In wastewater treatment, the clarification process removes suspended solids using gravity. The secondary function of a wastewater clarifier is to remove accumulated scum or floating matter on the surface. This blog post discusses the primary and secondary clarification processes, the types of wastewater clarifiers and how an experienced wastewater process engineer can help determine the right clarifier for your local wastewater treatment plant (WWTP).
Gravity primary clarification is the cheapest Biological Oxygen Demand (BOD) and Suspended Solids removal that can be implemented, and the design details are often overlooked. The primary clarification process, or sedimentation, is the first step in removing suspended solids, oil and grease from raw wastewater. Primary wastewater clarifiers separate settleable solids from wastewater flow before it progresses to biological treatment.
A primary clarifier's efficiency is measured by BOD and total suspended solids (TSS) removal efficiency. A well-designed primary clarifier can remove 25-35% of influent BOD and 50-65% of influent TSS. For optimal efficiency, POTWs should consider factors like hydraulic detention time and surface overflow rate as key design criteria for sizing primary clarifiers. Based on dry weather and wet weather conditions, optimal sludge blanket depth varies from 1-3 feet with primary sludge concentrations from 3-5%.
There are two general configurations for primary clarifiers:
Today, cloth media primary filtration is being implemented at a number of facilities and this small footprint technology can often be retrofitted into the space once occupied by rectangular primary clarifiers. As the technology matures, it has the promise of replacing gravity primary clarification.
The purpose of secondary clarification is to separate flocculated biomass from the treated liquid waste stream. A portion of the biomass is returned to the system, return activated sludge, and the excess is wasted, waste activated sludge. Today, the vast majority of secondary clarifiers are designed to be circular. The two most common configurations of circular secondary clarifiers are:
Clarifiers used for wet weather conform to the same principles as primary and secondary clarifiers in conventional scenarios. Wet weather produces significant grit washout from sewer and grit chambers to primary clarifiers. Secondary clarifiers may also experience extremely low sludge volume index. Increased and diluted influent plant flows affect the clarifier sludge blanket, creating transient flows that can diminish the performance of primary and secondary clarifiers.
When designing efficient clarifiers to handle wet weather, various ballasted flocculation processes warrant consideration. Such processes involve adding a coagulant and settling ballast (micro-sand) to the primary clarifier influent and installing inclined tubes (lamellas) in the clarifiers. Chemically enhanced primary treatments using coagulants force solids to coagulate more quickly and serve as a feasible alternative to increasing the biological treatment capacity for WWTPs during unexpected wet weather flows.
A high-solids separation facility can also be designed to use as a primary clarifier during wet weather. Increasing the design depth of clarifiers to a side water depth of at least 4.3 to 5 meters can help prevent sludge blanket washouts for WWTPs experiencing prolonged wet weather conditions.
There are several innovative technologies used to design primary, secondary and wet weather clarifiers and improve the effluent quality of treatment plants. A team of experienced wastewater process engineers can help you determine the best solution for your local WWTP.
Below is a list of general considerations when preparing to install new or replacement clarifiers.
At Fehr Graham, our team of wastewater process engineers has been helping communities across the U.S. improve the effluent quality of wastewater treatment plants. From designing collection systems, treatment systems and nutrient removal methods to ensuring environmental compliance or securing funding sources, we are your one-stop resource for wastewater treatment.