Bio-Gas in the Wastewater Sector

Milwaukee Metropolitan Sewerage District

Kevin Shafer, PEExecutive Director

Current MMSD Energy Situation Summary • Energy costs are significant to MMSD.• MMSD has significant carbon footprint.• Natural gas is most significant energy source:

• Financially• BTUs • Carbon footprint

• Reductions in energy use can save money.

JI Natural Gas

Purchased Energy• Energy Budget (2011): ~$13M• 16% of O&M budget• By Cost: 70% gas, 30% electricity• By BTU: 90% gas, 10% electricity

• Total BTUs purchased at WRFs is~10,000 Wisconsin residential homes

South Shore WRF Biogas• BioGas converted to electrical power by Engine Generators.• Air Used in Secondary Treatment• Heat Used in Digestion Process• Biosolids can be Transferred via 11-mile Pipeline.

Existing mixing system not effective ~ 40% VSR. Goal is to achieve 55% to 60% VSR.

Increase gas production ~ 325,000 ft3/da Additional ~ 1.4 MW on-peak power generated Value of increased gas production ~ $300,000/yr Additional heat recovery from engines for digesters

Reduce biosolids mass ~ 18% Increase effective capacity of digesters

Mixing technologies to be evaluated in design Pump and nozzle (good mixing, higher energy) Linear motion (fair mixing, less energy)

Bid October 2012 Complete 2016

SSWRF Digester Mixing

SSWRF Digester Mixing

SSWRF High Strength Waste Receiving Digester capacity increased with mixing project. Industrial wastes fed to digesters to make gas. Revenue for taking industrial wastes Airport deicing waste has been fed for ~ 10 years. HSW Receiving Facilities include:

Receiving station (AgriLife loadout station) Storage (Modify existing tank) Grinders and pumping (New)

Goal is > 60,000 ft3/da digester gas. Value of additional gas > $50,000/yr Minimum increase in digested biosolids

SSWRF High Strength Waste Receiving

Summary Current Renewable Energy Projects

~ 400,000 ft3/da digester gas ~ 0.9 MW (7,884,000 kwh/yr) ~ 1300 WI houses

Future Opportunities Digester feed thickening Others will be identified