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8.36 CHAPTER EIGHT

during backwashing. Center supports are typically used whenever trough length exceeds
14 ft (4.3 m).

FILTER DESIGN CRITERIA

The first step for the design engineer when designing filtration facilities should be to re-
view all current federal, state, and local laws and regulations that may be applicable. The
second step should be to review all applicable standards prepared by various organiza-
tions and associations.

Standards Set by Regulatory Agencies
The first design criterion to be considered for a filtration process is its ability to meet ap-
plicable water quality standards. The U.S. Environmental Protection Agency (USEPA)
has established the Surface Water Treatment Rule (SWTR) and the Enhanced Surface
Water Treatment Rules (ESWTR) as the controlling standards for filtration of surface wa-
ter and groundwater under the direct influence of surface water. The SWTR and ESWTR
require a public water supplier using a surface water or groundwater source deemed "un-
der the influence of surface water" to achieve 3-log (99.9%) removal and/or inactivation
of Giardia, 4-log (99.99%) removal and/or inactivation of viruses, and 2-log (99%) re-
moval of Cryptosporidium.
Credit for log removal is given to filtration processes based on their type, with the re-
maining required log removal to be achieved by disinfection. Conventional filtration is
usually given a 2.5-1og credit for Giardia removal; direct, slow sand, and diatomaceous
earth filtrations are usually given a 2-log credit; and all filtration processes are usually
given a 2-log credit for Cryptosporidium removal, provided turbidity limits are met.
Maximum turbidity levels are established for filtration performance standards. For con-
ventional and direct filtration, filtered water turbidity must be less than 0.3 ntu in 95% of
samples collected in a month. For slow sand and diatomaceous earth filtration, less than
1.0 ntu must be achieved in 95% of the samples. A level of 1 ntu must not be exceeded
at any time in conventional and direct filtration, and 5 ntu for slow sand and diatoma-
ceous earth filtration. These water quality limits must be considered in designing filtra-
tion facilities, as well as design of the entire treatment train.
In addition to drinking water quality standards, most states have adopted minimum
standards for designing filtration facilities, either with specific criteria or reference to other
design standards. The most widely known and adopted standard is Recommended Stan-
dards for Water Works, commonly known as the "Ten States Standards" (Great Lakes,
2003). This standard contains specific criteria on filter size, arrangement, allowable fil-
tration rates, structural details, hydraulics, materials of construction, filter washing, and
control systems. In some instances, states require pilot testing of treatment processes to
establish their validity and design criteria.
Standards that limit the recycle of waste washwater to the plant headworks and dis-
charge to surface water sources have also been established by the USEPA. Recycle should
generally be provided with some minimum level of pretreatment and be returned ahead
of the introduction of coagulants. Waste discharges to a waterway usually require the im-
plementation of a federal National Pollutant Discharge Elimination System (NPDES) per-
mit. If a sanitary sewer is available, local sewer use ordinances (based on wastewater treat-
ment plant discharge limits set by an NPDES permit) usually require pretreatment of the
waste washwater. State or local regulations may also require recycling of waste washwa-