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Mercury
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Facilities Loadings
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For more
information, contact David Eppstein by email at
deppstein@masco.harvard.edu,
or by calling 617-632-2860.
7.0 COMPARISONS OF OVERALL RESULTS
7.1 Introduction
In this section, we compare the estimated mercury loadings from the
five study groups to the MWRA total known industrial (permitted) loadings and to the MWRA
total system loadings. We also present normalized mercury discharge loadings for each of
the five study groups to simplify comparisons between the study groups.
The MWRA total known industrial (permitted) mercury loadings are the
loadings contributed by all industries that are monitored for mercury. The monitoring data
consists of flow estimates and analytical test results from self-monitoring events and
MWRA monitoring events.
The MWRA total system loadings include mercury loadings from all
sources including permitted industrial discharges, unpermitted residential and commercial
discharges, and ambient sources (such as groundwater infiltration into sewer pipes and
rainwater runoff in combined sewer areas). The total system loadings are calculated using
flow and concentration data from influent monitoring at the MWRA Deer Island and Nut
Island treatment plants.
7.2
Overall Results Comparisons - Total Industrial Loadings
The MWRA maintains nearly 1,100 sewer use discharge permits for
facilities in the Boston Metropolitan Sewer Service Area. Therefore, the 242 facilities
studied represent an important 22 percent of all MWRA-permitted facilities.
The MWRA uses a computer program, known as TRAC-IS, to track and
monitor the compliance record of its permitted sewer users. Through TRAC-IS, the MWRA can
produce an estimate of the overall "known" industrial mercury discharge loading
from all permitted sampling locations. TRAC-IS calculates loadings for only those sampling
locations that have database entries for both discharge flow and concentration test data.
This is the reason that we used the word "known" in the first sentence of this
paragraph.
In our study, we calculated Group Concentration Averages
(GCA’s) for sampling locations in each of the five study groups that had mercury
concentration test data. We then estimated mercury discharge loadings for sampling
locations that did not have mercury concentration test data by using the GCA’s. To
compare our results with the known industrial mercury discharge loadings, therefore, we
must subtract out all loadings calculated from GCA’s to determine our own
"known" loadings.
For FY 1995, our known study group mercury loadings are presented in a
spreadsheet identified as Table D-1 in Appendix D. Our known FY 1996 study group mercury
loadings are presented in a spreadsheet identified as Table D-2 in Appendix D.
For the full year of FY 1995, we found that our estimated known average
mercury discharge loading from all five study groups was about 0.009 pounds per day (first
half) and 0.047 pounds per day (second half) for an overall average of approximately 0.028
pounds per day. Similarly for all of FY 1996, our estimated overall known loading average
was about 0.042 pounds per day. In contrast, we noticed from available TRAC-IS reports
that the overall known industrial mercury sewer discharge loadings were about 0.053 pounds
per day in FY 1995 and 0.081 pounds per day in FY 1996.5
Thus, our five study groups possibly contributed 53 percent
(0.028/0.053 x 100) of the FY 1995 overall known industrial mercury loading to the sewers.
Similarly, our study groups possibly contributed 52 percent (0.042/0.081 x 100) of the FY
1996 overall industrial loading.
Since our five study groups appeared to contribute about 50 percent of
the known industrial mercury loadings while discharging only about 16 percent of the
estimated total industrial discharge flow, they may be relatively significant industrial
(permitted) contributors of mercury to the Boston Metropolitan Sewer Service Area. In
Section 7.4 of this report, we break down the percentage contributions of the individual
study groups to the total known industrial loadings.
In Table 3, we showed that the total estimated loadings from the five
study groups were from 0.016 pounds per day (minimum first half) to 0.077 pounds per day
(maximum second half). If we take the average for the year as 0.046 pounds per day, we
could say that our FY 1995 "known" estimated average loading of 0.028 pounds per
day was about 60 percent of the corresponding total estimated loadings of the five study
groups. Similarly, from Table 4, we could calculate the total estimated loadings for FY
1996 as 0.057 pounds per day. Thus, the "known" estimated loadings of 0.042
pounds per day represent about 70 percent of our corresponding total estimated loadings of
the five study groups. These percentages tell us that most of the loading estimates for
both FY 1995 and FY 1996 are based upon specific sampling location concentration data.
These percentages are unexpectedly high considering the relatively few
numbers of sampling locations that had available mercury concentration test data. The
percentages reflect, therefore, the likelihood that sampling locations with large
discharge flows (and thus large loadings) would have mercury concentration test data.
7.3
Overall Results Comparisons - Total System Loadings
For its Industrial Waste Report No. 12, dated October 1996, the MWRA
prepared estimates of mercury loadings in the sewage received at treatment plant headworks
from the sewer system of the Metropolitan Boston Service Area. The mercury loadings
received at the treatment plant are called total system loadings and are calculated daily
by multiplying measured concentrations by the associated daily flow and an
appropriate units conversion factor. The MWRA estimates were 0.77
pounds of total mercury per day in FY 1995 and 0.83 pounds per day in FY 1996. The MWRA
also estimated that the average treatment plant headworks flows were about 390 MGD in FY
1995 and 430 MGD in FY 1996.6
For comparison, our overall loadings estimates for the five study
groups, derived by averaging the minimum and maximum loadings over both half-year periods,
were 0.046 pounds per day in FY 1995 and 0.057 pounds per day in FY 1996. In addition, we
estimated that our five study groups discharged about 2.05 MGD.
Therefore, the mercury loadings contributions from all five study
groups were about 6 percent in FY 1995 and 7 percent in FY 1996 of total headworks mercury
loadings. These loadings contributions were made by wastewater discharge flows that
represented only about 0.5 percent of the total treatment plant headworks flow.
7.4
Overall Results Comparisons - Study Group Contributions
Broken down by study group and averaged for estimated minimum and
maximum mercury loadings over each of the two years of our study, the percentage
contributions of our five study groups to total industrial discharge mercury loadings and
MWRA total sewer system mercury loadings were approximately as follows:
TABLE 5
STUDY GROUP PERCENTAGE CONTRIBUTIONS
Facility Groups
(No. of Sampling
Locations) |
Known Mercury Discharge Loadings Averages as % of Total Known Industrial
Discharge Loadings |
Overall Mercury Discharge Loadings Averages as % of
Total System Loadings |
FY
1995 |
FY
1996 |
FY
1995 |
FY
1996 |
Clinical (64) |
4.5% |
6.0% |
0.5% |
0.8% |
Incinerator (3) |
3.4% |
6.3% |
0.2% |
0.6% |
Laundry (10 / 8) |
0.4% |
20.1% |
0.05% |
2.0% |
Other (85) |
3.4% |
2.5% |
0.4% |
0.4% |
Research (193) |
41.5% |
16.8% |
4.8% |
3.1% |
Totals (355 / 353) |
53.2% |
51.6% |
6.0% |
6.9% |
7.5 Overall Study
Results - Normalized
Normalized mercury discharge loadings for each type of study group can
be of help in judging the expected loadings from one individual discharge. For
normalization, we calculated average mercury loadings for each study group by taking known
loadings totals and dividing by the numbers of associated sampling locations. We elected
to express the average mercury loadings in units of pounds per day per 100 sampling
locations to avoid dealing with very small numerical loadings values.
Tables 6 and 7 summarize the normalization results for FY 1995 and FY
1996, respectively. The bar charts of Figures 3 and 4, respectively, present the
information graphically.
Ranked by highest-to-lowest average mercury loadings for the five study
groups, the FY 1995 results show that individual discharges could be listed in the
following order:
1st - Incinerator (1st half) or Research (2nd half)
2nd - Research (1st half) or Incinerator (2nd half)
3rd - Other
4th - Clinical
5th - Laundry.
The low ranking of Clinical facilities is somewhat surprising since it
was believed from the work of the Phase I Work Group that Clinical facilities would rank
high as potential sources of mercury discharge loadings. The reality may be that Clinical
facilities, while possibly having wastewater discharges with significant concentrations
of mercury, may have relatively low individual discharge flows that serve to produce low loadings
of mercury.
For FY 1996, the average loadings rankings varied from the first
half-year to the second half-year. This phenomenon is attributable to individual high
discharge concentrations in one of the half-year periods. Ranked by highest-to-lowest
average mercury loadings by study group, the FY 1996 results show that individual
discharges could be listed in the following order for the first half-year:
1st - Laundry (first by an order of magnitude)
2nd - Incinerator
3rd - Research
4th - Other
5th - Clinical facilities
The Laundry ranking as first in average mercury loadings (at about 0.5
pounds per day per 100 sampling locations) could have been expected from the very high
loadings of one facility during the first half-year period of FY 1996.
For the second half-year of FY 1996, the results show that the ranking
order would be:
1st - Incinerator (first by an order of magnitude)
2nd - Clinical
3rd - Research
4th - Laundry
5th - Other
Except for the second place ranking of the Clinical group, this order
is similar to that of all of FY 1995. As shown on Page 4 of Table D-2, one sampling
location in the Clinical group contributed about 60 percent of the estimated known group
loadings for the second half-year period of FY 1996. This high loadings contribution may
have been responsible for the elevated ranking of the Clinical group for the period.
From this brief review of normalized discharge loadings, we have gained
some insight into expected loadings from a typical study group facility. We also found
that the mercury concentration and the discharge flow are equally important factors in a
discharge loading value. Because of observed variabilities in the normalized discharge
loadings, however, we recommend that normalized loadings be calculated for additional
half-year periods to gain a better understanding of possible overall study group rankings
and long-term trends.
TABLE 6
FY1995
Average Mercury Loadings
Per 100 Locations
(see Figure 3)
| GROUP |
Minimum
1st Half |
Maximum
1st Half |
Avg
1st Half |
Rank *
1st Half |
Minimum
2nd Half |
Maximum
2nd Half |
Avg
1st Half |
Rank *
2nd Half |
No. of Tested
Locations |
| |
Lbs/Day |
Lbs/Day |
Lbs/Day |
|
Lbs/Day |
Lbs/Day |
Lbs/Day |
|
1st Half / 2nd Half |
CLINICAL: |
0.006 |
0.007 |
0.006 |
(4) |
0.011 |
0.012 |
0.012 |
(4) |
22 / 30 |
INCINERATOR: |
0.090 |
0.090 |
0.090 |
(1) |
0.030 |
0.030 |
0.030 |
(2) |
3 / 3 |
LAUNDRY: |
0.003 |
0.005 |
0.004 |
(5) |
0.001 |
0.006 |
0.004 |
(5) |
5 / 7 |
OTHER: |
0.001 |
0.008 |
0.005 |
(3) |
0.020 |
0.022 |
0.021 |
(3) |
13 / 14 |
RESEARCH: |
0.009 |
0.011 |
0.010 |
(2) |
0.067 |
0.071 |
0.069 |
(1) |
40 / 58 |
* Rankings by loadings values
FIGURE 3
TABLE 7
FY1996
Average Mercury Loadings
Per 100 Locations
(see Figure 4)
| GROUP |
Minimum
1st Half |
Maximum
1st Half |
Avg
1st Half |
Rank *
1st Half |
Minimum
2nd Half |
Maximum
2nd Half |
Avg
1st Half |
Rank *
2nd Half |
No. of Tested
Locations |
| |
Lbs/Day |
Lbs/Day |
Lbs/Day |
|
Lbs/Day |
Lbs/Day |
Lbs/Day |
|
1st Half / 2nd Half |
CLINICAL: |
0.011 |
0.013 |
0.012 |
(5) |
0.023 |
0.026 |
0.024 |
(2) |
29 / 26 |
INCINERATOR: |
0.069 |
0.069 |
0.069 |
(2) |
0.271 |
0.271 |
0.271 |
(1) |
3 / 3 |
LAUNDRY: |
0.533 |
0.536 |
0.534 |
(1) |
0.008 |
0.010 |
0.009 |
(4) |
6 / 5 |
OTHER: |
0.027 |
0.030 |
0.029 |
(4) |
0.002 |
0.006 |
0.004 |
(5) |
12 / 11 |
RESEARCH: |
0.028 |
0.031 |
0.029 |
(3) |
0.013 |
0.016 |
0.015 |
(3) |
58 / 69 |
* Rankings by loadings values
FIGURE 4
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