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

which conve.rt nitrate to nitrogen gases (61). Numerous microorganisms
are capable of carrying out this process, provided there is an organic
carbon source available. These organisms are less sensitive to environ-
mental conditions than the nitrifiers, but the process is temperature-
dependent. Process design and operational details for conventional de-
nitrification processes are discussed in reference (61).
Design and operational experience with onsite biological denitrification
systems is limited at this time (2)(65)(66). Several systems have been
suggested for onsite application, two of which are shown in Figure 6-
18. One employs a packed bed containing approximately 3/8-in. (l-cm)
stone that receives, on a batch basis, effluent from a nitrification
process. The nitrified wastewater flows to a dosing tank, where it is
held until a predetermined volume is obtained. Methanol (or other or-
ganic carbon source) is then added to provide a C:N ratio of approxi-
mately 3:l. After approximately 15 min, the wastewater is pumped up
through the anoxic packed stone bed. Effluent flows from the top of the
bed. Liquid retention times in the packed bed (based on void volume)
varying from 12 to 24 hours have been employed with good results (2).
Pumping may be provided by a l/3-hp submersible pump actuated by a
switch float within the sump. A small chemical feed pump controlled by
a timer switch may be used to feed the organic carbon source to the
sump. A 30% methanol solution may be used as the carbon source. Other
organic carbon sources include septic tank effluent, graywater, and
molasses. Metering of organic carbon source to the nitrified wastewater
requires substantial control to ensure a proper C:N ratio. Insufficient
carbon results in decreased denitrification rates, whereas excess carbon
contributes to the final effluent BOD (61). The use of an easily ob-
tainable, slowly decomposable, solid carbon source could also be con-
sidered. Peat, forest litter, straw, and paper mill sludges, for exam-
ple, could be incorporated as a portion of the upflow filter. Control
of the denitrification process using these solid carbon sources would be
difficult.
Another onsite nitrification-denitrification system that has been field
tested employs a soil leach field (66) (Figure 6-18). Septic tank ef-
fluent is distributed to a standard soil absorption field. An imperme-
able shield of fiberglass is placed approximately 5 ft (1.5 m) below the
distribution line. The location of this collector should be deep enough
to ensure complete nitrification within the overlying unsaturated soil.
The nitrified wastewater is collected on the sloped fiberglass shield,
and directed to a 24-in. (61-cm) deep bed of pea gravel contained within
a plastic liner (denitrifying reactor). The gravel bed is sized deep
enough to provide a hydraulic detention time of approximately 10 days
(based on void volume). Methanol or other energy source is metered to
the gravel bed through a series of distributors. The gravel bed is
vented with vertical pipes to allow escape of nitrogen gas evolved in
the process. Short-term experience with this system has been good.
Total nitrogen concentrations of less than 1 mg/l-N were achievable in
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