Design of Underground Storage Tanks Involving Water Collection for Water Reuse of Irrigation

Purpose: A Case Study for the Campus Green Area of UAB

By

Zhuo Li1, Robert W. Peters1, and Matthew Winslett2

1Department of Civil, Construction, and Environmental Engineering 2Facilities Management Department University of Alabama at Birmingham

Alabama Water Resources Conference 2013Orange Beach, AL

September 5-6, 2013

OverviewSignificance of Water Conservation at UAB Site DescriptionEstimation of Irrigation Water NeedsDesign of Rainwater Harvesting System Sensitivity AnalysisResults and DiscussionConclusions

Significance of Water Conservation at UAB

Water Consumption at UAB (Winslett, 2011): 2008-2009: 697,920 ccf (522,080,416 gallons) 2009-2010: 659,271 ccf (493,168,956 gallons)

Corresponding water and sewer costs at UAB:2008-2009: $7,025,0112009-2010: $6,907,892

An underground storage tank (UST) was installed in 2010 at the University Boulevard Office Building (UBOB).

Study InvestigationStudy investigation at Texas A&M University (TAMU)

For purpose of controlling storm runoff volume and landscape irrigation, Saour (2009) performed a feasibility study of implementing rainwater harvesting system (RHS). Uses an equation from TAMU to estimate water supply and demand Performed payback period with two scenarios of 20 and 14 years The results showed little effect on control of stormwater runoff volume

This study is similar to the project at UAB but the project at UAB is not concerned about reduction of stormwater runoff.

Site Description The Campus Green is

bordered by Blazer Hall, the Dining Commons, the Campus Recreation Center, and Heritage Hall.

Overall, the permeable and impermeable area are approximately 52% and 48%, respectively.

Source: Google Map, 2013

Estimation of Irrigation Water NeedsEffective precipitation

The mean value (inches) of last five-year precipitation data is used for estimation purpose:

Source: Birmingham Weather Forecast Office, 2011

Janu

ary

Febru

ary

Mar

chApr

ilM

ayJu

ne July

Augus

t

Septe

mbe

r

Octob

er

Novem

ber

Decem

ber

0

1

2

3

4

5

6

7

Mean Precipitation Value from Year 2008-2012

Month

Pre

cipit

ati

on, (i

nch

es)

Estimation of Irrigation Water NeedsEffective precipitation was estimated by Natural Resource Conservation

Service (NRCS) curve number method (SCS,1986) Assuming antecedent moisture condition (AMC) II. Hydrologic soil type B (SCS, 1982)

It was assumed that the measured value was used to calculate the runoff without considering estimation errors.

Land Type

Area

Square Feet Acres

Street and Roads (Paved Area) 220,000±11,000 5.1±0.25

Open Space (Grass and Trees) 469,000±24,000 10.8±0.55

Roof Area 207,000±10,000 4.8±0.23

Total 896,000±45,000 20.7±1.03

Estimation of Irrigation Water NeedsEvapotranspiration

The Blaney-Criddle formula (Blaney and Criddle, 1950) was used, and minimum crop factor of 0.6 was selected for turf .

Source: Birmingham Weather Forecast Office, National Weather Service, 2011

Janu

ary

Febru

ary

Mar

chApr

ilM

ayJu

ne July

Augus

t

Septe

mbe

r

Octob

er

Novem

ber

Decem

ber

0

5

10

15

20

25

30

Mean Temperature from Year 2008-2012

Month

Tem

pera

ture

, (°C

)

Estimation of Irrigation Water Needs Irrigation water needs= ETcrop – Pe (Brouwer and Heibloem, 1986).A well designed and operated irrigation can have efficiency ranges from

80% to 90 % (University of California Extension System, 2000).

Note: negative value indicates no additional water needs for irrigation beside rainwater

Janu

ary

Febru

ary

Mar

chApr

ilM

ayJu

ne July

Augus

t

Septe

mbe

r

Octob

er

Nov

embe

r

Decem

ber

-400,000

-200,000

0

200,000

400,000

600,000

800,000

1,000,000

Estimated Irrigation Water Needs, (gallons)

Month Irri

gati

on W

ate

r N

eeds,

(gallons)

Design of RHS at UAB The UAB Campus Recreation

Center pumps the groundwater in order to avoid being flooded.

The quantity of pumped groundwater is approximately 1.0 million gallons per year.

Assuming in each month, equal quantities of groundwater are pumped, hence 85,000 gallons per month.

Design of RHS at UAB Irrigation scheme

Most installers usually assume an efficiency of 75% to 90% (The Texas Manual on Rainwater Harvesting, 2005). Assuming 90% efficiency:

Month Irrigation Water Need, (gallons)

Collectable Rainwater, (gallons)

Groundwater, (gallons)

January 30,000 569,382 85,000February 173046 496,473 85,000

March 30,000 706,713 85,000April 439,619 488,373 85,000May 93,172 889,563 85,000June 819,852 396,947 85,000July 296,129 531,192 85,000

August 594,586 662,736 85,000September 30,000 444,396 85,000

October 598,105 521,548 85,000November 233,124 487,601 85,000December 30,000 535,050 85,000

Total 3,367,633 6,729,974 1,020,000

Design of RHS at UABTank size determination

Refers to UST at UBOB and situation in this project, the tank size was determined to be 60,000 gallons, which are two 30,000 gallon tanks.

Commercial water costs $3.21/CCF (Birmingham Water Works Board, 2013). Total water cost saving ~$ 13,284

Draco, Inc. Underground Water Tanks with Purpose of Landscape Irrigation

Size of Tanks (gal) Diameters of Tank (ft) Price of polyethylene Tank, ($)

10,000 10 15,75020,000 10 26,53730,000 12 37,90840,000 12 53,00550,000 12 62,080

Source: www.darcoinc.com/ (Darco Inc., 2013)

Tank Size Determination (Cont’d)Accessories and other cost Cost, ($)

30000 gallon polyethylene tanks 37,9082 HP Pump 585

60 GPM Filter 70Misc. such as landscaping, locating drains, connecting sprinklers, etc. 8,500

Dig tank hole and backfill 8,000Concrete tank support 4,000

Gravel around tank 25,000Piping to tank 12,000

Electrical/controls 10,000Subtotal Costs 106,063Overhead 10% 10,606

Engineering 15% 15,909Contingency 15% 15,909

Total 148,488Source: UAB Facilities Management Department, 2012

Tank Size Determination (Cont’d)If use one 60,000 gallon tank:

T Double costs if use two 30,000 gallon tanks

If only use a 30,000 gallon tank :T

If only use a 40,000 gallon tank:T

The optimize tank size can be 30,000 gallons.The UAB Facilities Management Department performed a

preliminary study that suggests using a 30,000 gallon tank. Smaller tank can make a little better payback but less of capacity for

efficient irrigation which is not preferable.

Design of RHS at UAB

Sensitivity Analysis A sensitivity analysis was performed in order to explore

the impact on tank size, payback period.The study investigated changes of±5%,±10% and ± 25% If the water supply can fully meet the demand, the tank

size will be reduced. Otherwise, it will remain the same.Three scenarios were studied:

Change of precipitationChange of ETET and precipitation increase or decrease at same time

Results and DiscussionSensitivity analysis shows no impact on tank size and overall

payback period.With a decrease in precipitation or an increase in

evapotranspiration, the payback period will be shorter.Overall, the designed rainwater harvesting can meet

approximately 86% of the total irrigation water requirement.The ongoing research project on recovery of condensed water at

UAB indicates good water quality that can be supplemented for irrigation.

To reduce the payback period, concrete water storage tanks or other cheap material-made tanks can be alternatives to decrease the capital investment.

The UST has a potential problem (algae formation) that can be controlled by disinfection and maintenance but leads to higher cost.

Conclusion The study provides a general estimation involving a feasibility

study of implementing a RHS at the UAB Campus Green.The estimation may be not highly accurate in some details

but generally it is reasonable providing results similar to the study at TAMU and UAB Facilities Management Department.

Based on the financial aspect, the payback period is a little long, ~12 years, indicating that rainwater harvesting is not economically viable for large scale implementation for irrigation purposes.

However, in an effort to make the campus “greener”, the RHS may be a viable approach.

Acknowledgements Sincere gratitude to Dr. Robert W. Peters for his valuable

time. Thanks offered to Mr. Matt Winslett for his strong

support by providing data need for this investigation.Thanks and appreciation to the Facilities Management

Department of UAB funding this study.

Thank you for your time.Questions?

Mean Daily Percentage (p) of Annual Daytime Hours for Different Latitudes

Latitude 60 55 50 45 40 35 30 25 20 15 10 5 0South North

Jul Jan 0.15 0.17 0.19 0.2 0.22 0.23 0.24 0.24 0.25 0.26 0.26 0.27 0.27Aug Feb 0.2 0.21 0.23 0.23 0.24 0.25 0.25 0.26 0.26 0.26 0.27 0.27 0.27Sep Mar 0.26 0.26 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27Oct Apr 0.32 0.32 0.31 0.3 0.3 0.29 0.29 0.29 0.28 0.28 0.28 0.28 0.27Nov May 0.38 0.36 0.34 0.34 0.32 0.31 0.31 0.3 0.29 0.29 0.28 0.28 0.27Dec Jun 0.41 0.39 0.36 0.35 0.34 0.32 0.32 0.31 0.3 0.29 0.29 0.28 0.27Jan Jul 0.4 0.38 0.35 0.34 0.33 0.32 0.31 0.31 0.3 0.29 0.29 0.28 0.27Feb Aug 0.34 0.33 0.32 0.32 0.31 0.3 0.3 0.29 0.29 0.28 0.28 0.28 0.27Mar Sep 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.27Apr Oct 0.22 0.23 0.24 0.24 0.25 0.25 0.26 0.26 0.26 0.27 0.27 0.27 0.27May Nov 0.17 0.18 0.2 0.21 0.22 0.23 0.24 0.25 0.25 0.26 0.26 0.27 0.27Jun Dec 0.13 0.16 0.18 0.2 0.21 0.22 0.23 0.24 0.25 0.25 0.26 0.27 0.27

Adapted from: Brouwer and Heibloem, 1986

The latitude of Birmingham is 33°31' 14" N, rounded to 33°.

Warm Season TurfThe grass type is warm season turf, which is suitable for

growing during the warm climate season. A minimum proper crop factor of 0.6 was selected for

calculation in order to conserve water.

Source: The University of Arizona Cooperative Extension, 2000

Source： SCS, 1986

Land Use Area, acres

CN Product, Area×C

N

Street and Roads (paved Area)

5.1 98 494.9

Open Space (grass and trees)

10.8 61 657.2

Total 15.9   1152.1

Steps of Calculating Effective RunoffThus, the composite CN was computed as:

The maximum possible retention for this area at AMC-II is:

The initial abstractions were estimated to be:

Because P>the depth of runoff (effective precipitation) was estimated as: