UNIVERSITY OF WISCONSIN SYSTEM
SOLID WASTE RESEARCH PROGRAM
Student Project Report
Dissolved Phosphorus Removal using Steel Slag By-Products
May 2015
Student Investigator: Matt Tlachac
Advisor: Dr. Daniel Keymer
University of Wisconsin-Stevens Point
Introduction
The Lower Green Bay and Fox River Valley has been designated an Area of
Concern (AOC) in the Great Lakes Region. The watershed has recently come under an
Environmental Protection Agency (EPA) approved Total Maximum Daily Limit (TMDL) to
reduce discharges of Total Suspended Solids (TSS) and phosphorus. Over half of
these discharges are known to be coming from agricultural lands. Means to reduce
surface run-off sources of TSS and phosphorus are well understood and local
conservation agencies are in the process of working with producers to improve and
expand these practices. However, there is currently no means of containing dissolved
phosphorus from drain tile lines. Initial United States Department of Agriculture (USDA)
Agricultural Research Service work indicates that dissolved phosphorus concentrations
in tiled lands testing excessive for phosphorus and/or that have receive heavy
applications of manure applications are likely to have phosphorus concentrations in
discharge of 1 mg L-1 or greater (Joern et al., 1998). The AOC has a number of areas
identified as having high soil P concentrations and there are many large dairy
operations in the area. The need for these farm operations to land apply their manure
often leads to over saturation of phosphorus in agricultural fields.
Phosphorus in the environment binds readily to a handful of very specific
elements due to phosphorus’ most common molecular form in the environment:
phosphate. Phosphates consist of one phosphorus atom bonded to four oxygen atoms
and carry an inherent 3- charge; often seen as PO43-. The negative charge often repels
the phosphate molecule away from slightly negative soil particles. But clay particles in
particular often have positive trivalent elements at their center, allowing for a strong
attraction between the 3- charge of the phosphate and the 3+ charge of the clay particle
site. Iron and aluminum are the two most popular trivalent clay components that strongly
interact with phosphorus. Calcium and magnesium are also commonly associated with
having strong attractive forces with phosphorus.
Steel slag is an industrial by-product often used for beneficial uses under
Wisconsin Administrative Code NR 538; however there are still many thousands of tons
of slag being landfilled each year from operational foundries across the state. Facilities
that use electric arc furnaces and have more basic conditions produce slags that tend to
have higher phosphorus removal capabilities (Jones, 2015). Basic conditions refer to an
environment that have a higher concentration of OH- molecules than H+ atoms, which
occurs at pH values above 7. The more basic slag, often referred to as electric arc
furnace slag (EAF slag), will also provide a similar environment to many soils that have
basic parent materials such as limestone. Iron, aluminum, calcium, and magnesium are
often common constituents in the composition of EAF slag, which gives the slag its
natural tendency to sequester dissolved phosphorus. Being able to utilize EAF slag as
an adsorbent for phosphorus could provide multiple benefits: diverting slag waste from
landfills and limiting nonpoint phosphorus discharges to surface waters.
The objective of this research project was to quantify the effect of contact time on
phosphorus adsorption to two separate sources of electric arc furnace (EAF) slag. The
hypothesized outcome was if the contact time was increased between the phosphorus
solution and the EAF slag, then higher amounts of phosphorus would be removed from
the water. We also hypothesized that the slag source with the higher iron concentrations
would be the more effective filter media for removing phosphorus.
Methods
In order to quantify the effectiveness of phosphorus removal due to adsorption
onto EAF slag, a phosphorus solution with a known concentration the design of this
experiment was testing two different times of contact between and the EAF slag in a
laboratory setting. Two different sources of EAF slag were also tested side by side in
the experiment for removal effectiveness. The times of contact tested were 10 and 30
minutes, chosen to approximate peak and average tile drain flow rates, respectively. A
stock solution containing phosphorus was prepared to a 0.0645M concentration in the
lab by dissolving dibasic potassium phosphate (K2HPO4) in distilled water. The
phosphorus stock solution was then mixed into 400 liters of tap water to achieve an
influent solution with 1.2 mg L-1 phosphorus, contained in two 55 gallon HDPE drums
connected in parallel to allow equal draw from each drum. The phosphorus solution was
pumped up through the reaction vessels containing the EAF slag using a peristaltic
pump. Flow rate was estimated from the vessel volume and slag porosity (∑) using the
following equations:
Vpore = Vvessel * ∑
HRT = Vpore/Q
where HRT is the hydraulic residence time and Q represents flow rate. The vessels
ranged from 3.116 L to 2.987 L in volume. Each trial lasted for a duration of 48 hours,
and 10 total effluent samples were taken during each trial. The sample schedule was as
follows: 0, 0.5, 1, 1.5, 2, 3, 5, 9, 21, and 48 hours into each trial. The early phase of
each trial was sampled more intensively due to the findings of past studies showing
decreased removal rates of EAF slag over time. Each slag sample was run in triplicate
vessels for each contact time to ensure more reliable data, for a total of twelve trials.
Effluent samples were refrigerated at 4 degrees C for no more than two weeks, before
being filtered through a 0.45 µm filter. The filtrate from the test trials was run through a
Lachat QuikChem Autoanalyzer to measure the soluble reaction phosphorus (SRP)
concentration.
The two slag sources varied in their particle size distribution. Slag source 1 from
Whitesville, IN, had been already crushed, whereas slag source 2, from Wisconsin
Rapids, had not been crushed yet. Manual size reduction of slag aggregates was used
to try and obtain a more homogenous mixture of particle sizes comprising the
adsorbent. In order to measure the distribution of particle sizes, a sieve analysis was
performed with mesh opening sizes as follows, from top to bottom, in millimeter
measurements: 12.7, 4.75, 2.36, 2.0, 1.18, and 1.0.
Performance metrics for the adsorption trials were calculated using formulas
derived by McGrath and Penn (2011). The maximum phosphorus (P) added calculation
determines what mass of phosphorus could be added to the slag filter before the
effluent phosphorus concentrations would rise above 99% of the original influent
concentration. Variables B and m were the same as explained in the Cumulative P
Removed (%) equation. The equation used to calculate it was:
𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑃𝑃 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 =
𝐿𝐿𝐿𝐿 𝐵𝐵
−𝑚𝑚
mg P kg-1
Cumulative P removed (%) is a calculation that determines what percentage of
the maximum phosphorus loaded into the slag filters would be sequestered by the
media. The integrated expressions containing variables B and m in the equation below
are taken from curves fit for P removal with incremental P addition (Figures 1-4) and x is
the maximum P added (calculated above):
𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑃𝑃 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 (%) =
𝑥𝑥
∫0 (𝐵𝐵𝐵𝐵^𝑚𝑚𝑚𝑚)𝑑𝑑𝑑𝑑
𝑥𝑥
mg P Cumulative P removed is also expressed as a mass value with units of mg/kg.
The equation does utilize a couple of the previously mentioned calculations. The
equation used was:
𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑃𝑃 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 = 𝑀𝑀𝑀𝑀𝑀𝑀 𝑃𝑃 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 ∗
(𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑃𝑃 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 (%)
�100)
The lifespan of each filter was calculated to determine how many days the filter
could last until all the phosphorus sequestering ability was spent. The equation used
was:
𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 = 𝑀𝑀𝑀𝑀𝑀𝑀 𝑃𝑃 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 ∗
𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 [𝑃𝑃]∗𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅
𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑜𝑜𝑜𝑜 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆
Results
EAF Slag 1 had a more homogenous composition of aggregates according to the
particle size distribution experiment. 78.61% of the material was between 12.7 and 4.75
mm in diameter. EAF Slag 2 had a higher diversity of particle sizes with 28.41%,
58.38%, and 9.89% in the 12.7+, 12.7-4.75, and 4.75-2.36 mm classes respectively.
Slag 2 was expected to have more diversity in its particle sizes due to the manual
crushing that was needed in order to obtain the more homogenous aggregate size.
However, Slag 1 had a higher proportion of aggregates smaller than 4.75 mm (20.3%
versus 13.2% for Slag 2). The results for the particle size distribution can be found in
Table 1 for both EAF slags.
The chemical composition of EAF Slag 1 revealed a high concentration of
phosphorus binding elements that were expected to be present. Iron and calcium were
the two largest components with 23.79% and 22.03% respectively. The next three
largest components were magnesium, manganese, and aluminum; with 6.04%, 4.09%
and 2.29%, respectively. Table 2 gives a complete chemical composition of Slag 1; a
composition of Slag 2 was not available at the time of the study.
The flow rate was derived from a calculation utilizing the porosity of the slag in
use. With an average porosity measurement of 0.467, the average pore space in the
reaction vessel came out to be 1.425 L. When the goal was to have a 10 minute
retention time, calculations told us we wanted a flow of 2.375 ml/s. When our goal was
to have a 30 min retention time, the flow calculation came to 0.792 ml/s.
The maximum P added calculation told us that EAF Slag 1 with a 30-minute
contact time would be able to handle the highest phosphorus loading rate. The
treatment condition that would accept the lowest phosphorus loading rate would be EAF
Slag 1 with a 10-minute contact time. The max P added calculation for EAF Slag 2 with
a 10-minute contact time seems to have been skewed by some measure within the
calculation. After comparing the rest of the data, Slag 2 with a 10-minute contact time
would have been expected to have the lowest allowable phosphorus loading rate.
The lifespan calculations for the different treatment conditions varied quite a bit.
The longest lifespan of 44 days belonged to Slag 1 with a 30-minute contact time, and
the shortest lifespan of 2 days belong to Slag 1 with a 10-minute contact time. Slag 2’s
lower readiness to remove phosphorus would explain why the filters would have a
longer lifespan. The needed mass of phosphorus to saturate every bonding site would
require a longer time to reach with Slag 2.
Within the same contact time treatment, EAF Slag 1 had a higher Cumulative P
Removal (%) in the 10-minute trials. Slag 1 removed approximately 20% of the
phosphorus on average, and Slag 2 removed approximately 4% consistently. The 30minute contact time treatment did not produce significant differences in percent
phosphorus removed between the two slag sources.
The mass of phosphorus removed per mass of slag had the highest result with
EAF Slag 1 with a 30-minute contact time with 117 mg P kg-1. The lowest removal rate
was seen using Slag 2 with a 30-minute contact time with a value of 9 mg P kg-1.
Even though replicate trials of each treatment condition were carried out, similar
results were rarely seen. Attempting to hold all variables constant proved to be more
difficult than thought. Water behavior and flow inside the reaction vessel also could
have varied from trial to trial, which could explain the variability witnessed in the
calculations. In future studies, more replicate trials will need to be completed in order to
give a more concise data set.
Discussion
Slag 1 was more than 3 times higher than Slag 2 in the 10-minute contact time
treatment, and also higher in the 30-minute contact time treatment. This conclusion also
applies to the cumulative mass of phosphorus removed. The particle size distribution of
the two EAF slags would support this conclusion as well due to Slag 1’s higher portion
of smaller size aggregates. The prevalence of those smaller size aggregates means
there is more surface area within the EAF slag media for the phosphorus to interact and
bind to those phosphorus attracting elements such as: calcium, magnesium, aluminum,
and iron. C. J. Penn et al. also found the concentrations of those 4 elements in
particular to be strong indicators of a filters phosphorus removal capacity (McGrath and
Penn, 2011) Table 2 confirms that those 4 elements are present in relatively high
concentrations within that slag.
The impact of contact time has two different stories when comparing the data
within each slag source. For Slag 1, the longer contact time had created a difference in
the calculated max P added, lifespan of the filters, and the cumulative mass of P
removed. The max P added values for the longer contact time suggest those trials could
have accepted over twice as much phosphorus loading than the shorter trial and still
produce a clean effluent. Finding that contact time is a significant factor is consistent
with what C. J. Penn found in his 2011 study of EAF slag flow through phosphorus
removal filters (McGrath and Penn, 2011). Slag 2 on the other hand did not exhibit such
predictable results. Although the removal percentage went up, many of the other
calculated factors do not follow the same trend. Even within the same treatment,
different trials produced varied results.
As with any project, a few limitations stood out as the trials were carried out and
unfolded. The lack of constant mixing in the 55-gallon drums could have led to uneven
loading rates of the dissolved phosphorus to the filters. With slag 2 not being pretreated
for homogenous aggregate size, manual crushing of the aggregates was required. I
believe this fact heavily influenced the difference in the particle size distribution
witnessed between the two slags. While running the trials with 30-minute contact times,
periodic observation of the discharge flows would lead one to believe that steady even
flows was not being achieved. The construction and design of the equipment was often
correct on paper, but would run into issues in reality. In order to avoid these issues,
running each trial separately; one bin at a time would be advised.
Future studies in the area of dissolved phosphorus removal using EAF slag
adsorption could research the ease and availability of phosphorus reclamation out of the
slag filters. If possible, the reclaimed phosphorus and nutrients could then be reapplied
to the field it came from, or a surrounding area in need of phosphorus, but be applied in
a more conservation minded method. Running multiple filters side by side, with some in
service, and some out of service in “reclaim” stages could be a potential layout. From
the data generated by this laboratory study, a pilot scale project should be conducted in
order to further investigate the feasibility of using EAF slag as a phosphorus removal
structure for agricultural tile drainage fluid.
Tables and Figures
Table 1. Particle size distribution for both EAF slags 1 and 2.
Slag 1
Particle Sizes (mm)
12.7 +
12.7-4.75
4.75-2.36
2.36-2
2-1.18
1.18-1
<1
Unaccounted
Total
%
weight
0.00
79.61
16.58
0.81
1.31
0.20
1.36
0.13
100.00
Slag 2
Particle Sizes (mm)
12.7 +
12.7-4.75
4.75-2.36
2.36-2
2-1.18
1.18-1
<1
Unaccounted
Total
%
weight
28.41
58.38
9.89
1.19
1.38
0.15
0.59
0.01
100.00
Table 2. Elemental composition of EAF slag 1. Results were provided by Whitesville Mill Services. Metal and
trace element concentrations were found using a inductively coupled plasma optical emission spectrometer.
Element
Aluminum
Antimony
Aresenic
Barium
Beryllium
Cadmium
Calcium
Chromium
Cobalt
Copper
Iron
Lead
Magnesium
Manganese
Molybdenum
Nickel
Potassium
Selenium
Silver
Sodium
Sulfur
Material
Slag 1
Concentration (PPM)
22869.00
<0.278
<0.036
395.00
0.80
3.33
220272.00
3200.00
6.51
66.50
237885.00
8.89
60375.00
40941.00
13.50
5.01
51.40
<0.037
<0.052
237.00
2495.00
Thallium
Titanium
Vanadium
Zinc
17.10
1502.00
641.00
41.40
Table 3 shows various calculations for the trials with 10 minute residence times. The Maximum P Added
calculation determines how much P could have been added to each slag filter before effluent P concentrations
rose about 1% of the original concentration. The lifespan calculation determines how many days the slag
filter could remain effective before becoming “spent”. Cumulative P Removal % is a calculation to determine
what percentage of the P added to the slag was sequestered by the filter. The mass of P removed during each
trial is quantified by the Cumulative P Removed calculation.
10 Minute Contact Time
Trial
Maximum P Added (mg)
Lifespan (days)
Cumulative P Removal (%)
Cumulative P Removed (mg/kg)
1
117.831
2.570
17.715
20.874
Slag 1
2
106.934
2.608
23.275
24.889
Slag 2
3
157.727
3.777
20.975
33.083
4
499.581
7.712
4.467
22.316
5
377.568
6.607
4.940
18.652
6
353.813
5.970
5.636
19.941
Table 4 shows various calculations for the trials with 30 minute residence times. The Maximum P Added
calculation determines how much P could have been added to each slag filter before effluent P concentrations
rose about 1% of the original concentration. The lifespan calculation determines how many days the slag
filter could remain effective before becoming “spent”. Cumulative P Removal % is a calculation to determine
what percentage of the P added to the slag was sequestered by the filter. The mass of P removed during each
trial is quantified by the Cumulative P Removed calculation.
Trial
Maximum P Added (mg)
Lifespan (days)
Cumulative P Removal (%)
Cumulative P Removed (mg/kg)
1
632.162
44.436
18.649
117.892
30 Minute Contact Time
Slag 1
2
3
4
493.400 441.344 201.621
33.265
30.292
12.613
18.877
18.478
16.059
93.139
81.552
32.378
Slag 2
5
75.017
4.593
15.651
11.741
6
69.690
4.182
13.118
9.142
P removed (%)
y = 96.94e-0.029x
R² = 0.9996
y = 78.237e-0.037x
R² = 0.7458
P added (mg/kg)
y = 110.51e-0.044x
R² = 0.9794
P removed (%)
Figure 1 shows slag 1’s ability to remove phosphorus when 10 minutes of contact is allowed. Trial A is
depicted with diamonds, has a trendline equation of Y=78.237e-0.037x and an R2 value of 0.7458. Trial B is
depicted with squares, has a trendline equation of Y=110.51e-.044x and has an R2 value of 0.9794. Trial C is
depicted with triangles, has a trendline equation of Y=96.94e-0.029x and has an R2 value of 0.9996.
-0.008x
-0.007x
= 16.954e
y = y14.055e
y = 12.157e-0.005x
= 0.7854
R² =R²0.8659
R² = 0.6731
P added (mg/kg)
Figure 2 shows slag 2’s ability to remove phosphorus when 10 minutes of contact is allowed. Trial D is
depicted with diamonds, has a trendline equation of Y=12.157e-0.005x and an R2 value of 0.6731. Trial E is
depicted with squares, has a trendline equation of Y=14.055e-0.007x and an R2 value of 0.8659. Trial F is
depicted with triangles, has a trendline equation of Y=16.954e-0.008x and has an R2 value of 0.7854.
P removed (%)
y = 83.524e-0.007x
-0.009x
y = 0.5234
84.826e-0.01x
R²
y ==82.553e
R² = 0.8921
R² = 0.4618
P added (mg/kg)
Figure 3 shows slag 1’s ability to remove phosphorus when 30 minutes of contact is allowed. Trial A is
depicted with diamonds, has a trendline equation of Y=83.524e-0.007x and an R2 value of 0.5234. Trial B is
depicted with squares, has a trendline equation of Y=84.826e-0.009x and has an R2 value of 0.8921. Trial C is
depicted with triangles, has a trendline equation of Y=82.553e-0.01x and has an R2 value of 0.4618.
P removed (%)
y = 68.995e-0.021x
R² = 0.1024
y = 66.749e-0.056x
R² = 0.8835
y = 53.107e-0.057x
R² = 0.8352
P added (mg/kg)
Figure 4 shows slag 2’s ability to remove phosphorus when 30 minutes of contact is allowed. Trial D is
depicted with diamonds, has a trendline equation of Y=68.995e-0.021x and an R2 value of 0.1024. Trial E is
depicted with squares, has a trendline equation of Y=66.749e-0.056x and an R2 value of 0.8835. Trial F is
depicted with triangles, has a trendline equation of Y=53.107e-0.057x and has an R2 value of 0.8352.
References
Fox, Garey, Derek Heeren, Joshua M. McGarth, Chad J. Penn, Elliot Rounds.
2012. Journal of Environmental Quality. Trapping Phosphorus in Runoff with a
Phosphorus Removal Structure. Vol 41: 672-679.
Joern, B. C., R. R. Simard, J. T. Sims. 1998. Journal of Environmental Quality.
Phosphorus Loss in Agricultural Drainage: Historical Perspective and Current Research.
Vol 27: 277-293.
Jones, Jeremy A. T. 2015. American Iron and Steel Institute. Electric Arc
Furnace Steelmaking. Webpage.
https://www.steel.org/Making%20Steel/How%20Its%20Made/Processes/Processes%20
Info/Electric%20Arc%20Furnace%20Steelmaking.aspx
McGrath, Joshua. M., Chad J. Penn. 2011. Journal of Water Resource and
Protection. Predicting Phosphorus Sorption onto Steel Slag Using a Flow-through
Approach with Application to a Pilot Scale System. Vol 3: 235-244.