It is mostly just a strongly organically (vegetable matter) contaminated effluent comprising which although more strongly contaminated than almost any other industrial effluent, still consists almost entirely of water.
The risk that leachate poses is that it will damage the environment if it escapes. Modern landfills are always designed to prevent liquid from leaching out and entering the environment. However, if they are not properly managed, the leachate is at risk for mixing with groundwater near the site, which can have polluting effects on surface water and groundwater. It has sometimes polluted wells, making it unfit to drink.
Leachate characteristics are determined by the waste disposed in the site, the landfill design, and the mode of operation at the landfill, as well as the age of the landfill itself. Young landfills (<5 years) tend to have an acetogenic leachate with a low pH as well as the highest levels of COD, BOD and heavy metals. In contrast older leachate leachate which is said to be a methanogenic will usually have slightly higher ammonia but less of the other contaminants seen in younger leachate. Ammonia is hard to treat in leachate, and at times (especially in arid climates) the salinity can also be a factor, no matter the age of the landfill.
In many landfills the leachate is collected at the base of the landfill and instead of being sent away for treatment at first, it is recirculated back through the landfill. This normally increases the rate at which the waste material decomposes, improves landfill gas production, and the leachate itself becomes older and easier to treat after recirculation.
Leachate Plant Operation
Landfill operators must always carefully monitor the groundwater and local streams to be sure that, the landfill remains fully sealed to the original design.
There are many landfills which have their own leachate treatment plants, and the best are designed so that their operation is automated. The operating condition of the plant is displayed at all times on a flow chart and instrumentation diagram, and the system is equipped with alarms to ensure that the operational staff can react quickly in the event of a malfunction.
Even without staff intervention the best leachate treatment plants are set up so that any operational faults stop the plant and the specific cause of the fault is displayed to the staff in charge to remedy before any pollution can occur.