To the site operator leachate recirculation would certainly appear like commonsense as some of the leachate recirculated will be taken in by the waste and not re-appear. The leachate will gain some treatment – especially a significant reduction in BOD and COD by anaerobic digestion within a methanogenic landfill. In addition, if leachate is being removed for treatment, the action of recirculation will aid the removal of leachate by decreasing the peak flows. Circulation throughout wet weather is smoothed by the delay which happens, while the recirculated leachate percolates through the waste and before it re-emerges at the base of the waste.
As mentioned earlier, during wet weather durations, recirculation within Municipal Solid Waste Landfills (MSW) could impermanently “remove” peak leachate streams, while they percolate within the waste and arrive again at a leachate collection point. This will certainly allow the leachate treatment facility throughput, or max off-site tankerage rates to be down-sized, and usually that will be more price efficient.
This process is frequently combined with a methane gas removal system. The enhanced rate of organic decomposition produced by the presence of more water, arising from the recirculation of leachate raises methane (biogas) output rates.
Achieving Leachate Recirculation
The best means of getting the leachate back into the waste at the surface of the landfill, is a matter of debate and opinions about the best method to do this vary. Leachate spraying is effective, but is not permitted by many environmental regulators. Nevertheless, in the past this has actually been very efficient at some sites, and if allowed it clearly represents the best potential of actual decrease of leachate to evaporation and evapotranspiration with vegetation.
Trickle irrigation can easily also decrease leachate. Leachate is passively aerated when it is directed to the surafce in pools or lagoons before it soaks back inot the waste, but again this may be banned due concerns that it may cause odours and encourage pests while on the surface. Also, it may enter the site surrounds and cause pollution of surface watercourses or groundwater if recirculation flows are not well controlled.
The Pivotal Role of Leachate Recirculation at Bioreactor Landfills
Bioreactor landfills are not permitted in many nations, but where they are they depend heavily on achieving good leachate recirculation, and “flushing” by recirculation is essential to the bioreactor landfill philosophy.
At bioreactor landfills, leachate recirculation rates and biodegradation parameters are very carefully controlled to accomplish faster stablizing of the waste mass. Fluid (leachate) injection into the waste mass is made use of to increase waste moisture content and thereby enhance waste destruction. An even distribution of recirculation flows which is hard to achieve, but is essential throughout the body of the waste, if the aimed-for good bioreactor self-purification of the waste is to be successful.
Leachate Recirculation at Normal Sanitary Waste Landfills
Common techniques for long-term leachate recirculation in MSW landfills include vertical injection wells and horizontal trenches. Unfortuanely, both of these approaches lead to non-uniform distribution of leachate. In addition, the amount of leachate that can be recirculated by these approaches is not adequate to get rid off all leachate usually produced by landfills found in humid regions. Non-uniform distribution of leachate leads to unequal landfill settlement and hence greater maintenance spending.
A horizontal blanket comprised of reasonably high hydraulic conductivity product(s) to recirculate leachate has been proposed by researchers. Saturated/unsaturated leachate flow modelling performed by the authors of one recent paper suggests that horizontal blankets composed of reasonably high hydraulic conductivity materials are capable of recirculating leachate at a rate up to an order greater than the traditional approaches. In addition, the horizontal blanket is capable of achieving relatively systematic leachate distribution which is expected to reduce uneven settlement in a landfill. If the blanket is composed of recycled materials such as scrap tires, this design option could be economical.
White, J. K., Beaven, R. P., Powrie, W. and Knox, K. (2011) Leachate recirculation in a landfill: some insights obtained from the development of a simple 1-D model. Waste Management, 131, (6)(doi:10.1016/j.wasman.2010.10.022). http://eprints.soton.ac.uk/53143/
White, J.K., Beaven, R.P., Powrie, W. and Knox, K. (2007) Leachate recirculation in a landfill: a simple 1-D model. In, 2nd International Workshop on Hydro-Physico-Mechanics of Wastes, Southampton, UK, 18 – 19 Apr 2007. http://eprints.soton.ac.uk/74230/