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Leachate Treatment: Frequently Asked Questions
Q1 What is leachate?
A. Leachate is a liquid, mostly water, which seeps out from the base of landfilled waste or composting material.
Q2 Why does leachate need to be treated rather than discharging to the environment?
A. Normal domestic waste leachate contains many nutrients to much higher strengths than for example domestic sewage. These are harmful to receiving watercourses. Leachate also contains many other substances which, depending upon the types of waste disposed of into the landfill, may be toxic to life, or may simply alter the ecology of the stream or watercourse, if not removed by treatment.
Q3 What is landfill?
A. Landfill is any form of waste tip, ranging from an uncontrolled rubbish "dump", to a full "containment" site engineered to high standards to protect the environment.
Q4 What is the most damaging or "difficult" substance to treat or remove from leachate.
A. For domestic landfill leachates ammoniacal nitrogen (NH4-N) is normally the most "difficult" constituent to remove. Landfills which have received significant quantities of hazardous / toxic materials may produce leachates which contain these substances, but for the majority of landfills these are not normally present in the leachate even when co-diposal of these substances may have occurred. Ammoniacal nitrogen is often present in leachate in concentrations of up to thousands of mg/l, and must be reduced to well below 5 mg/l before there is no danger to fresh water fish life.
Q5 Is leachate damaging if it seeps into the groundwater?
A. Yes, it may be, depending upon the nature of the local geology. If the contaminated water is far from any spring or other use of the groundwater this may not have any impact. Frequently, natural processes will themselves remove contaminants while still in the ground. Nevertheless, leachate contaminants of groundwater can be very damaging under certain circumstances, and groundwater contaminating discharges are not permissible in many countries, including Europe under EU Directives.
Q6 Why is Ammoniacal nitrogen "difficult" to remove?
A. Ammoniacal nitrogen is not effectively removed by physical aerobic processes under normal conditions (processes which take place in the presence of oxygen), and therefore simple aeration without the presence of a sludge biomass, or spraying / irrigation is seldom very effective in its removal. An aerobic biological process is needed for ammoniacal nitrogen "removal". When we refer to "removal" this normally comprises "nitrification" or the oxidation of NH4 to NO3 - itself a nutrient which may give rise to concern in an effluent.
Q7 Why treat leachate when it can be evaporated by spraying or irrigation, or why not just re-circulate it into the rubbish?
A. Most landfills can be operated satisfactorily for at least some periods in their lifetimes in this manner, and, if so and no significant negative environmental impact results, these methods are cheap and effective in avoiding damaging leachate discharges. Unfortunately, in those regions of the world where rainfall exceeds what can be effectively evaporated during winter or rainy seasons, this is often not feasible.
Q8 Is leachate suitable for land irrigation?
A. Weaker domestic waste leachates may be suitable for land irrigation, and providing that iron content is not excessive, so as to "blanket" leaf surfaces and inhibit photosynthesis, this may take place. Nevertheless, all leachates are saline to a greater or lesser extent and regular seasonal high rainfall is necessary to flush away salinity build-up.
Q9 If leachate from municipal waste landfills needs to be treated, how is this best done?
A. The simplest method if the site is near a suitable foul sewer is to discharge to the public sewer, and the leachate will be treated at a sewage treatment works. Alternatively, leachate must be tankered by road vehicle to a sewage treatment works. However, haulage costs make tankerage a costly option for most sites in anything but the lowest volumes, or short-term. Of course, care must be taken not to discharge any hazardous or odour producing chemicals to sewer and thus pre-treatment is frequently necessary before discharging leachate to a sewer.
Q10 Which substances, found in leachate, commonly cause problems if discharged to a public sewer?
A. Clearly, methane either conveyed in the sewer airspace, or in solution, must not be discharged to a sewer in view of explosion and asphyxiation risks this would entail. The maximum permissible safe concentration of dissolved methane in water (or leachate) is 1.4 mg/l. Most authorities responsible for sewers, require a factor of safety against methane hazard of 4 to 10, when setting the maximum permissible limit.
The discharge to sewer of sulphides, and a variety of other odour producing substances, must also be avoided as these odours may affect properties further down the sewer.
Discharge of sulphates in high concentrations, must be avoided in concrete sewers, in view of potential concrete attack.
In addition the discharge of all toxic and biologically inhibitory substances must be avoided.
It is advisable for expert technical advice to be sought on the suitability of any leachate discharges, for all but the most innocuous low concentration leachates, in view of the chemical complexity and variability of landfill leachates.
Q11 Okay, so you do not have a suitable sewer, or you cannot obtain a sewer discharge (Trade Effluent) consent. How do you treat your leachate?
A. The most cost effective method of full strength landfill leachate treatment is normally by extended aeration in a sequencing batch reactor. This process was developed by Aspinwall and Company (now part of Enviros Consulting) with assistance from UK DoE research projects over the past 20 years, and is now a well established and proven technology. Very high rates of ammoniacal nitrogen removal can be achieved in a single tank, and the process is robust and simple to operate. Chemicals are used merely for trace nutrient augmentation and, where necessary, for pH adjustment.
Published papers are available describing plants designed, installed and process commissioned by Enviros Consulting, free of charge by e-mailing: steve.last@enviros.com, and elsewhere on this website.
Q12 Is extended aeration suitable at all sites?
A.Extended aeration, with or without subsequent polishing stages, is suitable for most municipal waste leachates. It is not suitable where inhibitory substances are present at sufficient concentrations to prevent biological activity, or on its own when hazardous substances are present which would pass through the process unaltered.
Q13 What process can you combine with extended aeration in order to provide full treatment in circumstances where, even after the biological process, dangerous chemicals remain?
A.The list is large and requires professional expertise to apply to any particular leachate. The most popular due to low cost, is the use of a reed bed. (Copies of papers are available describing such use, free of charge by emailing steve.last@enviros.com). Other processes which have been applied to address specific leachate contaminants are:- Ozonation, Activated Carbon, Reverse Osmosis and Flash Evaporation.
Q14 What treatment does a reed bed provide?
A.A reed bed provides suspended solids and colour removal. It will also provide some reduction in the concentration of a wide variety of contaminants including ammoniacal nitrogen, BOD and COD, nitrate, iron, to name just a few. However, efficiency will vary seasonally with bed maturity etc. Copies of research papers are available free of charge by emailing steve.last@enviros.com). (Recent Enviros research work has lead to greater confidence in ammonical nitrogen removal rates such that in many instances we are now able to set design parameters for new reed beds with confidence.)
Q15 Nitrates are increasingly being monitored in discharges, and require to be removed. How can this be achieved to avoid this contaminant being present in the extended aeration effluent?
A. Enviros Consulting are leaders in the implementation of efficient denitrification as an additional process stage before discharging the extended aeration (nitrification) treated effluent. Nitrogen in nitrate, produced during initial nitrification of ammoniacal-N, is converted during denitrification to nitrogen gas, which is vented to the atmosphere in this process, so there are no harmful by-products. (Nitrogen is already present in air in substantial quantities.) The denitrification stage can frequently be achieved in the same tank, thus making this technique highly cost effective. However, a carbon/BOD source addition is normally required which will entail some additional running cost. Copies of published papers are available free of charge by emailing steve.last@enviros.com).
Q16 Where can I find out what leachate treatment processes are available?
A.There is some guidance we wrote for the DoE on the types of processes available, in an Appendix to Waste Management Paper 26B (HMSO, 1995), which is unfortunately now rather out of date. However, it is probably the first reference to read on the subject.
Q17 What are the comparative prices for different leachate processes?
There is nothing published that I know of which gives comparative prices between different leachate processes. The fact is that accurate comparative pricing between projects and processes is difficult because leachates vary so much in their strength and the nature of the contaminants. Not only that, the cost of the process is also very closely linked to the discharge water quality required.
We spend a lot of our time investigating leachate discharge routes, negotiating and applying for discharge consents on behalf of our Clients. Until the details, of both the raw leachate quality volume and strength and the discharge route are known, the cost of the necessary process requirements cannot be estimated, and pricing cannot be achieved accurately. Nevertheless, once we have entered into discussions with the EA and Water Company, and have good data and development information for the landfill, we undertake a cost benefit analysis comparing all the preferred treatment process and discharge/disposal route options.
If you look through our project profiles on this website, some prices are given - but hardly enough for an informed decision to be made! Tankerage is always the most expensive, with an average all-cost of £15/m3. "Simple" on-site biological (nitrification) treatment may cost no more than £0.50/m3 and may be cheaper than an untreated sewer discharge (especially when methane stripping costs and a volumetric charging element are included). Where denitrification is required for a very high quality/sensitive watercourse discharge, on-site biological treatment costs may rise to (say) as much as £5/m3. This is still an order of magnitude cheaper than processes such as Reverse Osmosis, which are disliked for other reasons in any event, but which cost £8/m3 to £15/m3
We can also give you the following guidelines:-
1. For all UK sites, a primary biological process is always the cheapest and environmentally the best practical (most sustainable) option. (Unless the site is one of the new hazardous waste landfills (post July 2004) one of very few older sites where sufficient co-disposal of hazardous and toxic waste has taken place, to render the leachate contaminated to the extent that biological action is inhibited by a hazardous or dangerous substance.) It may of course be necessary to add tertiary treatment to meet specific requirements, but biological treatment is normally accepted as the best primary treatment. 2. On a landfill site where leachate treatment is not going to be the primary activity, (nor will the site staff be highly skilled in operating highly optimised plant designs), it is not wise to choose any plant design dependent (minute by minute) on the correct calibration and operation of very sensitive water quality monitoring equipment. (If a watercourse is poisoned for just a few hours, or even less - it may take years to recover.) This is the case whatever the economics of the choice may suggest. The consequences of a treatment plant "failing", are severe. (Please also be aware that something like 1 in 5 of all treatment plants in the UK are currently classified as failing one or more of their consented parameters from time to time. The water industry, therefore, does not have a good record of complying with its promises.) 3. Successful selection of a leachate management strategy is a complex process for all but a few landfill sites, and frequently requires expert advice. Collection, treatment, and disposal are all linked, and in most cases need to be seen as an integrated system, with future long term changes in leachate strength and quality borne in mind during plant selection. Development of new landfill cells, and capping will have major effects on leachate quality, as will changing landfill practices. The consequences of a plant which fails and becomes unusable in wasted investment are massive. It does happen - we have modified, and are still modifying such plants currently, and have in the past "stripped out and started again" replacing inappropriate technology, at many sites!
Q14 How do I collect a reliable water quality sample?
A. We have prepared a separate flow chart from UK Environment Agency guidance to answer this question. See our Water Quality Sampling Flowchart.
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