Membrane operations wastewater treatment

A membrane is defined as an intervening phase separating two phases forming an active or passive barrier to the transport of matter. Membrane processes can be operated as (1) Dead-end filtration and (2) Cross-flow filtration. Dead-end filtration refers to filtration at one end. A problem with these systems is frequent membrane clogging. Cross-flow filtration overcomes the problem of membrane clogging and is widely used in water and wastewater treatment. 

The two water desalination applications described above represent the majority of the market for electrodialysis separation systems. A small application exists in softening water, and recently a market has grown in the food industry to desalt whey and to remove tannic acid from wine and citric acid from fruit juice. A number of other applications exist in wastewater treatment, particularly regeneration of waste acids used in metal pickling operations and removal of heavy metals from electroplating rinse waters [11]. These applications rely on the ability of electrodialysis membranes to separate electrolytes from nonelectrolytes and to separate multivalent from univalent ions. 

More progresses can be anticipated in the near future by promoting the integration of different membrane operations, including MCs and membrane bioreactor (MBRs), also for wastewater treatment. 

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. 

The first known commercial membrane-based liquid-liquid extraction system involved extraction of by-products from a wastewater stream using an aromatic solvent [102]. Before the membrane system was installed, the entire wastewater stream had to be incinerated leading to high costs for the gas fired incinerator per year. The membrane system lowered the contaminant concentration to adequate levels before the biological wastewater treatment plant, and saved significant operating cost. 

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. 

The experiments have proved that membrane distillation can be applied for radioactive wastewater treatment. In one-stage installation the membrane retained all radionuclides and decontamination factors were higher than those obtained by other membrane methods. The distillate obtained in the process was pure water, which could be recycled or safely discharged into the environment. It seems the process can overcome various problems of evaporation such as corrosion, scaling, or foaming. There is no entrainment of droplets, which cause the contamination of condensate from thin-film evaporator. Operation at low evaporation temperature can decrease the volatility of some volatile nuclides present in the waste, such as tritium or some forms of iodine and ruthenium. The process is especially economic for the plants, which can utilize waste heat, e.g., plants operating in power and nuclear industry. 

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