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

Anionic polyelectrolytes can be used in combination with the aluminum
and iron salts to improve settling, but may overly complicate the onsite
treatment system.
The required dosages of aluminum and iron compounds are generally re-
ported as molar ratios of trivalent metal salt to phosphate phosphorus.
Molar ratios currently used in practice today range from 1.5:1 to 4:1,
depending upon wastewater characteristics, point of addition, and de-
sired phosphorus removal (20)(62).
Adding aluminum or iron salts to the raw wastewater prior to the septic
tank has the advantage of using the existing septic tank for sedimenta-
tion (70). Aluminum or iron salts may be metered to the raw wastewater
with a chemical feed pump activated by electrical or mechanical impulse.
Mixing of the chemical with the wastewater is provided in the sewer line
to the septic tank. The quantity of metal salt added to.the wastewater
is dependent upon wastewater characteristics. Since the impulse to the
feed pump may come from any of a number of household events, it is not
possible to precisely adjust metal dosage. An average dose of salt
based on estimated phosphorus discharge is most practical.
Addition of iron or aluminum salts following the septic tank may also be
considered. A batch -feed system could be employed whereby a preset
chemical dose is provided when the wastewater reaches a preset volume in
a holding tank. Mixing may be provided by aeration or mechanical mixer,
followed by a period of quiescence. Additional raw wastewater flow
would be diverted to a holding tank until the precipitation-sedimenta-
tion cycle is completed. This system may be employed after the septic
tank and preceding the intermittent sand filter.
The processes briefly described above represent a few of the many chemi-
cal treatment processes that might be considered for onsite treatment.
They may be designed and constructed to fit the specific needs of the
site, or purchased as a proprietary device. Storage and holding of
chemicals must be considered in the design of these systems. Details on
chemical storage, feeding, piping, and control systems may be found
elsewhere (20)(62). Attention must be given to appropriate materials
selection, since many of the metal salts employed are corrosive in
liquid form.
b. Operation and Maintenance
Every effort should be made to select equipment that is easily operated
and maintained. Nonetheless, chemical precipitation systems require
semi-skilled labor to maintain chemical feed equipment, mixers, pumps,
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