Challenges in estimating the safety of emissions from waste incineration
Updated: Mar 21, 2023
Incineration of waste is the process of complete burning at temperatures above 800℃ under controlled air supply. The gases that are produced at the end of burning are then passed
through cleaning devices called scrubbers which essentially filter out pollutants. The gases that come out after passing through scrubbers could still contain harmful products of burning. To keep air pollution in check, regulatory agencies publish maximum allowed concentration on the basis of tests conducted to assess the impact of various pollutants on living beings. Operating an incinerator safely means that emissions need to be periodically tested so that their levels do not exceed these limits. Click here to know more
However, testing and controlling emissions and their impact present a variety of challenges.
A continuous emissions monitoring system is required to control the process and to ensure compliance.
Since the input varies widely, the emissions in the output also vary. A variety of standards need to be adhered to and running a process to meet all of these is quite difficult.
Another challenging feature is the corrosive nature of the gas that waste incinerators produce. To keep corrosion in check, the gases that are sucked into the gas analyzer (for chemical analysis) need to be heated to as high as 1000℃. This means the measuring equipment should be durable enough to handle the hot gases.
While the emissions may be within the limits to pass the air pollution tests, the pollutants may enter life forms via groundwater, soil and result in a phenomenon called biomagnification causing much harm.
Specificity or sensitivity?
Testing techniques can broadly be divided into chemical and biological monitoring methods. Techniques can also be divided based on how sensitive or specific they are. A sensitive technique is one that detects even very small quantities of the pollutant/substance being checked. A specific technique is one that can detect the presence of the pollutant even if it is present among a bunch of other confusingly similar substances. The sweet spot for a reliable test lies somewhere in between.
Chemical monitoring methods like GC MS (Gas chromatography with mass spectrometry)
are highly sensitive techniques which can measure very very small (<10 ppm or parts per million) quantities of analytes. It works by heating the sample being tested to the gas form and then detecting the nature and amount of the pollutant in the gas sample. This is done by comparing with a previously analyzed set called the standards. So as you may have guessed by now, if there is a completely new pollutant, its presence can be missed or wrongly estimated.
Biological monitoring techniques attempt to assess the impact of the pollutant on living beings, e.g. whether it can trigger adaptive mechanisms in a living organism or any other reactions. What are these adaptive mechanisms?
Cells may increase the production of antioxidants that help the organism manage the stress created by pollutant exposure
Cells increase the production of pumps that energetically push the pollutants out so that the kidney can excrete them away
Cells (inadvertently) end up dividing more to reduce the load of pollutant
How are these biological tests carried out?
To do this, cells such as H4IIE (which behave like liver cells) are used which act as a proxy for living organisms. The adaptation mechanisms are set in motion only when a sensor molecule (typically proteins called receptors) has enough exposure to a pollutant to stick to and start reacting. This means that the technique is not as sensitive as GC MS but is highly specific. In other words, it answers the question, “What happens to life when it encounters this pollutant?”
All these are complex multistep processes and cannot easily be recreated in a laboratory. So scientists rely on reporter assays. These assays work by monitoring a single marker gene whose appearance reports whether exposure has happened or not. An interesting advantage is that the test will be positive even if a new pollutant was present as long as it is able to produce the same reporter response as a previously recognized pollutant.
Both types of testing approaches- chemical and biological, need to be used together to get a true picture of the impact of pollutants. If the biological response seems too prominent then the chemical tests and analysis need to be repeated with more standards and a better estimate of the actual amounts needs to be done. On the other hand even if large quantities of a pollutant is detected chemically, there may be little or no adaptive response biologically.
The blog is by Sowmya Raghavan (Member SWMRT) , Support by Divya Tiwari (Member SWMRT)