Generation Of Disinfection By-products By Sodium Hypochlorite Disinfection In The Wastewater Tertiary Treatment

With the growing shortage of water resources, the recycling of water resource becomesone of the most important ways to resolve the problem of water shortages. The reuse of thesecondary effluent from municipal wastewater treatment plants can effectively alleviate theshortage of water resources. But the secondary effluent also contains a large number ofpathogenic microorganisms, trace amounts of toxic and hazardous pollutants,which will berisky to the use of the reclaimed water, so the ecological safety of the reclaimed water hasgained much concerns。Sewage disinfection plays a key role in killing pathogenic microorganisms, preventingthe spread of epidemic diseases and ensuring reclaimed water ecological safety. Comparedwith drinking water, urban sewage contains more pathogenic microorganisms and disinfectionbyproduct precursors, so it need higher doses of disinfectant to achieve discharge and reusestandards, which will result in higher generation amount and more types of disinfectionby-products(DBPs) than that in the drinking water, leading to potential risk to the safty useof the reclaimed water. Therefore, trihalomethanes (THMs) and dichloroacetonitrile (DCAN)were chosen as the objective matter to study the precursor and the generation amount ofdisinfection by-products in sewage disinfection by sodium hypochlorite Effect of differentreaction factors and different nitrogenous organic precursors on DBPs generation amountwere investigated in this paper. The results of this paper can provide a theoretical basis for thecontrolling of DBPs formation in the disinfection by sodium hypochlorite and protectingeffluent quality.The paper mainly included:(a) The effect of disinfection reaction factors on the DBPsformation.(b) The effect of nitrogen-containing inorganic ions in the secondary effluent onDBPs formation.(c) Influence of typical nitrogen-containing organic matter on DBPsgeneration potential. The results showed that:(1) The dosage of sodium hypochlorite was the most significant factor impacting theformation of DBPs. The generation amount of trihalomethanes (THMs) anddichloroacetonitrile (DCAN) increased with sodium hypochlorite dosage increased. Whensodium hypochlorite dosage was as much as15mg/L, THMs generation amount reached thepeak. And the extent of the impact of three kinds of THMs by sodium hypochlorite dosagefollowed such sequence: Chloroform> Bromodichloromethane> Dibromochloromethane.When sodium hypochlorite dosage was8mg/L and25mg/L, DCAN generation amount increased significantly.(2) DBPs generation amount increased firstly and then became stable with the increasingof disinfection time. THMs formation rate was the largest at the first one hour, and withreaction time prolonged, the generation rate tended to zero and stablized. The generationamount of DCAN reached a peak at reaction time of3.5h, and with time continued, DCANamount decreased gradually because of the hydrolysis reaction.(3) Increasing the reaction temperature accelerated the generation rate of trihalomethanes,a kind of disinfection byproducts. In this process, the effect of temperature on chloroformformation was the largest. Increasing the reaction temperature, on one hand, speeding up thereaction rate of sodium hypochorite with organic matter, on the other hand, it could speed upthe hydrolysis reaction of DCAN.Temperature had greater influence on the former. So thegeneration of DCAN was positively correlated with temperature.(4) pH affects DCAN generation amount greatly. With the increasing of pH, thegeneration amount of DCAN gradually decreases, while the generation of trihalomethanes(THMs) shows the opposite trend. Overall, pH has little effect on the generation of DBPs, thegeneration amount of DBPs remain basically unchanged.(5) With the increasing of Cl/N ratio, the generation of THMs showed a downward trend.In this process, changing amplitude of chloroform was more obvious. Increasing of Cl/N ratioresulted in the gradually increases of DCAN amount. The existence of ammonia in watercould promote the generation of THMs, however it had opposite influence on the generationof DCAN. With the increasing of nitrate, the generation of THMs gradually decreased andthen became stable. When the concentration of nitrate was10mg/L, the generation of THMsreached the maximum.(6) The existence of organic matter would promote the generation of disinfectionbyproducts during sterilization. The effect of four kinds of amino acids on disinfectionbyproducts THMs was as following: cysteine (7.15μg/L)> lysine (3.03μg/L)> leucine (2.44μg/L)> glycine (2.31μg/L). The effect of humic acids on disinfection byproducts followingthe order: Chloroform> Dibromochloromethane> Dichloroacetonitrile> Bromodichloro-methane.