Formation Of Assimilable Organic Carbon (AOC) During Oxidation Of Algal Suspension With Ozone And Ozone/Hydrogen Peroxide

Recent years, poor nutrients (mainly nitrogen and phosphorus) control in bothsanitary sewage and industrial wastewater effluent gave rise to the occurrence ofeutrophication in lakes and reservoirs. The seasonal algae bloom, owing to the highconcentration of nutrients in water, can adversely affect water treatments processes anddeteriorate the drinking water quality. Alagl suspension contained algae cells and a lotof algal organic matter (AOM), which was released by algae during the metabolism.AOM can be less removed by traditional water treatment processes, therefore, it maycause biological instability of the finished water in the distribution system. Ozone hasbeen applied into water treatment works for many years as oxidants or disinfectants.Ozone/hydrogen peroxide oxidation process, as one of the most mature advancedoxidation processes, has equally been used in water treatment processes. During theoxidation of algal suspension with ozone or ozone/hydrogen peroxide, if assimilableorganic carbon (AOC) forms in a large number, it will inevitably result in the regrowthof bacteria and finally affect bio-stability of water supply system.This research experimentize with microcystis aeruginosa, one of the mostcommon blue-green algae. Algae cell and algal organic matter, including extracellularorganic matter and intracellular organic matter, were characterized. The formation ofassimilable organic carbon in oxidation of algal suspension with ozone andozone/hydrogen peroxide was evaluated and compared.Algal organic matter was typically classified into the extracellular organic matter(EOM) and intracellular organic matter (IOM). Comparing with natural organic matter,algal organic matter contained less aromatic organic matter. EOM was enriched insoluble microbial by-products. While IOM had four typical EEM peaks, separatelyrepresented aromatic protein，soluble microbial by-product，humic-acid like and fulvic-acid like. Organic nitrogen in AOM contained higher molecular weight (MW), asorganic carbon in AOM included lower MW (less than1000Dalton). The concentrationof AOC in IOM was much higher than EOM, and it hardly vary during the algaegrowing phase.During the oxidation of algal suspension with ozone and ozone/hydrogenperoxide, the formation of AOC originated from two aspects. One was that algae cellswere destructed by oxidants and in the process AOC was released into the water and another was that ozone or ozone/hydrogen peroxide oxidized algal organic matter toincrease the AOC concentration of water. During the ozonation of algal suspension, afurther increase of the oxidant dose resulted in a further increase of AOC concentration,as it could destruct more algae cells and released more intracellular organic carbon.Both temperature and alkalinity could impact the formation of AOC during theoxidation process. Bicarbonate impaired the oxidation ability of the ozone or ozone/hydrogen peroxide system through capturing the hydroxyl radicals and weakened theformation of AOC.Comparing with the ozonation of algal suspension, ozone/hydrogen peroxideprocess yielded a relatively higher AOC concentration. Hydrogen peroxide acceleratedozone to decompose, yielding a higher concentration of hydroxyl radicals. Hydroxylradical could damage algae cell more effectively than ozone, discharged more IOMfrom algae cells. Oxidizing AOM with ozone/hydrogen peroxide formed AOC fasterthan oxidizing AOM with ozone alone, while no more AOC was yielded during theformer process. Therefore, producing a higher concentration of hydroxyl radicals todamage algae cells more efficiently could be the reason that ozone/hydrogen peroxidegenerated more AOC than ozone alone.Ozone and ozone/hydrogen peroxide could both significantly increase the AOCconcentration in EOM, as they oxidized microbial by-products, with high MW, intolower MW organic matter (less than1000Dalton). During the oxidation of IOM withozone and ozone/hydrogen peroxide, the AOC concentration in IOM was increased.Because oxidants reacted with organic nitrogen in IOM to yield more organic matterthat with a lower MW (less than1000Dalton). Moreover, organic carbon in IOMcontained comparatively lower MW than organic nitrogen. After oxidation, the MWdecreasing of organic carbon contributed to the formation of AOC in IOM.