Primary Treatment

Question 1

A properly operating primary sedimentation tank will remove nearly all of this.

Answer 1

Settleable Solids

Question 2

Typical removal efficiency for primary treatment:
BOD,
Total Suspended Solids (TSS)
Settleable Solids

Answer 2

Removal efficiency:
□ 20-30%
□ 40-60%
□ greater than 90%

Question 3

Function of flights in a rectangular clarifier

Answer 3

• Move settled sludge toward the sludge hopper
• Move floatable solids and scum toward the scum hopper

Question 4

Normal design hydraulic loading for primary sedimentation

Answer 4

1000 gal per day/sq ft of clarifier surface

Question 5

Characteristics of septic primary clarifier sludge

Answer 5

• Dark grey to black color
• Extremely offensive odor
• Low pH (less than 5)

Question 6

Objective of primary treatment.

Answer 6

Remove settleable and floatable solids

Question 7

Factors that affect the efficiency of a primary clarifier

Answer 7

• Hydraulic loading (excess flow can “wash out” the solids from a settling basin)
• Wastewater temperature (solids settle more rapidly in warm sewage)
• Wastewater strength (settleable solids flocculate poorly in dilute sewage)

Question 8

Function of inlet baffle in a circular primary clarifier.

Answer 8

• Direct flow uniformly through the clarifier AAAANNNNN
• Dissipate influent velocity

Question 9

Major source of odor when primary clarifier sludge is allowed to become septic.

Answer 9

Hydrogen sulfide gas (H2S)

Question 10

Control test used to estimate the efficiency of the primary clarifier and estimate the
volume of sludge that must be pumped from the clarifier

Answer 10

• Settleable solids test, utilizing an imhoff cone
• Compare the settleable solids results of the primary clarifier effluent and the
  raw sewage (influent); clarifier efficiency can be estimated.
• Observe the settled volume on the primary influent sample to estimate the
  sludge volume to be pumped.

Question 11

Pumping primary clarifier sludge too fast can cause this problem

Answer 11

Sludge “coning” in the sludge hopper, resulting in excess water being pumped along with
the sludge

Question 12

Indications of septicity and rising or rafting sludge in a primary clarifier.

Answer 12

• Bubbles appear across the clarifier surface, or appear above the location of the
  clarifier sweep arms or flights.
• Drop in sewage pH between the clarifier influent and the clarifier effluent.
• Clarifier D.O. is zero, and/or D.O. drops dramatically between clarifier influent and
  clarifier effluent

Question 13

Typical primary clarifier sludge characteristics.
Total solids
Volatile solids

Answer 13

Total Solids: 3 - 7%
Volatile solids: 65 - 75%

Question 14

Factors that can promote rising or rafting primary clarifier sludge

Answer 14

• Warm sewage temperature
• Too low sludge pumping rate (inadequate sludge removal)
• Excessively high raw sewage BOD;

Question 15

The cause of rafting or rising primary clarifier sludge.

Answer 15

Sludge decomposition and gasification. Septic primary clarifier sludge releases mostly
CO, with smaller amounts of methane and hydrogen sulfide gas

Question 16

Typical primary clarifier hydraulic detention time (HDT).

Answer 16

1-2 hours

Question 17

Conditions that may cause primary clarifier short circuiting.

Answer 17

• Uneven effluent launder weirs
• Missing or damaged influent baffles

Question 18

Another term for primary clarification.

Answer 18

Plain sedimentation

Question 19

The most direct method of determining the consistency of primary clarifier sludge.

Answer 19

Collect a sample from the primary clarifier sludge line during the middle of a pumping
cycle.

Question 20

The reason that clarifier effluent launderer weirs are “saw toothed.”

Answer 20

This design provides optimum weir cleansing velocity and even flow distribution.

Question 21

Factors that determine the hydraulic detention time in a settling tank.

Answer 21

Tank volume and flow rate (HDT = tank volume + flow rate, V/Q)

Question 22

Factors that contribute to efficient primary settling

Answer 22

• Warm sewage temperatures (the density of warm water is slightly less than cold water)
• Medium to high strength sewage (increases natural flocculation and settling)
• Proper baffling which reduces short circuiting
• Overflow rate at or below 1000 gpd/ft2 (the overflow rate measures the hydraulic
  loading to the clarifier)
• Regular removal of settled sludge to avoid septicity
• Pre-aeration (enhances flocculation of floatable solids and settleable solids)