Study Of Flocculation And Intergrated Membrane System For The Treatment Of Oily Wastewater

Oily wastewater contains large amount of oil, suspended solids, bacteria as well as high hardness and salinity, causing severe damage to ecological environment and production of oil fields. Flocculation and ultrafiltration (UF) can well remove pollutants in oily wastewater such as:suspended solids, oil and bacteria. Nanofiltration (NF) can reject ions such as:Na+、Cl-、Ca+、Mg2+、SO42-and reduce salinity and alkalinity, making the treated oily wastewater available for agricultural irrigation. Therefore, flocculation as pretreatment associated with ceramic membrane ultrafiltration-reverse osmosis to treat oily wastewater was depth studied.The pretreatment effect of single and compound flocculant on oil wastewater was researched. The experimental results showed that:diatomaceous has an better effect on oily wastewater than poly aluminium chloride (PAC) and poly ferric sulfate (PFS) using single flocculant. As for compound flocculant, a flocculant system of diatomaceous earth and PAC was optimal method to treat oil produced water. The optimal conditions were dosing quantity for diatomaceous earth of250mg/200ml, compound ratio for diatomite and PAC of25:1, pH value of7, mixing time of20minutes. The removal rates for TOC COD and turbidity were15.90%,89.43%and95.49%respectively. The removal rates were improved compared with the effect of single flocculant.In this paper, effect of transmembrane pressure, cross-flow velocity and temperature on permeate flux and effluent quality in the process of ceramic membrane ultrafiltration was studied respectively by single-factor experiment and Box-Behnken response surface methodology experiment. The results showed that:permeate flux was increased, while the removal efficiency of TOC、COD and turbidity were decreased with increasing transmembrane pressure, cross-flow velocity and temperature. The relationship among effect of the three factors on permeate flux and removal rate of TOC was concluded as follows:temperature> transmembrane pressure> cross-flow velocity. The optimal conditions were transmembrane pressure of1.60bar, cross-flow velocity of9.77L/s and temperature of40℃. Under these conditions, permeate flux and removal rate of TOC were319.68L/(m2·h) and93.68%respectively.The effect of transmembrane pressure, cross-flow velocity and temperature on permeate flux and effluent quality in the process of reverse osmosis was also studied respectively by single-factor experiment and Box-Behnken response surface methodology experiment. The results showed that:permeate flux was increased with increasing transmembrane pressure, cross-flow velocity and temperature. The removal efficiency of Na+、Cl-、Ca2+and SO42-were increased and then stabilized with increasing transmembrane pressure and cross-flow velocity, but discreased with increasing temperature. The relationship among effect of the three factors on permeate flux was concluded as follows:transmembrane pressure> cross-flow velocity> temperature. The relationship among effect of the three factors on removal rate of Cl" was concluded as follows:temperature> transmembrane pressure> cross-flow velocity. The optimal conditions were transmembrane pressure of26.03bar, cross-flow velocity of0.40m/s and temperature of39.2℃. Under these conditions, permeate flux and removal rate of TOC were29.61L/(m2·h) and94.58%respectively.In addition, membrane cleaning method of ceramic membrane and reverse osmosis was also studied in this paper. The optimal operation condition was that the ceramic membrane was first cleaned with1%C18H29O3SNa for20minutes, and then with1%NaOH for20minutes, finally with1%HNO3for10minutes, making the recovery rate of permeate flux reach97.2%. The polluted reverse osmosis was cleaned with commercial enzyme cleaning agent for40-60minutes, making the recovery rate exceed90%.