Study On Pretreatment Technology For Ultrahigh-salt Tuber Mustard By Micro-electrolysis And Electrolysis

The characteristic of the ultrahigh-salt tuber mustard wastewater were ultra-highsalt, low pH, high organic matter, high nitrogen and phosphorus and the heterocyclerefractory organic pollutants. At present, the tuber mustard wastewater was mainlytreated by biological process, but ultrahigh-salt and salinity fluctuation would affect thebiological treatment systemstability. Therefore, it is considered to meet the subsequentbiological treatment requirements through the the physicochemical pretreatment toreduce salinity, increase the pH, removal of refractory organics. Physicochemicaltreatments of ultrahigh-salt tuber mustard wastewater were researched, includingmicro-electrolysis, two-dimensional electrolysis and three-dimensional electrolysis. Thetreatment efficiency of “three micro-electrolysis-two-dimensional electrolysis” and“three micro-electrolysis-biochemical process’ were studied.The micro-electrolysis experiments found that under the condition of raw water pH,Fe/C volume ratio of1:1, Fe/H2O volume ratio of1:1and reaction time of30min,removals of COD, ammonia, phosphate, TN and salinity were36%-45%,34%-42%,97%-99.9%,34%-53%and22%-25%, meanwhile, the effluent pH increased1-2. Theseries of multi-level micro-electrolysis experiments showed that the removal efficiencyincreased significantly at the level of three. The orthogonal tests showed that each factoron the order of COD and ammonia removal rates were as follow: Fe/H2O volume ratio＞initial pH＞reaction time＞Fe/C volume ratio. The SEM analysis showed that thesurface of iron and carbon after reaction were covered by lots of sediment, after waterbackwash, the sediment decreased, the activity of iron and carbon recovered. The SEManalysis showed that the surface of iron and carbon after reaction increased Na, P and Cl.The XRD result of precipitation product indicated that phosphate was removed byFe3(PO4)2.The mechanism of micro-electrolysis had shown that organic pollutants weredegraded by the combined effects of chemical battery reaction, oxidation-reduction,filtration-adsorption, flocculation-precipitation and air floatation. The GC-MS resultsshowed that the organic pollutants of raw wastewater containing benzene ring werebiodegradable pollutants, including five kinds of heterocyclic compounds, one kind ofnitrogen and sulfur heterocyclic compounds, three kinds of phenols. The removal rate oforganic matter was53.8%by micro-electrolysis pretreatment, with heterocyclic compounds removal efficiency of80%. The organic molecular weight distributionexplained that the micro-electrolysis could convert macromolecular organics into smallorganic molecules, with the removal of some small organic molecules. Refractoryorganics in ultrahigh-salt tuber mustard wastewater were effectively removed bymicro-electrolysis.The two-dimensional electrolysis experiments found that under the optimalconditions of raw water pH, current density of156mA/cm2, electrode distance of1.5cm and electrode plate area/water volume ratio of0.8dm2/L, removals of COD,ammonia and phosphate were55.74%,99.77%and88.71%within120min. In addition,pH increased from5.07to9.54, the salinity decreased from7.02%to6.39%. Theanalysis of the current density on energy consumption and anode efficiency indicatedthat ammonia can use the electrolysis mode of pre-high and late-low current density.Under current density of156mA/cm2and electrolysis of30min, the ammonia energyconsumption was lowest96kW·h/Kg, the ammonia anode efficiency was highest8.47g/h·m2·A. Under current density156mA/cm2, the COD energy consumption was thelowest and the COD anode efficiency was the highest. The three-dimensionalelectrolysis tests found that under the condition of wastewater volume of600mL, rawwater pH, electrolysis current of8A, amount of activated carbon filling quantity of250g, distance between main electrode plates of6.5cm, electrolysis time of150min,removals of COD,TN and phosphate were76.47%,89.48%and97.81%. Under current8A, the COD energy consumption was lower and the COD anode efficiency was higher.The mechanism of electrolysis had shown that organic pollutants were degradedtogether in the direct oxidation and indirect oxidation. Through hydrogen peroxide andfree chlorine tests in the electrolysis process, found that hydrogen peroxide and freechlorine concentrations in ultrahigh-salt tuber mustard wastewater were significantlylower than in the sodium chloride solution, during the organic degradation processconsumed a lot of hydrogen peroxide and free chlorine, relating to the indirect oxidationprocess. The GC-MS results showed that the pollutants peak position of electrolyticeffluent were different of the raw water’s. The polycyclic organic matters werecompletely converted to organic compounds of a single ring or straight-chain structure.In the electrolysis process, biodegradable organic matters were changed into easilydegradable middleproduct. The organic molecular weight distribution showed that thetwo-dimensional electrolysis could convert macromolecular organics into small organicmolecules, with the removal of some small organic molecules. The wavelength scan of three-dimensional electrode indicated that a part of the organic matters were directlyoxidized to carbon dioxide. Refractory organics in ultrahigh-salt tuber mustardwastewater were also effectively removed by electrolysis.On the basis of micro-electrolysis, two-dimensional electrolysis andthree-dimensional electrode tests, the physicochemical combined process "threemicro-electrolysis-two-dimensional electrolysis" were further studied. The pilot testsof combined process showed that the removals of COD, ammonia, TN and phosphatewere79.41%,86.08%,81.71%and99.65%, pH increased from4.37to6.87, salinitydecreased from6.53%to2.09%. The effluent of physicochemical combined processwas suitable for biological treatment.The combination of physicochemical and biological tests showed that the threemicro-electrolytic pretreatment of water was conducive to the following anaerobic,aerobic and anaerobic-aerobic biological combination treatment.The effluent COD of“three micro-electrolysis-anaerobic biofilm-contact oxidation” combined process was230-420mg/L, achieved the three emission standard of “integrated WastewaterDischarge Standard”(GB8978-1996). The “micro-electrolysis-anaerobic biofilm-contact oxidation” combined process had a good prospect.