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9.10 CHAPTER NINE

on the roof. If funds are available, a permanent lighting system increases visibility and
eliminates hazards associated with portable lighting.
Because of high-humidity conditions, the lighting design must incorporate corrosion-
resistant materials such as aluminum and stainless steel. Lighting units must include in-
tegral gaskets and methods of sealing wire penetrations to minimize moisture within the
units. Conduit systems must use corrosion-resistant materials.
Lighting units must be mounted in a manner that minimizes damage from high water
levels or from operating cleaning equipment. Lighting levels on the order of 10 fc (100
lm/m 2) provide sufficient light to allow efficient and safe operations.
Ventilation. Little consideration was given to ventilation in older, covered slow sand
filters. Usually ventilation consisted of open hatches with portable ventilators installed
during maintenance operations. Although portable units increase ventilation, the actual
rates are generally below those required by code when engine-driven equipment is being
operated inside the filter unit.
For newer installations, ventilation rates for motorized equipment must be designed to
protect operators and should be similar to those provided in parking garages. Rates must
be set according to expected emissions of motorized equipment operated within the basin.
The number of air changes required is generally governed by state or federal regulations.
Filter Roofing System. Selection of a filter roofing system must consider capital ex-
pense, long-term reliability, headroom, availability of material, and site conditions. Cast-
in-place systems are generally the most expensive but are the most durable. Precast con-
crete slabs are less expensive than cast-in-place but have a slightly shorter life span.
Systems constructed of wood or steel experience significant deterioration or corrosion
problems and require replacement within a relatively short time. Obviously, the replace-
ment expense versus initial capital expense must be evaluated in design.
Methods of covering the roof system include shingles, metal roofing, single-ply mem-
brane, built-up roofing, and earthen fill. Again, each system has specific capital costs, and
not all are usable with each roof support system. An earth cover system can help blend a
facility into its surrounding but requires a strong support structure to carry the load.

Filter-to-Waste System. The design of a slow sand filter should provide the ability to
discharge filtered water to waste. The filter-to-waste system allows the operator to check
effluent quality after a filter cleaning and before bringing the filter back on line. It also
provides a method to "cleanse" filter media after resanding or other major reconstruction
work.
The filter-to-waste system must be designed to avoid a potential cross-connection. The
ideal system includes an air gap on the filter effluent to the drain pipe. However, many
regulators allow a hard piped connection to a drain system, with valving to direct water
to finished water storage or to waste. In the latter case, it is normally required to install
the filter-to-waste line at an elevation below the filtered water line and to provide an air
gap at the ultimate discharge point to minimize potential contamination.
Filter Draining. At some point in the filter cycle, head losses increase to such a level
that filter production is unacceptably low, and the filter must be taken off-line for clean-
ing. Because the rate is so low through the filter and the hydraulics between adjoining fil-
ters may not allow significant drop in filter level to occur, some form of separate drain
should be provided.
This drain should generally be installed to discharge filter supernatant either back to
the source or to a lagoon. The drain can also be connected to the filter-to-waste system
through an air gap, or it may be installed as a separate system. Cross-connection to the