Accumulation of organic solids in gravel-bed constructed wetlands

Abstract

The effects of wastewater loading rate and planting with Schoenoplectus validus (soft-stem bulrush) on the accumulation of organic matter were investigated in four pairs of gravel-bed constructed wetlands. The planted and unplanted wetland pairs, each supplied with a different hydraulic loading of dairy farm wastewaters pre-treated in an anaerobic and aerobic lagoon, had received cumulative suspended solids loadings (∼ 82% volatile) of between ∼ 1.6 and 5.4 kg m−2 over a 22 month period. Vertical and horizontal gradients of organic matter accumulation were sampled by stratified coring, and the impact of solids accumulations on wastewater residence times investigated using bromide as a conservative tracer. Mean accumulations of organic matter in the unplanted wetlands ranged between 0.4 and 2.3 kg m−2, while those in the planted wetlands reached mean levels of nearly 4 kg m−2. Highest levels were recorded in influent zones (up to 9.5 kg m−2) and in the upper 100 mm of the substratum. The effective porosity of the highest loaded wetlands was markedly reduced compared to that in the lowest loaded wetlands, with mean retention times decreasing to around half of their theoretical values (corrected for evapotranspirational water losses). The planted wetlands retained higher apparent gravel porosity, despite greater accumulations of organic matter. High evapotranspiration rates during hot summer days, markedly extended retention times and increased tracer dispersion.

Phosphorus Accumulation Pattern in a Subsurface Constructed Wetland Treating Residential Wastewater

“…In agreement with findings reported by Tanner et al [39] and Tang et al [40], the OM accumulation was highest in the inlet zone of the Sadowa wetland, but in some cases the OM content also increased in the outlet zone of the HSFCW (Figure 3). Contrary to our results, Tanner and Sukias [41] observed higher OM accumulation in the top layer (<10 cm) of planted and unplanted constructed wetlands. Drizo et al [42] found a significant inverse relationship between P concentration in wastewater flowing through a bed planted with macrophytes and distance from the inlet.…”

Evaluation of Organic Matter Removal Efficiency and Microbial Enzyme Activity in Vertical-Flow Constructed Wetland Systems

“…Fan et al found that plants did not play an important role in the removal of organic carbon [24]. However, Tanner and Sukias compared accumulation rates of organic matter in planted versus unplanted treatment beds, and found that the accumulation rates were higher in the planted systems [7].…”

“…There are some disputes on the function of wetland plants. Many studies have reported the importance of the presence of vegetation in subsurface flow-constructed wetlands, which might affect the development of clogging [7,8]. However, some studies have suggested that plants could benefit the Sub-surface Flow Constructed Wetlands (SSFCWs) by making the substrate more porous to increase the hydraulic conductivity [9,10].…”

Characterization of Clogging Material From Horizontal Subsurface Flow Constructed Wetland Systems

“…The highest thicknesses were observed in CWs cultivated with alligator weed. Some authors have shown the presence of this layer with thicknesses ranging from 10 to 30 cm (TANNER & SUKIAS, 1995;ALCABAR, 2010;KNOWLES et al, 2011). Much has been studied regarding the functions and benefits of plants in wastewater treatment: removal of nutrients, oxygen transfer to the substrate, support for the growth of biofilm of bacteria (rhizomes and roots), as well as improvement in substrate permeability (KADLEC & WALLACE, 2008).…”