Alternative technologies for the removal of micropollutants from wastewater

In addition to ozone and PAC, there are other established technologies available. These technologies have also been investigated for removal of micropollutants but studies have shown that they are either not really effective or very expensive to use for wastewater treatment.

"It has been shown that effective removal of hormones during drinking water treatment is mainly governed by adsorption mechanism on the membrane."

Nanofiltration

Nanofiltration (NF) is a pressure-driven separation process using membrane with pore size between 1 and 5 nm. NF membrane properties lies between ultrafiltration and reverse osmosis. Major focus on the use of NF membrane has been on environmental applications from water to wastewater treatment and reuse, apart from desalination. In surface and ground water, NF has been used to remove hardness and turbidity and, recently, for the removal of organic micropollutants including pesticides, hormones and pharmaceuticals and on groundwater contaminated with toxic heavy metal Arsenic. It has been shown that effective removal of hormones during drinking water treatment is mainly governed by adsorption mechanism on the membrane. 

NF membrane has been used to treat different types of industrial wastewater such as textile wastewater for possible water re-use. In wastewater, NF is a promising technique to remove PPCPs, with increased rejection of the compounds as the pH is increased. NF is considered as a relatively new technology for wastewater treatment yet due to its lower energy cost and higher flux compared to reverse osmosis, NF has been predicted to replace reverse osmosis in various applications. The dominant factors influencing the rejection mechanism of NF are the molecular size and charge of the compound.

Reverse osmosis

Reverse osmosis (RO) is a water purification technology that uses semipermeable membrane and high pressure (above osmotic pressure), allowing ideally only water molecules to pass through the membrane while leaving behind dissolved salts, organics, bacteria and other impurities. RO has the capability to remove over 99% of bacteria, viruses, organics, salts, and other water impurities. The removal of micropollutants by rejection in RO membrane is governed by different factors such as molecular size of the compound, charge, hydrophobicity, and hydrogen bonding capacity. RO has been applied in water reclamation and wastewater treatment plants as tertiary treatment to further remove organic micropollutants including pharmaceuticals and endocrine disruptors. 

RO has been shown to effectively remove organics (up to 100% total organic carbon reduction) resulting in high quality water which is appropriate for reuse. Molecular size of the compound and molecular weight cut off (MWCO) of the membrane are considered as the most important parameters influencing the removal of micropollutants. There are different types of RO membrane such as low energy, high rejection and high fouling resistant depending on its application. High fouling resistant membrane is more suitable for wastewater applications.

However with the high energy requirement and the cost of the membrane this may not be an appropriate treatment technology for municipal WWTPs.

UV

Ultraviolet (UV) light has been widely used for disinfection of drinking water and wastewater effluents. UV irradiation is also known to degrade organic compounds during water and wastewater treatment, either by direct photolysis or in combination with a chemical oxidant (UV-based advanced oxidation). The effectiveness of direct UV irradiation or photolysis to remove a wide variety of micropollutants ranged from 0 to 100%, depending on the reactivity of the compound. Degradation of micropollutants by direct photolysis has been found to depend on several factors such as the UV light source (low- or medium-pressure UV lamps), water pH, temperature and the nature and characteristics of the wastewater. 

The presence of natural organic matter in the wastewater is known to enhance the efficiency of UV treatment by producing hydroxyl (OH) radicals that reacts non-selectively with organic compounds. However for most compounds where significant removal cannot be reached with UV alone, a combination of UV irradiation with a chemical oxidant, a UV-based advanced oxidation process (AOP), could be considered. Among the most commonly used UV-based AOPs are the UV/H2O2 and UV/TiO2 systems. The OH radicals generated upon UV irradiation of the oxidant possess high oxidation potential making UV-based AOPs a promising technology for the removal of organic micropollutants in the wastewater.