Visitors to Hong Kong must rarely stop to think about the toilets. These aren’t the high-tech, multi-function, medical monitoring toilets of Japan that we’re talking about, but regular flushing commodes. And yet the toilets used in most of Hong Kong represent a remarkable technological achievement – and one with important lessons for some of the fastest growing cities in India . As a good monsoon fills up tanks and reservoirs across the country it also seems to wash away all the fears of water failure that underlie every Indian summer.Yet every year as the climate changes just a bit more and the demand for water grows a lot more, our dependence on this annual replenishment becomes increasingly dangerous. We need to plan for when the gift of the monsoon will no longer be enough. In parts of South India that time has come. The drought in the southern districts of Tamil Nadu, and even parts of Kerala which used to enjoy good rains, has become so bad that farmers are abandoning their fields and factories are being closed.Many reservoirs are now just expanses of baked earth and the borewells that are dug ever deeper and more desperately are pumping dry. And yet the demand for daily water use continues in densely populated cities like Chennai . People do economise, washing their cars less often, using wastewater for their gardens, but it’s hard to avoid some uses, most notably of toilets. Environmentalists quite rightly decry flushing toilets for literally sending gallons of scarce water down the drain, but once people are used to them it’s really hard to imagine switching to earth toilets and similar sustainable versions. Self-contained, minimal water using sewage might be possible on a farm, but hardly when you’re staying on the 10th floor of an apartment block.But this is where those toilets in Hong Kong are worth examining more closely. Because what casual users fail to realise is that the water that flushes through them is, for at least 85% of homes in the enclave, not fresh water, but seawater. Other cities use treated wastewater for sewage, or desalinated seawater, but Hong Kong is the only large city in the world that has built a whole sewage system around the use of minimally treated water drawn from the sea that surrounds it. It is a remarkable achievement that began back in the 1950s. With the Communists consolidating power on mainland China, refugees crowded into the then British enclave, putting unprecedented demands on the urban infrastructure. City planners realised that the existing water supply systems, which depended on rainwater flowing into a few existing reservoirs, would clearly fall short in the near future, especially since rapid construction was cutting into the older catchment areas. What’s left unsaid, but can be inferred, in accounts of Hong Kong water supply history is that the city’s isolated position at the edge of a hostile mainland must have made concerns about water supply even more acute. In similar situations places like Israel or Gibraltar have also developed innovative solutions to prevent being held hostage over water supply. Hong Kong came up with the idea of reservoirs built into the sea, wastewater treatment and using seawater for flushing toilets.This is not such a radical idea. Ships have used seawater for this purpose for ages, and the British tried using seawater in some coastal towns in the UK, an experience which was noted during their time in Bombay. In July 1894, for example, the Times of India (ToI) reported a meeting of the Municipal Corporation where Dr NN Katrak suggested that the authorities consider “the experiments carried out at Worthing, in England, with electrolysed seawater for flushing sewers and water closets… what the probable cost would be in making similar experiments in Bombay.”A paper in Water and Environment Journal (2006) by S.L.Tang, Derek P.T.Yue and X.Z.Li, 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 electrochlorination 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.” The seawater taken in is simply filtered and then has an electric current passed through which results in creation of a certain amount of sodium hypochlorite, which has disinfectant and bleaching properties. As Dr.Tze Ling Ng notes in a more recent paper in Water Policy (2015) journal the advantage of this is that “disinfection deactivates pathogenic microorganisms and suppresses biological growth in pipes.” Seawater treated in this way would seem to be particularly suited to use for flushing toilets – a position predicted by some a writer in ToI in 1898 who, in the wake of the plague of 1897, recommended that “large quantities of sea water should be introduced at the head of the sewers and drains….” envisioning a purging of the sewage system with seawater.Seawater does have some problems. It is corrosive, so can’t be used with regular pipes. Hong Kong uses metal pipes lined with an internal cement mortar lining, and even then Dr. Ng 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 freshwater system. Leaking seawater has also lead to the degradation of concrete and metal structures and fittings.” PVC pipes, particularly for the home connections, are now able to help with this, but they are also more prone to breaking. The really big issue is that seawater flushing requires a completely parallel system to supply it to toilets, though it can be mixed with other wastewater as it goes out. In practical terms this means Hong Kong has two types of plumbers – those who deal with regular issues, and those who can handle the seawater system. The capital cost of creating of such a system is also high and, unless one plan for and develops it as the city expands, as Hong Kong did, it will require much painful digging and installation work. This is what has always sunk the use of seawater. Reports in ToI show that, whenever the matter was raised in Bombay, the municipality balked at the cost, and turned to short term measures like cutting down on leakage in the existing network. In 1920, for example, 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.” Decreasing leakages and increasing efficiency of water use are all important and necessary measures, but in the end a basic supply is needed and we cannot forever hope to depend on the monsoons for this. Desalination and treating wastewater are possible solutions, but are expensive and energy intensive. Seawater is obviously only a possible solution along India’s coastline, but this includes huge cities like Mumbai and Chennai and very fast growing areas like Kochi and Goa.Dr Ng has done an analysis that compares the costs of using seawater and recycled wastewater for other large coastal cities around the world, including Buenos Aires, Chennai, Jakarta, Karachi, Los Angeles, Miami, New York, Osaka, San Francisco, Shanghai, Singapore, Sydney and Tokyo. The analysis uses formulae that multiply data on construction costs in these cities with estimates, based on Hong Kong’s system, of the size of seawater network that would be needed, and compares it with estimates of the cost wastewater recycling in these places.Such an exercise can only be speculative, but Dr Ng’s 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 density of these cities makes the system viable – and to that one can add the increasing demands of water from their populations and, particularly in Chennai, the lack of alternative supplies. “These results imply seawater flushing to be likely an economically viable (albeit unconventional) option for these cities that should be seriously considered by policymakers,” writes Dr.Ng. At the least there is a strong case for trial projects in the townships and large residential developments coming up close to the coast near the East Coast Road in Chennai or Navi Mumbai. (Pondicherry, whose water woes are as bad, though often overlooked, as Tamil Nadu, might make for another interesting site). Given the long term severity of India’s water woes we cannot afford to let possible solutions like seawater flushing just disappear down the drain.