Effects Of Filamentous Bacteria On The Activated Sludge Properties And The Filtration Characteristics Of Self-forming Dynamic Membrane (SFDM)

Sludge bulking in biological wastewater treatment processes often occurs and there is no exception in self-forming dynamic membrane bioreactor (SFDMBR). SFDMBR take advantage of the self-forming dynamic membranes (SFDMs)(i.e. initial "desired" sludge layers developed on the coarse-pore materials) to achieve required solid-liquid separation efficiency. Since the characteristics of SFDM are the significant factors affecting the filtration performance. The characteristics of SFDM are reasonably expected to be significantly affected by sludge properties, which is an important part of SFDM. As the main component of activated sludge, the content of filamentous bacteria is bound to influence some important properties of the activated sludge, thus affecting the filtration characteristics of SFDM. It is necessary to study the effects of filamentous bacteria on the activated sludge properties and the filtration characteristics of self-forming dynamic membranes (SFDMs) to provide important theoretical basis of the application and promotion of SFDMBR.In order to understand the influence of filamentous bacteria on the activated sludge properties, the twelve sludge properties were systematical investigated. Sludge properties includes volume moment mean diameter (Dvm), fractal dimension (Df), relative hydrophobic (RH), extracellular polymeric substance (EPS), polysaccharide (EPSc), protein (EPSp), EPSp/EPSc, viscosity, specific oxygen uptake rate (SOUR), zeta potential, sludge settlement index (SVI) and MLVSS/MLSS. The results showed that filamentous bacteria density had a strong positive effect on SOUR (rp=0.966, P=0.000), Dvm (rp=0.82, P=0.02) and viscosity (rp=0.765, P=0.045), but had a strong negative effect on EPSp/EPSc (rp=-0.781, P=0.038) and a moderate negative effect on Df (rp=-0.767, P=0.044). SOUR was correlated inversely to EPSp/EPSc (rp=-0.781, P=0.038), which indicated that the increase of filamentous bacteria content can improve the activity of sludge, leading to the increase in relative content of polysaccharide. In these characteristics of activated sludge, Dvm, viscosity, EPSp/EPSc and Df were the predominant factors affecting on the filtration characteristics of SFDM.This paper researched the influence of filamentous bacteria on the filtration characteristics of SFDM by a series of short-term filtration experiments about activated sludge with different filamentous bacteria density (FI) in SFDMBR. The results showed that the effluent turbidity was not affected by filamentous bacteria density, and when the SFDMBR run stable, the effluent turbidity was below1.5NTU. Filamentous bacteria content is the predominant factors affecting on the effluent flux and resistance of SFDMs. Relative to the SFDMs formed by the sludge with appropriate filamentous bacteria (FI=2, FI=3, FI=4), the SFDMs formed by the sludge with little filamentous bacteria (FI=0, FI=1) or excessive filamentous bacteria (FI=5, FI=6) had better filtration performance, the effluent flux is larger, the filtration resistance is small. According to the filtration characteristics of SFDMs, the filtration process could be divided into three stages:rapid formation stage, rapid growth stage and steady stage, and they changed as the concentrations of filamentous bacteria, indicating filamentous bacteria play an important role in the filtration process of SFDM.To further understand the reasons that caused these results, the characteristics of SFDM also were studied. And the results indicated that filamentous bacteria density had a strong positive effect on compressibility (rp=0.901, P=0.006) and Dvm (rp=0.947, P=0.001) of SFDM, but had a strong negative effect on EPSp/EPSc (rp=-0.825, P=0.022) and a moderate negative effect on D,(rp=-0.795, P=0.033). Filamentous bacteria mainly affected SFDM in three aspects:the type of material, particle composition and the structure of SFDM. Protein and polysaccharide was the main component of SFDMs regardless of FI, but the filamentous bacteria density had a strong relation with EPSp/EPSc whether in mixture or in SFDM. The SFDMs were made up of relatively large particle in sludge suspension. The structure of SFDMs formed by the sludge with little filamentous bacteria (FI=0, FI=1) was compact and dense; The structure of SFDMs formed by the sludge with excessive filamentous bacteria (FI=5, FI=6) was thickness and non-porous; the strueture of SFDMs formed by the sludge with appropriate filamentous bacteria (FI=2, FI=3, FI=4) was net and porous, which is better to filter. So in the filtration process, the growth of filamentous bacteria should be controlled to improve the filtration performance of SFDM.