Secondary Treatment - Wastewater Operators - page 270

Note: To ensure sufficient airflow to the filter, the underdrains should
never be allowed to flow more than 50% full of wastewater.
The effluent channel is designed to carry the flow from the trickling
filter to the secondary settling tank. The secondary settling tank pro-
vides 2 to 4 hours of detention time to separate the sloughing materials
from the treated wastewater. Design, construction, and operation are
similar to those for the primary settling tank. Longer detention times
are provided because the sloughing materials are lighter and settle more
slowly.
Recirculation pumps and piping are designed to recirculate (and
thus improve the performance of the trickling filter or settling tank) a
portion of the effluent back to be mixed with the filter influent. When
recirculation is used, pumps and metering devices must be provided.
7.3.3 filter Classifications
Trickling filters are classified by hydraulic and organic load-
ing. Moreover, the expected performance and the construction of the
trickling filter are determined by the filter classification. Filter classifi-
cations include standard-rate, intermediate-rate, high-rate, super-high-
rate (plastic media), and roughing types. Standard-rate, high-rate, and
roughing-rate are the filter types most commonly used. The standard-
rate filter has a hydraulic loading that varies from 25 to 90 gpd/ft 3 . It has
a seasonal sloughing frequency and does not employ recirculation. It
typically has an 80 to 85% BOD 5 removal rate and 80 to 85% TSS removal
rate. The high-rate filter has a hydraulic loading of 230 to 900 gpd/ft 3 .
It has a continuous sloughing frequency and always employs recircula-
tion. It typically has a 65 to 80% BOD 5 removal rate and 65 to 80% TSS
removal rate. The roughing filter has a hydraulic loading of >900 gpd/ft 3 .
It has a continuous sloughing frequency and does not normally include
recirculation. It typically has a 40 to 65% BOD 5 removal rate and 40 to
65% TSS removal rate.
7.3.4 standard operating Procedures
Standard operating procedures for trickling filters include sampling
and testing, observation, recirculation, maintenance, and expectations
of performance. The collection of influent and process effluent samples
to determine performance and to monitor the process condition of trick-
ling filters is required. Dissolved oxygen, pH, and settleable solids testing
should be performed daily. BOD 5 and suspended solids testing should be
done as often as practical to determine the percent removal.
The operation and condition of the filter should be observed daily.
Items to observe include the distributor movement, uniformity of distri-
bution, evidence of operation or mechanical problems, and the presence
of objectionable odors. In addition, normal observation for a settling
tank should also be performed.