Using waste to treat waste: Landfill leachate & biochar?

Waste companies use different strategies to handle landfill leachate, the unpleasant cocktail produced by waste in landfills. A relatively new strategy involving waste-based adsorbents joins the mix. 
Until fairly recently, landfills have been our go-to solid waste disposal method. These days however, there is growing interest in identifying practical and cost-effective ways to minimise the amount of waste going into landfills or open dumpsites– preference is given to waste recycling or valorisation (i.e, turning waste into a resource). Many of us already do this on a small scale, when we choose to DIY our own compost. Or transform this afternoon’s roast turkey into tomorrow’s sandwich filler. Or reuse our shopping bags. Or use clear beer bottles as peanut containers, as is often the case in Nigeria. On a larger scale, many countries are thinking hard about sustainable waste management. For example, an old pair of trousers has potential to be more than landfill material; it can be a source of heating if repurposed into insulation material. If it is incinerated, that old trouser also has potential to supply electricity and/or heat to a home.
But back to landfills: used globally, landfill types vary depending on the waste type, protective barrier materials used in their construction, and length of time they have been in existence. For example, a sanitary landfill system is typically used to dispose our municipal solid waste (e.g., household and commercial rubbish). As this waste decomposes however, gaseous and liquid (leachate) products are released in quantities and concentrations that are dependent on the waste type, season, and general landfill conditions, the liquid product (leachate). Leachate from typical municipal waste landfill is a mixture of organic compounds, ammonia and inorganic compounds. In other words, a nasty cocktail that is capable of polluting surface and ground water. Consequently, leachate collection and treatment is essential. Some strategies used by waste companies are briefly outlined below:

1. Physicochemical treatment: Quite like a ‘divide and conquer’ strategy that makes the most of the physical and chemical differences between the species present in leachate. For example, separating species by virtue of their different physical sizes (ultra- and nano-filtration), chemical sizes and structures (air stripping, oxidation, precipitation, ion exchange, adsorption).
2. Biological treatment: This is achieved by creating conducive environments for microbial organisms (microbes). For instance, some microbes perform best in the absence of oxygen and at certain temperatures and moisture levels. Biological treatment plants like activated sludge reactors take all of these into consideration. As a result, microbes help convert toxic nitrogen forms such as ammonia into molecular nitrogen via nitrification and denitrification reactions.

Each of these solutions face some challenges however, ranging from cost and technical complexities to the production of a new set of products which require further treatment. Adsorption, one of the strategies abovementioned, involves filtering leachate through zeolites or through enhanced coal- or wood-derived materials termed activated carbon. Owing to the high production costs involved with such adsorbents however, there is growing interest in using cheaper alternative adsorbents.
This brings us to biochar, a carbon-rich product that can be produced from a very wide variety of waste material. Researchers have shown that in some cases, biochars (enhanced or as-received) are capable of recovering nutrients and removing contaminants from various media – wastewater, soil, compost. This suggests that such biochars may also be capable of removing some of the problematic species present in landfill leachate. Such biochars would most likely require some form of modification to perform as effectively as commercial activated carbons. Recently, a group of researchers evaluated the potential for using a combination of biochar and municipal waste to treat landfill leachate and minimise greenhouse gas emissions. It is also worth considering how the contaminant-loaded biochars will be effectively disposed of.

It will be interesting to compare biochar performance with that of activated carbon on a gram per gram basis including the cost implications. It’s also preferable to assess their performances within controlled settings such as laboratories as well as onsite, reason being the former settings are not always accurate predictors of field performance. Intuitively, biochar adsorbents should cost less than activated carbon adsorbents since the starting material (waste) is of lower value.

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