In almost all fields, certain operations have gained common names through years of use and development. Although such names are not always logical, they are recognized and accepted because of their historical significance. Such is the case in environmental engineering. In fact, some of the names bear little resemblance to the process objectives and are even applied to more than one reactor configuration. For purposes of discussion, twelve common names have been chosen and are listed in Table 1.1. To relate those names to the classification scheme presented above. Table 1.2 was prepared. It defines each name in terms of the bioreactor configuration, the treatment objective, and the reaction environment. Many other named biochemical operations are used, but they can all be related to those described in Table 1.2.
Activated Sludge. Eight different types of activated sludge systems are listed in Table 1.2, suggesting that the name is not very descriptive. The common char-
Table 1.1 Common Biochemical Operations
Suspended growth reactors
Attached growth reactors
Biological nutrient removal
Upflow anaerobic sludge blanket
Rotating biological contactor Trickling filter Packed bed Anaerobic filter acteristic of all of them, however, is that they use a flocculent suspended growth culture of microorganisms in an aerobic bioreactor and employ some means of biomass recycle. Further examination of the table reveals that the primary treatment objective is the removal of soluble organic matter and oxidation of the carbon contained in it. Under appropriate conditions, nitrification will also occur, and thus it is listed as an objective for those systems in which it is most likely. Extended aeration activated sludge (EAAS) systems are often used on wastewaters that have not been treated in a physical operation to remove suspended organic matter. In that case, the insoluble organic matter becomes trapped in the biofloc and undergoes some oxidation and stabilization. Thus, that objective is marked for it. Most other activated sludge types are used on wastewaters from which settleable solids have been removed. As discussed earlier, however, those wastewaters still contain colloidal organic matter, most of which will be removed along with the soluble organic matter. Even though the colloidal material is insoluble and will be partially stabilized during treatment, the main event governing system performance is removal of the soluble organic matter, which is listed as the main treatment objective.
The first uses of activated sludge were on a batch basis. At the end of each aeration period suspended solids (referred to as sludge) were present and they were left in the bioreactor when the clear wastewater was withdrawn after settling. As this batch procedure was repeated the quantity of suspended solids increased, giving more complete removal of organic matter within the allotted reaction time. Although this increase in suspended solids with the associated improvement in removal activity was due to the growth of a viable microbial culture, the reason was unknown to the early researchers, who characterized the sludge as being "activated," thereby giving the process its name.1 Use of the batch process waned as larger facilities were required, but during the 1970s there was a resurgence of interest in the use of batch reactors because of the flexibility offered small installations. Now referred to as sequencing batch reactor activated sludge (SBRAS), many are in use treating both municipal and industrial wastewaters.
As the need to treat larger flows increased, the early batch operation was converted to continuous flow through the use of long aeration chambers similar to plug-flow reactors, followed by sedimentation and biomass recycle. Such systems are called conventional activated sludge (CAS). Various modifications of the plug-flow reactor were tried, among them introduction of the wastewater at various points along the tank, in what has been called step feed activated sludge (SFAS). In the mid-50s, various engineers began advocating the CSTR with cell recycle as an alternative to the CAS reactor because of its inherent stability. That stability, plus the advantages regarding the maintenance of the microbial community in a relatively constant physiological state, caused wide adoption of the completely mixed activated sludge (CMAS) process, particularly for the treatment of industrial wastewaters. The process, however, tended to produce sludges which did not settle as well as sludges from systems containing concentration gradients, so that today many bioreactor systems in use employ several small CSTRs in series before a large one, thereby achieving desired environmental conditions. Such systems are referred to as selector activated sludge (SAS) systems. Other innovations that require CSTRs in series, such as the use of high purity oxygen (HPOAS), have also been adopted. The history of the activated sludge process is very interesting and the reader is encouraged to learn
Table 1.2 Classification of "Named" Biochemical Operations
Removal of soluble organic matter
Stabilization of insoluble organic matter
Conversion of soluble inorganic matter
Aerob. Anaerob. Anoxic Aerob. Anaerob. Anoxic Aerob. Anaerob. Anoxic
Was this article helpful?