India is suffering "the worst water crisis in its history", according to a June report by government policy think tank NITI Aayog.
Worsening water shortages - for farmers, households, and industry - threaten the lives and incomes of hundreds of millions of Indians, and the economic growth of the country, the report said.
An estimated 163 million people out of India's population of 1.3 billion - or more than one in 10 - lack access to clean water close to their home, according to a 2018 report by WaterAid, an international water charity.
For Parthasarathy, that's a good business opportunity, and a chance to add to his fleet of 15 water trucks as the Tamil Nadu state government struggles to meet growing demand for clean water.
The port city of Chennai needs 800 million litres of water a day to meet a demand for water, according to official data.
At the moment, the government can provide only 675 million litres, according to the Chennai Metropolitan Water Supply and Sewerage Board.
Like many Indian cities, Chennai and its suburbs plug that gap by buying water, encouraging residents to dig backyard borewells, or using private wells.
In particular, Chennai depends on more than 4,000 private water tankers for its everyday water needs. The government also runs a "dial a tanker" service to meet demand.
According to the Chennai Private Water Tanker Lorry Association, which has more than 1,000 members, each tanker makes up to five trips a day, ferrying water from the outskirts of the city to apartments, hotels, malls, and offices.
Altogether, the tankers deliver 200 million litres of water a day to Chennai, according to the association.
"There are neighbourhoods that depend on tankers throughout the year with no access to government water pipelines," said Shekhar Raghavan, director of the charity Rain Centre, which encourages rainwater harvesting and water conservation in urban areas.
That, he said, has "given rise to the water mafia, which has total control over who will get how much water in the city".
Tankers identify good groundwater sources in agricultural areas in neighbouring districts, pay a small fee to access the water and then sell it for 50 times that cost, Raghavan said.
But what Raghavan called "indiscriminate" - and in many cases unauthorized - extraction of groundwater is creating growing problems as supplies run short.
In 2014, a state government directive restricted the use of such tanked-in water for commercial purposes, though the implementation of the order was erratic.
Hundreds were fined - but the amount of water being delivered has continued to grow, by about 25 per cent since 2014, the water lorry owners association estimates.
Drivers argue that they are providing a crucial public service and plugging leaks in government delivery mechanisms.
"Technically we are providing water free and helping people.
We are quenching the city's thirst," Parthasarathy said.
His company's water comes from 50 kilometres away from the city, and the fee charged to buyers is 2,000 rupees ($27.20) for 24,000 litres, he said.
That means "we only charge for transporting water," he said.
But critics say the firms' use of rural water is depriving people in those areas of sufficient water - and that fast-depleting water supplies mean it's time to rethink how water is managed in Chennai.
Some residents say they are being held to ransom by private water distributors, as they have no option but to pay and no prospect of being hooked to government-run water supplies soon.
"We are totally dependent on water lorries and they are taking advantage of that fact," said Raman Duraiswamy, convener of the Sholinganallur constituency welfare association, an umbrella body that has 600 resident associations as members.
"We have paid a lot in taxes and for the last five years have been waiting for the government to deliver on their promise of providing us piped water."
The associations are now contemplating going to court to try to fix what Duraiswamy called a "perennial problem".
Last month, the Madras High Court - Chennai's highest court - finally ruled on a petition brought by 75 drinking water bottlers against the 2014 Tamil Nadu water restrictions.
Those restrictions cut off the companies' access to groundwater in areas identified as "critical" and "dark zones", where the water table is overused or very low.
Bottlers had demanded exemption from the order, arguing that good monsoon rains would adequately replenish disappearing groundwater.
The court found otherwise.
In its decision, judges called groundwater the "backbone of India's drinking water and irrigation system" and ruled that companies extracting it for profit without permission were engaged in criminal "theft".
Overuse of rural groundwater was threatening food production and the country's food security, the court ruled.
The strongly worded order has stirred debate on the city's dependence on water tankers and packaged water - particularly as Chennai's government has yet to settle on alternatives.
R. Neelakandan, a spokesman for the Chennai water board, said feasibility studies were underway on new water sources and a new desalination plant was in the pipeline.
The plant would provide 150 million litres a day of water to Chennai and its suburbs, and is expected to be running in the next five years, he said.
But the alternatives "will take time and right now we are waiting for the (November) monsoon rains to fill up the dry lakes and ease the situation," he said in a telephone interview.