Biochemical Engineering Journal 29 (2006) 210–219 Sewage treatment in a combined up-flow anaerobic sludge blanket (UASB)–down-flow hanging sponge (DHS) system A. Tawfik a,∗ , A. Ohashi b , H. Harada b a National Research Centre, Water Pollution Control Department, El-Behoss Street, P.O. Box 12622, Cairo, Egypt b Department of Environmental System Engineering, Nagaoka University of Technology, Kamitomioka 1603-1, Nagaoka, Niigata 940-2188, Japan Received 8 May 2005; received in revised form 23 November 2005; accepted 27 November 2005 Abstract The performance of up-flow anaerobic sludge blanket (UASB) in combination with down-flow hanging sponge (DHS) system for sewage treatment at an average wastewater temperature of 15 ◦ C has been investigated for 6 months. The results showed that a combined system operated at a total HRT of 10.7h and total SRT of 88 days represents a cost effective sewage treatment process. The average COD total and BOD 5 total concentrations measured in the final effluent of the total system (UASB + DHS) amounted to 43 and 3.0 mg/l, respectively, corresponding to the overall removal efficiency of 90% for COD total and 98% for BOD 5 total . The total process provided a final effluent containing a low concentration of 12 mg/l for TSS. Eighty-six percent of ammonia was eliminated at space loading rate of 1.6 kg COD/m 3 d and HRT of 2.7 h. The calculated nitrification rate of the DHS system according to the nitrate and nitrite production amounted to 0.18 kg/m 3 d. The removal of F. coliform in the UASB reactor only amounted to 0.86 log 10 . On the other hand, the F. coliform concentration dropped substantially, i.e. by 2.6log 10 in the DHS system resulting only 2.7 × 10 3 /100 ml in the final effluent. The calculated average sludge production for UASB operated at an HRT of 8.0 h amounted to 30 g TSS/d, corresponding to sludge yield coefficient of 0.2 g TSS/g total COD removed, while it was indeed very low only 6.0 g TSS/d corresponding to sludge yield coefficient of 0.09 g TSS/g total COD removed, for DHS system. The DHS profile results revealed that in the first and second segment of DHS system, the COD total , BOD 5 total and TSS was eliminated, followed by the oxidation of ammonia in the next segments. © 2005 Elsevier B.V. All rights reserved. Keywords: Sewage; UASB; DHS; COD; Nitrification; F. coliform; DHS profile; OUR 1. Introduction In moderate climate countries, sewage treatment in an up- flow anaerobic sludge blanket (UASB) reactor has recently increased since the UASB configuration started to be efficiently applied for this purpose in the beginning of the 1980s. The treatment of sewage at 15 ◦ C was investigated by Mahmoud et al. [1] in a one-stage UASB reactor and a UASB–digester system. The latter consists of a UASB reactor complemented with a digester for sewage treatment and sludge stabiliza- tion. The UASB reactor was operated at a HRT of 6.0h and a temperature of 15 ◦ C. The digester was operated at 35 ◦ C. The UASB–digester system provided significantly higher COD ∗ Corresponding author. Tel.: +2 02 3351573; fax: +2 02 3351573. E-mail address: Tawfik8@yahoo.com (A. Tawfik). removal efficiencies than the one-stage UASB reactor. The achieved removal efficiencies in the UASB–digester system and the one-stage UASB reactor for total, suspended, colloidal and dissolved COD were 66%, 87%, 44% and 30%, and 44%, 73%, 3% and 5% for both systems, respectively. The stability values of the wasted sludge from the one-stage UASB reac- tor and the UASB–digester system were, respectively, 0.47 and 0.36 g CH 4 -COD/g total COD. Likely, the feasibility of treating municipal wastewater by a UASB reactor under low- temperature conditions was evaluated by Singh and Virarghavan [2]. COD removal efficiency ranged from 70% to 90% at an HRT of 6.0 h and a temperature of 11 ◦ C. The performance of the reactor was not satisfactory at temperature of 6.0 ◦ C, the average COD removal amounted to only 40%. Based on these results, the UASB reactor could be applied successfully for pre-treatment of domestic wastewater under low-temperature conditions, but additional post-treatment is required for removal 1369-703X/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2005.11.018