By: Robert T. McMillon and D.J. "Jody" Zabolio, III, P.E.
With waste production in the United States at an all time
high (more than 13.6 billion tons of organic waste is produced in this country
every year) there are not many wastewater treatment plants that can look at
naturally occurring organic waste and see a renewable source of energy.
However, the Village Creek Wastewater Treatment Plant in Fort Worth, Texas, has
done exactly that for the last 40 years. This last year, steps were taken that
could make Village Creek a net energy producer instead of an energy consumer.
The Village Creek plant serves more than 750,000 people and
numerous industries in 23 communities in north central Texas. Village Creek is
owned and operated by the City of Fort Worth and is the city's only sewage
treatment facility. Its permit allows the plant to process up to 166 million
gallons of wastewater every day.
Village Creek serves a major portion of the Dallas/Fort
Worth Metroplex, one of the fastest growing areas in the country. The plant
releases its treated water into the effluent-dominated Trinity River. During
the hot summers in North Texas, when the naturally flowing water in the Trinity
is at its lowest, 95 percent of the river's flow can be made up of effluent.
Award Winning
Village Creek has been a showcase since it opened in 1958.
The plant serves most of Tarrant County, where Fort Worth is located, as well
as a portion of the adjacent Johnson County. All of the products of the
wastewater process are reused. The water released from the plant flows down the
Trinity River into Lake Livingston, where it is processed as drinking water for
the city of Houston.
Village Creek uses a standard activated sludge wastewater
treatment process. The plant's large digester tanks use bacteria that convert
organic waste into substances that are more environmentally friendly. After the
treated water has been released, the organic byproducts are de-watered and the
Class A biosolids are applied to allowable agricultural land as fertilizer and
soil amendment.
Another byproduct of the organic treatment process is
methane gas. Most wastewater treatment plants simply burn off this gas, but not
Village Creek. When the plant first opened, it featured reciprocating internal
combustion engines rated at 1,620 and 1,760 horsepower. The methane gas
produced during the organic treatment process was captured and used to operate
the reciprocating engines to generate electricity. Eventually, the point was
reached where the plant could generate about 30 percent of its electricity
needs.
Over the years, governmental agencies and various industry
groups have recognized Village Creek for its efficient operations. In fact, in
1988 and again in 1998 the EPA bestowed Operating and Maintenance Awards of
Excellence on the plant. The 1988 recognition named Village Creek the best
large advanced wastewater treatment plant in the U.S.
Beyond Efficiency
The plant has not rested on its laurels. Over the last
several years, the internal combustion engines began to show their age.
Maintenance on the two units was becoming very expensive and their nitrous
oxide (NOx) emissions were above today's achievable limits. Over the more than
40 years of their service, the engines had been rebuilt several times. It was
only a matter of time before the engines would have to be replaced. Therefore,
several experts were brought in to help the plant move to the next level of
energy and operational efficiency.
Multatech Engineering, Inc., was retained to assist with the
engineering and planning process as well as TXU, a Dallas-based global energy
company, because of its many years of demonstrated expertise in energy
generation and distribution. As the plan for the upgrade of the plant's energy
generating capabilities unfolded, aggressive goals were set. The plan was not
only to meet all of the plant's own energy needs but, when possible, sell any
surplus energy generated on the open market.
Multatech did a feasibility study and determined that the
reciprocating internal combustion engines could be replaced with the more
energy-efficient gas-fired combustion turbines. Turbines would generate more
electricity from the same amount of methane gas. The turbines also would be
able to output more electricity relative to their size, and they would have
lower operating and maintenance costs, as well as lower nitrous oxide
emissions.
Two Solar Taurus Model 60 turbines that are each capable of
generating 5.2 megawatts of power were purchased. These models often are used
by manufacturing companies and industrial sites to self-generate or co-generate
electricity. Research showed that the turbines could generate anywhere from 90 to
95 percent of the electricity used during peak energy consumption periods in
the spring and fall of the year. The Village Creek plant consumes more
electricity during the rainy seasons in the region because this is when the
water pumps are running at peak levels. Fortunately, these also are the seasons
when residential consumers are not using a great deal of electricity.
Because of its expertise and significant experience
operating combustion turbines to generate electricity, TXU Energy, a subsidiary
of TXU, was retained to operate and maintain the turbines, and to act as the
energy manager for the facility. In addition to staffing the Village Creek
energy equipment with its experts, TXU Energy is able to continually monitor
the operations of the turbines from its control center in Dallas. TXU Energy's
computer-based systems are connected to the plant's equipment so that anything
out of the ordinary operating parameters will immediately alert a technician.
Technicians in the Dallas control center will work with operating personnel on
the ground at the plant to ensure efficient energy generation. A 20-year
contract with TXU Energy was signed and is renewable every year.
Even though it made sense to replace the old internal
combustion reciprocating engines with the more efficient gas-fired turbines, it
was realized that the Village Creek plant would not produce enough methane gas
to keep both turbines running at peak efficiency and generating 10.4 megawatts
of electricity. As a result, authorities are working with TXU Energy to bring a
long-term supply of methane gas from a nearby landfill to the Village Creek
plant. With these two sources of gas (the Village Creek plant and the landfill)
the turbines should be able to meet most, if not all, of its power needs. In
addition, during the summer and winter when the area is not experiencing much
rainfall and electricity needs are low, it is hoped that the turbines will
generate excess power that TXU Energy will be able to sell on the open market.
Recycling Heat Energy
In addition to capturing and using the methane gas produced
during the wastewater treatment process, a closed-loop system for the
high-temperature exhaust that is produced by the gas turbines has been
established. Rather than allow the heat to dissipate into the atmosphere, the
heat recovery system recycles this energy to maintain the required 95-degree
temperature in the 14 digester tanks as well as comfort heat for the
operational buildings.
The heat-recovery system directs the exhaust from the turbines
through a hot-oil circulating system that transfers the heat into an existing
hot-water circulation system to maintain temperatures in the digester tanks and
the buildings' heating system. The digester tanks must be maintained at 95°
to kill any pathogens that may be present and to produce the optimal amount of
methane gas from the bacterial decomposition process.
Getting Credit Where Credit Is Due
Efforts to reuse and recycle energy also benefit the Village
Creek Wastewater Treatment Plant. The Texas Electric Choice Act requires that
all retail electric providers, municipally owned utilities or electric
cooperatives participating in the competitive market must either own or
purchase capacity to produce energy from renewable sources. If the entity does
not own or purchase this capacity, it must purchase renewable energy credits on
the open market. Village Creek already is generating renewable energy credits
and TXU Energy is managing the sale of these credits.
In addition, the Village Creek plant will generate a
significant nitrous oxide reduction credit. Both the gas from the plant's
digester tanks as well as that from the nearby landfill are high in carbon
dioxide. As a result, the gas burns at a relatively low flame temperature. This
lower flame temperature reduces the nitrous oxide emissions from the process.
This is particularly important for this area because the EPA has designated the
region as a non-attainment area for NOx emissions.
What Lies Ahead
With the improvements made to the Village Creek Wastewater
Treatment Plant, the facilities will be able to handle the needs of the city
and the region currently served through at least 2010. At that time, it is
expected that population growth will facilitate either expanding the Village
Creek plant or building a second wastewater treatment plant farther upstream on
the Trinity River. Until then, Village Creek will continue to be an example of
how wastewater management and energy conservation can work hand-in-hand.
About The Author: Robert T. McMillon, assistant director, and D.J. "Jody" Zabolio, III, P.E., senior professional engineer, are with the Water Department, Pollution Control Division, for the City of Fort Worth, Texas.
