Leachate: Some Basic Facts
Leachate and landfill gas are usually both derived from a similar source: the decomposition of organic matter.
Companies offering solutions to the problem of landfill gas often, also, offer solutions for leachate. The parallel nature of both gaseous and liquid pollutant streams from a landfill site, however, belie the extent of the real differences.
In order to determine appropriate solutions for a landfill gas problem it is necessary to know the quantities of gas involved and the methane concentration. Such data can be obtained from monitoring boreholes and pumping trials.
The parallel with leachate is that it is necessary to know the following:
1. the anticipated flow rate which will require treatment;
2. the composition of the leachate at source;
3. the discharge composition required by the regulating authority.
From this information the technology needed to meet the discharge consent can be selected. There are estimated to be approximately twenty principal technologies that can be employed for leachate treatment.
Each of these may be combined in various modes with other standard chemical engineering unit processes, providing a huge range of viable options from which a solution must be engineered to optimise the balance between cost and quality.
1. The anticipated flow rate which will require treatment
Estimation of the leachate flow rate will depend upon the extent of the objective set when instituting leachate treatment. Where it is deemed necessary to intercept and treat a discrete leachate flow out of a site the rate will be readily available. Where it is planned to remove leachate from the body of the site and control the full leachate potential of the landfill in question it will be necessary to carry out a water balance study.
The leachate flow rate will be based largely upon the extent of water inflow into the site and not biological activity, as is the case in respect to landfill gas. Water inflow can be from rainfall and groundwater as well as biological activity.
Obtaining a reliable estimate of leachate production rates is a considerably more difficult exercise than estimating landfill gas production rates and, unless the site is adequately contained, will require the services of a skilled hydro-geologist. It is likely to involve field studies followed by pumping trials.
2. The composition of the leachate at source
The composition of abstracted leachate can be monitored during the course of such trials and used to form the basis of analysis. Samples should be taken in accordance with best practice and analysed in a suitably equipped laboratory.
3. The discharge composition required by the regulating authority
The extent to which leachate should be treated will be determined by the discharge consent that applies. Such a consent may be obtained from the Environment Agency, for discharge to ground water, or the local water company, for discharge to sewer. There may be other bodies with regulatory powers in certain situations.
It is important to determine the specific objective of any treatment process before chemical engineering analysis is commenced and it is essential that discharge consents be contractually agreed prior to the installation of a treatment plant.
Unlike landfill gas, where combustion is the near universal solution, the range of technologies applicable are directed specifically to treatment goals. If these change in the course of planning or construction a different method of treatment is likely to be needed, possibly resulting in major changes in the proposed plant.
The Method of Treatment
For further information on treatment methods available please refer to the section of the Product Index refering to leachate or the above link to treatment methods offered by Organics. These provide a basic description of the principle options for leachate treatment and the technology involved, the situations in which each can be applied and important aspects to be assessed whilst determining the comparative range of plant capital and operating costs.