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Guidance: With the expansion of city scale and the increase of population, the production of domestic waste is also growing rapidly. Landfill, composting, incineration and other methods of domestic waste treatment, will produce landfill leachate, if not treated and directly discharged into the environment, will cause serious environmental pollution.

Source: Tuhai Bridge

Taking the landfill leachate produced by a waste incineration power plant as an example, this paper analyzed the advanced treatment technology of landfill leachate.

Analysis of Leachate Characteristics in Waste Incineration Plant

Water volume characteristics of 1.1 leachate

Landfill leachate mainly comes from the infiltration of rainwater and surface water during storage and transportation, the water produced by fermentation and decomposition of garbage, and the water contained in garbage itself. It is generally considered that the yield of landfill leachate is 10%-20% of the garbage disposal capacity. The amount of leachate varies obviously with the seasons. In winter, it is 8%-10% of the amount of domestic waste, in summer, it is 12%-15% of the amount of domestic waste, and in rainstorm, it is as high as 20%-25% of the amount of domestic waste.

The different incineration processes also have certain effects on the amount of leachate. Using the circulating fluidized bed process, garbage is directly incinerated into the boiler after pretreatment, and there is no need to compost and store garbage, so the amount of leachate produced is relatively small.

Water quality characteristics of 1.2 leachate

Through the analysis of the leachate water quality characteristics of refuse incineration plant, it can be seen that the leachate of incineration plant has the following characteristics:

(1) Landfill leachate contains not only organic matter, but also oil, ammonia nitrogen, heavy metals and other pollutants. Water quality and water content are complex, and the concentration changes greatly.

(2) Landfill leachate from incineration plant is mostly produced on the day, without anaerobic fermentation, hydrolysis, acidification process, containing a variety of refractory organic matter, which has the characteristics of high concentration of COD and BOD5, high toxicity and difficult treatment.

(3) The proportion of microbial nutrients in landfill leachate is out of balance. When the system is debugged and started, a certain amount of potassium dihydrogen phosphate should be added to supplement phosphorus nutrients.

(4) The ammonia nitrogen value of leachate from refuse incineration plant is above 1.2g/L.

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(Pictures from the Internet)

2 Project Profile

The total recovery rate of the advanced treatment system is not less than 70%. The effluent water quality shall comply with the water quality standard of open circulating cooling water in the Water Quality of Industrial Water for Recycling and Utilization of Municipal Sewage (GB/T19923-2005). The concentrated water shall be treated by spray incineration without considering the separate treatment scheme of the concentrated water.

The project design adopts the technology of "mechanical filtration+regulating tank+mixed reaction sedimentation tank+anaerobic system+A/O system+membrane bioreactor(TMBR)+nanofiltration system(NF)+reverse osmosis system(RO)" to meet the problems of large variation of landfill leachate water volume, strong shock load resistance, high load treatment capacity, high ammonia nitrogen treatment capacity, high content of heavy metal ions and salt.

Technological design of 3-film

3.1 Membrane Bioreactor

A large number of microorganisms (activated sludge) are in full contact with the matrix (degradable organic matter in leachate, etc.) in the membrane bioreactor. They metabolize by oxidation and decomposition to maintain their growth, reproduction and degradation of organic pollutants. Membrane module is used to separate the mixture of leachate and sludge. The sludge is concentrated and returned to the bioreactor, thus avoiding the loss of microorganisms. Membrane module is equivalent to traditional secondary clarifier, but it overcomes many shortcomings of traditional secondary clarifier.

Membrane bioreactor (MBR) mainly consists of two parts: membrane module and bioreactor. According to the combination mode, MBR can be divided into external type and internal type (submerged type). For landfill leachate treatment, the use of built-in membrane bioreactor will cause many problems. Therefore, the design of this project uses external membrane bioreactor.

The tubular ultrafiltration membrane feed pump pumps the leachate from the aerobic tank to the tubular ultrafiltration membrane system for solid-liquid separation and concentration. The concentrated liquid is returned to the anaerobic tank, and the surplus part is flowed to the sludge storage tank. There are two loops in the tubular ultrafiltration membrane system. Each loop has four tubular ultrafiltration membranes with 8 mm diameter and PVDF inner surface. Each loop has a separate circulating pump, which provides a required flow rate along the inner wall of the membrane tube, thus forming turbulence, generating larger filtration flux and avoiding blockage of the membrane tube. Water treated by tubular ultrafiltration membrane enters the nanofiltration system after passing the test.

3.2 nanofiltration system

The total denitrification rate of external membrane bioreactor is over 99%. The effluent ammonia nitrogen and total nitrogen have reached the discharge standard. However, the COD and chroma of refractory organic matter are still beyond the standard. Because the effluent of tubular ultrafiltration membrane does not contain suspended substances and biodegradable organic substances, nanofiltration membrane was designed to treat the effluent of tubular ultrafiltration membrane in order to remove biodegradable organic substances and chroma.

Nanofiltration membrane is a new pressure-driven membrane separation technology between reverse osmosis and ultrafiltration. It has two characteristics: the separation of small organic molecules with hundreds of molecular weight in water and the Doonan effect for anions with different valences. The nanofiltration membrane does not intercept the valence ions, so the nanofiltration membrane can intercept the non-degradable macromolecule organic matter in the concentrated solution and discharge with the concentrated water.

The actual operating pressure of the nanofiltration membrane is about 7 bar, and the energy consumption is low, so the energy consumption of the nanofiltration membrane is low.

(Pictures from the Internet)

3.3 Reverse Osmosis System

Reverse osmosis system is the most important desalination device in this process. It has very high desalination capacity and can block all soluble salts and salts.