Anaerobic reactor design concepts for the treatment of domestic wastewater

Since the earlier anaerobic treatment systems, the design concepts were improved from classic reactors like septic tanks and anaerobic ponds, to modern high rate reactor configurations like anaerobic filters, UASB, EGSB, fixed film fluidized bed and expanded bed reactors, and others. In this paper, anaerobic reactors are evaluated considering the historical evolution and types of wastewaters. The emphasis is on the potential for application in domestic sewage treatment, particularly in regions with a hot climate. Proper design and operation can result in a high capacity and efficiency of organic matter removal using single anaerobic reactors. Performance comparison of anaerobic treatment systems is presented based mostly on a single but practical parameter, the hydraulic retention time. Combined anaerobic reactor systems as well as combined anaerobic and non-anaerobic systems are also presented. 1. Historical evolution of anaerobic treatment When the first anaerobic treatment systems were developed by the end of the 19th century, the design was not really adequate for good performance, possibly due to the misconception that the settleable solids were the most important sewage component to be removed (McCarty 1982). Although they are the most visible constituent, they account for only about 1/3 of the organic load in raw sewage, whereas another 1/ 3 is present in the form of colloids and the remaining 1/3 as dissolved matter. Much of the colloidal and soluble fractions were not removed because the design of the treatment systems was such that there was no possibility of contact between the non-settleable fractions and the bacteria that were active in the biological reactor. As a result the organic material removal efficiency was low, of the order of 30 to 40%. After the development of the first aerobic systems in the decade 1910–1920, another misconception was that the lower efficiency of the anaerobic systems was attributable to a lower metabolic capacity of anaerobic bacteria and therefore a long retention time would be necessary. This has confused sanitary engineers for a long time and to a large extent persists until today. In reality, the excellent performance of the aerobic process like activate sludge was mainly due to the intense contact of the active bacteria and the influent organic material, guaranteed by intense mixing by aeration. By contrast in classic anaerobic systems, mixing was unintentional. The high potential of anaerobic systems was shown by Young and McCarty (1969), who successfully operated an upflow anaerobic filter, treating concentrated mainly soluble rum distillery wastewater. Two very important improvements Reviews in Environmental Science and Bio/Technology (2006) 5:21–38 Springer 2006 DOI 10.1007/s11157-005-4888-y