Anaerobic processes as the core technology for sustainable domestic wastewater treatment: Consolidated applications, new trends, perspectives, and challenges

Anaerobic digesters have been responsible for the removal of large fraction of organic matter (mineralization of waste sludge) in conventional aerobic sewage treatment plants since the early years of domestic sewage treatment (DST). Attention on the anaerobic technology for improving the sustainability of sewage treatment has been paid mainly after the energy crisis in the 1970s. The successful use of anaerobic reactors (especially up-flow anaerobic sludge blanket (UASB) reactors) for the treatment of raw domestic sewage in tropical and sub-tropical regions (where ambient temperatures are not restrictive for anaerobic digestion) opened the opportunity to substitute the aerobic processes for the anaerobic technology in removal of the influent organic matter. Despite the success, effluents from anaerobic reactors treating domestic sewage require post-treatment in order to achieve the emission standards prevailing in most countries. Initially, the composition of this effluent rich in reduced compounds has required the adoption of post-treatment (mainly aerobic) systems able to remove the undesirable constituents. Currently, however, a wealth of information obtained on biological and physical-chemical processes related to the recovery or removal of nitrogen, phosphorus and sulfur compounds creates the opportunity for new treatment systems. The design of DST plant with the anaerobic reactor as core unit coupled to the preand post-treatment systems in order to promote the recovery of resources and the polishing of effluent quality can improve the sustainability of treatment systems. This paper presents a broader view on the possible applications of anaerobic treatment systems not only for organic matter removal but also for resources recovery aiming at the improvement of the sustainability of DST. 1. The emergence of anaerobic process for domestic sewage treatment The proposition of anaerobic processes as the core technology for domestic sewage treatment (DST) is quite recent. This proposition derives from a more general concept of sustainable environmental protection and resource conservation (EPRC) applied for wastewaters and solid wastes, as proposed by Lettinga et al. (1997). The application of anaerobic processes, however, can be traced to the time of the first engineered wastewater treatment plants (McCarty 1982). For instance, anaerobic processes have been extensively applied for the digestion of primary and secondary sludge in wastewater treatment plants based on conventional aerobic systems such as the activated sludge and trickling filter systems. The settleable solids fraction in the raw sewage (separated in primary settlers) corresponds to Reviews in Environmental Science and Bio/Technology (2006) 5:3–19 Springer 2006 DOI 10.1007/s11157-005-4630-9