Understanding Wastewater Primary Treatment Calculations for Optimal Performance (2023)


In the realm of wastewater treatment, achieving effective primary treatment is crucial for setting the stage for subsequent processes. This article delves into the intricacies of wastewater primary treatment calculations, focusing on key parameters that play a pivotal role in the efficiency of the primary treatment process.

Surface Loading Rate: A Critical Measure

Definition and Significance

Surface loading rate is a fundamental metric that gauges the volume of wastewater passing over a given area. In primary treatment, this rate directly influences the removal of settleable organic and floatable solids. The optimal surface loading rate, typically ranging from 300 to 1200 gallons per day per square foot (gpd/ft²), ensures effective primary clarification.

Calculation Example

Consider a circular clarifier with a diameter of 150 ft. With a flow of 4.85 million gallons per day (MGD), the surface loading rate can be determined by calculating the area of the clarifier and converting the flow to gpd.

[ \text{Area, ft}^2 = (0.785)(\text{Diameter})^2 ] [ \text{Area, ft}^2 = (0.785)(150 \, \text{ft})^2 ] [ \text{Area, ft}^2 = 17,662.5 \, \text{ft}^2 ]

Convert MGD flow to gpd: [ 4.85 \, \text{MGD} \times (1,000,000 \, \text{gal/1MG}) = 4,850,000 \, \text{gpd} ]

Surface Loading Rate: [ \text{Surface Loading Rate} = \frac{\text{Flow, gpd}}{\text{Area, ft}^2} ]

This calculation ensures a precise determination of the surface loading rate for the circular clarifier.

Weir Overflow Rate: Measuring Wastewater Flow

Understanding Weir Overflow Rate

A weir, a vital flow measurement device, plays a crucial role in assessing wastewater flow. Weir overflow rate, or weir loading rate, quantifies the volume of water leaving the settling tank per linear foot of weir. Design standards often dictate weir overflow rates between 10,000 to 20,000 gpd/ft² for optimal settling tank performance.

Calculation Example

Consider a circular clarifier with a diameter of 100 ft and a weir along its circumference. With an effluent flow rate of 2.22 MGD, the weir overflow rate can be calculated.

[ \text{Total feet weir} = 3.14 \times \text{Diameter, ft} ] [ \text{Total feet weir} = 3.14 \times 100 \, \text{ft} ] [ \text{Total feet weir} = 314 \, \text{ft} ]

Convert the flow from MGD to gpd: [ 2.22 \, \text{MGD} \times (1,000,000 \, \text{gal/1MG}) = 2,220,000 \, \text{gpd} ]

Weir Overflow Rate: [ \text{Weir Overflow Rate} = \frac{\text{Flow, gpd}}{\text{Total feet weir}} ]

This calculation provides a precise measure of the weir overflow rate for the circular clarifier.

Biochemical Oxygen Demand (BOD) and Suspended Solids (SS) Removal

BOD Measurement and Calculations

Biochemical Oxygen Demand (BOD) measurement is crucial for understanding the organic load in wastewater. Calculating BOD removal involves assessing the initial and final dissolved oxygen readings, with or without the need for additional seed for accurate results.

Calculation Example

Consider a 50 mL sample of wastewater influent with an initial DO reading of 10.2 mg/L and a final DO reading of 7.5 mg/L, without the need for seeding.

[ \text{BOD} = \text{Initial DO} - \text{Final DO} ] [ \text{BOD} = 10.2 \, \text{mg/L} - 7.5 \, \text{mg/L} ] [ \text{BOD} = 2.7 \, \text{mg/L} ]

This calculation determines the BOD in mg/L for the influent without the requirement of additional seed.

BOD and SS Removal Calculation

To calculate the pounds of BOD or suspended solids removed each day, the mg/L BOD or suspended solids removed and the plant flow are essential.

[ \text{BOD Removed, lb/day} = \text{BOD, mg/L} \times \text{Flow, MGD} \times 8.34 \, \text{lb/gal} ]

This formula allows precise determination of the daily pounds of BOD or suspended solids removed.


In summary, mastering the calculations associated with wastewater primary treatment is indispensable for optimizing treatment plant performance. From surface loading rates to weir overflow rates and BOD removal, each parameter contributes to the overall effectiveness of the primary treatment process. These calculations serve as valuable tools in the quest for efficient wastewater treatment, ensuring environmental sustainability and regulatory compliance.

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