proportional to the quantity of contaminants to be removed. Some techniques require pre-treatment of wastewater to ensure reliable performance, which could be potentially costly and technically demanding. For instance, presence of bicarbonate ions (HCO3−) can appreciably reduce the concentration of ·OH due to scavenging processes that yield H2O and a much less reactive species, ·CO3−. As a result, bicarbonate must be wiped out from the system or AOPs are compromised. It is not cost effective to use solely AOPs to handle a large amount of wastewater; instead, AOPs should be deployed in the final stage after primary and secondary treatment have successfully removed a large proportion of contaminants. Ongoing research also been done to combine AOPs with biological treatment to bring the cost down. Future Since AOPs were first defined in 1987, the field has witnessed a rapid development both in theory and in application. So far, TiO2/UV systems, H2O2/UV systems, and Fenton, photo-Fenton and Electro-Fenton systems have received extensive scrutiny. However, there are still many research needs on these existing AOPs. Recent trends are the development of new, modified AOPs that are efficient and economical. In fact, there has been some studies that offer constructive solutions. For instance, doping TiO2 with non-metallic elements could possibly enhance the photocatalytic activity; and implementation of ultrasonic treatment could promote the production of hydroxyl radicals. Modified AOPs such as Fluidized-Bed Fenton has also shown great potential in terms of degradation performance and economics. See also List of waste-water treatment technologies Fenton reaction Electro-oxidation Process engineering Water purification References Further reading Michael OD Roth: Chemical oxidation: Technology for the Nineties, volume VI: Technologies for the Nineties: 6 (Chemical oxidation) W. Wesley corner fields and John A. Roth, Technomic Publishing CO, Lancaster among other things. 1997, . (engl.) Water treatment Environmental engineering Environmental chemistry Green chemistry