Application of medium membrane method in leachate treatment

1 Harm of landfill leachate and characteristics of landfill leachate treatment by membrane method

Landfill leachate has the characteristics of high concentration of toxic and harmful substances, complex composition, high organic content, large fluctuation of water quality and quantity, difficult treatment and so on, which is very harmful to the environment. At present, the main treatment methods of landfill leachate are: into the municipal sewage plant treatment, landfill recycling treatment, waste incineration power plant incineration, waste residue retreatment and so on. Membrane process has been used more and more in landfill leachate treatment because of its simple equipment, easy operation and good effluent quality.

2 Introduction to the common membrane treatment process

At present, the common membrane process for landfill leachate treatment is mainly: biochemical method (A/O or A2/O) +MBR/ tube ultrafiltration + nanofiltration + reverse osmosis (+DTRO). In the common membrane process of landfill leachate treatment, the main functions of each process are as follows:

2.1 Non-membrane processes (such as traditional biochemical methods or some anaerobic/aerobic reactors, etc.) are usually used as pretreatment processes for membrane processes;

2.2 MBR: MBR process is the core of the whole landfill leachate treatment system, and is one of the main bodies to remove organic matter in landfill leachate. At present, the common MBR membrane components mainly include plate membrane components and hollow fiber membrane components. The two MBR components have advantages and disadvantages. Compared with hollow fiber membrane components, plate MBR membrane components have advantages such as lower transmembrane pressure difference, higher sludge concentration, lower pretreatment requirements, lower maintenance and cleaning frequency, no need for backwashing, and relatively simple operation. Hollow fiber MBR membrane module has the advantages of relatively high filling density, small membrane pool area and low investment in membrane module equipment. The concentration of organic matter in landfill leachate is higher. Under the condition of the same sludge load, the higher the concentration of activated sludge in MBR membrane tank, the stronger its ability to treat organic matter.

2.3 Ultrafiltration: ultrafiltration, due to its high filtration accuracy, can separate the microbial bacteria and sediments brought by the biochemical part from the sewage, in addition, ultrafiltration can also remove a part of the organic matter with a larger molecular weight in the wastewater. After biochemical treatment, the concentration of pollutants contained in the landfill leachate is often still high, and the water entering the ultrafiltration process often has higher turbidity, chroma, COD and heavier taste, therefore, after the ultrafiltration (common tube ultrafiltration) in the landfill leachate treatment process is used in MBR, as the pretreatment of NF and RO, the impurities in the water can be further removed. Ensure the stable operation of the subsequent process.

2.4 Nanofiltration: The main pollutants in the water produced by MBR or ultrafiltration process are usually organic matter, microorganisms, hardness, alkalinity and heavy metals. The nanofiltration process can remove most of the organic matter and inorganic salts in the water produced by MBR or ultrafiltration process, and the water produced can basically meet the discharge standard. The concentrated water of the nanofiltration process is generally returned to the landfill or further evaporation treatment; At present, the recovery rate of nanofiltration system in the process of landfill leachate treatment is generally relatively high (80%~85%), and the content of organic matter in the influent is high, which leads to the biggest problem faced by nanofiltration is membrane pollution and scaling. The life of membrane elements in nanofiltration treatment of landfill leachate is often low. As far as some landfill leachate treatment projects have been known, the water quality of most of the nanofiltration process can not meet the requirements of reverse osmosis influent, generally with high chroma and unpleasant taste, the treatment effect is not ideal.

2.5 Reverse osmosis process mainly plays a role in reducing the electrical conductivity and organic content of external drainage in the leachate treatment process. In addition, the reverse osmosis process can significantly intercept the ionic nitrogen (such as nitric acid, etc.) in the landfill leachate, reduce the total nitrogen value in the production water, and finally make the discharged water reach the national discharge standard. According to the current knowledge of the use of reverse osmosis at the landfill leachate treatment site, there are mainly the following problems:

a. Concentrated water reflux increases the system recovery rate: Reverse osmosis or nanofiltration processes often consider the way of concentrated water reflux to improve the system recovery rate, many landfill leachate treatment systems also use two-stage concentrated water reflux nanofiltration or reverse osmosis process, because landfill leachate intake is often high salt content and high organic matter characteristics, concentrated water reflux often leads to the further deterioration of nanofiltration or reverse osmosis system intake, accelerating the speed of membrane pollution. Then the service life of the membrane element is affected.

B. Use of circulating booster pump in section: In the landfill leachate treatment site that has been known, many reverse osmosis treatment systems are set up a single stage of concentrated water return (that is, the concentrated water of each stage is pumped through the intra-stage circulation booster pump in the internal circulation of this section, the amount of circulation in the segment is several times the water intake of the reverse osmosis system), so as to increase the inlet flow rate of the membrane element, prevent pollutants from deposition and contaminating the membrane element. However, a large amount of backflow of concentrated water will lead to the deterioration of the inlet water quality, thus aggravating the membrane pollution. In the landfill leachate system with poor water quality, backflow will lead to frequent membrane cleaning, which will affect the life of membrane elements.

c. Problems with meter setup: Because landfill leachate projects are generally small, and in order to improve recovery rates, two-stage designs are often used. However, in our field investigation of landfill leachate project, some system Settings monitoring data often have problems, such as two-stage system only set inlet and concentrated water pressure gauges, inter-stage pressure can not be monitored; Or the circulating booster pump in the setting section of a single membrane shell, but it cannot detect the pressure and conductivity into the membrane shell, which will cause the failure of the membrane system to be found in time during operation, and eventually lead to serious pollution or damage to the membrane components.

d. DTRO: The main function of DTRO is to further reduce the concentration of concentrated water in the system, but it will also cause the concentration of circulating water to further increase.

3. Analysis of landfill faults and suggestions for improvement

Through the field investigation, we found the following problems in the landfill membrane leachate treatment project:

3.1 Membrane pollution is fast, cleaning is frequent, operation faults are more, and the life of membrane components is generally short.

3.2 Serious corrosion of equipment, pipes and instruments.

3.3 Concentrated water recharge leads to an increase in intake salt content year by year, which ultimately makes it difficult for the reverse osmosis membrane system to operate.

3.4 The concentrated water reflux of the reverse osmosis system itself is also an important reason for the low life of the membrane elements of the landfill leachate project.

3.5 On-site instrument Settings often cannot meet the requirements of device operation monitoring.

From the findings of the current landfill operation project, the following aspects need to be considered for improvement:

a. Timely detection and replacement of equipment and instruments to ensure the accuracy of system monitoring data;

B. For projects that cannot discharge concentrated water, the method of evaporation crystallization can be considered, and the high-concentration wastewater can be converted into solid waste treatment. If there is a subsequent waste incineration project, the final concentrated water can be burned in an incinerator for treatment.

c. Timely chemical cleaning, if the system performance can not be restored, it is more considered to replace the film components to ensure that the water quality of each section is qualified.

d. For the system that cannot work properly due to the high salt content of the inlet water caused by recharge, seawater membrane or high-pressure reverse osmosis membrane elements can be considered to replace the original brackish water membrane elements, but the choice of this solution requires the corresponding transformation of the existing equipment, such as: Replace the high pressure pump to a higher head, a higher grade of pressure pipeline, whether the existing instrument range matches the high salt content, whether the existing agents can play a role in new conditions, whether new types of agents need to be replaced, etc., and this program can only be effective in a certain period of time, with the continuous recharge of concentrated liquid, the reverse osmosis of the water is constantly circulating and concentrating. Eventually, the salt content is too high to make the reverse osmosis system work. Therefore, to solve the problem of concentration of circulating liquid, it is necessary to consider its external transport or conversion into a solid waste removal system, rather than unlimited circulation and concentration in the system.

e. Advanced oxidation techniques can be considered to treat reverse osmosis or nanofiltration concentrated water to reduce the organic matter content in the recharge water or subsequent evaporation crystallization water.