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MEMBRANE BIOREACTORS (MBR) FOR LANDFILL LEACHATE TREATMENT Tony van Loggenburg*; Dr. Dirk Herold**, Christoph Kullmann** *AQUA AIR CAPE C.C., P.O. Box 38584, Pinelands 7430, Tel.: +27 (0)21 531 1603, Fax: +27 (0)21 531 1616, [email protected] **Koch Membrane Systems, Kackertstr. 10, 52072 Aachen, Germany, Tel.: +49 241 413260, Fax: +49 241 4132659, [email protected]

Over the last few years, there has been steady growth in the application of membrane bioreactor (MBR) technology for wastewater treatment. MBR systems with submerged membranes are increasingly used for some of the toughest wastewater treatment applications including sewage treatment for municipalities and treatment of wastewater for beverage, textile, food, paper and chemicals industries as well as landfill leachate treatment. The membrane bioreactor concept is a combination of conventional biological wastewater treatment and membrane filtration. The concept is technically similar to that of a traditional wastewater treatment plant, except for the separation of activated sludge and treated wastewater. In a MBR installation this separation is not carried out by means of sedimentation in a secondary clarification tank, but by membrane filtration. MBR systems are typically selected either due to the reduced footprint and civil construction costs, or due to the improved effluent quality that meets current and future discharge regulations. In addition, MBR effluent is often used for water reuse applications. MBR effluent may be used directly for reuse, or may be used as feed to a reverse osmosis (RO) system. The process design of a complete MBR depends on many factors such as the wastewater characteristics, the required effluent requirements, the membrane system used, and numerous environmental factors. All designs have a number of common fundamental parameters and these must be addressed before the physical design of the filtration system can commence. These parameters are: the design flux for the process, the number of required membrane trains, the number of modules per train, the type of permeate extraction, the type of membrane aeration, the cleaning protocol and the membrane system location. It is important to fix these design parameters early in the system design to avoid extra work at a later date. This presentation will discuss some practical aspects of MBRs in landfill leachate treatment. Data from an MBR system at the “De Wierde” landfill for household and industrial waste in the Netherlands will be shown.

MEMBRANE BIOREACTOR VS CONVENTIONAL TECHNOLOGY Conventional wastewater treatment consists of three steps; primary treatment, biological treatment and solids/liquid separation. The crucial part of the process is the solids-liquid separation. With a conventional sedimentation process, there is often insufficient removal of bacteria and suspended solids. For example, in one litre of typical secondary effluent there are ten million bacteria, which can be very difficult to remove. In an MBR, ultrafiltration membrane modules are submerged in the activated sludge to combine the biological step and the solid-liquid separation into a single process, see figure below. Since the membrane acts as a barrier, this improves the effluent quality. Also, the membrane barrier eliminates the secondary clarifier and allows the activated sludge to be more concentrated. This reduces the volume requirement for biological tanks, thus saving space and construction costs. Overall, the MBR process reduces footprint significantly

compared to the combination of wastewater treatment followed by sand filtration or ultrafiltration. The footprint savings due to the wastewater treatment plant alone can be as much as 50%, and there are additional footprint savings since the additional tertiary filtration steps are eliminated, see figure 1.

Figure 1: MBR vs. conventional activated sludge Due to the tough environment in which the membranes are operated, membrane module design is of the utmost importance for successful operation of the MBR. In recent years, MBR membranes have evolved, improving the deficiencies of the earlier MBR module designs. More importantly, the second generation MBR membrane modules are designed to be interchangeable. In other words, these systems are designed for retrofit installations so the original MBR modules can be easily removed and the new modules can be inserted in their place. EXAMPLE OF MBR TECHNOLOGY There are many different configurations of MBR technology. One example of a second generation module that optimizes both membrane and module design is a single header submerged hollow fibre UF module from the authors’ company, designated the PURON module from the Koch Membrane Systems. The patented module is designed to avoid the clogging and sludging that is an issue with some MBR module designs offered today. The module features hollow fibre membranes with a pore size of approximately 0.05micron. The lower ends of the membrane fibers are fixed in a header while the upper ends are individually sealed and are free to move laterally as shown in the figure below. All solids and particulates remain on the outside of the fibers while permeate is sucked out of the inside of the fibers by means of a vacuum, see figure 2.

Figure 2: Example of MBR module system The fibers are arranged in bundles and are submerged vertically into the activated sludge. To maintain the filtration rate of the membrane modules, air scouring is carried out at regular intervals. An air nozzle is integrated into the centre of the bundles to apply the air for scouring purposes. The central arrangement of the air nozzles inside the membrane bundles reduces the energy consumption, since the air is injected at the place where the risk of sludging is highest. The module design ensures that even hairs and fibrous compounds will be removed reliably from the system, so that a coarse pre-screen can be used, thus improving capital and operating costs. A special feature of these membranes is their enormous mechanical strength. This mechanical strength is provided by a braid inside the membrane material. The individual fibre bundles are connected in rows. Several of the rows are mounted into a frame made of stainless steel to form a module as shown in the figure below. The free moving fibers combined with central aeration ensure stable filtration during plant operation, long membrane life, and low operating costs by reducing the need for energy, cleaning and maintenance. Unlike most flat sheet and some hollow fibre membranes that do not allow back flushing, the membrane resists fouling and maintains permeability by introducing a small portion of the filtrate back through the fibre pores from the inside-out at timed intervals. The hollow fibers provide significantly higher membrane surface area and higher filtration capacity within the same module footprint compared to flat sheet membrane designs.

ECOPARK ‘DE WIERDE’ Prior to 2003 landfill “De Wierde” was a classical disposal site for organic, inorganic and specific industrial solid waste. ‘Classical’ suggesting that waste was collected, a minimum of sorting, and compacted into segregated landfill compartments. Percolate waters collected in the compartments was buffered as it transgressed from the acid phase to the methanogenic phase through the years, and up until 2003 all leachate from the De Wierde facility were transported several kilometres to a sister landfill called Weberpolder for further treatment. As the De Wierde facility expanded the leachate transport became a bottleneck

through the availability of the tanker wagon and the actual transport cost, and in 2001 a feasibility study was carried out via a waste treatment specialised company and the MBR technology was specified as the Best Available Technology or ‘BAT’ for leachates. This high tech approach to Percolate Treatment fitted into the business philosophy for De Wierde as major investments into Organic Waste Digestion and recycling facilities were also envisaged. The site realised at an early stage that the digestion towers would increase both the hydraulic and fouling load of the wastewaters which would in turn completely overload the existing transport to Weberpolder, and subsequently wastewater treatment at De Wierde was essential for future expansion as an Ecopark, see figure 3.

Figure 3: Ecopark De Wierde

Membrane Bioreactor Design In 2002 a tender was produced for a 20m3/h MBR to treat leachate arising from a new Organic Solid Digestion plant and Percolate from the existing landfill. The ratio was expected to be 15m3/h average percolate flow and 5m3/h digestion waters. The original design was based on an equalised (9000m3) wastewater of 3000 mg/l COD, 1000mg/l TKN (80% NH4) or 30,000 pe. The discharge required was extremely tight for this type of system and was set at