The Sanitary Landfill Leachate Recirculation Bioreactor
A sanitary landfill itself, will become a leachate recirculation bioreactor, once the site operator has been recirculating leachate for several months. The landfill will, to start with, be aerobic, but quite soon, especially if there is plenty of moisture from leachate recirculation, the waste will become anaerobic as the decomposition of the organic matter within the waste and the biochemical oxygen demand / chemical oxygen demand COD/BOD mops up all available oxygen.
Once the site has been anaerobic for a while, methanogenic bacteria will multiply and start to produce biogas. However, methanogens are relatively slow growing so the effect is not immediate and can take anywhere between 3 months and about 18 months, depending on a variety of factors.
Once a leachate recirculation bioreactor has become established in a landfill cell the recirculation of leachate through it provides what is effectively free leachate treatment. Of course, leachate recirculation will never in normal circumstances, within a sanitary landfill reduce the contaminants in the leachate to make it clean enough to be discharge to a watercourse, or for that matter even clean enough to send to a foul sewer in most instances.
However, the very young leachate which may for a very short period at the start of recirculation, have a COD of up to, of the order of, 100,000 mg/l of COD contamination, may well drop to no more than 5,000 mg/l to 10,000 mg/l. That is a big reduction and would avoid the cost of a large aeration energy input to satisfy that oxygen demand reduction if it was accomplished in a biological leachate treatment plant.
Leachate Recirculation Bioreactor Within a Bioreactor Landfill
So, far in this article we have discussed a leachate recirculation bioreactor when it is applied to sanitary landfills. It is a principle of all sanitary landfills that they are by definition designed to be filled rapidly, capped as soon as possible, and kept as dry as possible.
There are some countries where the opposite to the measures just mentioned, are applied. The concept of these landfills, which are known as “bioreactor landfills”, is that by ensuring that recirculation and other added water, wets the waste to an adequate moisture content for optimized aerobic composting to occur within the landfill, the landfill will be made environmentally safe. The waste is, in this type of landfill, left for a period before the next layer of waste is placed above, in an aerobic condition so that composting can occur un-aided within the landfill.
Where bioreactor landfill methods are practiced (for example in parts of Japan) the hope is that the waste in a landfill is effectively stabilized before the landfill is closed, allowing the landfill site area to be used for other land uses much more effectively than for a sanitary landfill. In addition the risk of water pollution from an escape of leachate many years after landfill closure would be much lower. Similarly, much less landfll gas, or none will be created, making the landfill safer from gas explosion risks.
Bioreactor landfills are quite rare on a global scale, but in principle this type of landfill would be one big leachate recirculation bioreactor.
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