A Study of Water Treatment at the Bhandup Water Treatment Plant, Mumbai.

Transcription

1 A Study of Water Treatment at the Bhandup Water Treatment Plant, Mumbai. Arshia Mehta May 2017 CAID

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3 The water treatment in Mumbai is carried out by the Municipal Corporation of Greater Mumbai. The Municipal Segment includes the workings of the city administration under the Government. The Water Treatment Plant (WTP) is spread over an area of 650 acres. This plant, treats the water brought from 7 dams, each about a 100 kilometers away, and then supplies that for drinking, industrial and agricultural purposes to the entire city of Mumbai. Bhandup has 7 sources of raw water- Upper Vaitarna, Modak Sagar, Tulsi, Tansa, Vihar and Bhatsa. Bhandup receives approximately 4,000 Million Litres per day (MLD) of water. Out of this amount, approximately 3,750 MLD gets treated each day. Approximately, 50% of this water supply (1910 MLD) comes from Bhatsa which is a major source. Each of these dam sites (with the exception of Vihar) is approximately 100 kilometers away at a higher level of elevation. Due to difference of elevation between these Dams and Bhandup WTP, the water flows under gravity, thus resulting in saving of energy (as pumping of large quantity of water is not required). The water from these dams are conveyed through an above-grade pipeline, which is about 108-inch in diameter. There is a great amount of pressure involved in transporting the water through the pipelines. Figure 1: Flow of Water through Mumbai 1

4 However, none of the water is stored in Bhandup. This treatment plant operates 24 X 7, 365 days a year and the water continuously moves from one stage of treatment process to another. In addition to the WTP at Bhandup, another water treatment plant, known as the Pandharpur Water Treatment Plant, located in Pandharpur, is where the raw water (natural, untreated water found in the environment) gets treated and sent to a reservoir, after which it gets delivered to the Eastern Suburbs of Mumbai. The basic process of water treatment at Bhandup is as follows : There are 6 main steps in which the raw water is treated at the Bhandup WTP. 1. Pre-Chlorination Step. This takes place at the Awai Chlorination Plant, which is located in the Awai Village, before the water reaches the Bhandup WTP. In this step, chlorine is injected in the gaseous form through cylinders in kg/hr. There is a fixed quantity of this dosage determined by the quality of water. The quality of water is tested through samples in the laboratory. 2

5 I was curious to see why the pre-chlorination dosage was given in addition to the usual post-chlorination dose. On further research, I realised that Pre-Chlorination is the addition of chlorine to raw (untreated) water. This produces residual chlorine which is useful in many stages of the treatment process. It is used to: improve the coagulation, control algae problems in the sedimentation plants, and reduce odour, taste and colour. It also helps reduce the load on filters and acts as a disinfectant. I was surprised to learn that the pre-chlorination treatment is less common in Western Countries. This is because a longer contact time with chlorine leads to the production of carcinogenic substances known as trihalomethane (THM) which causes huge health concerns. It is only used in those plants where THMs are not formed. 2. Sedimentation Step. The sedimentation process takes place in about 20 tanks which are circular in shape (44 metres in diameter). The raw water contains many impurities (e.g., sand, muck, silt, etc.) and hence it requires treatment. Traditionally these impurities were removed using alum, however as alum generates a lot of sludge which cannot be used and hence they now use an alternative to alum which is Poly Aluminium Chloride (PAC). The PAC is a yellow coloured liquid which is mixed with the water in very small quantities. This is mixed in the water through a dispenser, hanging on the top of the inlet channel. There is a continuous flow of water in the intake water channel, and the PAC is added drop wise. Thus only a very small quantity is mixed into a large amount of water. The next phase of treatment is suspended solids settlement in a clarifier. This is a large circular structure in which the water is circulated at a very slow pace. This acts like a large centrifuge. The impurities then get settled down and 80% of the sludge 3

6 is removed by opening out the outlet valve. This yields the partly treated water, consisting of only 20% sludge. This water is channelized to the next step for processing. I wanted to know what made PAC a better compound to treat impurities as opposed to aluminium sulphate (alum), as alum is a low cost alternative to the more expensive PAC compounds. Aluminium sulphate reacts with water to form aluminium hydroxide and sulphuric acid a byproduct. The aluminium hydroxide precipitates out of the solution and entraps neutralized charged dust particles (turbidity) as well as coagulates soluble colour and organics by absorption. The disadvantages of alum as I learnt during my visit and through further research is not solely based on the fact that it produces more sludge. Hence, I decided to compare the two. Aluminium Sulphate (Alum) coagulation ph range 5.5 to 6.5 supplemental alkalinity is required to achieve the optimal ph The residual aluminium formed after the treatment process often exceeds the acceptable limits Poly Aluminium Chloride (PAC) coagulation ph range5.0to8.0 PAC s important property is their basicity and higher the basicity, lower the consumption of alkalinity in the treatment process, hence supplemental alkalinity is not required The residual aluminium formed is generally lower, typically 0.01 to 0.05 mg/l large amounts of sludge is produced There is an increase in the amount of sulphate in the treated water, the increase is about from (3-5 mg/l to mg/l). lesser sludgeisproduced The increase in the amount of chlorine as compared to sulphate is much lesser. Table 1: Comparison of Alum and PAC 4

7 Thus through this comparison I realised that even though Poly Aluminium Chloride is a more expensive option, it leads to much better results and hence is used in the treatment of raw water over alum. 3. Filtration. Filtration occurs in an open aired rectangular tank. This contains three layers. Sand, Gravel and a layer of 8,200 nozzles. The filter sand is a natural filter which is brought from Godhra River, and is extremely fine. The water percolates through the three layers and the sludge remain on top. However as the sludge accumulates the rate of filtration decreases and hence the sludge is removed every 27 hours through a system called backwashing. The system of backwashing involves air pressure from below, onto the sand. However, there is a substantial head of water on top of the sand so the air pressure is balanced out by the water pressure downwards, which prevents the sand from getting removed. The sand only has sidewards movement. The particular pressure enables the sludge to get removed and this flows into an outlet chamber. A butterfly valve is installed to open the gates for water to flow out through the outlet, at an extremely slow rate to prevent the splurging of the sand upwards. This is a 15 minute process which is repeated every 27 hours, and is typically repeated after every 18 hours in the rainy season as the suspended solids (sludge) is more in the rainy season. 5

8 Figure 2: Representation of the Filter Tanks 4. Chlorine Contact Tank (CCT). The previous cycle was from up to down, so in order to keep the cycle going this cycle is from the top to the bottom. The filtered water is now sent to an underground tank known as the Chlorine Contact Tank. This is fine water, without impurities, but it yet contains bacteria and other germs. To get rid of these germs, the Post Chlorination Dose (disinfectant) is given to remove the bacteria that results in odourless, colourless, tasteless water. However the dosage is extremely important. It is important that the right amount of chlorine is given. Too much chlorine can be poisonous for the consumer and too little chlorine will not be enough to get rid of the bacteria and germs, thus again being harmful for the consumer. Hence an optimum dosage of chlorine must be given. From the CCT, the water is transferred for storage and distribution to a Master Balancing Reservoir. 5. Master Balancing Reservoir (MBR). There are two MBRs which are located at a height. The underground water is now pumped up using pumps but then from the MBRs the water 6

9 flows due to gravity (no pumping). At the Bhandup water treatment plant they try to use minimum power in order to conserve electricity. 6. Service Reservoir. There are 27 service reservoirs all around the city e.g., Malabar Hill, Bandalwar Hill Reservoir, Ghatkopar Lower and Higher Reservoir, Trombay Reservoir, Powai Reservoir, and other Western Suburb Reservoirs. At these service reservoirs it is again treated with small amounts of chlorine to remove any bacteria and germs that may have accumulated during the transportation. The treated water is then sent to the housing and industrial areas. The Supervisory Control and Data Acquisition (SCADA) Program is a computer observation system that helps them oversee the work at all the service reservoirs like the quantity, reservoir level and pressure. They measure this using a Flow meter and a Pressure meter. This helps them monitor the water in all parts of the city from Bhandup and also collect official data for reporting to the regulatory authorities. At Bhandup they measure and keep in mind the quality of the natural water at the lake as well. They measure the supply pattern, the lake level and if the lakes have sufficient useful content of water. The rainfall pattern, the rise and fall of the water table, the level of water coming into Bhandup, etc. This helps them ensure that they have a continuous supply of water 24 X 7, all throughout the year. The Bhandup WTP requires continuous power supply otherwise the entire city of Mumbai will not get water supply. This is why they have their own power station to generate electricity. It is also a very sensitive, controlled area to prevent any outsiders from entering and hampering Mumbai s water supply. During the visit we also visited the Chlorine Tank room, where chlorine gas was stored in large containers. This is commercially bought chlorine. The entire system is automated as chlorine is an extremely hazardous gas. We visited the laboratory. In the laboratory we observed that 7

10 the Water Treatment Operators (WTO) carry out various tests to determine whether or not the water after treatment is fit for drinking. The water is tested against the Water Treatment Standards ISO:10500:2012 to determine that water is fit for drinking depending on the turbidity, the ph, amount of chlorine and other metals and bacteria as well. The ph should be between 6.5 and 8.5 where 7 is the optimum ph of water. The turbidity should be approximately less than 1 NTU after the treatment has occurred. They use a turbidity meter to measure this. Apart from this they also used a compound which turned pink in the presence of chlorine. This sample was then put through the colorimeter to determine the amount of chlorine present in the sample. The bacteria and germs were seen through samples and agar put in petri dishes. They then use filtration and UV rays in an incubator to see how many germs are present and manually bacteria count this under a magnifying glass. The presence of other metals is determined by the atomic absorption spectrophotometry. The spectrophotometer is used to measure the transmission and opacity of light through the sample. Apart from measuring this data of whether the treated water is fit for drinking or not they also measure the standards of drinking water, water that can be used in our city. The laboratory also makes use of a sedimentation machine, where they determine the optimum dosage of chlorine required for chlorination, which is an integral step of the process. Thus the laboratory confirms that the water leaving Bhandup has been properly treated. The Water Treatment at Bhandup occurs at a large scale. The government has to invest huge amounts of money into the treatment of water, a necessity. Most of the funds for the treatment of water come through the tax revenues collected by the government. The citizens are also required to pay a price for water in order to cover the costs of providing these services. Note: The Bhandup Water Treatment Plant is categorized as a high-risk facility. Hence taking photographs was not permitted. 8

11 Works Cited 1. Treatments, The Water. Cholorination. Water Treatment Waste Water Treatment Water Treatment Process & Plant Design, 1 Aug. 2013, Sand Filtration Aquazur V. Degremont Drinking Water Production: Sand Filtration Aquazur, Mapping Mumbai's Water Supply, 9

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