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The Water System Design Manual (WSDM), used in conjunction with the regulations for public
water systems (Chapter 246-290 WAC), represents the fundamental basis for water system design
in Washington State. The approach taken in this manual for design of water systems and their
components has involved (1) development of performance standards rather than prescriptive
standards, (2) placement of mandatory requirements in WAC with corresponding reference made
to support “shall” or “must” statements in the WSDM, (3) provision of alternative design
approaches when a specific proposal meets considerations of “good engineering practice” and is
supported by documented justification, and (4) allowance for the customers of an individual water
system to participate in the determination of their own level or standard of reliable water service
under abnormal circumstances, provided that protection of public health is not compromised. The
WSDM is further intended to provide a basis for establishing the “standard of care” for engineering
professionals involved in water system design.

The WSDM has been developed to provide guidance predominately for the design of smaller
public water systems; typically those less than 1,000 service connections in size. The guidance
also has applicability to expanding water systems of any size and to systems classified as being
complex Group B systems. However, it is recognized, and expected, that existing local ordinances,
consumer service expectations, and existing water use data will normally serve to provide more
appropriate design criteria in most instances. It is generally recognized and advocated by DOH
that use of system specific information is a more accurate basis for design of water system
expansions than are any other criteria.

Separate water system design guidelines are available for simple Group B water systems not
requiring professional engineering expertise. The Board of Registration of Professional Engineers
and Land Surveyors determines what constitutes the practice of “engineering” in Washington State.
A court decision provided direction to the Board and to DOH by stating that “Non-professional
engineer designers (including laypersons and registered sanitarians) will not be permitted or
authorized to design public water systems except those serving fewer than 10 connections and
consisting solely of a simple well and pressure tank with one pressure zone, and not providing any
special treatment or having special hydraulic considerations…” Based upon these requirements,
it is expected that smaller more complex systems, as well as larger systems, will be designed by
Professional Engineers.

General Water System Reliability Considerations

This manual also provides recommendations regarding concerns over the sometimes complex and
controversial issue of water system reliability. Although there appears to be no consensus opinion
as to all the defining characteristics of a “reliable” system, a 1997 American Water Works
Association Research Foundation (AWWARF) project entitled Managerial Assessment of Water
Quality and System Reliability was conceived and published to initiate a national dialogue on the
integral nature of water quality and water quantity reliability. Forty utility managers from around
the nation participated in a consensus “American Assembly” process to develop an American
Assembly Statement on the topic. Excerpts from their report are included in this discussion;

however, the reader is directly referred to the report for more details on both the process and the

The report basically states, “water supply reliability has historically involved assuring the
quantity of available supplies. The avoidance of service interruptions due to droughts, storage
and distribution system failures, or natural disasters has been the main focus of attention. The
water industry is facing significant new reliability challenges in assuring the quality of delivered
water. At the same time, water suppliers are facing competitive challenges to overhaul their
business approaches to providing water service in order to meet ambitious standards of efficiency
and customer service. Assuring water quality reliability is a subtle and technically complicated
concern that has implications for utility management extending from source to tap and throughout
every functional area of utility operations.”

An important concept associated with reliability in the AWWARF publication was stated by
reference to previous work done by the Board on Infrastructure and the Constructed Environment
of the National Research Council (NRC) in 1993-94, whereby using a consensus process a
framework was developed for evaluating utility infrastructure performance. The NRC produced
the following definition: “Performance – the degree to which infrastructure provides the services
that the community expects – may be defined as a function of effectiveness, reliability, and cost.
Infrastructure that reliably meets or exceeds community expectations, at an acceptably low cost, is
performing well.”

Effectiveness was defined as “the ability of the system to provide the services the community
expects, as described by: (1) capacity and delivery of services, (2) quality of services delivered, (3)
the system‟s compliance with regulatory concerns, and (4) the system‟s broad impact on the
community.” Reliability was defined as “a recognition of the various uncertainties inherent in
infrastructure services and, more formally, as the likelihood that infrastructure effectiveness will
be maintained over an extended period of time, or, as the probability that service will be available
at least at specified levels throughout the design life of the infrastructure system.”

In formulating this definition of infrastructure performance, the NRC has captured the management
context of reliability. Reliability is an aspect of performance. Two important points are also made
clear in the discussion presented along with this definition by the NRC:

   1. Performance is recognized as the outcome of management‟s “optimization,” involving
      trade-off relationships between effectiveness, reliability, and cost.

   2. It is acknowledged that the objectives driving this optimization must be grounded in
      processes which document the community‟s expectations for service quality and delivery.

These points were underscored in the AWWARF publication, and enlarged upon by presentation of
two additional themes:

   1. “Utility management does not have complete freedom to act in optimizing the trade-offs
      between effectiveness, reliability, and cost. Depending on various nuances of a utility‟s

       institutional structure, there are some structural limits on the scope of management
       abilities to optimize performance. Other super-imposed limits arise from history (e.g.
       deteriorated infrastructure) or from changing economic and demographic conditions such
       as declining average incomes in older cities or new urban growth.”

    2. “The objectives of the optimization are driven not only by expressed community desires,
       but also by external forces such as SDWA regulations.”

Relative to customer attitudes, the AWWARF publication indicates that, “where customer
attitudes towards reliability have been examined, there are likely to be differences between
quantity and quality in terms of willingness to pay. On the quantity side, it is widely known that
the customer demand is elastic – i.e., customers will accept either some degree of service
interruption (outdoor use restrictions, etc.) and/or they will curtail such uses to some extent if
presented with appropriate peak season rate structures. On the quality side, customer demand is
believed inelastic. Survey research shows very strong customer desire for safety. The concept of
the reliability or interruptability of the level of safety provided is a question that many utilities do
not even bother to ask. It is believed that customer desire for aesthetically appealing and
completely safe water is strong and absolute. Many respondents to the participant survey said
simply „100%‟ in response to the question on water quality reliability goals.”

The AWWARF publication discussion indicates the complexity faced when addressing reliability,
whether it is quantitative or qualitative. This manual, as well as the DOH drinking water
regulations, presents an approach that provides for flexibility in responding to consumer
expectations related to water quantity (without compromising public health protection) while
concomitantly addressing the less flexible consumer expectations related to reliability of water
quality. The AWWARF and NRC publications reference utility operations and management, and
their influence and impacts on reliability. This manual, while also recognizing the importance of
those aspects, is more directed toward reliability afforded through good design and construction of
water supply and delivery facilities.

WAC 246-290-420 states, “All public water systems shall provide an adequate quantity and
quality of water in a reliable manner at all times consistent with the requirements of this chapter.”
Reliability relates to the dependability the system exhibits and the degree of confidence consumers
have regarding its ability to deliver water to the point of use when it is desired. Reliable and safe
drinking water of an adequate quality is of paramount importance and safeguards intended to
assure this are spread throughout the drinking water regulations. It is often more difficult for
consumers to see the link between the provision of an adequate quantity of water and the
relationship to public health protection.

Some aspects of a reliable quantity of water are closely linked to public health protection, (such as
providing for sufficient water for drinking, cooking, and daily sanitary needs). While providing
this quantity of water, distribution systems need to also maintain adequate pressures to prevent
contamination from cross-connections under all reasonably anticipated demand scenarios. It is this
link between providing the quantity desired while maintaining minimum pressures to mitigate

contamination potentials in the distribution system that is not always clear to the consumer. When
extended to include all demands placed upon a water system (i.e., irrigation, car washing,
recreation, etc.), consumers may find this link of public health protection to water quantity even
more obscure. Protection of public health under full-use situations is mainly done by assuring,
through design and operation, that adequate system pressures are maintained. The consumer
relates to a level of expectation associated with the delivery pressure of the water for routine uses.
Low-pressure situations are noticed by consumers and may give rise to complaints. The purveyor
protects both the health of the receiving population and the level of expectation for the consumer
by maintaining adequate pressure in the system

Reliability is also associated with availability of water, particularly with the degree of consumer
expectation for uninterrupted levels of service. A limited amount of water available under unusual
circumstances (i.e., drought conditions, line breaks, unscheduled power outages, etc.) may lead to
some curtailment of uses for outside demands, or acceptance of a period when water demands must
be controlled. The degree of consumer acceptance during such periods is expected to vary from
community to community and situation to situation. A reliable system is one that is designed and
operated such that it accommodates the needs and expectations of the consumers under all
conditions of operation.

System reliability to provide adequate levels of service can be separated into two basic
components, source reliability and facility reliability.

Source Reliability

Source reliability relates to the dependability of drinking water sources (surface water bodies,
springs, and groundwater aquifers) to provide an adequate or desired quantity of water over a given
period of time. For surface waters, reliability of supply usually entails probabilistic analysis of
historical data related to rainfall, snowpack, runoff, and flow rates, especially during years of
extended periods without significant precipitation (drought). Such an analysis leads to predictions
of frequencies for years when water availability may be expected to be limited. These are normally
expressed as the 1 in 10, 20, 50, or even 100-year recurrence intervals for water availability, based
on historical events.

Superimposed upon predictions of probabilistic availability for water is another component
associated with the consumer’s understanding and acceptance of restricted water uses in times of
low availability. Since it may not be reasonable to expect that water will always be present in
quantities sufficient for unrestricted use, some utilities have adopted a standard of 98% reliability
for their surface sources. This standard implies that for 98 years out of 100, the source will be able
to provide sufficient water for consumer uses without restriction. For the other two years, it would
be expected (and accepted) that water use would need to be curtailed. If a lower standard were
adopted, more frequent curtailment would be required.

There may be additional times, (i.e., over the 98% level), during any 100-year time frame that
consumers will be asked to curtail water use to some degree. Utilities must often make decisions
for prospective curtailment actions months ahead of time based upon data available at the time.

(e.g., a spring under extended periods of low precipitation may be followed by a summer with
extended periods of greater than normal precipitation, or vice versa, or for some systems the snow
pack levels, depended upon to provide adequate late-summer source quantities, may melt more
rapidly than normal, etc.). As a result of these possible scenarios and the point in time at which
management decisions need to be made, consumers may sometimes be asked to curtail use when it
is later determined to have been unnecessary. Or, at other times they are not asked to curtail when
it is later determined that a sooner curtailment request would have been warranted. Regardless of
whether or not the curtailment request was indeed necessary, the frequency of the requests to
consumers will impact the perception they have regarding the reliability of the water supply.

It is appropriate that individual communities choose their own level of source reliability for surface
water dependent systems. When establishing a local standard, potential impacts to consumers
should be expressed in terms that can be readily understood so that informed decisions can be
made regarding acceptable levels of source reliability.

For groundwater sources, there are also reliability impacts associated with climatic changes;
however, the impact is generally not as rapid and usually not as great. Source reliability for ground
waters is associated with the estimated sustainable yield of an aquifer. This is attained through
pumping tests and hydrogeological analysis of the aquifer. The extent of the analysis is usually
related to the size of the utility and its willingness to expend resources to gain the necessary data.
Pumping test procedures for wells are outlined in Chapter 7 of this document.

Facility Reliability

Facility reliability relates to the dependability of public water system facilities, such as pumps,
storage tanks, and pipelines, in delivering the desired quantities of water over given periods of
time. The frequency and duration of service interruptions to consumers, and the cost required to
minimize such interruptions, have direct impacts on consumer expectations. Correspondingly,
consumer expectations will drive many decisions regarding improvements that provide higher
levels of reliability for the system.

Consumers may accept interruptions to water service for one or two days a year because of system
flushing, cleaning, maintenance or repair. They may, however, find it unacceptable, to be out of
water periodically each month for more than three or four hours at a time, or at any time for more
than a day or two. Considering possible events, the community and designer need to weigh the
greater engineering and construction costs for more reliable facilities against the costs of the
impacts associated with the loss of service. For each event that could result in diminished levels of
service, an estimate of the probability that the event could occur and its associated duration needs
to be made. Once these estimates are made, they then can be shared with consumers to determine
the desired level of reliability and the associated costs.

The following are some options and recommendations related to facility reliability under some
situations that could be experienced by some utilities. A general assumption is made that all
facilities are being properly operated and maintained.

Conceivably, the most reliable system would have multiple sources (also a “Ten-States Standards”
recommendation) of supply, each delivering water to different entry points in the distribution
system and to different locations in the system. The source pumps would be controlled
automatically by level controls in multiple gravity storage tanks and would have standby power
facilities that would be automatically activated when power supply to the area is interrupted. This
combination of sources would be able to meet peak system demands without relying on storage and
be able to replenish depleted storage tanks, while meeting normal demands, within a 24-hour
period. When the largest source is not available, the remaining sources would be able to meet the
average daily demands of the system. Storage would be sized to provide a standby volume at a
pressure of at least 20 psi - sufficient to accommodate the needs of all consumers throughout the
duration of any problem. Finally, the water system would have an alarm system such that the
operator would be notified of abnormal conditions, such as an overflow or a very low water level
condition in the reservoir, a malfunctioning pump, a system control failure, etc. Most systems do
not have, nor may be realistically expected to have, all of these features to insure maximum
reliability. Clearly, as more of these features are incorporated into the design and construction of
the system facilities, there will generally be an increase in the reliability of water service.

Considerations Relative to Increasing Reliability

Multiple sources of supply provide increased reliability since if one source, or its corresponding
pump and/or controls is taken off-line, fails, or becomes contaminated, service can still be
provided. Dispersing the locations of sources would lower the probability of a contamination
plume impacting more than one source. It also allows for smaller pipelines, as the water supply is
closer to the point of use.

If the capacity of the sources were sufficient to provide for peak day service, it would enable a
purveyor to isolate any storage tank at any time for inspection, cleaning, or repair without
interrupting service to consumers. There would also be greater assurance for fire protection with
higher-capacity sources providing a more rapid replenishment of storage tanks following times of
service outage. Recognizing the constraints on the availability of additional water resources and
associated water rights, DOH recommends that a system not be designed on a basis of 24-hour
pumping to meet peak day demands. Having source capacity of 120% to 130% of projected peak
day demands allows for a factor of safety and the ability to react to unexpected demand scenarios.

Multiple pumps in a well would be more reliable than a single pump (i.e., water could still be
provided even if any individual pump failed). Service would be interrupted for a short time,
however, if the pumps are on the same riser pipe. When the pipe column is pulled to repair or
replace the failed pump, the remaining pumps would also be off. Multiple pumps in a well, or a
number of closely spaced wells drawing from the same aquifer, are not as reliable as dispersed

Geographically dispersed sources are more apt to be served by different power grids leading to less
disruption from localized power outages. Also, if a single well or well field were to become
contaminated, the system would be totally without a potable supply until treatment is provided or
another source becomes available.

Gravity storage tanks allow for delivery of water, including fireflow, when neither power nor
sources are available. (A source may be taken off-line for maintenance or repair, a pump or control
failure, a problem with the transmission pipe, or because it may be contaminated). Having more
than one tank allows for one to be taken out of service for inspection, cleaning, or repair without
interrupting service to consumers.

One factor a community needs to consider is whether or not the sewer system relies on power
for pumping and, as such, may also be nonfunctional during a power outage. Provision of
water service under this situation may result in sanitary sewer backups or overflows increasing
the risk to public health. If a system is capable of delivering water during a power outage, the
community should also provide for the collection and pumping of wastewater under those

If gravity storage from a low to moderately high reservoir (less than 70 feet) is not feasible due to
topography, the purveyor should evaluate the alternative of structurally elevating storage for the
system before deciding on ground level storage with booster pumping facilities. DOH
recommends having standby power available when relying on pumps to deliver water and maintain
system pressure. An on-site generator that starts automatically when power supply is interrupted is
preferable, especially if the provision of fireflow relies on pumps. The generator should be sized to
provide the startup power requirements [kilowatts (kW)] for the pump(s), which usually exceed the
running power requirements. Another alternative is an engine driven pump. This alternative
requires consideration of transporting and storing fuel for the generator or engine. The purveyor
may choose to use a portable generator that can be moved from site to site as needed. In this case,
it is beneficial to have the connection and transfer switch established in advance so that it is quick
and easy to use the portable generator. Reliable power to the site may also be provided by the
power supplier through multiple primary leads from separate substations.

A summary of DOH recommendations on general system reliability is included in Chapter 5 of this
manual. Recommendations related to specific system components are also located in the chapters
addressing those specific components.

Reliability is also a consideration in longer-range planning activities for a water system.
Plans to ensure long-range water system reliability should, at a minimum, address (1) water
shortage response activities, (2) long-term adequacy of water rights for meeting growth
expectations of the system, and (3) conservation as a mitigating practice to reduce the
frequency or degree of curtailment when water availability is marginal. See the DOH Water
System Planning Handbook for further information and detail regarding these concepts for
longer-range system reliability.