The special collection on Onsite and Decentralized Wastewater Management Systems is available in the ASCE Library (https://ascelibrary.org/jswbay/onsite_decentralized_wastewater_systems).Onsite and decentralized wastewater treatment systems treat 20%–25% of the wastewater in the US, particularly in low-density residential and commercial areas. While sewerage systems are expected to expand over the next decades, it remains cost-prohibitive to connect many rural and suburban areas to centralized treatment facilities. Benefits of onsite and decentralized wastewater systems include their low cost, simple operation, and opportunities for groundwater recharge and recovery of water, energy, and nutrients close to their source of generation. However, these benefits must be balanced against risks to public health and water quality, particularly in light of increasing urbanization and climate change. This ASCE special collection contains 12 papers addressing topics including the fate and transport of pollutants in soil-based systems, passive biofilters that achieve high nutrient removal performance with low operations and maintenance (O&M) requirements, and systems that can recover nutrients and water for reuse close to the source of wastewater generation (Fig. 1).Soil-Based Treatment SystemsSeveral of the papers in this special collection address issues with conventional onsite systems, which consist of a septic tank, drain field, and soil-based treatment system. Dong et al. (2019) applied a HYDRUS Constructed Wetland 2D model to simulate soil-based wastewater treatment system performance. The model was calibrated and validated using water content, chemical oxygen demand (COD), ammonia, and nitrate data from laboratory-scale drain fields that were fed with either domestic wastewater or a mix of domestic and food service wastewaters. The drain fields removed COD and ammonia, while nitrate export was observed due to limited denitrification. The model provided a good fit to the experimental data and should provide a useful tool for elucidating transformation mechanisms in soil treatment systems under varying conditions.The majority of prior research on nutrient transformations in soil-based treatment systems has focused on small systems serving a single residence. However, many onsite systems serve schools, hospitals, restaurants, and other businesses with high flows. Humphrey et al. (2019) compared nitrogen transformations in low-flow systems with gravity distribution and high-flow systems with low-pressure pipe distribution and different soil types. The results provide insights into which flows and soil types should be targeted for implementation of advanced nitrogen removal systems.Soil-based treatment systems require adequate vertical distance between the bottom of the drain field and the seasonal high groundwater table to treat the wastewater before it reaches the groundwater aquifer. However, many regions are experiencing water table elevation changes due to changes in precipitation patterns, sea level rise, recharge from onsite wastewater systems, or imports of out-of-basin potable water. Cox et al. (2019) carried out an analysis of historical groundwater table data for coastal areas in southern Rhode Island. The results point to potential risks to public health and the environment due to inadequately treated septic system effluents from rising groundwater tables.Fernández-Baca et al. (2020) compared methane, carbon dioxide, and nitrous oxide fluxes from drain field soils with control (non-drain field) soils under both dry and wet weather conditions. The authors also quantified key functional genes involved in nitrogen and methane cycling using quantitative polymerase chain reaction (qPCR). Greenhouse gas trends were similar for drain field and control soils under dry weather conditions. After a simulated rain event, drain field soils had higher emissions for nitrous oxide and carbon dioxide than control soils, but not for methane. The results indicate that microbial communities in onsite wastewater drain fields are capable of mitigating greenhouse gases production under dry conditions, but these systems are net producers of these gases during rain events.Kohler et al. (2020) used data from the Boulder County Public Health Department permit database to assess the resiliency of onsite wastewater treatment systems after an extreme (1,000-year) storm event. The authors analyzed the resilience of 123 “failed” systems that required replacement of septic tanks and/or soil treatment areas after the 2013 storm compared with 150 systems that required similar repairs prior to the storm. Resilience was evaluated by measures of fragility (extent of loss of function), rapidity (time required to restore function), and resourcefulness (costs of repairs and related losses). Both the frequency of failure and the time required to recover system function increased poststorm. The authors provide suggestions for improving onsite system resilience as extreme precipitation events become more prevalent under climate change.Passive Nutrient Reduction SystemsIn many regions of the country onsite wastewater treatment systems are significant contributors to eutrophication and nitrate contamination of groundwater. Advanced treatment units have been developed that are similar to centralized systems in their design and operation, requiring multiple pumps, blowers, chemical feed, and sludge separation and disposal systems. Many of these systems have high energy and chemical requirements and have been shown to be too complex to be operated successfully by homeowners or part-time operators and septic system installers with limited training. To address this issue, low cost, low complexity passive nitrogen reduction systems (PNRSs) have been developed that are similar to conventional onsite systems in their O&M requirements. These systems typically include an unsaturated biofilter to promote organic carbon oxidation and nitrification followed by a saturated biofilter containing a slow-release solid electron donor, such as sawdust or elemental sulfur, to promote denitrification. The biofilters can either be set up in biofilm reactors or as soil-based systems with layers of porous media materials below the drain field.Four of the articles in this special collection built on the onsite wastewater testing infrastructure and expertise at the Massachusetts Alternative Septic System Test Center (MASSTC), which has been operated by the Barnstable County Health Department since 1999. These papers contribute to our understanding of the physical, chemical, and biological transformation process in soil-based PNRSs and tested out novel methods for nondestructively monitoring these systems. Langlois et al. (2020) characterized microbial communities from three MASSTC PNRSs with different media compositions. Pan lysimeters were used to collect samples for chemical analysis and 16S rRNA sequencing. Principal coordinate analysis showed extensive overlap between the microbial communities, despite differences in matrix materials, depths, and seasonal variations in performance. Graffam et al. (2020) used oxygen imaging to investigate redox dynamics within bench-scale soil-based PNRSs set up using matrix material from a MASSTC system. Pulsed dosing of artificial wastewater was used to mimic typical dosing patterns in onsite systems. The results highlight the role of matrix composition on oxygen dynamics in the unsaturated zone and its effect on nitrification performance. Waugh et al. (2020) investigated the fate of nitrate and microbial communities in bench-scale incubations set up with matrix materials collected from different depths of a MASSTC PNRS. Mass balances and qPCR analysis of nirK genes showed that denitrification was the dominant pathway for nitrogen removal, with little nitrous oxide production. The rate of denitrification could be enhanced by methanol addition, suggesting that the availability of readily degradable organic carbon released from the sawdust media limited denitrification. Wehrmann et al. (2020) evaluated the biogeochemical sequestration of phosphorus in a soil-based PNRS operated at MASSTC. High levels of phosphorus removal were observed in the system, with most of the removal observed in the unsaturated layer due to sequestration into Fe and Al oxyhydroxides, formation of Fe- and Al-P minerals, such as vivianite, and incorporation into organic matter. Phosphorus sequestration was enhanced by Fe(II) release by dissimilatory iron reduction in anoxic zones that developed near the distribution system. Contrary to prior studies, phosphorous adsorption played a minor role in phosphorus removal.Rodriguez-Gonzalez et al. (2020) developed a hybrid adsorption and biological treatment system (HABiTS) at a Florida site to address issues of variable performance of PNRSs under transient loading conditions. In HABiTS, ion exchange materials (natural zeolite minerals and scrap tire chips) were incorporated into the biofilters as biofilm carriers. When loading rates were high, ammonium and nitrate were adsorbed by the ion exchange media. When loading rates were low, nitrogen-containing ions desorbed from the ion exchange materials, which were bioregenerated by the attached biofilms. Addition of recirculation and pre-denitrification to the unsaturated biofilters significantly improved overall nitrogen removal performance. The project also differed from other PNRS studies in this special collection in that the authors used elemental sulfur pellets as a slow-release electron donor in the saturated biofilter rather than lignocellulosic material (i.e., sawdust).Onsite/Decentralized Nutrient and Water RecoverySeveral of the papers in this special collection focused on the recovery of nutrients (nitrogen, phosphorus, potassium) from urine for use as agricultural fertilizers and onsite water reclamation. Urine makes up less than 1% of wastewater volume yet contributes the majority of nutrients to wastewater (roughly 80%, 50%, and 70% of N, P, and K in domestic wastewater). Therefore, separate collection of urine for fertilizer production has the potential to recover valuable fertilizer products, reduce the energy costs and environmental damage caused by conventional fertilizer production, and decrease concentrations of nutrients in the mainstream wastewater, reducing its eutrophication potential and cost of treatment. Jagtap and Boyer (2020) compared ammonia stripping–acid absorption with ion exchange using clinoptilolite for recovery of nitrogen from struvite-precipitated urine. The authors also evaluated distillation for potassium recovery from urine for potash and nonpotable water recovery. Integration of struvite precipitation, followed by ammonia stripping–acid absorption of ammonia as ammonium sulfate and then distillation successfully recovered separate phosphorus, nitrogen, and potassium products with nutrient concentrations greater than or equal to commercially available fertilizers.Hazard et al. (2020) operated a steam distillation system for recovery of ammonium from source-separated urine at varying urine:steam ratios. The full-scale prototype system included carbonate addition, steam generation, steam distillation, and condensation of ammonia. A mass balance analysis showed that nitrogen recoveries greater than 90% as ammonium in the reactor distillate were achieved when urine:steam ratios were less than 7. The energy requirements for urine:steam ratios between 6.5 and 7.5 were competitive with the Haber-Bosch process, which is the conventional process for nitrogen fertilizer production.The characteristics of source-separated urine are highly variable, depending on the types of urine-collecting toilets used, diets of the users, and storage and transport conditions. Separation of urine for nutrient recovery also results in changes in mainstream wastewater characteristics and treatment plant costs, for example, by reducing aeration requirements for nitrification. Bisinella de Faria et al. (2020) developed a dynamic influent generator that generates influent flow rates and concentrations [e.g., various fractions of COD, total Kjeldahl nitrogen (TKN), total phosphorus (TP)] for both stored collected urine and the remaining mainstream wastewater. The generator was analyzed for a case study of a city with a population of 100,000 and varying urine retention fractions (0%, 20%, 50%, 80%). Both Activated Sludge Model 1 (ASM1) and Sumo1 from Dynamita. were used for dynamic simulation of a modified Ludzack-Ettinger process. The results indicated that lower energy costs and greater effluent stability can be achieved even with modest-level urine retention (20%).The special collection also includes one forum (Lackey et al. 2020) on the topic of decentralized nonpotable water reuse. Forums include essays on topics of interest to ASCE journal readers and do not necessarily reflect the views of ASCE or the Journal’s editorial board. Lackey et al. synthesized information from reports by the San Francisco Public Utilities Commission, National Academies of Sciences, Engineering, and Medicine, and the US Water Alliance, other published work, and interviews with public health and utility representatives. The authors discuss the trends, challenges, and opportunities of building-scale water source (e.g., gray water, black water, stormwater, condensate) reuse (e.g., for toilet flushing, washing, cooling, and heating). Benefits of onsite reuse include increased water and energy sustainability and resilience for communities. Challenges include a lack of standardized regulatory approaches and guidance and a lack of trained operators and inspectors for these systems. Research needs on technical questions are also identified in this forum.ConclusionsThe Journal of Sustainable Water in the Built Environment aims to disseminate research findings, challenges, and opportunities on water issues throughout the developed landscape, including onsite and decentralized water and wastewater systems. The Journal is a natural fit for this special collection addressing issues of design, performance, sustainability, and resilience of onsite and decentralized wastewater treatment systems. We encourage future submissions on this topic to this Journal. Further related research work may be added to the special collection later.AcknowledgmentsWe thank our authors for their outstanding contributions to this special collection and the reviewers for all their hard work helping to improve the quality of the submitted manuscripts.References Bisinella de Faria, A. B., M. Besson, A. Ahmadi, K. M. Udert, and M. Spérandio. 2020. “Dynamic influent generator for alternative wastewater management with urine source separation.” J. Sustainable Water Built Environ. 6 (2): 04020001. https://doi.org/10.1061/JSWBAY.0000904. Cox, A. H., G. W. Loomis, and J. A. Amador. 2019. “Preliminary evidence that rising groundwater tables threaten coastal septic systems.” J. Sustainable Water Built Environ. 5 (4): 04019007. https://doi.org/10.1061/JSWBAY.0000887. Dong, Y., S. I. Safferman, and A. P. 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