Measuring Plant Performance


To evaluate how well a plant or unit process is performing, performance efficiency or percent (%) removal is used. The results obtained can be compared with those listed in the plant's operations and maintenance (O&M) manual to determine if the facility is performing as expected. This chapter presents sample calculations often used to measure plant performance or efficiency.

The efficiency of a unit process is its effectiveness in removing various constituents from the wastewater or water. Suspended solids and biochemical oxygen demand (BOD) removal are therefore the most common calculations of unit process efficiency.

In wastewater treatment, the efficiency of a sedimentation basin may be affected by such factors as the types of solids in the wastewater, the temperature of the wastewater, and the age of the solids. Typical removal efficiencies for a primary sedimentation basin are as follows:

• Settleable solids 90-99%

• Suspended solids 40-60%


Note: The calculation used for determining the performance (percent removal) for a digester is different from that used for performance (percent removal) for other processes. Care must be taken to select the correct formula.

.. „ , (Influent Concentration - Effluent Concentration) x 100 % Removal = (5.1)

Influent Concentration

Problem: The influent BODs is 247 mg/L, and the plant effluent BOD is 17 mg/L. What is the percent removal?


247 mg/L


Equation 5.1 is also used to determine unit process efficiency. The concentration entering the unit and the concentration leaving the unit (e.g., primary, secondary) are used to determine the unit performance:

.. „ , (Influent Concentration - Effluent Concentration) x 100

Influent Concentration

Problem: The primary influent BOD is 235 mg/L, and the primary effluent BOD is 169 mg/L. What is the percent removal?


235 mg/L


The calculation used to determine percent volatile matter reduction is more complicated because of the changes occurring during bio-solids digestion:

problem: Using the digester data provided below, determine the percent volatile matter reduction for the digester:

Raw biosolids volatile matter = 74% Digested biosolids volatile matter = 54%


%VM Reduction (0 74 - 0 54) X100 = 59°% 0.74 - (0.74 X 0.54)


Use the following information for the chapter review questions:

Plant influent Flow = 8.25 MGD Suspended solids = 350 mg/L BOD = 225 mg/L Primary effluent Flow = 8.35 MGD Suspended solids = 144 mg/L BOD = 175 mg/L Active sludge effluent Flow = 8.35 MGD Suspended solids = 17 mg/L BOD = 24 mg/L Anaerobic digester Solids in = 6.6% Solids out = 13.4% Volatile matter in = 66.3% Volatile matter out = 49.1%

5.1 What is the plant percent removal for BODS?

5.2 What is the plant percent removal of TSS?

5.3 What is the primary treatment percent removal of BODS?

5.4 What is the primary treatment percent removal of total suspended solids (TSS)?

5.5 What is the percent volatile matter reduction in the anaerobic digestion process?


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