In nature, microbial communities in fluid environments attach to solid substrates (e.g., rocks, sand granules) to form biofilms. Biofilms are aggregations of microorganisms attached to a submerged surface, affording a protective matrix for complex community interactions. These biofilms also increase the resistance of microbes to environmental perturbations (e.g., temperature changes, toxins).
In engineered bioreactors, permanent fixed surfaces (aka “biological attachment surfaces” and “microbial support media”) are commonly provided as attachment sites for development of biofilms. Such engineered bioreactors include those used in water and wastewater treatment facilities, toxic waste remediation processes, pharmaceutical and chemical manufacturing processes, and renewable fuel production.
Operational advantages to the use of such microbial support media include: (1) increased system stability and reliability; (2) increased microbial productivity; (3) decreased reactor size requirements; (4) minimization of microbial loss when product is removed from the reactor; and (5) significantly greater resistance to environmental stresses such as temperature or pH fluctuations and exposure to toxins.
Types of Microbial Support Media. There are two broad classifications of microbial support media commonly used in industry, namely (1) “Random Media” and (2) “Modular Media.”
Random Media (also known as “Dumped Media”) comprises individual pieces of media which are randomly placed into a bioreactor. Random Media can comprise a wide variety of material including, but not limited to, lava rock chunks and various shapes of synthetic media (e.g., perforated balls, saddles, pall rings).
There are a number of problems inherent in the use of Random Media as a microbial support media, including: (1) Random Media can have an inconsistent pore space between individual microbial support media pieces, thereby resulting in uneven hydraulic flow through the microbial support media, increased backpressure and reduced substrate contact with the attached biofilm; (2) Random Media typically has relatively low specific surface area for microbial attachment; and (3) due to its configuration, uneven biomass buildup can occur in Random Media, this buildup resulting in a long-term increase in pore blockage and hydraulic short circuiting.
Modular Media (also knows as “Sheet Media”) typically comprises formed sheets, usually made of PVC plastic, which are joined together. There are two main types of modular media, namely, simple vertical tubes and vertical cross-flow tubes. In vertical cross-flow tubes, the tubes intersect at opposite angles (e.g., sixty degrees), thereby increasing mixing. Both types of modular media were developed originally for air systems. In biological applications, they are currently used primarily in down-flow aerobic trickling filter systems for wastewater treatment.
Modular media poses a number of problems, particularly when applied to upflow anaerobic reactors, namely that (1) the tubular media has poor mixing characteristics and tends to be susceptible to aperture blockage due to biomass buildup, particularly at the interface where modules are stacked, and (2) the cross flow media has good mixing characteristics in high fluid velocity situations such as down-flow trickling filters, however, in the low upward fluid velocities characteristic of anaerobic reactors, laminar flow occurs, reduced mixing characteristics occurs, and reduced contact of substrate with the attached biofilm occurs.