Perspective on the Synthesis of Unit Operations and Process (UOP) Concepts with Hydrologic Controls for Rainfall-Runoff

Since the passage of the 1972 Clean Water Act, control and treatment of rainfall-runoff (storm water) discharges have advanced from a challenge that was understood only well enough to realize the difficulties associated with the application of conventional treatment design to becoming our most recent water treatment and reuse challenge. Since the National Pollutant Discharge Elimination System (NPDES) Storm Water Phase I permitting regulations in the 1980s, there has been a proliferation of Best Management Practices (BMP). However, experience over the last decade has demonstrated that there continues to be a significant gap in knowledge between BMP design/analysis/monitoring and the fundamental unit operations and processes (UOP) that can demonstrate treatment viability as a function of the hydrologic, physical, and chemical characteristics of rainfall-runoff loadings. Such knowledge is critical to the success of a new generation of control strategies, BMPs, sustainable urban development (SUD) or low impact development (LID) concepts that will develop in response to ecological, environmental, and regulatory conditions, for example, the recent Phase II Storm Water Final Rule. Success requires the integrated knowledge of fundamental hydrologic, physical, and chemical properties of rainfall-runoff loadings combined with a foundation of UOP concepts and principles. This synthesis is critical whether the objective is hydrologic control, water chemistry control, water reuse, or, as is often the case, a combination of these. Without such a foundation, terms such as “best management practices” or “control strategy” have little meaning at best and, at worst, are a euphemism for doing something, usually without an assessment plan and usually at the expense of significant resources. As this environmental discipline, research area, and industry evolve, it is recommended that we develop a synthesis between the well-established UOP principles and well-established hydrologic controls while recognizing the uniqueness of rainfall-runoff control, treatment, and reuse.

Compared to the treatment of other waters (for example, wastewater or drinking water), rainfall-runoff poses unique challenges. The hydrologic, physical, and chemical phenomena associated with rainfall-runoff are deterministically complex, spatially diffuse, highly unsteady, and the arrival of such events and associated loadings have deterministic and stochastic components. When periodic or stochastic loadings from anthropogenic activities are combined with the stochastic arrival of rainfall-runoff events, control and treatment become significantly more challenging. For example, the interaction of rush-hour traffic and a rainfall-runoff event results in water chemistry and, to a certain degree, in water quantity loadings that are different than if the same event occurred under low traffic conditions. Although the relationship between water quality and quantity behavior can be deterministic to a significant extent, quality and quantity parameters also can vary by orders of magnitude across a single event.

Rainfall-runoff, impacted by anthropogenic activities, transports significant loads of dissolved, colloidal, and particulate solids in a complex heterogeneous mixture that includes metals, nutrients, and pathogens, as well as inorganic and organic compounds. Loads and concentrations of these constituents are significantly above ambient background levels and, for many land uses, can exceed surface water discharge criteria on an event or long-term basis. Rainfall-runoff mobilizes and transports a wide gradation of particulate matter ranging in size from smaller than $1\mu \mathrm{m}$ to greater than $\mathrm{10,000}\mu \mathrm{m}$ , as well as gross solids and trash in source area watersheds. Solutes partition between dissolved and particulate phases, as well as distribute across the particulate and colloidal gradation. Partitioning and speciation are also highly dynamic and driven by hydrology and coupled water chemistry. These complexities result in interesting and challenging behavior for representative monitoring as well as UOP design.

Most of us who teach, conduct research, consult, regulate, and practice in the environmental professions have been trained to think about UOPs in terms of wastewater, process water, or drinking-water design. Although the fundamental analyses of UOP mechanisms, for example, sedimentation, separation, oxidation, chemical precipitation, and ion exchange, etc., do not change for rainfall-runoff, the nature of the loadings changes dramatically. A comparison between municipal dry weather wastewater flows and rainfall-runoff illustrates the point.

Despite our best efforts, a plethora of fundamental research, and practical experience with wastewater UOPs, the characteristics of wastewater and the performance of many wastewater UOPs are still based on TSS and ${\mathrm{BOD}}_5$ , nutrients/pathogens not withstanding. Acknowledging diurnal behavior and variability in fluxes, centralized wastewater UOPs are loaded by reasonably steady flow and steady loadings. In addition, the removal of grit, sediment, and trash is carried out by preliminary UOPs upstream of the primary clarifiers. Therefore, a TSS and ${\mathrm{BOD}}_5$ basis to examine UOPs of primary/secondary clarification and biological oxidation may be reasonable for domestic or municipal wastewater. In addition, automated and grab sampling methods have evolved so that TSS and ${\mathrm{BOD}}_5$ can be reasonably sampled across UOPs such as primary or secondary clarifiers. Over the past 50 years, our understanding of UOPs developed in large part because of the extensive knowledge base generated from wastewater characteristics and conventional UOP treatment trains for centralized wastewater treatment.

However, the conditions for rainfall-runoff UOPs are significantly different. Most rainfall-runoff loadings are largely a function of hydrology, land use/infrastructure/surface characteristics, and anthropogenic activities that impact the watershed from which the loadings are generated. Flow and loading variability in rainfall-runoff can change by orders of magnitude during an event. In most urban conditions, these anthropogenic loadings to UOPs contain metals, nutrients, pathogens, a very wide size gradation of particulate and gross solids, and these loadings are largely inorganic. Depending on the season and land use, these rainfall-runoff events can also contain biogenic loadings. Transport and treatment are largely driven by hydrology. Loadings and treatment of these loadings are not amenable to TSS and ${\mathrm{BOD}}_5$ bases. Many of these loadings that contain metals can be inhibitory to biological processes. The residuals captured by these rainfall-runoff UOPs are not as amenable to biological oxidization or reduction and, depending on UOP management of residuals, can represent a significant disposal or reuse material.

Monitoring and verification of rainfall-runoff UOPs also present significant challenges and currently represents a controversial and healthy debate on a number of selected issues. These issues include the need for UOP mass balances; how mass balances can be achieved; how to representatively sample in order to examine UOP mechanisms and performance; what is the appropriate frequency, volume, and methodology of influent, effluent, and separated residual sampling; verification of event-based and life-cycle UOP behavior; and the contentious issue of automated samplers versus manual/grab sampling. Because rainfall-runoff phenomena can exhibit a significant degree of unsteady behavior, these issues are of critical importance for many in the profession who are involved in rainfall-runoff UOPs. Although monitoring and mass balances for rainfall-runoff UOPs are generally much more difficult than for centralized wastewater, the trend away from BMPs toward UOPs should bring the fundamental requirements of representative sampling, mass balances, and verification into more evolutionary focus. Recent developments for scientifically based monitoring, mass balance, performance, and verification criteria for UOPs by regulatory agencies are encouraging. Although rainfall-runoff control is a relatively young environmental discipline and the path forward may not always be clear and perfectly straight, these developing scientifically based criteria represent a positive direction. If parallel monitoring and mass balance debates occurred with respect to wastewater UOPs, these predate the author, although this does not suggest that there are not current monitoring and mass balance challenges for wastewater UOPs.

Depending on watershed characteristics, the event hydrology, and the UOP system, the large volume of runoff and peak flow rate generally cannot be successfully treated during the event without some form of hydrologic control; although a number of UOP exceptions have been developed with large hydraulic or volumetric capacity. For flows driven by hydrology, diffuse control generally makes sense and is advantageous, yet decentralized and on-site treatments present challenges related to operation, maintenance, verification, and sustainability. Currently, nearly all rainfall-runoff UOPs are decentralized or on-site systems. In the U.S., we recognize the general evolution towards centralized wastewater treatment from decentralized or on-site treatment systems although sustainable decentralized treatment has definite benefits. However, a parallel evolution of rainfall-runoff UOPs, whether in the form of end-of-pipe treatment trains, LID, or SUD systems, toward centralized rainfall-runoff treatment is more challenging. A simple evaluation of the volumetric and hydraulic flow requirements for most UOP systems, even for a high frequency return event such as a 1-year or 2-year event, helps put the challenge of treatment in perspective. It is generally recognized that in most cases hydrologic control can be most effective on a diffuse basis. Such diffuse hydrologic control has clear benefits for water quantity parameters while also providing thermal benefits, water-quality benefits, and reduced UOP requirements.

Development of BMPs, initially based on controls of hydrologic or water quantity parameters, has occurred over the past 3 decades in the U.S. Hydrologic controls such as detention, retention, infiltration, and evaporation developed scientifically to the extent that there now exists reasonably well-established understanding, predictive capabilities, models, and management strategies. Hydrologic, hydraulic, and water quality transport models are now a critical component of modern environmental disciplines. Although there has also been development and understanding with respect to rainfall-runoff treatment, treatment control lags behind hydrologic control. Many BMPs are still thought of as “black boxes” where only “in” and “out” monitoring occurs to establish a percent reduction in a constituent or an effluent concentration. It is sometimes said that best management practices are rarely “best,” rarely “managed,” and rarely “practiced.” Although progress has been made and there are excellent BMPs in use, it is time to move forward and begin to focus on fundamental concepts, principles, mechanisms, and models for treatment, control, and reuse. The unit operation and process framework provides an excellent opportunity for continued progress.

This editorial is not intended as a review of the UOP framework. There are many classic chemical, environmental, and mechanical engineering texts that provide an excellent presentation of this framework. Therefore, it is worth indicating what such a framework nomenclature provides for rainfall-runoff treatment, control, and reuse. The nomenclature “unit operation and process” alone provides an initial mechanistic interpretation, a benefit over the nomenclature “best management practice.” The first level of distinction allows one to discern between a physical operation and a chemical process in rainfall-runoff treatment. For example, hydrodynamic separators for particle separation are classified as unit operations, immediately suggesting a physical mechanism of treatment. In another example, hydrous oxide media systems for ion exchange with ionic metal species are classified as unit processes, immediately suggesting a chemical mechanism of treatment. In another example, chemical precipitation media for phosphorus that also provide physical filtration for suspended solids and, therefore, combine physical and chemical mechanisms would be identified as a unit operation and unit process system. Although this appears intuitive to those with a background in unit operation and process theory, the UOP framework provides an important mechanistic framework and method to examine rainfall-runoff treatment compared to the generic classification as a BMP.

Rainfall-runoff UOPs have an important future, as an environmental discipline, as a research direction, and as an industry. As an industry, the market for rainfall-runoff UOPs is doubling every 3 to 4 years and is currently a $100+ million treatment industry. Rainfall-runoff treatment, control, and reuse will become the environmental industry of this century in the U.S. Eventually, we will realize that although we must develop structural rainfall-runoff UOPs and these will become a permanent component of rainfall-runoff treatment, control, and reuse, structural UOPs alone (as a combination of decentralized and centralized treatment) are not economically sustainable. Source control must be an integral part of rainfall-runoff treatment, control, and reuse. In addition, the success or failure of rainfall-runoff UOPs is dependent in large part upon our understanding and application of hydrologic processes and control as integral parts of physical and chemical unit operations and processes.