Phosphorus Saturation Potential:  A Parameter for Estimating the Longevity of Constructed Wetland Systems

Abstract

Phosphorus (P) adsorption capacities of materials derived from batch experiments can vary by several orders of magnitude depending on the method used, leading to potential misinterpretation of the P retention capacity on a long-term basis and unrealistic estimations of constructed wetland systems (CWS) longevity. The objective of this study was to determine if the P saturation of the material in a column could be used for this purpose with an improved accuracy. A 278-d column experiment with a synthetic P solution was conducted to investigate the long-term P retention capacity of electric arc furnace (EAF) steel slag up to its P saturation point. EAF slag showed a high affinity for P, reaching a saturation value of 1.35 g of P kg(-1). Investigations of the regeneration of the P adsorbing capacity by this material showed that, after 4 weeks of water desaturated resting, EAF steel slag was able to increase its initial P adsorptive capacity to 2.35 g of P kg(-1). A sequential P fractionation experiment was performed to quantify the proportion of P bound to mineral compounds in EAF. From the most loosely bound to the most strongly bound P fraction, P was associated with resin extractable (14%), Fe extractable (0.5 M Na2CO3, 47%), Al extractable (0.1 M NaOH, 1%), Ca extractable (1 M HCl, 12%), and Ca in a stable residual pool (concentrated hot HCl, 26.5%). X-ray fluorescence analyses of EAF steel slag chemical composition revealed that the continuous application of a P solution resulted in 75% and 59% increases in K2O and P2O5 respectively; Al2O3 and FeO increased by 8%, while the portion of CaO remained unchanged. The investigated properties (P retention potential, regeneration of P adsorption, P fractionation) provide useful data about the suitability of slag material as a media for long-term P removal and should enable an improved prediction of the longevity of full-scale CWS.

Enhanced phosphorus removal in a waste stabilization pond system with blast furnace slag filters

“…Considering that the P adsorption capacity of BFS correlates significantly with media chemical composition (e.g., Ca, Mg, S and Si) and operational conditions (e.g., pH) [13], it seams that P adsorption capacity of the media should be tested for each particular case, in order to determine the longevity of the BFS filters P-removal capacity. However, there is not a reliable laboratory-based method for predicting longevity of filters and further work is needed in this area [14].…”

Potential and limitations of phosphate retention media in water protection: A process-based review of laboratory and field-scale tests

“…Further evidence in support of this proposed mechanism was that the retention occurred at alkaline pH, and the dissolution of Ca from the material occurred during the experiment. Drizo et al (2002) investigated the P removal efficiency of electric arc furnace slag by packing the material in a column (height 15 cm, internal diameter 10.3 cm) with a pore volume of 0.6 l. A phosphate solution of 350-400 mg P l -1 was passed through the column at a percolation rate of 1.73 l d -1…”

The Use of Blast Furnace Slag for Removal of Phosphorus from Wastewater in Sweden—A Review

“…For over a decade, Drizo et al [22] have been advocating that estimating the maximum P adsorption capacity of the filter media using standard 24 h batch experiment and Langmuir and Freundlish isotherms leads to unrealistic estimates of the materials P adsorption capacities. Yet, researchers continue to employ this method, which results in frequent failures in the field [22].…”

“…In this case, melter slag was used in a full-scale treatment plant in New Zealand. It is however difficult to compare these results since different researchers have performed their investigations in different ways, a fact that has been stressed by Drizo et al [22] and Johansson Westholm [30]. …”