Landfill leachate is mainly the result of precipitation of water into the layers of buried waste, and biochemical reactions of waste that has dangerous substances and pollutants that lead to the contamination of surface and groundwater resources. Therefore, it must be collected and treated properly. The investigation of various biological methods in leachate treatment, their advantages and disadvantages, and their effect on reduction of COD (chemical oxygen demand) are the objectives of this study. Reviewed processes include anaerobic and aerobic sequencing batch reactor, up-flow anaerobic sludge blanket, moving-bed biofilm reactor, membrane bioreactor, and aerated lagoons, lead to reduction of biodegradability pollutants in different circumstances. The present study has indicated that the most and the least reduction of COD has been through aerated lagoon (95%) and moving-bed biofilm reactor (8%), respectively.
In recent decades, increasing population, industrialization and excessive consumption by humans, has increased waste production significantly [
Biological processes are highly effective in the treatment of young landfill leachate, while their impact is reduced in old leachate treatment due to the resistance of contaminants to biodegradation [
This process involves the biological degradation of organic and inorganic materials in the absence of oxygen [
Advantages | Disadvantages |
---|---|
Lower dosage of phosphorus required as a growth factor for anaerobic bacteria | Heavy metals can prevent digestion |
Lower excess sludge production | Ammonia toxicity |
Lower energy usage | Sensitive to changes in temperature and pH, as well as various toxic substances existing in influent of the reactor or byproducts produced in the reactor |
Useable biogas production | |
Removal of more than 90% of BOD | |
Higher organic loading rates | Remain high concentration of ammonia in effluent |
Lower operating costs | Long time for start up |
ganic acids into carbon dioxide (CO2) and methane (CH4) [
1) Anaerobic sequencing batch reactor
Anaerobic sequencing batch reactor (ASBR) process can be seen as a suspended growth process with reaction and solid-liquid separation operations in a pond, similar to sequencing aerobic reactor (SBR). ASBR includes four steps: a) feeding, b) reaction, c) sedimentation, d) discharge/effluent discharge. During the reaction, the mixing is done intermittently for a few minutes every hour to ensure uniform distribution of nutrients and solids. Biological reactions start during feeding stage. After the feeding, with any concentration of micro-organisms, F/M (food to microorganism) ratio is high, which provides the driving force for metabolic activity and converting waste into biogas [
2) Up-flow anaerobic sludge blanket
Up-flow anaerobic sludge blanket (UASB) is a modern anaerobic treatment process that can have high efficiency and little hydraulic retention time (HRT) [
Aerobic treatment results in the removal of biodegradable organic pollutants and nitrification of ammonium nitrogen (
1) Aerobic biological treatment―attached growth system
In this way, microorganisms, as a biofilm or microbial membrane, operate snugly on the solid surface inside the reactor. The solid body is called Media and is made of stone, sand, ash, plastic and other synthetic materials. Attached growth systems are either fixed (completely submerged) or float (some parts inside the fluid and the other parts outside the fluid in the air) [
a) Moving-bed biofilm reactor
Moving-bed biofilm reactor process is based on the use of suspended polymer (plastic) carriers which continuously move inside the aerobic tank and active biomass grows on its surface [
b) Membrane bioreactor
Membrane bioreactor systems is a combination of biological (by the bioreactor) and physical (separation of water from solids by membrane) process [
advantages | disadvantages |
---|---|
Low hydraulic retention time (HRT) and high sludge retention time (SRT) | High bed costs |
low sludge production | Requires high concentration of dissolved oxygen in biofilm reactors for denitrification |
high resistant to toxic materials | |
constant active biological biomass | Congestion problems in the fixed beds |
Low sensitivity to temperature and amount of sludge production | |
nitrification in low temperature | |
Removal of high amount of nutrients | |
Nitrification and denitrification can simultaneously occur on both exterior and interior sides of a biofilm |
the membrane bioreactor process, aerobic bioreactor unit, through supplying oxygen leads to COD oxidation and nitrification and anoxic unit in the absence of oxygen leads to denitrification. The MBR System with high sludge retention time (SRT) leads to the growth of nitrifying bacteria that has significant influence on the nitrification process [
2) Aerobic biological treatment―suspended growth system
a) Sequencing batch reactors
In this type of treatment system, all stages including aerobic treatment, sludge settling and clarification are done in a tank. Sequencing batch reactor (SBR) treatment system is less affected by changes in the amount of organic load and ammonium nitrogen concentration. Despite the good performance and the flexibility of this system, its usage produces large volumes of sludge and poor clarification [
b) Aeration Lagoon
One of the easiest processes in landfill leachate treatment is aerated Lagoon, which are aerated through surface aerators or deep aeration diffuser. In this way, the process occurs chemically, and using biological oxidation [
According to investigations, raw leachate can be treated well in this system. Re-
moval of 64% - 85% of COD (the initial concentration of 17,000 mg/L) in volume and specific loading rate, which is variable between 0.4 - 9.4, is possible. Approximately, 83% of the removed COD is converted to methane and the rest is converted to sludge [
In a study, by the use of UASB process, the maximum COD removal efficiency with organic loading rate of 5 g/L/day, was 71% and no reduction in the amount of
[
Moving-bed biofilm reactor performance is an effective method for the biological treatment of landfill leachate. 90% nitrogen and 20% COD have been removed. The high degree of nitrogen removal can be achieved in a high volume of nitrification and denitrification [
1526 to 3182 and then 1586 mg/L, and the average output COD from aerobic MBBR increased from 938 to 1045 and then to 1318 mg/L. The total COD removal efficiency declined from 94% to 92%. Anaerobic MBBR system played an important role in the removal of COD which reduced the total COD from 91% to 80%. Aerobic MBBR increased COD removal efficiency from 3% to 12%. By reducing anaerobic HRT from 4 to 2.5 days, minor changes occurred in the removal of COD in both reactors. In the third stage, along with the reduction of HRT, organic loading rate is increased. Average COD output from anaerobic and aerobic MBBR system increased from 578 to 1134 and from 347 to 471 mg/L, respectively. The total COD removal efficiency declined from 95% to 94%. The total COD removal rate in anaerobic MBBR system decreased from 92% to 86% and in aerobic MBBR has increased from 3% to 8%. The results have indicated that the anaerobic-aerobic system can be effective in leachate treatment [
During an investigation, membrane bioreactor system with aeration rate of 35 g O2/m2/day resulted in 80-83% reduction of COD with initial concentration of 1000 - 3000 mg/L [
control the deposition rate. In the fourth stage, with increasing HRT, the proportion of microorganisms to food increased more than normal and resulted in unstable operations. According to
SBR process, with the amount of 2.5 to 4 mg/L dissolved oxygen and pH 6.5 - 8.5, eliminates 87% of COD and 96% of
Using aerated lagoons results in the removal of 80 and 75 % ammonia and COD with initial concentration of 965 and 1740 mg/L removed from old landfill leachate during long retention time between 11 and 254 days, respectively. Partial of 80% removal of nitrogen in aerated Lagoon was done by ammonia evapora-
initial concentration of COD (mg/L) | COD removal (%) | resources |
---|---|---|
Anaerobic sequencing batch reactor | ||
17,000 | 64 - 85 | [ |
6000 | 80 | [ |
Up-flow anaerobic sludge blanket (UASB) | ||
45,000 - 90,000 | 71 | [ |
6000 | 77 | [ |
Moving-bed biofilm reactor (MBBR) | ||
800 - 2000 | 20 | [ |
7500 - 7900 | 8 | [ |
Membrane bioreactor (MBR) | ||
1000 - 3000 | 80 - 83 | [ |
1550 | 63 | [ |
14,000 | 80 | [ |
aerobic sequencing batch reactor (ASBR) | ||
4000 | 87 | [ |
3200 | 30 | [ |
Aerated lagoons | ||
1740 | 75 | [ |
5518 | 97 | [ |
tion [
In this study, various methods of biological treatment of landfill leachate such as anaerobic sequencing batch reactors, aerobic sequencing batch reactors, up-flow anaerobic sludge blanket (UASB), moving-bed biofilm reactor (MBBR), membrane bioreactor (MBR), and aerated lagoons were examined. As mentioned, the minimum and maximum removal of COD by biological reactors were 64% and 85%, 30% and 87%, 71% and 77%, 8% and 20%, 63% and 83%, 75% and 95%, respectively. According to the results, the highest and lowest removal of COD was achieved by using aerated Lagoon (95%) and moving-bed biofilm reactor (8%), respectively.
Payandeh, P.E., Mehrdadi, N. and Dadgar, P. (2017) Study of Biological Methods in Landfill Leachate Treatment. Open Journal of Ecology, 7, 568- 580. https://doi.org/10.4236/oje.2017.79038