Effect of the liquid upflow velocity on thermophilic sulphate reduction inacidifying granular sludge reactors

The effect of the superficial liquid upflow velocity on the acidifying and sulfate reducing capacity of thermophilic (55 degreesC; pH 6.0) granular sl udge bed reactors treating partly acidified wastewater was investigated. A comparison was made between a UASB and an EGSB reactor, operated at an upfl ow velocity of 1 m.h(1) and 6.8 m.h(1), respectively. Both reactors were in oculated with a mixture of mesophilic sulphidogenic, thermophilic sulphidog enic and thermophilic methanogenic sludge (ratio 2:1:1). They were fed a sy nthetic wastewater containing starch, sucrose, lactate, propionate and acet ate and a low sulphate concentration (COD/SO42 ratio of 10). At the end of the experiment, the sulphate level of the influent was slightly increased t o a COD/SO42 ratio of 8. The reactors were operated at a hydraulic retentio n time of about 5 h and the imposed volumetric organic loading rates (OLR) ranged from 4.9 to 40.0 g COD 1(1)d(1). When imposing an OLR of 40.0 % COD 1(1)d(1), the acidification efficiency dropped to 80% and the sulphate redu ction efficiency decreased to 50% in the UASB reactor. In the EGSB reactor, the sulphate reduction efficiency dropped to 30% directly following the OL R increase to 40 g COD 1(1)d(1), but recovered rapidly to 100% (at an OLR o f 35 g COD 1(1)d(1)) until the end of the experiment. In the UASB reactor, there was a net acetate and propionate production. At the higher organic lo ading rates, propionate was converted to n-butyrate and n-valerate. These b ack reactions did not occur in the EGSB reactor, in which an active methano genic population developed, leading to a net acetate removal (up to 502) an d a high gas loading rate (up to 8.5 11(1)d(1)). In both reactors, the effl uent 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 degreesC). The biogas (including CH4 and CO2) production rates in t he UASB were very low, i.e. < 31 biogas 1(1) (reactor) d(1), resulting in n egligible amounts (< 20 %,) of H2S stripped from the reactor liquid. In the EGSB reactor, the biogas production rates reached up to 8.511(1)d(1), resu lting in H2S stripping efficiencies up to 75%.