A properly operating primary sedimentation tank will remove nearly all of this.
Settleable Solids
Typical removal efficiency for primary treatment:
BOD,
Total Suspended Solids (TSS)
Settleable Solids
Removal efficiency:
□ 20-30%
□ 40-60%
□ greater than 90%
Function of flights in a rectangular clarifier
- Move settled sludge toward the sludge hopper
- Move floatable solids and scum toward the scum hopper
Normal design hydraulic loading for primary sedimentation
1000 gal per day/sq ft of clarifier surface
Characteristics of septic primary clarifier sludge
- Dark grey to black color
- Extremely offensive odor
- Low pH (less than 5)
Objective of primary treatment.
Remove settleable and floatable solids
Factors that affect the efficiency of a primary clarifier
- 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)
Function of inlet baffle in a circular primary clarifier.
- Direct flow uniformly through the clarifier AAAANNNNN
- Dissipate influent velocity
Major source of odor when primary clarifier sludge is allowed to become septic.
Hydrogen sulfide gas (H2S)
Control test used to estimate the efficiency of the primary clarifier and estimate the
volume of sludge that must be pumped from the clarifier
- 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.
Pumping primary clarifier sludge too fast can cause this problem
Sludge “coning” in the sludge hopper, resulting in excess water being pumped along with
the sludge
Indications of septicity and rising or rafting sludge in a primary clarifier.
- 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
Typical primary clarifier sludge characteristics.
Total solids
Volatile solids
Total Solids: 3 - 7%
Volatile solids: 65 - 75%
Factors that can promote rising or rafting primary clarifier sludge
- Warm sewage temperature
- Too low sludge pumping rate (inadequate sludge removal)
- Excessively high raw sewage BOD;
The cause of rafting or rising primary clarifier sludge.
Sludge decomposition and gasification. Septic primary clarifier sludge releases mostly
CO, with smaller amounts of methane and hydrogen sulfide gas
Typical primary clarifier hydraulic detention time (HDT).
1-2 hours
Conditions that may cause primary clarifier short circuiting.
- Uneven effluent launder weirs
- Missing or damaged influent baffles
Another term for primary clarification.
Plain sedimentation
The most direct method of determining the consistency of primary clarifier sludge.
Collect a sample from the primary clarifier sludge line during the middle of a pumping
cycle.
The reason that clarifier effluent launderer weirs are “saw toothed.”
This design provides optimum weir cleansing velocity and even flow distribution.
Factors that determine the hydraulic detention time in a settling tank.
Tank volume and flow rate (HDT = tank volume + flow rate, V/Q)
Factors that contribute to efficient primary settling
- 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)
