Anaerobic sequencing batch reactor ASBR

The ASBR process is able to handle soluble influent streams and also those with higher TSS (Dague et al., 1992; Schmit and Dague, 1993). The ASBR is a non-steady state or pseudo-steady state, anaerobic treatment system. By definition of non-steady state, substrate conversion rate and biomass production rate of the system vary during the cycle. An intermittent feed and decant regime results in alternating high/low substrate (feast/famine) conditions in the reactor. One of the most important operation characteristics of the ASBR is a high substrate concentration at the end of the feed cycle and a low substrate concentration at the end of the react cycle. The reactor cycle includes four steps: feed, react, settle and decant (Arora, 1985; Sung and Dague et al., 1995) (Fig. 23.3).

During feeding, substrate is added to the reactor. Normally, the volume of waste being added to the reactor during feeding is the same as the volume being decanted as effluent. At the end of feeding, the reactor is mixed to distribute the waste throughout the liquid volume. The second step in the cycle is to react. Proper mixing during this step is found to be important in the conversion of organic substrate to biogas. The time

Fermentation, biogas and biohydrogen production 629 Biogas or liquid recycle

Supernatant

Settled biomass

Supernatant

Settled biomass

Settle Decant

Settle Decant

Biogas

Decant ^ ports

Treated Waste waste feed

Biogas

Decant ^ ports

Treated Waste waste feed

Fig. 23.3 ASBR cycle.

required for this step depends on several parameters such as substrate composition, substrate strength, required effluent quality, biomass concentration and reactor temperature (Sung and Dague, 1992; Hur et al., 1998). In the third step, mixing is shut off to allow biomass solid separation while the reactor itself acts as the clarifier. The time required for clarification varies, depending on biomass concentration and its ability to settle, and ranges from a few minutes to one hour. The settling time needs to be short to wash out the poorly settling biomass, but not so short that granular biomass is washed out of the reactor (Angenent and Dague, 1995). The last step is the decanting of the effluent out of the reactor. The decant step takes place after sufficient solids separation has occurred. The time required for this step is governed by the total volume to be decanted during each cycle and the decanting rate. The total volume is dependent on the HRT and the volume of the reactor. Since the ASBR is a closed system, a reduced pressure results when effluent is withdrawn unless a provision is made for biogas backflow. To overcome this, a separate gas bag is installed to maintain constant pressure. While decanting, the gas bag decreases in volume, refilling again during the feed step. Once the decant step is completed, the reactor is ready to be fed another batch of substrate (Sung and Dague, 1995).

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