Landfill leachate mainly comes from the water holding capacity of garbage itself, the liquid produced by garbage decomposition and the rainwater brought in during garbage collection and transportation. Landfill leachate has the characteristics of high ammonia nitrogen content, high concentration of organic pollutants, high salinity, complex components, large fluctuation range of water quality and quantity, which makes it difficult to treat.
I. Characteristics of Landfill Leachate
1. High concentration of organic pollutants and good biodegradability
The CODcr of landfill leachate is as high as 20 000-60 000 mg/L and BOD5 is 10 000-30 000 mg/L, which belongs to high concentration organic wastewater. Most of the organic compounds in leachate are soluble organic compounds. About 90% of the soluble organic carbon in leachate is composed of short-chain volatile fatty acids. The main components are acetic acid, propionic acid and butyric acid, followed by gray yellow enzymatic acid with polycarboxyl and aromatic carboxyl groups. Therefore, the biodegradability of leachate is better.
2. High concentration of ammonia nitrogen
The concentration of ammonia-nitrogen in leachate can reach 1000-3000mg/L. Nitrogen in leachate mostly exists in the form of ammonia-nitrogen, accounting for 75-90% of total nitrogen.
3. High Salt Content
The salt content in leachate is usually more than 10 000 mg/L. Membrane treatment will result in low water yield due to excessive osmotic pressure. It is difficult to start up, low load, unstable operation or even unable to operate because of excessive salt content in ordinary biochemical treatment.
4. Water quantity and water quality fluctuate greatly
The amount of leachate is affected by the type of municipal solid waste collection and transportation system, the composition of refuse, rainfall and other factors. The daily output of leachate is about 5%~40% of the garbage. The variation range of pollutant concentration is 3 to 5 times.
II. Common Technical Routes for Leachate Treatment
1. Biochemical + Advanced Oxidation + Advanced Treatment
Leachate has high concentration of organic pollutants and good biodegradability. Biochemical treatment process is the most thorough and economical process to treat high concentration organic wastewater. It can degrade organic pollutants in a large scale under relatively economic conditions, and at the same time, it can play a role in nitrogen and phosphorus removal, which makes the overall treatment cost of leachate more economical. Because the leachate also contains many refractory macromolecular organic compounds, some “inert COD” which can not be biodegraded and adsorbed will always be retained after biochemical treatment. Engineering practice shows that the CODcr of leachate can be reduced to less than 1000 mg/L by various biochemical treatment processes. The removal rate is very considerable, but the effluent can not meet the discharge standard directly.
2. Biochemical + Membrane Processing
After biochemical treatment, the leachate is further treated by membrane process, which is the most commonly used leachate treatment method at present. The effluent quality of this process is good and can meet the standard of reuse water. It also has a high anti-variability ability for the fluctuation of leachate water quality and quantity, and has a high operational stability. After membrane separation, the effect of pollutants is obvious, and the separated effluent can meet the corresponding national discharge standards. Membrane technology has the advantages of continuous operation, high degree of mechanization and easy management. The instability of water quality has little influence on the effect of membrane treatment.
3. Membrane process or evaporation treatment
The disc-tube reverse osmosis DTRO membrane has the characteristics of good anti-pollution, high flux and long service life. The front end of the disc-tube reverse osmosis DTRO membrane can directly treat the leachate only through sand filtration protection. Even in the case of high turbidity, high SDI value, high salinity and high COD, it can operate economically, effectively and steadily.
MVC evaporation process for leachate treatment has the advantages of fast start-up, low energy consumption, low concentration ratio and small occupation area. The problems of evaporation process are as follows: firstly, the condensate contains volatile hydrocarbons, volatile organic acids and ammonia, which need further treatment to reach the standard, and the cost of treatment is relatively high; secondly, when COD in leachate is relatively high, the content of reactor is easy to foam, which directly affects the quality of effluent and concentration multiple, and can be solved by adding defoamer, and the cost of ammonia-nitrogen conversion is high; When transferred to the condensate and followed by ion exchange treatment, the resin replacement frequency is high.
III. Technical Key Points and Difficulties
1. Accurate prediction of design water quantity and quality
Accurate prediction of design water quantity and quality is the basis of Engineering design. The daily production of landfill leachate should take into account the residence time of garbage in the aggregate pit, main components and local rainfall. The water quantity and quality of landfill leachate can refer to the operation data of garbage in the same area.
2. Biochemical treatment
The COD of landfill leachate is high, and the energy consumption of aeration system is too high when aerobic process is used directly. Therefore, the leachate should be treated by aerobic process after reducing the concentration of organic pollutants through anaerobic reactor.
3. Selection of Membrane System
The selection of membrane system is influenced by the design effluent standard.
In recent years, the environmental protection requirement of newly-built garbage is getting higher and higher. Many newly-built garbage require the reuse of leachate after treatment and the requirement of “zero discharge”, which puts forward higher requirements for the design of leachate treatment system. With the complex composition of landfill leachate and the high concentration of pollutants, it is difficult to treat, and the treatment standard is constantly improving. The treatment of landfill leachate should be considered from the whole landfill, and experts in the field of water treatment are actively exploring more treatment processes of landfill leachate.
Luoyang Building Material and Architectural Design and Research Institute is committed to the innovation and development of technology and equipment in building materials and construction industry. It has a double-level design and Research Institute which integrates scientific research and development, design, general contracting, supervision, technical consultation and technical service of building materials and construction engineering. The garbage gasification power generation technology developed by our institute has high efficiency and continuity, gas De-Coking and purification, automatic control of one-key operation, strong adaptability, modular design, zero emission and waste heat.