Fragmented limestone wastes as a constructed wetland substrate for phosphorus removal

Abstract

This study investigated the potential of fragmented Moleanos limestone (FML) as substrate in constructed wetlands for phosphorus removal. Being a waste material which results from industries of extraction and production of decorative stones for construction activities, FML is quite inexpensive. Laboratory experiments were performed in a packed bed column with recycle to test the FML's capacity to sorb phosphorus from a phosphate solution. The experimental data were found to follow the Langmuir model better than the Freundlich model. From the isotherm parameters obtained it can be concluded that the sorption capacity of FML is similar to those quoted in the literature for mineral materials. A subsurface flow constructed wetland filled with FML and planted with Phragmites australis was monitored during 20 months, at a mean hydraulic load of 40 ± 4 L/(m² day). Results demonstrated that average phosphorus removal efficiencies of 61% ± 7 can be achieved during the trial period, and a global first order kinetic constant of 0.062 ± 0.001 m/day was fitted to experimental data using the well mixed serial tanks model. Morphological aspects of reed growth were also studied and indicated high plant yield. The use of FML as a substrate media in constructed wetlands can be a promising solution in developing a cost effective, environmental friendly and sustainable technology for phosphorus removal, particularly where this material is readily available.

Introduction

The use of constructed wetlands (CW) for wastewater treatment has become common practice worldwide, including wastewater treatment in small agglomerations, treatment of specific industrial waste material or as a tertiary (advanced) treatment in urban wastewater treatment plants. As for the application of CW as an advanced treatment, the usual aim is to remove nutrients (phosphorus and nitrogen compounds) which are detrimental to the balance of more sensitive receiving media where such effects as euthrophication may occur. The processes of removing nutrients through CW include adsorption by the support matrix (bed filling), precipitation, sedimentation and stabilisation through plant and microbial growth (Kadlec and Wallace, 2008, Vohla et al., 2011, Vymazal, 2007). In the particular case of phosphorus compounds removal, the appropriate choice of filling material is a key factor, since the physical-chemical processes such as phosphate precipitation and adsorption of phosphorus compounds on its surface are pointed out as important phosphorus (P) removal mechanisms. The needs for high capital investment resulting from the cost of the filling material as well as the need for large installation areas have been the major obstacles against the development and use of this environment friendly technology (Guan et al., 2009, Valipour et al., 2009). The most cost-effective materials are usually found among industrial waste and by-products as well as among natural minerals. This investigation is thus pertinent as it involves evaluation of the sorption ability of fragmented Moleanos limestone (FML) and verification of its technical viability as a filling material to be used in constructed wetlands. The Moleanos limestone is used as a decorative material in civil construction, extraction and processing of this rock produce huge amounts of waste materials every year which are used in civil engineering works or deposited in landfills. The use of this material for CW filling, especially if CW are located in the neighbourhoods of extraction or processing sites, constitutes a valuation of debris and an economical advantage for the implementation of CW.