By: Eng. Ayman Afifi March 2009. Water Consumption The consumption or use of water, also known as water demand, is the driving force behind the hydraulic.

By: Eng. Ayman Afifi

March 2009

Water Consumption The consumption or use of water, also known as water demand, is

the driving force behind the hydraulic dynamics occurring in water distribution systems.

Anywhere that water can leave the system represents a point of consumption, including a customer's tap, a leaky main, or an open fire hydrant.

The three basic demand types are:1. Customer demand is the water required to meet the non-emergency

needs of users in the system. 2. Unaccounted-for water (UFW) is the portion of total consumption

that is "lost" due to system leakage, theft, un-metered services, or other causes.

3. Fire flow demand is a computed system capacity requirement for ensuring adequate protection is provided during fire emergencies.

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Water Consumption Determining demands is not a straightforward process like

collecting data on the physical characteristics of a system. Some data, such as billing and production records, can be collected directly from the utility but are usually not in a form that can be directly entered into the model. Once this information has been collected, establishing consumption rates is a process requiring study of past and present usage trends and, in some cases, the projection of future ones.

After consumption rates are determined, the water use is spatially distributed as demands, or loads, assigned to model nodes. This process is referred to as loading the model. Loading is usually a multi-step process that may vary depending on the problem being considered. The following steps outline a typical example of the process the modeler might follow:

1. Allocate average-day demands to nodes.2. Develop peaking factors for steady-state runs.3. Estimate fire and other special demands.4. Project demands under future conditions for planning

and design.

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Baseline DemandsMost modelers start by determining baseline

demands to which a variety of peaking factors and demand

multipliers can be applied, or to which new land developments and

customers can be added. Baseline demands typically include

both customer demands and unaccounted-for water.

Usually, the average day demand in the current year is the

baseline from which other demand distributions are built.

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Data Sources 1. Pre-Existing Compiled Data

The first step in finding demand information for a specific utility should always include researching for the utility's existing data. Previous studies, and possibly even existing models, may have a wealth of background information that can save many hours of investigation.

However, many utilities do not have existing studies or models, or may have only limited resources to collect this type of information. Likewise, models that do exist may be outdated and may not reflect recent expansion and growth.

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Data Sources 2. System Operational Records

Various types of operational records are available that can offer insight into the demand characteristics of a given system. Treatment facility logs may provide data regarding long-term usage trends such as seasonal pattern changes or general growth indications. Pumping logs and tank level charts contain data on daily system usage, as well as the changing pattern of demand and storage levels over time.

Water distribution systems may measure and record water usage in a variety of forms, including:

Flow information, such as the rate of production of a treatment or wellfacility.

Volumetric information, such as the quantity of water consumed by a customer.

Hydraulic grade information, such as the water level within a tank.

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Data Sources 3. Customer Meters and Billing Records

If meters are employed throughout a system, they can be the best source of data for

determining customer demands. Customers are typically billed based on a volumetric measure

of usage, with meter readings taken on a monthly or quarterly cycle. Using these

periodically recorded usage volumes, customers' average usage rates can be computed. Billing

records, therefore, provide enough information to determine a customer's baseline demand, but not enough to determine fluctuations in demand

on a finer time scale such as that required for extended-period simulations.

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Data Sources 4. Spatial Allocation of Demands

Although water utilities make a large number of flow measurements, such as those at

customer meters for billing and at treatment plants and wells for production monitoring,

data are usually not compiled on the node-by-node basis needed for modeling. The modeler

is thus faced with the task of spatially aggregating data in a useful way and

assigning the appropriate usage to model nodes. 8

Cont. Data Sources The most common method of allocating baseline demandsIs a simple unit loading method. This method involves counting the number of customers [dunums of a given land use, number of fixture units, or number of equivalent dwelling units] that contribute to the demand at a certain node, and then multiplying that number by the unit demand [for instance, number of gallons (liters) per capita per day] for the applicable load classification. For example, if a junction node represents a population of 200, and the average usage is 120 l/day/person, the total baseline demand for the node would be 24,000 l/day.

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Cont. Data Sources Another approach to determining the baseline demand

for individual customers involves the use of billing records. However, rarely does a system have enough

recorded information to directly define all aspects of customer usage. Even in cases where both production

records and full billing records are available, disagreements between the two may exist that need to

be resolved.

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Example - Demand Allocation In a detailed demand allocation, a key step is

determining the customers assigned to each node. The figure below demonstrates the allocation of customer

demands to modeled junction nodes. The dashed linesrepresent the boundaries between junction associations.

For example, the junction labeled J-1 should have demands that represent nine homes and two commercial establishments. Likewise, J-4 represents the school, six

homes, and one commercial building.

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Example - Demand Allocation

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Unaccounted-For Water Ideally, if individual meter readings are taken for every customer, they should exactly equal the amount of water that is measured leaving the treatment or well facility. In practice, however, this is not the case. Although inflow does indeed equal outflow, not all of the outflows are metered. These "lost" flows are referred to as unaccounted-for water (UFW).

There are many possible reasons why the sum of all metered customer usage may be less than the total amount of water produced by the utility. The most common reasons for discrepancies are leakage, errors in measurement, and un-metered usage. Ideally, customer demands and unaccounted-for water should be estimated separately. In this way, a utility can analyze the benefits of reducing unaccounted-for water. There are many possible reasons why the sum of all metered customer usage may be less than the total amount of water produced by the utility. The most common reasons for discrepancies are leakage, errors in measurement, and un-metered usage. Ideally, customer demands and unaccounted-for water should be estimated separately. In this way, a utility can analyze the benefits of reducing unaccounted-for water.

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Unaccounted-For Water

1. Leakage 2. Meter Under Registration 3. Un-metered Usage

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Unaccounted-For Water 1. Leakage

Leakage is frequently the largest component of UFW and includes distribution losses from supply pipes, distribution and trunk mains, services up to the meter, and tanks. The amount of leakage varies from system to system, but there is a general correlation between the age of a system and the amount of UFW. Newer systems may have as little as 5 percent leakage, while older systems may have 40 percent leakage or higher. Leakage tends to increase over time unless a leak detection and repair program is in place.

Other factors affecting leakage include system pressure (the higher the pressure, the more leakage), burst frequencies of mains and service pipes, and leakage detection and control policies. These factors make leakage very difficult to estimate, even without the complexity of approximating other UFW causes. If better information is not available, UFW is usually assigned uniformly around the system.

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Unaccounted-For Water 2. Meter Under Registration Flow measurement errors also contribute to UFW. Flow measurements are not always exact, and thus metered customer usage may contain inaccuracies. Some flow meters under-register usage at low flow rates, especially as they get older.

3. Un-metered Usage Systems may have illegal connections or other types of un-metered usage. Not all un-metered usage is indicative of water theft. Fire hydrants, blow-offs, and other maintenance appurtenances are typically not metered.

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