SUSTAINABLE SEWAGEMelissa 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.

• 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).

• 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 biogasOnsite 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 energy-intensive 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 SustainabilityHarness and boost biogas (2006)•Ten 30kW Capstone Micro-turbinesUnison Solutions Gas Conditioning EquipmentTwo 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-turbinesUnison 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