Study On Technologies Of Mitigating Membrane Fouling In Membrane Bioreactors

Membrane bioreactor (MBR) is a novel wastewater treatment technology. This technology has a rapid development and a wide concern because of water resource deficiency and water quality deterioration. Compared with the conventional wastewater treatment technologies, MBR has a higher quality effluent and a handy operational system. Whilst there is a technical problem which has not been solved satisfactorily, that is membrane fouling. Membrane fouling shortens membrane life, increases operation costs, and prevents MBR from wide application, especially in developing area. The aim of this research is to develop one or two handy operational technologies which can mitigate membrane fouling with lower costs, and to popularize the MBR technology in China. This dissertation studied physiochemical processes to mitigate membrane fouling by using ferric salts, which includes membrane pre-coating and mixed liquor flocculation, and analyzed characteristics of membrane foulants in molecular weight distribution. Two parallel MBRs treating domestic wastewater were operated to observe the pre-coating effects. Before MBRs operated, membrane A was pre-coated with flocs of ferric hydroxide, while MBR B was not. After 253 days comparison, effluents from two MBRs both reached the related standard in stable period, and furthermore fouling rate of membrane A was lower than that of membrane B. Ferric salts flocculation can remove non-degradable organics and mitigate membrane fouling. The results showed that the CODCr difference of mixed liquor and effluent has a close relationship with the trans-membrane pressure, and main part of CODCr in mixed liquor is caused by non-degradable organics. It is also found that intermittent ferric salts dosing decreased CODCr of mixed liquor in MBR and increased membrane flux. When feed water is typical domestic wastewater and concentration of ferric salts is controlled to less than 60 mg/L, the microorganisms in activated sludge will not be adversely affected. Molecular weight distribution of membrane foulants was analyzed with the system combined with steric exclusion chromatography (SEC) and refractive index detector (RID). The results showed that the matter with macro molecule (>10000 Dalton) is the main part of membrane foulants, whose proportion is about 90%. Moreover membrane fouling rate and proportion of macro molecule matter in mixed liquor has a linear correlation. In addition proper dosage of ferric salts in mixed liquor was proved to be an effective way to remove macro molecule matter.