Very High Strength Leachate
The definition of “very high strength leachate” varies, but when higher than normal strength leachates are found in a landfill, the most common causes are:
- young (acetogenic) leachate
- a site where leachate has been allowed to stand in the waste for a prolonged period, often for many years
- a site which has accepted large quantities of liquid wastes (ie in excess of good UK practise for the co-disposal of liquid wastes, when co-disposal was permitted prior to July 2003).
In addition, we are sometimes approached with data which is claimed to indicate very high strength, but in reality only contains a large quantity of high organic content suspended solids as a result of poorly controlled sampling technique. To check this, always include the analysis of suspended solids in the test suit you analyse. (The best indication of leachate strength will normally be that obtained from filtered samples.)
1. Young Acetogenic Leachate
While mixed leachates from a mature landfill comprising of many cells at different stages of decomposition will not normally exceed a highest concentration of about 5,000 mg/l of Chemical Oxygen Demand (COD), acetogenic leachate from a newly formed landfill area may peak at
100,000 mg/l of COD, and exceed 10,000 mg/l for a significant period. Ammoniacal nitrogen in such leachates will not be nearly as high, but may be seen at up to (say) 2,000 mg/l.
The highest values of COD imply very high investment for treatment, and yet at most sites such high CODs are transient and short lived. It seldom makes economic sense to build treatment facilities to cater for a leachate feed comprising solely of this strength of leachate as such loadings would rarely, if ever, provide pay-back on the necessary very high investment, before natural decomposition in the waste in itself reduces these CODs by “orders of magnitude”.
Always seek expert advice on leachate characterisation before designing a leachate treatment facility to handle such leachates, and avoid the pitfalls made by others.
2. Leachate has been allowed to stand
When leachate stands in waste in a containment (lined) landfill, for periods of years or more its strength is normally seen to increase. This is assumed to occur due to diffusion effects through for example unbroken plastic bags, etc. CODs seen are not as high as for acetogenic leachate, but ammoniacal nitrogen may be two to five times as strong as for acetogenic leachate, and as the ammoniacal nitrogen is harder to treat, and has a large oxygen demand per molecule than COD, the effect on treatment plant cost is high. Another concern is that diffusion effects may have allowed abnormally high levels of dangerous substances (such as heavy metals, herbicides and pesticides) to build-up in the leachate.
Again, under such circumstances dangerous substances may inhibit treatment or be banned from discharge in the effluent. Leachate strength may also reduce quite rapidly as draw-down of leachate levels becomes established.
Always seek expert advice on leachate characterisation before designing a leachate treatment facility to handle such leachates, and avoid the pitfalls made by others.
3. High Rates of Liquid Waste Co-disposal
Under such circumstances, the leachate strengths will vary according to the quantity and nature of the wastes deposited (liquid and solid) and options for leachate treatment will need very careful assessment.
More than possibly in any other instance, the site operator should seek expert advice on leachate characterisation before designing a leachate treatment facility to handle such leachates, to avoid the many pitfalls which exist.
Summary
In a nutshell and in simplistic terms what happens in a landfill is specific to each site, (bearing in mind that each and every landfill is a unique bioreactor) - thus it is in all instances of very high strength leachates particularly important to do treatability trials for leachate, before the start of the process design.
Do always expect to see an initial increase in both COD and ammoniacal nitrogen (NH4) in the leachate from young landfills (and new cells in all landfills), due to the breakdown of complex organics containing Carbon and and Nitrogen, including carbohydrates and proteins.
The COD increases rapidly in the early stages of degradation as the organic acids are formed, and also decreases dramatically as the landfill becomes methanogenic and the acids are converted consumed - so a mature landfill should have relatively low leachate COD when compared with the acetogenic stage. The carbon leaves the mature landfill in the methane (CH4) and carbon dioxide (CO2) components of the biogas. (Biogas has normally low N concentrations for methanogenic landfills with leachate pH around 7.5 to 8).
The ammonia increase in the early stages of landfill decomposition and thereafter remains relatively high during the life of the landfill. Concentration does come down gradually as the landfill becomes mature and N is removed by way of leaching, during the leachate extraction process.
Treatability trials may be essential for all high strength leachates.
Always seek expert advice. Leachate characterisation is a highly specialised subject. Enviros will respond to requests for priced proposals. Click here and complete our contact form.
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