Is below method correct way to calculate average BOD ?

The facility is presumed to produce residential equivalent strength sewage during daily activities and wastewater with high organic strength and elevated FOG (fat, oil and grease) characteristics during events. This high strength situation is associated with food preparation. The characteristics of domestic and restaurant raw and pretreated wastewater are shown in table below.

Type of Wastewater BOD (mg/l)
Raw Domestic 200-290 (Ave=245)



Raw Restaurant 1000-2000
Pre-treated Restaurant 101-880(Avg=506) flow out of grease trap


The above average strength of pretreated restaurant waste (BOD=506 mg/l) was assumed when kitchen waste passes through grease interceptor. The strength of the wastewater will reduce more in septic tank when it mixes with raw sewage with low concentration of daily activities. The average BOD in septic tank for 7 days cycle was calculated as follows:

Daily Average Flow for 5 Days= 1220 gallon
Facility Day Off=52 gallon
Flow During Event= 4740 gallon

Average BOD={(506)(4740)+(1220)(245)(5)+ (52)(245)}/ 10892= 359 mg/l