ReviewSequential anaerobic–aerobic treatment for domestic wastewater – A review
Introduction
Conventional aerobic technologies based on activated sludge processes are dominantly applied for the treatment of domestic wastewater due to the high efficiency achieved, the possibility for nutrient removal and the high operational flexibility (Gavrilescu and Macoveanu, 1999). Nevertheless, the high capital and operational costs that coincide with the introduction of these technologies impose significant financial constraints on expanding the sewage treatment coverage, particularly in the low income countries. Therefore, to smooth the progress of sanitation services including conveyance and treatment, reliable, unsophisticated and cost-effective treatment technologies should be adopted. Moreover, in countries of limited water resources like Jordan, treated wastewater is accounted for in the national water budget for mainly agriculture usage. Hence, extending sanitation services would result in the development of new urban wastewater ‘reuse’ schemes. Subsequently, agricultural use of treated sewage will stimulate the (peri)urban food production and will reduce the amounts of fresh water allocated to agriculture.
Anaerobic (pre-)treatment of domestic wastewater can serve a viable and cost-effective alternative (Lettinga, 1995) due to its relatively low construction and operational cost, operational simplicity, low production of excess sludge, production of energy in form of biogas and applicability in small and large scales. Moreover, owing to its compactness it can be located near or even inside the area of wastewater collection, stimulating (peri-) urban reuse. Since anaerobic treatment is a pre-treatment method, an adequate post treatment system is required to reach to local standards for discharge and/or agricultural reuse (Elmitwalli et al., 2003, Tawfik et al., 2005, Chernicharo, 2006). Treatment of domestic wastewater in sequential anaerobic–aerobic processes exploits the advantages of the two systems in the most cost-effective set-up. In comparison with conventional aerobic technologies, the combined anaerobic–aerobic system consumes distinctly less energy, produces less excess sludge and is less complex in operation (van Haandel and Lettinga, 1994, von Sperling and Chernicharo, 2005).
In the anaerobic system, solids are entrapped and organic matter is converted into biogas consisting mainly of methane and carbon dioxide. Organically bound nitrogen is converted to ammonium and sulfate is reduced to hydrogen sulfide. Sludge production in anaerobic systems is low and the excess sludge is already digested and can be directly dewatered, typically by drying beds. Regarding the microbiological indicators, coliform removal efficiency is low in anaerobic systems (Keller et al., 2004, Pant and Mittal, 2007). However, helminth eggs are removed more effectively, particularly in the upflow anaerobic sludge blanket (UASB) reactor (Gerba, 2008). Anaerobic effluent’s residual concentration of suspended solids and organic matter is polished in the aerobic system, along with ammonium oxidation to nitrite/nitrate via nitrification. Depending on the type of process and the operational conditions, aerobic treatment provides about 1–2 log pathogens removal (von Sperling and Chernicharo, 2005).
Nitrogen level adjustments can be incorporated in sequential anaerobic–aerobic system through partial recirculation of the nitrified aerobic effluent to the anaerobic reactor for denitrification to take place in conjunction with anaerobic digestion. In the integrated anaerobic reactor part of the organic carbon content in the raw wastewater serves as carbon source for denitrification and the rest is converted to methane. The proposed set-up is particularly of interest for concentrated wastewaters and/or lower ambient temperatures as under those conditions the volumetric design is not limited by the hydraulic loading rate (van Lier, 2008), i.e. there is already a volumetric spare capacity available to accommodate the recirculated flow.
To put the sequential anaerobic–aerobic treatment options on view and to state their feasibility and efficiency in domestic wastewater treatment, a desk review of the researched anaerobic–aerobic systems was performed with accentuation on high rate systems. The sequential systems were classified according to the mode of growth in the aerobic reactor i.e. suspended growth versus attached growth systems.
Section snippets
Sequential anaerobic-suspended growth aerobic systems
In the spectrum of suspended growth treatment processes, Activated Sludge (AS) is the most common configuration. By definition, the basic AS process consists of two basic units: (1) a reactor in which the microorganisms responsible for treatment are kept in suspension and aerated and (2) a liquid solids separation unit. An essential feature of the process is recirculation of part of solids removed from the liquid solids separation unit back to the aeration unit to maintain a high concentration
Sequential anaerobic-attached growth aerobic systems
Attached growth systems can be classified into three general processes: (1) non-submerged attached growth processes, (2) submerged attached growth processes and (3) processes with suspended packing for attached growth.
Other combined processes
Other, and likely more expensive, systems such as the aerobic membrane biological reactors (Sheng-bing et al., 2003) and the dissolved air flotation (Odegaard, 2001, Pinto Filho and Brandao, 2001, Reali et al., 2001, Tessele et al., 2005) have been proposed in literature for treatment of domestic wastewater in conjunction with anaerobic reactors.
Treatment of concentrated sewage
The superiority of sequential anaerobic–aerobic systems over conventional aerobic systems is more profound with treatment of concentrated sewage. In countries of limited per capita share of water, like many of the Middle Eastern countries, the COD content of the produced sewage range between 1500 and 2000 mg l−1 (Halalsheh, 2002, Mahmoud, 2002). Treatment of such concentrated sewage via conventional aerobic systems is highly expensive, especially with respect to operational costs. Pilot trials in
Conclusions
Literature results obtained so far clearly show the effectiveness of the sequential anaerobic–aerobic systems in domestic wastewater treatment and consolidate their advantage over the conventional aerobic systems. The significant contributions of anaerobic treatment in the sequential systems’ overall performance emphasize the discernment of reduction in energy consumption and excess sludge production upon replacing conventional aerobic systems by sequential anaerobic–aerobic systems. However,
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