Investigation into the kinetics of constructed wetland degradation processes as a precursor to biomimetic design

Abstract

In this study, biomimetic principles were incorporated into a kinetic study of a pilot-scale, horizontal subsurface-flow constructed wetland (6.0 m × 1.0 m × 0.5 m) in Leipzig, Germany. The bed contained glacial gravel (4–8 mm) planted with Phragmites australis. Construction was completed in October 2013 and experiments commenced in August 2015. During establishment, the system was fed with only municipal tap water (165 L∙d⁻¹). The Phragmites root system had penetrated to the bottom of the wetland within 18 months. To break into the constructed wetland ‘black box’, the system was divided into a three-dimensional grid of sample ports. Initially, the wetland was physicochemically characterized (prior to addition of nutrients from an external source) in order to quantify the natural, baseline state. Thereafter, an impulse-response tracer test was conducted, using a fluorometer, for continual measurement of uranine concentration. 100% tracer recovery was achieved. The RTD was multi-modal – indicating by-pass flow – and showed long tailing due to mixing, diffusive effects and dead zones. Kinetic performance was investigated via monitoring total organic carbon and total nitrogen degradation, with a continual feed of artificial domestic wastewater (110 mg∙L⁻¹ COD). 93% reduction in TOC and TN was achieved for 5 weeks (11 November – 08 December 2015), despite high inflow loading (69.9 g∙m⁻³∙d⁻¹ TOC; 28.1 g∙m⁻³∙d⁻¹ TN) and colder temperatures. There was a general decline in reaction rate and rate constant from late October to early December. The average rates of TOC and TN removal were 65.08 ± 2.16 g∙m⁻³∙d⁻¹ and 26.22 ± 0.68 g∙m⁻³∙d⁻¹, respectively (Tanks-In-Series model). These results are the first set in a series. Continual observation and repetition of these experiments into long-term operation will deepen understanding of the internal development and performance of constructed wetlands, as is in line with the biomimetic approach, and provide the basis of a framework for improved wetland design.