Elsevier

Water Research

Volume 43, Issue 8, May 2009, Pages 2240-2250
Water Research

Removal of phosphorus from agricultural wastewaters using adsorption media prepared from acid mine drainage sludge

https://doi.org/10.1016/j.watres.2009.02.010Get rights and content

Abstract

Excess phosphorus in wastewaters promotes eutrophication in receiving waterways. A cost-effective method for the removal of phosphorus from water would significantly reduce the impact of such wastewaters on the environment. Acid mine drainage sludge is a waste product produced by the neutralization of acid mine drainage, and consists mainly of the same metal hydroxides used in traditional wastewater treatment for the removal of phosphorus. In this paper, we describe a method for the drying and pelletization of acid mine drainage sludge that results in a particulate media, which we have termed Ferroxysorb, for the removal of phosphorus from wastewater in an efficient packed bed contactor. Adsorption capacities are high, and kinetics rapid, such that a contact time of less than 5 min is sufficient for removal of 60–90% of the phosphorus, depending on the feed concentration and time in service. In addition, the adsorption capacity of the Ferroxysorb media was increased dramatically by using two columns in an alternating sequence so that each sludge bed receives alternating rest and adsorption cycles. A stripping procedure based on treatment with dilute sodium hydroxide was also developed that allows for recovery of the P from the media, with the possibility of generating a marketable fertilizer product. These results indicate that acid mine drainage sludges – hitherto thought of as undesirable wastes – can be used to remove phosphorus from wastewater, thus offsetting a portion of acid mine drainage treatment costs while at the same time improving water quality in sensitive watersheds.

Introduction

Eutrophication is a serious water pollution concern and phosphorus (P) is one of its main causes (U.S. Geological Survey, 1999). Legislation limiting P use in household products and mandating treatment of industrial and municipal wastewaters has decreased P released from these sources (Litke, 1999). One of the largest remaining P sources is agriculture, especially with the advent of large-scale animal feeding operations (Sharpley et al., 1999). The largest of these animal feeding operations are now regulated for point source discharges under the National Pollutant Discharge Elimination System (NPDES) (U.S. EPA, 2003). An efficient and economical method of P removal from these agricultural wastewaters is critical to their long-term sustainability and would significantly decrease the flow of P into the environment.

Phosphorus is typically removed from municipal and industrial wastes through the addition of aluminum or iron salts such as alum. These salts precipitate when mixed with neutral waters to form a heavy floc blanket that settles through the water column. Phosphorus removal occurs by a combination of mechanisms, including adsorption by the aluminum or iron oxide floc, and direct precipitation of aluminum or iron phosphate. For complete removal of P, two to three times the stoichiometric requirement of aluminum or iron is usually needed (Metcalf and Eddy, Inc, 1991). Based on recently published costs for alum and iron salts (Chemical Market Reporter, 2006), the cost of these chemicals generally prevents the use of this technology for many agricultural wastewaters. Calcium compounds such as lime are sometimes used for P removal, but require an elevated pH to achieve good P removal, which is not cost-effective for large flows to be discharged into the environment. In the treatment of municipal wastewaters by biological processes, P serves as an essential nutrient for the growth of the microorganisms. Therefore, P concentration in these wastewaters is typically elevated, although the process can be optimized for removal of P (U.S. EPA, 2007). However, it is often more common to add a tertiary chemical treatment for removal of P, such as iron or aluminum salts.

Because of the drawbacks of traditional P removal technology, several investigators have developed novel compounds for the removal of P from wastewaters. Gel-based adsorbents have been developed for use in aquaculture applications (Kioussis et al., 1999), and were found to be reusable for several cycles of sorption/regeneration. Blaney et al. (2007) describe a P sorption media consisting of iron oxide nanoparticles embedded in an anion exchange resin that apparently shows favorable P sorption capacities and rates. Genz et al. (2004) investigated the use of granulated ferric hydroxide and activated alumina of P removal from membrane bioreactor effluents. Granulated ferric hydroxide was found to show a higher capacity than the alumina for P. Although each of these materials showed promise for P removal, the cost of the materials would be expected to be relatively high, as they were all prepared from chemical reagent sources.

Due to the cost of using commercial reagents for P removal, several research groups have investigated the use of raw minerals and industrial waste products for P sequestration. These include calcium and iron oxides from steelmaking slags (Drizo et al., 2006), calcium and aluminum silicate minerals (Gustafsson et al., 2008), iron and aluminum oxides from water treatment residuals (Makris et al., 2004), and coal combustion fly ash (O'Reilly and Sims, 1995). These wastes are high in compounds such as iron, aluminum and calcium oxides that have an affinity for P. However, these waste products may not be readily available in many geographical areas, and transport costs could prevent their utilization. An alternative source of iron and aluminum oxides that is widely available in coal and metal mining regions is the waste generated by the neutralization of acid mine drainage.

Acid mine drainage (AMD) is formed by the oxidation of sulfur in minerals associated with coal and metal deposits, such as pyrite (FeS2), to form sulfuric acid (Stumm and Morgan, 1996). The acid then solubilizes other metals present in the host rock, usually including aluminum and manganese. Typical treatment for AMD flows is neutralization with alkaline materials, such as limestone, lime, caustic or ammonia (Evangelou and Zhang, 1995). The neutralization of the acid results in precipitation of iron and aluminum as metal hydroxide sludges with high water content. Collection and disposal of these waste sludges incur a major operating cost for AMD treatment facilities. Development of alternate uses such as P sequestration for the waste sludge would decrease AMD treatment costs as well as prevent release of P into the environment.

The use of AMD sludges for P sequestration has recently been investigated by the U.S. Department of Agriculture – Agricultural Research Service and the U.S. Geological Survey (Adler and Sibrell, 2003). Test results showed that the sludges were effective at P removal from both aerated and oxygen-deficient waters. The sludge was also effective as a soil amendment – addition of a few percent sludge to a high-P soil decreased the water extractable P by 90%. Other investigators have also examined the use of AMD sludges through various contacting strategies for the removal of P from wastewaters. Evenson and Nairn (2000) described treatment of a wastewater in a wetland enriched with AMD sludge products. Wei et al. (2008) mixed AMD sludge as a slurry with wastewater to effect P removal and found good uptake of the P, but a solid/liquid separation step was needed before releasing the treated water. Penn et al. (2007) considered several phosphorus sorbing materials and selected acid mine drainage residuals for treatment of agricultural ditch water. Recent studies in the authors' laboratory have demonstrated that many AMD sludge materials, once dried, will hold their shape when rewetted. Thus these materials would be utilizable in packed bed contactors for direct water treatment. Drying of the sludge at or near the AMD treatment site would also markedly decrease the transport cost of the sludge media, due to the significant reduction in weight that would be obtained. A patent has recently been granted in the United States for this technology (Sibrell, 2007). The purpose of this paper was therefore to further investigate the use of dried AMD sludge (Ferroxysorb) as an adsorption media for the removal of P from wastewaters. Specifically, P adsorption capacities and removal rates were determined for several different sludge sources, with varying solution compositions and temperatures. We also investigated wastewater column treatment and stripping of the sorbed P from the Ferroxysorb media.

Section snippets

AMD sludges tested

AMD sludges from six different coal mine drainage sites in Pennsylvania were tested. Sites for this study were selected because they represent a wide range of water chemistry and treatment conditions. Coal mine discharges were preferred because the concentrations of impurities such as arsenic, cadmium, copper, and zinc are generally lower in coal mine discharges than those from metal mines. Therefore, the resultant sludges would be expected to be less likely to cause contamination of the

P sorption

Based on earlier published results, most AMD sludges containing aluminum and iron hydroxides have a strong affinity for P in water solutions (Adler and Sibrell, 2003). Adsorption isotherms for the removal of P from water with three of the sludge samples described earlier are shown in Fig. 1. The removal of P from aqueous solution by the sludge is consistent with an adsorption process, as shown in Fig. 1, where the data have been fitted to a Freundlich adsorption isotherm according to the

Summary and conclusions

The use of Ferroxysorb media prepared from AMD sludges for the removal of P from agricultural wastewaters was investigated. Six different AMD sludges were tested and showed that P sorption capacity and kinetics were influenced by sludge composition and physical properties. Sludges containing a combination of both Fe and Al performed better than other types tested. The amount of contamination of the sludge with soil from the sludge pond liner also influenced sorption capacity by diluting the

Acknowledgements

We would like to thank the following people for their assistance in procuring samples of sludge from acid mine drainage facilities. Bill Beacom of the Babb Creek Watershed Association sampled the Babb Creek facility, Brent Means of the Office of Surface Mining arranged for sampling of Glen White and Ace treatment systems, and Wade Cowder of the Pennsylvania Department of Environmental Protection arranged for sampling of the Toby Creek and Brandy Camp systems.

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