Anaerobic Digestion for Environmental Processes

Anaerobic Digesters

Presently there are three categories of anaerobic treatment systems. The first category is the conventional anaerobic digester, which includes two basic designs and another that combines the two. The standard rate digester is the most basic treatment system. It mixes the waste is solely by the movement of gas up through the solid matter and into the top of the tank; there is no external mixing. This process is highly inefficient, for it utilizes only 50 percent of the total waste volume, and requires a very long solids retention time (SRT), usually greater than 30 days (Owen, 212).

To improve upon the standard rate digester, engineers created the high-rate digester which incorporates external mixing to the process. This additional mixing improved the process tremendously by reducing the required SRT to between 6 and 30 days while increasing the organic loading rate approximately 5 times. The two-stage digester is a combination of the high and standard rate digesters, placing the high-rate digester before the standard rate digester. This arrangement is done to "thicken" the waste in the second step and aid in the collection of digester gas. Nevertheless, this system frequently fails to completely separate the waste, making this arrangement inefficient and impractical (Metcalf & Eddy, 612).

Anaerobic contact processes were developed to overcome the problems associated with conventional reactors. This second category of anaerobic digesters is similar to the aerobic activated sludge process, because it includes a set of reactors in series, often with recycle. There are two system configurations for anaerobic contact processes. A schematic of these processes is in Figure 2. The upflow anaerobic process is a large reactor which allows the waste to flow up from the bottom and separates the waste into 3 zones. At the very top is the biogas zone where the gas is collected. Bacteria digest waste in the lowest portion of the upflow reactor; the bioreactor zone. In between these two stages is the clarifier zone where the which exports the stabilized waste (Owen, 220).

The anaerobic activated sludge process consists of a bioreactor and clarifier in series. When influent waste is pumped into the bioreactor, bacteria are allowed to digest the waste, and biogas is created. The gas is then collected at the top of the bioreactor in a variety of ways. The effluent waste, often called mixed liquor, is sent to a clarifier where the larger solids are allowed to settle out of solution. A portion of these sediments are returned to the bioreactor in order to maintain an adequate level of biomass in the reactor. The liquid effluent from the clarifier is then ready for disposal.

With these processes it is often necessary to allow large clusters of bacterial growth to develop so that a sufficient level of bacteria remains in the system. The bacterial clusters either settle in the clarifier of the activated sludge process and are recycled, or they settle to the bottom of the upflow clarifier reactor. This buildup of biomass can be achieved by maintaining a large SRT in the system. Due to the recycling of the anaerobic activated sludge process and the settling nature of the upflow bioreactor, both these systems support a long SRT while simultaneously providing a low hydraulic resonance time (HRT). Thus, they are able to handle a high waste flowrate and a high organic loading rate while operating at 65- 95% efficiency (depending upon the type of waste). Although anaerobic contact processes require energy for pretreatment of the sludge, their ability to utilize almost the total waste volume in creating methane makes this process self sufficient (Owen, 222).

The last classification of anaerobic reactors are the submerged media anaerobic reactors (SMARs). These reactors are similar to the upflow bioreactor yet they contain an additional internal media which supports bacterial growth. The Static-Bed SMAR uses either rocks or synthetic media as the support material. Because the media provides a stationary adherence place for the bacteria, the bacteria are able to grow and fill the cracks between the support media, thus creating the ability to maintain a large stock of biomass. As waste influent flows up through the system it is digested by the bacteria which cling to the support media (Owen, 224).

The Fluidized-Bed SMAR is an improved version of the S-B SMAR. It uses smaller particles (.5 to 1 mm in diameter) as it's support material while separating a portion of the effluent for recycle. The packed particle volume is expanded by 5-20% during typical operation. This expansion allows for bacterial growth between the particles and also provides a large surface area on which the biomass can grow. The recycling of the effluent evenly distributes the solids in the bed and returns any live bacteria that separated from the media and entered the clarifier zone (Owen, 225).

Both SMAR systems are able to resist system stress failure due to changing conditions and other factors. They can also support short HRT's while maintaining large SRT's. Although operating the SMAR reactors at higher temperatures will improve efficiency, speed and increase the amount of sludge utilized by the bacteria, it is not necessary to pre-heat the influent waste. In both systems the gas and liquid effluent are separated at the top of the reactor (Owen, 223).

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