Performance Of PAC/Membrane Hybrid Process For Treating Micro-polutted Water At Low Temperature

In northern China，especially the Northeast，it’s difficult to treat micro-pollutedwater at low temperature in Winter. Remval efficiency of organic matters andammonia nitrogen in micro-polluted water at low temperature is extremely limited,which is treated by traditional water treatments. So，it’s particularly important todevelp a cost-effective，environment-friendly and easy-to-operate technology totreat micro-polluted water at low temperature.A high concentration powdered activated carbon （PAC）/microfiltration （MF）hybrid process was established to treat organic matters and ammonia nitrogen inmicro-polluted water at low temperature，taking advantage of adsorption of organicmatters onto PAC，excellent removal efficiency of ammonia nitrogen by biologicalactivated carbon （BAC），outstanding separation of microbes, virus and PAC. Newmethods of mitigating membrane fouling was brought out by the research onmechanisms of membrane fouling.The research mainly includes three parts: establishment of high concentrationPAC/membrane hybrid process and determination of operational parameters,removal efficiency and influencing factors, and influencing, control techniques andmechanism of membrane fouling. The aim of the research is to provide a newmethod of treating micro-polluted water at low temperature for northest China.Humic acid （HA） adsorbed by PAC and micro-polluted water treated by MF atlow temperature （10oC） were studied to investigate the performance of the separatetechnology. The results showed that: PAC was ideal to adsorb HA at lowtemperature and the removal efficiency of turbidity by MF was excellent at lowtemperature. High concentration PAC/membrane hybrid process was established andthe operational parameters were also determined as: PAC dosage of50g/L，HRT of2h，29.5-min-filtraiton with16.6L/（m²Â·h） and then0.5-min-backwash with115L/（m²Â·h），aeration rate of0.25m3/h and everyday2.5%PAC bulk replacementafter100days operation.Performance and influencing factors （characteristics of the feed water, start-upmethods and temperature） of membrane hybrid process were studied. The results showed that: removal efficiency of ammonia nitrogen was95%, removal rate ofdissolved organic carbon （DOC）, ultraviolet adsorption at254nm (UV₂₅₄), totalorganic carbon （TOC） and permanganate index was70.8%,88.4%，76.3and58.9%,respectively, removal efficiency of trihalomethane formation potential （THMFP）was81.7%and removal rate of turbidity was above99%. Characteristics of the feedwater （ammonia nitrogen, organic matters and turbidty） had little effect on theperformance of the hybrid membrane process.The start-up time at low temperaturecould be shortened to25days by inoculating with200mL biological PAC （BPAC）bulk from other reactors, however, removal efficiency reduced and irreversiblemembrane fouling increased. Low temperature reduced the activity of the microbes,however, removal efficiency of ammonia nitrogen at low temperature was4%lowerthan that at room temperature, because high concentration of PAC provide theconvenient environment for microbes to resist the bad conditions （low temperature）.The reaction of organic matters adsorbed by PAC was exothermic, so removal rateat low temperature was higher than that at room temperature.Membrane fouling in hybid membrane process for treating micro-pollutedwater at low temperature was elaborated, using attenuated total reflection fouriertransform infrared spectroscopy （ATR-FTIR）, atomic force microscope （AFM）,scanning electron microsope analysis to observe characteristics of membranesurface after different chemical cleaning. The results showed that: chemicalreversible membrane fouling accounted for83%of total membrane fouling. Themain organic fouling matters were protein, polysaccharide and humic acid, and themain inorganic fouling matters were Ca²⁺and Fe³⁺. The shape and hydrophilicitiy offouled membrane returned to the extent of the new membrane after chemicalcleaning in turn （NaOH, NaClO, HCl and H₂O₂） and membrane fouling obtainedeffective control.Influencing factors （start-up methods, temperature and replacement of PACbulk） were investigated by analysis of trans-membrane pressure （TMP）, distributionof membrane fouling resistance and characteristics of the bulk, and new techniques(aeration, backwash with deionized water and input with Ca²⁺in the feed water) tomitigate membrane fouling was brought up. The results showed that: when highconcentration of PAC in the reactor lost adsorption ability, no PAC bulk replacement or everyday1%PAC bulk replacement and input with200mL BPACbulk, would increase irreversible membrane fouling, because the concentration ofSMP in PAC bulk increased. Average particle size distribution decreased and SMPin PAC bulk increased at low temperature, accelerating irreversible membranefouling. Backwash with deionized water could restore the repulsion betweenorganic matters and membrane surface and then reduce irreversible fouling withinthe slow growth period. Particle size distribution of PAC bulk could be increasedand concentration of SMP could be reduced through input with Ca²⁺in the feedwater, lead to mitigation of irreversible fouling during the rapid growth period.