Enhancement of thermophilic anaerobic digestion of thickened waste activated sludge by combined microwave and alkaline pretreatment

Pretreatment of thickened waste activated sludge （TWAS） by combined microwave and alkaline pretreatment （MAP） was studied to improve thermophilic anaerobic digestion efficiency. Uniform design was applied to determine the combination of target temperature （110-210°C）, microwave holding time （1-51 min）, and NaOH dose （0-2.5 g NaOH/g suspended solids （SS）） in terms of their effect on volatile suspended solids （VSS） solubilization. Maximum solubilization ratio （85.1%） of VSS was observed at 210°C with 0.2 g-NaOH/g-SS and 35 min holding time. The effects of 12 different pretreatment methods were investigated in 28 thermophilic batch reactors by monitoring cumulative methane production （CMP）. Improvements in methane production in the TWAS were directly related to the microwave and alkaline pretreatment of the sludge. The highest CMP was a 27% improvement over the control. In spite of the increase in soluble chemical oxygen demand concentration and the decrease in dewaterability of digested sludge, a semi-continuous thermophilic reactor fed with pretreated TWAS without neutralization （at 170~C with 1 min holding time and 0.05 g NaOH/g SS） was stable and functioned well, with volatile solid （VS） and total chemical oxygen demand （TCOD） reductions of 28% and 18%, respectively, which were higher than those of the control system. Additionally, methane yields （L@STP/g-CODaded, at standard temperature and pressure （STP） conditions of 0°C and 101.325 kPa） and （L@STP/g VSadad） increased by 17% and 13%, respectively, comoared to the control reactor.

Risk assessment of Giardia from a full scale MBR sewage treatment plant caused by membrane integrity failure

Membrane bioreactors（MBR） are highly efficient at intercepting particles and microbes and have become an important technology for wastewater reclamation. However, many pathogens can accumulate in activated sludge due to the long residence time usually adopted in MBR, and thus may pose health risks when membrane integrity problems occur.This study presents data from a survey on the occurrence of water-borne Giardia pathogens in reclaimed water from a full-scale wastewater treatment plant with MBR experiencing membrane integrity failure, and assessed the associated risk for green space irrigation. Due to membrane integrity failure, the MBR effluent turbidity varied between 0.23 and 1.90 NTU over a period of eight months. Though this turbidity level still met reclaimed water quality standards（≤ 5 NTU）, Giardia were detected at concentrations of 0.3 to 95 cysts/10 L, with a close correlation between effluent turbidity and Giardia concentration. All β-giardin gene sequences of Giardia in the WWTP influents were genotyped as Assemblages A and B, both of which are known to infect humans. An exponential dose–response model was applied to assess the risk of infection by Giardia. The risk in the MBR effluent with chlorination was9.83 × 10-3, higher than the acceptable annual risk of 1.0 × 10^-4. This study suggested that membrane integrity is very important for keeping a low pathogen level, and multiple barriers are needed to ensure the biological safety of MBR effluent.