feasibility and revenue of rain water harvesting in davao city

FEASIBILITY AND REVENUE OF RAIN WATER

HARVESTING SYSTEMS (RWHS) IN DAVAO

AN ANALYSIS USING SIMULATION

Simulating revenue of RWHS for different variables over 23 years of rainfall data, combined with

literature to study feasibility and most effective implementation.

Part of Bachelor Built Environment Third year internship research water management at

Rotterdam University of Applied Science.

Project Feasibility of Rain Water Harvesting Systems Document name B.01 Version 4.2 Organizations HELP Davao, TropICS, Ateneo de Davao, DOST XI Website http://davao.vincentoskam.nl Author Vincent Oskam Date, Location 21 December 2012, Davao, Philippines

Region XI Davao

http://davao.vincentoskam.nl/

Feasibility and revenue of Rain Water Harvesting Systems (RWHS) in Davao An analysis using simulation

Vincent Oskam, 2012 http://davao.vincentoskam.nl D.ID B.01…. 2 of 10

I. INTRODUCTION Davao City is one of the largest cities in the

world based on land surface. The tropical

climate in the southern Philippine island

Mindanao receives more than 1000 mm of

rain per year. The short duration intense

rainfall leads to overload of the drainage

systems, while wasting the valuable free

source of (rain)water.

Without treatment, rainwater can be used for

non-health purposes like CR flushing.

This study uses simulation to assess the

effectiveness and feasibility of Rain Water

Harvesting System (RWHS) implementation

in the Davao City area.

KEY FINDINGS RWHS revenue is greatly dependent on the

factors and variables catchment area, usage

and tank size. The rainfall can only be stored

up to the maximum tank capacity, revenue is

limited by the effective usage of the RWHS.

These variables lead to far lower revenue

than expected with popular formulas. They

also reduce the mitigation effect on drainage

load significantly. Maximum retention

capacity is the tank size divided by the

catchment area. For a 1000 m2 surface with 2

m3 tank it is maximum 2 mm for that 1000

m2.

Tanks are high investments and the RWHS is

expected to have long Return Of Investment

(ROI) time and relatively low revenue.

ORGANIZATION OF THE PAPER Chapter II explains the methods used for

simulation, chapter 1. Covers the basics of

RWHS and chapter 2. Shows the simulation

results. Finally chapter 3. contain conclusions.

II. METHODS To ensure replicability and reliability of this

research, the used methods are described

below.

LITERATURE As basis for this feasibility study, literature on

RWHS is consulted defining the different

strategies, design criteria and best practices.

As primary basis the (Texas water

development board, 2005) publication is used.

All cited works are referred to using APA 6 in

chapter Works cited at the end of this

pulication.

SIMULATION To calculate revenue RWHS of different tank

sizes and daily usage are simulated over 23

years of rainfall data from Pag-Asa Davao: 07˚

07’N, 173˚ 39’E 18m +MSL. For the years:

1985-1986, 1989-1995 and 1997-2010.

Simulation is done using Microsoft Excel

2010 © with the following formulas.

→

→

→

→

∑

∑

Storage in reservoir on day n

Storage in reservoir on day n-1 (the day before)

Volume of the reservoir Precipitation on day n

Usage on day n

Revenue on day n Average daily revenue

∑ Total of days n

When daily usage is higher than available in

the storage tank, the maximum available is

assumed.



1. BASICS, POLICY AND

PRACTICE

The source of water is the rainfall on the

catchment area, that can effectively be

collected in the storage. Rainfall minus

evaporation, splash-out,

inverted run-off, etc.

The area that is used to catch

the rainwater is called the

catchment area. The catchment area will need

sufficient slope and facilities to discharge the

water to the storage.

The storage is used as buffer for

the non-rainfall events. The

advantage of elevated storage is

that it can be used without the help of an

extra pump. Note that storage of large

amounts of water result in high weight on

buildings, construction stability and safety

should be taken into consideration.

Collected rainwater can be used

untreated for non-quality demanding

purposes, like toilet flushing or gardening. It

is not to be used for drinking, dishes etc

without proper threatment.

To use the collected rainwater

as drinking water or other

quality demanding purposes, it

should be properly filtered and treated to

ensure safety and health. 1

1 Consult expert before using for health purposes.

Illustration 1.1



If gravitational flow of the collected

water into the building is not

necessary, undesirable or

impossible, a storage tank on

ground level or underground can be used.

The water can be used for gardening under

gravitational flow or as slow storm water

discharge or aquifer recharge area.

To use water from storage on

ground level, a pump can be used for

water transport into the building.

The pump will have to be strong

enough to overcome resistance in

the pipes and gravitational force to the

highest connection in the building. That is

why preferably elevated storage is used to

use the gravitational flow of water.

Ground level storage can be used

without a pump for gardening, for

example in dry periods.

The harvested rainwater can

also be stored with the sole

purpose of temporarily

retaining storm water, prevent soil erosion

and/or aquifer recharge.

RWHS IN POLICY RWHS is covered or connected by national

laws, ordinances and guidelines.

NATIONAL LAW In March 17 of 1989 Republic act 6716 was

approved, inter alia stating that “The

Department of Public Works and Highways

(DPWH) shall … undertake construction of

water wells, rainwater collectors, development

of… …every barangay in the country shall have

at least one additional potable water source.

…”. (Republic Act No. 6716, 1989)

LOCAL LAW Davao City Rainwater Ordinance of 2009

states inter alia that “All construction of new

commercial and industrial buildings including

major renovations … public and private

buildings must include an RCS. … RCS must be

integrated in existing government buildings,

specially public school and public markets.

…”.2,3 (Rainwater ordinance 0298-09, 2009)

INTERNATIONAL GUIDELINE World Health Organizations publication:

Guidelines for Drinking-water Quality third

edition inter alia covers local government’s

responsibility on drinking-water supply and

water quality: “… Local health authorities play

an important role in managing water

resources and drinking-water supplies. … Local

authorities will also give specific guidance to

communities or individuals in designing and

implementing community and household

drinking-water systems and correcting

deficiencies… Management of household and

small community drinking-water supplies

generally requires education programmes

about drinking-water supply and water quality

… include: Water hygiene awareness, basic

technical training…, …, a system of continued

support, follow-up and dissemination of the

water quality programme to achieve and

maintain sustainability.” (World Health

Organization (WHO), 2008)

NATIONAL GUIDELINES Administrative order No. 2007-0012 from the

Department of health states inter alia: “…

Local Health Authority at the municipal or city

level shall identify the list of paramters that

will be examined to determine the potability of

drinking water … undertake a systematic

assessment of all the parameters listed in the

2007 Philippine National Standars for

Drinking Water (PNSDW 2007)… ”.

(Department of Health, 2007)

2 RCS: Rainwater Catchment System 3 Status and reviews of the ordinance must be checked before basing conclusions on this citation.



2. REVENUE SIMULATION

DEFINING FACTORS Revenue, TCO and ROI is dependent on

several proportional and inversely

proportional factors.

PRICE OF WATER To calculate financial revenue, Total Cost of

Ownership (TCO) and Return Of Investement

(ROI) the reduction in water bill is calculated

assuming a reduction in tap water usage

equal to the RWHS revenue.

Price of water in Davao City is dependent on

the monthly usage, increasing with higher

usage.

11-20 m

3

21-30 m

3

31-40 m

3

41-50 m

3

51-up m

3

14.40 18.60 24.70 36.00 36.00

Table 1 Price of tap water in Davao City in PHP 2013 (WDInfo, 2011)

COST OF IMPLEMENTATION Initial investment in material en services.

Material Cost

Water tank 1m3 21 125.00 PHP Water tank 1.5m3 26 125.00 PHP Water tank 2m3 32 000.00 PHP Assumed 2 days of plumbing services

700.00 PHP

Assumed various plumbing materials

3000.00 PHP

Cost of yearly maintenance

30 minutes per week or 26 hours per year

Table 2 Indication of investment cost (ICE Hardware shop, 2012)

CATCHMENT AREA SIZE The rain water harvesting potential is

proportional dependent on the catchment

area size. For every square meter the

potential is calculated in liters as 1 * [rainfall

in millimeter].

100 m2 500 m2 1000 m2

5 mm 0.5 m3 2.5 m3 5 m3 10 mm 1 m3 5 m3 10 m3 20 mm 2 m3 10 m3 20 m3 40 mm 4 m3 20 m3 40 m3 60 mm 6 m3 30 m3 60 m3 80 mm 8 m3 40 m3 80 m3

Table 3 Indication of RWHS potential

STORAGE TANK SIZE The maximum potential rain water

harvesting is limited by the size of the storage

tank. After the tank is filled, precipitation can

no longer be stored and will flow over via

relief valves to the storm water drainage

system.

DAILY USAGE As rain water, untreated, cannot be used as

replacement of potable water, revenue is

limited to the non-health usage purposes of

harvested rain water. For example flushing

toilet flushing and gardening.

An American Standard© toilet generally has a

6.06 liter (1.6 gallon) tank. Allowing for 165

flushes per square meter rain water

harvesting. Or assuming 2 toilet visits per

office day per FTE 4 , 82 FTE can be

accommodated.

4 FTE Full Time Employee



SIMULATING REVENUE Using the simulation process described in

chapter Methods, the complex relationship

between the variables is shown. It is not

possible to devise a simple formula to

calculate the revenue.

CATCHMENT AREA The rain water available in the storage tank is

dependent on the catchment area, larger

catchment areas require less rainfall to fill the

tank, increasing rainwater availability.

Illustration 2.1

USAGE Harvested rainwater without treatment can

only be used for non-health purposes.

Therefor the revenue is limited by the daily

usage. Revenue is equal to the amount of

harvested rainwater actually being used.

Revenue increases with daily usage, until

usage is equal to tank size.

Illustration 2.2

TANK SIZE The tank is used to store water for the days

without or little rain. The larger the tank, the

more dry days it can buffer. When average

daily usage is low, the increase in revenue

slows down for larger tanks.

Illustration 2.3

Illustration 2.4 on the following page shows

an overview of several different factors and

variables.



0

0.1

0.2

0.3

0.4

0.5

0.6

S50U120

S50U240

S200U240

S200U500

S500U500

S500U1000

S1000U1000

S1000U2000

Dai

ly r

eve

nu

e in

cu

bic

me

ters

(S)urface area in square meters (U)sage in cubic meters a day

0.5 M^3

1 M^3

1.5 M^3

2 M^3

FINANCIAL REVENUE Financial revenue is proportional to the price

of water per cubic meter.

Illustration 2.5

RETURN OF INVESTMENT (ROI) ROI depends on daily revenue and

implementation cost. Implementation cost

increases with tank size.

Illustration 2.6

Illustration 2.4



3. CONCLUSIONS Rain Water Harvesting System (RWHS)

implementation is mandated by national law

and local ordinance. Due to high

implementation cost, maintenance and low

revenue, filtering rain water to health

purpose water is not advised. However,

should rain water be used for health purposes

national Department of Health and World

Health Organization guidelines states the

local government is responsible for

guidelines assuring quality.

RWHS revenue calculated by advised

formulas, using average rainfall, overestimate

the actual revenue simulated over rainfall

data in Davao. The complex relation between

catchment area, tank size and maximum non-

health usage potential, result in lower actual

revenue.

The size of the tank defines the amount of

water that can be stored in dry or little

rainfall days, larger tanks result in higher

revenue. Larger catchment areas need less

rainfall to fill the tank, thus also resulting in

higher revenue. The revenue is equal to the

amount rain water actually being used for

non-health purposes, higher usage potential,

i.g. the number of toilet flushes a day.

The relation between variables is to complex

to be calculated with a simple formula. For

actual revenue calculations and best

implementation practices a custom

simulation is advised. The simulation

program used in this paper is available on

http://davao.vincentoskam.nl . It must be

noted that simulation over the Pag-asa Davao

rain gauge does not necessarily provide an

accurate prediction for all locations in Davao

and not at all for areas outside of Davao.

On general it can be said that larger tank sizes

are advised for buildings with catchment

areas greater than 500 m2 and maximum

potential usage of 500 liters per day.

On the whole, RWHS is not very revenues for

smaller catchment areas. When implemented

on small catchment areas, larger tank sizes

will result in relative low increases in

revenue while implementation cost almost

doubles between a 1 m3 and 2 m3 tank.

It must also be noted that RWHS will only

result in mitigation of the storm water

drainage until the tank is filled. High intensity

rainfall events will quickly fill the RWHS

tanks, actual mitigation during high intensity

rainfall is therefor expected to be low as long

as tank sizes remain small.

http://davao.vincentoskam.nl/



III. Works cited

Ruelo, R. (2012, November). Water usage and

maintenance DOST. (V. Oskam,

Interviewer)

Republic Act No. 6716. (1989, 5 17). AN ACT

PROVIDING FOR THE CONSTRUCTION

OF WATER WELLS, RAINWATER

COLLECTORS, DEVELOPMENT OF

SPRINGS AND REHABILITATION OF

EXISTING WATER WELLS IN ALL

BARANGAYS IN THE PHILIPPINES.

Philippines. Retrieved 12 24, 2012,

from

http://www.gov.ph/1989/03/17/rep

ublic-act-no-6716/

Rainwater ordinance 0298-09. (2009). Davao

City Rainwater Ordinance of 2009.

Davao City, Philippines: City Council.

Ateneo De Davao University. (n.d.).

Central_911. Retrieved 11 3, 2012,

from WMDAS:

http://202.91.163.247/wmdas/ws

Bacongco, K. (2012, 01 26). Davao water

distrcit justifies price hike. Retrieved

10 12, 2012, from philSTAR.com

Mindanao:

http://www.philstar.com/nation/arti

cle.aspx?publicationsubcategoryid=20

0&articleid=771462

CSG Network. (n.d.). Water Consumption

Calculator. Retrieved 5 11, 2012, from

CSGNetwork.com:

http://www.csgnetwork.com/wateru

sagecalc.html

Davao City Water District. (2010-2011).

Water bill. DCWD.

de Amado, J. (2007, 05 19). The rainwater

havesting system. Inquirer lifestyle.

Retrieved 10 30, 2012, from

http://showbizandstyle.inquirer.net/l

ifestyle/lifestyle/view/20070519-

66830/The_rainwater_harvesting_sys

tem

de Amado, J. (2007, 05 19). The rainwater

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Retrieved 10 30, 2012, from

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ifestyle/lifestyle/view/20070519-

66830/The_rainwater_harvesting_sys

tem

de Vera, E. B. (2012, 03 25). DENR urges

adoption of rainwater harvesting.

Manila Bulletin, p. 1. Retrieved 10 30,

2012, from

http://www.mb.com.ph/articles/249

501/denr-urges-adoption-rainwater-

harvesting#.UI8ntcVLP5M

DENR. (1994). DENR Administrative order NO.

26-A.

Department of Health. (2007). Administrative

order No. 2007-0012 . Philippine

National Standards for Drinking Water

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http://www.lwua.gov.ph/downloads_

10/Philippine%20National%20Stand

ards%20for%20Drinking%20Water

%202007.pdf

Department of Public Works and Highways

(DPWH). (n.d.). Blueprints DOST XI

Davao building cnr. Dumanlas and

Friendship road. Davao, Philippines.

DOST. (n.d.). What we do. Retrieved 09 19,

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and Technology:

http://dost.gov.ph/index.php?option

=com_content&view=article&catid=5

8%3Aabout-dost&id=381%3Awhat-

we-do&Itemid=82#vision

DPWH Davao. (n.d.). Proposed three story

building DOST XI. Davao.

Dr. Sales, A. (2012, 10). Rain water harvesting

for new DOST Davao building. (V.

Oskam, Interviewer)



Google. (2012). Davao. Retrieved 11 4, 2012,

from Google Maps:

http://goo.gl/maps/yQIlS

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Rainwater harvesting. Retrieved 10

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http://greenroofphilippines.com/rain

water-harvesting-new/

ICE Hardware shop, C. (2012, 10 15). Price of

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Interviewer)

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Interviewer)

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Davao, Philippines.

Pag-asa. (1994). Surface observation 28-02-

1994. PAGASA FORM 1001-AB, 1.

Retrieved 11 2, 2010

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Texas manual on Rainwater

Harvesting. Austin, Texas: Texas

Water Development Board.

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Gerston, J., & Colley, S. (2005). The

Texas Manual on Rainwater

Harvesting. Austin, Texas: Texas

Water Development Board.

The Philippine Star. (2010, 4 18). Campaign

on rain water harvesting to be

launched. Philippine Star, p. 1.

Vierhout, M. M. (2012). PUM Expert's Final

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%20to%20construct%20a%20rainw

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