Ground and Water: Theory to Practice

Proceedings of the 55th Canadian Geotechnical and 3rd Joint IAH-CNC and CGS Groundwater Specialty Conferences, Niagara Falls, Ontario, October 20-23, 2002

Edited by D. Stolle, A.R. Piggott and J.J. Crowder and published by the Southern Ontario Section of the Canadian Geotechnical Society

1123

REMOVAL OF PHOSPHATE AND WATERBORNE PATHOGENS FROM

WASTEWATER EFFLUENT USING PERMEABLE REACTIVE MATERIALS

D.J.A. Smyth, Department of Earth Sciences, University of Waterloo, Waterloo, Ontario

D.W. Blowes, Department of Earth Sciences, University of Waterloo, Waterloo, Ontario

C.J. Ptacek, Department of Earth Sciences, University of Waterloo, Waterloo, Ontario

M.J. Baker, Shell Canada Ltd., Calgary Alberta

G. Ford, Septech Environmental, London, Ontario

S. Foss, Massachusetts Alternative Septic System Test Center,Barnstable County Health and Environmental

Department, Barnstable County, MA

E. Bernstene, Massachusetts Alternative Septic System Test Center,Barnstable County Health and Environmental

Department, Barnstable County, MA

ABSTRACT

Over the past decade, extensive laboratory testing and field studies have demonstrated the effective removal of

phosphate from groundwater and wastewater using Basic Oxygen Furnace (BOF) slag, a waste material from the steel

production process. The BOF slag contains significant concentrations of calcium and iron oxyhydroxides, which create

high pH conditions following interaction with water. A laboratory column containing 10% BOF oxide by weight in silica

sand has been operating continuously since 1993. With an influent concentration of approximately 3 mg/L PO4-P, the

column has discharged water containing less than 0.2 mg/L PO4-P on a continuous basis. Two recent field applications

have confirmed the removal of phosphate from domestic (North Bay, 1999) and communal (Massachusetts Alternative

Septic System Test Center, October 2001) wastewater by BOF slag material. The applications have also demonstrated

the capability of the treatment systems to remove waterborne pathogens, including coliform bacteria and E. coli, from the

wastewater effluent. . A second chamber containing organic carbon has been added to provide pH buffering of the

effluent from the Cape Cod system.

RÉSUMÉ

Depuis une décennie, plusieurs tests en laboratoire et études de terrain ont démontré l’efficacité d’enlever les

phosphates de l’eau souterraine ou usée par l’utilisation de scories de Fourneau à Oxygène Basique (FOB), un résidu

provenant de la production de l’acier. Les scories de FOB contiennent des concentrations significatives en calcium et

hydroxydes de fer qui créent des conditions de pH élevées suivant leur interaction avec l’eau. Une colonne de laboratoire

contenant 10% de FOB par masse en sable siliceux a opéré en continu depuis 1993. Avec une concentration initiale

d’eau d’environ 3 mg/L PO4-P, la colonne a déchargé l’eau contenant une concentration inférieur à 0.2 mg/L PO4-P sur

une base continue. Deux applications de terrain récentes ont démontré le potentiel des scories de FOB pour soustraire

les phosphates de l’eau domestique (North Bay, 1999) et usée de communauté (Massachusetts Alternative Septic

System Test Center, October 2001). Les applications ont aussi démontré la capacité des systèmes de traitement de

retirer les pathogènes transmissibles par l’eau comme les bactéries coliformes et E. coli, de l’effluent d’eau usée. Une

deuxième chambre contenant de la matière organique fut rajoutée pour fournir un tampon pour l’effluent du système de

Cape Cod.

1. INTRODUCTION

In situ treatment of dissolved contaminants in

groundwater by permeable reactive barriers (PRBs) has

developed as an effective and cost-efficient approach to

the remediation and management of contaminated sites

over the past decade (Blowes and Ptacek, 1992; Blowes

et al., 1997; Powell et al., 1998). PRBs provide passive

interception of contaminated groundwater and the

destruction or removal of the contaminants by a

combination of physical, chemical or biological processes

within or down-gradient of the reactive materials.

O’Hannesin and Gillham (1998) and Yamane et al. (1995)

applied PRBs containing zero-valent iron for the treatment

of chlorinated solvents in groundwater. Blowes et al.

(1999abc) used zero-valent iron in a PRB to remove

chromium (VI) and chlorinated solvents from groundwater

prior to its discharge to a river. Benner et al. (1997; 1999)

used organic carbon in a PRB to enhance the activity of

sulfate-reducing bacteria and promote the treatment of an

acid-mine drainage plume from sulfidic mine tailings

through the precipitation of metal sulfide minerals and the

generation of alkalinity. Robertson et al. (2000) used

similar organic reactive materials to effectively remove

nitrate by promoting microbially mediated denitrification in

groundwater impacted by domestic septic systems and

agricultural wastes.

Baker et al. (1998) and Blowes et al. (1999; 2000)

recognized the potential for Basic Oxygen Furnace (BOF)

slag to remove contaminants such as phosphate, arsenic

and selenium, and water-borne bacteria and viruses from

groundwater. BOF slag is a poorly sorted mixture of

material ranging in grain size from silt to fine gravel, and is

a non-metallic waste byproduct of steel production, and is

generated at several steel plants throughout North

America. BOF slag contains various oxides and silicates

of iron, calcium, magnesium and aluminum, but in
Ground and Water: Theory to Practice

1124

particular is rich in iron and calcium oxyhydroxides.

Typically, the interaction between BOF slag and water

results in elevated pH conditions of as high as 12, but no

other measurable water quality impacts. Proctor et al.

(2000) identified antimony, cadmium, total chromium,

manganese, molybdenum, selenium, silver, thallium, tin

and vanadium at concentrations in excess of background

soil in the United States in samples of BOF slag as part of

a review of 58 active steel mills. Toxicity Characteristic

Leaching Procedure (TCLP) tests using leach solution at

pH 2.8 for their samples generated leachate from BOF

slag of non-hazardous characteristics BOF slag is

inexpensive and shipping and transport have a strong

influence on its overall cost. BOF slag is commonly used

as aggregate in roadbed and other construction projects.

McRae et al. (1999) evaluated BOF slag and other

reactive materials for the treatment of arsenic and

selenium in groundwater. Both arsenic and selenium were

removed to very low levels for more than 125 pore

volumes of flow in a dynamic test of a column that

contained a mixture of silica sand, limestone and 10%

BOF oxide by weight. The influent concentration of total

arsenic (1:1 ratio of As(III) and As(V)) was 1 mg/L.

Arsenic was non-detectable in the effluent (<0.018 mg/L).

In a subsequent unreported column test conducted at the

University of Waterloo (UW), BOF slag in a 1:1 mixture

with gravel decreased concentrations of arsenic from 4

mg/L in the influent to less than 0.01 mg/L in the effluent.

The test was conducted using low pH (<4) groundwater

from an industrial site in the eastern United States. In the

early summer of 2002, construction for the installation of a

full-scale BOF-slag PRB for the treatment of arsenic in

groundwater was initiated at an industrial site in the

central United States.

This paper will focus on the use of BOF slag in PRB

systems for the treatment of phosphorus and waterborne

pathogens in groundwater. The work was initiated in the

laboratory in the early 1990s, and has expanded to

include several field-scale applications, some of which are

active.

2. PHOSPHATE AND WATER-BORNE PATHOGENS

IN GROUNDWATER

Phosphorus is an important constituent of domestic and

agricultural wastewater. Concentrations of phosphate in

wastewater may range between 5 and 15 mg/L PO4-P.

The natural capacity of subsurface soil and rock materials

to attenuate the mobility of phosphate is limited. Thus,

phosphate can be a mobile contaminant in groundwater.

This mobility is a particular concern where domesticwastewater or agricultural organic-waste systems are

located in close proximity to surface water. Groundwater

discharge or surface runoff can contribute phosphate

loading to the receiving surface water. Phosphate is

typically the limiting nutrient present in surface water.

Increased inputs of phosphate can increase the growth of

algae and other plants and upset the ecological balance

of aquatic habitats.

Domestic wastewater and agricultural wastes are

prevalent sources of bacteria and viruses in groundwater.

The site-specific conditions and hydrogeologic setting

associated with the introduction of wastewater to the

subsurface have strong influences on the potential

survivability and transport distance of pathogens in the

subsurface. For example, Bales et al. (1995), in a

controlled tracer test in a sand aquifer in Cape Cod, found

detectable concentrations of bacteria persisting for tens of

metres down-gradient of a source, but detectable virus

concentrations were restricted to within several metres of

the source. DeBorde et al. (1998), however, found

transport distances for both bacteria and viruses of

several tens of metres from a school septic system in a

sand and gravel aquifer. Survival times of more than nine

months for the viruses were also reported. The potential

impact of water-borne pathogens is a particular concern in

areas of groundwater use for water supply purposes (US

EPA, 2000).

3. PHOSPHATE TREATMENT

Baker et al. (1998) evaluated the removal of phosphate

from water by a variety of metal oxides in laboratory batch

experiments. The metal oxides included BOF oxide, BOF

slag and activated alumina. In batch tests, the oxides

were incorporated in reactive mixtures containing 50 wt%

silica, 45 wt% crushed limestone and 5 wt% metal oxide.

Approximately 50 g of each reactive mixture in 500 mL

solutions caused concentrations of phosphate to decrease

from approximately 10 mg/L PO4-P to less than 0.01 mg/L

mg/L PO4-P in periods of several hours.

 On the basis of the batch-test results, dynamic column

tests were initiated. Two columns containing the BOF

oxide, crushed limestone and silica sand mixture and

activated alumina, crushed limestone and silica sand

mixture in the same proportions as in the batch tests were

set up. Flow to the BOF oxide column averaged 1.3 pore

volumes per day, and the residence time for water in the

column was approximately 0.9 days. Flow to the activated

alumina column was approximately 0.6 pore volumes per

day, with an average residence time of approximately 1.7

days. The influent concentration of phosphate was

approximately 3.3 mg/L PO4-P.

The activated alumina column removed phosphate to

concentrations of less than 0.01 mg/L PO4-P for a period

of two years (approximately 400 pore volumes) beginning

in 1995. The pH of the effluent increased from

approximately 7.5 to 9 during this period. Although the

column provided excellent treatment performance, the

potential utility of activated alumina in field applications is

limited because of costs. Grain-surface analysis indicated

the removal of phosphate by activated alumina was

attributed to sorption (Baker et al., 1998).

Flow to the BOF oxide column was initiated in 1993. The

pH of the effluent was initially 9.75, but decreased to

approximately 9 after 1,450 pore volumes of flow in mid1997. Removal of phosphate by the BOF oxide column to
Smyth et al.

1125

concentrations of less than 0.01 mg/L PO4-P was

achieved at early time, but effluent concentrations of

phosphate increased to less than 0.4 mg/L PO4-P after 40

pore volumes of flow. After four years of continuous flow

the BOF oxide mixture removed in excess of 80% and

typically achieved 90% removal of the influent phosphate.

Mineralogical analysis on samples of the reactive solids

attributed the removal of phosphate by BOF oxide to

sorption and the subsequent precipitation of low solubility

calcium-phosphate minerals such as hydroxyapatite. A

salt-tracer test confirmed there was no apparent change

of the hydraulic characteristics of the material of the

column after four years of treatment. After approximately

eight years and 2,900 pore volumes of flow in early 2002,

the effluent from the BOF oxide column continued to have

a pH of 9.2 and a phosphate concentration of 0.3 mg/L

PO4-P or less.

Flow of 0.7 to 0.9 pore volumes per day was initiated in a

third column containing a mixture of silica sand (40 wt%),

limestone (35 wt%) and BOF slag (25 wt%) in 1994. Flow

to this and the BOF oxide column has been maintained

continuously to the present. The effluent from the BOF

slag column had a pH of 10.85 and a phosphate

concentration of less than 0.1 mg/L PO4-P after

approximately 1,500 pore volumes of flow in early 2002.

The peak concentration of phosphate in the effluent from

the BOF slag column was 0. 5 mg/L PO4-P, but this level

of phosphate in the effluent was not persistent.

4. INITIAL FIELD TRIALS

Baker et al. (1997) evaluated the use of BOF oxide for the

removal of phosphate in two field trials. In the first, a

stream of effluent from the primary clarifier of a municipal

sewage treatment plant was directed through a cylindrical

column (0.5 m in diameter and 0.5 m in height) containing

a mixture of BOF oxide, limestone and silica sand. The

clarifier effluent had been pretreated by an oxidizing

biofilter. Under flow rates of approximately 0.75 pore

volumes per day, effective phosphate removal was

attained for four months. Concentrations of total and

orthophosphate in the influent were 3.93 and 2.5 mg/L

PO4-P, respectively. Average concentrations in the

effluent were 0.14 mg/L (Total-P) and 0.05 mg/L (OrthoP), indicating treatment efficiencies of in excess of 95 %

removal.

In the second trial, the BOF oxide mixture was

incorporated in the treatment chamber of a funnel-andgate system that intercepted the plume from a septic

system at a rural school. The treatment chamber was 2.0

m in width in the direction perpendicular to groundwater

flow, 0.7 m in thickness, and 1.0 m in depth, and was

installed at and below the water table. Monitoring

indicated that there was some variability in the

groundwater flow characteristics in the vicinity of the

system and the concentration of phosphate entering the

treatment chamber. The BOF-oxide mixture reduced

phosphate concentrations from as much as 2 mg/L PO4-P

in the influent to approximately 0.2 mg PO4-P within the

treatment chamber.

5.0 CURRENT FIELD DEMONSTRATIONS

BOF slag has been used in several systems to remove

phosphorus from wastewater since the initial

demonstrations. These demonstrations have included

septic systems for single-family households, or equivalent,

and communal systems for rural hotels and campground

facilities. The BOF slag is mixed with gravel or crushed

stone to generate a material with a hydraulic conductivity

of more than 10-2 cm/sec.

A BOF slag treatment chamber was installed as part of

the septic system at a single-family household in North

Bay, Ontario in the fall of 1999. The effluent from the

septic system is treated in an oxidizing sand filter.

Ammonia is converted to nitrate and the organic content

of the wastewater is decreased in the sand filter. The

effluent from the sand filter is captured and introduced to

the BOF slag chamber, which contains a 1:1 mixture of

BOF slag and gravel. The effluent from the BOF slag

chamber is discharged through screened pipes to the

vadose zone. The water table occurs at a depth of

approximately one metre below ground surface in the

vicinity of the infiltration gallery. The temperature, pH,

phosphate and e-coli content of the effluent from the

septic system, sand filter and BOF chamber have been

monitored once or twice a month by the City of North Bay,

on behalf of the Ontario Ministry of Environment.

Monitoring results as a function of time are shown on

Figures 1a-d. Temperature is similar in all components of

the wastewater treatment system, and reflects seasonal

variations of air temperature. The pH of the effluent from

the BOF slag chamber has consistently been above 12.

The results indicate sustained and excellent removal of

phosphorous and waterborne pathogens from the septic

system effluent by the BOF slag chamber. Phosphate in

the effluent from the BOF chamber has been less than

0.05 mg/L PO4-P. E. coli have been non-detectable (less

than 5 or 20 CFU (Colony Forming Units)/ 100 mL) in all

but one sample. No special maintenance of the BOF slag

chamber has yet been required. The pH of the shallow

groundwater in three monitoring wells approximately a

meter down-gradient of the infiltration gallery ranged from

6.24 to 6.93 seven months after the system was installed.

The pH buffering has been achieved through geochemical

interactions with the aquifer materials and CO2 in the

vadose zone in the vicinity of the infiltration bed. This

observation is consistent with that observed at the zero-valent

iron PRB in Elizabeth City (Blowes et al., 1999b). Monitoring of

that PRB system has shown that buffering of the elevated pH

(>10) conditions created by zero-valent iron to background

levels occurs in the groundwater within transport distances of

several meters down-gradient of the barrier.
Ground and Water: Theory to Practice

1126

Figure 1: Monitoring data as a function of days since

operation of a domestic system for a single-family

residence was initiated. The data represent effluent from

the septic tank (raw water), effluent from the aerobic sand

filter (sand filter) and effluent from the BOF-slag filter

(BOF filter). The data include pH (Figure 1a), temperature

(Figure 1b), phosphate (Figure 1c) and E. coli (Figure 1d).

At the Massachusetts Alternative Septic System Test

Center, a BOF-slag chamber (PhosphexTM) was installed

in October 2001. The BOF slag is enclosed in a concrete

tank. The chamber receives approximately 330 U.S.

gallons (1,240 L) of communal wastewater that has been

aerobically pre-treated daily. The wastewater has a

residence time of approximately a day in the chamber.

The wastewater has a BOD5 between 3 and 7 mg/L, pH of

7 and phosphate (as PO4) of 3.75 mg/L. E. coli are

present at between 1,000 and 20,000 CFU/100 mL. The

 pH of the effluent has ranged from 11.53 to 11.97, but is

gradually decreasing with time. Phosphate concentrations

in the effluent have been non-detectable (<0.05 mg/L as

PO4). Total phosphorus has also not been detected in the

effluent (< 0.025 mg/L P). E. coli have also not been

detected in the effluent (<5 CFU/100 mL). The elevated

pH of the effluent has been identified as a concern for

discharge to surface water, and to groundwater in some

jurisdictions, therefore a second chamber was added to

the treatment system in June 2002 to evaluate

approaches to buffer the effluent pH.

6. SUMMARY

BOF slag is an effective material for use in PRB systems

for the removal of phosphorus and pathogens such as E.

coli from groundwater and wastewater. Laboratory

columns containing BOF oxide have provided sustained

removal of phosphorus for a period of more than eight

years. At the field-scale, BOF slag and gravel mixtures

have achieved removal of phosphate to concentrations of

less than 0.05 mg/L PO4-P, and the removal of E. coli in

wastewater from domestic septic-system effluent to nondetectable levels (<5 CFU/ 100 mL). The excellent

treatment levels could provide significant benefits in

decreasing the negative impacts of on-site wastewater

treatment systems on groundwater and receiving surface

water systems. The

BOF slag is generated at several steel-production facilities

in North America. The cost of BOF slag is comparable to

that of typical construction aggregate materials. The

combination of excellent treatment potential and

inexpensive cost makes BOF slag a good candidate for in

situ treatment of selected contaminants in groundwater.

Although the tendency for BOF slag to generate effluent

with elevated pH precludes its use in treatment systems

that discharge directly to surface water, buffering of the

pH to background levels can be achieved within short

transport distances in situations where discharge to the

subsurface is feasible. Alternately, pH buffering of the

effluent could be achieved in an additional treatment

chamber prior to discharge to surface water or

groundwater.

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Fig. 1a: pH with Time

0 200 400 600 800 1000

pH 0

2

4

6

8

10

12

14

Raw pH

Sand pH

BOF pH

Fig. 1b: Temperature with Time

0 200 400 600 800 1000

Temperature (oC) 0

5

10

15

20

25

30

Raw Water Temperature

Sand Filter

BOF Filter

Fig. 1c: Phosphate-P with Time

0 200 400 600 800 1000

Phosphate-P (mg/L) 0

5

10

15

20 PO 4-P in Raw Water

PO 4-P in Sand Filter

PO 4-P in BOF Filter

Fig. 1d: E. Coli with Time

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0 200 400 600 800 1000

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1e+0

1e+1

1e+2

1e+3

1e+4

1e+5

E. Coli in Raw Water

E. Coli in Sand Filter

E. Coli in BOF Filter
Smyth et al.

1127

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