Understanding the Biorealis Systems Anaerobic Methane Digester

The process by which anaerobic bacteria decompose organic matter into methane, carbon dioxide, and a nutrient-rich sludge involves a step-wise series of reactions requiring the cooperative action of several organisms. In the first stage, a variety of primary producers (acidogens) break down the raw wastes into simpler fatty acids. In the second stage, a different group of organisms (methanogens) consume the acids produced by the acidogens, generating biogas as a metabolic byproduct. On average, acidogens grow much more quickly than methanogens. They are also much hardier organisms, able to survive a broader range of temperature and pH conditions.

Problems with


As a result of these dependencies, single-stage digesters, where both species are cultured together in the same environment, are inherently unstable. In such systems, any imbalance between the two organisms creates a positive feedback (runaway) situation which can quickly bring the entire process to a halt. That is, a slight drop in methane production rate without a corresponding reduction in substrate production will cause volatile acids to accumulate, causing the pH to drop, killing more methanogens, causing further accumulation of acids...and so on, until the system crashes.

Conventional single-stage digesters are not only unstable, they are also relatively inefficient. For example, if the system is operated at a hydraulic retention time (HRT) conducive to growth of acidogens (2-3 days), any methanogens present will be washed out of the system faster than they can reproduce, preventing them from ever becoming established. On the other hand, if the system is operated at an HRT conducive to growth of methanogens (20-25days), the faster growing acidogens will be maintained at the low growth rates of the endogenous growth phase - essentially at starvation level.


Systems as 

Basis for 

In living organisms, the digestive process is maintained with great stability and efficiency in spite of widely varying nutrient loading, and hydraulic flow rates. These characteristics derive from 1) basic physical configuration, and 2) a responsive "control system".

In higher animals, digestion is a multi-stage process in which organic material is first macerated and mixed with enzymes (as in the mouth), hydrolyzed in an acid bath (stomach), then passed through an anaerobic plug flow reactor with extensive surface area for bacterial colonization (intestine). The entire process is controlled by a highly responsive and self-adapting, fine-grain control system (i.e. brain, nervous system) able to precisely regulate temperature, pH, peristalsis, etc.

Biorealis Systems, Inc. (BSI) digesters are designed to emulate these characteristics of natural living systems, gaining improvements in both efficiency and stability over conventional single-stage designs.  The basic components include (1) an acid-phase digester (i.e. stomach), (2) a methane- phase digester (intestine), and (3) a microprocessor based controller (brain).

BSI Digester 
BSI digesters are custom designed for the application, sized and configured for the estimated quantity and content of wastes to be digested.  A typical unit consists of an insulated, heated, gas-tight tank partitioned into two chambers: the first enclosing about ¼ of the total volume, the second about three times larger than the first, or ¾ of the total.  Typical appurtenances include a heat exchanger, auxiliary heating system, access hatches, sludge removal ports, pipe fittings and valves, etc.  An electronic controller allows adjustment of temperature and HRT to provide optimum performance for varying wastes and temperatures.
Digesters may be configured as a single large tank, or multiple smaller modules manifolded together. Factors to be considered in the final design include space requirements, relative cost (i.e. cost of tanks vs added piping, fittings & controls), redundancy, and surface area heat loss. 

The ideal feedstock is a 6-8% slurry with a Carbon to Nitrogen ratio of about 30:1. Incoming waste material should be macerated, and, ideally,  as close to the operating temperature (95 degF) of the digester as possible. 

The acid-phase digester is operated as a continuous stirred tank reactor (CSTR) at an HRT of about three days - optimum for the rapid growth of acid-forming bacteria, but preventing, or limiting the growth of methanogens. Gas produced in this tank consists primarily of carbon dioxide. Tank contents are stirred intermittently by pumped influent entering the tank at a velocity high enough to swirl the contents.

The second phase digester is optimized for growth of methanogens by operating it at an HRT of about 9-10 days.  Concentrated volatile fatty acids flow from the acid-phase digester to the methane- phase digester via a short plug-flow "duodenum", where they are immediately mixed with the methanogen- rich contents. Methane tank stirring is typically provided by recirculating biogas through a bubble diffuser located near the bottom of the tank.

Support and

Biorealis Systems can help with design, construction and installation, and provide continuous remote monitoring and maintenance support of installed systems. For further information, go to the discussion area, or contact us with your specific requirements. For information on how to design and build a pilot-scale digester suitable for on-site treatment of toilet wastes, follow the links below. 


Design and Build Your Own Small Scale Digester

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