Mesophilic submerged anaerobic membrane bioreactor for pulp and paper secondary sludge treatment

Abstract

The production of pulp and paper is associated with the generation of large quantities of wastewater that has to be purified to avoid severe pollution of the environment. Wastewater purification in pulp and paper mills uses sedimentation, biological treatment, chemical precipitation, flotation and anaerobic treatment, and the specific combination of techniques is determined by the local conditions. Wastewater treatment generates large volumes of organic solid waste (primary and secondary sludge) that after dewatering can be incinerated and thus used for bio-energy production. Sludge is currently viewed as biofuel of poor quality due to its high water content, and some mills treat it solely as a disposal problem. In this study, a new technology Submerged Anaerobic Digestion Membrane Bioreactor (SAnMBR) was developed in order to treat pulp and paper mill sludge, which can not only improve the quality of effluent after pretreatment, but also can recycle the value of PPMSS (pulp and paper mill secondary sludge). In this thesis, a laboratory-scale SAnMBR was operated for 330 days under 37 °C to explore the feasibility of PPMS treatment at different hydraulic retention times (HRTs), besides, alkali pretreatment was adopted to PPMS in order to explore its effects on performance of SAnMBR for the treatment, included biogas production, biogas yield, solids destruction ratio and effluent COD. COD removal efficiencies were all excellent (over 95%). The solids reduction ratio was in the range of 20%-50% and there was no significant difference in solids reduction ratio under different HRTs and alkali pretreatment. The biogas production rate decreased with an increase in HRT before and after alkali pretreatment had no significant impact on biogas production rate, and these two factors also influenced extracellular polymeric substances (EPS), soluble microbial products (SMP) and solids destruction ratio. Membrane performance was affected by PSDs of supernatant and SCOD, alkali pretreatment caused sludge deflocculation and a larger amount of fine colloidal flocs, which resulted in increasing of membrane fouling rate. However, the results shown that the biogas production in lower HRT was better comparing with it in higher HRTs, and quality of effluent was kept excellent during the experimental period, the pretreatment increased membrane fouling rate, and it had no significant effect on biogas production. SAnMBR was feasibility from this study, but membrane fouling should be an issue.