The Tala tank, also spelled Tallah tank (Bengali pronunciation: [ˈʈala tæŋk]), is a water tower in Kolkata, West Bengal, India. Construction started in 1909 and it was inaugurated in May 1911 by Edward Norman Baker, the Lieutenant Governor of Bengal. The tank, which is owned by Kolkata Municipal Corporation, is fed by Palta Water Works near Barrackpore. More than 110 years after construction, the tower remains the major water supplier to the city of Kolkata.

The water tower, which is claimed to be the world's largest overhead water reservoir, covers 3–4 acres (12,000–16,000 m²), has a capacity of 9.9 million imperial gallons (45,000 cubic metres), stands 110 ft (34 m) off the ground and weighs 44 thousand tonnes – including water – at maximum capacity. The tank has four individually isolated chambers and a single pipeline for the water source from Palta and to send the water supply to the city.

The steel was imported from the United Kingdom and is of similar quality to that which was used to build the RMS Titanic. It has survived multiple calamities including the 1934 Nepal–India earthquake, World War II Imperial Japanese aerial bombings from 1942 to 1944 and Cyclone Amphan in 2020.

The water tower has undergone renovations since its centenary, under the consultancy of IIEST Shibpur, Jadavpur University, IIT Kharagpur and Central Electrochemical Research Institute at an estimated cost of ₹250 million (equivalent to ₹420 million or US$5.3 million in 2023). The renovations were carried out one chamber at a time to prevent interruptions in the city's water supply.

Before pumped water supply to Kolkata began in 1820, the city's water was mainly sourced from the Lal Dighi^{[lower-alpha 1]} at B. B. D. Bagh, which the British authorities considered was the only hygienic water body in the area. In 1820, as demand for water grew, a small pump house was built at Chandpal Ghat to draw water from the Hooghly River. Water was supplied to a limited area through open brick aqueducts.^[3] The first drinking water was supplied to the city with the establishment of a small water treatment plant at Pulta Water Works, which started operating between 1864 and 1870 at Pulta (now Palta), a neighbourhood near Barrackpore and then 32 km (20 mi) north of Calcutta (as the city was then called – now the neighbourhood is part of North Kolkata).^[4][5] The supply reached the city via Tala and the first pipeline was installed in 1868^[6] at a cost of ₹660,000 (equivalent to ₹290 million or US$3.6 million in 2023), having a capacity of 6,000,000 imperial gallons (27,000,000 litres) to cater a population of approximately 400,000.^[3]

Initially, Palta Water Works fed an underground reservoir at Wellington Square (now Subodh Chandra Mallick Square) in the city centre. It had a capacity of 790 thousand imperial gallons (3,600 cubic metres) which gradually became insufficient for the growing population. Supply from underground reservoirs proved to be challenging in terms of cost and time, and because of the very flat landscape in and around the city there was no suitable place to build a high reservoir. To ensure uninterrupted water supply to the city, the Assistant Engineer of Calcutta Corporation (now Kolkata Municipal Corporation or KMC), Arthur Peirce, conceived what became Tala tank in 1901.^[7] The Chief Engineer of KMC, W. B. MacCabe, designed the tank in 1902. Having a nine-million-gallon water reservoir overhead was a daring proposition.^[8] Edward Norman Baker, Lieutenant Governor of Bengal, commissioned the tank on 18 November 1909.^[9]

Philanthropist Babu Khelat Ghosh donated around 7 bigha^[10] [3–4 acres (12,000–16,000 m²)] of land for the construction.^[11] The lane adjoining the tank was later named Khelat Babu Lane. M/s T K Mukherjee and Co.^{[lower-alpha 2]} did the piling work. The foundation of reinforced concrete was built by Rajendra Nath Mookerjee's company M/s Martin and Co. It was heavily ballasted and compressed using steamrollers. A barrier wall was built with two rows of 25–30 ft (8–9 m) long piles and concrete around the plinth so it did not spread.^[8] Raw materials, wood and special anti-corrosive steel plates were brought from Burma (now Myanmar) and Middlesbrough, England,^[15] respectively, and fabrication works were carried out on-site.^[16] M/s Clayton, Son and Co. of Leeds, England carried out the work of fabricating and erecting the steel columns and tank. The flat top cover was made up of 66 millimetres (2.6 in) thick fine concrete, which was provided by Arracon Co. and Babu Kali Sunkar Mitter.^[17] The construction work was completed by 12 January 1911 at a total cost of ₹1.1 million (equivalent to ₹360 million or US$4.6 million in 2023).^[6][7] With new main pipelines and installations, the total cost of the tank was around US$1.55 million in 1911 (equivalent to $43.05 million in 2023).^[18]

Connections from Palta waterworks to the Tala tank were made with 42 in (110 cm) diameter cast iron pipes that were laid along Barrackpore Trunk Road. Pumps in a large building at Palta drew water from the river and lifted it into large masonry tanks and the water reached the city under gravitational force at a rate of 11.5 inches per mile (18 cm/km).^[4][19] The tank was designed to work as a water reserve for the main lines in the city.^[8] It connected the Wellington Square reservoir via Circular Road and Dhurramtollah (now Dharmatala).^[18]

Tala Tank is claimed to be the largest overhead reservoir in the world^[20] was constructed on the 250-year-old^[40] Barrackpore Trunk Road. It is a square, 110 ft (34 m) high, 321 ft (98 m) wide and 16 ft (4.9 m) deep structure,^[9] that has a floor area of 321 sq ft (29.8 m²).^[8] It is made of Burmese teak wood^[7] and steel of the same quality that was used in RMS Titanic.^[41] Because the soil's bearing capacity is limited due to silt below Kolkata, the tank was built upon a 2.5 ft (0.76 m) monolithic concrete bed weighing 26 thousand tonnes. The structure's piling uses 20–25 ft (6–8 m) logs with fragmented bricks (or "Jhama Khoa" in Bangla).^[8] The 288–295, 88.5 ft (27 m) columns, which are embedded on a plinth that supports the tank, are made of galvanised steel. This steel structure has 49 four-legged trussed grouped columns, 14 six-legged trussed grouped columns, one nine-legged trussed grouped columns and one seven-legged trussed grouped columns supporting the source and supply pipe. The bottom of the reservoir rests on wooden sleepers above the columns with no bolts or welding.^[7] The columns are braced in groups of four but no groups are inter-braced to provide flexibility to the structure and prevent it from vibrating.^[42] It has American roofing material, supported with steel beams (resting on steel joists) that helps keep the water cool and clean.^[43][44] The eaves are covered with brass gauze to deny access to birds and insects.^[8][17]

The tank is unique; it acts as a balancer.^[8] Having a single 60 in (150 cm) diameter mild steel riveted water source and a supply pipe that fills the tank during off-peak hours^{[lower-alpha 3]} and stores water in four equally sized, isolated chambers and supplies it to the city and other major underground reservoirs according to demand. The water pressure from Tala assists the pumps to fulfil the main line's demand.^[18] The chambers were made independent by cross frames to individually facilitate maintenance in each chamber without hampering the water supply.^[20] The pipes open into a common compartment, which is fitted with eight penstock gates (two gates for each storage chamber).^[17] The total capacity of the tank is 9.9 million imperial gallons (45,000 cubic metres). It weighs 36 thousand tonnes empty and 48.5 thousand tonnes when full.^[8] The whole tank with the support weighs 44 thousand tonnes while the steel components weigh 8,500 t.^[6][7] Tala Tank can store enough water to satisfy the needs of Kolkata and the adjacent residential-cum-business district Salt Lake City for two days.^[16]

• Water supply and sanitation in India