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SMARTFERM® Dry AD Technology
Zero Waste Energy can design facilities for 5,000 TPY to 100,000 TPY of almost any organic material utilizing SMARTFERM proprietary technology. SMARTFERM is a state-of-the-art dry anaerobic digestion system for organic waste processing. Like other dry fermentation, it is well suited for the production of biogas from stacked solid organic waste in a non-continuous batch process. Unlike other dry fermentation technologies, SMARTFERM offers a sub-grade percolate tank, which significantly reduces percolate and biogas process piping runs as well as the size of motors and fans to move them. The sub-grade percolate tank also greatly enhances the overall thermal efficiency of the process. The SMARTFERM design allows for a thermophilic mode of operation and is offered on two platforms: a shop fabricated steel digester system or as a cast-in-place concrete digester system.
All SMARTFERM systems can be designed to include biogas-processing technology for combined heat and power (CHP) generation for both onsite use and export as well as compressed natural gas (CNG) for use to fuel waste transport vehicles. In addition, in-vessel composting (IVC) options can provide partial or complete sanitizing of compost for the wholesale or retail market.
SMARTFERM is offered on two platforms based on the amount of organic waste to be processed: shop fabricated steel digesters or as a cast-in-place concrete digesters (see below for details). Our technology is built in the US specifically for the waste industry.
After the organic waste is loaded into the digester, the digester hatch device is closed with a gas seal to ensure that process conditions are maintained and biogas cannot escape. Each digester contains an in-floor aeration system activated immediately after the door is sealed. The aeration system pumps outside air into the organic waste material, creating aerobic conditions that self-heat the material to process temperatures before anaerobic conditions are created using energy from the system. This provides significant energy savings. Aerobic digestion generally lasts no more than 12 hours.
Following the initial aeration of the organic material, the aerobic bacteria consume the remaining oxygen in the digester to establish anaerobic conditions. Under anaerobic conditions, the organic waste is finely sprayed with conditioned process water containing thermophilic micro-organisms (“percolate”) that decompose the waste and produce biogas. This percolate is pumped in a closed loop between the digesters and the heated and insulated percolate tanks located beneath the dry digester area.
Percolate is sprinkled on the material for approximately 20 days causing the production of biogas. Percolate is collected in a drainage system, screened for solids in a specially designed weir called a “sand trap” and overflows back to the percolate tank where it is recharged with the thermophilic organisms required for digestion. High quantities of organic acids arising during the beginning of the process are stored and degraded in the percolate tank to ensure proper pH balance. This makes the percolate tanks very important for biogas production in ZWE’s facilities. The required thermophilic process temperature in the digesters is maintained through accurate process control of the percolate tanks, which regulate percolate temperature.
The production of biogas begins quickly after percolation. Biogas is collected in an embedded piping system and stored in roof-mounted double-membrane bladders. Stored biogas is available for nomination into the CNG system or a CHP system.
Before the digester doors are opened at the end of the batch process, on the 21st day percolate circulation is ended to immediately reduce biogas production. Air is also circulated through the floor aeration system and collected in an exhaust piping system that initially combusts low quality biogas (i.e. methane content of 20% and less) in an Environmental Control Device (ECD). The digester air is then processed in a bio-filter when the methane content is below 1-2%. In total, the shutdown process is a 4- to 5-hour procedure.
Shop Fabricated Steel Digesters
The shop fabricated alternative is a readily executable small-scale dry anaerobic digestion technology system. This design option addresses the increasing needs of customers to treat and process organic waste substrates where they are generated or managed and to generate renewable energy onsite. The shop-fabricated delivery approach provides comparatively low facility capital costs for annual throughput as low as 4,000 tons per year. This is particularly true in locales where the cost of concrete is relatively high.
A basic prefabricated SMARTFERM features steel fabricated and insulated digesters, requiring a minimal amount of space. Each digester has a specially designed hatch that provides a gas-tight seal to ensure anaerobic conditions are properly maintained during processing. The base system also contains a percolate tank (either below ground or above ground), mechanical electrical container, containerized combined heat and power system, package bio-filter, external biogas storage bladder and environmental control device. Typical on site construction time is 6 to 8 weeks.
Cast-in-Place Concrete Digesters
The cast-in-place (CIP) concrete SMARTFERM digester system combines the SMARTFERM’s modular mechanical and electrical systems design with on-site construction of concrete digesters. This approach allows for larger scale digesters and systems, while achieving the same benefits of shop fabrication for all of the critical piping, mechanical, electrical, and control systems. The modular concept also permits flexibility if demand increases, waste composition changes or economic parameters change.
ZWE’s patented SMARTFERM dry anaerobic digestion technology contains unique features that make it the most cost-effective and productive AD system available for organics processing:
- Moisture addition and removal not required. Dry solids can be in excess of 50% of the organic waste input.
- The 21-day batch average cycle time is up to 25% more productive than other dry systems.
- Digesters are biologically self-heated through the air system and re-circulation of the liquid percolate through the material, effectively minimizing energy usage.
- The liquid percolate contains the necessary biological constituents and proper pH balance to negate the use of previously digested material (re-circulation) needed to start subsequent batches.
- Plants require a smaller footprint than traditional systems, creating opportunities for urban applications and lowering infrastructure and operating costs.
- Thermophilic mode of operation improves biogas production and downstream renewable energy applications.
- Odor is controlled through the injection of oxygen into the digester at the end of the process. This strips odors (primarily hydrogen sulfide) and yields a superior product. Exhaust air produced from the shutdown process is oxidized in a biofilter for further odor mitigation.
- In the thermophilic mode of operation, the resulting digestate is free of pathogens (e.g., salmonella) and considered a compost product per the US EPA’s Process to Further Reduce Pathogen’s (PFRP) sanitation requirements.