Combining submerged membrane technology with anaerobic and ...

Combining UASB and MBR for the treatment of low-strength wastewater at environmental temperatures4.4. Results and discussion4.4.1. System performance4.4.1.1. General resultsThe system was operated at ambient temperature, and wastewater temperatureschanged with seasons (21.0-17.5 °C). The pH of the effluent from UASB was around 6.7.Aerobic chamber and permeate pH varied from 6.7 to 7.7 and from 7.0 to 8.2, respectively,depending on the system performance.Despite operating in psychrophilic conditions volatile fatty acids (VFA) concentrationin the UASB effluent was below minimum detection limit of the method used (20 mg·L -1 )during the whole experimentation. Methane reached more than 70% of the biogascomposition during the whole operation. Biogas production in the UASB chamber wasdetected during the four experimental periods, with an average production rate of 46.3±9.5L·d -1 , depending on OLR applied and temperature. Biogas production yield was around0.15 m³ methane·kgCOD eliminated-1. The upflow velocity in the UASB reactor was around 0.15m·h -1 , below maximum value of 1 m·h -1 typically recommended for sewage treatment inconventional UASB systems (van Haandel and Lettinga, 1994).4.4.1.2. Organic matter removalFigure 4.2 depicts the evolution of the COD concentration and OLR applied to theUASB and the MBR reactor as well as the total COD removal percentage and the CODremoval percentage achieved in the UASB reactor. As can be observed on figure 4.2b, theoverall COD removal was normally above 97.5 % during the four operational periods. Themost part of the organic matter removal took place in the UASB reactor, with percentagesabove 80 %, except from days 75 to 90, when some micropollutants degradationexperiments were carried out. The addition of methanol in the feeding in order to dissolvethis micropollutants led to a drastic increase on COD concentration in the feeding (figure4.2c) and provoked a slight shock load in the UASB reactor, decreasing COD removalpercentage in this reactor (figure 4.2b) and increasing OLR applied to the MBR (figure4.2a). With the exception of these days in period I, COD concentration in the permeatewas always lower than 15 mg·L -1 . Differences between soluble COD concentration in themembrane chamber and the COD concentration in the permeate were observed during themost part of the operation, indicating that the membrane retained a fraction of colloidalmatter present in the mixed liquor.115