Primary Treatment (Sedimentation) - Wastewater Operators - page 250

Scum must be removed from the surface of the settling tank fre-
quently. This is normally a mechanical process but may require manual
start-up. The system should be operated frequently enough to prevent
excessive buildup and scum carryover but not so frequently as to cause
hydraulic overloading of the scum removal system. Settling tanks require
housekeeping and maintenance. Baffles (prevent floatable solids, scum,
from leaving the tank), scum troughs, scum collectors, effluent troughs,
and effluent weirs require frequent cleaning to prevent heavy biological
growths and solids accumulations. Mechanical equipment must be lubri-
cated and maintained as specified in the manufacturer's recommenda-
tions or in accordance with procedures listed in the plant's operations
and maintenance (O&M) manual.

Process control sampling and testing are used to evaluate the per-
formance of the settling process. Settleable solids, dissolved oxygen,
pH, temperature, total suspended solids, and BOD 5 , as well as sludge
solids and volatile matter, testing is routinely carried out.

6.2.3 operator Considerations

Before identifying a primary treatment problem and proceeding
with the appropriate troubleshooting effort, the operator must be cog-
nizant of what constitutes normal operation (is the system operating as
per design or is there a problem?). Several important items of normal
operation can have a strong impact on performance. In the following
section, we discuss the important operational parameters and normal
observations.

6.2.3.1 Primary Clarification: Normal Operation

In primary clarification, wastewater enters a settling tank or basin.
Velocity reduces to approximately 1 fpm. Solids heavier than water set-
tle to the bottom, while solids lighter than water float to the top. Settled
solids are removed as sludge and floating solids are removed as scum.
Wastewater leaves the sedimentation tank over an effluent weir and
moves on to the next step in treatment. Detention time, temperature,
tank design, and condition of the equipment control the efficiency of
the process.

Note: Again, notice that the velocity is based on minutes instead of sec-
onds, as was the case in the grit channels. A grit channel velocity of 1
fps would be 60 fpm.

6.2.3.2 Primary Clarification: Operational Parameters

• flow distribution —Normal flow distribution is indicated by flow

to each in-service unit being equal and uniform with no indica-
tion of short-circuiting. The surface loading rate is within design
specifications.