The use of advanced water treatment technologies to reclaim wastewater has evolved with each new generation of facilities. Membrane technology—typically thought of in terms of water treatment—has begun to emerge as the most significant advancement in wastewater treatment in the past 20 years, and its presence is expected to become wide spread in the future. Early use of membrane treatment for wastewater appeared more than 25 years ago, but over the past 5 years there has been a rapid increase in the volume of wastewater that is treated to exceptional water quality standards with membranes, typically for reuse purposes. In fact, today there are about 70 full-scale municipal wastewater treatment facilities using membrane technologies, about 40 of them in the United States.
The potential to use membranes exists whenever they provide the ability to remove contaminants that cannot be removed by other technologies, remove contaminants at less cost than other alternatives, or require less land area than competing technologies. For wastewater treatment applications, membranes are currently being used for the tertiary removal of dissolved salts, organic compounds, phosphorus, colloidal and suspended solids, and human pathogens, including bacteria, protozoan cysts, and viruses. Membrane technologies for wastewater treatment include:
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Membrane bioreactors—usually microfiltration (MF) or ultrafiltration (UF) membranes immersed in aeration tanks, or implemented in external pressure-driven membrane units, as a replacement for secondary clarifiers.
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Low-pressure membranes—usually MF or UF membranes, either as a pressure system or an immersed system, providing a higher degree of solids removal following secondary clarification.
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High-pressure membranes—nanofiltration or reverse osmosis pressure systems for treatment and production of high-quality product water suitable for indirect potable reuse and high-purity industrial process water.
The biggest single technical challenge with the use of membranes for wastewater treatment is the high fouling that occurs universally. Membrane fouling appears to be mainly due to colloids, soluble organic compounds, and bacteria that are present in secondary effluent and are typically not well removed with conventional pretreatment methods. Membrane fouling results in an increase in feed pressure and requires frequent cleaning of membranes. This leads to a reduction in overall facility efficiency and a shorter membrane life. Other technical barriers include the difficulty and expense of managing the concentrate from high pressure membranes, and the undefined ability of membranes to effectively remove all chemical contaminants and pathogens of concern that are found in municipal secondary effluent.
These barriers are slowly being overcome through extensive industry research and should pave the way for highly efficient membrane applications to help handle the increased demand for potable water while also proactively addressing the increase in wastewater effluent production. Treating wastewater with membranes is a viable option when considering urban reuse, agricultural reuse, industrial reuse, groundwater recharge, salinity barriers, and augmentation of potable water supplies, or to meet very low effluent water quality limits for nutrients.