Modeling kinetics and isotherms of functionalized filter ... · PDF fileIsotherms of Functionalized Filter Media for Nutrient ... Department of Civil, Environmental and Construction

Modeling Kinetics andIsotherms of FunctionalizedFilter Media for NutrientRemoval from StormwaterDry PondsFahim Hossain, Ni-Bin Chang, and Marty WanielistaDepartment of Civil, Environmental and Construction Engineering, University of Central Florida, Orlando, FL;[email protected] (for correspondence)

Published online 29 December 2009 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.10415

Sorption media with mixes of some recycled mate-rials, such as sawdust and tire crumbs, combinedwith sand/silt and limestone, become appealing innutrient removal for promoting urban stormwatermanagement with sustainability implications. Thisarticle aims to present a specific type of functional-ized filter medium and examine its physicochemicalprocess for nutrient removal with the aid of Langmuirand Freundlich isotherms and isolated filtrationkinetics. Within a suite of batch tests, pollutants ofconcern include ammonia, nitrite, nitrate, orthophos-phate, total dissolved phosphorus, etc. The potentialfor application in stormwater management facilities,such as dry ponds, was emphasized in terms of lifeexpectancy and removal efficiency. When comparedwith the natural soil that was selected as the controlcase in the column tests, our ‘‘green sorption mediamixture’’ with respect to three types of sorption,including adsorption, absorption, and ion exchange,proved relatively effective in terms of removing mostof the target pollutants under various influent wasteloads. Sensitivity analysis with respect to the pH val-ues and initial concentrations simultaneously or sep-arately was presented in the end to enhance the engi-neering reliability analysis. It shows that with the

inclusion of limestone, drastic changes of pH valuescan be buffered well, so that the impacts on the ulti-mate removal efficiency of ammonia and nitratescan be isolated solely to the changes of initial concen-trations. � 2009 American Institute of Chemical EngineersEnviron Prog, 29: 319–333, 2010Keywords: sorption media, stormwater manage-

ment, filtration kinetics, isotherm test, green infra-structure, engineering sustainability

INTRODUCTION

Nutrients such as ammonia, nitrite, nitrate, andphosphorus are common contaminants in waterbodies all over the world. High nitrogen and phos-phorus contents in stormwater runoff have impededthe reuse potential and impacted ecosystem integrityand human health. Nitrate may be toxic and cancause human health problems such as methemoglobi-nemia, liver damage, and even cancers. Phosphorusmay trigger eutrophication issues in fresh waterbodies, which could result in toxic algae and eventu-ally endanger the source of drinking waters. Nutrientremoval is very important for the sustainability of theaquatic ecosystem and environment. All thesenutrients have acute and chronic harmful outcomesfor human beings and ecosystems directly or� 2009 American Institute of Chemical Engineers

Environmental Progress & Sustainable Energy (Vol.29, No.3) DOI 10.1002/ep October 2010 319

indirectly. According to the United States Environ-mental Protection Agency (USEPA), unionized ammo-nia is very toxic for salmonid and nonsalmonid fishspecies [1]. Fish mortality, health, and reproductioncan be hampered by the presence of 0.10–10.00 mg/Lof ammonia [1]. Nitrate is more toxic than nitrite andcan cause human health problems such as liver dam-age and even cancers [2, 3]. Nitrate can also bindwith hemoglobin and result in oxygen deficiency inan infant’s body called methemoglobinemia [4]. Ni-trite, however, can react with amines chemically orenzymatically to form nitrosamines that are verystrong carcinogens [5].

Nitrogen and phosphorus compounds are mostfrequently measured to indicate nutrient loadings innatural and built systems. They are found in urbanstormwater runoff primarily from highways [6].Nitrates normally result from vehicular exhaust on theroad itself and adjacent soils from fertilization of land-scaped areas beside the roads and the neighboringresidential areas [7, 8]. On the other hand, whenurban regions gradually expand due to regional de-velopment, centralized sewage collection, treatment,and disposal is often unavailable for both geographicand economic reasons. Thus, decentralized or onsitewastewater treatment systems (OWTS) may be neces-sary to protect public health. Nationwide, wastewatereffluent from OWTS can represent a large fraction ofnutrient loads to groundwater aquifers. However,wastewater effluents reclaimed from secondary waste-water treatment plants and reused as irrigation watermay result in the same level of environmental impact.

Nitrogen, particularly nitrate–N, easily moves fromterrestrial ecosystems into surface and ground waters,including lakes, streams, rivers, and estuaries [9–11].According to USEPA, the maximum contaminant levelof nitrate and nitrite levels in drinking water shouldnot be >10.00 mg/L nitrate��nitrogen (NO3

2��N) and1.00 mg/L nitrite��nitrogen (NO2��N), respectively[12]. For effective stormwater management, a biore-tention or bioinfiltration pond is a relatively newurban stormwater best management practice (BMP)[13]. The use of differing filter media in wet and drybioretention ponds to help promote nutrient removalis an appealing engineering approach to deal withthe increasing trend of higher nutrient concentrations.Large-scale implementation with different filter mediato remove nutrients will be popular in the future [14,15]. The main purpose of this research is to examinethe sorption capacity and model the isolated filtrationkinetics of selected mixes of filter media for nutrientremoval using Langmuir and Freundlich isothermsand column tests. Pollutants of concern mainlyinclude ammonia, nitrite, nitrate and orthophosphate(OP), total dissolved phosphorus (TDP), etc. Filtermedia reviewed include but are not limited to tirecrumb, sawdust, activated carbon, iron amended res-ins, orange peel, peat, leaf compost, naturally occur-ring sands, zeolites, coconut husks, polymers, soybean hulls, etc. The goals of this study are thus to:(1) present specific functionalized filter media for nu-trient removal via a systematic literature review, (2)examine the adsorption, absorption, and ion

exchange capacity for nutrient removal separately asa function of the physicochemical conditions, (3) esti-mate the life expectancy of the selected filter mediawhich we denote as ‘‘green sorption media,’’ (4)understand the isolated filtration kinetics of selectedfilter media mixtures in the physicochemical process,and (5) discuss the field implementation potential.These efforts should collectively demonstrate thetrend and potential for application of green sorptionmedia leading to the promotion of sustainable civilinfrastructure systems in urban regions.

LITERATURE REVIEW

Nutrient Concentrations in Stormwater andGroundwater in Florida

Stormwater runoff is one possible source of nitro-gen, among others such as septic tanks and land-based application of reclaimed wastewater or fertil-izer, which can contribute to elevated nitrate andnitrite concentrations in the Upper Floridian aquifer.Both the Florida Department of EnvironmentalProtection (FDEP) and St. Johns River Water Manage-ment District (SJRWMD) found that maximum totalphosphorus (TP), orthophosphate, total nitrogen(TN), ammonia��nitrogen (NH3��N), and nitrate��nitrogen (NO3��N) 1 nitrite��nitrogen (NO2��N)were 0.30 mg/L, 0.27 mg/L, 1.30 mg/L, 0.05 mg/L,and 0.05 mg/L, respectively, in stormwater runoff[16]. Because of the collective impacts of stormwaterrunoff from both urban and agricultural areas andonsite wastewater treatment plants, this result clearlydemonstrated that nitrate concentrations haveincreased in many Upper Floridian aquifer springssince the 1950s. Phelps [17] reported that nitrate con-centrations ranged from less than 0.02–12.00 mg/L,with a median of 1.20 mg/L, based on 56 Upper Flo-ridian aquifer wells sampled in Marion County during2000–2001 [17]. It is known that nitrate concentrationshave exceeded 1.00 mg/L in recent years at somesprings in Lake, Marion, Orange, Seminole and Volu-sia Counties according to Phelps et al. [18] and St.Johns River Water Management District [19]. Increas-ing trends in nitrate concentrations were documentedin Volusia County springs, such as DeLeon and Gem-ini Springs [18] and Blue Spring [19].

Stormwater Best Management PracticesA number of devices, collectively known as struc-

tural BMP, were employed to treat contaminatedstormwater [20]. Nutrient in stormwater, groundwater,and wastewater can be removed by using physico-chemical and microbiological processes. The formerinclude activated carbon absorption, ion exchangewith synthetic resins, reverse osmosis, and electro-dialysis, whereas the latter consists of nitrification anddenitrification triggered by either autotrophs or heter-otrophs. Bioinfiltration processes with differing sorp-tion media have been gaining popularity due to theircost-effectiveness [21]. Within the context of bioinfil-tration, including stormwater trench, ponds, infiltra-tion biowells in low-impact development (LID), and

320 October 2010 Environmental Progress & Sustainable Energy (Vol.29, No.3) DOI 10.1002/ep

exfiltration facilities, three types of physicochemicalprocesses are of interest. They are adsorption,absorption, and ion exchange. Besides, two impor-tant processes that result in the transformation ofnitrate are nitrification and denitrification in themicrobiological process. Although both may contrib-ute to nutrient removal, most of the filter media areexpected to improve solid–liquid contact and preventchanneling during physicochemical and microbiologi-cal processes. In general, the higher surface area ofclay/silt in natural soil might provide more contactarea for the solid to absorb and more space for bacte-ria to grow. As a consequence, the hypothesis in thisstudy is that the functionalized filter media using aproper mixture of recycled materials, such as tirecrumb, sawdust, fine sand, clay, or silt might have ahigher capacity for nutrient removal. Comparativestudies for a quantitative process-based understand-ing of the filtration kinetics with respect to differingmaterial mixes in both physicochemical and micro-biological processes would help to sustain therenewed interest for the bioinfiltration BMP in storm-water management. However, the interesting andchallenging research question is how to examine theadsorption, absorption, and ion exchange capacityfor nutrient removal restricted to the physicochemicalconditions solely in the case of the hydraulic reten-tion time (HRT) not being sufficiently long to bringabout complete microbiological transformation duringthe storm events in the BMP facilities.

Sorption Media Used for Nutrient RemovalIn water chemistry, sorption is brought about by

physical van der Waals forces and/or bindingbetween the pollutant chemical complexes on thesurface of the media. Adsorption occurs normally atthe media surface, either its interior or immediateexterior surfaces within a porous media. Yet absorp-tion takes place when the pollutant penetrates to themolecular level of the sorption media. Within thesorption process, the pollutant and media becomehomogenous in structure although no chemical reac-tions occur resulting in alteration of the media’schemical structure. However, ion exchange is notstrictly a sorptive process as it involves the replace-ment of ions. For example, potassium or sodium inthe media may be commonly replaced by variousmetals, making the flow to receiving waters lessharmful or harmless. Some nutrients, such as phos-phorus, removed by inorganic media are likely to bein the form of sorption/precipitation complexes. Thedistinction between adsorption and precipitation isthe nature of the chemical bond forming between thepollutant and sorption media. Yet the attraction ofsorption surface between the pollutant and the sorp-tion media causes the pollutants to leave the aqueoussolution and simply adhere to the sorption media. Inthe context of using various green sorption media fornutrient removal, it might appear that sorption is fol-lowed by precipitation or occurs at the same time inthe same physicochemical process. For example, theinitial reaction of phosphorus removal by sorption

media appears to be adsorption with slow alterationof the complex to a precipitate. When limestone ispresent, however, removal of phosphorus by calciumcompounds such as calcium carbonate may involvesorption followed by precipitation. These processesalso occur in natural soils though with differentdegrees. Most pollutants, including bacteria andviruses, in stormwater runoff are likely to be hydro-phobic. Therefore, significant shifts in any one offour chemical conditions, including the backgroundconcentration of the pollutant, dissolved oxygen con-centration, pH values, and salinity, may cause achange of thermodynamic equilibrium between sorp-tion media and aqueous solution.

As long as the HRT is long enough, a microbe-min-eral interface can be initiated for either or both nitrifi-cation and denitrification. Nitrification which changesammonia to nitrite and later nitrate may be triggeredby some nitrifiers in an aerobic environment. The ni-trate would be eventually lost from the stormwatertreatment system after being converted by denitrifiersto nitrogen gas in an anaerobic environment. Still, thedenitrification process must count on the presence oforganic matter, such as a carbon source (i.e., electrondonors). Pollutants removed by the adsorption processin green sorption media may subsequently desorb.The adsorption, absorption, ion exchange, and precip-itation processes are actually intertwined and dynamic.For example, ammonia maybe sorbed by clay in biore-tention filters filled with sorption media. However, thenitrate that is quite soluble may be sorbed first andthen leached from the clay during subsequent stormevents rejuvenating the sorptive sites for incomingnutrients such as ammonia. During this stage, absorp-tion/adsorption capacity would, in turn, become avail-able, which is limited in each type of media. Theremight be competition for N, P and metal removalsimultaneously when they sorb onto the same sites.They each prefer different or even mutually exclusiveconditions (e.g., pH, dissolved oxygen, temperature,etc.). Besides, some of the nutrient removal mecha-nisms are either reversed or promoted by anaerobicconditions. For example, adsorption/precipitation ofdissolved phosphorus with ferric/ferric oxide embed-ded in the sorption media can occur only in an aerobicenvironment and the complex dissolves under anae-rorobic conditions. When the sorption media is satu-rated, an aerobic environment is most likely to bemaintained for quite a while in the wet ponds. How-ever, both anaerobic and aerobic conditions wouldappear alternately in the dry ponds so that the denitri-fication process returns after storm events affecting thesorptive process. This would complicate the nutrientremoval process in the sense that we cannot count onthe denitrification process as the major nutrient re-moval mechanism. Consequently, a more profoundstudy with respect to the physiochemical processes,such as adsorption, absorption, and ion exchange,would become of interest.

Removal of ammonia, nitrite, nitrate, and phospho-rus by sawdust, tire crumb, sand, clay, and otherorganic media can be achieved using natural,functionalized, and/or engineered filter media. Using


sorption media for the removal of both nitrogen andphosphorus species has been well documented [22].In recent years, Debusk et al. [23], Kim et al. [24],Clark et al. [25], Boving and Zhang [26], Hsieh andDavis [13], Ray et al. [20], and Seelsaen et al. [27] car-ried out experiments to remove nutrients from storm-water runoff using a suite of sorption media. Theyinclude but are not limited to sawdust, peat, compost,zeolite, wheat straw, newspaper, sand, limestone,expanded clay, wood chips, wood fibers, mulch,glass, ash, pumice, bentonite, tire crumbs, expandedshale, oyster shell, and soy meal hull. Table 1presents a list of sorption media used in previouswork to remove nutrients from stormwater.

METHODOLOGY

The overall evaluation of sorptive processes iscommonly comprised of two aspects: performanceand capacity. Performance in terms of filtrationkinetics refers to the efficiency of the process and theconcentration of the resulting effluent. Capacity refersto how much of the pollutant can be removed beforethe green sorption media must be replaced, which isrelated to the isotherm tests.

Isotherm Study for the Sorption Media MixtureAn adsorption isotherm can be produced by

exposing a known quantity of adsorbate to variousdosage of adsorbent in a batch test. The isothermgives us am idea about when a certain amount of ad-sorbent reaches the equilibrium condition with afixed mass of adsorbate. Adsorption also depends onthe solubility of adsorbent. The adsorption strength isinversely proportional to solubility [41]. In our experi-ment, about 800.00 g filter media mixture was pre-pared by using 50.00% sand, 20.00% limestone,15.00% sawdust, and 15.00% tire crumb. A knownconcentration of adsorbate solution (i.e., 1.00 mg/L)was prepared from stock solution. Three hundredmilliliter of that solution was transferred into eachErlenmeyer flask and five flasks were used. Then50.00 g of media mixture was taken in flask 1, 100.00g in flask 2150.00 g in flask 3, 200.00 g in flask 4, and250.00 g in flask 5 simultaneously. The top of theeach flask was covered by parafilm to minimize out-side disturbance during the waiting period. All theflasks were kept on a shaking platform (Innova 2000,New Brunswick Scientific) with 50 rpm for a certaintime (variable for each different adsorbate). After thewaiting period, the flasks were removed from theshaking platform and samples were collected fromthe flasks. The test temperature was in between 22and 238C (i.e., in room temperature). Isotherm curvesfor ammonia, nitrate, nitrite, OP, and TDP were cre-ated via this procedure.

Ammonia��nitrogen (NH3��N; Fisher Scientific)solution was prepared from anhydrous NH4Cl (driedat 1008C), nitrate (NO3��N) solution was preparedfrom KNO3 (dried at 1058C for 24 h) and nitrite(NO2��N) solution was prepared from NaNO2 fromFisher Scientific. All solutions were freshly preparedto avoid possible contamination. In some cases am-

monia (100.00 mg/L), nitrate (10.00 mg/L), and stand-ard phosphorus stock solutions (50.00 mg/L) werepurchased commercially from HACH (Loveland, CO).All the glassware was washed with HCl (i.e., 1:1 solu-tion) before starting the experiments.

The Freundlich and Langmuir isotherm equationswere used to analyze the data. The Langmuirisotherm is obtained by plotting a graph between 1/qand 1/C and the Freundlich isotherm by plottingbetween log q and log C. Overall, the following twoequations were applied in this study.� Freundlich isotherm equation [5],

log q ¼ logK þ 1

nlogC (1)

� Langmuir isotherm equation [5],

1

q¼ 1

qmKads

1

C

� �þ 1

qm(2)

where q is the sorbed concentration (mass adsorbate/mass adsorbent), qm is the maximum capacity ofadsorbent for absorbate (mass adsorbate/massadsorbent), C is the aqueous concentration of adsorb-ate (mass/volume), Kads is the measure of affinity ofadsorbate for adsorbent (unitless), and K is the Mea-sure of the capacity of the adsorbent (unitless).

Life Expectancy of the Sorption MediaThe isotherm testing enables us to determine the

life expectancy of filter media in BMP operation. Thislife expectancy can be determined with respect toeach type of pollutant of concern in the study. First,the maximum capacity of adsorbent for a particulartype of adsorbate may be retrieved from the corre-sponding isotherm plot. The life expectancy of filtermedia depends on the amount of media used in aspecific system, the concentration of nutrient instormwater and flow rate of stormwater. If we knowthe concentration of nutrient in and flow rate ofstormwater, the amount of nutrient per year in storm-water can be calculated, and thus the life expectancyof media may be easily deduced.

Removal Efficiency, Kinetics, and Head LossA laboratory column test method is a physical

model which attempts to simulate a portion of thereal world subsurface environment under a controlledset of experimental conditions. Five Plexiglas col-umns with a diameter of 5.00 cm (2.00 inches) andlength of 30.00 cm (1.00 foot) were prepared. All thefive columns were tied with a wooden frame. Alljoints of the columns were made leak proof usingpipe thread sealant. The top and bottom of the col-umn were closed but removable screw caps wereused to enable addition and removal of media. A fil-ter with glass beads (diameter of 4.00 mm) wasplaced at the bottom to prevent the outward flow offiner particles from the column during the collectionof samples. Although the column is 30.00 cm long,the media was only filled up to about 22.50 cm (9.00


Table 1. Sorption media used by different researchers to treat stormwater.

No. Sorption media

Additionalenvironmental

benefits via otherpollutant removal

Physical/chemicalproperties References

1 Sandy coastal soil [28]2 Compost Oil and greases,

heavy metalsMaple and elm leaf

compost[29]

3 Peat Cu, Cd, Ni [23]WollastoniteLimerockSand with quartz

4 Alfalfa D < 4.00 mm [24]Leaf mulch compost D < 2.00 mmSawdust D < 2.00 mmWheat straw D < 4.00 mmWood chips D < 2.00 mmNewspaper D (average) < 4.00 mmSulfur For large particles,

D 5 2.00–2.36 mmand for smallparticles,D 5 0.60–1.18 mm

Limestone D 5 0.60–1.18 mm5 Crushed piping materials Organics [30]6 Iron sulfide [31]7 Peat Cu, Fe, Pb, Zn [25]

Carbon sand, enretechsand, or sand

ZeolitesActivated carbon

8 Natural sand (bankfiltration)

[32]

9 Lignocellulosic material Basically pine barkchips

[33]

10 Clay Cd, Pb, Ni [34]11 Zeolites [35]12 Opoka Microorganisms [36]13 Waste medium density

fiberboard (MDS)sawdust

[37]

14 Wood fibers Polynucleararomatichydrocarbons

Aspen wood fiberscomposed of51.00% cellulose,26.00% hemicellulose,21.00% lignin, and1.00% ash

[26]

15 Mulch Lead, TSS, oil,and grease

[13]Soil Sandy loamSand Sand

16 Zeolites Cu, Pb, Zn [15]Pure quartzitic sand

17 Allophane [38]ChitinPumice Iron (18.20%),

aluminum (13.70%),calcium (12.70%),magnesium (7.30%),and others

Bentonite 4.00–8.00% calciumcarbonate


inches) from the bottom. Tygon (Saint-Gobain, no.16) tubes were added to both the top and bottom ofthe column for delivering the flow of influent andeffluent. Influent was flowed to the column from areservoir by using a peristaltic pump (Master flex L/S,Cole-Parmer instrument).

The kinetics for nitrate, nitrite, OP, TP, and TDPwere derived from the column study. Kinetics wasderived for each species with different influent con-centrations that mimic the actual fluctuations instormwater dry ponds. Filtration kinetics gives anidea about the velocity of a chemical reaction andhelps us to estimate the duration of the nutrient re-moval process by sorption media. It provides us withan estimate of the residence time and volume of areactor. For example, it is expected that lime-stone (CaCO3), as calcium (Ca21) ion, will help toremove phosphorus in the form of hydroxyapatite(Ca5(OH) (PO4)3). The approximate chemical reac-tion is shown as follows [42],

CaCO3 ! Ca2þ þ CO�3 (3)

3HPO2�4 þ 5Ca2þ þ 4OH� ! Ca5ðOHÞðPO4Þ3 þ 3H2O

(4)

A schematic diagram of the columns setup is givenin Figure 1. Four columns were loaded with 580.00 gof media mixture and the fifth one, the control, wasloaded with natural soil collected from the Hunter’sTrace pond in Marion County, Florida. The reason forsuch separation of testing in different columns withrespect to different chemical species is to avoid thecrosscontamination by the different chemical species

of interest. The surface areas of sorption media wouldplay an important role for the adsorption, absorption,and the growth of microbes for nitrification/denitrifica-tion. Yet this testing would not consider the impact ofthe growth of microbes for nitrification/denitrification.It was expected that sorption processes may dominatethe system in the first few hours allowing us retrievethe kinetics information without interference. The sub-sequent abiotic test ensured that this assumption heldthroughout the tests.

To simulate practical conditions as far as possible,no pretreatment of the sorption media and naturalsoil was carried out. The stormwater was collectedfrom UCF campus. The influent concentration of thestormwater was then controlled by spiking fromstock solution (i.e., augmentation). The influent con-centration portfolio for all testing species comprised5.00 mg/L, 2.50 mg/L, and 0.50 mg/L although itmight vary by 65.00% in actual testing due to theinstability of augmentation. The experiment wasdone in a batch mode. The five columns wereflushed three times by the experimental solution priorto the beginning of the experiment to maintain a con-sistent working environment. Flushing is expected toremove some off the possible contaminants fromsorption media mixture before starting the experi-ment. After flushing, the valve at the bottom of eachcolumn was closed to retain the nutrient laden solu-tion into the media. Samples, except for ammoniaand TN, were collected after 1.00, 3.00, and 5.00 hgenerally by opening the valve at the bottom of thecolumn. For ammonia and TN, the sample collectiontime was 0.50, 1.00, and 1.50 h. Each time, about60.00 mL of sample was collected from each columnfor study of the kinetics. The samples were diluted in

Table 1. (Continued).

No. Sorption media

Additionalenvironmental

benefits via otherpollutant removal

Physical/chemicalproperties References

Steel slagLime stoneZeolites

18 Hard wood mulch Cu, Cd, Cr, Zn, Pb,dichlorobenzene,naphthalene, fluoranthene,benzopyrene

Silver maple, Norwaymaple, Red oak,and Cherry mulch, size4760 micron,

[20]

19 Wood fibers Zn, Cu D 5 4.00 mm [27]SandZeolitesGlass D 5 4.00 mmAshCompost

20 Iron sulfide [39]21 Metallic iron D 5 0.006–0.01 mm;

surface area 0.31 m2/g[40]

Clinoptilolite Fe D 5 0.18–4.00 mm

D is the diameter of the media.


case of higher concentration during the chemicalanalysis to avoid exceeding the detection limit.

Kinetics studies play a significant role in the designof an optimized reactor to produce the desired prod-uct. In most studies, it is common to initially assumefirst-order reactions (see Eq. 5a), and the rate con-stant k (h21) is calculated from the slope of the lineof ln[C0]/[C] versus reaction time. Integration of equa-tion results in

�dC=dt ¼ k½C� and ln½C0�=½C� ¼ kt (5a)

where C0 is the influent concentration (ie., nutrient inthis case).

The rates may be calculated from a linear regres-sion of ln[C0]/[C] versus reaction time for the reduc-tion of ammonia, nitrite, nitrate, OP etc. if first-order kinetics is fairly well followed. If first-orderreaction is not a good fit, a second-order reactionmay be assumed as the kinetics by a similarapproach in which graphs between 1/C versus time

for each species may be plotted for identification(see Eq. 5b).

�dC=dt ¼ k½C�½Hþ� and 1=½C� ¼ 1=½C0� þ kt (5b)

A list of methods used in the chemical analysis isshown in Table 2. A HACH 2800 spectrophotometeris used to determine the effluent concentration ofnutrients by using Powder pillows (purchased fromHACH Company, Loveland, CO). The pH values weremeasured using an Accumet research pH meter (typeAR 50-duel channel). In these columns, however,both nitrification/denitrification and sorption mecha-nism may participate in the removal process.

A sensitivity analysis with respect to the pH valuesin the water bodies and initial concentrations ofammonia and nitrates was conducted to observe theeffect of varying pH values on different nutrientsgiven that the limestone may behave as a buffer. It isexpected that the change of pH values will play animportant role, especially in adsorption process.

Table 2. Analytical methods for the determination of chemical species in effluent.

Chemical species Title of method Method no.

Ammonia as nitrogen Salicylate method Method 8155Nitrate as nitrogen Cadmium reduction method Method 8192, 8171Nitrite as nitrogen Diazotization method Method 8507Total nitrogen Persulfate digestion method Method 10071Total dissolved phosphorus Acid persulfate digestion method Method 8190Total phosphorus Acid persulfate digestion method Method 8190Orthophosphate PhosVer 3 (ascorbic acid) method Method 8048

Figure 1. Schematic diagram of the column setup (Here CP 5 control panel; columns 1–4 are filled with theproposed media mixture and column 5 is filled with soil collected from Hunter Trace pond as control). [Colorfigure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]


Initial pH values were adjusted from 3.00 to 5.00,7.00, and 9.00 and the initial concentrations from 0.50to 2.50 and 5.00 mg/L in the sensitivity test. The HRTwas 5.00 h for nitrate and OP and 45.00 min for am-monia.

It was a major concern during the experiment toelucidate whether the removal process of nutrientsfrom stormwater is mainly a physicochemical or amicrobiological process. An abiotic test was con-ducted to confirm the type of process that wouldprove that sorption mechanism is the dominant one.A stock solution of 2,000.00 mg/L of HgCl2 was pre-pared for abiotic control. Nine milliliters of HgCl2was added to every 1.00 L of influent. The HRT was5.00 h for nitrate and OP and 1 h for ammonia,respectively. The abiotic test was conducted for am-monia in response to the presence of nitrifiers,whereas it is conducted for nitrate and phosphorus inresponse to the presence of denitrifiers and phospho-rus accumulating bacteria, respectively. All other con-ditions remained identical (i.e., as in the kinetic anal-ysis). Extreme care was taken to use HgCl2 duringthe experiment since it is a hazardous substance.

The head loss of the column was also measured asa critical engineering design parameter. For this rea-son, two new columns with the same size as those inthe column test were built. Each column has threeholes: One is at the top, another one is at the bottom,and the other is in the middle. The distance betweentop and bottom holes is about 22.86 cm and the mid-dle hole is about 11.43 cm below the top one. A tube(inner diameter of 5.00 cm) as piezometric tube wasconnected with each hole using glue. The water wasdirected to flow continuously into the column from a

reservoir about 120.00 cm above the floor of theroom and column bottom is about 10.16 cm abovethe floor. The reading was taken 15.00 min after com-mencement of water flow.

RESULTS AND DISCUSSION

Isotherm Study for the Sorption Media MixtureFrom Tables 3 and 4, it can be seen that the value

of n is >1 for nitrate and TDP. When n 5 1 or lower, itindicates that all types of adsorbent have an equal af-finity for the adsorbate. When n > 1, the affinitydecreases with increasing adsorption density [5]. Themaximal capacity of adsorbent for adsorbate is alsoshown by qm. The isotherm plots (Figures 2–6) areshown later to provide an idea of the overall scenario.

Life Expectancy of the MediaSuppose that 300,000.00 g of media is used in a

BMP system to remove OP in the runoff. Based onour isotherm test of OP, the maximum waste load is0.01 mg nutrient/mg filter media. So the maximumamount of OP that can be adsorbed is 2310.00 g(0.01 mg/mg 3 300,000.00 g). Assuming that stormwater has an average OP concentration of 1.00 mg/Land the average storm water flow is about 378.50 Lper day (100.00 gal per day), then the total amountof OP will be about 138.15 g/yr [i.e., (100 3 365 33.78 3 1.00)/1000]. As a result, the life expectancy ofthe media mixture for OP removal would be about16.74 yr (2,310.00/138.15). This life expectancy mayvary according to the type of media used, the wasteloads in stormwater, and the intensity, frequency and

Table 3. Parameters of the Langmuir isotherm for different species.

SpeciesIsotherm equation

for Langmuir R-square value 1/(qmKads) 1/qm (mg/mg)

NH3��N y 5 10,233.000x 2 8880.700 0.94 10,233.00 28880.70OP y 5 272.850x 2 129.740 0.97 272.85 2129.74NO3��N y 5 128.740x 1 1030.000 0.80 128.74 1030.00NO2��N y 5 229,620.000x 2 229,133.000 0.84 229,620.00 2229,133.00TDP y 5 101.120x 1 137.000 0.74 101.12 137.00

y 5 1/q; x 5 1/C.

Table 4. Parameters of the Freundlich isotherm for different species.

SpeciesIsotherm equationfor Freundlich R-square value 1/n log K K (mg/mg)

NH3��N y 5 3.951x 2 3.213 0.95 3.95 23.21 0.001OP y 5 1.293x 2 2.215 0.96 1.29 22.22 0.006NO3��N y 5 0.231x 2 3.043 0.85 0.23 23.04 0.001NO2��N y 5 34.571x 2 3.389 0.75 34.57 23.39 0.00041*TDP y 5 0.771x 2 2.268 0.75 0.77 22.27 0.005

y 5 log q; x 5 log C.*Very small number that might be ignored.


Figure 2. The isotherm study for ammonia. (a) The Langmuir isotherm plot and (b) the Freundlich isothermplot.

Figure 3. The isotherm study for orthophosphate. (a) The Langmuir isotherm plot and (b) the Freundlichisotherm plot.

Figure 4. The isotherm study for nitrate. (a) The Langmuir isotherm plot and (b) the Freundlich isotherm plot.


duration of the stormwater in the study area. Basedon the same rationale, Table 5 summarizes all the rel-evant life expectancies of the functionalized filtermedia with respect to each type of pollutant of con-cern in this study. It appears that the effective re-moval of nitrogen species will probably be more amicrobiological than physicochemical process.

Filtration KineticsA great difference of removal efficiency was

observed across differing column tests and the iso-

therm studies. Nutrients cannot flow through the flaskin the isotherm test as there is no inflow or outflowin the flask setup. In the column tests, however, themedia, such as sawdust, may release (i.e., desorption)some nutrients absorbed in the early stage, whichcould ultimately impact the total removal efficiency.For this reason, we conducted the test by flushing thecolumn three times at the beginning of each run towash out the contributed nutrients possibly from saw-dust in previous testing. Findings in the filtrationkinetics analysis showed that if the influent concen-tration is lower ammonia, the sorption media canremove it in a relatively more efficient way. Theremoval efficiency may even reach 100.00% withwaste load concentrations of 0.50 and 2.50 mg/L after1.00 and 1.50 h of HRT, respectively. When theammonia concentration was up to 5.00 mg/L, theremoval efficiency was about 64.00% after 1.50 h ofHRT. Given that the ammonia concentration is nor-mally not very high in stormwater, the proposedsorption media mixture should work well in terms ofremoving ammonia from stormwater runoff. Theremoval efficiency of nitrate was about 95.36%,81.34%, and 65.68% after 5.00 h of HRT when the

Figure 5. The isotherm study for nitrite. (a) The Langmuir isotherm plot and (b) the Freundlich isotherm plot.

Figure 6. Isotherms for total dissolved phosphorus (TDP). (a) The Langmuir isotherm plot and (b) theFreundlich isotherm plot.

Table 5. Isotherm test of sorption media mixture fordifferent nutrient species.

Species Life expectancy (yr)

Ammonia as nitrogen 0.244Orthophosphate 16.737Nitrate as nitrogen 2.108Nitrite as nitrogen 0.009Total dissolved phosphorus 15.851


influent waste loads were 0.50, 2.50, and 5.00 mg/L,respectively. Results indicated that the removal effi-ciency was about 94.14% and 98.72% when the influ-ent waste loads were 0.50 and 2.50 mg/L, respec-tively. However, it was reduced to 65.40% when theinfluent waste load was as high as 5.00 mg/L. Withthis observation, we conclude that the filter mediamixture is efficient and effective for the removal ofboth nitrate and nitrite at lower influent concentra-tions (i.e., 0.50 and 2.50 mg/L) that covers most ofthe cases in real world stormwater management sys-tems. With such an analysis, it is certain that the pro-posed sorption media mixture can remove TN.

OP (70.00–90.00%) is the main component of TP.The removal of OP was 79.50, 94.39, and 97.50% af-ter 5.00 h HRT, when the influent concentrationswere 0.50, 2.50, and 5.00 mg/L, respectively. The re-moval of OP increased with increasing influent con-centrations indicating that the proposed media mix-ture may perform well if the stormwater has higherphosphorus concentration. The same tendency wasobserved for the cases of TDP and TP removal. Theremoval of TDP was 86.30, 96.06, and 98.17%, whenthe influent concentrations were 0.50, 2.50, and 5.00mg/L after 5.00 h HRT. The removal of TP was>99.00% independent of the magnitude of influentconcentrations. Hence, it is confirmed that the pro-posed media mixture will be effective in removingother forms of phosphorus.

The removal efficiency of nutrients of the naturalsoil was also observed for comparison. The resultsconfirmed that natural soil is not capable of removingnitrate given—only 19.20% nitrate was removedwithin 5.00 h HRT with influent concentration of

0.50 mg/L. But natural soil seems to be quite effectivein removing ammonia. The removal efficiency of am-monia was about 98.68% and 96.20% within 1.50 hHRT when the influent concentration was 0.50 and5.00 mg/L, respectively. However, natural soil cannotadsorb the ammonia and nitrate for a long time andsome desorption was frequently observed. Naturalsoil can adsorb some nitrite at lower influent concen-tration. The removal efficiency of OP by natural soilwas not good enough at lower influent concentration.The results showed that it can only remove 19.40% ofOP at an influent concentration of 0.50 mg/L. Butboth TP and TDP had a removal efficiency of>75.00% in our tests.

Modeling the filtration kinetics would help the en-gineering design. We assume that the proposed filtermedia in these experiments may follow either first-order or second-order filtration kinetics. The regres-sion equations, R-square values, and rate constantsmay be determined from the appropriate plots. Wefound out that it is very difficult to determine thekinetics for ammonia as it is removed very quickly bythe media. Overall, the OP, nitrate, and nitrite followthe second-order filtration kinetics. In the case of OPtesting, second-order filtration kinetics may bederived with respect to a good R-square value of0.70–0.94. The minimum R-square value for nitrate is0.88 and for nitrite is 0.81.

Tables 6 and 7 summarize all kinetic analysesbased on the proposed sorption media and naturalsoil. With these R-square values, it can be concludedthat all the water quality constituents follow second-order filtration kinetics more closely. This is mainlydue to the collective impact of both the influent con-

Table 6. Summary of kinetics for the proposed sorption media mixture.

Species

Initialconcentration

(mg/L)

First-order

equation

R-squarevalue forfirst-orderequation

K valuefor first-order

equation(h21)

Second-orderequation

R-squarevalue forsecond-order

equation

K valuefor

second-order

equation(L/mg h)

Nitrate 5.00 y 5 0.230x 0.65 0.75 y 5 0.074x 1 0.193 0.99 0.072.50 y 5 0.330x 0.99 0.33 y 5 0.302x 1 0.391 0.92 0.300.50 y 5 0.749x 0.26 0.25 y 5 9.516x 1 2.000 0.88 9.52

Orthophosphate 5.00 y 5 0.521x 0.28 0.52 y 5 1.637x 1 0.201 0.86 1.642.50 y 5 0.420x 0.11 0.42 y 5 1.511x 1 0.389 0.69 1.510.50 y 5 0.105x 0.78 0.11 y 5 1.340x 1 1.754 0.94 1.34

Nitrite 5.00 y 5 0.222x 0.83 0.22 y 5 0.072x 1 0.198 0.92 0.072.50 y 5 0.897x 0.99 0.89 y 5 5.088x 1 0.402 0.82 5.090.50 y 5 0.683x 0.65 0.68 y 5 6.736x 1 1.879 0.93 6.74

TP 5.00 y 5 1.328x 0.78 1.33 y 5 11.275x 1 0.202 0.96 11.282.50 y 5 1.314x 0.52 1.31 y 5 19.46x 1 0.413 0.73 19.460.50 y 5 0.954x 0.94 0.95 y 5 27.53x 1 1.68 0.75 27.53

TDP 5.00 y 5 0.942x 0.44 0.94 y 5 2.089x 1 0.199 0.91 2.092.50 y 5 0.692x 0.74 0.69 y 5 1.715x 1 0.405 0.86 1.720.50 y 5 0.519x 0.23 0.52 y 5 3.454x 1 2.045 0.36 3.45

For the first-order equation, y 5 ln[C0]/[C] and x 5 t; for the second-order equation, y 5 1/[C] and x 5 t.


centrations and the pH values. Apparently, the pro-posed media mixture exhibits better removal effi-ciency in terms of all chemical species of concern(i.e., ammonia, nitrate, nitrite, TN, TP, TDP, and OP).Our justification is that ammonia, nitrate, nitrite, andTN were mainly removed by sawdust and tire crumbvia adsorption and/or some ion exchange whereasTP, TDP, and OP were mainly removed by tire crumband limestone via absorption and precipitation.

Abiotic TestFinally, the chemical analysis for the abiotic test

confirmed that the nutrients removal process in ouranalysis was mainly a physicochemical process. After5.00 h of HRT, the removal efficiency of nitrate andOP was about 83.32 and 92.20%, respectively, withan initial concentration of 0.50 mg/L. The ammoniaremoval efficiency was about 100.00% after 1.50 h ofHRT and the same initial concentration. All of theremoval efficiencies remain almost to those weobserved in the kinetics analysis. Since we did notseed or add sludge into the column to foster anyamenable microbial environment and the natural andcitrus grove sand was heated up to 1058C prior touse, it is not possible for bacteria to grow in such ashort HRT in the media. In other words, no nitrifica-tion/denitrification process was triggered in our test.Aerobic and anaerobic conditions are very importantto trigger biological nitrification/denitrification. ThepH of the effluent was 6.50–8.00.

pH Values and Sensitivity AnalysisThe pH of effluent varied between 7.00 and 8.00

in the media columns, and 6.00 and 7.50 in the natu-ral soil column (i.e., the control case) at room tem-perature. Room temperature was between 22 and248C. These pH values have important effect on filtra-tion kinetics. If the medium is acidic, it cannot retain

the nutrients sufficiently long and desorption startsvery soon after adsorption. On the other hand, somesorption media can retain the nutrient well even inbasic pH environments. Again, basic pH is favorablefor the precipitation reactions between phosphorusand limestone. In summary, the proposed sorptionmedia can quickly remove the nutrient from storm-water runoff whereas the natural soil can remove partof the nutrient if the HRT is long enough. However,the latter is not practicable because the stormwaterwould reach groundwater quickly via seepage flow ifthe vadose zone is shallow. For this reason, it is bet-ter to use sorption media to remove nutrient fromstormwater runoff before it reaches groundwateraquifers.

OP can be removed through the precipitation reac-tion by calcium ions. This precipitated portion maysettle on the surface of sorption media and restrictthe further adsorption process. It can be assumedthat neutral pH will be best for nutrient removal bysorption media. With the aid of sensitivity analysis asshown in Figures 7–9, it was observed that theremoval efficiency increased with increasing pH fornitrate and ammonia. In Figures 7–9, the plots revealthat when the pH value is close to 5–7, the removalefficiency of OP reaches its maximum. Overall, theremoval efficiency showed a decreasing trend withincreasing pH values for OP if the pH values were>7. Our evidence clearly showed that OP removalwas well-established at pH � 7.00 and started todecrease after pH 5 7.00. The results in Figure 7 par-ticularly confirm that basic pH values are not alwaysfavorable for the precipitation reactions betweenphosphorus and limestone since additional factors,such as initial concentrations, might also be influen-tial.

Both nitrate and ammonia are soluble in water.Nutrients may be dissolved in water at acidic pHand adsorbed on solid surface at higher pH. The

Table 7. Summary of kinetics for the natural soil (Hunter’s trace soil).

Species

Initialconcentration

(mg/L)

First-order

equation

R-squarevalue forfirst-orderequation

K valuefor first-order

equation(h21)

Second-orderequation

R-squarevalue forsecond-order

equation

K valuefor

second-order

equation(L/mg h)

Nitrate 5.00 y 5 0.230x 0.65 0.75 y 5 0.074x 1 0.1903 0.99 0.070.50 y 5 0.066x 0.22 0.07 y 5 1.635x 1 2.146 0.21 1.64

Orthophosphate 5.00 y 5 0.577x 0.39 0.58 y 5 0.443x 1 0.202 0.71 0.440.50 y 5 0.036x 0.84 0.04 y 5 0.065x 1 1.650 0.82 0.07

Nitrite 5.00 y 5 0.146x 0.25 0.15 y 5 0.039x 1 0.197 0.31 0.040.50 y 5 0.652x 0.88 0.65 y 5 6.101x 1 1.820 0.96 6.10

TP 5.00 y 5 1.003x 0.75 1.00 y 5 3.344x 1 0.221 0.91 2.090.50 y 5 0.336x 0.85 0.34 y 5 1.425x 1 1.981 0.97 1.43

TDP 5.00 y 5 0.953x 0.41 0.95 y 5 1.946x 1 0.176 0.46 1.950.50 y 5 0.620x 0.33 0.62 y 5 5.502x 1 2.083 0.66 5.50

For the first-order equation, y 5 ln[C0]/[C] and x 5 t; for the second-order equation, y 5 1/[C] and x 5 t.


adsorption/absorption of ammonia and nitrate in thefilter media depends on both pH values and initialconcentrations. The pH values would normally varyfrom 7.00 to 8.00. In general, the acidic pH shouldpromote desorption whereas the basic pH shouldfavor the adsorption. There might be competitionbetween adsorption and desorption process in thesystem at acidic pH. It is expected that the adsorptionprocess would prevail facing increasing pH and ulti-mately show the higher removal efficiency. In anyinstance, with the inclusion of limestone as one ofthe ingredients in the filter media, the pH values arebuffered to the maximum so that the nutrient removalremains almost same over all pH levels tested in thesensitivity analysis.

Head LossThe head loss was calculated based on the afore-

mentioned procedure in a batch run. It presents thepermeability rate in the system. Within the natural

soil column, the head loss was about 57.15 cm ofwater (22.50 inches of water) and in filter media col-umn the head loss was about 83.82 cm in water(34.00 inches of water). Storm water detention pondsor dry ponds are areas that are normally dry, butfunction as detention reservoirs during storm events.The head loss information may be used to design theessential depth of the dry ponds so as to help thestorm water pass through the pond via infiltrationbefore overflow. Their volume should at least beequal to the average runoff event during the year.The removal of nutrients in these dry ponds could beworse than that in wet ponds.

CONCLUSIONS

In stormwater management, dry ponds have dualpurposes in both quality and quantity control. With-out having specific filter media, typical removal effi-ciencies in dry ponds would vary between 10.00 and20.00% [43]. This study proved that the functionalizedfilter media may effectively and efficiently removemost of the nutrient species within an appropriateHRT via both adsorption and the more dominantabsorption processes. The life expectancy of the pro-posed media is reasonably long for removing phos-phorus species thus ensuring system reliability ingreen infrastructures. However, this is not the casefor nitrogen removal. Microbiological effects (i.e., ni-trification and denitrification effects) need to be care-fully considered to complete the design goal. Thecolumn test in this study was set up to test its poten-tial for nutrient removal in dry ponds where storm-water impact is in a batch mode. During field appli-cation, the proposed sorption media mixture can bewrapped in geotextile and laid at the bottom of theriprap apron area in dry pond systems or placed aspart of the fore bay. In any circumstance, the assur-ance of HRT would be a major challenge in fieldapplication because the time for the intermittent flow

Figure 8. Variations of removal efficiency of nitratewith the varying pH values and initial concentrations.

Figure 9. Variations of removal efficiency of ammoniaas a function of varying pH values and initialconcentrations.

Figure 7. Variation of removal efficiency of OP withthe varying pH values and initial concentrations.


(i.e., infiltrate) to pass through the media layer mustconstitute the legitimate HRT. Otherwise, the removalmechanism would be limited as a physicochemicalprocess. In field applications, the design of thicknessof the media layer at the bottom of the dry pondsmay be examined further with regard to the treatmentefficiency data.

ACKNOWLEDGMENTS

The authors appreciate and acknowledge the sup-port of the Southwest Florida Water Management Dis-trict and the professional advice and guidance ofChris Zajac from the District.

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