Phosphorus removal by blast furnace slag and cement...

300 © IWA Publishing 2011 Water Quality Research Journal of Canada | 46.4 | 2011

Phosphorus removal by blast furnace slag and cement

clinker – flow cell studies for estimation of sorptive

capacity for use with constructed treatment wetlands

Anamika Sikdar Paul and Bruce Anderson

ABSTRACT

Blast furnace slag and cement clinker were explored in long-term flow cell experiments for

estimation of their phosphorus (P) removal efficiencies. A local gravel, typically used in constructed

treatment wetlands, was used as a control medium. The experiments examined the removal of

phosphorus from a solution initially containing 4 mg P/L. The slag and clinker were nearly 100%

efficient due to very high sorptive capacities. The control gravel medium removed 50% of the influent

phosphorus. Results from this study indicate that the use of blast furnace slag in constructed

wetlands or filter beds is a promising solution for P removal via sorption mechanisms.

doi: 10.2166/wqrjc.2011.112

Anamika Sikdar Paul (corresponding author)AMEC Earth and Environmental,160, Traders Blvd (E),Suite 110, Mississauga, ON,L4Z 3K7 CanadaE-mail: [email protected]

Bruce AndersonDepartment of Civil Engineering,Queen’s University,Kingston ON,Canada,K7L 3N6 Canada

Key words | adsorption, clinker, constructed wetland, phosphorus, slag, sorption capacity

INTRODUCTION

The adverse effects of eutrophication due to the presence of

phosphorus in surface waters are well established (Orive

et al. ). The OntarioWater Resources Act 1990 Guideline

F-5 sets the total phosphorus limit of 1 mg/L formunicipal and

private sewage treatment systems discharging into a water-

body. Sunny Creek Estates (SCE) is a mobile home village

using a combination lagoon–CW system to treat its sewage

and discharges directly into the Bay of Quinte. The Bay of

Quinte on the north eastern shore of Lake Ontario is a recog-

nized Area of Concern. The Bay of Quinte Remedial Action

Plan (BQRAP) set an objective of 0.3 mg/L of total phos-

phorus and a further stringent guideline of 0.1 mg/L is

presently being considered. Currently, the SCE treatment

system does not achieve compliance with respect to effluent

concentrations of phosphorus, and better treatment is needed.

Conventional technologies for removal of phosphorus

from point source wastewater discharges are physical pro-

cesses (settling, filtration), chemical precipitation (with

aluminum, iron and calcium salts) and biological processes

that rely on biomass growth (bacteria, algae, plants) or

intracellular bacterial polyphosphates accumulation

(Bashan & De-Bashan ). Long-term studies and

increased operational experience indicate that phosphorus

removal is variable or inconsistent (Richardson & Craft

; Reed & Brown ; Wood ) in subsurface con-

structed wetlands (CW) that can be attributed to the

complexity of phosphorus removal mechanisms, and the

lack of consideration of these complexities in design.

The major factors that make P removal by the wetlands

particularly difficult are the type, quantity and diversity of

the influents that need to be treated. The principal phos-

phorus removal mechanism, adsorption/precipitation,

being a finite process, requires the P saturated substrate to

be replaced after a certain operational period (Faulkner &

Richardson ; Mann & Bavor ; Drizo et al. ;

Shilton et al. ). Given these, the sorption and deso-

rption of phosphorus in constructed wetlands is impacted

not only by the physical and/or chemical characteristics of

the substrate media, but also by phosphorus loading, hydrau-

lic conditions, temperature, time and dissolved oxygen.

When designing a CW for P removal, the selection of the

material to be used as thewetland substrate (rootingmedium)

plays a crucial role (Mann & Bavor ; Drizo et al. ,

; Johansson & Gustafsson ). A potential medium

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301 A. S. Paul & B. Anderson | Phosphorus removal by blast furnace slag and cement clinker Water Quality Research Journal of Canada | 46.4 | 2011

for a constructed wetland, if rich in Fe, Al and Ca ions, would

be considered for application in the treatment process. A gen-

eral summary of the media mentioned in the literature as

having been tested as wetland soils are blast furnace slag,

light expandable clay aggregates, cement clinker, crushed

brick, activated aluminum oxide, half burned dolomite,

maerl, siliceous sedimentary rock opaka, peat amended

with steel wool, red mud, mussel shells and shale.

When tested in batch and column laboratory experiments

at both low and high P concentrations, blast furnace (BF)

slags, waste products from iron production demonstrated

high P removal efficiencies (Mann & Bavor ; Baker

et al. ; Johansson & Gustafsson ). The results

observed by other researchers (Drizo et al. ) confirmed

that steel slag showedexcellent removal efficiencywhen com-

pared to the other investigated materials. Rosolen ()

tested 17 different materials in batch tests, and it was noted

that slag material and clinker showed the highest potential

of achieving efficient P removal. Van Weelden () and

Laska () evaluated P removal efficiencies by zebra

mussel shells, which are primarily made up of calcium com-

pounds, demonstrating satisfying sorption potential.

Although batch tests provide a good indication of the

material’s capacity to retain P, when making a selection of

potential substrates to be used in designing constructed wet-

lands and/or filters for P removal it is necessary to conduct

long-term column or pilot-scale experiments (Drizo et al.

; Shilton et al. ). These will allow evaluation of

the influential factors for both sorption and desorption in

these test systems.

The main focus of this research was to develop and

implement a program of experimental investigations into

the behavior of phosphorus sorption in the media surface

under normal laboratory conditions representative of those

occurring in the field. The work would sufficiently charac-

terize the media for eventual application in a filter or as

the rooting media in the wetland.

The research program was conducted in three stages.

The first phase involved conducting batch studies and inves-

tigating the reaction kinetics of the batch studies. The last

phase of the program was conducted to investigate the

forms of sorbed phosphorus within the media. The results

of the first and last phase of the program are being presented

in separate papers. The present paper presents the results of

the second phase of the research program. During the

second stage of the research program, long-term column

experiments using custom designed flow cells were con-

ducted for estimating the P removal efficiencies of slag

and clinker media. In addition, local gravel was used as a

control medium and the amount of P retained by each

medium was determined.

MATERIALS

Sample preparation

The media were washed, sorted and dried prior to commen-

cing formal laboratory investigations. Emphasis on local

availability, amount of processing required, natural abun-

dance and cost effectiveness were considered during

media selection. The slag was obtained from National Slag

Limited, Hamilton and clinker from Essroc Cement,

Picton. Gravel used as a control medium was being used

in the wetland in SCE.

The size used for all tests was 0.5–8 mm. The size ranges

for the media reported in the literature are 2.5–10 mm for

slag (Drizo et al. ), 2–10 mm for fine gravel (Seo et al.

), 8–16 mm for gravel (Prochaska & Zoubilis 2006),

2–5 mm for very fine gravel (Xu et al. ), 0.2–3.2 mm

for sand (Arias et al. ) and 3.5–10 mm for gravel

(Garcia et al. ). As such, the size range used here allowed

for some comparison to the previous studies. An unprocessed

(unwashed and not sorted) fraction of each material was

retained for subsequent media characterization studies.

METHODS

Laboratory-scale experiments were conducted to examine

the physical and hydraulic properties, and the phosphorus

retention mechanisms, of the three candidate media.

Media characterization

Thehydraulic and geometric designof thewetlandfilterwill be

strongly influenced by the physical properties of the selected

medium. Information on media particle size distribution,


along with porosity and hydraulic conductivity, can be used to

predict the movement of water through the filter.

Standard laboratory tests were performed to determine

the following:

• Particle size distribution analysis (ASTM D 422 )

• Bulk density determination (Black )

• Particle density determination (ASTM D 854 )

• Porosity calculations (Vomicil )

• Soil alkalinity (pH) measurements (Clesceri et al. ;

ASTM D 4972 )

• Hydraulic conductivity determination (ASTMD5856 )

Hydraulic conductivity

Constant head tests were performed using an up-flow per-

meameter to determine the hydraulic conductivity (K)

values of the media, as shown in Figure 1. The media were

placed in the permeameter and lightly compacted. For

each test, K was determined using Darcy’s Law for uncon-

fined flow through a porous medium as outlined in

Equation (1) (Reed & Brown ):

K ¼ VLAht

(1)

where V is the fluid volume (m3), K is the hydraulic conduc-

tivity (m/s), h is the differential head across the sample (m),

Figure 1 | Constant head test apparatus and permeameter for hydraulic conductivity

tests.

L is the sample length (m), A is the cross-sectional area of

media sample (m2) and t is the time (s).

Horizontal flow cell experiments

Batch testing is a convenient procedure for comparing the

performance potential of various sorbent materials; how-

ever, bench scale test results are only indicative of the

performance under static test conditions, and should not

be extrapolated to model field performance (Benefield

et al. ). To overcome some of the limitations in data

interpretation and application inherent to batch shaker test-

ing, flow cell experiments were conducted using horizontal

flow filter cells to investigate phosphorus retention proper-

ties of gravel, slag and clinker media.

Procedure

Figure 2 shows the testing apparatus used in the flow cell

studies. Approximately 5,100 cm3 of the media was placed

in the middle chamber of a three chamber plexiglass hori-

zontal flow cell. Figure 3 shows the dimensions of a flow

cell and the location of sampling wells. The total length of

each flow cell was 775 mm and the length of the middle

chamber was 600 mm. The lengths of the inlet and outlet

chambers were 110 and 65 mm, respectively. The height

and the width of the flow cell were 95 and 100 mm,

respectively.

An influent flow of 10 L/day was maintained continu-

ously throughout the study. The influent P concentration

Figure 2 | Horizontal flow filter cells (from left: slag, gravel and clinker).

Figure 3 | Details of the flow cell.


of 4 mg/L (±0.001) was passed continuously through the

flow cells. The influent concentration was based on

observed wetland influent concentrations and those noted

in the literature, which ranged from 30 to 1 mg/L (Drizo

et al. , , ; Johansson & Gustafsson ;

Arias et al. ; Metcalf & Eddy Inc. ).

At regular intervals, liquid from the sampling wells in

the flow cell was collected from fixed sampling locations

during the test. For the gravel flow cell, samples were col-

lected from location A (at 10 cm), B (at 25 cm), C (at

40 cm) and D (at 50 cm) from the inlet. For the slag and clin-

ker flow cells, the samples were collected from location A

(at 15 cm), B (at 30 cm) and C (at 45 cm) from the inlet.

The samples were analyzed for orthophosphate by

Quickchem method 10-115-01-1-A (Diamond ).

Samples were collected at intervals of 3 days for gravel

and 4 days for slag and clinker. For each sampling period,

incremental treatment volumes were calculated by multiply-

ing the flow rate by the length of the interval period. After a

period of 10 weeks, the flow cell study was terminated.

Treatment profiles

The phosphorus concentration profiles in the liquid phase

were developed along the length of each filter cell, and rep-

resented the amount of phosphorus being sorbed within the

media as time passed.

Sorption capacity

Orthophosphate removal was calculated based on percent

removal, R (%), of the incoming concentration. The phos-

phorus removed during a sampling interval period was

calculated based on the total load delivered to the filter

since the last sampling period and is given as follows:

P ¼ R100

� (Vi � Vs(i�1)) �Cin (2)

where P is the total phosphorus load (mg), Vi is the volume

treated to date (L) and Vs(i�1) is the total volume treated at

the previous sampling period (L) and Cin is the influent P

concentration (mg/L) in flow cell.

A section of the flow cell, Ms (kg) is the region between

the two adjacent sampling wells. The mass of a section of

media was estimated by using the percentage of the total

length of the filter in relation to the total mass of the filter.

The removal of phosphorus per unit mass for a given

time period, Xi (mg/kg), was the ratio of P to Ms. Finally,

the cumulative phosphorus sorptive capacity of each filter

cell was calculated in each section of the cell.

RESULTS AND DISCUSSION

Media characterization

Particle density (ρs), bulk density (ρb), porosity (η), hydraulic

conductivity (K) and soil (pH) for the three media are sum-

marized in this section.

Grain size distribution

Figure 4 shows the grain size distribution of the three media

plotted as %-finer on a grain size distribution curve. The

majority (∼70%) of the gravel was retained on the smaller

sieves (<2 mm) whereas the majority of the slag (∼82%)

Figure 4 | Plot of %-finer of media grain particles as a function of sieve size.

Table 1 | Physical characteristics of candidate media

Bulkdensity

Particledensity Porosity Hydraulic


and clinker (∼77%) were retained on the larger size

(>2 mm) sieves. Gravel had an effective grain size (d10) of

0.5 mm, coefficient of uniformity (Cu) of 3.40 and a coeffi-

cient of curvature (Cc) of 0.75. Clinker had an effective

grain size of 0.9 mm, coefficient of uniformity (Cu) of 4.44

and a coefficient of curvature (Cc) of 1.74. Slag had an effec-

tive grain size of 1.2 mm, coefficient of uniformity (Cu) of

2.92 and a coefficient of curvature (Cc) of 1.37. Coefficient

of curvature (Cc) and coefficient of uniformity (Cu) are

derived from effective grain sizes d10, d30 and d60.

All three media are classified as poorly sorted sand (SP)

under the Unified Soil Classification system based on the

values of Cc and Cu (ASTM D 2487 ). Recent Danish

guidelines as mentioned in Arias et al. () recommend

a d10: 0.3–2 mm, d60: 0.5–8 mm and Cu< 4 to ensure ade-

quate hydraulic conductivity. Except clinker (Cu¼ 4.44) all

other criteria were satisfied by the media. All three media

were expected to show good performance for phosphorus

sorption, based on studies such as that of Xu et al. (),

who noted that the phosphorus sorption capacity was influ-

enced by the physico-chemical characteristics of the media,

wherein finer grain size resulted in higher phosphorus

sorption.
Medium (g/cm3) (g/cm3) (%) pH conductivitya (m/s)
Gravel 1.603 2.6737 40.03 7.8 0.0056 (±0.001)

Slag 1.163 2.5874 55.06 10.8 0.25 (±0.07)

Clinker 1.004 3.1460 68.08 11.4 0.19 (±0.02)

aAverage of four tests.

Density and porosity

The observed results from the bulk density, particle density

and porosity values were consistent with values reported

by Rosolen (). Table 1 summarizes the average values

for the bulk density, particle density, porosity, pH and

hydraulic conductivity for the three media. Porosity affects

the fluid flowrate through the media, and a higher porosity

may correspond to a greater surface area and, therefore,

more exposed sites for phosphorus adsorption. The design

range for porosity recommended by Kadlec & Knight

() is 35–45%. Clinker was the most porous material.

Gravel was observed to have the recommended design por-

osity whereas clinker and slag had greater porosity than

recommended for use as a wetland medium for phosphorus.

However, as is discussed later, the high porosity values of

clinker and slag did not act in a detrimental way for phos-

phorus removal.

Hydraulic conductivity

Hydraulic conductivity tests showed that slag had the

highest conductivity, followed closely by clinker, and

Table 2 | Hydraulic loading parameters and effluent pH for flow cells

MediumPorosity(η)

Linear fluidvelocity, vx,(cm/h)

Hydraulicresidence time(h) Rea pH

Gravel 0.40 12.30 4.88 0.05 7.4–7.6

Slag 0.55 8.94 6.71 0.08 9.3–10.5

Clinker 0.68 7.23 8.30 0.06 9.5–11.0

aCalculated using linear fluid velocity.


finally the gravel. The recommended design range for the

hydraulic conductivity for the constructed wetlands

medium given by Reed & Brown () is 0.0012–0.12 m/s

(100–10,000 m3/m2 per day). Gravel had a hydraulic con-

ductivity of 0.0056 m/s which is within the recommended

design range given by Reed & Brown (). Even though

the hydraulic conductivities of clinker and slag were

higher than the recommended range, they still proved to

be good media for phosphorus removal as noted later.

This was probably due to the fast reaction times of the phos-

phorus sorptive reactions, which would be beneficial in

downsizing the filter bed.

Though the laboratory studies use consistent and repro-

ducible testing techniques, field conditions seldom resemble

laboratory conditions. Hydraulic conductivities calculated

in the laboratory could be reduced by as much as a factor

of ten in the field (Kadlec & Knight ). Calder et al.

() reported that the hydraulic conductivities at their

field site were reduced considerably, by a factor greater

than 10, from the laboratory derived design values.

pH

pH is an important factor for the phosphorus reactions as it

controls the type of chemicals taking part in the reactions

and also affects the dissolved oxygen content of the receiv-

ing water body indirectly (Drizo et al. ; Reddy &

D’Angelo ; Garcia et al. ). The effluent from the

gravel flow cell was able to meet the regulatory pH require-

ment whereas the effluent from the slag and the clinker flow

cell had high pH and did not meet the regulatory pH

requirement of 6–9.5 (Environmental Protection Act –

O. Reg. 560/94, Ontario Ministry of Environment ).

Thus, it may be necessary to have a neutralization process

(e.g. secondary filter with a low pH media like acidic peat)

after the application of slag or clinker for sorption in order

to regulate the final effluent pH.

The physical characteristics all influence the effective-

ness of a medium for the removal of phosphorus (Drizo

et al. ; Prochaska & Zouboulis ). However, these

effects are usually on a short-term basis. For long-term phos-

phorus removal to be enhanced and sustainable, additional

properties (e.g. chemical composition) of the medium and/

or operational parameters of the wetland or filter become

important (Arias et al. ). The additional properties and

operational parameters can be evaluated using flow cells

as described in the next section.

Horizontal flow cell experiments

Horizontal flow cell experiments were conducted to investi-

gate the phosphorus retention properties of the three media

(gravel, clinker and slag).

Hydraulic loading parameters

The linear fluid velocity (vx) and hydraulic residence time

(HRT), which are dependent on the porosity, were calcu-

lated. Table 2 summarizes the hydraulic residence time,

linear fluid velocity, Reynolds number (Re) in the flow

cells, the porosity of the media and the range of pH at the

outlet of the flow cells.

The linear fluid velocity was different in each flow cell,

due to the different porosity of each medium. The flow

cell tests were conducted using a constant flow rate of

10 L/day, which resulted in the hydraulic residence times

of a few hours. The focus of this research was to have com-

parable loading conditions (10 L/day), to allow comparison

of the tests results between the media. Any field setup using

residence times similar to that used in the present work (e.g.

10 h) should remove comparable amounts of phosphorus

from the wastewater.

The retention times of slag and clinker are greater than

those of gravel, but even if the residence time was increased

for gravel, the performance probably would not change due

to the inherently poorer sorption characteristics of the

medium. Rosolen () used a hydraulic retention time of

approximately 1 day. Calder et al. () had approximately


6 days retention time for similar gravel; even then the gravel

(comparable medium to the present study) performed poorly

(P removal – 25%) as compared to the slag (P removal –

65%). Interestingly, Toet et al. () reported that the

increase of hydraulic retention times from 0.3 to 9.3 days

did not increase the phosphorus removal. Based on the

above studies, for the present study, hydraulic retention

time was considered not to be an influential factor for sorp-

tion. Future work focusing on the study of hydraulic

retention times is required to confirm the assumption. The

low values of Reynolds number (Re< 1) shows that the

flow is Darcian in nature. The pH observed in the flow

cells shows that the outflow from the slag and clinker are

alkaline in nature.

Phosphorus sorption profiles

Soluble phosphorus concentrations were monitored at the

sampling locations by collecting and analyzing samples

from these locations in each cell. As mentioned earlier, for

the gravel flow cell, samples were collected from location

A (at 10 cm), B (at 25 cm), C (at 40 cm) and D (at 50 cm)

Figure 5 | Phosphorus sorbed by candidate media flow cells.

from the inlet. For the slag and clinker flow cells, the

samples were collected from location A (at 15 cm), B (at

30 cm) and C (at 45 cm) from the inlet.

Phosphorus sorption profiles (mg of P sorbed per L of

influent solution) were developed for each flow cell, and

are shown in Figure 5. The outflow phosphorus concen-

tration for the gravel flow cell stabilized at approximately

1.75 mg/L after 2 weeks which indicates that the concen-

tration of sorbed phosphorus was 2.25 mg/L. The same

trend was noted for the samples taken from all four

sampling locations in the cell. The amount of phosphorus

sorbed was higher close to the inlet than in the later sections

of the flow cell. This was not unexpected as phosphorus was

steadily being sorbed along the length of the flow cell, leav-

ing less phosphorus in the solution in the latter sections. The

gravel showed good initial performance until about 20 days,

after which all concentrations were similar. Although the

gravel removed approximately 50% of the influent phos-

phorus, it did not meet the regulatory requirement of

1 mg/L. It should be noted that gravel was used as a control

medium for studying the hydraulic properties and was not

really expected to be a good medium for phosphorus


removal. A similar sourced gravel used by Rosolen () in

her work also showed good performance initially (until

about 15 days) and this medium achieved equilibrium as

indicated by a constant concentration of phosphorus in

the effluent. Near saturation (effluent values 90% of influ-

ent) was noted by 36 days.

In the case of slag, the sorbed phosphorus was more

than 3.95 mg/L during the entire test period as indicated

by the outflow concentration of less than 0.05 mg/L.

During the initial period, the concentration of sorbed phos-

phorus fluctuated slightly in the first section but, after 20

days, it was more than 3.5 mg/L consistently until the con-

clusion of the test. The sudden performance improvement

in the first section (at around 20 days), unlike anything men-

tioned in the literature, was unanticipated. This

performance improvement would be significant, with the

filter now being compliant in the first section as well.

Although this initial high effluent concentration of phos-

phorus (approximately 1.5 mg/L) is not a matter of

concern as this happened only in the first section, it does

warrant some discussion. This could be caused by fines

being washed out of the filter, and consequent opening up

of the micropore sorption sites. This might become impor-

tant if one would like to shorten the length of the filter for

other applications (e.g. to optimize capital costs). This may

indicate that an acclimation period might be required for

shorter slag filters in the field before they become compliant.

However, the need for the acclimatization period can not be

really explained or confirmed by the information in the lit-

erature. It is important to note that the flow cell outflow

concentration was not effected during the test period

(<0.05 mg/L) and remained below the recommended limit

of 0.3 mg/L (the Bay of Quinte objective) and 0.1 mg/L

(the proposed Bay of Quinte objective).

For the clinker flow cell, the concentration of outflow

phosphorus was less than 0.02 mg/L during the entire test

period and in all sections, showing that the amount of

phosphorus sorbed was more than 3.98 mg/L. There was

a small discrepancy observed during the test at day 40,

but given the high amounts of sorbed phosphorus

observed, this was not considered of any consequence.

The outflow phosphorus concentration was even below

0.1 mg/L (the proposed Bay of Quinte objective) for clin-

ker. The system never achieved saturation during the test

period. The effluent phosphorus concentration from the

first section was very low, which indicates that most of

the phosphorus was being sorbed in this section.

Clinker was considered the best medium of the three

tested, as this flow cell had excellent overall performance

shown by the consistently low phosphorus outflow

concentrations.

Saturation is said to occur when the effluent concen-

tration reaches 95% of the influent concentration

(Benefield et al. ). In the present study, saturation was

never reached during the entire test period for all media,

due to the high sorption capacity of the media. As men-

tioned earlier, Rosolen () found that by increasing the

influent concentration from 8 to 22 mg/L, her gravel finally

achieved saturation (outflow concentrations of ∼21 mg/L)

within 1 week. The slag and clinker used in the same

study did not reach saturation even at the high dosing.

Drizo et al. () reported that steel slag was nearly

100% efficient in removal of phosphorus from an influent

concentration of 20 mg/L for 114 days. After increasing

the influent concentration to 400 mg/L for 21 days, the

steel slag still did not reach saturation. Higher phosphorus

concentrations such as 400 mg/L are not representative of

the typical influent concentrations in the field for domestic

wastewater applications, and this level of dosing would

not be useful for estimating filter performance. Therefore,

increasing the influent phosphorus concentration to achieve

saturation of the media was not conducted in this research.

None of the filters in the present work reached satur-

ation that leads to an inability to predict the ultimate

capacity (x/m)u of the media. Calder et al. () also

observed that the field filters never reached saturation. It

would therefore be difficult to predict the longevity of the fil-

ters and only an approximation could be made as discussed

in the following section.

Sorption capacity

The cumulative amounts of phosphorus removed per unit

mass (x/m, mg/kg) by the three media are plotted over

time in Figure 6. All three media show higher amounts of

phosphorus sorbed in the first section. This leaves less phos-

phorus in the latter sections for subsequent removal. In the

gravel and clinker flow cells, there was no difference in

Figure 6 | Phosphorus retained by sections of the flow cell media.


the mass sorbed between the latter sections, whereas in the

slag flow cell there was a small difference in the latter

sections.

Figure 6 shows that clinker was the best of the three

media, as it retained the most phosphorus (maximum

1,500 mg/kg) by the end of the test period, with slag follow-

ing close behind (1,200 mg/kg). Gravel sorbed a maximum

of approximately 700 mg/kg of the influent phosphorus

(less than 50% compared to clinker) and, as mentioned,

this medium failed to meet the regulatory requirements for

effluent phosphorus compliance concentration. In other

words, the phosphorus retained at the end of the test could

be considered as a very conservative estimate of the sorption

capacity of the medium, since none of the flow cells reached

saturation levels. In the first section, the removal efficiency of

clinker was >99% whereas for gravel it was approximately

50–60%. Slag showed a removal efficiency of >98%. These

results compare favorably with other studies in the literature.

Prochaska & Zouboulis () noted that gravel had

low phosphorus sorption capacity whereas their slag had

an extremely high sorption capacity. Drizo et al. ()

noted that their slag removed >99% of the influent

phosphorus. There have not been many studies on cement

clinker as a medium for phosphorus removal, although

some studies using coal ash demonstrated high phosphorus

removal efficiency (Kirk et al. ; Gray & Schwab ).

Arias et al. () noted in a study of 13 Danish sands (com-

parable to the gravel in the current study) that the removal

efficiency of sands all decreased significantly to levels

<50% after 12 weeks.

Rosolen () calculated the ultimate sorption

capacities, (x/m)u (mg/kg), based on the batch isotherm

tests for all three media. The (x/m)u for gravel was 15 mg/kg,

for slag it was 9,361 mg/kg and for clinker it was

14,438 mg/kg, respectively, for an influent concentration

of 8 mg/L. When compared to the conservative sorptive

capacity of gravel estimated in the present study, it was

higher than those estimated by Rosolen (). Therefore,

the sorption capacities for slag and clinker would be

expected to be very high compared to gravel and to the esti-

mated isotherm sorption capacities by Rosolen.

Since the flow cells did not achieve saturation in the pre-

sent study, a very conservative estimate of the volume of

medium required to achieve treatment for one year for the


slag and clinker is shown in Table 3. The amount of media

needed to reach the desired concentration of 1 mg/L for a

slag medium filter was assumed to be the mass present in

the first two sections, whereas for clinker it was assumed

to be the mass present in the first section only. The

measured concentration of the effluent was below 1 mg/L

for slag after the second section (<0.23 mg/L) and for clin-

ker after the first section (<0.02 mg/L). The observed

effluent concentration from the gravel flow cell was

∼2 mg/L gravel at the conclusion of the test and this was

used for the calculation of the volume of the medium.

Rosolen () also calculated a conservative estimate

of the filter volumes for the three media for an influent con-

centration of 8 mg/L. As her slag and clinker flow cells did

not reach saturation, the sorption capacity was recorded as

the highest value of sorption capacity achieved after the first

section of the filter for the slag and the clinker. The esti-

mated volume for gravel was 8,260 m3, for slag it was

54 m3 and for clinker it was 8 m3, respectively.

Table 3 indicates that the slag and clinker were much

better media with high sorption capacities, as compared to

gravel, for phosphorus removal. Despite the conservative

nature of these estimates, both slag and clinker demonstrated

excellent potential for use in the design of a reasonably sized

filter. The costs of filter replacement and spent filter disposal

or regeneration should be considered in any design exercise,

and the smallest filter with the longest lifespan would be the

most effective choice. Water quality considerations (e.g. pH

of the effluent) would also need to be addressed in the overall

impact assessment of the filters.

The criteria to evaluate the system should include the

estimated sorption capacity of the medium along with the

required effluent concentrations, influent flow rates and con-

centration (and expected variability), physical hydraulic

characteristics or a combination of these operational factors.

Table 3 | Sorption capacity and estimated filter volumes

Medium Pinitial Peffluent

Sorption capacityfrom flow cells(mg/kg)

Volume of mediafor one yearoperation (m3)

Gravel 4 mg/L ∼2 mg/L 700 193

Slag 4 mg/L <1 mg/L 1,200 130

Clinker 4 mg/L <1 mg/L 1,500 65

CONCLUSIONS

In conclusion, blast furnace slag and cement clinker showed

very high efficiencies (nearly 100%) in removal from an influ-

ent (4 mg P/L) over a 9-week period. The slag medium

accumulated 1,200 mg P/kg of slag whereas cement clinker

accumulated 1,500 mg P/kg of clinker, presumably through

the processes of specific adsorption onto metal hydroxides

and precipitation as hydroxyapatite. The maximum amount

ofP that could be removed byeithermediawasnot determined

but would exceed the values mentioned earlier. The control

medium, local limestone gravel accumulated 700 mg P/kg of

gravel. The maximum amount of sorption in this gravel

would be close to the value of 700 mg P/kg of gravel since

this medium appeared to reach a quasi-equilibrium condition.

Theoutflowconcentrationswere less than0.3 and0.02 mg P/L

from the slag and clinker flow cells, respectively. However,

the pH of the outflowwas alkaline in both flow cells. The out-

flow concentration from the gravel flow cell was 1.75 mg P/L

and the pH of the outflow was neutral. Overall, gravel

removed far less as compared to either clinker or slag media

and did not meet the discharge guideline of 1 mg P/L. Slag

and clinker showed very high sorption capacities and high

outflow pH values. Mixing peat with either media could

reduce the pH of the outflow without affecting the sorption

capacities. Also, it is expected that slag and clinker will have

an impact on the effluent quality with respect to heavy metals

present in the wastewater. Therefore, it is recommended that

future studies should investigate the impact of slag and clinker

on the removal of heavy metals from wastewater.

For design purposes, flow cell studies provide better and

more realistic estimates of ultimate sorption capacities than

do isotherm tests. Simple jar testing (e.g. isotherm testing) is

not enough to select the best medium. Flow cell testing simu-

late (in the small scale)field filter operation and performance.

These tests are important to predict the sorption capacity of

the medium for the long-term removal of phosphorus.

ACKNOWLEDGEMENT

The financial assistance provided by the Natural Sciences

and Engineering Research Council of Canada (NSERC) is

gratefully acknowledged.


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