Intermittent water distribution networks: A tale of two cities

Intermittent water distribution networks can be varied and complicated

Water distribution networks that supply water intermittently are complicated and varied and often result in poor service and poor water quality. Intermittent supply can lead to instances of reduced pressure in the piped water networks, which can increase the risk of contamination. Estimates suggest that intermittent water supplies (IWS) pose the risk of contamination of water supplies among approximately 10 billion people-hours/ day globally. 

The most common indicator of the quality of an intermittent system is the continuity of water supply i.e. the average hours per day (or week) during which water is supplied informs this study titled 'Learning from intermittent water supply schedules: Visualising equality, equity, and hydraulic capacity in Bengaluru and Delhi, India' published in Science of The Total Environment.

Supply continuity can affect how much water consumers can withdraw from the system and the risk of contamination. Infrequently and/or inconveniently timed water supplies make people store more water to use and water storage can be expensive, increases water age, and leads to contamination and growth.

While water that is provided in consistent volumes and according to a predictable schedule is ideal, even this water, delivered to different neighbourhoods according to different supply schedules by different times of day, with different frequencies, and/or with different supply continuities can impose considerable burden on the consumers. 

In IWS that satisfy consumers, the water supply schedule is such that the hydraulic capacity of the network is maintained to support water flow rates, and water flows through the network quickly and continuously. Networks with insufficient hydraulic capacity may be unable to support continuous operations even with unlimited water. 

There is very little information related to how IWS systems function in cities in India and their water supply schedules continue to be unexplored.

IWS systems in Delhi and Bengaluru

This study visually analyses and compares the water supply schedules within and between two large Indian cities namely Delhi and Bengaluru, that are the 4th and 13th largest IWS systems globally.

The study aims at:
•    Comparing the complexity and variability of intermittent supply schedules with current, average-based benchmarking metrics for IWS
•    Quantify the equality between neighbourhoods in the two cities in terms of  water supply continuity and  water storage capacity
•    Explore equity between neighbourhoods in terms of water access, water quality, supply continuity, schedule-necessitated water storage capacity, and the percentage of households with pipe connections to the network 
•    Find out if city-wide peak factors and hydraulic capacity can support increased water supply continuity in the cities.

The study finds that:

The current benchmarking that focuses on average water supply continuity is inadequate to understand the outreach of IWS among consumers.

Intermittent water supply schedules are varied and complex and affect consumers. However, this variety and complexity is obscured by current benchmarking efforts that focus on a utility's average supply continuity in hours per day of supply. 

The updated International Water Association's manual of best practice for performance indicators for water supply services recommends the use of population-weighted averages for assessing water supply continuity.

While averages are reflective of a system's average effect on water consumers, such averages still fail to capture the vast heterogeneity and inequality present in most IWS. For example, while the population weighted, average supply schedule continuities in Delhi and Bengaluru are 4.3 and 3.0 hours/day, the supply continuities vary from 1 to 9 hours/day in Delhi and of 43 min/day and 7.5 h/day in Bengaluru.

Equality is evaded in water supply schedules in both cities

A water supply pattern can limit the water consumers access in two ways. If the supply continuity is short, it can prevent consumers from withdrawing as much water as they need. On the other hand, if the time between water supplies is long, it prevents consumers from storing more water as they would like to utilise during the gap.

Shorter supply continuity and infrequent supply can impose water quality risks. For example, short supply continuity increases the fraction of time without supply during which contaminants can seep into the network while infrequent supplies increase the duration of time during which water must be stored thus increasing the chances of contamination. 

Two percent of consumers in Delhi are scheduled to receive water continuously and 87 percent are scheduled to receive water at least once per day while 0.1 percent receive water daily, but for only 20 min. In Bengaluru, 54 percent of the consumers are scheduled to receive water every other day and 10 percent  once per week. Only 0.04 percent consumers in Bengaluru are scheduled to receive water continuously and  0.5 percent are scheduled to receive water for at least 12 h/day. The least supplied 0.02 percent of consumers in Bengaluru are scheduled to receive water only 30 min once per week. In total, 1.5 percent of consumers in Bengaluru are scheduled to receive water for 3 h/week or less – an average supply continuity of < 26 min/day. 

Bengaluru consumers have to store large amounts of water due to inconvenient timings of water supply

While water supply in Delhi is scheduled at least twice daily to the majority (58 percent) of supplied consumers and at least daily to 87 percent of consumers, the median consumer in Delhi requires less than half a day's water storage. In contrast, 91 percent of residents in Bengaluru are scheduled to receive water every other day or less frequently necessitating a median storage of almost two days (45 h) – four times larger than in Delhi. 

The required storage in Delhi is further minimised due to the timings of water supply that is scheduled in the morning and evening.  Almost two thirds of Delhi (63 percent) is scheduled for supply between 6 and 7 am and 46 percent is scheduled for supply between 6 and 7 pm.

In contrast, the timing of Bengaluru's supply is much more uniform with 10 to 15 percent of consumers being supplied with water at any given time of the day, necessitating more storage than in Delhi. In addition to the inconvenience and costs of larger storage requirements in Bengaluru, the prevalence of night-time supplies means that many residents are forced to either wake up during the night to manage their water collection or purchase automatic water-tank-management systems. 

Thus supply frequency – the number of days per week of water supply – which varies substantially between Delhi and Bengaluru and irregular spacing in some supply schedules determine the storage needs of the consumers in both cities. The required storage capacity and water age is much larger in Bengaluru than in Delhi, but this burden is more equally distributed in Bengaluru.

Affluent consumers cope with the long periods between supplies by avoiding storage and installing large underground, ground-level, and/or rooftop tanks and thereby gaining more water from the same system as compared to less affluent consumers, or by installing and operate powerful suction pumps.

The quality of service is inequitable in both cities

Wealth and quality of water services varies spatially in each city with the peripheral supply regions having lower piping and varied but lower quality of service and better services concentrated in wealthier regions in terms of better supply continuity, smaller required storage, and more households connected to the network. These are relevant for each dimension of service in Bengaluru, but only for access to the water network in Delhi.  

Gauging hydraulic capacity from supply schedules 

Much of Delhi receives water concurrently and Delhi's network is scheduled to convey 3.8 times its average flow at 6 am in the morning. In contrast, Bengaluru's supply schedules peak at 10 AM and induce peak flows of only 1.3 times its average flow because the city also supplies water at night. 

Continuously operated networks must have sufficient capacity to convey as much water as consumers want and to accommodate variations in consumer demand, including its peak during the maximum hour of the year. Delhi's high peak factor (3.8) and water production (274 LPCD) combine to suggest its network is capable of conveying water at a rate of over 1000 LPCD – which can be good for continuous supply, even if demand increases. 

Bengaluru's water production is limited (106 LPCD) and its peak factor is lower with a hydraulic capacity of at least 138 LPCD, which is sufficient to meet the average consumer demand expected in cities by Indian guidelines of 135 LPCD (CPHEEO, 1999). However, it does not guarantee sufficient capacity to accommodate variations in average demand or greater then expected increase in water demand.  

Thus Bengaluru's low peak factor and prevalent nocturnal supplies are reflective of capacity limitations in Bengaluru's water treatment facilities, transmission network, and/or storage reservoirs.

The study found that:

• Scheduled supply continuity is 45 percent higher in Delhi than in Bengaluru on average, but in both cities some areas are scheduled to receive water continuously while other areas receive only 20 min or less per day. 
• Delhi's schedules necessitate four times less storage than in Bengaluru, but this storage is less equally distributed. In both cities, access to the pipe network is inequitably divided and poorer regions have fewer households with piped connections. Wealthier regions have longer supply continuities and reduced storage requirements. Delhi has ample hydraulic capacity to support continuous water supply while night time supply practices in Bengaluru lead to insufficient treatment, transmission, and/or reservoir storage capacity. 
• The study recommends that researchers, operators, and regulators should use water supply schedules, equality, equity, and hydraulic capacity in predictable intermittent water supplies through indicators such as water supply continuity, the hours of storage their supply pattern requires consumers to have, and the equality of both these metrics.
• The study recommends four tools that leverage water supply schedule data such as comparing supply patterns, equality, equity, and hydraulic capacity to evaluate water supply systems by presenting case studies of Delhi and Bengaluru.

The paper can be accessed here