CALCS - STEP Tank Sizing - 2023.09.05 22-005 Tahoma
Terra Commercial
S.T.E.P. Tank Sizing Worksheet Hatton Godat Pantier, Inc.
3910 Martin Way E, Suite B
Olympia, WA 98506
T (360) 943-1599
F (360) 357-6299
Bldg
No.
1
Bedrm
2
Bedrm
3
Bedrm
Total
Units Population Daily Flow (gals)
Tank Size
(Gal)
8 ft Dia.-
Capacity (Gal)
A 4 8 0 12 20 974 4,772 6,000
B 4 8 0 12 20 974 4,772 6,000
C 4 20 0 24 42.5 2,072 7,817 6,000
D 4 8 0 12 20 974 4,772 6,000
E 4 8 0 12 20 974 4,772 6,000
F 4 8 0 12 20 974 4,772 6,000
G 0 4 8 12 27.5 1,751 5,756 6,000
H 0 2 4 6 13.75 916 3,952 6,000
Total 24 66 12 102 183.75
Unit Type
9/7/2023 H:\Office\JOBS\2022\22-005 Tahoma Commercial\PERMIT\Calculations\Water Sewer Calc\STEP Tank Sizing 06.20.2023.xlsx
Tahoma Terra Commercial
STEP Sewer Tank Sizing Calculation
9/7/2023
Typical Unit Flow Rates:65 gpd/person
Three bedroom unit design flow based upon: 163 gpd/unit 2.5 people (TRPC Statistics)
Two bedroom unit design flow based upon: 121.9 gpd/unit 1.875 25% Reduction
Single Bedroom unit design flow based upon: 81 gpd/unit 1.25 50% Reduction
Minimum Tank Volume = 1.75 x daily flow (per TCEH pump chamber standards)
Maximum Daily Flow = 974 gallons
Minimum Tank Size = 1,705 gallons
Daily flow to the tank 974 gal 8 three bedroom units, 4 two bedrom units
Reserve Space (Vr)(@ peak flow ~ 6.0 hrs) 974 gal 15-in tank to HWA per Detail 7-22.1 or PF=4
Operating Zone (Voa)(per City detail 0.33 ft) 423 gal 4-in Lead Pump ON to OFF per Detail 7-22.1
Scum Layer (VSC)870 gal ((5.24 x O&M Interval)+12.04) x Population
Clear Zone (Vcz)(settling time 18 hrs) 750 gal (50 x Population x Settling Time)/24
Sludge Layer (Vsl)(pump interval 6 yr) 1754 gal ((8.15 x O&M Interval)+38.82) x Population
Total tank volume required 4772 gal
6000 gal
City's Max Allowable STEP Tank Size
Septic Tank Volume
VERIFY MINIMUM TANK SIZE: Using storage and reserve volumes recommended by
Xerxes "Septic Tank Sizes for Large Flows", for primary tanks on a 6 year maintenance cycle
yields the following:
Bldg A
1
Tahoma Terra Commercial
STEP Sewer Tank Sizing Calculation
9/7/2023
Typical Unit Flow Rates:65 gpd/person
Three bedroom unit design flow based upon: 163 gpd/unit 2.5 people (TRPC Statistics)
Two bedroom unit design flow based upon: 121.9 gpd/unit 1.875 25% Reduction
Single Bedroom unit design flow based upon: 81 gpd/unit 1.25 50% Reduction
Minimum Tank Volume = 1.75 x daily flow (per TCEH pump chamber standards)
Maximum Daily Flow = 974 gallons
Minimum Tank Size = 1,705 gallons
Daily flow to the tank 974 gal 12 two bedroom units
Reserve Space (Vr)(@ peak flow ~ 6.0 hrs) 974 gal 15-in tank to HWA per Detail 7-22.1 or PF=4
Operating Zone (Voa)(per City detail 0.33 ft) 423 gal 4-in Lead Pump ON to OFF per Detail 7-22.1
Scum Layer (VSC)870 gal ((5.24 x O&M Interval)+12.04) x Population
Clear Zone (Vcz)(settling time 18 hrs) 750 gal (50 x Population x Settling Time)/24
Sludge Layer (Vsl)(pump interval 6 yr) 1754 gal ((8.15 x O&M Interval)+38.82) x Population
Total tank volume required 4772 gal
6000 gal
City's Max Allowable STEP Tank Size
Septic Tank Volume
VERIFY MINIMUM TANK SIZE: Using storage and reserve volumes recommended by
Xerxes "Septic Tank Sizes for Large Flows", for primary tanks on a 6 year maintenance cycle
yields the following:
Bldg B
1
Tahoma Terra Commercial
STEP Sewer Tank Sizing Calculation
9/7/2023
Typical Unit Flow Rates:65 gpd/person
Three bedroom unit design flow based upon: 163 gpd/unit 2.5 people (TRPC Statistics)
Two bedroom unit design flow based upon: 121.9 gpd/unit 1.875 25% Reduction
Single Bedroom unit design flow based upon: 81 gpd/unit 1.25 50% Reduction
Minimum Tank Volume = 1.75 x daily flow (per TCEH pump chamber standards)
Maximum Daily Flow = 2,072 gallons
Minimum Tank Size = 3,625 gallons
Daily flow to the tank 2072 gal 12 two bedroom units
Reserve Space (Vr)(@ peak flow ~ 2.8 hrs) 974 gal 15-in tank to HWA per Detail 7-22.1 or PF=4
Operating Zone (Voa)(per City detail 0.33 ft) 423 gal 4-in Lead Pump ON to OFF per Detail 7-22.1
Scum Layer (VSC)1848 gal ((5.24 x O&M Interval)+12.04) x Population
Clear Zone (Vcz)(settling time 18 hrs) 844 gal (50 x Population x Settling Time)/24
Sludge Layer (Vsl)(pump interval 6 yr) 3728 gal ((8.15 x O&M Interval)+38.82) x Population
Total tank volume required 7817 gal
Number of Tanks 2
Total Tank Volume 6000 x 2 = 12,000 gal
City's Max Allowable STEP Tank Size
Septic Tank Volume
VERIFY MINIMUM TANK SIZE: Using storage and reserve volumes recommended by
Xerxes "Septic Tank Sizes for Large Flows", for primary tanks on a 6 year maintenance cycle
yields the following:
Bldg C
1
Tahoma Terra Commercial
STEP Sewer Tank Sizing Calculation
9/7/2023
Typical Unit Flow Rates:65 gpd/person
Three bedroom unit design flow based upon: 163 gpd/unit 2.5 people (TRPC Statistics)
Two bedroom unit design flow based upon: 121.9 gpd/unit 1.875 25% Reduction
Single Bedroom unit design flow based upon: 81 gpd/unit 1.25 50% Reduction
Minimum Tank Volume = 1.75 x daily flow (per TCEH pump chamber standards)
Maximum Daily Flow = 974 gallons
Minimum Tank Size = 1,705 gallons
Daily flow to the tank 974 gal 12 two bedroom units
Reserve Space (Vr)(@ peak flow ~ 6.0 hrs) 974 gal 15-in tank to HWA per Detail 7-22.1 or PF=4
Operating Zone (Voa)(per City detail 0.33 ft) 423 gal 4-in Lead Pump ON to OFF per Detail 7-22.1
Scum Layer (VSC)870 gal ((5.24 x O&M Interval)+12.04) x Population
Clear Zone (Vcz)(settling time 18 hrs) 750 gal (50 x Population x Settling Time)/24
Sludge Layer (Vsl)(pump interval 6 yr) 1754 gal ((8.15 x O&M Interval)+38.82) x Population
Total tank volume required 4772 gal
6000 gal
City's Max Allowable STEP Tank Size
Septic Tank Volume
VERIFY MINIMUM TANK SIZE: Using storage and reserve volumes recommended by
Xerxes "Septic Tank Sizes for Large Flows", for primary tanks on a 6 year maintenance cycle
yields the following:
Bldg D
1
Tahoma Terra Commercial
STEP Sewer Tank Sizing Calculation
9/7/2023
Typical Unit Flow Rates:65 gpd/person
Three bedroom unit design flow based upon: 163 gpd/unit 2.5 people (TRPC Statistics)
Two bedroom unit design flow based upon: 121.9 gpd/unit 1.875 25% Reduction
Single Bedroom unit design flow based upon: 81 gpd/unit 1.25 50% Reduction
Minimum Tank Volume = 1.75 x daily flow (per TCEH pump chamber standards)
Maximum Daily Flow = 974 gallons
Minimum Tank Size = 1,705 gallons
Daily flow to the tank 974 gal 12 two bedroom units
Reserve Space (Vr)(@ peak flow ~ 6.0 hrs) 974 gal 15-in tank to HWA per Detail 7-22.1 or PF=4
Operating Zone (Voa)(per City detail 0.33 ft) 423 gal 4-in Lead Pump ON to OFF per Detail 7-22.1
Scum Layer (VSC)870 gal ((5.24 x O&M Interval)+12.04) x Population
Clear Zone (Vcz)(settling time 18 hrs) 750 gal (50 x Population x Settling Time)/24
Sludge Layer (Vsl)(pump interval 6 yr) 1754 gal ((8.15 x O&M Interval)+38.82) x Population
Total tank volume required 4772 gal
6000 gal
City's Max Allowable STEP Tank Size
Bldg E
Septic Tank Volume
VERIFY MINIMUM TANK SIZE: Using storage and reserve volumes recommended by
Xerxes "Septic Tank Sizes for Large Flows", for primary tanks on a 6 year maintenance cycle
yields the following:
1
Tahoma Terra Commercial
STEP Sewer Tank Sizing Calculation
9/7/2023
Typical Unit Flow Rates:65 gpd/person
Three bedroom unit design flow based upon: 163 gpd/unit 2.5 people (TRPC Statistics)
Two bedroom unit design flow based upon: 121.9 gpd/unit 1.875 25% Reduction
Single Bedroom unit design flow based upon: 81 gpd/unit 1.25 50% Reduction
Minimum Tank Volume = 1.75 x daily flow (per TCEH pump chamber standards)
Maximum Daily Flow = 974 gallons
Minimum Tank Size = 1,705 gallons
Daily flow to the tank 974 gal 12 two bedroom units
Reserve Space (Vr)(@ peak flow ~ 6.0 hrs) 974 gal 15-in tank to HWA per Detail 7-22.1 or PF=4
Operating Zone (Voa)(per City detail 0.33 ft) 423 gal 4-in Lead Pump ON to OFF per Detail 7-22.1
Scum Layer (VSC)870 gal ((5.24 x O&M Interval)+12.04) x Population
Clear Zone (Vcz)(settling time 18 hrs) 750 gal (50 x Population x Settling Time)/24
Sludge Layer (Vsl)(pump interval 6 yr) 1754 gal ((8.15 x O&M Interval)+38.82) x Population
Total tank volume required 4772 gal
6000 gal
City's Max Allowable STEP Tank Size
Bldg F
Septic Tank Volume
VERIFY MINIMUM TANK SIZE: Using storage and reserve volumes recommended by
Xerxes "Septic Tank Sizes for Large Flows", for primary tanks on a 6 year maintenance cycle
yields the following:
1
Tahoma Terra Commercial
STEP Sewer Tank Sizing Calculation
9/7/2023
Typical Unit Flow Rates:65 gpd/person
Three bedroom unit design flow based upon: 163 gpd/unit 2.5 people (TRPC Statistics)
Two bedroom unit design flow based upon: 121.9 gpd/unit 1.875 25% Reduction
Single Bedroom unit design flow based upon: 81 gpd/unit 1.25 50% Reduction
Minimum Tank Volume = 1.75 x daily flow (per TCEH pump chamber standards)
Maximum Daily Flow = 1,751 gallons
Minimum Tank Size = 3,064 gallons
Daily flow to the tank 1751 gal 12 two bedroom units
Reserve Space (Vr)(@ peak flow ~ 3.3 hrs) 974 gal 15-in tank to HWA per Detail 7-22.1 or PF=4
Operating Zone (Voa)(per City detail 0.33 ft) 423 gal 4-in Lead Pump ON to OFF per Detail 7-22.1
Scum Layer (VSC)1196 gal ((5.24 x O&M Interval)+12.04) x Population
Clear Zone (Vcz)(settling time 18 hrs) 750 gal (50 x Population x Settling Time)/24
Sludge Layer (Vsl)(pump interval 6 yr) 2412 gal ((8.15 x O&M Interval)+38.82) x Population
Total tank volume required 5756 gal
6000 gal
City's Max Allowable STEP Tank Size
Bldg G
Septic Tank Volume
VERIFY MINIMUM TANK SIZE: Using storage and reserve volumes recommended by
Xerxes "Septic Tank Sizes for Large Flows", for primary tanks on a 6 year maintenance cycle
yields the following:
1
Tahoma Terra Commercial
STEP Sewer Tank Sizing Calculation
9/7/2023
Typical Unit Flow Rates:65 gpd/person
Three bedroom unit design flow based upon: 163 gpd/unit 2.5 people (TRPC Statistics)
Two bedroom unit design flow based upon: 121.9 gpd/unit 1.875 25% Reduction
Single Bedroom unit design flow based upon: 81 gpd/unit 1.25 50% Reduction
Minimum Tank Volume = 1.75 x daily flow (per TCEH pump chamber standards)
Maximum Daily Flow = 916 gallons
Minimum Tank Size = 1,603 gallons
Daily flow to the tank 916 gal 12 two bedroom units
Reserve Space (Vr)(@ peak flow ~ 6.4 hrs) 974 gal 15-in tank to HWA per Detail 7-22.1 or PF=4
Operating Zone (Voa)(per City detail 0.33 ft) 423 gal 4-in Lead Pump ON to OFF per Detail 7-22.1
Scum Layer (VSC)598 gal ((5.24 x O&M Interval)+12.04) x Population
Clear Zone (Vcz)(settling time 18 hrs) 750 gal (50 x Population x Settling Time)/24
Sludge Layer (Vsl)(pump interval 6 yr) 1206 gal ((8.15 x O&M Interval)+38.82) x Population
Total tank volume required 3952 gal
6000 gal
City's Max Allowable STEP Tank Size
Bldg H
Septic Tank Volume
VERIFY MINIMUM TANK SIZE: Using storage and reserve volumes recommended by
Xerxes "Septic Tank Sizes for Large Flows", for primary tanks on a 6 year maintenance cycle
yields the following:
1