Large-scale Composting & Digestion Technology Options

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1 Large-scale Composting & Digestion Technology Options COMPOST MATTERS IN BC FEBRUARY 27, 2014 SCOTT GAMBLE, P.ENG. CH2M HILL CANADA LTD.

2 Passively Aerated & Turned Systems Static Piles Bunkers Windrow Turned Mass Bed Passively Aerated Windrows (PAWs) In these systems, oxygen is supplied through passive aeration. In some cases, mechanical agitation is used to re-establish free air space within the composting piles and speed up the degradation process.

3 Actively Aerated Systems Aerated Static Piles (ASP) Covered Aerated Static Piles Enclosed ASP (tunnels) Static and Agitated Containerized Systems Channels and Agitated Beds Rotating Drum (small-scale and large-scale) Air is supplied to the composting pile in these systems, typically using a network of air pipes underneath the pile. There are many variations in the design of aeration systems. Some systems also provide agitation to re-establish free air space.

4 How Technology Influences Processing Times

5 Aerated System Basics Continuous vs Intermittent

6 Aeration Floors and Plenums Pipe on Grade In-floor/Below-floor - Exposed pipe - Pipe and spigot - Trenches - Raised plenum (e.g. BacTee)

7 Aerated Static Pile Method Piles are built overtop a network of aeration pipes. Air is forced through the piles using high-pressure centrifugal fans. Piles are sometimes contained within 3-sided bunkers. Piles are not turned, or they may be torn apart and rebuilt once or twice during the active composting phase. Aeration fans are turned on/off by timers, or run continuously. Fan operation can be controlled by temperature or oxygen sensors.

8 Covered Aerated Static Piles (CASP) Covers provide litter control and shed precipitation. Engineered covers also provide odour control. Typically use individual aeration fans for each pile. Timer and/or temperature control.

9 Static Container Systems Pre-mixed materials are loaded into the container. Containers moved with a roll-off truck. Positive aeration. End doors with seals for loading and unloading. Very modular, but more than 15 bins becomes unmanageable. Typically designed with a 7 to 14 day retention time.

10 Agitated Container Systems Pre-mixed materials are loaded into one end. Moving floor, spinners or augers move the material along the length of the container. Positive aeration. Typically designed with a 7 to 21 day retention time.

11 Tunnel Systems ASP in a box. Materials are typically loaded into the tunnel with a wheel loader. Positive aeration with infloor aeration pipes are typical. Aluminum doors with seals. Typically designed with an active composting time of 2 to 4 weeks. Often have complex aeration systems and computer controls.

12 Discharge Agitated Bay Systems Highly automated with low labour requirements Positive or negative aeration. Appropriate for food waste and L&YW. Not typically used for biosolids. Active composting time typically 3 to 4 weeks. Turning frequency is typically every 1 to 3 days. Turning cycle takes from 8 to 20 hours depending on size of the bay. Path of Turner Through Bay Receiving Older Material Newer Material

13 Channel Systems Material is loaded into the front end of the channel. Turner travels from back to front. Material is thrown 10 to 20 ft towards the back of the channel with each pass of the turner. Typically used with positive aeration. Length of channel and turning frequency determines retention time. 10 to 28 days is typical.

14 Drum Systems Two size ranges: large and small Large-scale drums use technology borrowed from cement industry. Drums rotate slowly (e.g. 1 to 2 rpm). Slight incline combined with rotation causes material to move from end to end. Air injected at discharge end. Typically 1 to 7 day retention time. Large-scale drums are normally combined with another technology for active composting.

15 Anaerobic Digestion Process The AD process occurs in distinct stages: different groups of microorganisms convert the organic waste materials into successive products that ultimately result in biogas production. Hydrolytic Bacteria Acidogenic Bacteria Acetogenic Bacteria Methanogenic Bacteria H2, CO2 Complex Organic Compounds Carbohydrates, Proteins, Fats Simple Organic Compounds Sugar, Amino Acids, Fatty Acids Organic Acids and Alcohols CH4, CO2 Biogas Acetic Acid Acetate Hydrolysis Acidogenesis Acetogenesis Methanogenesis Optimal ph 4.8 to to to to 7.2 Hydrolysis involves the breakdown of large and complex organic material into small organic molecules under acidic conditions. Acidogenesis occurs when fermentative microorganisms break the hydrolyzed materials down into a range of different organic acids and alcohols. Acetogenesis is further fermentation of organic acids and alcohols to form short-chain volatile fatty acids (VFAs) and hydrogen (H 2 ). Methanogenesis involves the conversion of byproducts from previous steps into biogas.

16 General Types of AD Technologies Wet digesters are designed to handle materials that are dissolved or suspended in water. In High-solids digesters, the materials are either pumped into a digester tank as a slurry or stacked in place (e.g. with front-end loaders). When stacked in place, water is percolated through the materials to distribute nutrients and microorganisms; they are not submerged in a tank. High Solids Less than 80% Moisture Content Wet (Low Solids) Greater than 80% Moisture Content High-Solids Stackable Less than 60% Moisture Content High-Solids Slurry 60 to 80% Moisture Content

17 Wet System (BTA) Dry Slurry (DRANCO) Dry Stackable (Harvest Power)

18 Digestate Management Digestate is the solid or semi-solid material left over at the end of the digestion process. In wet (low-solids) and high-solids-slurry digestion systems, the digestate is the solid or semi-solid material extracted from the bottom of the digestion tanks. In high-solids-stackable digestion systems, digestate is the solid material removed from the digestion tunnels. A typical quantity of digestate for all digester types is 0.85 tonnes of dewatered digestate for each tonne of wet SSO added to the digester. It is important to estimate digestate quantities, dewatering requirements (if any), and how much centrate will be generated from dewatering operations. Digestate is typically composted, but is sometimes land applied.

19 Biogas 1 tonne of SSO typically produces between 100 and 150 m 3 of biogas. The biogas is primarily composed of methane and carbon dioxide. The methane concentration of biogas is dependent upon feedstock composition, digester biology, and operating conditions. Biogas is typically 100% saturated with moisture and needs to be dehydrated before it can be used in any application. Hydrogen sulphide (H 2 S) can be transformed within an engine into acids that can cause corrosion of the engine. H 2 S levels need to be below 1000 ppmv, and for some applications may need to be lower.

20 Biogas Utilization From a conversion and utilization standpoint, the methane content of the biogas is what determines energy and reuse potential. Biogas generated from MSW organics typically contains in the order of 60% methane. Analyses of the revenue potential from biogas use should be based on the Actual yields rather than the theoretical yield.

21 Supporting Infrastructure An organic processing facility includes of a number of components in addition to the active composting and anaerobic digestion systems: Feedstock Receiving Area Amendment Storage Area Compost Curing Area Finished Compost Storage Area Residuals Storage Areas Leachate Management Infrastructure Contaminated Stormwater Management Infrastructure Uncontaminated Stormwater Management Infrastructure Roadways and Parking Lots Administration and Staff Areas Maintenance and Parts Storage Legend 1. Enclosed Feedstock Receiving Area 2. Outdoor Feedstock Receiving Area 3. Amendment Storage Area 4. Feedstock Preprocessing 5. Anaerobic Digestion System 6. Enclosed Composting System 7. Biofilter 8. Biogas Refining 9. Compost Curing Area 10. Finished Compost Screening and Storage Area 11. Residuals Storage Areas 12. Leachate Storage Tank 13. Contaminated Stormwater Management Pond 14. Administration and Staff Facility 15. Maintenance Warehouse

22 Questions? Scott Gamble CH2M HILL Edmonton, AB