Impacts Of Water Hardness And Aluminum Coagulants On The Potable Reuse Treatment Of Reclaimed Water

Distinguishing from existing Indirect Potable Reuse（IPR）processes,a new process for potable reuse of reclaimed water is proposed in this paper.The new process means that the B2-grade reclaimed water is discharged into the environmental buffer zone（non-drinking water source）,after the self-purification of the water body,it is further treated with drinking water and finally introduced into the supply of drinking water.Compared with the IPR,on the one hand,the new process gives full play to the self-purification function of environmental buffers and reduces the risk of drinking water source pollution.On the other hand,the produced water is more acceptable to users and appears to have lower disposal costs and a smaller carbon footprint.After secondary treatment,tertiary treatment（microfiltration,ultrafiltration,ozone,etc.）and disinfection can be used to produce B2-grade reclaimed water.The quality of B2-grade reclaimed water shall meet the requirements of the standard"Water Quality of Landscape Environment for Municipal Sewage Reuse"（GB/T 18912-2019）,but the discharge threshold of water hardness has not been specified in this standard.Because the treatment process of B2-grade reclaimed water cannot effectively remove the hardness,high-hardness B2-grade reclaimed water may be discharged into the environmental buffer.Hardness may not have a positive effect on further treatment for potable water.Therefore,different hardness process influent（raw water supplemented by reclaimed water）was synthesized in this paper,the influence of different hardness on the deep treatment process of Coagulation-Ultrafiltration-Nanofiltration（CUF-NF）was studied,and the influence of different aluminum coagulants on CUF-NF process was also studied.These results show that:（1）For the coagulation stage,the coagulant polyaluminum chloride(PACl₂₅)has better purification efficiency but a smaller floc size under the same hardness of test water.The impacts of hardness increase on flocs are various,including charge neutralization,compression of double electric layer,and adsorption bridge.Increasing hardness can promote the formation and growth of flocs,and facilitate the removal of organics（especially hydrophobic components）by sediment.（2）For the coagulation-UF stage,PACl₂₅ in the coagulation stage has better purification efficiency and higher UF membrane flux under the same hardness of test water.Because increasing the water hardness can reduce the content of organic matter（especially hydrophobic components）and increase the ionic strength in the supernatant,the pollution of the UF membrane can be alleviated to obtain a larger membrane flux,and the pollution of the Al Cl₃-UF membrane can be alleviated more significantly.Meanwhile,increasing the water hardness can improve the purification efficiency in the coagulation-UF stage.The DOC and DBPs-FP in the UF filtrates corresponding to the higher hardness test water are lower,but the hardness is still higher because the UF membrane cannot effectively reject Ca²⁺/Mg²⁺.（3）For the coagulation-UF-NF stage,PACl₂₅ at the same hardness of test water can still obtain better purification efficiency（but Al Cl₃ has better hardness removal）and higher NF membrane flux.Increasing the hardness of test water will lead to the NF membrane rejecting more Ca²⁺/Mg²⁺,which will cause the change of electrostatic repulsion on the membrane surface and aggravate the pollution of the NF membrane,and this aggravating effect is more significant in Al Cl₃-UF-NF.Meanwhile,increasing the hardness of test water can improve the water purification efficiency in the coagulation-UF-NF stage.The DOC and DBPs-FP in the NF filtrates corresponding to the higher hardness test water are lower,and the hardness can reach the drinking water sanitation standard（<450 mg/L,express as Ca CO₃）.The reuse of reclaimed water for drinking can effectively relieve the pressure on the water resources supply,so it is of great significance to put forward the appropriate technology for the reuse of reclaimed water for drinking.Although the final effluent from the new process does not meet the hygienic standard of drinking water,healthy and hygienic drinking water can inevitably be obtained by improving the advanced treatment process of drinking water.However,the production purpose of B2-grade reclaimed water is not for drinking and reuse,so this paper studies the influence of hardness on subsequent advanced treatment technology,to provide a reference for improving the quality index of B2-grade reclaimed water.The study of the effect of aluminum coagulant can provide new insights for the advanced treatment of drinking water.