The influence of sludge characteristics from fullscale MBR in membrane filterability

Dissertação apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para a obtenção do grau de Mestre em Engenharia do Ambiente,perfil Sanitária

In wastewater treatment, membrane bioreactor systems (MBR) offer several advantages when compared with conventional treatment processes. A small footprint, the production of a high quality effluent, and low sludge production are some of the advantages of this “recent” technology. However, membrane fouling is still a major drawback, increasing the operational costs since intensive aeration and physical/chemical cleaning are required. This thesis aims to study which characteristics/components present in the sludge constitute an influence in the filterability of activated sludge, which is a measure of fouling propensity. A research was conducted between September 2008 and February 2009, in which five fullscale MBR were visited. Three different experiments occurred during this period: the Blank Characterization experiment to study the characteristics of sludge from MBR; the Dilutions experiment to see the effect of different MLSS in membrane filterability, using sludge dilutions with permeate; and the Solids Concentration experiment to study the impact of MLSS in filterability through manipulating hydraulic retention time (HRT) in a full-scale MBR. In all experiments the filterability of the sludge was measured through the Delft Filtration Characterization method developed by TUDelft (Evenblij et. al., 2005). This method allows measuring the additional resistance in the membrane during membrane filtration. Also, when possible, other parameters were measured in the sludge such as MLSS, particles in the ranges 2-100 μm and 0.4-5.0 μm, soluble microbial products (SMP), and viscosity. A relationship between filterability and each one of the other parameters was tried to explain the membrane performance. The results showed that no single or direct correlation between filterability and MLSS existed. From the analysis of sludge from a full-scale MBR, a three-way relationship was observed between filterability, SMP and temperature. Higher concentrations of SMP were observed at lower temperatures, while at the same time filterability showed worst results. This was also confirmed by the analysis of the diluted samples, though at a lower level. The particles in the range 0.4-1.0 μm demonstrated a significant relationship with SMP. Although SMP particles are considered to be smaller than the observed range, particle counting in the range 0.4-1.0 μm seems to be a good indicative of SMP levels in the activated sludge. The Solids Concentration experiment showed that an improvement in membrane filtration occurred when hydraulic retention time (HRT) was changed from 17 hours to 30.8-40.8 hours at the same time that MLSS varied between approximately 14.3 and 18.2 g/L. However, it cannot be assured that MLSS by itself played a major role in filterability.