Experiences with a top layer of gravel to enhance the performance of vertical flow constructed wetlands at cold temperatures

摘要

In a first phase of this study it was shown that the Austrian effluent standards for organicnmatter could not be met in winter for vertical flow (VF) beds designed for and loaded with 27 gnCOD.m22n.d21n(3m2nper person equivalent). The aim of this second phase of the study was toninvestigate, if the performance of a constructed wetland can be enhanced, i.e. if the effluentnrequirements can be met, when an additional gravel layer (15 cm, 4–8mm) is added on top ofnthe main layer of the VF bed. The hypothesis was that this top layer would increase the thermalninsulation and consequently the temperatures in the filter bed during cold periods, thus resultingnin higher removal efficiencies during winter. Two VF beds were operated in parallel; one bednwith such a 15 cm top layer, one without. Otherwise the construction of both beds wasnidentical: surface area of about 20m2n, 50 cm main layer (grain size 0.06–4mm, d10 ¼ 0.2mm;nd60 ¼ 0.8mm), planted with common reed (Phragmites australis). The beds were intermittentlynloaded 4 times per day with mechanically pre-treated wastewater (hydraulic loading: 47mm.d21n;nmedian value of the influent concentration: 505mg COD.L21n). Despite a better performancenduring the first winter, the bed with additional top layer showed in general a very unstablenperformance. It is assumed that the main reason for this was that the oxygen transfer wasnreduced by the additional top layer so far that suspended organic matter could not anynlonger be degraded in between loadings. Therefore clogging of the filter occurred.