SUSTAINABLE SEWAGE
Melissa K. Scanlan
Associate Dean & Associate Professor
Vermont Law School
Energy and Sewage
• CO2 from fossil fuel combustion =
largest source of U.S. greenhouse gas emissions
(approx. 78 % of total GWP- weighted emissions in 2012).
• Accelerate transition to renewable energy by moving large
systems off of fossil fuels.
• Status Quo = municipal wastewater facilities meet energy
needs through fossil fuel combustion and spend a large
portion of their budgets on external purchases of energy.
• U.S. wastewater facilities spend around $4 billion/yr for
purchased energy.
Energy Demands to Treat Sewage
Sustainable Sewage
• Goal = Transform energy intensive process, powered by
fossil fuels, into net energy producers, powered by waste.
• WERF: “The energy contained in wastewater and biosolids
exceeds the energy needed for treatment by ten-fold.”
Two Steps to Sustainability
STEP 1 = Efficiency
• Treating municipal sewage to remove pollutants is an
energy intensive process.
• First step in becoming net energy producers is to make
the facility more energy efficient.
Step 2 = Harness Sewage Power
• Use the energy contained in the wastewater and its clean up process to
meet the facility’s remaining energy needs.
• Municipal wastewater contains three types of energy that can be
harnessed: thermal, hydraulic, and chemical.
1.
Thermal energy or the heat energy contained in the wastewater,
which is governed by the specific heat capacity of water.
1.
Hydraulic energy of two types. Potential energy is the energy due
to the water elevation while kinetic energy is the energy from moving
water (velocity).
1.
Chemical (calorific) energy or the energy content stored in the
various organic chemicals in the wastewater. The organic strength is
typically expressed as a chemical oxygen demand (COD) in mg/L.
Best Opportunity for Sewage Power
• Biosolids = 18,000 kJ/kg (8,000 Btu/lb) on a dry weight
basis.
• Anaerobic Digestion = convert the volatile solids to biogas
Onsite power generation units convert gas to electricity.
Heat from the power generation units = recovered and used to heat
the digesters and the facility.
+ Adding high strength organic waste, such as fats, oils and grease
from food production, to co-digest with biosolids
 boost biogas production.
• After removing contaminants, biogas can be used as a vehicle
fuel, injected into a natural gas pipeline, or the methane converted
into liquid biofuel – methanol.
Other Sources of Sewage Power
• Sewage = good medium to grow algae
create bioreactors to harness energy
• Algae used to produce biofuels (including methane,
biodiesel, ethanol, hydrocarbon chains, and hydrogen).
Other Sources …
Sewage Heat Recovery
• Capture energy inherent in wastewater using heat-
exchange technology.
1.
Energy can be harvested at the wastewater treatment
plant and used to heat facility  with the added benefit
of reducing the temperature of the clean water before
it’s returned to a natural water body.
1.
Heat exchangers placed directly in sewers to harvest
the energy—in the form of heat—to offset heating
demands at nearby land uses.
Case Studies
• Strass im Zillertal (Austria)
• Sheboygan, Wisconsin (USA)
• Milwaukee, Wisconsin (USA)
Strass, Austria
• Waste Volume: peak winter flow 10 MGD of municipal
sewage
• Most energy efficient plant in Austria
• Net energy producer:
• (2005) Consumed 7,860 kWh/day and
Produced 8,490 KWh/day of electricity
Strass Sustainability
Step 1: Efficiency
• Nitrogen removal is the most costly and most energyintensive part of wastewater treatment.
• (2002) implemented DEMON® process for
deammonification  major energy savings
Step 2: Harness Sewage Power
• Key = biogas digesters producing electricity
• (2001) installed higher-efficiency co-generation engine
that provides 340 kW of electrical power from biogas
Sheboygan, Wisconsin, USA
• Waste Volume: 10 MGD of municipal sewage
• Currently produces 90% of its annual electrical needs and
85% of its heat needs
• Biogas Produced: 500,000 ft3/day @ 65% methane
• Electrical Energy Produced:
16,500 kWh/day (at peak biogas production)
700 kW power from biogas
• Thermal Energy Produced:
55 million BTU/day
Sheboygan Sustainability
Harness and boost biogas
(2006)
• Ten 30kW Capstone Micro-turbines Unison Solutions
Gas Conditioning Equipment Two Cain Heat Recovery
Units
(2008)
• Co-digestion of high strength wastes (from food
processing and ethanol production)
• Boosted biogas production 150%
(2010)
• Two 200kW Capstone Micro-turbines Unison Solutions
Gas Conditioning Equipment Two Cain Heat Recovery
Units
Milwaukee, Wisconsin, USA
Waste Volume Capacity: 600 MGD of municipal sewage
• Two sewage treatment facilities:
• Jones Island and South Shore
• 2011 Purchased Energy
• Energy Budget (2011): ~$13M (16% of O&M budget)
• By Cost: 70% gas, 30% electricity
• By BTU: 90% gas, 10% electricity
• Total purchased BTUs = ~10,000 Wisconsin residential
homes
• Goal = energy independence by 2035.
South Shore Facility (Milwaukee)
• Step 1  reduce energy
•
•
•
•
•
•
demands
install new high efficiency
blowers, new instrumentation
and control systems and
power monitors.
Step 2  harness energy
produced on site
**2012 = 2/3 energy produced
on site
New digester mixing systems
to increase biogas production
and produce more electricity
to power the facility.
Add high strength wastes to
generate more biogas.
Five new engine/generators
fueled by biogas & power
the sewage treatment
process
Jones Island (Milwaukee)
• Step 1  energy
efficiency
• Install new
instrumentation, control
systems, and power
monitors.
• Step 2  harness
energy produced on site
• Install 20 kw
photovoltaic solar array
• Fuel 3 new turbine
generators with methane
from regional landfill
(pipeline)
Conclusion
• Design municipal sewage facilities for energy efficiency and
harness energy potential in waste.
• Transition off fossil fuels for these major systems.
• Should this be a component of sustainability planning for
municipal sewage treatment globally?
• What are the policies and laws that can accelerate and support
this transition?
Contact Melissa K. Scanlan: mscanlan@vermontlaw.edu