Converting cooking grease to methane gas
by Irwin Rapoport
Restaurants across America generate a massive amount
of waste cooking grease, oils and fats on a daily basis,
much of which either ends up in landfills or is poured
down drains where the substances are either processed
in wastewater treatment plants, coat and clog sewers
or contaminate the eco-system.
Efforts to re-use this resource include using grease
as a transportation fuel source for vehicles with engines
modified to burn it, but this idea still has limited
Disposing of this grease is becoming harder as more landfills
no longer accept it and cities that formerly processed
grease from outside cities and counties, now only accept
locally produced grease. Hence, disposal fees are increasing,
which impacts cities and restaurants.
However, Riverside, California, with nearly 300,000 residents,
is utilizing the grease collected from restaurants to
increase production of methane gas, which is used to
power the city’s water treatment plant.
A local waste management company collects the grease
from area restaurants, pre-treats it (rendering and screening
the grease) and then transports it to the City’s wastewater
treatment plant for injection into the plant’s anaerobic
digesters. Riverside, like most major metropolitan areas,
requires restaurants to properly and environmentally
dispose of the grease they generate. Thus the collection
service is something restaurants would be in need of
regardless of the program.
“Restaurant grease is an untapped source of energy that
is not being harvested,” said David Wright, general manager
of Riverside Public Utilities.
Wright’s department allocated more than $16,000 to the
public works department, which in April 2005, initiated
the pilot phase of the grease-to-gas-to-power program
to increase production of a highly renewable fuel. The
program was developed by Riverside’s wastewater staff
that learned about the possibility of using grease from
industry publications that were following ongoing experiments
at the time.
“Following a test period that lasted for several months,
analysis of the data supported the economic viability
of the program and it was continued on a full-time basis
in April 2006,” said Richard Pallante, Riverside’s Wastewater
Systems manager and one of the project designers.
“It has been very valuable to the community,” said Kevin
Street, Riverside’s regulatory compliance manager. “We
are not the only city doing this. There are some industries
that have already implemented similar technology, but
we are one of the few cities in the country to have such
a grease-to-gas-to-power program.”
Street stressed that pre-treatment of the grease is essential,
as it removes some of the larger debris which could negatively
impact the injection process if it enters the system.
“It also provides a mechanism to ensure consistency with
the quality of product that we are getting,” he said.
“In the wastewater treatment business, anaerobic digestion
of solid waste is common practice. The breakdown of solids
is enhanced with the addition of restaurant grease as
is methane gas production. For us, adding grease to the
digesters not only increased the amount of methane we
produce, but also the quality.”
The additional methane gas produced from restaurant grease
now allows the city to run two of its three cogeneration
engines at the plant around the clock, producing an average
of 1.5 to 1.7 megawatts of power. Through cogeneration,
the waste heat produced by combusting methane in engines
producing electricity is recovered, which allows the
city to reduce its purchase of natural gas.
Riverside anticipates the grease-to-gas program will
save the city millions of dollars by reducing the amounts
of electricity and natural gas purchased to run the wastewater
treatment plant. In terms of actual savings, studies
showed that between May and November 2006, the city saved
$85,000 a month in natural gas expenses by using restaurant
grease as a cogeneration fuel.
In-house staff was responsible for the design, construction
and launch of the program. Developing the program cost
the city about $100,000.
Thus far more than 40 cities across the United States
have contacted city officials to learn more about the
Riverside’s program has been recognized and this has
translated into prestigious awards such as the American
Public Power Association Energy Innovation Award (2007),
the California Utilities Association Efficiency Award
(2007), The League of California Cities Helen Putnam
Award for Planning and Environmental Quality (2007) and
the Southwest Coast Air Quality Management District’s
20th Anniversary Clean Air Award (2008).
“In October we had folks from other agencies on-site
taking pictures and being briefed on our program,” said
Street. “One of the major attractions of what we have
done here is that with a relatively small budget, a little
investment on-site with the staff at the plant and the
resources that we had available, we were able to put
up a functioning system for a relatively low cost. Our
system may have some limitations, but as far as starting
out as a public pilot project and moving to full production,
it has been extremely successful.”
Street’s explanation matches the view that many advocates
of alternative energy stress – that the equipment and
technology needed to generate such energy is currently
Street believes that Riverside’s success can be replicated
in many cities, provided the proper planning and infrastructure
to match the annual grease supply are put in place.
Riverside operates two 1.3 million gallon digesters in
conjunction with its grease injection system. The two
digesters collectively consume an average of 32,000 gallons
of grease per day.
To further enhance energy production at the plant, the
city recently installed a fuel cell which uses digester
produced methane as its primary fuel. The fuel cell has
increased on-site power production to up to 3 MW.
“This will allow the wastewater treatment plant to be
even closer to energy independence,” said Street. “Even
though we are only using grease at this time, it’s important
to note that there are other waste streams that can be
injected into the digesters which may have similar benefits.”
Such sources include materials high in sugar and grease
Riverside, which in 2005 adopted a Green Action Plan
in regards to everything from power generation to tree
planting, has kept its citizens informed about the success
of the grease-to-gas program through the city’s website,
public forums and council meetings.