Leachate Formation

LEACHATE FORMATION

Leachate can pollute both groundwater and surface water supplies. The degree of pollution will depend on local geology and hydrogeology, nature of waste and the proximity of susceptible receptors. Once groundwater is contaminated, it is very costly to clean it up. Landfills, therefore, undergo siting, design and construction procedures that control leachate migration.

1 Composition and properties   

Leachate  comprises  soluble components  of waste and its degradation products enter water, as  it  percolates  through  the  landfill.  The  amount  of leachate generated depends on:

water availability;   

landfill surface condition;   

refuse state;   

condition of surrounding strata.   

The  major factor,  i.e.,  water  availability , is affected  by precipitation, surface runoff,  waste  decomposition  and  liquid  waste  disposal.  

2 Control    

The best way to control leachate is through prevention, which should be integral to the  site design.  In  most  cases,  it  is  necessary  to  control liquid  access, collection  and  treatment, all of which can be done using the  following landfill liners: 

Natural  liners:  These  refer  to  compacted  clay or  shale,  bitumen  or  soil sealants, etc., and are generally less permeable, resistant to chemical attack and have  good sorption properties. They generally do not act as true containment barriers, because sometimes leachate migrates through them.

Synthetic  (geo-membrane)  liners:  These  are  typically made  up  of high  or medium density polyethylene and are generally less permeable, easy to install, relatively strong and have good Deformation characteristics. They  sometimes expand or shrink according  to temperature and age.             

3 Treatment             

Concentrations  of various substances  occurring in leachate are too high to be discharged

to  surface  water  or  into  a  sewer system. These  concentrations, therefore, have to be

reduced  by  removal,  treatment  or both. The various treatments of leachate include: 

Leachate  recirculation:  It  is one of the simplest forms of  treatment. Recirculation of leachate  reduces  the hazardous  nature  of leachate  and helps wet the waste, increasing its potential for biological degradation.          

Biological treatment: This removes BOD, ammonia and suspended solids. Leachate  from land  filled  waste  can  be  readily  degraded by biological means, due  to high  content of volatile  fatty  acids (VFAs).  The common methods are aerated  lagoons (i.e.,  special devices  which  enhance  the aerobic process es of degradation of organic substances over the entire depth  of the tank) and activated sludge process, which differs from aerated lagoons  in that discharged sludge is recirculated  and is often used for BOD  and ammonia removal. While under  conditions  of  low  COD,  rotating  biological  contactors  (i.e.,  biomass is brought into contact with circular blades fixed to a common axle which is rotated) are very effective in removing ammonia. In an anaerobic treatment system, complex organic molecules are fermented in filter. The common types are anaerobic filters, anaerobic lagoon and digesters.

Physicochemical treatment: After biological degradation, effluents still contain significant concentrations of different substances. Physicochemical treatment processes could be installed to improve the leachate effluent quality. Some of these processes are flocculation-precipitation. (Note that addition of chemicals to the water attracts the metal by floc formation). Separation of the floc from water takes place by sedimentation, adsorption and reverse osmosis.