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

Nitrification of septic tank effluents occurs readily within intermit-
tent sand filters (see Section 6.3). Field expfrience ingicstes that
intermittent sand filters loaded up to 5 gpd/ft (0.02 cm /m /d), and
properly maintained to avoid excessive ponding (and concomitant anaero-
bic conditions), converts up to 99% of the influent ammonia to nitrate-
nitrogen (2). Aerobic biological package plants also provide a high
degree of nitrification, provided solids retention times are long and
sufficient oxygen is available (see Section 6.6).
The biological denitrification (nitrates to nitrogen gases) of waste-
water follows a nitrification step (61). There has been little experi-
ence with long-term field performance of onsite denitrification pro-
cesses. Ideally, total nitrogen removal in excess of 90% should be
achievable, if the system is properly operated and maintained (61).
C. Design- and Construction Features
Septi;*Tanis: There are no septic tank design requirements specifically
estab lshe to enhance high levels of nitrogen removal. Designs that
provide excellent solid-liquid separation ensure lower concentrations of
nitrogen associated with suspended solids.
Nitrification: Biological nitrification is achieved by a select group
of aerobic ml'croorganisms referred to as nitrifiers (61). These organ-
isms are relatively slow-growing and more sensitive to environmental
conditions than the broad range of microorganisms found in biological
wastewater treatment processes. The rate of growth of nitrifiers (and
thus the rate of nitrification) is dependent upon a number of parame-
ters, including temperature, dissolved oxygen, pH, and certain toxi-
cants. The design and operating parameter used to reflect the growth
rates of nitrifiers is the solids retention time (SRT). Details of the
impact of temperature, dissolved oxygen, pH, and toxicants on design SRT
values for nitrification systems are outlined in reference (61). In
brief, biological nitrification systems are designed with SRT values in
excess of 10 days; dissolved oxygen concentrations should be in excess
of 2.0 mg/l; and pH values should range between 6.5 and 8.5. Toxicants
known to be troublesome are discussed in reference (61).
Details of the design and construction of intermittent sand filters and
aerobic package plants are found in Sections 6.3 and 6.4. In general,
designs normally employed for onsite application of these processes to
remove BOD and solids are sufficient to encourage nitrification as well.
Denitrification: Biological denitrification is carried out under anoxic
conditions in the presence of facultative, heterotrophic microorganisms
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