Advanced AHS Digestion Process
The Advanced High Solids (AHS) digester system overcomes the fundamental obstacles of existing
single-phase anaerobic digesters by creating an environment in which all primary groups of bacteria
can exist in symbiotic relationships and carry out the acid and methane phases involved in digesting
organic wastes.

The AHS digester separates the two distinct acid and methane phases into separate tanks or
“reactors”, allowing each group of bacteria to thrive in the optimum environment in terms of
temperature and pH.  This is the source of the high efficiency of the AHS digester.
Products & Processes
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1) Processes
2) Advantages
3) Commercial  
      Digester
Additionally, the conventional plug-flow digesters and completely mixed digesters used in the past to digest solid wastes
are energy-intensive in operation and exhibit problematic material handling characteristics.

AHS Design Advantages over Other Digester Systems
Several fundamental problems with existing digesters were identified: solutions to these problems were identified and
incorporated into the design of the AHS.  The following lists the fundamental design problems in existing digesters and
their solutions:

1)  
AHS utilizes no mechanical mixing and stirring.  Proven, highly reliable, non-mechanical hydraulic mixing system that
uses water in the tanks for mixing, resulting in a highly reliable operation and maintenance costs are considerably lower.
Most other digesters rely on mechanical mixing/stirring, which results in unscheduled operating interruptions from
mechanical component failure. In addition, mechanical mixing systems require more energy, substantially raising
operating costs.

2)  
AHS is size-scalable providing flexibility because the design is not based on custom internal mechanical subsystems.
This provides total flexibility in sizing each system, allowing for cost-effective system expansion as processing needs
grow. Most digesters are inflexible, relying on custom-built internal mixing systems that limit the tank dimensions to the
size for which the mixers were designed. This requires new mixers to be constructed if the tank volume is to be
changed.  The use of mechanical mixing often requires extensive pretreatment of the waste materials and system
scalability becomes problematic and costly.  If a facility requires more capacity, additional tank and mixing arm systems
must be installed, increasing system capital and operating costs.

3)  
AHS is flexible processing high solids and liquids.  OPS has performed  lab and engineering design improvements,
which allows the AHS system to process high liquids, high solids up to 40%, or a combination of both.  The AHS system
will digest a wider variety of biomass materials including food-processing waste, agricultural crop residues, animal waste
streams, municipal green waste and ethanol production by-products.  Many digesters can only process high-liquid waste
streams, ones with very low percentages of organic solids.  Typically, most systems require that there be less than five
percent organic solids in the waste stream.

4)  
AHS uses commercially available components. Incorporating standard components allows for  a more reliable
system, with minimal downtime and substantially lower operating and maintenance costs. Other digesters rely on custom
manufactured components, and extensive downtime can result when these components fail.  Long periods usually pass
before replacement parts can be obtained. In addition, systems with custom components historically require a much
higher operating and maintenance budget.

5)  
AHS allows both batch solids digestion and continuous biogas production. This design uses multiple tanks operating
in parallel, providing an optimal environment for each bacterial process.  Each tank is a stand-alone, and is loaded at a
different time.  If the biological process in a tank were to crash, it can be taken off-line and the other tanks will continue
to operate. Since the organic wastes are loaded at different times, although each tank generates biogas at a different
level, the gas is collected in one manifold and the resulting flow is nearly constant. Most other digesters are single stage
systems, which require that all bacterial processes take place in a single tank.  There are two fundamental faults with
these systems.  First, if the process fails, the entire system is inoperative.  Secondly, biogas production from a single-
tank batch-loaded system is cyclic; gas production varies considerably, following the loading cycle of the waste stream.

Air Quality Benefits of Advanced Anaerobic Digestion
Biogas produced in the AHS digester is treated to remove moisture and sulfur compounds such as H2S.  The generation
of carbon dioxide in the AHS digester does not increase this greenhouse gas in the atmosphere, as does the generation
of carbon dioxide in the conversion processes of petroleum products.  This is because in the AHS digester process,
most of the carbon from the digested vegetation is recycled back to the growing vegetation without increasing
atmospheric carbon dioxide.  The sustainable, renewable methane from the anaerobic process, when properly oxidized
in an internal combustion engine or heating register, is essentially a non-polluting fuel.

This environmental benefit of the AHS digester is the reduction of methane released into the atmosphere. When
vegetation and animal manure decompose naturally, methane is always emitted.  Live vegetation does not absorb and
convert free methane into cellulosic material as it does with free carbon dioxide.  Therefore, naturally degrading organic
waste increases the methane released to the atmosphere.  However, the organic material converted in the AHS digester
sequesters the generated methane until it is converted to carbon dioxide in the combustion process of an internal
combustion engine or heating register, then absorbed back into living vegetation.

Advanced Anaerobic High Solids (AHS) Digester Process
The AHS Digester is a two-stage, hybrid system with a dry, sequentially-loaded, batch first stage and a wet, attached-
growth second stage (methanogenic) with leachate recycling between the stages (Hartman, 2004; Zhang and Zhang,
1999). Leachate recirculation prevents solids from fouling the “wet” methanogenesis digester and because the batches
are phased, the leachate contains a relatively constant concentration of organic acids.

The AHS digester system overcomes the fundamental obstacles of existing single-phase anaerobic digesters by
creating an environment in which all primary groups of bacteria can exist in symbiotic relationships and carry out the
acid and methane phases involved in digesting organic wastes.

The AHS digester separates the two distinct acid and methane phases into separate tanks or “reactors”, allowing each
group of bacteria to thrive in the optimum environment in terms of temperature and pH.  This is the source of the high
efficiency of the AHS digester.  

Additionally, the conventional plug-flow digesters and completely mixed digesters used in the past to digest solid wastes
are energy-intensive in operation and exhibit problematic material handling characteristics and are subject to variations
in pH and organic loading rates.

AHS Operational Design Advantages
Currently there are over 3,000 high solids commercial digester systems in operation in Germany and they incorporate a
first generation two-stage process. The German design loads the material into the first stage-mixing tank for 7 plus
days; then the material is moved into the first stage digester fermentation reactor for a period of 14 up to 60 days based
on the specific system design. The material is finally moved into the second stage digester reactor for a period of 7 to
30 days. The shortest total retention time system evaluated was 28 days with some systems requiring up to 90 days
retention time.

The OPS AHS design combines the first stage mixing process with the first stage digester fermentation reactor. By
incorporating the OPS patented self cleaning auger strainer process and hydraulic mixing system in the first stage
digester reactor, the mixing and blending of new materials can be combined into one process, thus reducing the
retention time and provide a more homogenous material.
Digester Type
Solids
Handling
Total Solid
Concentration
in Feedback
Solids
Loading
Biogas
Production
Methane
Content in
Biogas
O&M
Costs
AHS
Batch
30%
High
Continuous
60-75%
Low
Two-Phase
Continuous
5-10%
Medium
Continuous
40-60%
Medium
SEBAC
Batch
>10%
Medium
Discontinuous
40-60%
Low
Plug Flow
Continuous
5-10%
Low
Continuous
50-70%
Medium
Complete Mixed
Continuous
2-10%
Low
Continuous
50-70%
High
AHS Two-Stage Circulation Diagram
The loading process for typical first generation systems requires a set amount of tons of pre-mixed materials per each
feeding cycle 24 hours per day 7 days per week. The OPS design allows material to be loaded as it is delivered and
does not require mixing prior to loading. The basis of this process is that the OPS design will load new materials into one
reactor for select number of days.  Since the reactors will be completely hydraulically mixed, all new materials will be
uniformly blended with the existing materials and liquid in the reactor. The operational requirement is that materials
loaded into each tank contain a specified overall carbon to nitrogen ratio range that will provide optimal conditions in the
digester reactors.

Evaluated first generation digesters incorporate various types of internal mechanical mixing systems.  Interviews with
plant operators indicated that these mechanical mixing systems were prone to breakdown and failure. Repairs required
shutting down the reactor and having to work inside the harsh environment to complete repairs. Recently a similar
two-stage digester in Italy incorporated the hydraulic mixing into the first stage fermentation reactor. System
performance was monitored and compared to the mechanically mixed process. Operational results showed a 22 %
increase in system efficiency with only a 4% increase in electrical use for a net 18% gain in efficiency.
The OPS design circulates only water from the first stage fermentation reactors to the second stage digester reactor.  
This process greatly reduces the movement of high solids materials and provides a much more stable environment
within the system.

With the combination of the first two stages and the controlled circulation of digester water between reactors, the OPS
systems requires just 16 day retention time, greatly reducing the foot-print of the current first generation designs.

AHS Digester Commercial Demonstration System
Through extensive engineering and design efforts, OPS has constructed a commercial AHS demonstration system with
50,000-gallon total reactor volume on the campus of UC Davis, with major grant sponsorship through the California
Energy Commission.  In addition, the California Integrated Waste Management Board, UC Discovery Grant Program,
and the Propane Education Research Center have sponsored through contributions.
In 2008, OPS completed the construction of the full-scale commercial demonstration commercial digester facility at UC
Davis and began full-scale operational testing programs that ran for almost eleven months. OPS worked with Dr.
Ruihong Zhang and her research team to monitor every aspect of the commercial plant digester including both
biological and mechanical performance.  

The designs and techniques used in the construction of the digester followed commercially proven industrial standards.
Nothing depended upon unproven technological developments in its design, and only commercially available
components went into its construction.  

After the test run completion in late 2009, OPS began evaluating the extensive amount of operational and performance
data obtained during the test period.  To demonstrate how much data was collected for evaluation, the computer
controls program collected a reading from every single pump, motor, meter, fluid level, biogas flow, power use, heating
and every other piece of instrumentation associated with the commercial plant every minute, 24/7 for over ten months.
AHS Digester Commercial Demonstration Plant
AHS Digester Commercial Demonstration Plant -- Feedstock Materials
The designs and techniques used in the construction of the digester follow commercially proven industrial standards.
Nothing depends on unproven technological developments in its design, and only commercially available components
go into its construction.  

The testing results were better than expected, especially from the engineering design and mechanical operation.  We
suffered only one mechanical problem during the over ten months of testing and that was with a modified boiler.  The
digester system operated as anticipated and this allowed us to enter into the commercial digester engineering stages.

AHS Digester Process
The AHS Digester is a two-stage, hybrid system with a dry, sequentially-loaded, batch first stage and a wet, attached-
growth second stage (methanogenic) with leachate recycling between the stages (Hartman, 2004; Zhang and Zhang,
1999). Leachate recirculation prevents solids from fouling the “wet” methanogenesis digester and because the batches
are phased, the leachate contains a relatively constant concentration of organic acids.

The AHS digester system overcomes the fundamental obstacles of existing single-phase anaerobic digesters by
creating an environment in which all primary groups of bacteria can exist in symbiotic relationships and carry out the
acid and methane phases involved in digesting organic wastes.

The AHS digester separates the two distinct acid and methane phases into separate tanks or “reactors”, allowing each
group of bacteria to thrive in the optimum environment in terms of temperature and pH.  This is the source of the high
efficiency of the AHS digester.  

Additionally, the conventional plug-flow digesters and completely mixed digesters used in the past to digest solid wastes
are energy-intensive in operation and exhibit problematic material handling characteristics and are subject to variations
in pH and organic loading rates.
AHS Competitive Advantages
The AHS process offers significant advantages over existing anaerobic digestion technologies.  These advantages stem
from innovative design features that provide optimum environmental conditions for the system’s microorganisms,
resulting in significantly faster and more efficient conversion of both high-solid and high-liquid organic feedstock into
biogas and value-added byproducts.

Compared with the most recently proposed digester for solid-waste conversion, a sequential batch anaerobic
composting (SEBAC) reactor (Chenowyth, 1991), the AHS two stage process has demonstrated superior performance.  
It processes more solids per unit of digester volume and produces a higher methane content biogas at a more constant
rate (Zhang & Zhang, 1998).
AHS Digester Two-Phased Closed-Loop System