Mengistu Geza

Watershed-scale impacts of nitrogen from on-site wastewater systems: parameter sensitivity and model calibration.

A numerical watershed model was used to evaluate the potential influence of various point and nonpoint sources including on-site wastewater systems (OWS) on stream nitrate concentration in Turkey Creek Watershed, Colorado. A watershed analysis risk management framework model was used for this study, and was calibrated to observed stream nitrate concentrations using an automatic calibration tool. Parameter sensitivity analysis was done to select critical parameters for calibration and to reduce uncertainty in the simulated results. Sensitivity analysis of nitrate transport and transformation parameters showed that stream nitrate concentration is highly sensitive to cation exchange capacity, nitrification rate, base saturation of ammonium, initial concentration of ammonium in the soil, and some of the crop growth related parameters. The calibrated model was used to evaluate scenarios related to OWS including the impacts of population growth and new development and impacts of conversion of OWS to conventional sewers. The results showed that there would be a significant increase in stream nitrate concentration with increasing population. Conversion of OWS to sewers increased stream nitrate concentration but decreased nitrate concentration in the bottom soil layer indicating that OWS are beneficial with respect to stream nitrate concentration but may increase nitrate concentrations in groundwater.

STUMOD—a Tool for Predicting Fate and Transport of Nitrogen in Soil Treatment Units

A typical onsite wastewater treatment system consists of a septic tank and a soil treatment unit to treat wastewater before it is discharged through the vadose zone to an aquifer. A tool was developed for the purpose of predicting the fate and transport of nitrogen in soil treatment units (STUMOD or Soil Treatment Unit Model). STUMOD calculates nitrogen species concentrations and the fraction of total nitrogen reaching the aquifer or a specified soil depth. Input data include parameters for hydraulics and nutrient transport and transformation. An analytical solution is used to calculate the profile of pressure based on Darcy’s equation and the relationships between suction head, unsaturated hydraulic conductivity, and soil moisture. Chemical transport is based on simplification of the advection–dispersion equation. STUMOD is relatively simple to use but accounts for important processes such as ammonium sorption, nitrification, and denitrification. STUMOD accounts for the effect of soil moisture content (a surrogate for redox conditions) on nitrification and denitrification reactions. The model has provisions to handle the influence of temperature and organic carbon content on nitrogen transformation. Model outputs, generated based on input parameters obtained from extensive literature review, were compared to a numerical model and data from laboratory tests and field sites. Both measured data and STUMOD outputs show a relatively higher removal in clayey soils compared to sandy soils. Consistent with literature data for most soils, STUMOD predicted ammonium conversion to nitrate within the first foot below the trench infiltrative surface.

Review of Flowback and Produced Water Management, Treatment, and Beneficial Use for Major Shale Gas Development Basins

Produced water management is a major challenge to oil and natural gas exploration and production, in particular for unconventional gas development. Rapid development of shale gas production, regulations, water scarcity, concerns of environmental impacts, and limited disposal options are driving operators to consider, plan and change their current hydraulic fracturing and produced water management methods. Development of beneficial use for various applications has become a new trend for sustainable shale gas production. Treatment and beneficial use of flowback and produced water can enhance gas recovery and economic viability of gas production. It also minimizes potential environmental impacts, reduces demands on local water supplies, and meets regulatory requirements. However, there are many factors affecting the feasibility of beneficial reuse including water quality and quantity, appropriate treatment technologies, economics, disposal options and capacities, environmental impact, and regulations. The objective of this study is to evaluate the challenges and prospect of beneficial use through review of existing studies on flowback and produced water management in major shale plays.

New Conceptual Model for Soil Treatment Units: Formation of Multiple Hydraulic Zones during Unsaturated Wastewater Infiltration.

Onsite wastewater treatment systems are commonly used in the United States to reclaim domestic wastewater. A distinct biomat forms at the infiltrative surface, causing resistance to flow and decreasing soil moisture below the biomat. To simulate these conditions, previous modeling studies have used a two-layer approach: a thin biomat layer (1-5 cm thick) and the native soil layer below the biomat. However, the effect of wastewater application extends below the biomat layer. We used numerical modeling supported by experimental data to justify a new conceptual model that includes an intermediate zone (IZ) below the biomat. The conceptual model was set up using Hydrus 2D and calibrated against soil moisture and water flux measurements. The estimated hydraulic conductivity value for the IZ was between biomat and the native soil. The IZ has important implications for wastewater treatment. When the IZ was not considered, a loading rate of 5 cm d resulted in an 8.5-cm ponding. With the IZ, the same loading rate resulted in a 9.5-cm ponding. Without the IZ, up to 3.1 cm d of wastewater could be applied without ponding; with the IZ, only up to 2.8 cm d could be applied without ponding. The IZ also plays a significant role in soil moisture distribution. Without the IZ, near-saturation conditions were observed only within the biomat, whereas near-saturation conditions extended below the biomat with the IZ. Accurate prediction of ponding is important to prevent surfacing of wastewater. The degree of water and air saturation influences pollutant treatment efficiency through residence time, volatility, and biochemical reactions.

GIS-based nitrogen removal model for assessing Florida’s surficial aquifer vulnerability

Florida’s aquifer system exhibits spatially variable hydrogeological characteristics including shallow depth to aquifer and karst features. These characteristics contribute to groundwater vulnerability to nitrogen contamination and thus warranting vulnerability studies that allow zonation of areas into different levels of susceptibility to contamination from land use practices. A geographic information system (GIS)-based nitrogen fate and transport model (GIS-N model) was developed to assess aquifer vulnerability to contamination by examining the fate and transport of ammonium and nitrate from onsite wastewater treatment systems (OWTS). The GIS-N model analyzes fate and transport of nitrogen through the unsaturated zone using a simplified advection–dispersion equation. Operational inputs considered in this model include wastewater effluent ammonium or nitrate concentration, hydraulic loading rates, and OWTS locations. The GIS-N model considers two different modeling approaches: single step and two step. The single-step model considers a denitrification process assuming all the ammonium is converted to nitrate before land application, while the two-step model uses ammonium as an input and considers nitrification followed by denitrification. The resulting maps were classified into vulnerability zones based on the Jenks’s natural breaks in the data histogram. It was revealed that groundwater vulnerability from OWTS is sensitive to the depth to water table, first-order reaction rates, and parameters controlling the time and amount of conversion. Nitrate concentration is highest in areas with shallow water table depth. The vulnerability maps produced in this study will facilitate planners in making informed decisions on placement of OWTS and on groundwater protection and management.

Quantitative Tools to Determine the Expected Performance of Wastewater Soil Treatment Units Guidance Manual, Toolkit User's Guide and Visual-Graphic Tools

DEC1R06a: Development of Quantitative Tools to Determine the Expected Performance of Unit Process in Wastewater Treatment Units Onsite wastewater treatment system (OWTS) systems are an important part of the wastewater treatment and water management infrastructure in the U.S. Thus, proper OWTS selection, design, installation, operation and management are essential. While OWTS vary widely in their design and implementation, most systems are conventional OWTS that on the soil treatment unit (STU) for wastewater constituent treatment, hydraulic capacity, and eventual recharge to water resources. While there is considerable concern about potential water quality degradation associated with OWTS, current permitting and design focus mainly on ensuring that the hydraulic loading is not excessive. The STU provides an effective and sustainable means for wastewater reclamation, but occasional water quality degradation has been experienced. The likely cause for this is an incomplete understanding of treatment processes in various STUs, and the lack of available tools for assessing the performance of the STU. The overall goal of the project was to provide a toolkit to assess STU performance to enable evaluation and design of expected STU performance for important wastewater constituents over a relevant range of OWTS operating conditions. The toolkit is appropriate for a wide range of users, and includes an implementation protocol for different tools of varying complexity. Specific project objectives were to:   1. identify the best practices, available data, data gaps, and promising tools and techniques utilized in STU design and performance,   2. develop and test tools for performance-based STU design,   3. develop a protocol for using the tools,   4. refine the tools and protocol using data from laboratory studies, field sites, and numerical modeling, and   5. provide a final tool-kit and protocol to aid system designers and decision makers assess the expected STU performance. DEC1R06b: Quantitative Tools to Determine the Expected Performance of Unit Process in Wastewater Treatment Units: Toolkit Users Guide Onsite wastewater treatment systems (OWTS) are an important part of water management infrastructure in the United States. Thus, proper OWTS selection, design, installation, operation and management are essential. To aid this life-cycle, a toolkit was developed to enable evaluation and design of expected STU performance. The toolkit is comprised of this Guidance Manual, a companion Toolkit Users Guide, individual tools, and supplemental information. This framework provides detailed information to less experienced users while enabling more experienced users to start directly with STUMOD or other tool implementation referring to limited sections of the Guidance Manual or Users Guide. The toolkit was developed for a wide range of users faced with different needs of varying complexity when evaluating treatment of nitrogen, microbial pollutants (bacteria and virus), and organic wastewater contaminants (OWCs). Progressing through simple to more complex tools ultimately guides the user to the simplest tool that is appropriate, but discourages using a tool that is too simple for the decision at hand. The simplest tools include look-up tables and cumulative frequency distributions to direct the user to available pertinent information. Nomographs enable initial screening and quick insight into expected nitrogen removal based on the predicted output from STUMOD. Cumulative probability graphs illustrate modeling results in a risk-based framework while numerical model simulations demonstrate the usefulness of complex tools. Finally, two spreadsheet tools were developed for nitrogen transport, N-CALC and STUMOD, allowing the user to evaluate a range of STU operating conditions, soil hydraulics, and/or treatment parameters, as well as the relative influence of these factors on performance. DEC1R06c: Quantitative Tools to Determine the Expected Performance of Unit Process in Wastewater Treatment Units: Visual-Graphics Tools This file includes the visual-graphic tools: nomographs, cumulative probability graphs, and scenario illustrations. Chapter 1.0 includes nomographs illustrating the fraction of total-nitrogen remaining with depth. Chapter 2.0 includes cumulative probability graphs that illustrate the likely range of treatment outcomes. Chapter 3.0 includes HYDRUS simulation outputs that illustrate various operational scenarios. Finally, a list of visual-graphic tools is provided to aid in locating the visual-graphic tool of interest. This is a separate document that must be used in conjunction with the Guidance Manual and Users Guide. The companion Guidance Manual is organized into four Chapters describing the toolkit and providing guidance for tool selection and use. The fundamental assumptions that were incorporated and a detailed description of the tool development for these visual-graphic tools are provided in the companion Users Guide. Additional tools provided as separate files include STUMOD and N-CALC as MicrosoftTM Xcel files. This title belongs to WERF Research Report Series . In both the nomograph and the cumulative probability graphs, treatment information provided by these tools is based on data generated by numerical models that can incorporate complex and robust treatment and operating conditions. The parameters used for nomograph development are summarized in Table VG-1. Table VG-2 provides a definition for each parameter. Because the choices for representative OWTS conditions are limited, the user must decide how their OWTS system fits within the limited treatment estimations displayed by the graphics. Nomographs and cumulative probability graphs were developed for the following fixed operating conditions:   •Effluent Quality    ○Standard Effluent = representative of septic tank effluent (STE) as 60 mg-N L-1 as ammonium-nitrogen plus 1 mg-N L-1 as nitrate-nitrogen    ○Nitrified Effluent = representative of aerobically treated STE to achieve nitrogen reduction and transformation as 15 mg-N L-1 as nitrate-nitrogen   •Hydraulic Loading Rate (HLR)    ○2 cm d-1    ○5% Ksat   •Regional Temperature Range    ○Frigid/Cryic = Average Range 0 to 8 oC, Annual Mean 4.5 oC    ○Mesic = Average Range 8 to 15 oC, Annual Mean 11.5 oC    ○Thermic = Average Range 15 to 22 oC, Annual Mean 18.5 oC    ○Hyperthermic = Average Range 22 to 29 oC, Annual Mean 25.5 oC ISBN: 9781843393955 (eBook)