POWER November 2016 - 42

REGULATORY COMPLIANCE
Next-Generation FGD Wastewater
Bioreactor Technology Introduced
for ELG Compliance
Compliance with one of the latest federal environmental regulations, the Effluent
Limitations Guidelines (ELG), can be both costly and complicated. A
newly available option offers features and capabilities that may make it a
more attractive alternative than conventional approaches.
James Peterson PE
I
n September 2015, the U.S. Environmental
Protection Agency (EPA) finalized
a new rule affecting flue gas desulfurization
(FGD) wastewater discharges
from coal-burning steam electric power
generation units by setting stringent federal
limits on total suspended solids (TSS),
arsenic, mercury, nitrate/nitrite, and selenium
(Table 1).
The final Effluent Limitations Guidelines
(ELG), as it is known, establishes
effluent limitations based on best available
technology economically achievable
(BAT) and is based on " technological
availability, economic achievability, and
other statutory factors and is intended to
reflect the highest performance in the industry. "
The ELG rule identifies a technology
basis for BAT for FGD that includes
a " chemical precipitation system [Figure
1] that employs hydroxide precipitation,
sulfide precipitation (organosulfide), and
iron coprecipitation, followed by an anoxic/anaerobic
fixed film biological treatment
system designed to remove heavy
metals, selenium and nitrate. "
In a biologically based approach to wastewater
treatment, specialized microorganisms
are cultured in an engineered bioreactor to
maintain environmental conditions that facilitate
transformation of contaminants from
Table 1. Steam Electric Effluent
Limitation Guidelines for flue gas
desulfurization wastewater (existing
sources). Source: EPA
Element
Arsenic (ppb)
Mercury (ppt)
Selenium (ppb)
Nitrate-nitrite (ppm as N)
42
30-day
average
8
356
12
4.4
Daily
maximum
11
788
23
17
Biomass + CO2
Selenate
www.powermag.com
Selenite
Selenium
POWER | November 2016
a soluble and toxic form to a solid product
that can be safely handled and disposed. In
the case of selenium, microbial respiration is
performed by certain bacteria when nitrate, a
more energetically favorable electron acceptor,
is depleted, and bacteria respire selenate
as a substitute terminal electron acceptor to
produce selenite.
The process continues with a decrease
in oxidation reduction potential (ORP)
through selenite to form an insoluble precipitate
of elemental selenium (Figure 2).
Under the right conditions, this precipitate
can be retained in the bioreactor and periodically
wasted from the system in a concentrated
form.
1. Meets EPA technology basis. This diagram shows the full process of biological
treatment following physical/chemical precipitation for flue gas desulfurization (FGD) wastewater
provided by Frontier Water technology. Source: Frontier Water
Lime
Clean flue gas out
H2O
Air
Flue gas
Limestone
FGD scrubber
To FGD
WWTS
Hydrocyclone
Dewatering
Equalization
tank
Recycled
sludge
Clarification
Filtrate back to
equalization
Dewatering
Sludge cake
Gravity
filtration
Equalization
Biological
treatment
Ultrafiltration Discharge
Disposal
2. Good bacteria at work. Microbial respiration is used to achieve selenate reduction
to elemental selenium. Source: Frontier Water
CNP + Micro
1st stage RXN
gypsum desaturation
2nd stage
RXN
Polymer 2nd stage
Organosulfide
Coagulant
Sludge
thickener

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POWER November 2016

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