Treatment Wetlands

About this book

Wetlands are among the most productive and biogeochemically complex ecosystems on earth, concentrating biological activity at the intersection of terrestrial and aquatic environments. For decades, engineers and ecologists have harnessed this natural productivity to treat polluted water — constructing wetland systems that replicate and accelerate the processes by which natural marshes, bogs, and fens cleanse water that passes through them. Treatment Wetlands, Second Edition, by Robert H.

Kadlec and Scott D. Wallace, published by CRC Press in 2009, stands as the most comprehensive technical reference available for the design, construction, operation, and evaluation of engineered wetland systems for water quality improvement. Building on the landmark first edition co-authored by Kadlec and Robert Knight in 1996, this updated volume incorporates over a decade of additional monitoring data, refined design models, and expanded coverage of system types and pollutant categories.

The book addresses an exceptionally wide range of applications. Constructed treatment wetlands are used worldwide to treat municipal wastewater effluent, agricultural runoff, industrial process water, stormwater, acid mine drainage, landfill leachate, and contaminated groundwater. They range in scale from small on-site systems serving individual dwellings to vast engineered marshes receiving millions of litres per day from urban wastewater treatment facilities.

Their appeal lies in combining effective pollutant removal with low energy consumption, low operational complexity, low cost relative to conventional treatment technologies, and the additional ecological and amenity values that vegetated systems can provide. Kadlec and Wallace organise their treatment of the subject around the two primary system configurations that dominate practice worldwide. Free water surface (FWS) wetlands — also called surface flow wetlands — allow water to flow across the surface of a shallow basin planted with emergent macrophytes such as bulrushes, cattails, and reeds.

The water surface is exposed to the atmosphere, permitting oxygen transfer and solar-driven processes, and the shallow depth supports rooted vegetation whose stems and leaf litter provide attachment surfaces for microbial biofilms. FWS systems are particularly effective for secondary or tertiary treatment of municipal effluent, wildlife habitat creation, and large-scale applications where land is available. Subsurface flow (SSF) wetlands route water through a porous medium — typically gravel or crushed rock — rather than across an open surface.

In horizontal SSF systems, water enters at one end of a gravel bed and travels horizontally through the medium to an outlet at the opposite end, remaining below the surface throughout. Vertical flow systems deliver water intermittently to the surface of the bed, allowing it to percolate downward through the medium before draining from the base. Subsurface flow systems eliminate the exposed water surface, reducing mosquito breeding concerns and odour issues that can affect FWS systems in sensitive locations.

They also support distinct microbial communities within the anoxic and anaerobic zones of the gravel matrix, enabling pollutant transformation processes that are less accessible in surface flow environments. Hybrid constructed wetland systems — combining FWS and SSF stages in deliberate sequence — have emerged as a powerful design strategy for achieving treatment objectives that neither system type can meet alone. A common configuration pairs vertical flow SSF cells, which efficiently oxidise ammonia through nitrification, with horizontal flow SSF cells or FWS stages, where the anoxic conditions necessary for denitrification convert nitrate to atmospheric nitrogen gas.

This sequential arrangement can achieve near-complete nitrogen removal — a target that is difficult to approach with a single wetland type — while also addressing biochemical oxygen demand, suspended solids, and pathogens. The pollutant removal mechanisms reviewed in the book are numerous and interconnected. Suspended solids are removed primarily by sedimentation and filtration.

Biochemical oxygen demand is reduced through aerobic microbial decomposition and, in SSF systems, anaerobic degradation. Nitrogen undergoes a complex sequence of transformations: organic nitrogen is mineralised to ammonia, ammonia is oxidised to nitrate by nitrifying bacteria in aerobic zones, and nitrate is reduced to nitrogen gas by denitrifying bacteria in anaerobic zones. Phosphorus removal is achieved through plant uptake, microbial assimilation, and — most significantly for long-term performance — chemical precipitation and adsorption to calcium, aluminium, and iron compounds in the substrate.

Pathogens are reduced through sedimentation, ultraviolet exposure in surface systems, predation, and natural die-off. The book provides detailed coverage of the design equations and empirical datasets underpinning each removal process, along with analysis of the factors — temperature, hydraulic loading rate, vegetation type, substrate chemistry — that govern performance variability. A distinctive strength of Treatment Wetlands is its empirical foundation.

Kadlec and Wallace draw on an extraordinarily large dataset of operational systems spanning multiple continents, climates, and treatment objectives, presenting performance statistics that reflect the real-world range of outcomes rather than idealised laboratory conditions. The book's design guidance is accordingly calibrated against this empirical record, with conservative safety factors recommended for systems where regulatory compliance is critical. For practitioners in sustainable water management, civil and environmental engineering, landscape ecology, and green infrastructure planning, this volume provides an indispensable technical foundation for understanding and applying one of the most promising and versatile tools in the sustainable treatment of water.