Geoscience Reference
In-Depth Information
environmental laboratories. Water and wastewater laboratories usually perform a number of differ-
ent tests. Lab test results provide the operator with the information necessary to operate the treat-
ment facility at optimal levels. Laboratory testing usually includes determining service line flushing
time, solution concentration, pH, COD, total phosphorus, fecal coliform count, chlorine residual,
and BOD (seeded), to name a few. The standard reference for performing wastewater testing is con-
tained in Standard Methods for Examination of Water & Wastewater (Clesceri et al., 1999).
In this section, the focus is on standard water/wastewater lab tests that involve various calcula-
tions. Specifically, the focus is on calculations used to determine the proportioning factor for com-
posite sampling, flow from a faucet estimation, service line flushing time, solution concentration,
biochemical oxygen demand, molarity and moles, normality, settleability, settleable solids, biosol-
ids total, fixed and volatile solids, suspended and volatile suspended solids, and biosolids volume
index and biosolids density index.
24.15.2 C omposite s ampling p roCedures and C alCulations
When preparing oven-baked food, the cook sets the correct oven temperature and then usually
moves on to some other chore. The oven thermostat maintains the correct temperature, and that is
that. Unlike the cook, in water and wastewater treatment plant operations the operator does not have
the luxury of setting a plant parameter and then walking off and forgetting about it. To optimize
plant operations, various adjustments to unit processes must be made on an ongoing basis.
The operator makes unit process adjustments based on local knowledge (experience) and on lab
test results; however, before lab tests can be performed, samples must be taken. The two basic types
of samples are grab samples and composite samples. The type of sample taken depends on the spe-
cific test, the reason the sample is being collected, and requirements in the plant discharge permit.
A grab sample is a discrete sample collected at one time and one location. It is primarily used
for any parameter whose concentration can change quickly (e.g., dissolved oxygen, pH, temperature,
total chlorine residual) and is representative only of the conditions at the time of collection.
A composite sample consists of a series of individual grab samples taken at specified time inter-
vals and in proportion to flow. The individual grab samples are mixed together in proportion to the
flow rate at the time the sample was collected to form the composite sample. The composite sample
represents the character of the water/wastewater over a period of time. Because knowledge of the
procedure used in processing composite samples is important (a basic requirement) to the water/
wastewater operator, the actual procedure used is covered in this section:
1. Determine the total amount of sample required for all tests to be performed on the com-
posite sample.
2. Determine the average daily flow of the treatment system.
Key Point: Average daily flow can be determined by using several months of data, which will pro-
vide a more representative value.
3. Calculate a proportioning factor:
Totalsample volu
me required(mm)
Proportioning factor (PF)
=
(24.170)
No.ofsamples
×
Averagedai
ly flow (MGD)
Key Point: Round the proportioning factor to the nearest 50 units (e.g., 50, 100, 150) to simplify
calculation of the sample volume.
4. Collect the individual samples in accordance with the schedule (e.g., once/hr, once/15 min).
5. Determine flow rate at the time the sample was collected.
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