How home water usage & storage volume is determined?

 by  Maryam Hussain

An adequate supply of water to fulfill the needs of kitchen, drinking and sanitation is a primary requisite for good health. When a town water supply is not available, then the owner of the property must develop a quality water storage and supply system for the use of residents of his property. 

While estimating the water storage requirements, consideration should be given to the peak daily water usage, the maximum-day demand, the capacity of the normal and standby pumping equipment, the availability and capacity of second power supply, an estimated duration of power failure, and the duration of time required for repairs with respect to the time of power and equipment failure.

A preferred minimum storage capacity would be a two-day use or a maximum-day usage. Three days of storage is the preferred one. All distribution reservoirs should be covered, should be provided with an overflow, and should be provided with a drain, access manhole, and a meshed air vent.

Other things being equal, a large-diameter shallow tank is preferable over a deep tank of the same capacity. The tank with larger area to volume ratio is less expensive to construct, and water pressure fluctuations on the distribution system are less. The cost of storage compared to the decreased cost of pumping and the greater reliability of the water supply system will be additional factors in making a decision. Let us demonstrate these thoughts through some illustrations.

Illustration-1: 

Estimate the daily water consumption by an adult person who uses water in toilet tap & flush tank, bathtub or shower, toilet washbasin, kitchen dishwashing, laundry machine, summer drinking, meals & breakfast cooking & tea making.

Solution:

Table-1

How to estimate domestic water consumption per person
SN	Usage	Estimated use of water	Estimated time duration of Use	Estimated number of usages per day	Volume calculations
					Reference calculation	Result (gallons per day)
1.	Toilet tap & flush tank,	3.2 gal per time	-	05 times	3.2 x 5	16
2.	Bathtub or shower	3.5 gpm	15 minutes per use	once	3.5 x 15	52.5
3.	Washbasin	1 gpm per time	3.75 minutes per use	08 times	1 x 3.75 x 08	30
4.	Dishwashing	1.5 gpm per time	2 minutes per use	06 times	1.5 x 2.0 x 06	18
5.	laundry machine	7.5 gal	-	once	7.5	7.5
6.	Drinking	0.5 gpd	-	-	0.5	0.5
7.	Cooking & tea	0.25 gpd	-	-	0.25	0.25
8.	Total					124.75 gpd

Therefore, the total daily water consumption by a person is 124.75 gallons

Tip#1: Compute the required water volume for a family of 4 identical persons

Total daily water consumption by a family of 4 persons (Total-1) is 4 x 124.75 = 499 gallons

Table-2

 

How to estimate domestic water consumption per house
SN		Estimated use of water	Estimated time duration of Use	Estimated number of usages per day	Volume calculations
					Reference calculation	Result
1.	Garden	2.5 gpm	15 minutes per use	once	2.5 x 15	37.5
2.	A Car wash (per vehicle)	40 gpd	-	once	40	40
3.	Total -2					77.5 gpd

 

Tip#2: Compute the total required daily water volume for a house

The total daily water consumption by the whole family V_w = Total-1 + Total-2

=  499 gpd  +  77.5 gpd                                                                                                  

= 576.6 gallons

V_w = 792 liters = 0.792 m³

Tip#3: Compute 3-day storage volume

A three days required storage will be V_s = 3 x 0.792 m³ = 2.376 m³

Tip#4: Compute the tank volume including 25 % void or empty volume

V_t = 1.25 x 2.376 m³ = 2.97 m³

Illustration-2

For the above-estimated water volume (V_s) for a house, calculate the height of the water column for the following two schemes & conclude which scheme gives the lesser loss in a bath tap water pressure, located 5m below w.r.t tank bottom, when the volume of water in the tank reduces by 25 %; recommend better scheme. Assume the temperature of water = 25 Celsius for both schemes.

Scheme a). width of tank = W = 1 m,     breadth of the tank = b = 2 m

Scheme b). width of tank = W = 1 m,     breadth of the tank = b = 1 m

Solution:

Applicable equation:

The volume of water in the tank:               V = W x b x h

Where h = water column height in the tank

Constant for the given case

Z  = elevation of tank bottom w.r.t bath tap = 5 m

V_c = cut off volume of water in the tank = 0.75 x V_s = 1.782 m³

Scheme a). width of tank = W = 1 m,     breadth of the tank = b = 2 m

With tank containing V_s = 2.376 m³ volume of water, the calculated water column (h-a1) in the tank is = 1.188 m

With tank containing V_c = 1.782 m³ volume of water, the calculated water column (h-a2) in the tank is = 0.891 m

Reduction of pressure observed in bath tap =  1.188 - 0.891 = 0.297 m

Scheme b). width of tank = W = 1 m,     breadth of the tank = b = 1 m

With tank containing V_s = 2.376 m³ volume of water, the calculated water column (h-b1) in the tank is = 2.376 m

With tank containing V_c = 1.782 m³ volume of water, the calculated water column (h-b2) in the tank is = 1.782 m

Reduction of pressure observed in bath tap = 2.376 – 1.782 = 0.594 m

Conclusions:

a). Scheme-a will provide water at almost constant flow rate & pressure, to the consumer, when the storage volume changes from 100 % to 75 %.

b). With scheme-2, the consumer in the bathroom will observe a prominent change in water supply flow rate & pressure when the storage volume changes from 100 % to 75 %. Moreover, the consumer has to continually manipulate the tap to adjust the pressure & flow rate during the bath & this will be a source of disturbance & waste of time.

Recommendations:

a). If you are going to refurbish your house’s water storage tank or going to construct a new house including the overhead-elevated water storage tank, then first of all estimate the water consumption & provide an appropriate safety factor of 1.25 to the estimated volume by going through the details in the illustration-1.

b). Then follow the method of scheme-a of illustration-2 to provide inherent comfort of usage to the consumer.