Introduction

The best solution to the potential leachate problem is to prevent its development. Landfill leachate generation cannot in practice be entirely avoided. However, a tight soil cap and/or liner on completion can greatly minimize the possibility; however, a leachate-free landfill may not be entirely desirable.

Leachate control measures for groundwater and surface-water quality protection must be incorporated in the site design and monitoring started before operation. A water balance for the landfill disposal facility should be established to serve as a basis for the design of leachate control and surface runoff systems, taking into consideration heavy rainfall, landfill, cap construction, in addition to runoff, infiltration, and evapotranspiration.

Leachate from existing community sanitary landfills and from industrial waste storage and disposal sites can be expected to contain organic and inorganic chemicals characteristic of the contributing community and industrial wastes. Household hazardous wastes may include small amounts of cleaning solvents, paints and paint thinners, oils, pesticides, and drugs if not restricted or their sale prohibited.

Leachate Generation

The precipitation less runoff, transpiration, and evaporation will determine the amount of infiltration. Infiltration and percolation will, in the long term, after field capacity has been reached, determine the amount of leachate, if any, produced, assuming groundwater and lateral flow are excluded. A major factor is a cover material that is carefully graded, which ideally permits only limited infiltration and percolation to support vegetative cover and solid waste decomposition, with optimal runoff but without erosion to prevent significant leachate production.

The soil cover should have a low permeability with low swell and shrink tendency upon wetting and drying. Runoff depends on rainfall intensity and duration, permeability of the cover soil, surface slope (4 percent, not greater than 30 percent for side slopes), condition of the soil and its moisture content, and the amount and type of vegetative cover. Evapotranspiration during the growing season for grasses and grains may be 20 to 50 in.

Leachate Control

It should be noted that if all infiltration is excluded and the solid waste kept dry, biodegradation by bacteria, fungi, and other organisms will cease and the solid waste will be preserved in its original state. Bacterial activity will generally cease when the moisture content drops below 14 to 16 percent. The maintenance of an optimal amount of moisture in the fill, as in controlled composting (an aerobic process), is necessary for biodegradation (an anaerobic process in a landfill), methane production, final stabilization, and possible future recycling of the solid waste or reuse of the site.

The objective in the design of landfill liners is to minimize or eliminate the infiltration of leachate into the subsurface soils below the landfill so as to eliminate the potential for the groundwater contamination. A number of liner designs have been developed to minimize the movement of leachate into the subsurface below the landfill. One of the many types of liner designs is shown in the figure below.

Leachate Recirculation

Waste biodegradation and stabilization of the biodegradable organic matter in a landfill can be accelerated by leachate recirculation. Controlled leachate recirculation, including nutrient addition to maintain optimum moisture and pH, can enhance anaerobic microbial activity, break down organics as shown by reduced total organic carbon (TOC) and chemical oxygen demand (COD), convert solid waste organics to methane and carbon dioxide, and precipitate heavy metals. Complete biological stabilization can be achieved in four to five years. Heating of recirculated leachate to $86^oF$ ($30^oC$) has been found to accelerate the stabilization process.

Leachate Treatment

Leachates containing a significant fraction of biologically refractory high-molecular-weight organic compounds (i.e., those in excess of 50,000) are best treated by physicochemical methods, such as lime addition followed by settling. Leachates containing primarily low-molecularweight organic compounds are best treated by biological methods, such as activated sludge. Combinations of these methods may be required to achieve permit requirements and stream discharge standards. In the final analysis, the treatment required will depend on the composition of the fill material, leachate volume and characteristics, and the water pollution control standards to be met.