design variations in aerated static pile composting systems · design variations in aerated static...

Design Variations in Aerated Static Pile Composting Systems

Scott Gamble, P.Eng.

Compost Council of Canada 2014 Annual Conference

Halifax, Nova Scotia

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• ASP composting originated at the USDA research facility in Beltsville, MD in the early 1970’s.

• ASP’s have remained a mainstay for composting dewatered biosolids.

• There were many “departures” in SSO composting technologies in the 1980’s and 1990’s.

• ASP systems are the building block from which many of the common higher tech systems have evolved (e.g. channels, tunnels, static containers).

• During the past ten years we have seen a revival of ASP systems.

What are Aerated Static Piles?

Beltsville ASP circa 1975

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Where Do ASP’s Fit in the

Technology Spectrum?

Passively Aerated Systems Actively Aerated Systems

Passively Aerated Static Piles

Passively Aerated Windrows

Bunker

Windrow

Turned mass bed

Aerated Static Piles

(uncovered and covered)

Tunnels

Static Containers

Agitated Containers

Channels

Agitated beds

Rotating drum

Environment Canada Technical Document on MSW

Organic Processing, 2013

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Advantages Disadvantages

• Pile configurations and height reduce

the space requirements of the facility.

This can translate into lower land costs

and civil development costs.

• Positive aeration provides better odour

control than passively aerated systems.

Negative aeration with biofilters

provides even better odour control.

• Smaller surface area (relative to

windrows) reduces the impacts of cold

weather and rain infiltration.

• Significantly shorter active composting

times than passively aerated systems.

• Capital cost of forced-aeration

equipment.

• Over-aeration can remove moisture and

slow biological decomposition process.

• Feedstock preprocessing requires a

higher degree of care; feedstocks must

be properly sized, well mixed, and

moistened.

• More operator skill is required to

manage the aeration systems.

• Aeration systems generally require

three-phase electrical supply.

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What Feedstocks can be Handled?

• Leaf and yard waste (Harvest

Power pre 2008)

• Food waste (Jepson Prairie)

• Biosolids (Clearwater, Kelowna)

• Digestate (Harvest Power)

• Abattoir waste (Spa Hills)

• Specified Risk Materials

(Lethbridge Research Station)

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Basic ASP Components

Aeration Header

Centrifugal Fan

Biocover (e.g. compost, screening overs)

or Fabric Cover

Aeration Lateral

• Pile Configurations

• Aeration Laterals

• Fan Controls

• Aeration Method

(air direction)

• Aeration Zoning

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Common ASP Design Variations

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Discrete Piles

Pile Configurations

Bunker Systems

Extended Piles.

Pile heights typically

range from 6 to 11

feet high.

Aeration Laterals - Above Grade

Aeration Laterals - Below Grade

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• The simplest way to controls aeration fans is

on an intermittent basis using a timer. A typical

timer cycle involves 2 to 5 minutes of run time

during each 15 to 20 minute cycle.

• Fans can also be operated on an intermittent

basis using temperature feedback. Fans turn

on when temperatures reach a high level set

point, and off when a low level set point is

reached

• Fans can also be operated on an intermittent

basis using temperature feedback. Fans turn

on when oxygen reaches a low level set point,

and off when a high level set point is reached.

Fan Control – Intermittent Aeration

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• Aeration fans operated on a

continuous basis are normally

controlled using a VFD.

• The fan speed can be set

manually on the VFD keypad,

or remotely via a PLC.

• Fans/VFD’s wired to a PLC can

also be programmed to ramp

up and down “automagically” in

response to temperature or

oxygen readings.

Fan Control – Continuous Aeration

Basic Aeration Methods

Positive Aeration

Negative Aeration

Bidirectional Aeration

Control Dampers

Dual Fan System

Single Fan System

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Aeration Zoning

It is important for operators to

understand the air flow limits of

their fans/ ducting system, and

how their PLC logic distributes

air between zones.

• In a multi-zone system, each

aeration fan supplies air to two or

more compost piles or zones.

• Fans typically operate continuously

and air flow to/from each zone is

controlled with dampers.

• Dampers can be controlled

manually or “automagically” with a

PLC.

• Systems can be positively or

negatively aerated, or bidirectional.

Basic Multi-zone Aeration

Positive Aeration

Negative Aeration

Bidirectional Multi-Zone Aeration

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• Pile configuration (length, width) is a balance between

available space and site traffic/layout, duct and fitting

costs (for headers and laterals) and fan costs.

• Pile sizing should also consider daily/weekly feedstock

quantities, delivery schedules, and site operational

schedules.

• In bunker systems, end wall vs side wall lengths should

be optimized. Roof truss lengths and column locations

are a factor when enclosing ASP piles in buildings or

pole barns.

• Longer piles (e.g. > 100ft) may require a special lateral

designs (e.g. varying diameter and/or perforation

pattern).

ASP Size Considerations

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• Ducting materials must match the anticipated air temperatures. Ducting

designs must account for expansion/contraction, fan vibration, and

construction tolerances.

• Duct sizes, fittings and transitions must be appropriate to the air volumes

so that system pressures are reduced.

• Condensate and leachate drainage must be considered in the ducting

design and aeration fan selection. The ducting design should also allow

for easy maintenance and cleanout of ducting runs.

• Ducting and fans in negative and bidirectional aeration systems must be

designed to resist corrosion.

• Aeration fans must provide appropriate aeration rates and meet pressure

requirements of the system.

Aeration Design Considerations

Thank you!

Scott Gamble, P.Eng.

CH2M HILL (Edmonton)

tel: 780-628-4496

email: scott.gamble@ch2m.com

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