Electricity And Fuel From Wastewater
Wastewater
Technology Provides Electricity, Fuel
By
Michele Braas, Project Engineer, RETTEW; Holly White, Communications
Specialist, RETTEW; and Tom Darby, Executive Director, Hermitage Municipal
Authority
Most of the technology used
for wastewater treatment is not new, but the application of these existing
technologies combined with out-of-the-box thinking, can provide a valuable
source of renewable energy.
In Hermitage, PA, near the state’s border with
Ohio, municipal leadership have been working with environmental engineers to
accomplish just that.
What began in
2005 as a mandate to resolve inflow and infiltration issues transformed into a
wastewater treatment plant revitalization.
The project
included installing a new biosolids recycling system and generating enough
electricity to sustain the entire facility.
Today, the
leadership is working on a process to use the system to provide fuel for the
municipality’s fleet as well.
The History
The Hermitage Municipal Authority owns a sewage treatment
facility within the City of Hermitage, in Mercer County, Pennsylvania.
City personnel operate the
treatment site, which serves Clark Borough, the City of Hermitage, Jefferson
Township, Shenango Township, South Pymatuning Township, and Wheatland Borough.
The facility operates with
about 5 million gallons per day (MGD), serving a customer base which includes
several industrial users.
The plant discharges all
treated wastewater to the Shenango River.
Many
municipalities experience inflow and infiltration from aged collection and
conveyance systems, and unfortunately, Hermitage was no stranger to these
challenges.
After experiencing several
sanitary overflows, Hermitage entered into a Consent Order and Decree with the
Pennsylvania Department of Environmental Protection (PA DEP).
The order required facility
upgrades to eliminate those overflows.
While
working to meet the consent order, Hermitage elected to use this opportunity as
to refocus the municipal leaders into a comprehensive plan for their wastewater
treatment needs in the future.
The leadership split its
upgrades into two phases. Phase I would construct new tankage and headworks
facilities to reduce overflows, while Phase II would convert the new tankage
into treatment facilities and upgrade the remainder of the facility.
These
upgrades would total $40 million and provide enhanced wastewater treatment
capacity as well as Net Zero renewable results.
They were to include two
equalization basins, new headworks, and a new ultraviolet disinfection system.
The design would also include
an advanced anaerobic digestion complex to stabilize sludge and produce biogas.
Such a process would reuse
the biogas as energy, contributing to significantly reduced operating costs.
The upgrades resulted in
lifting the PA DEP’s consent order, as well as preparing the treatment plant
for the next 40 years of use.
Wastewater Treatment
Center Improvements
The Authority and City first undertook design and construction
of the equalization tanks.
Both came online in 2008,
designed for conversion in Phase II into sequencing batch reactors.
At
that time, Hermitage also updated the headworks facilities.
Today, following improvements
from both phases, the facilities now include a course bar screen, followed by a
fine screen at the headworks.
Next, the wastewater is
exposed to grit removal, and afterwards is processed through three sequencing
batch reactors.
Ultraviolet light disinfects
the effluent, which is then discharged to the Shenango River.
The facility is permitted for
a hydraulic loading of 7.7 MGD, but can process up to 42 MGD during intense wet
weather situations.
Net Zero Biosolids
Along with its wastewater treatment, Hermitage also now provides
treatment of waste-activated sludge (WAS) that’s generated within its process.
The site also processes waste
delivered by third-party users, including food waste, such as expired dairy
products and liquid residual wastes.
The
system treats all wastes via a temperature-phased anaerobic digestion process
that allows for the destruction of volatile solids as well as the production
and capture of biogas.
The Authority uses the biogas
for energy generation, while the volume of sludge not converted to biogas is
treated to the level of an Environmental Protection Agency 503 Class A
Biosolid.
The biosolids are then used
by local companies and agricultural businesses as fertilizer.
Sludge to Biosolids
Processing: Preliminary Treatment
 |
A gravity belt
thickener
reduces the volume of sludge. |
Two gravity-belt thickeners
provide mechanical sludge thickening to reduce the volume of sludge — by about
84 percent — fed to the anaerobic digestion process.
Through the belt thickeners,
the total solids are increased to between five and six percent.
The thickened WAS is then
ready to be mixed with the third-party food and liquid wastes.
Tanker trucks deliver liquid
residual waste on a daily basis, which is then unloaded into the thickening and
dewatering building.
The wastes pass through a
flow meter, moving into the food waste hydrolysis tank.
Trucks also arrive regularly
with packaged food waste, such as expired dairy and drink products, still in
the original packaging.
The Authority maintains
equipment to remove the cardboard and plastic packaging, with the contents then
conveyed to the hydrolysis tank.
All cardboard and plastic
materials are then recycled at another location.
The combined food and liquid
waste then passes through a screen, flow meter, and gravity thickener.
Sludge to Biosolids
Processing: Anaerobic Digestion
After preliminary treatment, WAS and combined residual wastes
are mixed and sent to a temperature-phased anaerobic digestion process.
The digestion process first
allows for material hydrolysis, or decomposition, to occur.
Then volatile acid
fermentation, also called acidogenesis and acetogenesis, takes place, which is
the transformation of sugars, fatty acids, and amino acids into carbonic acids,
alcohols, hydrogen, carbon dioxide, and ammonia.
Finally, the substance goes
through methane formation, also known as methanogenesis.
These
steps transpire by first conveying the waste to two sequencing feed tanks.
As more sludge arrives in the
feed tanks, the waste is regularly transferred to the thermophilic digester,
and then later to one of three mesophilic digesters.
The
digesters are part of an Infilco Degremont Inc. 2PAD system for
the co-digestion of the combined sludge.
The Infilco process uses a
temperature-phased approach combining thermophilic and mesophilic digestion.
This phased approach
separates process stages, making it more efficient and effective at volatile
solids destruction as well as resulting in biogas generation.
At Hermitage, the sludge resides
in the single thermophilic digester for about two days and can stay in one of
the mesophilic digesters for about 10 days.
The piping transferring the
sludge from the sequencing tanks to the thermophilic digester is adjacent to
and aligns with the piping used to transfer between the thermophilic digester
and mesophilic digesters.
During simultaneous batch
transfers from the sequencing tanks to the thermophilic digester and the
thermophilic to the mesophilic digester, heat is transferred into the piping.
That heat is then used to
preheat the raw sludge arriving in the thermophilic digester.
Also,
as the sludge passes in adjacent pipes, heat transfers from the
higher-temperature waste, which is leaving the thermophilic digester to the
lower-temperature waste, which is arriving in the thermophilic digester.
This results in a decreased
temperature of the waste arriving at the mesophilic digester, moving from
approximately 135 degrees Fahrenheit to 95 degrees Fahrenheit.
The
thermophilic digester operates in the temperature range of 122 to 150 degrees
Fahrenheit.
This range shortens process
time by aiding hydrolysis and acidogenesis, as the fats and sugars break down
and become other elements.
Because of its high
temperature, the digester promotes pathogen destruction, a requirement in
producing Class A Biosolids.
Thermophilic digestion also
enables enhanced volatile solids treatment by creating an environment for
certain types of bacteria to live, which in turn contributes to a higher level
of gas production in the next phase of the process.
After
residing in the thermophilic digester for 48 hours, the sludge is then
transferred to one of three mesophilic digesters.
 |
| One of the three Infilco mesophilic digesters |
From this process comes
biogas, in quantities large enough to help supply the facility’s energy needs.
Hermitage collects the biogas from the mesophilic digester for use.
Biosolids
Reuse
Not all of the solids are converted to biogas. After the
digestion process stabilizes the solids, a belt filter press dewaters the
substance to separate the liquids from the solids.
The liquid, also called
filtrate, cycles back to the start of the wastewater treatment plant.
The City then sends the
solids, now qualified as Class A Biosolids, off site for reuse.
Area businesses and residents
use the recycled waste as a soil amendment, by applying it to agricultural
land, or in landscaping work.
Biogas Treatment and
Use
Hermitage sends the biogas collected from the mesophilic
digester to be cleaned for electric generation.
Any excess gas is burned off
via flare in a specially designated area.
The
biogas to be used in energy generation requires a multi-step purification
process.
First, the gas passes through
an iron sponge system to remove hydrogen sulfide with a proprietary media.
Then the gas undergoes a
process to remove moisture.
Lastly, a set of carbon
filter beds remove siloxanes from the biogas supply.
The treated biogas is then
sent to a combined heat and power (CHP), or cogeneration, internal combustion
engine and generator.
 |
| The facility’s internal combustion engine and generator. |
When combined with natural
gas, the CHP unit provides 100 percent of the heat for the digestion process
and the entire facility’s electrical power needs.
Net
Zero Wastewater
Through
the recent upgrades, Hermitage has converted its wastewater treatment
facilities from a system that experienced overflows to a system that is
environmentally responsible.
The current self-sustaining
operations provide its own power while meeting a community need for beneficial
reuse of residual wasted and expired food products.
More to Come
The
City and Authority have made tremendous strides in energy and solids
management, and are continuing in their commitment to being environmentally
responsible by initiating a Phase III for their wastewater treatment
facilities.
For
the next phase, Hermitage is investigating additional uses for its generated
biogas.
The agencies contracted with
RETTEW to provide a preliminary evaluation on the viability of converting
biogas to compressed natural gas (CNG).
The CNG would then be used as
fuel to power CNG vehicles for the municipality’s use.
Preliminary results
demonstrated that Hermitage could generate substantial savings — in excess of
$100,000 annually — by switching to a CNG-derived form of biogas.
Phase
III of the upgrades will provide equipment to convert the biogas to CNG, as well
as alter the City and Authority’s fleets to CNG vehicles.
The biogas currently treated
and sent to the CHP will be compressed and sent through a pressure swing
absorption vessel for carbon dioxide removal.
Hermitage will then compress
the gas again, and keep it in a cascade storage system.
After being odorized, the
final product will be dispensed through a fast-fill dual dispenser.
CNG dispensers operate like a
typical gasoline dispenser, with the ability to track volume and cost and a
keypad for user operation.
The
agencies are currently pursuing funding opportunities to finance the biogas
conversion to CNG and subsequent use.
The project analysis, design,
and permitting are slated to begin in 2015.
The Story
When
the personnel at The City of Hermitage and The Hermitage Authority began to
address an inflow and infiltration issue years ago, they began the journey to
renewable energy.
Now, the agencies are looking
to generate their entire electrical power needs as well as fuel for their fleet
vehicles, all while continuing to meet the needs of local residents and
businesses.
Hermitage is an example of
how far a commitment to advanced water treatment technology can go.
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