Pilot Study On UF-RO Process For Advanced Treatment Of Coastal WWTP Effluent

Due to geographical restrictions,most of pipelines in coastal areas are laid near coastline.With the development of seafood processing industry and pipeline leakage,influent TDS&C1^-of WWTP are significantly high and changing periodically with the season,which restricts the applications of reclaimed effluent such as landscaping.Aiming to produce reclaimed water meeting the requirements of landscaping in southern area,a pilot scale UF-RO was for used for advanced treatment of coastal WWTP effluent.In this paper,the variation regularity of influent salt content was monitored through conductivity.Then,pretreatment techniques such as coagulation,sand filtration,activated carbon filtration and bag filter were optimized and compared.Moreover,UF-RO system was optimized with consideration of water quality,water-yielding capacity,anti-pollution of the membrane and other factors.Finally,membrane selection and their operating mode were studied.When using coagulation and sand filtration for pretreatment,polyaluminium chloride?PAC?was proved to be better than polyferric sulfate?PFS?.31%of COD_cr and 80%of turbidity were removed with the optimal PAC dosage of 12mg/L.Generally,the proper cleaning cycle of sand filter tank was 6 h,and should be shortened to 3h if influent became worse.In order to control the breeding of algaes and bacteria which resulting in membrane fouling,sodium hypochlorite was added with optimal dosage of 2.4mg/L.As free chlorine which oxidized membrane material could be detected if more than 2.4mg/L of Na Cl O was added.Since activated carbon filter exhibited poor organic matters removal performance and probably leaded to follow-up security filter blocking by activated carbon powder,activated carbon filter was deprecated and NaHSO₃ was used to restore residual chlorine.The turbidity of ultra filtration effluent maintained well at 0.1 NTU.Meanwhile,SDI₁₅of ultra filtration effluent was around 1.5,which met the influent requirements of reverse osmosis(SDI₁₅?5).The highest removal ratio of COD_cr by ultrafiltration was 12%.while it almost could NOT remove TP,NH₃-N,Cl^-and conductivity.Ultrafiltration effluent could be directly reused for landscaping when WWTP influent was lower than 500 mg/L.The content of NH₃-N,TP,CODcr and turbidity of UF-RO effluent was lower than 0.1mg/L,0.1 mg/L,10 mg/L,0.1NTU.As chloride content of WWTP effluent reached about6000mg/L,the desalination rate of reverse osmosis membrane was more than 97%,and chlorine in the RO effluent was less than 180mg/L,which was lower than 250mg/L as the requirement of recycled water.Water flux and desalination rate increased as the influent pressure increased for all the three reverse osmosis membranes under 0.8MPa-1.0MPa.Under the same pressure,the effluent flux was B menbrane>A menbrane>C menbrane,while the desalination rate was A menbrane>C menbrane>B menbrane.For example,the water flux of A membrane at 1.0MPa was only 0.16m³/h with desalination ratio of 99.7%;the water flux of B menbrane at 0.8MPa reached 0.28 m³/h withdesalination ratio of 98%;the water flux of C menbrane at 1.0MPa was 0.28 m³/h with desalination ratio of 99.3%.It was conformed that water produced by all three RO membranes could guarantee the reused water requirement,and B membrane was better.T Furthermore,2 sections of reverse osmosis membrane operation mode was more stable than that of 1 section.Moreover,offline Na OH cleanwas best than the other methods.About preservation of reverse osmosis membrane,no relevant protective measures was needed for short-term break,but NaHSO₃ or other reducing agent must be added to protect membanres for long-term pause.