Using Anaerobic Digestion to Produce Renewable Energy

Jactone Arogo Ogejo, Ph.D., P.E. Assoc. Professor and Extension Specialist

VBN Annual Conference 2013 Farmville, VA September 10, 2013

What is Anaerobic Digestion

A fermentation process mediated by bacteria in oxygen free environments resulting in the production of methane (CH4) carbon dioxide (CO2) and other minor gases Process occurs naturally in: • marshes; sediments; wetlands; guts of

ruminants and some insects

Engineered systems are used at WWTP, farms, landfills

• Electricity for on-farm use (avoided cost)

• Electricity to grid • REC & Carbon Credits • Nutrient trading • Tipping Fees • Sale of separated liquid and solids

Revenues

Animal Bedding • Locally generated

Fertilizer • 90% less odor • Reduce P & K (separation) • N – form bioavailable to plants • On-farm use and/or sold • Water Quality (e.g. reduction in

pathogens, leaching of N)

Products and Use

Environmental benefits

• Reduced odor • Capture of potent GHG

Manure (dairy, swine)

Food waste

Feedstock (organic matter)

Horticultural and pet food

DIGESTER

GENERATOR

Technology

Feedstock and products of anaerobic digestion

Types of Anaerobic Digesters

Wet type Low solids < 15% TS

Dry type High solids 15 – 50 % TS

Anaerobic digesters are currently used at wastewater treatment plants and on farms around the US including some installations in Virginia

According to (http://www.biogasdata.org)

In the U.S. • 1238+ plants produce biogas • 837+ use biogas for energy • 292+ generate electricity from biogas • 74+ deliver electricity to the grid • 25+ deliver biogas to pipelines

The potential to generate renewable energy from wastewater is significant

Biogas is produced in about 18 wastewater treatment facilities with daily flows > 2 MGD Smallest: Town of Christiansburg - 2.2 MGD

Largest: Henrico Co. Water Reclamation facility - 40

MGD

In Virginia……..

Listing of anaerobic digester installations at wastewater treatment plants in Virginia with indications of how the biogas produced is used (source: biogasdata.org).

Facility Plant flow (MGD)

Digester Temp Biogas use

Alexandria combined sewer system, Alexandria 35.0 Mesophilic Flared, drive machinery, heat digester, HVAC, injected in pipeline Chesapeake-Elizabeth WPCF, Virginia Beach 20.8 Mesophilic Flared Town of Christiansburg, Montogemery 2.2 Mesophilic Flared, heat digester, electricity from micro turbine Falling Creek Sewage Treatment Plant, Chesterfield 7.5 Mesophilic Flared, heat digester Henrico County Water Reclamation Facility, Henrico 40.0 Mesophilic Flared HRSD-Atlantic Sewage Treatment Plant, Virginia Beach 34.65 Mesophilic Flared HRSD-Nansemond Sewage Treatment Plant, Suffolk City 17.0 Mesophilic Flared, heat digester, HVAC, James River WPCF, Newport News 13.99 Mesophilic Flared Leesburg Water Pollution, Bath 5.0 Mesophilic Heat digester, HVAC Moores Creek Regional STP, Charlottesville 10.0 Unknown Unknown North River Wastewater Treatment Plant, Rockingham 12.0 Mesophilic Drive machinery, heat digester Peppers Ferry STP, Pulaski 4.5 Mesophilic Flared, drive machinery, heat digester, HVAC, injected in pipeline Proctors Creek WWTP, Chesterfield 16.0 Mesophilic Flared, heat digester, HVAC UOSA – Centreville, Fairfax 30.0 Thermophilic Flared, heat digester, HVAC, injected in pipeline Waynesboro STP, Waynesboro 2.45 Mesophilic Flared, heat digester, electricity from combustion engine Western Virginia Water Authority, Roanoke City 35.0 Mesophilic Flared, drive machinery, heat digester, electricity from combustion engine York River WPCF, York 6.66 Mesophilic Flared

About 200 on-farm anaerobic digesters in the US

Only one dairy farm has a digester in Virginia

On-Farm Anaerobic Digester

Chatham, VA 1,200 milking 1st AD installed on dairy farm in VA

Evaluation Criteria Association of State Energy Research

Technology Transfer Institutes (ASERTTI) protocol

Objectives

Evaluate AD over 1 year ~

• Waste stabilization

• Quantity & quality of biogas produced

Document operation & maintenance of AD

Volume

Total and soluble chemical oxygen demand (COD)

Total (TS) and volatile solids (VS)

Composition Use

Evaluation

Period: May 2011 – August 2012 • bi-weekly May 2011 to September 2011 • monthly October 2011 – August 2012

The Dairy Energy INC. Digester

Every 30 min.

Every 90 min.

Solids

Material Flow

Digester

Design Parameters • Type: 2-Stage mixed plug flow

• Temp: mesophilic ~ 101 °F

• Design HRT: 28 days

• Design by DVO Inc. WI

• Depth: 14 ft liquid and 2 ft gas storage

Acid Chamber

Met

hane

C

ham

ber

158 ft

75 ft

Influent

Effluent

Met

hane

C

ham

ber

Every 30 min.

Every 90 min.

Solids

Products: Biogas is converted to electricity using a GENSET and fed to the grid (360 KW)

Every 30 min.

Every 90 min.

Solids

Products: sometimes the biogas is used in a boiler

Every 30 min.

Every 90 min.

Solids

Products: Any excess biogas or when engine or boiler is not running is flared

Every 30 min.

Every 90 min.

Solids

Products – Digester effluent

Waste Stabilization

Influent

Effluent Separated Liquids

45%

32%

Influent

Effluent Separated Liquids

51% 55%

TS: 7.5%

VS: 6.8%

VS: 3.9%

VS: 2.9%

TS: 5.3%

TS: 4.2%

Waste Stabilization

Solids reduction Total: 30% Volatile: 42%

Biogas Production

Biogas Use & Quality

Nutrient content of manure fed to digester

• Consistent solids content, pH, and nutrients

• Average pH of the raw manure is about 7.7

• Total nitrogen to phosphorus (P2O5) is 2.6 (wet wt)

• Ammonia nitrogen is about 42% of total nitrogen

The digester effluent

• The digestate is sent to the solids separator • The N, P, and K concentration consistent values following digestion. • The ratio of total nitrogen to total phosphorus is about 2.7 (wet

weight) basis, similar to raw manure N:P • No change in total N and P concentrations between raw and

digested manure. • Ammonium-N is 51% of the total nitrogen in digested manure –

increase of 10% compared to raw manure

Solids Separator • Mechanical screw press separator

with 0.5 mm slot openings

• Separated solids have 25-30% dry matter and used as animal bedding or fertilizer (contains some nutrients)

• For every 1000 gal (approx. 8,300 lbs.) of digestate processed, 760 lbs. (wet basis) of solids are

produced.

• The separated liquid is used as liquid fertilizer.

Separated liquid and solids nutrient content

Liquids

Solids

Nutrients Partitioning • Solids: 18, 23, & 9% of N, P,

and K, respectively. • Liquid: 82, 77, and 91% of

N, P, and K, respectively

Service & Maintenance

• Oil Change every 550 hours of engine run time ( 45 minutes)

• 10% electricity consumed by digester operations (pumps, separator) • Routine daily monitoring 20 minutes

450 KW Capacity

350 KW Produced

10% Consumed by Digester Operations

Successful AD performance

The evaluation showed ….

Recommendations

Investigate co-digestion (dairy manure + other feedstocks)

Monitoring & Evaluating ADs necessary to assure effective waste treatment & biogas production

Why is there only one digester on a dairy farm in VA?

• Size of dairy farms: 700 Dairies in VA, average

herd size of ~90

• High capital cost: On-farm systems are not cost effective for the size of farms in VA

• Green energy and carbon credits: Policies not well established

AD in VA – What is the Answer?

• Co-Digestion/Comingling/Blending feedstocks

• Centralized AD System

• Improved process efficiency

• New technology

To make AD work in small farms in Virginia we have set goals

1. To determine the optimum mix of selected

organic residuals to produce maximum quality and quantity of gas

2. Best digester configuration for maximum gas production

3. Figure out what will make the economics work for anaerobic digesters in small farming communities

Feedstocks have different biogas yield

The idea is ….

individually

Combined

Studies in our laboratory have shown that blending dairy manure (DM) with poultry processing wastewater (PPW) increases the quantity and quality of biogas produced

A – 100% DM; B – 33% DM; C – 50% DM; D – 67% DM; E – 100%PPW

Examples of High Solids Anaerobic Digestion Systems (HSAD)

Based in California – handle a diverse mixed feedstock of up to 50% Total Solids

Source. R.H. Zhang, UC Davis

Source. R.H. Zhang, UC Davis

Source: W. Beadle, BIOFERM

Source: W. Beadle, BIOFERM

Source: ORBIT website

Some challenges and barriers to the use of anaerobic digesters

• Economics: high capital cost, limited capital

resources, economics (real and perceived) does not justify the investment

• Working with public utilities: electric and gas • Separation/sorting organic feedstock – for dry

fermentation digesters • Air permitting and transportation and use of

mixed substrates in specific localities • Lack of community and utility leadership or

interest in green power

Thank You

Source: Moffatt B., AgStar Conference 2007

Contact: Jactone Arogo Ogejo Biological Systems Engineering Virginia Tech (540) 231 6815 arogo@vt.edu