Physico-chemical screening of phosphate-removing substrates for use in constructed wetland systems

Abstract

The objective was to provide selection criteria for substrates that would enhance phosphate removal from waste water in a constructed wetland system (subsurface horizontal flow). Measured properties of seven substrates (bauxite, shale, burnt oil shale, limestone, zeolite, light expanded clay aggregates (LECA) and fly ash) were: pH, cation exchange capacity (CEC), hydraulic conductivity, porosity, specific surface area, particle size distribution and phosphate (P) adsorption capacity. Fly ash and shale had the highest P adsorption values, followed by bauxite, limestone and LECA. Longer-term experiments in which synthetic waste water was passed over shale and bauxite gave maximum P uptake values of 730 and 355 mg P kg⁻¹, respectively. X-ray fluorescence measurements showed that substantial precipitation of P had occurred on the shale surfaces. On the basis of these measurements it was concluded that, of the seven materials examined, shale had the best combination of properties as a substrate for constructed wetland systems (CWS).

Introduction

Recent work has emphasised the potential importance of natural and constructed wetland systems (CWS) for purifying waste water. These systems are useful for reducing biochemical oxygen demand (BOD), suspended solids (SS), ammonium (N) and phosphorus (P) in sewage (Moshiri, 1993; Kadlec and Knight, 1996; Vymazal et al., 1998). Such systems are becoming widely accepted as a suitable low-cost technology for small to medium-sized communities (Moshiri, 1993; Vymazal et al., 1998). Studies of the removal of P by wetland systems have been carried out in the U.S.A. (Moshiri, 1993; Kadlec and Knight, 1996), Australia (Mann and Bavor, 1993) and several countries in Europe: Denmark (Schierup et al., 1990), Norway (Zhu et al., 1997), U.K. (Green, 1997), Czech Republic (Vymazal, 1995) and more recently, Sweden (Johansson, 1997) and the Netherlands (Schreijer et al., 1997).

Results from several studies have shown that immobilisation of P in CWS occurs through substratum adsorption, chemical precipitation, bacterial action, plant and algal uptake and incorporation into organic matter (Kadlec and Knight, 1996; Vymazal et al., 1998). Of these, the substrate may play the greatest role, and could very well be the factor that is most amenable to control. Consequently, it is important to select those substrates with the highest P adsorption capacity, which is dependent upon chemical and physical properties of the material (Zhu et al., 1997; Drizo, 1998). Such materials might include minerals with reactive Fe or Al hydroxide or oxide groups on their surfaces, or calcareous materials which can promote Ca phosphate precipitation (Drizo et al., 1997; Johansson, 1997; Zhu et al., 1997). Apart from Fe, Al and Ca minerals, the rate of P adsorption is controlled by substrate E_h, pH and adsorptive surface area (Vymazal et al., 1998). Fine grained materials have large surface areas and therefore the potential to enhance P adsorption capacity (Zhu et al., 1997). However, such materials often have low hydraulic conductivity which leads to the occurrence of overland flows and insufficient contact between wastewater and substrate within the CWS (Kadlec and Knight, 1996; Drizo, 1998). Therefore, the materials should be sufficiently permeable to prevent surface channelling of the CWS (Kadlec and Knight, 1996). In addition, they should be cheap and locally available in order to reduce the costs of CWS construction.

This paper describes investigations of a range of candidate substrates, all meeting the above criteria. Comparison of seven materials is presented, based on measurements of pH, cation exchange capacity (CEC), hydraulic conductivity, porosity, specific surface area and particle size distribution, and phosphate adsorption capacity. These parameters all influence the effectiveness of a substrate for the removal of phosphates, on a short-term basis. Also, to estimate the capacity of substrates, laboratory investigations of the P saturation point are reported. Finally, X-ray fluorescence spectrometric analysis was used to demonstrate the presence of P on the surface of the materials. The properties investigated in the later two experiments influence the effectiveness of the substrate for the removal of phosphates, on a long term basis.