South Plant Grease Co-Digestion Study

Local sewer agencies have a need to
address restaurant grease trap waste.
In response, there is the potential of a
renewable energy fuel source and
opportunity to invest in sustainable
waste management technologies.
Wastewater Treatment Division (WTD)
34 local jurisdictions feed
wastewater to King County
WTD’s system
3 major treatment plants
serve 1.5 million across 420
sq. miles
600 employees manage the
treatment of about 175
million gallons of sewage
each day
Our challenge: integrating
sustainability into urban
sanitation systems
In Evaluation of Emerging Technologies
Level of technology development
Applicability to WTD system
Potential benefits to WTD facilities or environment
Potential impacts to WTD facilities or environment
Potential for recovery of valuable resources
Consistency with King County policies & directives
A Business Need for Energy Management
WTD represents 58% of all
King County government’s
facility energy use
Target #1: Achieve a 10% normalized net
reduction in energy use
Target #2: Produce, use or procure renewable
energy equal to 50% of use
Target #3: Maximize the cost-effective
conversion of waste to energy
2010 Wastewater Treatment Energy Use by Type
WTD Already Produces
610,000 MMBtu’s of Digester Gas
South Plant
Digester gas scrubbed. 20% of it used on site for
process heat. 65% sold directly to PSE [85% use, 15%
West Point
New cogen plant expected to generate 18,000+
mega-watt hours of ‘renewable energy’ every year,
nearly 1/3 of the plants yearly electricity consumption
Plant will use 60% of digester gas to produce
electricity for sale to SCL – two engines, capable of
producing 2.3 MW each (combined 4.6 MW installed power)
44% of digester gas energy used on-site for process
needs including raw sewage pumping [total adds to
>100% as cogen process also generates all process heat required for the
Why South Plant for Brown Grease?
Relatively easier plant
access for trucks visiting
brown grease receiving site
- must be self serve
Centrally located in Renton
Digester capacity
Local Sewer Agencies’
Proposed Preferred Pumper Program
If PPP moves forward:
 Potential for “new” brown
grease coming on market
 Standardization and
improved record-keeping
 Would need public &
private involvement to
leverage resources/support
beneficial reuse
2006: Food Waste Co-Digestion Study
 Increased energy production
 Uncertain supply with private composter contracting for
restaurant food waste
 High onsite cost to unload and store incoming waste;
screen out metals and glass; mix and grind
 2006 report recommended looking at liquid waste as
potentially more cost-effective
WTD Approach to Study
Experience available: nationally, agencies/consulting firms
have implemented full-scale grease co-digestion projects
Plants currently accepting brown grease by truck (partial list):
Riverside, CA
East Bay Municipal Utility District, CA
Millbrae, CA
Oxnard, CA
Watsonville, CA
So. Bayside Authority (Redwood City), CA
Lincoln, NE
Pinellas County, FL
2012: Brown Grease Co-Digestion Study
Q1: How much brown grease can
the current wastewater
processing facilities manage?
Q2: How much would an
appropriately-sized brown
grease receiving facility
Q3: What are the estimated
operating costs and
Q1: How much grease can current wastewater
processing facilities manage?
Capacity of solids processing
facilities (digesters, biosolids
mngt., etc.) - OK
Capacity of biogas handling
systems could be limited by
waste gas burners - ??
Capacity of biogas-to-energy
systems - OK
Practical operational limit to
daily truck deliveries onsite maximum 10–20 trucks/day
Q2: How much would an appropriately-sized
brown grease receiving facility cost?
Representative site selected
Identified facilities needed
Truck offloading facilities, grease receiving
Screening, heating, mixing, pumping
Modular design to facilitate
expansion if needed
Capital cost estimate: $2M
Q3: Identify estimated operating costs and
Cost estimates prepared for facility to manage
15,000 - 30,000 gpd (10 – 20 trucks/day)
Cap’l recovery:
O&M cost:
$400,000/yr to $500,000/yr
Energy revenue:
$100,000/yr to $200,000/yr
Tipping fee required to break even
Next Steps
Market assessment
How much brown grease is currently being
collected by haulers?
Can a sufficient quantity of grease be
secured to recover costs?
Is the PPP moving forward?
Further verify key grease
co-digestion process
Seek grant funding
WTD Considerations Moving Forward
Must be cost-effective for ratepayers
Ability to help WTD meet energy targets
Need assurances that supply won’t disappear
Is this something better done by private sector?
Brown grease can create storage and pumping
Must be O&M cost neutral
Ensure trucks don’t create safety & security issues
Digester and scrubber capacity is not unlimited
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