Membrane unit operations wastewater treatment

In this chapter the possibility of integrating different membrane unit operations in the same industrial cycle or in combination with conventional separation systems is analysed and discussed. Many original solutions in water desalination, agro-food productions and wastewater treatments are reviewed highlighting the advantages achievable in terms of product quality, compactness, rationalization and optimization of productive cycles, reduction of environmental impact and energy saving. 

Use of membrane techniques for wastewater treatment has been commercialized for several decades. Ceramic membranes always play an important role, especially when the operating conditions are too harsh for polymeric membranes. From an engineering viewpoint, packing more membrane area into a unit of a smaller size is constantly pursued. Ceramic membranes of a hollow-fiber configuration are no doubt a great choice due to their substantially higher surface area/volume ratio. 

Some of the largest plants for seawater desalination, wastewater treatment and gas separation are already based on membrane engineering. For example, the Ashkelon Desalination Plant for seawater reverse osmosis (SWRO), in Israel, has been fully operational since December 2005 and produces more than 100 million m3 of desalinated water per year. One of the largest submerged membrane bioreactor unit in the world was recently built in Porto Marghera (Italy) to treat tertiary water. The growth in membrane installations for water treatment in the past decade has resulted in a decreased cost of desalination facilities, with the consequence that the cost of the reclaimed water for membrane plants has also been reduced. 

Part II covers the unit operations of flow measurements and flow and quality equalizations pumping screening, sedimentation, and flotation mixing and flocculation filtration aeration and stripping and membrane processes and carbon adsorption. These unit operations are an integral part in the physical treatment of water and wastewater. 

Many conventional wastewater treatment processes that have long been in use are now considered impractical because they require a large amount of space, a large number of unit operations, and are affected by problems associated with odor and other emissions. Recent years have seen an increasing trend toward process intensification, which has led to the development of advanced membrane processes that are simple to construct and operate, have well-defined flow patterns, better dispersion effects, relatively low power consumption, lower emissions, and high mass-transfer performance, which are compact and recyclable. 

A cost analysis of an extractive membrane bioreactor (EMB) for wastewater treatment has been reported by Freitas dos Santos and Lo Biundo [6.24]. The EMB studied was similar with those reported in Chapter 4. Calculations were carried out for a feed flowrate of 1 m h of wastewater polluted with dichloromethane at a concentration of 1 g l A minimum pollutant removal rate of 99 % and 8000 h of operation per year were considered. As expected, the analysis indicated that the costs are strongly dependent on the pollutant flow entering the bioreactor to be transformed. Two key parameters, namely the total membrane area required and the external mass transfer coefficient, were studied. The results show that the costs and membrane area decrease significantly as the mass transfer coefficient increases from 0.5 x 10 to 2.0 x 10 m-s (these values are typical for large units, while laboratory measured values harbor around 5x10 m-s [624]). Using a mass transfer coefficient of 1.0 x 10 m s the authors calculated the costs and the membrane area required for different wastewater flowrates. These results are shown in Fig. 6.3. 

Consequently, membrane bioreactors are an example of the combination of two unit operations in one step for example, membrane filtration with the chemical reaction. In a typical membrane bioreactor, as weU as acting as a support for the biocatalyst, the membrane can be a very effective separation system for undesirable reactions or products. The removal of a reaction product from the reaction environment can be easily achieved thanks to the membrane selective permeability, and this is of great advantage in thermodynamically unfavourable conditions, such as reversible reactions or product-inhibited enzyme reactions. A very interesting example of a membrane bioreactor is the combination of a membrane process, such as microfiltration or ultrafiltration (UF), with a suspended growth bioreactor. Such a set up is now widely used for municipal and industrial wastewater treatment, with some plants capable of treating waste from populations of up to 80 000 people (Judd, 2006). 

The membrane flux is the flow rate of permeate per unit area of membrane surface and typically proportional to the TMP. The flux for a new membrane, operating with water only, is referred to as the clean water flux, and serves as a useful benchmark. Clean water flux rates for membranes that are used for wastewater treatment may be of the order of 3—4 m /m /day. 

Biocatalytic membrane reactors can also be used in production, processing, and treatment operations. The trend toward environmentally friendly technologies makes these units particularly attractive because of their ability to operate at moderate temperature and pressure and to reduce the formation of by-products. Enzymes, compared to inorganic catalysts, generally permit greater stereospecificity and higher reaction rates under milder reaction conditions. Relevant applications of biocatalytic membrane reactors include production of new or better foodstuffs, in which desired nutrients are not lost during thermal treatment novel pharmaceutical products with well-defined enantiomeric compositions and wastewater processes. 

---