Ecotechnologies for the Treatment of Variable Stormwater and Wastewater Flows

The occurrence of variable stormwater and wastewater flows, mostly precipitation driven, brings with them the challenge of both peak flows and pollutant loads. Wastewater treatment systems can be divided into those that are specifically designed and operated to deal with variable flows, and those that presume more steady-state operation, only coping with peak flows as anomalies for short periods of time. To date, the different types and scales of variability and the impact of this variability on functioning and treatment performance have neither been well characterised nor properly dealt with for the design of suitable treatment systems. In this book, ecotechnologies are defined as processes for the treatment of variable wastewater flows thatThis book focuses on treatment systems compliant with these definitions, but which are also specifically designed for variable flows.

A wide range of ecotechnologies has been applied to treatment of variable stormwater and wastewater flows. Stormwater ponds and basins were already introduced as common ‘end-of-the-pipe’ treatment solutions in the 1960s, almost parallel to the first attempts to develop structured wastewater treatment with the help of plants, inspired by natural wetlands. Constructed wetlands specifically designed for the treatment of variable flows emerged in the 1990s and were joined by a growing group of vegetated filter systems, named bioretention filters, raingardens or retention soil filters, all following the principle of gravity-driven wastewater filtration. This chapter provides a general overview of these treatment facilities, including swales and buffer strips. Although the latter ones are gravity-driven filtration systems, they are commonly used for the treatment of road runoff and are highly adapted to fit into their landscape structure, they are described in a separate section. Each section includes references to detailed design and operation guidelines.

When nutrient loads are discharged into surface waters with variable stormwater and wastewater flows, surface water pollution is impaired. Nutrients can lead to oxygen depletion and eutrophication of surface waters, including excessive plant and algae growth. Popular examples of structures harmed by excessive nutrient inflow are the Baltic Sea or the Great Barrier Reef in Australia. Hence, removing nutrients, especially nitrogen and phosphorus compounds, is a major target when variable flows should be treated. This chapter gives an overview of the available removal mechanisms and the potential efficiencies of different treatment facilities. While particle-bound nutrients can be removed via sedimentation processes, dissolved nitrogen and phosphorus compounds cannot as they differ in their biochemical degradation: the adsorption capacity for nitrogen compounds is often renewable, whereas the uptake of phosphorus compounds is limited over time. Hence, treatment facilities need to be able to address the different requirements.

A variety of ecotechnologies have shown promising yet variable results in reducing faecal microbial contaminants under challenging operational conditions. But relatively limited work has been conducted to investigate and understand faecal microbe removal in these systems under highly fluctuating hydraulic and contaminant loading. In most instances, ecotechnology-based systems such as sedimentation ponds, constructed wetlands and bioretention filters have proved effective for treating episodic discharges and demonstrated performance resilience removing faecal microbial contaminants with modest to good efficiency particularly where inflow concentrations are high. However, microbial removal may depend greatly on the type of microorganism, treatment system design and operational factors. Design characteristics such as type of filter material and depth, presence of a submerged zone, type of vegetation and operational conditions such as inflow concentration, and antecedent dry periods in combination with temperature changes can all affect the removal of faecal microbes. Factors influencing survival, fate and behaviour of retained faecal microbes are still poorly understood. These knowledge gaps need addressing in order to fully evaluate microbial removal from fluctuating contaminated flows and more accurately interpret faecal indicator bacteria-based water quality and potential health risks associated with discharge from these ecotechnology-based systems.

Metals were the first priority pollutants to be widely investigated in stormwater. In solid phase, they are often attached to very fine particles. The dissolved fraction creates considerable environmental problems as it is the most bioavailable fraction. Hence, removal of both fine and dissolved particles plays a major role in the treatment of polluted runoff. Ecotechnologies specifically designed to remove metals should be able to address different treatment mechanisms. However, the exhaustion of sorption capacity reduces the lifespan of treatment facilities. Additionally, metal concentrations fluctuate extremely—spatially, seasonally and over time—which poses another challenge for further increasing removal efficiencies. While soil- or sand-based systems should be designed in a way that the filter material can be exchanged, newer developments such as Floating Treatment Wetlands show promising removal capacities as the installations bind metals in sludge sediments, which can be removed from time to time. The different treatment mechanisms, aforementioned developments and techniques as well as their removal capacities will be discussed in this chapter.

Emerging contaminants became a major topic in water treatment when laboratory detection methods for concentrations at a nanogram-scale improved approximately two decades ago. Research on using ecotechnologies to remove emerging contaminants in variable stormwater and wastewater flows has been conducted for more than a decade, but so far, not all removal mechanisms are well understood and only few setups have been investigated. This chapter summarises the current knowledge, focussing on pesticides and emerging contaminants listed on the watch list of the European Union. However, large-scale investigations are still rare and further research will have to be conducted in this field to enable practitioners to provide recommendations for design and maintenance of treatment facilities in the field of ecotechnologies.

Constructed wetlands (CWs) subjected to variable loads present a series of challenges for designers and researchers. Classical design approaches (e.g. rule of thumbs or first-order kinetic model) are not suited to properly estimate the removal efficiencies of CWs under varying flows. The internal removal processes of CWs are expected to be influenced by the variation of influent pollutant concentrations and hydraulic loads for particular CW applications (e.g. stormwater or combined sewer overflow treatment). A powerful tool to properly study and design CWs under varying flows is given by mathematical model. Either for design or research purposes, mathematical models have been developed to simulate CWs subjected to varying flow and are revised in this chapter. Models used to simulate the hydraulic behaviour as well as the treatment performances of variable-flow CWs are reviewed. Moreover, future perspectives of mathematical models in this field are analysed in terms of design-support tools, process-based model for design purposes, and limitation for a wider application.