Environmental Engineering Reference
In-Depth Information
Water Resources
Withdrawals of surface water and/or groundwater are expected to continue dur-
ing the operations phase of both biomass power plants and biofuel production and
refinery facilities. The amount of water needed depends on the type of facility. In
a typical biomass power plant, the primary consumptive use of water will be to
support the cooling system used to condense spent steam for reuse. Once-through
cooling systems require large quantities of water to be withdrawn from and returned
to a surface body of water. Wet recirculating cooling systems recycle cooling water
through cooling towers where some portion of water is allowed to evaporate and
must be continuously replenished. Wet recirculating cooling systems also periodi-
cally discharge small volumes of water as blowdown and replace that amount with
freshwater to control chemical and biological contaminants to acceptable levels. A
third type of cooling system, the dry cooling system, condenses and cools steam
using only ambient air and requires no water to operate; however, some dry cooling
systems can also be hybridized into wet/dry systems that use minimal amounts of
water, which is allowed to evaporate to improve performance. Other consumptive
uses of water at a biomass power plant include the initial filling and maintenance of
the steam cycle, sanitary applications to support the workforce, and a wide variety of
incidental maintenance-related industrial applications.
Most uses of water at a biomass power plant will ultimately result in the genera-
tion of some wastewater. Blowdown from both the steam cycle and the wet recir-
culating cooling system will represent the largest wastewater stream and, because
water in both the steam cycle and the cooling system undergoes some chemical
treatment, the discharge will contain chemical residuals. Its temperature will also
be elevated. Water discharged from once-through systems does not undergo chemi-
cal treatment, but the temperature of the discharge will be elevated. All wastewater
discharges from biomass power plants can be directed to a holding pond for evapo-
ration, cooling, and further treatment but are likely to be eventually discharged to
surface waters. The Clean Water Act requires any facility that discharges from a
point source into water of the United States to obtain a National Pollutant Discharge
Elimination System (NPDES) permit. The NPDES permit ensures that the state's
water quality standards are being met.
Water is used in a wide variety of applications for biofuel production and refin-
ing facilities and can be consumed at rates as high as 400 gallons per minute (gpm).
Some water used in production and refining activities can be recovered and recycled
to reduce the demand on the water source. Algae production ponds can be large but
are very shallow (about 12 inches). Only a small volume of water would need to be
added to replace any evaporation. Bioreactors for algae production are closed sys-
tems and require very little additional water. As much as 100 gpm of wastewater can
be discharged from a biofuel production and refining plant. The effluent discharge
temperature would be at or slightly above ambient temperature and would often con-
tain small amounts of chemicals. As with wastewaters from biomass power plants,
such discharges can be directed to lined holding ponds for further treatment or dis-
charged directly to a surface water body under the authority of an USEPA-issued
NPDES permit.
