Research On The Deep Treatment For The Concentrated Water Of Ultrafiltration-Reverse Osmosis Double Membrane System

With the increasing demand for the environmental protection, petrochemical enterprises are required to reduce the discharge capacity of effluent even to zero. As a result, increasing the rate of recycle water is widely studied in recent years. In order to reduce the discharge capacity, using ultrafiltration-reverse osmosis double membrane technology is the common way for refinery to purify oily waste water. But the said technology can only produce about 40-50% of recycle water purification, the other is still need further treatment in order to achieve discharge standards. To get more purified water recycled and a higher recovery of the wastewater treatment systems, membrane distillation process is used in this paper to treat the effluent water from Shengli oilfield petrochemical factory, which is treated by ultrafiltration-reverse osmosis double membrane system first. Chemical oxidation method is also used to deal with the concentrated waters of ultrafiltration-reverse osmosis double membrane and membrane distillation respectively, in order to reduce the COD value to discharge standards.According to the properties of the concentrated water of double membrane system, the orthogonal experiment is designed to optimize the operation conditions of vacuum membrane distillation process. The optimal operation condition is that temperature is 80℃, vacuum degree is 0.08 MPa, liquid flow is 1.5 L·min^-1, and material liquid enrichment 2 times is advisable. After treated by the said experiment condition, the water's COD value and hardness value are 3mg·L^-1 and 1.12 mg·L^-1 respectively. With the increasing of feeding temperature, vacuum degree and liquid flow, membrane flux also get an increasing. The greatest effect factor is feeding temperature, the vacuum degree takes the second place, the last one is feeding flow.Chemical oxidation method is used in this thesis to oxidise the concentrated waters of ultrafiltration-reverse osmosis double membrane and membrane distillation respectively. The effects of H₂O₂ content, FeSO₄·7H₂O content, pH value, reaction time and reaction temperature on the degradation efficiency of organic compounds by Fenton process were investigated. Results showed that Fenton process acted efficiently in the water treated by double membrane when H₂O₂ content is 0.455 ml, FeSO₄·7H₂O content is 0.278g,pH value is 2.5, reaction time is 120min. In this case, the COD removal rate can reach a high point of 55.62%. The result also shows that this process acted efficiently in the water treated by membrane distillation when H₂O₂ content is 0.68 ml, FeSO₄·7H₂O content is 0.417 g,pH value is 2.5, reaction time is 120min. In this condition, the COD removal rate can reach the point of 49.56%. It was also acknowledged that the removal rate of COD is almost independent of reaction temperature.The economic cost of using two processes to deal with the concentrated water of ultrafiltration-reverse osmosis double membrane system with 0.35 million t·a^-1 scale, 75% productivity of water and 4 years operation cycle is compared in this thesis. The result shows that the cost of chemical oxidation-membrane distillation process and membrane distillation-chemical oxidation process is 12.9 yuan·ton^-1 and 10.5 yuan·ton^-1 respectively. After treated by membrane distillation- chemical oxidation process, the COD value of concentrated water can reduce to 100mg·L^-1, and the water productivity can reach 75%, meeting the national standard for petrochemical enterprises'effluent. So it can be acknowledged that membrane distillation-chemical oxidation process has a better applicability to deeply treating concentrated water.