#2 Is everything right as rain?
Desalination plants, and what happened to rainwater harvesting in Mumbai?
Recently, the Brihanmumbai Municipal Corporation (BMC) announced plans to set up a new desalination plant by 2025 in partnership with IDE Water Technologies, to increase its capacity to supply water to the city of Mumbai. Currently, the BMC supplies around 3900MLD (million litres per day), but the demand is for around 4200MLD. The proposed plant is expected to provide around 200MLD, which will go some way towards reducing the deficit in demand (which would have increased by 2025).
Why this approach should make you salty
Desalination can proceed via a number of different techniques such as distillation, reverse osmosis, and electrodialysis, which are explained briefly here. Reverse osmosis is the dominant technology used, with distillation coming a distant second1.
A common feature of all these methods is their high economic costs, both in terms of initial investments and subsequent energy requirements. For this reason, desalination projects have largely been restricted to high income countries in the Middle East (which also have access to energy reserves), or island countries with limited alternative means of generating potable water. Note that the cost would have to include the setting up of a network of pipelines from the plant to the city. The plant in Mumbai is estimated to cost around Rs 1600 crore.
Another downside is that desalination is inherently a centralized system. This involves higher infrastructure and maintenance costs, along with single-point-of-failure issues. (The current set-up of supplying water to Mumbai from various dams and lakes is also centralized, and the recent supply reduction due to flooding at the Bhandup filtration plant is an example of single-point-of-failure effects.)
Another major challenge with desalination is the safe disposal of salty “brine”, an effluent produced in the process. Desalination takes in seawater, and separates it into streams — a freshwater stream that is the output, and a concentrate waste stream2 that has varying salinity depending on the input salinity, but is around 5 percent salt for seawater (which is around 3.5 percent salt). Even more concerning is the presence of toxins such as chlorine and copper that are used during treatment processes. To get an idea of the scale of brine production, note that, in 2019, while desalination produced, on average, 95 million cubic metres of freshwater, it produced 142 million cubic metres of brine¹.
The methods used for disposal of this brine effluent include direct discharge into oceans, surface water or sewers; deep well injection and brine evaporation ponds3. In the case of coastal projects, as will be the case in Mumbai, it is possible that waste will be disposed untreated directly into the sea. (To be fair, IDE Water Technologies state that their plant will be “environmentally sustainable”, so this is something that can be looked into once they release their initial plan for the plant.)
When hypersaline water is continuously discharged into seas and oceans, there are grave risks to the ecology of the marine ecosystem. As Jones et al¹ state:
The high salinity of brine causes elevated density in comparison to the salinity of the receiving waters, which can form “brine underflows” that deplete dissolved oxygen (DO) in the receiving waters. High salinity and reduced DO levels can have profound impacts on benthic organisms, which can translate into ecological effects observable throughout the food chain.
Concerningly, most of the recent improvements in sustainable brine management are rather expensive to implement. It will be important to ensure that in the quest for “cheaper” operation, we do not damage the Arabian Sea ecosystem, as that could lead to serious knock-on effects.
What about rainwater harvesting?
Mumbai is a city that receives an average annual rainfall of about 2200mm. The area of Mumbai is about 437 square kilometres. This means that a crude estimate (in a 2003 BMC report) of the amount of rainfall falling over Mumbai annually can amount to around 2400MLD of water! This is a significant fraction of the 4200MLD currently demanded by Mumbai residents, though it is also a significant overestimate of the amount of rainwater that can actually be harvested, since not all of Mumbai is developed and terraced or otherwise able to harvest rainwater. The BMC report estimates that the total supply from rainwater harvesting (RWH) can plausibly amount to around 600MLD.
The cost of installing a RWH system in an existing building was estimated to range between Rs 2000 and Rs 30,000 in 2015, with prices going down if it was planned for at the construction stage. Some inexpensive techniques for collecting rainwater at the individual level can be found here.
In 2002, the BMC mandated that all new building constructions covering over 1000 square metres must compulsorily harvest rainwater. In 2007, the rule was made even more stringent, requiring all new constructions covering over 300 square metres to set up RWH facilities. The BMC will deny occupancy certificates to buildings that do not have such facilities.
This sounds great! So why are we still short on water?
Raining on this parade
BMC estimates show that only around 36 percent of new constructions approved between 2007 and 2015 have implemented RWH systems on their premises. This is a huge step down from the expected outcomes of the BMC mandates. Janak Daftari, a water conservationist, says that
developers install an inadequately-designed tank to harvest water, merely for the sake of getting clearances from the BMC. …it is ineffective for harvesting and provides little relief from water stress.
This is an example of unintended consequences arising from not adequately accounting for stakeholders working towards their own self-interests. While constructing a fully functional system is expensive, it is easy (and more profitable) to install the bare minimum for compliance.
Maintaining an RWH system is also a non-trivial task. It is very important to prevent contaminants and pollutants from getting into the RWH storage tanks, and the tanks and system require servicing before every monsoon. There are examples of housing societies where within a few years, the harvested rainwater soon started turning saline and brackish, which started corroding pipes and rusting cars that were washed with the harvested water. The simple solution then was to stop using the RWH system.
A possible patch to counteract the installation of sub-par equipment could be to have a review process, whereby the BMC revisits these new constructions at intervals after issuance of the occupancy certificate and tests the working of the RWH system and the stored water quality. This will clearly need a huge capacity increase from the BMC, and it is also not clear who is to be penalized for test failures — should it be the developer for installing sub-par equipment, or should it be the society, for not maintaining and operating the system appropriately? There might even be external factors like pollution seeping in from a nearby source.
Are there other alternatives?
The 2003 BMC report has suggested various possible alternatives for expanding Mumbai’s water supply, such as the use of ground water and recycling, along with desalination and RWH which we looked into above. The report leans towards RWH being the most viable alternative for Mumbai.
Mumbai’s water supply pipelines are also extremely old, dating back to the British Raj. As such, maintenance is an expensive task, and leaks often develop which lead to water wastage. There is also some amount of illegal tapping which reduces the amount reaching the city. Proper policing of these pipelines, even simply via water pressure monitors placed at regular intervals, could help in this regard.
Demand reduction
Beyond the monitoring, the focus has always tended to be on increasing the supply. Instead, for the rest of this article, we will look into reducing the demand. Economics suggests that reducing the demand while keeping the supply fixed will reduce the quantity demanded in equilibrium.
Economics is a lot about studying peoples responses to incentives. The trouble with making something compulsory is that it also incentivizes people to come up with creative ways to sidestep the mandate (as we saw regarding RWH systems above).
Water charges are often billed to societies as a whole rather than to individuals. The benefit of charging for water individually is that each individual gets to see the amount they are paying for water rather than simply paying for it as part of a larger society maintenance bill. Water bills also provide an opportunities to nudge people into being more frugal, via comparing their usage to previous months and years.
The BMC currently spends Rs 19.44 to supply 1000 litres of water. However, it charges a subsidized rate of only Rs 5.22 per 1000 litres to housing societies, and Rs 4.33 per 1000 litres to slums. One way to reduce dependence on the centrally supplied water is to raise the price. This encourages water to be used for more valuable uses, and reduces wastage.
One major sticking point in raising prices will be that raised prices will disproportionately affect the poorer sections of society. This can be countered by using telescopic rates for pricing (which again works best when bills are individual rather than housing society based). That is, we have low (or even zero) rates for the first few litres of water, that are absolutely essential for every citizen. Then, we have progressively higher rates for water as the usage goes up. This will allow those who can’t do without more water to pay more for it, which makes economic sense as they value it more.
Another issue is that the raised prices will also increase incentives towards stealing water and illegally tapping it. Here is where the increased policing and pressure monitoring mentioned above will come in handy.
The BMC proposed telescopic rates along similar lines in October 2020. Their proposal was to provide 750 litres per household at the subsidized rate mentioned above. Usage between 750-1000 litres would be at four times the subsidized rate, usage between 1000-1250 litres would be at five times the subsidized rate, and usage above that would be at six times the subsidized rate.
Theoretically, this seems like a step in the right direction. In fact, stopping at 6X could be considered to have the effect of encouraging households who land in that bracket to be more wasteful since the rate is not increasing anymore. So rates could instead keep being increased every 250 litres.
The other question was whether the 750 litre subsidy was too high. The BMC calculates this assuming 5 people per household, and 150 litres per person. Using a single bucket for a bath is twenty litres, and using the shower is more. The standard flush tank size in India is ten litres. These are areas where there can be reduction in usage by the people. However, changing standards such as the ten litre tank might be tough.
If the BMC wants to incentivize RWH system adoption, it could gradually reduce the 750 litre subsidy, and use the increased revenues to provide subsidies for the installation and maintenance of RWH systems. This could help increase ground water levels in Mumbai, which is a positive externality.
The BMC proposal, however, was shot down across party lines, which shows how something that might make economic sense, might not make political sense. To quote Jean-Claude Juncker,
“We all know what to do, but we don’t know how to get re-elected once we have done it.”
Edward Jones, Manzoor Qadir, Michelle T.H. van Vliet, Vladimir Smakhtin, Seong-mu Kang, “The state of desalination and brine production: A global outlook”, Science of The Total Environment, Volume 657, 2019, Pages 1343-1356, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2018.12.076. (https://www.sciencedirect.com/science/article/pii/S0048969718349167)
Wenten, I. Gede, et al. "Integrated processes for desalination and salt production: A mini-review." AIP Conference Proceedings. Vol. 1818. No. 1. AIP Publishing LLC, 2017.
José Morillo, José Usero, Daniel Rosado, Hicham El Bakouri, Abel Riaza, Francisco-Javier Bernaola, Comparative study of brine management technologies for desalination plants, Desalination, Volume 336, 2014, Pages 32-49, ISSN 0011-9164, https://doi.org/10.1016/j.desal.2013.12.038. (https://www.sciencedirect.com/science/article/pii/S0011916414000071)