On-Site Wastewater Treatment and Disposal Systems - Forced ...

7.2.4.6 Considerations for Multi-Home and Commercial
Wastewaters
Designs of the mound system for larger flows follow the same design
principles as for smaller flows. In cluster systems serving more than
five homes, however, peak flow estimates can be reduced because of flow
attenuation, but contributions from infiltration through the collection
system must be included. Peak flow estimates should be based on the to-
tal number of people to be served (see Chapter 4). Rates of infiltra-
tion vary with the type of collection sewer used (19)(20).
With commercial flows, the character of the wastewater is an important
consideration. Proper pretreatment is necessary if the character is
significantly different than domestic wastewater.
Modifications to the design of the mound system may be desirable for
larger flows on sloping sites or in slowly permeable soils. In both
instances, the absorption area should be broken up into a series of
trenches or smaller beds. This is beneficial on sloping sites because
the beds can be tiered to reduce the amount of fill required (see Figure
7-14). Depths of fill material below beds should not exceed 4 to 5 ft
(1.3 to 1.7 ml because differential settling will cause the bed to set-
tle unevenly. If the system is tiered, each trench or bed must be dosed
individually. This can be done by automatic valving or alternating
pumps or siphons.
In sites with slowly permeable soils, breaking the absorption area into
smaller trenches or beds helps distribute the effluent over much wider
areas. Spacing of the beds or trenches should be sufficient so that the
wastewater contributed from one trench or bed is absorbed by the natural
soil before it reaches the lower trench or bed (see Table 7-10). The
beds or trenches should be as long as the site allows. A long bed, bro-
ken into several shorter systems, each served by a pump or siphon, is
preferred over two or more short parallel beds, especially in soils
where the effluent moves downslope.
Flexibility in operation should also be incorporated into systems serv-
ing larger flows, since a failure can create a significant problem.
Alternating bed. 'systems should be considered. A three-bed system is
suggested where each bed contains 50% of the required absorption area
(19). Two beds are always in operation, providing 100% of the needed
infiltrative surface. The third bed is alternated into service on a
yearly schedule. Thus, each field is in service for two years and
"rested" for one year to rejuvenate. The third bed also acts as a
standby unit in case one bed fails. The idle fields can be put into
service immediately while the failed bed is rehabilitated.
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