Functions of slags and gravels as substrates in large-scale demonstration constructed wetland systems for polluted river water treatment

Abstract

The choice of substrates with high adsorption capacity, yet readily available and economical is vital for sustainable pollutants removal in constructed wetlands (CWs). Two identical large-scale demonstration horizontal subsurface flow (HSSF) CWs (surface area, 340 m(2); depth, 0.6 m; HLR, 0.2 m/day) with gravel or slag substrates were evaluated for their potential use in remediating polluted urban river water in the prevailing climate of northwest China. Batch experiments to elucidate phosphorus adsorption mechanisms indicated a higher adsorption capacity of slag (3.15 g/kg) than gravel (0.81 g/kg), whereby circa 20 % more total phosphorus (TP) removal was recorded in HSSF-slag than HSSF-gravel. TP removal occurred predominantly via CaO-slag dissolution followed by Ca phosphate precipitation. Moreover, average removals of chemical oxygen demand and biochemical oxygen demand were approximately 10 % higher in HSSF-slag than HSSF-gravel. Nevertheless, TP adsorption by slag seemed to get quickly saturated over the monitoring period, and the removal efficiency of the HSSF-slag approached that of the HSSF-gravel after 1-year continuous operation. In contrast, the two CWs achieved similar nitrogen removal during the 2-year monitoring period. Findings also indicated that gravel provided better support for the development of other wetland components such as biomass, whereby the biomass production and the amount of total nitrogen (TN; 43.1-59.0 g/m(2)) and TP (4.15-5.75 g/m(2)) assimilated by local Phragmites australis in HSSF-gravel were higher than that in HSSF-slag (41.2-52.0 g/m(2) and 3.96-4.07 g/m(2), respectively). Overall, comparable pollutant removal rates could be achieved in large-scale HSSF CWs with either gravel or slag as substrate and provide a possible solution for polluted urban river remediation in northern China.

Wetlands for wastewater treatment and subsequent recycling of treated effluent: a review

“…Wetland media could be sand, gravel, rock, or organic material such as soil and compost, which provide the primary support for the wetland plants and microorganism growth, enhancing biodegradation of wastewater pollutants in addition to its impact on system hydrology mechanisms (Tietz et al 2007 ; Meng et al 2014 ). Moreover, wetland substrates remove contaminants from the wastewater by ion exchange, adsorption, precipitation, and complexation (Dordio and Carvalho 2013 ; Ge et al 2015 ), enhancing the effluent quality to meet reuse standards in agriculture. However, the chemical composition of wetland substrate can affect the system efficiency.…”

Domestic Wastewater Depuration Using a Horizontal Subsurface Flow Constructed Wetland and Theoretical Surface Optimization: A Case Study under Dry Mediterranean Climate

“…However, the only sustainable mechanism for TP removal is plant uptake and subsequent harvesting. It has been accepted traditionally that the amount of P that can be abated by harvesting constitutes only an insignificant fraction of the amount of the P load in the system [58], but recently it has been demonstrated that a well-developed Phragmites australis can assimilate up to 75.1% of total dissolved P influent [14].…”

“…When P abatement is required, it is necessary to incorporate an alternative treatment. Different alternatives like the use of slags, which have been studied recently, remove P mainly via CaO-slag dissolution followed by Ca 2+ phosphate precipitation [14] or the use of an "irregularly shaped" HF-CW as described by Gorra et al [65].…”

A review on the removal of conditioning chemicals from cooling tower water in constructed wetlands

“…Adsorption has been shown to be an important removal mechanism for antiscalants and corrosion inhibitors. The CW substrate composition defines the adsorption capacity of the CW (Ge et al, 2015;Sakadevan & Bavor, 1998). recently published an overview of commonly used substrates to enhance adsorption processes in CWs, such as natural clay minerals, industrial by-products and activated carbon.…”