|
Mercury
Work Group
Phase I Reports >> Infrastructure Report
Executive Summary | End-of-Pipe Report |
Operations Report | Infrastructure Report
For more information,
contact David Eppstein by email at
deppstein@masco.harvard.edu,
or by calling 617-632-2860.
4.0 MERCURY ACCUMULATION IN BIOMASS
Preventing mercury from entering the waste stream will not ensure
the elimination of mercury being accumulated within the special
waste piping system. The combination of organic waste products
being discharged, bacteria, temperature and humidity within a
piping system provides a natural environment for biomass growth
and a vessel for the accumulation of mercury onto the walls of
the waste pipe.
Biomass evolves from organic matter such as blood products, urea,
soaps, chemical reagents, bacteria, virus, infectious wastes,
etc., that have been discharged into the piping system. The combination
of these organic substances forms a growth on the side of the
piping system.
Within a trap, the growth can be more pronounced due to the lower
flows of wastes over the sides of the pipe thus creating a continuous
liquid "incubator" where oxidation or dehydration of
bacteria will not occur.
Within a flowing pipe, the biomass growth occurs principally below
the liquid level with lesser biomass present above that line.
A hardened skeleton of carbon, oxidized soap products containing
elements such as potassium and dried blood products is formed
and strongly adheres to the wall of the piping material.
The presence of organic material allows for the formation of methyl
and dimethyl groups which, when combined with mercury, increases
the solubility of the now formed methyl and dimethyl mercury compounds
into organic substances such as the biomass.
The two species of methyl mercury groups accumulate in the biomass
and can concentrate to significant levels. As larger biomass
materials are eventually dislodged and carried away in the waste
stream, a non-compliance issue will most likely occur due to "slugs"
of mercury laden biomass being discharged. It should be noted
that biomass formation also occurs within neutralization reactor
vessels thereby increasing the potential for further mercury accumulation.
Concentration levels of mercury 1,000 times that found in wastewater
have been reported as absorbed into organic masses. The presence
of large amounts of biomass could lead to a higher mercury concentration
potential.
Deaconess-Glover, a New England Deaconess Hospital affiliate institution,
performed a small pilot test to determine if biomass solids were
contributing to its mercury effluent violations. The pilot study
was performed on an area of plumbing during non-operating hours:
city water was poured down a laboratory sink at a 1 minute, 3
minute and 5 minute interval and allowed to travel through the
piping where samples were then collected at three corresponding
time intervals. After the rinsing pilot test was completed, a
biomass sample was removed from the conveyance piping and analyzed
for mercury. The results are as follows:
|
Influent City water at interval
|
1 minute = < 0.0002 ppm
|
|
|
3 minute = < 0.0002 ppm
|
|
|
5 minute = < 0.0002 ppm
|
|
Effluent samples at interval
|
1+ minutes = 0.037 ppm
|
|
|
3+ minutes = 0.0064 ppm
|
|
|
5+ minutes = 0.015 ppm
|
|
Biomass at 28.6 % solids contained
|
9700 ppm
|
This was one of the first pilot tests which provided evidence
that mercury laden biomass was being dislodged by wastewater flows
and eventually becoming a part of the effluent.
Figure 4.1 and 4.2 illustrate how mercury wastes combined with
organic material is, in turn, absorbed by the biomass where it
accumulates within the piping system. A numeric key and general
description of the accumulation process, for figures 4.1 and 4.2,
is presented in Figure 4.3.
There is a tendency for greater amounts of dimethyl mercury to
form within piping systems where the waste water is elevated in
temperature and where heats of formation of chemical reactions
may occur within the piping systems. Additionally, dimethyl mercury
may vaporize at lower temperatures due to the interaction with
more unstable chemical compounds within the piping system or due
to negative pressures within the laboratory room where the point
of discharge occurs.
Since vaporized dimethyl mercury is water soluble, it can emanate
from the trap back into the room. Traps should be of the deep
seal type and be always sealed to help ensure complete protection
against possible vapor "drawback."
FIGURE 4-1: Mercury Accumulation Process in a Special Waste Trap
Source: Flow Tech Associates, Inc., Detail 1
FIGURE 4-2: Mercury Accumulation Process in a Special Waste Pipe
Source: Flow Tech Associates, Inc., Detail 2
FIGURE 4-3: Numeric Key and General Description
of Mercury Accumulation
Process
Source: Flow Tech Associates, Inc., Detail 3
RETURN
TO INFRASTRUCTURE REPORT
TABLE OF CONTENTS
|
