title: Effect of the liquid upflow velocity on thermophilic sulphate reduction in acidifying granular sludge reactors.
abstract: The effect of the superficial liquid upflow velocity on the acidifying and sulfate reducing capacity of thermophilic (55 degrees C; pH 6.0) granular sludge bed reactors treating partly acidified wastewater was investigated. A comparison was made between a UASB and an EGSB reactor, operated at an upflow velocity of 1 m.h-1 and 6.8 m.h-1, respectively. Both reactors were inoculated with a mixture of mesophilic sulphidogenic, thermophilic sulphidogenic and thermophilic methanogenic sludge (ratio 2:1:1). They were fed a synthetic wastewater containing starch, sucrose, lactate, propionate and acetate and a low sulphate concentration (COD/SO4(2-) ratio of 10). At the end of the experiment, the sulphate level of the influent was slightly increased to a COD/SO4(2-) ratio of 8. The reactors were operated at a hydraulic retention time of about 5 h and the imposed volumetric organic loading rates (OLR) ranged from 4.9 to 40.0 g COD l-1d-1. When imposing an OLR of 40.0 g COD l-1d-1, the acidification efficiency dropped to 80% and the sulphate reduction efficiency decreased to 50% in the UASB reactor. In the EGSB reactor, the sulphate reduction efficiency dropped to 30% directly following the OLR increase to 40 g COD l-1d-1, but recovered rapidly to 100% (at an OLR of 35 g COD l-1d-1) until the end of the experiment. In the UASB reactor, there was a net acetate and propionate production. At the higher organic loading rates, propionate was converted to n-butyrate and n-valerate. These back reactions did not occur in the EGSB reactor, in which an active methanogenic population developed, leading to a net acetate removal (up to 50%) and a high gas loading rate (up to 8.5 l l-1d-1). In both reactors, the effluent sulphide concentration was always below 200 mg l-1, of which about 90% was present as undissociated H2S (under the given conditions--pH 5.8-6.1 and 55 degrees C). The biogas (including CH4 and CO2) production rates in the UASB were very low, i.e. < 31 biogas l-1 reactor d-1, resulting in negligible amounts (< 20%) of H2S stripped from the reactor liquid. In the EGSB reactor, the biogas production rates reached up to 8.5 l l-1d-1, resulting in H2S stripping efficiencies up to 75%.
authors.affiliation: Sub-Department of Environmental Technology, Agricultural University of Wageningen, 'Biotechnion'-Bomenweg, 2, PO Box 8129, 6700 EV Wageningen, The Netherlands.
authors.forename: P N
authors.initials: PN
authors.lastname: Lens
authors.forename: D
authors.initials: D
authors.lastname: Korthout
authors.forename: J B
authors.initials: JB
authors.lastname: van Lier
authors.forename: L W
authors.initials: LW
authors.lastname: Hulshoff Pol
authors.forename: G
authors.initials: G
authors.lastname: Lettinga
mesh_terms: D000693:Anaerobiosis; D019149:Bioreactors; D002241:Carbohydrates / Q000032:analysis / Q000737:chemistry*; D003720:Densitometry; D005740:Gases; D006862:Hydrogen Sulfide / Q000032:analysis / Q000737:chemistry; D008697:Methane / Q000737:chemistry*; D008962:Models, Theoretical; D009038:Motion; D010084:Oxidation-Reduction; D010100:Oxygen / Q000032:analysis / Q000737:chemistry; D012722:Sewage / Q000737:chemistry*; D013463:Sulfuric Acid Esters / Q000737:chemistry*; D013696:Temperature; D014835:Volatilization; D014865:Waste Disposal, Fluid / Q000295:instrumentation* / Q000379:methods
chemical_list: D002241:Carbohydrates; D005740:Gases; D012722:Sewage; D013463:Sulfuric Acid Esters; D008697:Methane; D010100:Oxygen; D006862:Hydrogen Sulfide