By O. Oyedele Adeosun, Obafemi Awolowo University
Providing sufficient water of appropriate quality and quantity has been one of the most important issues in human history. Most ancient civilizations were initiated near water sources. As populations grew, the challenge to meet user demands also increased.
People began to transport water from other locations to their communities. For example, the Romans constructed aqueducts to deliver water from distant sources to their communities.
Today, a water supply system consists of infrastructure that collects, treats, stores, and distributes water between water sources and consumers. Limited new natural water sources, especially in the southwest region of the USA, and rapidly increasing population has led to the need for innovative methods to manage a water supply system. For example, reclaimed water has become an essential water resource for potable and nonpotable uses. Structural system additions including new conveyance systems and treatment and recharge facilities and operation decisions, such as allocating flow and implementing conservation practices, are made with the present and future demands in minds. As additional components and linkages between sources and users are developed, the complexity of the water supply system and the difficulty in understanding how the system will react to changes grows.
Many efforts on the development of a water supply system have been made through for sustainable water supply. However, the complexity of system limited the site specific application at the first era. As water demands pressures raise increasingly on the existing water supply system, many studies attempted to develop a general water supply system to assist decision makers to design more reliable systems for a long range operation period. These attempts also include the optimization of total system construction and operation cost. Under given situations such as pipeline maintenance, non-revenue water, advanced metering infrastructure, the ultimate goal of this paper is to ensure water distribution system challenges are overcome and supply water sources to users reliably in a more sustainable and timely manner as a long-term plan.
Water Distribution Systems
The purpose of distribution system is to deliver water to consumer with appropriate quality, quantity and pressure. Distribution system is used to describe collectively the facilities used to supply water from its source to the point of usage.
Requirements of Good Distribution System
Layouts of Distribution Network
The distribution pipes are generally laid below the road pavements, and as such their layouts generally follow the layouts of roads. There are, in general, four different types of pipe networks; any one of which either singly or in combinations, can be used for a particular place. They are: Grid, Ring, Radial and Dead End System.
Grid Iron System:
It is suitable for cities with rectangular layout, where the water mains and branches are laid in rectangles.
The supply main is laid all along the peripheral roads and sub mains branch out from the mains. Thus, this system also follows the grid iron system with the flow pattern similar in character to that of dead end system. So, determination of the size of pipes is easy.
The area is divided into different zones. The water is pumped into the distribution reservoir kept in the middle of each zone and the supply pipes are laid radially ending towards the periphery.
Dead End System:
It is suitable for old towns and cities having no definite pattern of roads.
Until the early 1990s, there were no reliable and standardized methods for accounting for water losses. Leakage management performance was measured in terms of “unaccounted-for water.” Since this term had no generally accepted definition, there was wide room for interpretation. Unaccounted-for water was typically expressed as a percentage of system input, which is already problematic.
Given this situation, utility performance could not be measured or compared, realistic targets could not be defined, and performance against targets could not be tracked reliably.
While this situation still exists in many countries, significant progress has been made to address these past shortcomings. Over the last 20 years, a number of organizations from around the world have developed a suite of tools and methodologies to help utilities evaluate and manage water losses in an effective manner.
One recommendation of the WLTF (Water Loss Task Force) was to use the term “non-revenue water” instead of “unaccounted-for water.” NRW (non-revenue water) has a precise and simple definition. It is the difference between the volume of water put into a water distribution system and the volume that is billed to customers. NRW comprises three components as follows:
Physical (or real): losses comprise leakage from all parts of the system and overflows at the utility’s reservoirs. They are caused by poor operations and maintenance, the lack of active leakage control, and poor quality of underground assets.
Commercial (or apparent): losses are caused by customer meter under registration, data handling errors, and theft of water in various forms.
Unbilled authorized consumption: includes water used by the utility for operational purposes, water used for firefighting, and water provided for free to certain consumer groups.
Although it is widely acknowledged that NRW levels in developing countries are often high, actual figures are elusive. Most water utilities do not have adequate monitoring systems for assessing water losses, and many countries lack national reporting systems that collect and consolidate information on water utility performance. The result is that data on NRW is usually not readily available. Even when data is available, it is not always reliable, as some poorly performing utilities are known to practice “window dressing” in an attempt to conceal the extent of their own inefficiency.
Lost water can be calculated as (A + L + R) [d] × flow rate [m3/d] = water lost [m3]
The volume of water lost from an individual pipe burst does not only depend on the flow rate of the event, but is also a function of run time. This is often overlooked. The leak run time consists of three components:
Many water utilities in Asia practice passive leakage control, meaning that they repair only those leaks that are visible. This is clearly not enough since 90% of the leaks are usually not visible on the surface. This means it takes far too long, often many years, until the utility is even aware that there is a leak. Since awareness time largely determines the volume of water lost from a pipe burst, utilities need a strategy to reduce awareness time.
The most traditional and basic method is to have a team of leak detection specialists who check all pipes on a regular basis. Since leak noise can be detected, this work is done with a wide range of listening devices, ranging from simple mechanical listening sticks to electronic ground microphones or even leak noise correlators. Leakage inspectors use this equipment to listen to the network and identify problems, much like doctors use stethoscopes. If every part of the network is surveyed once a year, the average leak run time (awareness time) is 6 months. To reduce awareness time, the survey frequency can be increased. However, leak detection efforts will still not be well targeted. To be able to determine how much water is lost in specific parts of the network, the network must be split in hydraulically discrete zones and the inflow to these zones must then be measured. By computing the volume of leakage in each zone, leak detection specialists can better target their efforts. Clearly, the smaller the zone, the better the information and the efficiency of leak detection. The smallest zones are called District Metered Areas (DMAs). A DMA is hydraulically discrete and ideally has only a single inflow point. The inflow and corresponding pressure is measured and monitored on a continuous basis. Ideally, when the entire distribution network is split into DMAs, the utility has several advantages. For instance:
Furthermore, DMAs (District Metered Area) can be helpful in managing pressure. At the inflow to the DMAs, pressure reducing valves can be installed, and the pressure in every DMA can be adjusted to the required level. There is no ideal size for a DMA. The size, whether it is 500 or 5,000 service connections, is always a tradeoff. The decision has to be made on a case-by-case basis and depends on a number of factors (e.g., hydraulic, topographic, practical and economic).
The size of DMAs has an impact on the cost of creating them. The smaller the DMA, the higher the cost. This is because more valves and flow meters will be required and maintenance is costlier. However, the benefits of smaller DMAs are that:
Topography and network layout also play an important role in DMA design and size. Therefore, there will always be DMAs of different sizes in a distribution network. An important influencing factor is the condition of the infrastructure. If mains and service connections are fragile, then bursts will be more frequent and the optimal DMA will be relatively small. On the other hand, in areas with brand new infrastructure, DMAs can be larger and still manageable.
According to the recommendations of the International Water Association’s (IWA) Water Loss Task Force, if a DMA is larger than 5,000 connections, it becomes difficult to discriminate small bursts (e.g., service connection bursts) from variations in customer night use. In networks with very poor infrastructure conditions, DMAs as small as 500 service connections might be warranted. A calibrated hydraulic model should always be used for DMA design irrespective of the size of the DMAs.
Water losses from larger diameter pipes can be quite significant, especially in the Asian context with predominantly low-pressure systems, where leaks will not come to the surface and remain unnoticed for many years. Leaks on large diameter pipes are always difficult to detect and often specialized equipment is required (e.g., inside pipe inspection and leak detection). These techniques are costly but might be economically well justified where water availability is limited and every cubic meter of water recovered can be sold to existing or new customers.
ADVANCED METERING INFRASTRUCTURE
Commercial losses are nearly always less in volume than physical losses, but this does not mean that commercial loss reduction is any less important. Commercial loss reduction has the shortest possible payback time, as any action immediately results in an increase in billed volume and an increase in revenues. Commercial losses consist of three main elements:
Metering: Minimizing customer meter under-registration requires substantial technical expertise, managerial skills, and upfront funding. Customer meter management should be undertaken holistically, best described by the term “integrated meter management.”
In this effort, utilities should seek to select appropriate meter types and prepare tailored specifications. This can prove difficult, especially where procurement laws and regulations encourage purchasing the cheapest products on the market.
A number of meter manufacturers produce meters that “on paper” meet the specifications but deteriorate at an amazing rate in the field. This is one of the major obstacles for sustained improvement of customer meter accuracy. Contributing to this problem is the lack of good quality meter testing facilities, especially when it comes to larger diameter meters, and the lack of experience in how to best utilize such facilities. This makes it easy for manufacturers to supply meters from second class quality manufacturing batches with little risk that the utility would ever find out.
Another common problem is the reluctance to invest in high quality but more costly meters for large customers. Normally, the top accounts of a utility generate such a large portion of their revenues that any investment in more advanced meters can be economically justified. The payback time is often just a matter of months. Yet, many water utilities opt to maintain and calibrate old meters over and over again instead of taking appropriate action and installing new meters.
Billing system issues: The billing system is the only source of metered consumption data that can help determine the volume of NRW through an annual water audit. However, most billing systems are not designed to retain the integrity of consumption data. Rather, they are designed to deliver accurate bills to customers and correctly account for the bills. However, there are many day-to-day processes in operating a billing system that have the potential to corrupt the integrity of the consumption data, depending on the design of the particular system. Issues that can affect consumption volumes include
Water theft: While meter under-registration is more of a technical problem, water theft is a political and social issue. Reducing this part of commercial losses is neither technically difficult nor costly, but it requires making difficult and unpleasant managerial decisions that may be politically unpopular. The reason is that illegal connections are nearly always wrongly associated with only the urban poor and informal settlements. However, water theft by high-income households and commercial users, sometimes even large corporations, often accounts for sizable volumes of water lost and even higher losses of revenue.
In addition to illegal connections, other forms of water theft include meter tampering and meter bypasses, meter reader corruption, and illegal hydrant use. Another common problem is “inactive accounts.” In cases where a customer’s contract has been terminated, the physical service connection, or at least the tapping point on the main, still exists and is easy to re connect illegally. A stringent inactive account management and verification program can easily solve this problem.
Water distribution system should be based on a pipe layout that is suitable and have no or less water stagnation within the pipe to avoid tuberculation, encrustation and sediment deposits
Through a wealth of specialized publications and software development is now well understood that water distribution system management is technically difficult, but with current technologies, software systems, and highly specialized equipment (flushing and scraper), this is simply not the case anymore.
Water utilities will also need to practice appropriate design of system expansions/distribution (e.g., new network parts already constructed as DMAs) and use higher quality works, materials, and equipment. In addition, regulators and policy makers should require water utilities to do periodic water audits and regularly publish detailed water distribution system data, which can then be independently audited.
Again, water distribution system management should not be a one-time activity. Although an intense and comprehensive water distribution system reduction program is suitable to reduce the backlog of required water distribution system reduction measures, it should not lead to a sustainable low level of water distribution system unless water distribution system management becomes part of the normal day-to-day activities of the water utility.
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Image credit: "69/365," Drongowski © 2011, used under an Attribution-ShareAlike 2.0 Generic license: https://creativecommons.org/licenses/by-sa/2.0/