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

have also been employed in some units. While filters may produce highquality
effluent, they are very susceptible to both internal and
external clogging. The behavior of clarifiers is dependent upon biomass
settling properties,
D5sign2peak hydraulic
solids loading
overflow rates
rate, and hydraulic overflow ra es.
should be less than 800 gpd ft 5 (32
m /d/m 1; $nd at averag flo design values normally range from 200 to
400 gpd/ft (J to 16 m'/d,$rn'l Solids loading rates are usually less
than $0 lb/ft /d ($45 kg/m /dl'based upon average flow and less than 50
lb/ft /d (242 kg/m /d) based upon peak flows.
f. Biomass Return
Once separated from the treated wastewater, the biomass must be returned
to the aeration tank or be wasted. Air lift pumps, draft tubes working
off the aerator, and gravity return methods are normally used. Batch
units and plants that employ filters do not require sludge return.
Rapid removal of solids from the clarifier is desirable to avoid deni-
trification and subsequent floatation of solids. Positive sludge return
should be employed in package plants since the use of gravity return
systems has generally proved ineffective (2)(20).
Removal of floating solids from clarifiers has normally been ignored in
most onsite package plant designs. Since this material results in
serious deterioration of the effluent, efforts should be made to provide
for positive removal of this residue. Reliance on the owner to remove
floating scum is unrealistic.
!I* Biomass Wasting
Most onsite package plants do not provide for routine wasting of solids
from the unit. Some systems, however, do employ an additional chamber
for aerobic digestion of wasted sludge. Wasting is normally a manual
operation whereby the operator checks mixed liquor solids and wasted
sludge when mixed liquor concentrations exceed a selected value. In
general, wasting should be provided once every 8 to 12 months (2)(35).
h. Controls and Alarms
Most aerobic units are supplied with some type of alarm and control sys-
tem to detect mechanical breakdown and to control the operation of elec-
trical components. They do not normally include devices to detect
effluent quality or biomass deterioration. Since the control systems
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