Key Points

1. Submerged attached growth bioreactors (SAGBs) provide high biomass densities, resulting in compact bioreactors that require significantly less land area than traditional suspended and attached growth systems.

2. Submerged attached growth bioreactors can be used for aerobic and anaerobic removal of biodegradable organic matter, for nitrification, and for denitrification. Research is underway on the development of SAGBs that will allow phosphorus accumulating microorganisms to grow.

3. Flow through the media in a SAGB can be upward, downward, or horizontal. Furthermore, granular media can be either packed or fluidized. Most SAGBs rely primarily on the attached biomass and are designed and operated with hydraulic residence times (HRTs) sufficiently short to minimize the growth of suspended biomass. However, combined suspended and attached growth (CSAG) processes use clarifiers to recycle suspended biomass, so that both suspended and attached biomass are utilized for treatment.

4. Packed bed bioreactors are usually used to treat wastewaters containing both soluble and particulate organic matter, such as domestic wastewaters. Fluidized bed biological reactors (FBBRs) are usually used to treat wastewaters containing mostly soluble organic matter, such as industrial wastewaters and contaminated groundwaters. Combined suspended and attached growth processes are usually used to upgrade existing suspended growth bioreactors.

5. The performance of SAGBs is often characterized by correlating it with the total organic loading, which is the mass flow rate of biodegradable organic matter divided by the bioreactor media volume.

6. The surface loading can also be used to characterize the performance of SAGBs. This parameter is particularly useful for characterizing the performance of attached media in CSAG processes.

7. The total hydraulic loading (THL) for many SAGBs must be controlled within specified limits. For packed beds, the THL must not exceed certain maximum values to avoid excessive headloss and unreasonable backwash volumes. For FBBRs, the THL must be sufficiently large to maintain the media in a fluidized condition, yet not so large as to wash out the bioparticles.

8. The solids retention time (SRT) can be used to characterize the performance of FBBRs using inert media such as sand, and the suspended growth component of a CSAG.

9. Many factors, such as pH, temperature, and dissolved oxygen concentration affect the performance of SAGBs. Their effects are similar to those observed with other attacheed growth processes.

10. Design procedures for SAGBs use the performance correlations described in items 5 through 8, above. Excess biomass production rates and oxygen requirements are estimated using net process yield factors and oxygen stoichiometric coefficients, similar to those used in the initial design of suspended growth systems.

11. Appropriate pretreatment is required to ensure long-term performance of SAGBs. Clogging of media and/or nozzles can result from debris or fibrous materials present in the wastewater. Pretreatment of municipal wastewater generally consists of primary clarification, either with or with out chemical enhancement. A high-rate biological process may also be used.

12. Experience with CSAG processes indicates that placement of the media can be important in determining the effectiveness of the biomass that develops on it.

0 0

Post a comment