resources , water resource management have turned to water reclamation and reuse to sustain agricultural activities. Reclaimed water use is not only limited to agriculture but widely used in other purposes such as irrigating landscapes, nurseries and greenhouses, flushing toilets, and replenishing groundwater aquifers. In 2006, an estimated 9.8×106 m3 d -1 of treated municipal wastewater was used in the United States. Studies have documented the presence of many micro-contaminants such as pharmaceuticals and personal care products, pesticides, phenolicestrogens, surfactants, biocides, and disinfection by-products in reclaimed water. These contaminants may accumulate in the area irrigated by reclaimed water and may result in contamination of the soil and plants. Along with the combined use of pesticides in greenhouses, it is possible for receiving waters to receive a wide range of pollutants. This is especially true in rural areas where contaminated runoff from farmland contributes a significant proportion of the pesticide load. The overall objective of this study was to promote slow sand filtration columns as a cost effective engineered solution to treat emerging contaminants. Specifically we simulated a greenhouse irrigation system that contains emerging contaminant concentrations of 400 ng L -1 based on the range of literature values for concentrations of emerging contaminants found in reclaimed nonpotable wastewater , a concentration higher than the average to be able to adequately detect the compounds in our study. This pilot scale study is a preliminary study to see how viable SSF columns can be to remove emerging contaminants. Results of the present study can be combined with previous studies of using SSF columns to remove both pathogens and contaminants provided by reclaimed water and pesticide use. The removal efficiencies of 14 selected pharmaceuticals and personal care products , 7 pesticides, 3 plasticizers, and 2 detergents/emulsifers, and the trends after the project’s initial start and declogging maintenance removal rates thereafter were examined. Chemical residues have been ubiquitous in the environment as they are found in many environmental matrices, from sewage water, effluent water from wastewater treatment plants, river water, to even drinking water. These compounds can come from sources such as households, nurseries, wastewater treatment plants, factories, hospitals, and any other facility dispensing chemical waste such as pharmaceuticals, personal care products, antibiotics, plasticizers, pesticides, etc. This group of compounds, known collectively as “emerging compounds” or “compounds of emerging concern” , is notorious for their occurrence in the environment and their complexity existing in particular environmental matrices. Even at their low concentrations they can affect human health and environmental health. Analytical techniques using Gas Chromatography-Mass Spectrometry or Liquid Chromatography-Mass Spectrometry have paved the way for determining the concentrations of CECs even at their trace amount. GC-MS was first used to determine pharmaceuticals and personal care products in the environment in 1976. Advances to the quantification of CECs in environmental samples have increased detection sensitivity and reliability. CECs come in a wide variability in their concentrations, polarities, and thermal labilities.