With the slowdown of economic activity due to Covid-19, we witnessed an increase in clarity of waterbodies that were previously polluted. For instance, Venice’s usually dark and murky waterways have turned noticeably cleaner as shown in Figure 1.

A view shows clearer waters by a gondola in a Venice canal on March 17, 2020 as a result of the stoppage of motorboat traffic, following the country's lockdown within the new coronavirus crisis.

Figure 1: A View Shows Clearer Waters by a Gondola in a Venice Canal on March 17, 2020, following the Country’s Lockdown (Pattaro, 2020)

Likewise in Malaysia, the once-heavily polluted Pinang River that runs through Georgetown, Penang is now jade green with algae although it was once pitch black. As what Universiti Sains Malaysia’s Professor Dr Chan Ngai Weng, who researches environmental management and hydrology, said, ” With the MCO, the dumping has stopped. You can see now how a river cleans itself if the pollution load is low.” (Coronavirus: Penang river green again, thanks to MCO, 2020).

Clearly, when pollution load remains low in the absence of human activity for an extended period of time, waterbodies have the ability to clean themselves up, a process known as self-purification.

Self-purification comprises of a series of complex physical, chemical and biological processes.

1. Chemical Processes

  • Redox reaction
  • Complexation reaction
  • Coagulation reaction

2. Physical Processes (Mondal, n.d.)

  • Dilution
  • Sedimentation
  • Re-suspension
  • Filtration
  • Gas Transfer

3. Biological Processes

  • Decomposition of organic substances to basic elements
  • Uptake of substances up the food chain

As there are many processes and talking about how each of them works will take quite a bit of time, I will use the case study of Venice canal to explain how some of these processes work.

In the case of Venice canal, suspended solid pollutants from sources such as domestic and industrial wastewater and runoff from agricultural or urban activities increases turbidity of the water. However, with the Covid-19 lockdown in place and the subsequent reduction of motorboat traffic flow on the canal, these solid pollutants are allowed to settle out and that explains the increased in clarity of the water. Here, sedimentation as one of the nature’s method in removing suspended particles from the water course, have clearly acted its way in self purifying the canal’s water.

As for the Pinang River, it is mainly fouled by organic substances, with all kinds of oil from workshops and factories that emulsified and mixed with organic matter from the wastewater of markets, hawkers and so on. Hence, the main process involved in the self purification is the decomposition of these organic substances by aerobic and anaerobic microorganisms, depending on the availability of dissolved oxygen in water. As the normal growth of flora and fauna was observed in the Pinang River after the lockdown, the breakdown of organic pollutants is most likely aerobic by nature  (Negulescu, 1985)

From these two case studies, what we have learnt is that waterbodies are always constantly carrying out the self cleaning process and it is the activity of human which increases the rate of pollution and causes a breakdown in the nature’s mechanism of self purification. Simply put, sometimes rather than coming up with high cost methods in cleaning up pollutants in waterbodies, the best way is to reduce the introduction of pollutants into these waterbodies.

In short, Prevention Can Be Better than Cure.

 

References:

1. Clifford, C., 2020. The Water In Venice, Italy’s Canals Is Running Clear Amid The COVID-19 Lockdown — Take A Look. [online] CNBC. Available at: <https://www.cnbc.com/2020/03/18/photos-water-in-venice-italys-canals-clear-amid-covid-19-lockdown.html> [Accessed 24 July 2020].

2. The Straits Times. 2020. Coronavirus: Penang River Green Again, Thanks To MCO. [online] Available at: <https://www.straitstimes.com/asia/se-asia/penang-river-green-again-thanks-to-mco> [Accessed 24 July 2020].

3. Mondal, P., n.d. Self Purification Of Natural Water Systems | Water Management. [online] Your Article Library. Available at: <https://www.yourarticlelibrary.com/water/self-purification-of-natural-water-systems-water-management/28273> [Accessed 24 July 2020].

4. Negulescu, M., 1985. Self Purification. Developments in Water Science, [online] 23, pp.53-70. Available at: <https://doi.org/10.1016/S0167-5648(08)71087-6> [Accessed 24 July 2020].