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A UASB-digester system: option for anaerobic treatment of low temperature municipal wastewater

General information
A UASB-digester system is invented for low temperature municipal wastewater treatment (Mahmoud et al., 2004). This system treats wastewater in an upflow anaerobic sludge blanket (UASB) reactor at a short hydraulic retention time (6 hours). The UASB sludge is recirculated over a heated sludge digester where the wastewater suspended chemical organic demand (COD), captured in the UASB reactor, is converted to methane. The stabilized digester sludge is returned to the UASB reactor where it continues to capture wastewater organic solids, and at the same time supplies methanogenic biomass to the UASB reactor for conversion of the wastewater soluble COD. The UASB-digester system includes the sludge digester which enables a low excess sludge production, but the recirculation of the UASB sludge to the digester consumes energy. So far, only limited studies show the feasibility of the UASB-digester system for municipal wastewater treatment at 15°C. However, the temperature of municipal wastewater in moderate climate zones can be as low as 10°C. L. Zhang further assessed its feasibility at temperatures below 15°C.

Contents
The world-wide numerously applied activated sludge process can provide good effluent quality but consumes high amounts of energy and is characterized by high operational cost (Verstraete et al., 2009). Large fractions of dissolved organic materials are converted to biomass, consuming considerable energy, which still requires further treatment. As a result, the energy consumption of a conventional wastewater treatment plant due to aeration is high, 0.6 kWh per m3 of wastewater, which accounts for about half of the total energy consumption (McCarty et al., 2011).

Anaerobic municipal wastewater treatment can be an alternative to reduce energy consumption and operational cost (Siegrist et al., 2008), but is applicable especially at higher temperature climates in tropical countries. Low temperature is still a challenge for anaerobic wastewater treatment of municipal wastewater because of low hydrolysis rate of the influent organic matter and the low methanogenic activity, converting hydrolyzed material into biogas.

Many different kinds of anaerobic reactors have been studied to deal with these problems caused by low temperatures. An energy friendly municipal wastewater treatment plant for moderate climate regions, specifically a UASB-digester system is introduced here. Among anaerobic reactors, a UASB-digester is a promising system as it can provide relatively high chemical oxygen demand (COD) removal efficiencies and energy production in the form of methane. Furthermore, it provides stabilized excess sludge compared with other two phase systems like anaerobic filter (AF) - anaerobic hybrid (AH) reactor (AF-AH) or hydrolytic upflow sludge bed (HUSB) - UASB reactor (HUSB-UASB) (or expanded granular sludge bed (EGSB) instead of UASB).

A pilot scale UASB-digester was studied to treat domestic wastewater at temperatures of 10-20°C at an HRT of 6 h in the UASB reactor and 15 h in the digester. Besides, Co-digestion was applied for improving soluble COD removal efficiency of a UASB-digester system, operated at low temperatures and treating domestic sewage with a high dissolved/suspended COD ratio (Zhang, L., et al., 2013); Effects of sludge recriculation rate and sludge transferpoint on the performance of the UASB-digester system are studied (Zhang, L., et al. 2012); Anaerobic digestion steps like hydrolysis and methanogenesis related to the system are also studied (Zhang et al., 2016).

literatures

 * 1)     Zinia, N. J. and C. Kroeze (2015). "Future trends in urbanization and coastal water pollution in the Bay of Bengal: the lived experience." Environment, Development and Sustainability 17(3): 531-546.
 * 2)     Verstraete, W., et al. (2009). "Maximum use of resources present in domestic "used water"." Bioresource Technology 100(23): 5537-5545.
 * 3)     McCarty, P. L., et al. (2011). "Domestic wastewater treatment as a net energy producer-can this be achieved?" Environmental Science and Technology 45(17): 7100-7106.
 * 4)     Siegrist, H., Salzgeber, D., Eugster, J., Joss, A. 2008. Anammox brings WWTP closer to energy autarky due to increased biogas production and reduced aeration energy for N-removal. in: Water Science and Technology, Vol. 57, pp. 383-388.
 * 5)     Zhang, L., et al. (2013). "Co-digestion to support low temperature anaerobic pretreatment of municipal sewage in a UASB-digester." Bioresource Technology 148: 560-566.
 * 6)     Zhang, L., et al. (2012). "The effect of sludge recirculation rate on a UASB-digester treating domestic sewage at 15 °C." Water Science and Technology 66(12): 2597-2603.
 * 7)     Zhang, L., Hendrickx, T.L.G., Rijnaarts, H.H.M., Zeeman, G. 2016. Anaerobic Treatment of Municipal Wastewater in a UASB-Digester System, Vol. Doctor, Wageningen University. The Netherlands.