Biosand filters (BSFs) are point‐of‐use water treatment systems that can provide safe and affordable potable water to households in developing countries. However, little information is available on the dynamic performance of BSFs or for selecting local materials to use as filter media. In this study, water quality dynamics, biofilm characteristics, and hydraulic performance were investigated in two full‐scale BSFs. During four months of operation with sewage‐contaminated surface water, average log10 removals of 1.7, 1.2, and 0.6 were observed for Escherichia coli, total coliforms, and total heterotrophs, respectively. Average removal efficiencies for turbidity, ultraviolet absorbance at 254 nm, and total organic carbon were 86, 36, and 27%, respectively. Dynamic analysis showed that significant removal of indicator organisms and total organic carbon occurred during the pause period between charging the BSFs. Biofilm characterization revealed that more particle‐associated biomass exists within the BSF column compared with the schmutzdecke layer.

Stormwater biofiltration systems can alleviate eutrophication by reducing nitrogen loadings from stormwater runoff. These systems can be modified to further enhance nitrogen removal by incorporating a submerged layer with a carbon source to promote denitrification. The objectives of this research were to compare the nitrate (NO À 3 ) removal performance of various media mixtures under varying environmental conditions and to estimate the longevity of carbon media in denitrifying biofilter microcosms. Media types evaluated consisted of mixtures containing sand, gravel, and/or eucalyptus wood chips. Negligible NO À 3 removal and high concentrations of total Kjeldahl nitrogen (TKN), phosphate (PO 3 À 4 ), and dissolved organic carbon (DOC) were observed from carbon-containing media that were left unsaturated before analysis. High removal of NO À 3 (85-100%) and low concentrations of TKN and DOC were observed when carbon-containing media were acclimated under saturated conditions. A mixture of gravel and eucalyptus-wood media exhibited high NO À 3 removal rates, low final DOC concentrations, and was selected for further analysis. Results indicated that carbon-containing biofilters should include gravel instead of sand media and be designed to maintain saturation to increase NO À 3 removal rates and decrease TKN, PO 3 À 4 , and DOC export. Results from the gravel-wood media were then imported into a novel model developed to estimate media longevity. Based on observed DOC dissolution rate, estimated longevity of the gravel-wood media in denitrifying biofilters was greater than a decade. Results indicate that permanently saturated carbon-containing media will supply a sustainable source of carbon over the functional lifespan of stormwater biofilters.

Two important and large non-point sources of nitrogen in residential areas that adversely affect water quality are stormwater runoff and effluent from on-site treatment systems. These sources are challenging to control due to their variable flow rates and nitrogen concentrations. Denitrifying bioreactors that employ a lignocellulosic wood chip medium contained within a saturated (anoxic) zone are relatively new technology that can be implemented at the local level to manage residential non-point nitrogen sources. In these systems, wood chips serve as a microbial biofilm support and provide a constant source of organic substrate required for denitrification. Denitrifying wood chip bioreactors for stormwater management include biofilters and bioretention systems modified to include an internal water storage zone; for on-site wastewater, they include upflow packed bed reactors, permeable reactive barriers, and submerged wetlands. Laboratory studies have shown that these bioreactors can achieve nitrate removal efficiencies as high as 80–100% but could provide more fundamental insight into system design and performance. For example, the type and size of the wood chips, hydraulic loading rate, and dormant period between water applications affects the hydrolysis rate of the lignocellulosic substrate, which in turn affects the amount and bioavailability of dissolved organic carbon for denitrification. Additional field studies can provide a better understanding of the effect of varying environmental conditions such as ambient temperature, precipitation rates, household water use rates, and idle periods on nitrogen removal performance. Long-term studies are also essential for understanding operations and maintenance requirements and validating mathematical models that integrate the complex physical, chemical, and biological processes occurring in these systems. Better modeling tools could assist in optimizing denitrifying wood chip bioreactors to meet nutrient reduction goals in urban and suburban watersheds.

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