United States
Environmental Protection
Agency
Auxiliary and Supplemental Power
Fact Sheet: Viable Sources
INTRODUCTION
This fact sheet describes the use of Auxiliary and
Supplemental Power Sources (ASPSs), which can
provide Wastewater Treatment Plants (WWTPs)
with a secondary power source in the case of a
blackout or other problem resulting in a loss of
power. In other cases the utility provider may use
this power to supplement other sources of power on
a continuous basis. In order to be effective, these
ASPSs should provide the power necessary to run
the WWTP efficiently and effectively, and should
also have a short start-up time if they are to be used
in an emergency.
Most WWTPs have electric power connections to
at least two independent power substations, such
that if power from one substation fails (i.e., due to
a localized storm or the downing of a local power
line), the WWTP could receive power from the
other substation. However, if the entire grid fails
(such as it did for much of the northeast and the
Great Lakes states in August 2003), having power
feeds from separate substations that are all
connected to the same main grid will not meet the
auxiliary power needs to keep many WWTPs
operating during such a failure. Without an
adequate reliable auxiliary power source, many
WWTPs will discharge untreated sewage into the
receiving waters.
There are a number of different types of ASPSs that
can provide reliable power to WWTPs on either a
continuous or emergency basis. These include:
• Internal Combustion Engine Driven
Generators (diesel, natural gas, or bio-
gas)
• Microturbines
Fuel Cells
Solar Cells
• Wind Turbines
Some of these technologies can also be used by the
wastewater utilities to supplement their commercial
power sources. Technologies such as fuel cells,
solar cells, wind turbines, and bio-gas driven
generators can provide renewable energy on a
continuous basis, while diesel or natural gas power
generators have been used to reduce peak energy
demands on a short term basis.
Planning for auxiliary power must take into account
the expected flow rates at the WWTP during the
time of the power failure in order to ensure that
sufficient auxiliary power will be available to meet
the normal operating needs of the WWTP.
Planners should also consider other factors that
could affect the amount of power required by the
WWTP to remain operational, such as potential
weather conditions (wet weather can increase storm
water flow to the WWTP in combined systems),
collection system pump station operation, and
whether drinking water is distributed during the
power failure (this function requires increased
pump capacity, and could be a factor for combined
water/wastewater utilities). If the technology is
planned to supplement commercial power, other
considerations, such as continuous operating costs,
energy market trends, and long range fuel price
projections, may need to be factored in.
In addition to general considerations related to
evaluating auxiliary and supplemental power
sources, there are also technology specific
considerations that must be evaluated. These
include:
-------
Reliability: ASPSs must provide reliable
auxiliary power under adverse conditions.
ASPSs should be available for immediate
service (i.e., warm up quickly) and be
available for the time period for which they
are needed without interruption. In some
case, auxiliary power may be needed for
extended periods of time (i.e., 48 hours or
more), and sufficient fuel must be available
for long term operation.
Cost: ASPS technologies range widely in
costs which will be a major factor in a
utility's selection of the best options for
providing auxiliary or supplemental power.
Costs should be weighed against many
other factors, including the expected life,
annual maintenance, and reliability of the
technology, as well as potential economic
and environmental costs associated with an
extended power failure at the POTW.
Appropriateness: ASPSs should have
sufficient capacity to operate primary
treatment and disinfection for all
wastewater flows for at least 24 hours after
a power interruption. For discharges to
sensitive water bodies, capacity to operate
additional unit processes (i.e., advanced
treatment) may be required by state
regulatory authorities.
Security: When possible, ASPSs should
be located on-site, because it is easier for
most wastewater utilities to protect on-site
power supplies than it is to protect
transmission lines and substations that feed
the plant or remote pumping stations.
Environmental Factors: The goal of
insuring an adequate auxiliary power
supply is to protect human health and the
environment in the event of a power
interruption. An auxiliary power supply
should be adequate to prevent raw sewage
from coming in contact with the public or
discharging to sensitive receiving waters.
However, spills or leaks from underground
fuel tanks used to store fuel for ASPSs can
create a risk to the ground water or the
environment. In addition, some of the older
gas or diesel engine driven generators
produce air emissions that are harmful to
public health.
• Safety: One significant obstacle to the
installation of on-site electricity generation
at WWTPs is the safety risk associated with
the operation of such equipment. Operators
must be trained to safely operate and
maintain the equipment. There may also be
concerns with fuel storage and handling.
For example, large above ground fuel or gas
storage may pose a risk to public health
from an accident or terrorist attack.
Internal Combustion Engine Driven Generators
Electric generators can be furnished with engines
that can run on diesel fuel, natural gas, or bio-gas.
In many cases the engine can be provided with duel
fuel capability. All of the engines currently being
manufactured are required to meet Clean Air Act
(CAA) emissions requirements as stated in sections
89-90, Chapter 40 of the Code of Federal
Regulations. Some states have additional
requirements that restrict the use of some auxiliary
or supplemental power sources. States are required
to be as strict in environmental regulations as the
federal government, but can be more strict if
needed to meet local air quality restrictions (like
emissions in California). While older engines can
contribute to air pollution problems, today high
efficiency, low emission engines are available for
most generators.
Microturbines
Microturbines are a new, innovative technology
based on jet engines (more specifically the turbo
charger equipment found in jet engines) that use
rotational energy to generate power. Microturbines
can run on bio-gas, natural gas, propane, diesel,
kerosene, methane, and other fuel sources, making
them suitable for a variety of applications. From an
environmental standpoint, these new machines take
up less space, have higher efficiencies, and generate
lower emissions than reciprocating engines. If
operated from a natural gas pipeline, no on site gas
storage is needed, thus reducing safety concerns.
Solar Cells
-------
Solar cells, also known as photovoltaic (PV) cells,
convert sunlight directly into electricity. They are
often assembled into flat plate systems that can be
mounted on rooftops or other open areas. Solar
cells require only sunlight (a renewable energy
source) as fuel, and have no emissions. They
generate electricity with no moving parts and
require little maintenance, making them ideal for
remote locations. However, solar cells are
dependant on weather. If there is no sun there is no
energy generated. If used as an auxiliary source of
power, some type of storage system (i.e., batteries)
must be provided.
Fuel Cells
A fuel cell is an electrochemical device similar to
a battery. While both batteries and fuel cells
generate power through an internal chemical
reaction, a fuel cell differs from a battery in that it
uses an external supply that continuously
replenishes the reactants in the fuel cell. A battery,
on the other hand, has a fixed internal supply of
reactants. The fuel cell can supply power
continuously as long as the reactants are
replenished, while the battery can only generate
limited power before it must be recharged or
replaced. Most types of fuel cells can operate on a
wide variety of fuels including hydrogen, digester
gas, natural gas, propane, landfill gas, diesel, or
other combustible gas. In some cases, such as in a
WWTP, methane (sludge gas) from anaerobic
digesters can be reused in the fuel cell instead of
flaring off the excess gas. Other advantages of fuel
cells include few moving parts, modular design and
negligible emission of pollutants.
Wind Turbines
Wind turbines convert wind into mechanical energy
and electricity. A generator is equipped with fan
blades and placed at the top of a tall tower. The
tower must be tall in order to harness the wind at a
greater velocity, free of turbulence caused by
interference from ground obstacles such as trees,
hills, and buildings. Generally, individual wind
turbines are grouped into wind farms containing
several turbines. The power generated from wind
farms can be inexpensive when compared to other
traditional power production methods. The cost to
generate the electricity from wind farms decreases
as the size of the farm increases. Wind turbines do
not produce any harmful emissions nor do they
require any fuel product for operation. However,
wind turbines do require periodic maintenance,
which can present a safety problem, since most
turbines are mounted on tall towers. There is also
concern about construction and other activities
below each turbine, although the land can generally
still be used for animal grazing or farming.
Problems with birds flying into the turbine
propellers have been reported, however newer
designs have reduced this problem.
REFERENCES
1. Auxiliary and Supplemental Power Fact
Sheet: Solar Cells, EPA 832-F-05-011,
US EPA September 2005.
2. Auxiliary and Supplemental Power Fact
Sheet: Fuel Cells, EPA 832-F-05-012,
US EPA September 2005.
3. Auxiliary and Supplemental Power Fact
Sheet: Wind Turbines, EPA 832-F-05-013,
US EPA September 2005
4. Auxiliary and Supplemental Power Fact
Sheet: Microturbines, EPA 832-F-05-014,
US EPA September 2005.
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for use by the U.S. EPA.
EPA 832-F-05-009
Office of Water
March 2006
------- |