Study by research group at IIT-Delhi says contaminants may affect subsoil and groundwater if not treated cautiously

As Delhi’s civic bodies prepare to biomine their landfill sites, a research group at IIT-Delhi —professors Manoj Datta, G.V. Ramana, T.R. Sreekrishnan and research scholars Mohit Somani and Garima Gupta — working in the areas of geo-environment and biochemical engineering, offer their view on the project in an email interview to Sidharth Ravi. Excerpts:

What are some of the difficulties that might come up during a biomining exercise and what would be their possible consequences?

Biomining is an appealing solution as it offers the option of progressive reduction in height of waste dumps and their eventual disappearance. A small part of the recovered material can be used for recycling or generating energy. The bulk [more than 50% to 60%] of the segregated material produced by biomining looks like soil and gravel. Unfortunately, they contain several contaminants which were present in the original waste, even though the waste has undergone biodegradation as well as washing, due to infiltration of rainwater over the years. Reuse or disposal of this ‘contaminated soil-like material’ is the biggest challenge. In several countries, this material, often referred to as ‘secondary soil’, have specific guidelines for their off-site applications.

What are the findings of your research on leachate characteristics of ‘soil-like material’ from Indian landfills?

Our study on four-five Indian dump sites has shown that the soil-like material from mining of MSW [municipal solid waste] releases brownish leachate when rainwater percolates through it. The leachate has high soluble salts and elevated heavy metals in comparison to the local soil. These findings are supported by results of overseas researchers.

If excavated material is used without further processing or use of liners what would happen?

If the soil-like material is spread in thin layers, say a few inches or less than 30 cm, its impact is likely to be minimal. However, if it is placed in thickness of several metres, as in filling low-lying areas or embankments, or tens of metres, as in deep-mined pits, the contaminants can affect the subsoil as well as the groundwater through colouration, increase in salinity and elevation of heavy metals. In such cases, the importance of liner systems, cover systems and leachate collection systems cannot be under emphasised.

What kind of difficulties may be faced in filling a mine with excavated landfill material?

The base of deep pits, created by mining in the past, is close to the groundwater table. The fractures and fissures in the base rock act like small conduits interconnected to the groundwater. Placing excavated and segregated soil-like material from landfills on the base of these pits, in close proximity of the water table, without polluting it, is a daunting task. Single liners such as clay, geomembranes or geosynthetic clay liners alone would not be enough to stop leakages. Double composite liner systems, with leachate collection as well as leakage detection systems (similar to those used at the base of hazardous waste landfills) would be desirable. Placing such liners along the steep side slopes of the pits as well as along the jagged base of the pit would be complex and expensive.

Any other observation on the exercise being undertaken by the municipal corporations of Delhi…

Biomining is a slow process involving the use of rotating screens (trommels) to segregate the material. A critical step relates to drying of the wet excavated waste by spreading and aeration (sun-drying) prior to screening. This is a natural process which takes time. Trommel screening of dry waste is desirable for good quality of segregation, but it produces dust. If the waste is not dried adequately, it forms lumps and clods which clog the screens of trommels. A fine balance is needed to prevent dust emissions or the entire operation has to be performed under a gigantic cover. The process can be hastened by using several trommel screening plants concurrently, if adequate land area is available. But it is still likely to take several years to process the entire waste at any large dump site of 50 m height, keeping in view the fact that operations are interrupted during the monsoon season.