A visit to Hong-Kong and you’ll notice the remarkable technological achievements they’ve achieved to solve the sewage issues some of the fastest-growing cities in India face today. These regular yet strategically planned public toilets. As soon as the first rains hit our soil and our tanks and reservoirs are optimized, the citizens seem to instantaneously forget the water crisis they were facing just a few months ago. 

With global warming showing its effects and the climate cycles encountering a major shift, Indians in the near future should expect monsoons that no longer relinquish the ever-expanding water demands. The unfortunate first signs of this phenomenon being in action is southern farmers are abandoning their fields and factories due to lack of a good monsoon.

Lessons India should adopt from Hong Kong's sewage system - Sheet1
Stonecutters Island Sewage Treatment Works_ ©Drainage Services Department Hong Kong Copyright 2016

As this ever-increasing water demand further rises, borewells and reservoirs are forced to run dry, disrupting the water table. While people are careful and aware of this impending threat, the economic use of water for toilets, today, doesn’t look like a feasible option. Switching to more sustainable options seems like a long-shot, for public toilets as well as ones used for private use. But this is where the lesson from Hong Kong steps in. What users fail to realize is that 85% of the flushed water is seawater and not fresh. 

Hong Kong is the only city in the world to build an entire sewage system dependent on the untreated seawater that surrounds it. Understanding Hong Kong’s political and geographical stand-point, they’ve managed to devise a system that works independently of its context by constructing reservoirs and treatment plants in the sea to support their sewage and drainage needs.

Lessons India should adopt from Hong Kong's sewage system - Sheet2
Hong Kong’s largest ever environmental infrastructure project_ ©Drainage Services Department Hong Kong Copyright 2016

The History of the Solution

According to the Economic Times (Doctor, 2017), this is not such a radical idea. In July 1894, the Times of India (ToI) reported a meeting of the Municipal Corporation (Doctor, 2017) where Dr. NN Katrak suggested that the authorities consider “the experiments carried out at Worthing, in England, with electrolyzed seawater for flushing sewers and water closet and what the probable cost would be required in conducting similar experiments in Bombay.”

A paper in Water and Environment Journal (2006) by S.L.Tang, Derek P.T.Yue, and X.Z.Li (Li, 2006), which compares the engineering costs of freshwater, recycled water, and seawater systems in Hong Kong explains what seems to be a similar system: “In Hong Kong, an electro chlorination technique has been widely applied for the disinfection of such flushing water instead of conventional chlorination processes because seawater contains a large amount of sodium chloride.” 

To put it into context, the seawater is filtered and then an electric current is passed through. This reaction results in sodium hypochlorite which has disinfectant and bleaching properties. In a more recent paper in Water Policy (2015) journal (Ng, 2015), As Dr.Tze Ling Ng notes the advantage of this is that “disinfection deactivates pathogenic microorganisms and suppresses biological growth in pipes.”

Required Investments

While this seems like a feasible option for our coastal cities, seawater demands different infrastructural needs. Regular pipes can’t be used as they may corrode because of the chemical configuration of seawater. To respond to this issue, Hong Kong uses metal pipes lined with an internal cement mortar lining, and even then, Dr. Ng (Ng, 2015) notes that it has been found that “the average frequency of pipe bursts for the seawater system to be three times greater than for the city’s fresh water system. 

Leaking seawater has also led to the degradation of concrete and metal structures and fittings.” The major issue, however, is that the flushing supply system needs to be completely separated from all other water supply linesthis means having a dual supply system. This results in a requirement of skilled plumbing labor as well that can resolve these issues for future maintenance. The monetary investment for the required infrastructure is quite high and it needs a well-planned, city-wide execution strategy to reduce construction costs. 

According to ToI reports, whenever the matter was raised in Bombay, the municipality deflected short-term measures like cutting down on leakage in the existing network. In 1920 (Doctor, 2017), one corporator said that the “proposal to utilize sea-water for road-watering and flushing would take years again to come into operation but Bombay was sick enough of the water famine to be fed any longer on empty hopes.”

Lessons India should adopt from Hong Kong's sewage system - Sheet3
World’s deepest sewage tunnel_ ©Drainage Services Department Hong Kong Copyright 2016

Way Forward

Dr. Ng (Ng, 2015) has done an analysis that compares the costs of using seawater and recycled wastewater for other large coastal cities around the world. The analysis uses formulae that multiply data on construction costs in these cities with estimates, based on the size of seawater network that would be needed, and compares it with estimates of the capital required for wastewater recycling in these cities.

While this exercise can be assumed speculative, Dr. Ng’s (Ng, 2015) conclusion is still striking: three cities top the list for places where a seawater system makes a great deal of sense: Chennai, Mumbai, and Shanghai. The population density of these cities makes the system viable along with the increasing demands of water from their citizens and, particularly in Chennai, the lack of alternative supplies. “These results imply seawater flushing to be likely an economically viable option for these cities that should be seriously considered by policymakers,” writes Dr.Ng (Ng, 2015). 

As a concluding remark, and understanding India’s Water scarcity issues, possible and viable solutions like seawater flushing need to be given a fair thought. And if found to be potential alternatives, research, and development regarding the same should be put in motion.

References

  1. https://economictimes.indiatimes.com/news/politics-and-nation/what-india-can-learn-from-hong-kong-sewage-system/articleshow/59815238.cms?utm_source=contentofinterest&utm_medium=text&utm_campaign=cppst
  2. Comparison of engineering costs of raw freshwater, reclaimed water and seawater for toilet flushing in Hong Kong – Tang – 2006 – Water and Environment Journal – Wiley Online Library
  3. http://repository.ust.hk/ir/Record/1783.1-67588 
Sharvari Rajwaday
Author

Designing for change has always been an aspiration. This is me doing the same through words. My interests in the field include Research, Sustainable strategies and Urban design and hope to work on similar lines in the future.

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