Innovative Strategies to Achieve Low Total Phosphorus Concentrations in High Water Flows

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Innovative Strategies to Achieve LowTotal Phosphorus Concentrations in HighWater FlowsBrooke K. Mayer a b , Daniel Gerrity b c , Bruce E. Rittmann b d ,Daniel Reisinger e & Sherry Brandt-Williams fa Civil, Construction and Environmental Engineering, MarquetteUniversity, Milwaukee, Wisconsin, USAb School of Sustainable Engineering and the Built Environment,Arizona State University, Tempe, Arizona, USAc Department of Civil and Environmental Engineering andConstruction, University of Nevada Las Vegas, Las Vegas, Nevada,USAd Swette Center for Environmental Biotechnology, BiodesignInstitute, Arizona State University, Tempe, Arizona, USAe Camp Dresser McKee, Denver, Colorado, USAf St. John's River Water Management District, Palatka, Florida, USAAccepted author version posted online: 22 Mar 2012.Publishedonline: 05 Feb 2013.

To cite this article: Brooke K. Mayer , Daniel Gerrity , Bruce E. Rittmann , Daniel Reisinger & SherryBrandt-Williams (2013): Innovative Strategies to Achieve Low Total Phosphorus Concentrations in HighWater Flows, Critical Reviews in Environmental Science and Technology, 43:4, 409-441

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Critical Reviews in Environmental Science and Technology, 43:409–441, 2013ISSN: 1064-3389 print / 1547-6537 onlineDOI: 10.1080/10643389.2011.604262

Innovative Strategies to Achieve LowTotal Phosphorus Concentrations

in High Water Flows

BROOKE K. MAYER,1,2 DANIEL GERRITY,2,3 BRUCE E. RITTMANN,2,4

DANIEL REISINGER,5 and SHERRY BRANDT-WILLIAMS6

1Civil, Construction and Environmental Engineering, Marquette University, Milwaukee,Wisconsin, USA

2School of Sustainable Engineering and the Built Environment, Arizona State University,Tempe, Arizona, USA

3Department of Civil and Environmental Engineering and Construction, University ofNevada Las Vegas, Las Vegas, Nevada, USA

4Swette Center for Environmental Biotechnology, Biodesign Institute, Arizona StateUniversity, Tempe, Arizona, USA

5Camp Dresser McKee, Denver, Colorado, USA6St. John’s River Water Management District, Palatka, Florida, USA

Eutrophication caused by excess phosphorus (P) loading poses aserious environmental risk to freshwater bodies around the world.While conventional P-removal technologies often satisfy maximumeffluent levels of 1,000 μg-P/l, the resulting environmental P con-centrations can still contribute to eutrophication. The challengeremains to achieve low total P levels of ≤ 10 μg-P/l in very largewater flows. This issue is often exacerbated by the presence of unre-active organic phosphorus. The authors critically assess innovativedevelopments in advanced oxidation, adsorption, biological up-take, and ion exchange for their ability to achieve very low total Pconcentrations in high-flow systems. Adsorption appears to have thegreatest potential for near-term implementation. Biological uptakeand ion exchange show promise based on laboratory-scale researchand may be long-term options. Pretreatment using advanced oxi-dation may be valuable in converting organic P to the more readilyremovable orthophosphate form.

This article not subject to US copyright law.Address correspondence to Brooke K. Mayer, Construction and Environmental Engineer-

ing, Marquette University, P.O. Box 1881, Milwaukee, Wisconsin, 53201-1881, USA. E-mail:[email protected]

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410 B. K. Mayer et al.

KEY WORDS: adsorption, biological uptake, eutrophication, ionexchange, phosphate, phosphorus

1. INTRODUCTION

Phosphorus (P) is considered the limiting nutrient for phototrophic growth inmost fresh waters (Ragsdale, 2007; Wetzel, 1983). In excess, P accelerates thegrowth of phytoplankton (suspended algae and cyanobacteria) to extraor-dinary levels, which can lead to eutrophication (Organization for EconomicCooperation and Development, 1982; U.S. Environmental Protection Agency[USEPA], 1983). Eutrophication, or excessive nutrient enrichment, is a seriousglobal concern with vast ramifications, including additional drinking watertreatment, decreased biodiversity, shifts in species composition and domi-nance, limited recreational use, loss of fisheries, and restricted navigation(USEPA, 1998).

Waters worldwide are experiencing major increases in P concentra-tions. For example, P fluxes to oceans have increased approximately 2.8-foldsince the Industrial Revolution (Bennett et al., 2001), and over 400 coastaldead zones can be found at the mouths of rivers discharging P (Diaz andRosenberg, 2008; Selman et al., 2008). Surveys in the United States and theEuropean Union (EU) estimate that 78% and 65% of their coastal areas, re-spectively, exhibit symptoms of eutrophication. Inland waters are equallyat risk. According to the U.S. Environmental Protection Agency (1998), eu-trophication is the biggest overall source of impairment of the nation’s riversand streams, lakes and reservoirs, and estuaries. Economic damages from theeutrophication of freshwaters in the United States alone have been estimatedat $2.2 billion annually (Dodds et al., 2009). In the EU, approximately 50% ofall lakes have total P (TP) concentrations in excess of 25 μg-P/l (Bogestrand,2004), which may pose a risk of eutrophication.

U.S. guidelines strive to limit eutrophication by establishing maximumTP concentrations of 50–100 μg-P/l in streams and 25 μg-P/l in lakes andreservoirs (USEPA, 1986). According to an international assessment by theOrganization for Economic Cooperation and Development (OECD; 1982),the trophic state of a lake or reservoir is determined by a combination ofits detention time and influent TP concentration. For example, a lake witha detention time of 10 years would likely remain oligotrophic if its inflowconcentration were less than about 25 μg-TP/l, while it would be at risk ofeutrophication if its inflow concentration were around 100 μg-TP/l.

Since P concentrations as low as 20 μg-P/l may stimulate algal pro-duction (Dillon and Rigler, 1974; Schindler, 1977), water bodies whose olig-otrophic status is of high importance may require limits much lower thantypical regulations or guidelines. For example, the maximum TP concentra-tion allowed in the Florida Everglades is only 10 μg-P/l (Everglades Forever

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Strategies to Achieve Low Total P Concentrations 411

Act, 1994). The same limitation applies to discharges to the Spokane River(Blaney et al., 2007). Likewise, guidelines for the Great Lakes range from 5to 15 μg-TP/l (Nielson et al., 1995). According to the international guidelinesof the OECD (1982), oligotrophic lakes with detention times <1 year mayrequire inflow TP concentrations less than about 15 μg-P/l.

Sources of P include runoff from urban and agricultural lands, naturalcontributions from soil and atmospheric deposition, and point sources suchas municipal and industrial wastewater treatment plants (WWTPs; Aertebjerget al., 2001; Selman et al., 2008; USEPA, 1983). Efforts have generally focusedon implementing increased treatment and/or source control to reduce P con-centrations in wastewater effluents. For example, the use of P-based productssuch as detergents, fertilizers, toothpaste, pesticides, corrosion inhibitors, andflame retardants has been restricted (Osmond et al., 1995; Woods et al., 2008).Efforts to limit P discharges through source reductions as well as advancedwastewater treatment have lowered environmental P concentrations (Etnieret al., 2005). To meet typical eutrophication thresholds, advanced wastew-ater treatment must be used to achieve effluent concentrations from 100 to2,000 μg-P/l (Blaney et al., 2007; Hammer and Hammer, 1996; Urgurlu andSalman, 1998). Despite having low TP concentrations (<100 μg-P/l), someP-limited environmental waters are still susceptible to algal blooms.

It is widely recognized that conventional treatment strategies, includ-ing adsorption, biological nutrient removal, precipitation, and crystallization,are unable to reduce P concentrations below approximately 100 μg-P/l dueto thermodynamic and kinetic limitations (Blaney et al., 2007; Cooper et al.,1993; Jenkins et al., 1971; Jenkins and Hermanowicz, 1991; Kuba et al., 1993).Thus, meeting a more stringent standard for P removal appears to be impos-sible with current technology. Furthermore, conventional processes performpoorly when the P concentration is initially low (<100 μg-P/l) in influentflows (Kuroda et al., 2000; Sedlak, 1991) and when TP is comprised of asignificant organic phosphorus (OP) component (Heerboth, 2007; McKelvie,2005; Murphy, 2007; Turner et al., 2005).

Scenarios with high process flow rates (e.g., large rivers) and low ini-tial P concentrations present a considerable environmental technology chal-lenge. Any water user that takes in and returns large volumes of water maydischarge a high TP load, even if the concentration is at typically low am-bient water levels. Power plants, irrigators, and water-management districtspresent examples of high-flow dischargers. Because such flows contributelarge volumes to freshwater systems, they yield relatively high environmen-tal P loadings unless the TP concentration is reduced to a very low level(e.g., ≤ 10 μg-P/l). Conventional P-removal technologies cannot achieve thisgoal, and the problem is exacerbated when the water contains significant OP.

The authors of the current study are associated with a particular exam-ple, the St. Johns River Water Management District (SJRWMD) in northeasternFlorida. The SJRWMD is one of several agencies responsible for a cumulative

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annual removal goal of 84 metric tons of TP from the St. Johns River Basin,as set forth in the St. Johns River Algal Initiative (Reisinger et al., 2008).Treatment strategies for the SJRWMD must account for high surface waterflows (≈ 50 MGD or 2.6 m3/s) and low ambient TP concentrations (≈ 90 μg-P/l), the majority of which is OP. Because conventional technology cannotachieve a very low TP concentration in a very large flow, we investigated awide range of innovative alternatives.

This article provides a comprehensive, critical review of innovative P-removal technologies described in peer reviewed literature and patents orcommercially available. The technologies were evaluated on the basis of thefollowing criteria: (a) applicable to treatment of high-flow surface water sys-tems, (b) able to remove TP to less than 10 μg-P/l, (c) able to remove OP,and (d) likelihood of commercial availability. The technologies addressedin this review employ a variety of removal mechanisms, including physico-chemical adsorption, biological removal, ion exchange, advanced oxidation,and a combination thereof. Many of the processes have been tested at thelaboratory- or pilot-scale, but have yet to be demonstrated at a larger scale.Several technologies have been used to treat other contaminants (e.g., salin-ity, organic carbon, organopesticides) or in alternative applications, but thesenovel approaches may also have potential in P removal applications.

Traditional P-removal strategies with a significant body of publishedresearch (summarized in the Appendix) were considered for context, butwere not included in this review as they cannot meet the established treat-ment goals. These traditional technologies commonly target effluent con-centrations of 1,000–2,000 μg-P/l, with best available technologies achieving100 μg-P/l (Blaney et al., 2007; Hammer and Hammer, 1996).

2. PRETREATMENT BY ADVANCED OXIDATION

Many innovative treatment technologies target orthophosphate removal, buttheir efficacy for OP removal is uncertain. In wastewater, detergents andfertilizer runoff are often responsible for inorganic phosphorus loadings,while human excretions and the degradation of organic pesticides are of-ten responsible for the organic fractions. Relative fractions of OP are widelyvariable in aquatic systems, ranging from low levels of <10% (Ayoub andKalinian, 2006) to being the dominant constituent at >50% (Turner et al.,2005). Yet, OP remains poorly understood and has largely been ignored andgrouped in the nonreactive and nonbioavailable TP component (McKelvie,2005). Accordingly, OP is not targeted by conventional treatment technolo-gies. However, the significant bioavailability of OP and recent evidence thatit is less refractory than previously believed has spawned recognition of itsrole in eutrophication (Heerboth, 2007; McKelvie, 2005).

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Pretreatment with advanced oxidation processes (AOPs) may convertOP to the more readily removable orthophosphate form, thereby achievinghigher TP removal when coupled with an innovative P-removal technology.Organic compounds are quickly degraded using AOPs (e.g., ozone/peroxide,UV/peroxide, titanium dioxide photocatalysis, and Fenton’s reaction) basedon the production of nonspecific free-radical species, such as hydroxyl radi-cals (•OH). In addition to conversion to orthophosphate, pretreatment withAOPs will also increase the bioavailability of the bulk organic matter, whichmay enhance performance of downstream P-removal processes (e.g., in-creasing adsorbent life and reducing membrane fouling). However, severalunresolved issues must be addressed.

First, commercially available AOPs have not been tested for the con-version of P to orthophosphate in complex, heterogeneous organic matter.While AOPs have been evaluated in relation to the destruction of specificP-based contaminants, such as organophosphorus pesticides (Badawy et al.,2006; Daneshvar et al., 2004; Wu et al., 2009), scientific evidence is insuf-ficient to fully characterize the efficacy of converting nonspecific OP to or-thophosphate. Furthermore, the non-specific nature of AOP destruction mayintroduce significant competition between TP and organic content withinwater matrices, thereby reducing the efficiency of OP conversion.

Second, AOPs are generally energy- and cost-intensive processes, whichmay preclude their use solely on the basis of operational costs and/or detri-mental environmental impacts such as carbon emissions. Selective partialoxidation to release orthophosphate could make AOPs more efficient interms of cost and energy, thereby providing a means to improve TP removalin conjunction with an innovative P-removal technology.

Ozone is a powerful oxidant capable of transforming the molecularstructure of organic compounds. Ozonation often increases the bioavailabil-ity and reactivity of organic compounds (Masten and Davies, 1994), whichmay improve P removal. For example, Aquafiber Technologies Corporation(Orlando, FL) has used ozonation as a pretreatment for their biological pe-riphyton P-removal process. Ozone AOPs rely on combinations of ozone,hydrogen peroxide (H2O2), and/or ultraviolet (UV) light to improve treat-ment performance by generating •OH radicals (Masten and Davies, 1994).In addition to ozone, UV photolysis (which is usually insufficient to degradecontaminants on its own) can be supplemented with H2O2 to generate •OHradicals. In contrast to some of the ozone-based AOPs, UV/H2O2 may requirepretreatment to increase UV transmission in water.

Titanium dioxide (TiO2) is a commercially available nanoparticle thatgenerates a variety of radical species when irradiated with UV light (λ<387 nm), including superoxide (O2•) radicals for reduction and •OH radi-cals for oxidation (Gerrity, 2008). The electron transfer inherently regeneratesthe TiO2 catalyst and allows the photocatalytic process to repeat. The pres-ence of a solid phase photocatalyst in the water also allows for adsorption of

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target contaminants directly onto the TiO2 nanoparticles (Gerrity et al., 2009).Ceramic membrane filters have been used primarily to separate TiO2 fromthe effluent, but they may also provide an element of physical removal forany orthophosphate or particulate phosphorus adsorbed on the TiO2 surface.

Fenton’s reaction uses H2O2 and ferrous iron for the generation of radi-cals, as shown by the primary reaction Fe2+ + H2O2 → Fe3+ + OH· + OH−

(Crittenden et al., 2005). Since the optimal pH for the reaction is between2 and 4, pH adjustment often makes this process impractical for large-scaleapplications. However, Fenton’s reaction may offer an appealing synergisticbenefit for P removal. Since the iron is a true catalyst and is not consumedby the AOP, the residual iron could be used for downstream P adsorption,precipitation, and removal.

3. ADSORPTION

Physicochemical adsorption refers to the mass transfer of liquid phasecontaminants to solid phase adsorbents on the basis of surface energy(Crittenden et al., 2005). Phosphorus removal by adsorption during coag-ulation followed by sedimentation and filtration was first used in the 1950s(Morse et al., 1998). The adsorption removal mechanism, employed in coag-ulation and fixed-bed filtration configurations, remains the best establishedand most widely used mechanism for P removal (Karapinar et al., 2004;Morse et al., 1998).

Significant benefits include the relatively low costs, wide availability,and high adsorption capacity of conventional adsorbents such as aluminumsulfate (alum), ferric chloride, and lime, as well as byproduct adsorbentssuch as fly ash, steel slag, and red mud (Lan et al., 2006; Mortula et al., 2007;Xiong et al., 2008). Additionally, adsorption processes have the potential foradsorbent recycling and phosphate recovery for beneficial use in fertilizersand detergents (de-Bashan and Bashan, 2004).

Drawbacks of physicochemical adsorption include high costs (opera-tion, maintenance, sludge handling, and disposal), large reactor volumes,and possible effluent neutralization requirements (Blaney et al., 2007; Xionget al., 2008). Moreover, thermodynamics and kinetics limit conventional ad-sorbents to 75–90% P removal under favorable conditions (Bowker et al.,1987; Robertson, 2000; Tomson and Vignona, 1984). Although this may besufficient for conventional applications, it is inadequate for P-sensitive waterbodies with very low TP goals.

Continued research and development of adsorbents may offer a suit-able alternative to conventional adsorption. The following sections elaborateon basic principles, advantages, disadvantages, and development status ofinnovative adsorbent technologies.

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3.1 Engineered Adsorbents3.1.1 HYBRID ANION EXCHANGE—PHOSXNP FROM SOLMETEX

SolmeteX (Northborough, MA) recently commercialized a patented hybridanion exchange (HAIX) technology originally developed at Lehigh University(SenGupta and Cumbal, 2007; SenGupta and Zhao, 2000). The system,PhosXnp, consists of an ion-exchange resin impregnated with hydrated ferricoxide (HFO) nanoparticles, which provide P-adsorption capacity. The sur-face charge of the ion exchange resin allows anions to enter, while repellingcations. Phosphate removal is unaffected by competition for sorption sitesfrom common anions (e.g., SO4

2–, CO23–, F–, Cl–) since it diffuses into the

inner sphere and is bound to the impregnated iron while competing anionsform complexes with the outer sphere (Blaney et al., 2007).

The HAIX material is suitable for fixed bed applications as it is capa-ble of multiple regeneration cycles. Bench-scale experiments using HAIX totreat synthetic and actual wastewater were performed in 11-mm-diameter,250-mm-long glass columns at a superficial liquid velocity of 2.5 m/hr andan influent concentration of 260 μg-P/l. Phosphate breakthrough occurredafter nearly 4,000 bed volumes (BV), with an effluent concentration of ap-proximately 50 μg-P/l after 7,000 BV and 250 μg-P/l after 15,000 BV (Blaneyet al., 2007).

3.1.2 BLUEPRO

The BluePRO technology from Blue Water Technologies, Inc. (Hayden, ID)uses a moving-bed filter containing HFO–coated sand. The HFO coating isscoured away by the moving bed, but is continuously regenerated throughferric chloride addition. The waste iron material is removed from the systemand can be recycled upstream of the reactor, thereby increasing P removalwith pretreatment. A two-stage BluePRO reactor has reportedly achieved ef-fluent P concentrations of <10 μg-P/l (BluePRO). Blue Water Technologies,Inc. also markets a BluePRO Plus configuration that incorporates advancedoxidation as a pretreatment step to convert OP in the target water matrix toa more reactive form. Both systems are commercially available in modularconfigurations (Blue Water Technologies).

3.1.3 POLYMERIC HYDROGELS

Polymeric hydrogels have shown potential for phosphate removal dur-ing tests in aquaculture applications. A water-soluble linear polymer,poly(allylamine hydrochloride) or PAA•HCl, was used to synthesize hy-drophilic polymer networks using epichlorohydrin (EPI) as a crosslinkingagent. These networks, called hydrogels, can absorb large amounts of waterwhile remaining insoluble. Laboratory-scale tests of P removal from aqua-culture wastewater (influent phosphate concentrations ranging from 40,000to 80,000 μg/l) were performed in 40–100-ml volumes containing 0.1–1.1 g

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of ground, dry gel particles. The hydrogels demonstrated efficient bindingof phosphate ions, with a maximum phosphate binding capacity of 47 mg/gpolymer at pH 7.0. Phosphate binding decreased as the pH shifted awayfrom neutral. The results showed that the gels were >98% efficient in re-moving PO4-P within three hours of reaction. The gels were successfullyregenerated and reused for at least five treatment cycles. No fouling wasobserved in the presence of other organic and inorganic species (Kioussiset al., 1999; Kioussis et al, 2000). Ground hydrogel particles could be usedin a continuous flow process such as packed columns or fluidized beds;however, the ability of the gels to reduce P loading in water with low initialP concentrations and at larger scales has yet to be established.

3.1.4 ASAHI KASEI CHEMICALS ADSORBENT

Asahi Kasei Chemicals (Tokyo, Japan) recently reported the development ofa highly efficient adsorbent featuring small beads with surface microporesand an internal network of submicron pores, thereby providing a large sur-face area to promote P adsorption. The adsorbent has reportedly achieved<10 μg-P/l at flow rates ten times higher than suitable for most commonlyavailable adsorbents. Faster adsorption may result in lower system costs,reduced space requirements, and improved high-flow treatment capabilities(Asahi Kasei Chemicals, 2007).

The adsorbent was used in pilot-scale tests conducted in conjunctionwith the Japan Sewage Works Agency to verify performance in secondarytreated municipal wastewater (Fitzpatrick et al., 2009). Total P concentrationsin the secondary effluent were reduced from 100–2,100 to 20–40 μg-P/l,respectively, using packed columns operated at a space velocity of 15 hr–1

(Midorikawa et al., 2008). The Asahi Kasei Chemical high-speed adsorbenthas potential to quickly transition to a viable, commercially available optionto satisfy high flow, ultra-low P limitations.

3.2 Alternative Adsorbents

Alum, ferric chloride, and lime are commonly used adsorbents with a largebody of research substantiating their P-removal capabilities (Morse et al.,1998). Other common adsorbents, such as granular activated carbon (GAC),have demonstrated limited potential for phosphate adsorption and removal(Mortula et al., 2007; Sigworth and Smith, 1964). In addition to these commonadsorbents, numerous alternative materials demonstrate significant adsorp-tion capacities. Recent testing of various innovative adsorbent materials hasdemonstrated promising results for TP removal from high surface water flowrates.

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3.2.1 TITANIUM DIOXIDE

Titanium dioxide (TiO2) is a white powder commonly used as a paint pig-ment, sunscreen additive, antifogging agent, semiconductor material, or pho-tocatalyst for water treatment (Gerrity, 2008). It has also been used as anadsorbent/crystallization agent for P removal. Macroporous TiO2 was usedas a seed crystal for crystallization of metal phosphates. Metal phosphatessuch as hydroxyapatite (HAP, Ca5(OH)(PO4)3) or struvite (NH4MgPO4) canbe crystallized as a means of removing P from aqueous solutions. In a studyconducted by Nagamine et al. (2003), the TiO2 (Sakai Chemical Industry Co.Ltd.) acted as a sorbent, trapping P within the pores, rather than as a seed inthe traditional crystallization sense. The TiO2 used in the study was designedwith a unique fibrous morphology in the micrometer scale and a double-porestructure in the macropore region to facilitate sorption. Synthetic wastewaterat an influent concentration of 2,000 μg-P/l was tested in a laboratory-scalereactor at a flow rate of 0.3 ml/min. The system removed a maximum of 90%P and maintained an average of 50–60% removal over 100 days (Nagamineet al., 2003). While P compounds crystallized out of solution as a result ofTiO2 treatment, the dominant removal mechanism appeared to be adsorp-tion of the compounds in the macroporous TiO2 structures, which may beamenable to low-TP, high-flow applications.

3.2.2 SCHWERTMANNITE

Iron oxides have demonstrated strong P-adsorption properties (Parfitt et al.,1975). Additionally, some iron oxide adsorbents, such as Schwertmannite(Fe8O8(OH)6SO4), can be efficiently separated from aqueous solution usingmagnetic filtration. Schwertmannite is a highly reactive nanoscale crystallinematerial characterized by high adsorption capacity, high solubility, and highmetastability. It has been shown to successfully remove phosphate fromaqueous solutions under neutral pH conditions. Acid-activated Schwertman-nite (173 mg/l) was used to treat synthetic wastewater in a semicontinuouslaboratory-scale column experiment. A magnetic filter was used to collect theSchwertmannite slurry. Adsorption varied as a function of influent P concen-tration. At an influent concentration of 100,000 μg-P/l, the linear Langmuirisotherm parameters were q = 44 mg/g and K = 0.00735 g/l. In comparisonto other adsorbents, activated Schwertmannite demonstrated 1.4–68 timeshigher P-adsorption capacity. The activated Schwertmannite was success-fully regenerated three times, resulting in <6% loss in adsorption capacity(Eskandarpour et al., 2007).

3.2.3 RAW DOLOMITE

Dolomitic rock is used extensively by the construction industry for aggregate,cement, and building stones (Haddad, 1990). Accordingly, existing quar-ries may be able to produce low-cost, readily obtainable raw dolomite for

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use as an adsorbent in water treatment processes. Laboratory tests evalu-ated the P-adsorption potential of crushed raw dolomite passed through astandard number 200 sieve. A fluidized bed reactor consisting of a 1.5-cm-diameter, 120-cm-long glass column was packed with 28.5 g of powderedraw dolomite with an average bed depth of 10 cm. Using P-spiked distilledwater and synthetic groundwater (influent concentration of approximately280,000–340,000 μg-PO4/l) at a flow rate of 5 ml/min, the effluent P concen-tration remained below the detection limit for approximately 300 BV. For tapwater, P breakthrough occurred after 150 BV. Tests using secondary-treatedwastewater effluent with initial concentrations of 500–1,000 μg-PO4/l sus-tained approximately 100% P removal for 28–94 BV. The adsorptive capacityranged from 0.025 to 0.072 mg-P/g dolomite (Ayoub and Kalinian, 2006).

3.3 Byproduct Adsorbents

Metal oxides, which are characterized by high adsorption capacities, arefound in numerous industrial byproducts, thereby making byproducts attrac-tive candidates for use as alternative adsorbents in water treatment processes.The reuse of waste products is also an important element of green engineer-ing and sustainable design. As byproducts, these adsorbents may be relativelyinexpensive compared to commercial alternatives, but large-scale availabilityand consistent supplies may be difficult to guarantee.

3.3.1 STEEL SLAG

Steel slag containing iron oxides and alumina is the magnetically separatedindustrial waste from steel factories. Batch experiments used 7.5 g/l of dried,ground, and sieved (0.35 mm) steel slag in 100 ml of solution. With an initialconcentration of 50,000 μg-P/l, 99% P removal was achieved (Lan et al.,2006). Laboratory tests were conducted using secondary municipal effluentat a flow rate of 8.4 ml/min through 20-cm-diameter, 20-cm-high packedPVC columns. Phosphate adsorption increased with temperature. For aninfluent TP concentration of 1,000–3,400 μg-P/l, approximately 74% removalwas achieved, resulting in a maximum adsorption capacity of 5,300 μg-P/l(Xiong et al., 2008). Greater than 50% P removal was observed for batchexperiments using steel slag in 30 ml of solution over a 48-hr period withinitial concentrations of 0–200,000 μg-P/l (Sakadevan and Bavor, 1998).

Shilton et al. (2006) described the performance of a full-scale steel slagactive filter (targeting precipitation and/or adsorption) used to treat effluentfrom a pond system in New Zealand over a 10-year period. In the first yearof operation, the filters achieved 77% TP removal, which correlated closelywith the 72% removal obtained in preliminary column-scale experiments.The filters reduced the mean influent concentration of 8,200 μg-P/l to aneffluent concentration of 2,300 μg-P/l during the first five years of operation.However, performance declined following the initial five-year period. After

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10 years, the average effluent TP concentration was slightly higher than theinfluent concentration. The maximum P-adsorption capacity for the steel slagwas 1,230 μg-TP/g slag (Shilton et al., 2006).

3.3.2 BLAST FURNACE SLAG

Another industrial byproduct containing large amounts of aluminum, iron,and calcium is blast furnace slag, which is derived from the separation ofiron from ore (Johansson and Gustafsson, 2000; Sakadevan and Bavor, 1998;Yamada et al., 1986). Slag has demonstrated P-adsorption capacity in batch,column, and field-scale experiments (Johansson and Gustafsson, 2000). Blastfurnace slag from the Sydney Steel Corporation in Canada reduced the in-fluent P concentration in deionized water from 2,500 μg-P/l to approxi-mately 1,300, 500, 400, and 250 μg-P/l using concentrations of 4, 8, 12, and16 g/l slag, respectively. In secondary municipal wastewater effluent, 16 g/lslag reduced the total P concentration from 3,700 μg-P/l to approximately2,000 μg-P/l (Mortula et al., 2007). In batch experiments performed in 30 mlof solution over a 48-hr period, Sakadevan and Bavor (1998) demonstrated>50% P removal for initial concentrations of 0–200,000 μg-P/l. Phosphateadsorption on blast furnace slag is pH dependent, with reports of optimalperformance between pH 7 and 8 (Yamada et al., 1986) and above pH 9(Johansson and Gustafsson, 2000). Laboratory-scale tests showed that P re-moval improved as concentrations of soluble calcium increased, suggestingthat the major removal mechanism may be precipitation of hydroxyapatite(Johansson and Gustafsson, 2000).

Pilot- and field-scale studies have been conducted using the furnaceslag-based patented Phosphex system consisting of an upflow filter followinga recirculating sand filter developed by the University of Waterloo (Waterloo,Ontario, Canada). The system uses a proprietary reactive media consistingof furnace slag, metal oxides, and limestone. Lab and field studies haveachieved > 90% P removal in multiyear tests (Phosphex). While success-ful, Phosphex was designed for wastewater matrices with high influent Pconcentrations. Thus, its scalability and ability to achieve low TP levels inhigh-flow surface water systems has yet to be established.

3.3.3 ACTIVATED RED MUD

Red mud is an alkaline sludge formed in large volumes during alumina refin-ing. Its low cost and high P-adsorption capacity have attracted considerableattention (Baker et al., 1998; Li et al., 2006; Pradhan et al., 1998). Acid or heatactivation of red mud has significantly enhanced P removal from aqueoussolution. At pH 7, batch experiments in 20 ml of solution using activatedred mud yielded nearly 99% phosphate removal for an initial concentrationof 155,000 μg-P/l (Li et al., 2006). Similarly, in batch experiments performedin 100 ml of solution using 2,000 μg/l of acid-activated red mud, Pradhan

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et al. (1998) observed 80–90% phosphate reduction for initial concentrationsof 30,000–100,000 μg-PO4/l.

Baker et al. (1998) evaluated the use of permeable reactive mixturesconsisting of red mud (targeting adsorption), limestone (targeting precipi-tation), and silica sand (permeable matrix) for the simultaneous adsorptionand precipitation of phosphate. Reactive mixtures can be employed in var-ious treatment configurations, including horizontal treatment layers withinconventional septic systems; self-contained, single-pass, flow-through mod-ules following primary treatment; or vertical in situ barriers. Batch tests usingthe reactive mixture were performed in 500 ml of solution over 10 hr. Thered mud mixture reduced influent phosphate concentrations from approxi-mately 8,500 μg-P/l to approximately 4,500 μg-P/l. In comparison, activatedalumina mixtures yielded effluent P concentrations below the detection limitafter 10 hr. Tests were also performed in 20-cm columns. Using an iron ox-ide reactive mixture, effluent P was <10 μg-P/l for up to 40 pore volumes,and an average phosphate reduction of >90% was observed. For the acti-vated aluminum oxide reactive mixture, effluent P remained below 10 μg-P/lthroughout the experiment (approximately 400 pore volumes; Baker et al.,1998).

3.3.4 DRIED ALUM RESIDUAL

Aluminum sulfate (alum [Al2(SO4)3]) is the most common coagulant used inwater treatment (Crittenden et al., 2005). Alum coagulation produces largevolumes of alum-rich residual, which may be recycled as a byproduct adsor-bent after drying. Mortula et al. (2007) examined the P-removal efficacy ofoven-dried, air-dried, and freeze-dried alum residual collected from the LakeMajor Water Treatment Plant, Halifax Regional Municipality, Canada. In directcomparison batch experiments using deionized water and secondary munic-ipal effluent, oven-dried alum residual was the most efficient P adsorbent.It reduced influent P concentrations in deionized water from 2,500 μg-P/lto approximately 800, 400, 200, and 100 μg-P/l using concentrations of 4,8, 12, and 16 g/l oven-dried alum residual, respectively. In secondary mu-nicipal wastewater effluent, 16 g/l oven-dried alum residual reduced the TPconcentration from 3,700 μg-P/l to approximately 500 μg-P/l (Mortula et al.,2007).

3.3.5 IRON OXIDE TAILINGS

The P-removal potential of iron oxide tailings (average particle size of70 μm) derived from a mineral processing industry in Canada was stud-ied in a laboratory-scale experiment conducted by Zeng et al. (2004). Thetailings contained significant amounts of iron oxides, which are efficient Padsorbents (Parfitt et al., 1975). The tests were performed in a 2.54-cm-diameter, 25-cm-high plexiglass columns at a flow rate of 0.08 ml/min. In-fluent flows of phosphate solution (20,000–22,000 μg-P/l) and hog manure

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solution (40,000–50,000 μg-P/l) were reduced to below the detection limitand 5,000 μg-P/l, respectively, after 30 days. The phosphate adsorption ca-pacity was 8.6 mg-P/g at pH 3.2, which decreased with increasing pH (Zenget al., 2004).

3.3.6 FLY ASH

Fly ash is the major solid waste byproduct resulting from the combustionof coal in power plants (Li et al, 2006; Ugurlu and Salman, 1998). The lowcost and ready availability of fly ash has led to extensive studies of its po-tential as an adsorbent targeting heavy metals, dyes, cadmium, chromium,arsenic, and phosphate. Bench-scale experiments were performed in a5-cm-diameter, 65-cm-high column containing 32 g of fly ash. Using influentconcentrations of 20,000, 50,000, and 100,000 μg-P/l, > 99% P removal wasachieved (Ugurlu and Salman, 1998). Acid and heat activation of fly ash maysignificantly enhance P removal (Li et al., 2006).

3.3.7 CEMENT KILN DUST

During the production of cement, fine kiln dust is transported by fluegasses to a collection filter. In a comparison of various industrial byproducts,Mortula et al. (2007) examined the efficacy of cement kiln dust collected fromLafarge Cement Plant, Brookfield, Nova Scotia, Canada. The dust primarilyconsisted of small (<0.1 mm) CaO and SiO2 particles. It reduced the influ-ent P concentration in deionized water from 2,500 μg-P/l to approximately1,200, 250, 100, and 50 μg-P/l using concentrations of 4, 8 12, and 16 g/lcement kiln dust, respectively. In secondary municipal wastewater effluent,16 g/l cement kiln dust reduced the TP concentration from 3,700 μg-P/l toapproximately 1,800 μg-P/l (Mortula et al., 2007).

3.3.8 OIL SHALE ASH

The combustion of oil shale (an organic-rich sediment) at power plants pro-duces a solid waste called oil shale ash. Batch experiments using oil shale ashfrom Estonia indicated that for influent concentrations of 1,600–98,000 μg-P/l, 67–85% P removal was achieved. The removal efficacy was independentof particle size and appeared to be governed by chemical precipitation ratherthan adsorption. The observed P-binding capacity of 65 mg-P/g was inde-pendent of oil shale ash volume (Kaasik et al., 2008).

3.3.9 BONE CHAR

Bone char is carbonized bone consisting of carbon, calcium carbonate, and aporous hydroxyapatite matrix, all of which combined yield a high adsorptivecapacity. Laboratory-scale batch experiments performed in 250 ml of solu-tion demonstrated that 0.45-mm-particle-size bone char (Tate and Lyle NorthAmerica, Decatur, IL) was capable of reducing the influent P concentrationin deionized water from 2,500 μg-P/l to approximately 600, 400, 250, and

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250 μg-P/l using concentrations of 4, 8, 12, and 16 g/l of bone char, re-spectively. In secondary municipal wastewater effluent, 16 g/l of bone charreduced the TP concentration from 3,700 μg-P/l to approximately 800 μg-P/l(Mortula et al., 2007).

3.4 Adsorption Comparison

Adsorption processes targeting P removal are well established, highly ef-fective treatments with potential for near-term implementation. Numerousparameters can be used to quantify the efficacy of P adsorption, making thecomparison of different adsorbents in different water matrices difficult in theabsence of direct head-to-head testing. The Freundlich adsorption isotherm,as described by Crittenden et al. (2005), is often used to model phosphateadsorption, thereby enabling comparisons. The Freundlich isotherm modelwas used to compare those adsorbents for which Freundlich parameterswere provided in the literature (listed in Table 1). The following assump-tions were made to complete the model: flow rate of 50 MGD (158 m3/min;a high flow rate), empty bed contact time of 15 min (Crittenden et al., 2005),filter loading rate of 3.27 gpm/ft2 (0.13 m/min based on the 3 to 4 gpm/ft2

range recommended by Crittenden et al., 2005), and an influent TP concen-tration of 100 μg-P/l (low influent P). The model neglects competition foradsorption sites, particularly with natural organic matter; thus, the results aretheoretical maxima (i.e., best-case scenarios).

The Freundlich isotherm model was used to calculate the theoreticalbed life for each adsorbent, as illustrated in Figure 1. Theoretical bed lives

FIGURE 1. Predicted bed life for potential adsorbents based on parameters from the literature.

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ranged from 1 to 855 days, but the data from comparable studies often variedwidely. For example, parameters from Sakadevan and Bavor’s (1998) studyof steel slag resulted in a bed life of 855 days, but parameters from other steelslag studies (Lan et al., 2006; Xiong et al., 2008) gave values ranging from4 to 10 days. Despite the wide variability of the results in Figure 1, they sug-gest that iron- and aluminum-based adsorbents (e.g., steel slag, blast furnaceslag, activated red mud) are more effective than carbon-based adsorbents(e.g., bone char, GAC, cement kiln dust) for P removal.

4. ION EXCHANGE

Undesirable ions can be exchanged for solid phase ions based on ion affinityin a process known as ion exchange (Crittenden et al., 2005). Compared toadsorption, ion exchange provides a more selective means of separating ionsfrom solution. Ion exchange has recently gained recognition in P-removalapplications (Lan et al., 2006). The following sections review innovative ion-exchange processes for P removal.

4.1 Capacitive Deionization

Capacitive deionization is a relatively novel technique developed for de-salination. In capacitive deionization, a charged electric field is applied to areactor, which causes ions to accumulate on oppositely charged carbon elec-trodes. Sakakibara (2004) patented a method for removing P using electricfield deionization, but information on commercial availability was not readilyavailable. Capacitive Deionization Technology (CDT) Systems manufacturescapacitive deionization systems at a commercial scale. They have initiatedseveral demonstration projects for desalination, which is their target market.Although the P-removal capabilities of capacitive deionization are not cur-rently emphasized by CDT, phosphate ions may be removed from solutionby direct capture of the ions at the electrodes, as well as by P precipitationresulting from the accumulation of multivalent metal cations (inner layer) andhydroxide or phosphate ions (outer layer) at the cathode (CDT; Dietz, 2004;Farmer et al., 1995). Laboratory experiments at Lawrence Livermore NationalLaboratory (Farmer et al., 1995) indicated that efficiencies as high as 76%removal were obtained in tests with sodium phosphate (Na3PO4). Capacitivedeionization appears to be a promising technology with commercial-scalemanufacturing; however, information related to commercial P-removal effi-cacy and energy consumption was not available. The primary advantages ofthis process are reduced byproduct waste and energy efficiency, but furtherwork specific to P ions may be required to extrapolate these results fromdesalination to P removal.

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4.2 Layered Double Hydroxide Compounds

Layered double hydroxides (general formula: [M(II)1-xM(III)x(OH)2]x+[Ax/nn–]

·mH2O, where M(II) and M(III) are the divalent and trivalent metal ionsand An– is the interlayer anion), also called LDH, have been shown toselectively remove phosphate through adsorption and ion exchange (Koilrajand Kannan, 2010). Iron-based LDH can be used to achieve P removal forwater treatment applications. Phosphorus removal results from ion exchangebetween phosphate and carbonate ions. The released metal cations (Mg2+,Ca2+, and Fe3+) and/or their hydroxides also effectively enhance P removalby adsorption followed by precipitation. Laboratory-scale tests used driedand crushed (<250 μm diameter) iron-based LDH in columns operated at aflow rate of 20 ml/hr and a feed volume of 3,000 ml/g LDH to remove 80%of the influent P from river water at an influent concentration of 200 μg-P/l(Seida and Nakano, 2002).

Koilraj and Kannan (2010) found a maximum phosphate uptake around91 mg-P/g LDH using Zr4+ ion-incorporated ZnAlZr LDH. The influent phos-phate concentration ranged from 10,000 to 250,000 μg-P/l, 1 g/l LDH wasused, and the pH was 5.5 (Koilraj and Kannan, 2010). Other ZnAl LDH eval-uations in wastewater demonstrated an equilibrium adsorption capacity ofabout 50 mg-P/g under circumneutral to slightly alkaline conditions (Chenget al., 2009). In tests comparing different molar solutions, Mg/Al at a molarratio of 2 removed 99.35% of the P in a 1 mmol/l PO4

3– solution (Lv et al.,2008).

LDH expands in the presence of water, resulting in loss of flow throughthe filter (Jin and Fallgren, 2006). Additional research to evaluate hybrid filtermaterials may overcome this issue and improve the commercial viability ofthis technology. Another process concern is that P exchange decreases inwater and wastewater systems where other competing anions are typicallypresent.

4.3 Zr(IV)-Loaded Resins

Metal-loaded (e.g., Cu(II), Co(II), Fe(III), Al(III), Y(III), La(III), and Mo(VI),Ti(IV), Zr(IV)) ligand exchangers have recently been promoted for theiranion selectivity and trace nutrient removal capabilities in aqueous solu-tion. Zhu and Jyo (2005) evaluated the P-removal capacity of a phospho-ric acid resin immobilized with Zr(IV). Laboratory-scale tests performed ina 7-mm-diameter column reduced the phosphate concentration in seawa-ter and river water from nearly 300,000 μg/l to <10 μg/l at flow rates of10–100 hr–1 in space velocity. The resins worked efficiently over a widepH range (2.03–8.21), with a sorption capacity of 345 mol/l (Zhu and Jyo,2005).

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4.4 Magnetically Stabilized Fluidized Bed

The separation of proteins from cell lysate mixtures is commonly performedusing magnetic separation in biotechnology and proteomics. This technologymay also be applicable to liquid-solid separation. In one particular patentedprocess, magnetized ion exchange particles are stabilized in a fluidized bed.Protein adsorption occurs on the surface of the magnetized ion exchangeparticles (Noble et al., 1992). It may be possible to remove OP using mag-netically stabilized fluidized beds to target protein separation. However, thefeasibility of this application is currently undetermined, and its potential forlarge-scale applications is unknown. An additional consideration is whethercell lysis is required prior to protein capture. Cell lysis as a pretreatmentstep would likely pose a significant challenge for the treatment of high waterflows.

4.5 Ion Exchange Comparison

Ion exchange technologies show promise for P removal at the bench scale,but have not yet been developed into commercially viable processes withreported P efficiency or process requirements. The drawbacks of this tech-nology are the generally narrow pH range, necessity of OP conversion toinorganic P ions through AOPs or cell lysis, decreased P removal in thepresence of other common anions, and the swelling of LDH filters withouta mitigating material. Most evaluations have been completed on wastewa-ter and high-P river water. Additional work on lower P concentrations isrequired prior to use in low P scenarios.

The advantages of this technology are the low level of waste byproductand possibly lower energy requirements. Further, desorption of the P ionsmay permit recycle from the concentrated waste stream for use as a plantgrowth amendment. The HAIX (Blaney, 2007) effectively combines ion ex-change with adsorption for P removal and has been evaluated at low Pconcentrations. Hybrid processes such as this may overcome ion exchangeobstacles, increasing commercial applications.

5. BIOLOGICAL UPTAKE

Biological treatment of water and wastewater is a common practice thathas been documented for decades (de-Bashan and Bashan, 2004; Rittmannand McCarty, 2001). Biological P removal relies on the uptake of P as anutrient for biomass synthesis, which requires significant growth and accu-mulation of heterotrophic microorganisms. Accordingly, biological P removalis commonplace in wastewater treatment (Chae et al., 2006; Franzreb andHoll, 2000; Tam and Wong, 2000), where organic substrate concentrations

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(i.e., biochemical oxygen demand [BOD]) are sufficiently high. High-flow,low-P waters normally have little BOD, which severely limits the usefulnessof traditional technologies for biological P removal. Thus, newly developedbiological processes capable of treating high surface water flows to achievelow TP concentrations are of considerable interest. The following sections re-view innovative biological processes that do not demand substantial growthand accumulation of heterotrophic biomass.

5.1 Immobilized Photosynthetic Microbes

The heterotrophic bacteria commonly used for biological treatment requirehigh organic nutrient loads, which are typically associated with wastewa-ter. Alternatively, for water matrices with relatively low levels of organics,it may be more appropriate to use phototrophic organisms (e.g., photosyn-thetic bacteria and algae), which use sunlight as their energy source andCO2 as their carbon source. One of the major concerns associated with bio-logical treatment is maintaining separation between the microbial cells andthe treated water (Sawayama et al., 1998b). Technologies such as activatedsludge followed by sedimentation or membrane filtration, upflow anaerobicsludge blanket methods, and microbial immobilization have been employedin an effort to facilitate downstream separation of microbes from the treatedwater (Sawayama, 1998b).

Cyanobacteria are one group of photosynthetic microorganisms capableof removing inorganic nutrients such as phosphate. In the absence of agita-tion, the filamentous flocculating cyanobacteria Phormidium bohneri rapidlysettle out of solution, enabling straightforward separation of the microbesfrom treated effluent (Talbot and de la Noue, 1993). Laliberte et al. (1997)investigated the use of P. bohneri for the removal of phosphate from do-mestic wastewater. Tests were performed in 24-l aerated bioreactors in directsunlight. At influent concentrations of 2,400–4,000 μg-PO4/l, phosphate wascompletely removed after 50 and 75 hr of growth, respectively. P. bohneridemonstrated high biomass productivity at 23–57 mg of dry weight/l/daycombined with phosphate removal rates as high as 20 mg/l/day. In addi-tion to phosphate uptake by the bacteria, chemical precipitation was be-lieved to account for a large portion of P removal. Precipitation occurs as abyproduct of photosynthesis since the uptake of CO2 during photosynthesiscan increase the pH to 9–11, favoring precipitation of calcium phosphate(Laliberte et al., 1997).

Recent studies demonstrated that immobilization of microbial cells onor within a matrix (e.g., polymers, fibers, ceramics) effectively prevents cellwashout, separates microbes from treated effluent, and offers greater opera-tional flexibility. Immobilization of the tropical cyanobacterium Phormidiumlaminosum on hollow cellulose fibers demonstrated success in the removalof phosphate ions from water. At a residence time of 48 hr, the influent

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phosphate concentration of 6,620 μg-P/l was reduced to <40 μg-P/l in sec-ondary treated sewage at 43C (Sawayama et al., 1998a).

Rhodobacter is another group of photosynthetic bacteria capable ofremoving P in an immobilized bioreactor. Using the purple nonsulfur bac-terium R. capsulatus immobilized on cellulose beads, 6% of phosphate wasremoved from a diluted growth media at 35C with a residence time of 10 hr(Sawayama et al., 1998b). Batch treatments using porous ceramic blocks withimmobilized R. sphaeroides S, R. sphaeroides NR-3, and Rhodopseudomonaspalustris removed 77% of phosphate from synthetic wastewater after 48 hr.In semicontinuous treatment, approximately 90% removal was observed afterapproximately 20 days (Nagadomi et al., 2000).

Biological treatment systems also commonly use microalgae. Immobi-lized microalgae can yield higher nutrient removal efficiencies comparedto their freely suspended counterparts (de-Bashan and Bashan, 2004). Im-mobilized Chlorella cells grew slower than suspended cells, but exhibitedincreased metabolic activity (Lau et al., 1997; Tam and Wong, 2000). Theimmobilization of C. vulgaris in calcium alginate beads resulted in a re-duction in P concentrations from about 6,000 μg-P/l to <500 μg-P/l after4 hr of treatment in a 5-l laboratory-scale reactor. Removal of 94% of phos-phate was observed in the aerated bioreactor after 8 hr of treatment. Themajor nutrient removal mechanisms were algal uptake and adsorption on thebeads, but precipitation likely contributed to removals since pH increased asphotosynthesis progressed (Tam and Wong, 2000).

The development of a novel twin-layer immobilization bioreactor of-fers potential advantages over gel entrapment of microalgae, includingelimination of cell leakage and lower costs. In the twin-layer system, mi-croalgae were immobilized by self-adhesion on a wet, microporous, ul-trathin substrate layer. A second layer, consisting of macroporous fibroustissue (glass fibers) provided growth medium for the microbes. Immo-bilization of C. vulgaris in a twin-layer bioreactor resulted in a reduc-tion of more than 90% of phosphate from a secondary synthetic wastew-ater within nine days using a 2-l laboratory-scale reactor (Shi et al.,2007).

Additional studies explored the possibility of improving nutrient re-moval using synergistic combinations of microorganisms. One study used C.vulgaris microalgae in tandem with the growth-promoting bacteriumAzospirillum brasilense. A. brasilense is typically used to promote the growthand yield of crops, but also appears to enhance the performance of unicel-lular plants such as microalgae. Coimmobilization of the two species in algi-nate beads significantly increased the removal of phosphate from syntheticwastewater compared to microalgae alone. The phosphate concentration wasreduced from 12,000 to 3,000 μg-PO4/l after 48 hr (de-Bashan and Bashan,2004; de-Bashan et al., 2002).

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Immobilized photosynthetic microbes clearly have potential for P re-moval. However, the technology is in its early stages, and it would be essen-tial to conduct larger-scale testing to validate system performance, preferablyusing the water matrix of choice to evaluate microbial growth and survivalunder site-specific conditions.

5.2 Phosphate-Binding Protein

Natural systems may provide valuable biomimicry prospects and insight intothe collection and uptake of trace environmental P. For example, microor-ganisms in P-limited environments have evolved efficient phosphate assim-ilation mechanisms. The diatom Didymosphenia geminata flourishes in theP-limited (<20 μg/l) waters of Rapid Creek, South Dakota. Through secre-tion of enzymes, D. geminata is able to directly access pools of OP (Maurer,2008).

Furthermore, large numbers of the high-affinity phosphate binding pro-tein, PstS, are expressed in a variety of microorganisms in low-P environ-ments (Hong et al., 1999; Poole and Hancock, 1984). These proteins may beused in engineered systems to bind soluble phosphate. Kuroda et al. (2000)observed P removal using PstS proteins (purified from Pseudomonas aerug-inosa) immobilized on plastic substrate (N-hydroxysuccinimide-activatedsepharose, Amersham Pharmacia Biotech, Bucks, England). The immobi-lized proteins were packed into 50-mm × 70-mm laboratory-scale columnsand tested using pond water containing 15 μg-P/l. The effluent P concen-tration was below the detection limit (Kuroda et al, 2000), indicating thatthe PstS protein has the potential to significantly reduce initially low TPconcentrations in water. Since many organisms express the protein, purifi-cation is theoretically possible; however, the feasibility of large-scale proteinpurification is currently undetermined.

One group of organisms that expresses the phosphate binding proteinPstS is marine picocyanobacteria (e.g., Prochlorococcus, Trichodesmium,Crocophaera), which survive in the low-P Sargasso Sea (Dyhrman et al.,2007). Cells can uptake inorganic P at a maximum rate of 5.4 nmol/mg cellprotein-min (Kuroda et al., 2000). The cyanophages that infect these specieshave also been found to stimulate PstS protein expression. The expression ofthe PstS gene that occurs when cyanophages infect their hosts may enhanceP acquisition (Dyhrman et al., 2007; Sullivan et al., 2005). Biological uptakeof P using PstS-expressing microorganisms in P-limited water matrices istheoretically possible, but this application is currently untested.

The use of microbes expressing the PstS protein in water treatment ap-plications would require that the microorganisms survive in a reactor. Sinceonly certain microbes express the PstS protein, including marine cyanobac-teria, their use in fresh water systems may be limited. Researchers recently

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developed a novel virus capture system that relies on the expression of po-liovirus receptors on the surface of E. coli cells (Helmy, 2009). The bacterialprotein receptors bind the viruses, thereby enabling their separation fromsolution. In theory, the PstS protein could be expressed on the outside ofbacteria to similarly bind and collect P from water matrices, thereby sup-plementing natural metabolic uptake. The current development status of Premoval using the PstS protein is theoretical, albeit promising. However, re-strictions on the possible release of genetically engineered microbes into theenvironment may hinder implementation of this technology.

5.3 Biological Uptake Comparison

Biological P removal is commonly used in wastewater treatment becauseanaerobic/aerobic cycling can be controlled, and the nutrient and microbialloadings are sufficient to drive the process. On the other hand, environmen-tal waters with high flow rates generally are not conducive to conventionalbiological P removal since the loading of biological organic matter is low.The use of photosynthetic microbes may be a more appropriate alternativedue to their reliance on sunlight rather than aqueous organic matter for en-ergy. Photosynthetic P removal can be implemented in either suspended orattached (i.e., natural biofilm or immobilized on a substrate) configurationsand has demonstrated success in laboratory studies. In addition, biological Premoval may be perceived as a green alternative since no chemical sludgesare generated. If properly designed, the process has the potential to be car-bon negative and perhaps even co-produce valuable end products such as Pfor agriculture and energy in the form of biodiesel (Rittmann, 2008). Beforefull-scale implementation, future studies must provide a direct comparison ofspecies, configuration, water matrix, and cost to identify the optimal systemfor biological P removal in high-flow applications. Although feasible, usingphosphate-binding proteins affixed to engineered substrates or the mem-branes of genetically engineered bacteria appears to be less practical due tothe novelty of the concept.

6. DISCUSSION

Water bodies around the world are jeopardized by excessive P-loadingsreleased by anthropogenic activities. Numerous conventional P-removaltechnologies—including adsorption, biological nutrient removal, precipita-tion, and crystallization—have been researched and successfully applied inbench-, pilot-, and full-scale applications. Yet the remaining environmentalTP concentrations, often less than 100 μg-P/l, can still contribute to eutroph-ication, particularly when flowrates are large. Existing treatment strategiesare unable to achieve very low TP concentrations (e.g., ≤ 10 μg-P/l), par-ticularly when challenged with high surface water flows containing low

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initial P concentrations (possibly with a significant OP component), and loworganic-substrate availability. Such flows yield relatively high environmentalP loadings unless the TP concentration is reduced to a very low level, thusinspiring the development and implementation of novel approaches to Premoval.

This work reviewed a wide range of innovative processes with potentialto achieve very low TP levels in high water flows. Each of the technologieswas assessed using four criteria: (a) applicable to treatment of high-flowsurface water systems, (b) able to remove TP to less than 10 μg-P/l, (c)able to remove organic P, and (d) likelihood of commercial availability. Asummary of the technologies and their potential with respect to these criteriais provided in Table 2. With respect to the last criteria, very few of thesystems have currently been tested at the full-scale level or are commerciallyavailable.

Since the processes were evaluated independently, the wide range ofconditions, experimental methodologies, and measured parameters preventsa direct comparison of the results. Prior to full-scale implementation, direct,head-to-head testing of prospective technologies in applicable site-specificsystems would be required to further explore their capabilities and limita-tions while assessing their relative efficacy under comparable conditions.While direct comparison is not possible here, this review highlights severalkey trends. Adsorption processes appear to have the greatest potential fornear-term implementation, with the use of byproduct adsorbents introducingan added benefit of green engineering and sustainable design. Recent inno-vations in ion exchange and biological uptake configurations show promisebased on laboratory-scale testing and may be long-term options.

6.1 Applicability to High-Flow Surface Water Systems andCommercial Availability

Adsorption is a well-established, generally low-cost option for P removal. Itis easily scaled to large water flows, with key considerations being processfootprint, cost and availability of adsorbent material, and disposal of chemicalsludges. A large number of innovative adsorbent materials and configurationsare available and it may be possible to recover P for reuse from the resultingsludges. Byproduct materials may also be used to limit material costs.

Ion exchange is another well-established technology, but commercialapplications do not currently focus on P removal. One possible concern forhigh-flow ion exchange installations is the generation of concentrated wastestreams. However, these waste streams present an opportunity to recoverconcentrated P in a more appropriate form for reuse compared to sludgesfrom adsorption processes. Hybrid ion exchange processes (such as theHAIX) that take advantage of ion exchange and other removal mechanismssuch as adsorption appear to have special promise for TP removal.

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TABLE 2. Summary of Innovative Total Phosphorus Removal Strategies

Technology

High-flowsurface water

systemsLow TP

<10 μg-P/l)

Ability toremove

organic P

Likelihood ofcommercialavailability

(current scaleof testing)

AdsorptionEngineered

Hybrid anionexchanger

Possible Yes Limited Commerciallyavailable

BluePRO Yes Yes Limited FullPolymerichydrogels

Possible Not yetestablished

Limited Bench

Asahi Kaseiadsorbent

Yes Yes Limited Pilot

Alternative MaterialsTitaniumdioxide


Limited Bench

Schwertman-nite

Possible Unknown Limited Bench

Raw dolomite Possible Yes Limited Bench

Byproducta

Iron-based Possible Possible Limited Bench, FullAluminum-based

Possible Possible Limited Bench, Full

Carbon-based Possible Unlikely Limited Bench

Ion ExchangeCapacitive

deionizationYes Untested Limitedb Pilot (for

desalination)Layered double

hydroxidesPossible Not yet

establishedLimitedb Bench

Magneticallystabilizedfluidized bed

Limited Untested Limitedb Bench

Zr(IV)-loadedresins

Possible Yes Limitedb Bench

Biological UptakeFlocculating

cyanobacteriaPossible Yes Yes Bench

Immobilizedbacteria


Yes Bench

Immobilizedmicroalgae


Yes Bench

Microbes fromlow-Penvironments

Limited Untested Yes Theoretical

Immobilized PstSprotein

Possible Yes Yes Bench

Expression ofPstS protein

Limited Untested Yes Theoretical

aEfficacy of byproduct adsorbent is highly variable depending on source of material.bMust convert to inorganic phosphorus prior to removal.

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Biological uptake by photosynthetic microorganisms or engineeredP-targeting proteins is the least well-characterized approach in terms ofcommercial availability and use in high-flow systems. Photosynthetic mi-croorganisms may have good potential in terms of scalability and for thegeneration of valuable coproducts; however, sensitivity to growth conditions(i.e., temperature and sunlight) provides uncertainty. The use of engineeredproteins or organisms may be a significant technical and social hurdle forlarge-scale implementation.

6.2 Ability to Achieve Low Total Phosphorus Concentrations

Although additional comparative testing using lower input P concentrationsis needed, the performance of several adsorbents for achieving very lowTP concentrations (<10 μg-P/l) was notable, including HAIX, BluPRO, theAsahi Kasei Chemicals adsorbent, and raw dolomite. Although not directlycomparable, the results from the literature seem to indicate that aluminum-and iron-based adsorbents perform better for P removal than carbon-basedmaterials.

For ion exchange, Zr(IV)-loaded resins successfully achieved the lowTP goal, but other technologies either have not yet been directly testedfor P removal or have not been tested at relevant influent P concentrations.Despite a lack of results, ion exchange is a promising technology for removalto low TP concentrations, since it can specifically target P.

Several biological uptake systems have demonstrated the ability toachieve very low TP concentrations at the bench scale, including floccu-lating cyanobacteria and immobilized PstS protein. A possible advantage ofbiological uptake is that the microbes and protein seem to be optimized forinitially low P concentrations.

6.3 Ability to Remove Organic Phosphorus

The ability to remove OP was generally limited in all reviewed technologies.Adsorption generally targets inorganic P ions, although some OP may alsobe removed. Ion exchange cannot directly remove OP as it relies on theexchange of inorganic phosphate ions. Accordingly, an AOP must be usedto convert OP to inorganic P, which introduces cost concerns at a large scale.Overall, biological uptake processes are the most promising for the directremoval of OP.

ACKNOWLEDGMENTS

This review was initiated, funded, and supported by the St. John’s River WaterManagement District (SJRWMD). The authors would like to acknowledge the

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valuable direction provided by Mary Brabham of the SJRWMD and PatrickVictor of CDM.

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APPENDIX A

Conventional Phosphorus Removal Technologies Not Considered in theReview

Removal Mechanism Technology

Adsorption / Filtration • Adsorption/Clarification/Filtrationusing alum or ferric chloride and/orpolymers• Adsorption on activated alumina• Direct filtration• Dissolved air flotation/Filtration• Electrocoagulation• High-rate sedimentation• High-rate two-stage filtration• Limestone treatment• Low-intensity chemical dosing ofstormwater treatment areas (STAs)• Microfiltration• Ultrafiltration

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Removal Mechanism Technology

Biological • Algal filtration/Turf scrubbers• Emergent macrophyte STAs• Fish harvesting• Fixed film bioreactor• Floating plants• Managed wetlands• Periphyton STAs• Submerged vegetation• Wetlands (STAs)• Activated sludge• Biofilm processes• Membrane bioreactor

Precipitation /Crystallization

• HYPRO advanced chemicalprecipitation• Crystalactor R©

• Struvite precipitation•; Crystallization in a fluidized bed• Magnetite-seeded precipitation• RIM-NUT (REM-Nut) also relies onion exchange

Other • Aquifer Storage and Recovery• Deep Well Injection• Overland Flow• Percolation Ponds•Sediment Dredging

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Innovative Strategies to Achieve Low Total Phosphorus Concentrations in High Water Flows

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