Leachate Quantity
“How much leachate will a particular site produce?” - is always a difficult question. Clearly, climate and particularly rainfall will be a big influence on this, but site operational practice (size of cells, rate at which completed cells are capped etc), and the potential for dry incoming waste to absorb leachate will all be important.
For a new landfill the only method available will be to model the site water balance from site opening to closure on the basis of a wide ranging set of assumptions. The resulting choice of plant size will then require the application of good judgement, based upon a sensitivity analysis and a range of "scenarios".
Sizing a plant for an existing landfill site will be easier if good leachate generation rate data is available, but it will usually also be necessary to take into account future development plans, and the likely changing nature of the incoming wastes as recycling, and waste pre-processing alter the nature of the waste residues being landfilled.
In most areas of the world a primary aim of waste management practise since the 1970s, or earlier, has been to minimise the quantity of leachate generated. In the 1990s it was realised that, now that proven leachate treatment technologies are available for leachate treatment, this was not essential to minimise the environmental impact of leachate. In fact by minimising the generation of leachate we are severely extending the period before landfills will become harmless to the environment. No landfill will forever remain intact, and even the best designed containment systems will eventually fail. The concern must be that this will put an excessive burden on future generations, and that our current actions in sealing, and keeping dry vast tonnages of waste are not sustainable.
Even a fairly simplistic analysis of the current very slow rate of "flushing" of lined and capped  landfills, shows that the rate of waste degradation and leaching will be extremely slow, with projected stabilisation times of several hundreds of years and longer for very deep sites. Click here to download a copy of an WordTM (.doc) article (from The Surveyor, 1993) which quite simply explains in layman’s terms why current flushing rates will mean that modern landfills will remain a liability to future generations for hundreds of years, and therefore current landfill practices (which will remain the national landfilling policy throughout much of the developed world for the foreseeable future), are not sustainable.
Waste degradation could be accelerated by circulating fluids through the waste in a controlled manner, and operating the landfill as an engineered flushing bioreactor. This concept, which is promoted in the UK by Waste Management Paper 26B (HMSO, London 1995), offers significant environmental and economic benefits and is consistent with the aims of sustainable waste management policy (Making waste work: a strategy for sustainable waste management: HMSO, London, 1995).
Government and landfill industry research has been undertaken on this subject. The first issue to establish was whether the permeability of the well compacted wastes in most landfills would be sufficient for accelerated flushing to take place. This resulted in the construction of the Pitsea compression cell which has been used to mimic the highly compressed waste conditions at the base of landfills. We recommend that those interested visit the web page of SUnRISE which is based in the Department of Civil and Environmental Engineering at the University of Southampton to find out more about, "The influence of mechanical and hydrological properties of waste on sustainable waste management practices".
Clearly, leachate quantity is a major consideration in the cost of leachate management, and while currently all UK/ EU landfill operators operate on the principle of minimisation it should perhaps be otherwise. Wherever sufficient water sources are available to resource the greater flushing flow rates necessary, landfills would be better operated as sustainable flushing bioreactors.
Up. See also Leachate Treatment - an Overview of Processes.
|
Newsletter