At the current time Membrane Bioreactors (MBR) are the primary focus of wastewater treatment technologies. MBR is a combination of biological activated sludge process for the removal of nutrients and a membrane filtration for separation of biomass and treated water. In industrial wastewater treatment MBRs have proven to be a particularly reliable technique [Cornel et al., 2006]. The biological degradation is the same as in conventional activated sludge process (CAS) [DWA, 2005]. Main research in the field of MBR takes place in the understanding and minimization of membrane fouling and blocking [e.g. Judd, 2006] and in process optimization (e.g. to reduce the energy demand; Krause, 2005]. The advantages of this process is a small footprint, flexibility and solid free effluent which can be directly reused or – if purification is required –more easily conditioned compared to conventional systems. On the other hand, MBRs show a higher energy demand, require a higher level of automation and the membranes must be cleaned due to fouling and scaling.
The most energy efficient choice is submerged membrane modules, where the modules are submerged directly into the activated sludge tank. The fouling control is achieved by an air scour at the membrane surface by an additional (coarse) bubble aeration (= crossflow-aeration). The necessary shear velocity is created by the movement of the bubbles close to the membrane surface.
Microdyn-Nadir has developed a new submerged membrane module for MBR application (Bio-Cel®) with the objectives of a high operational reliability, low capital and operational cost. New Bio-Cel® Module pilot plants are in operation, one of them treating wastewater from a chemical park in Wiesbaden (Germany).
Wastewater characteristics and design of pilot plant
The wastewater at the chemical park is discharged by more than 80 companies in the pharmaceutical industry, electric chemical company, methyl-cellulose-, offset printing-, dye-, synthetic resin- and polymer membrane manufacturers. Overall the wastewater is heterogeneous and contains solvents, methyl-cellulose residues, pharmaceutical residues, pigment remainders, nitrates and nitrides and other salts. The chemical oxygen demand (COD) of the effluent is about 4,700 mg/l (± 1,000 mg/l).
The chemical wastewater is pumped after neutralisation from the primary sedimentation into the pilot plant. The treatment steps of the pilot MBR are fine screening (0.5 mm), denitrification (4 m³) and nitrification (8 m³). The total tank volume is about 12 m³. The membrane module with an active surface area of 60 m² is submerged into the nitrification tank. The plant is equipped for chemical cleaning. The permeate is discharged back into the WWTP. In Figure 1 the pilot plant is depicted schematically.