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Cetak

Rainwater Harvesting System

on . Posted in Sistem Penuaian Air Hujan

 

Introduction

Note: There are no translations available.

Even in humid tropical Malaysia where rainfall is abundant (annual rainfall 2400mm for Peninsular Malaysia), water fluctuations due to a lack of rainfall do happened as in the 1998 drought which brought unpleasant water supply disruptions for Klang Valley folks.

The Study House at Taman Wangsa Melawati, Kuala LumpurFollowing this Water Crisis, the Minister of Housing and Local Government on 7 May 1998 has expressed the Government’s interest for houses to be designed for collecting rainwater. In 1999, the Ministry of Housing and Local Government has produced a Guideline on Installing a Rainwater Collection and Utilization System.

In support of the Government's interest in rainwater harvesting, NAHRIM through collaboration with other government agencies such as Department of Irrigation and Drainage (DID), Department of Local Government, Universiti Teknologi Malaysia (UTM), Universiti Sains Malaysia (USM) and Universiti Malaya (UM) is currently pursuing R&D on rainwater harvesting focusing on hydrologic and hydraulic design, system design and performance, installation and operational costs and water quality aspects.

Consequently, NAHRIM is currently carrying out a study for a double storey terrace house located at Taman Wangsa Melawati, Kuala Lumpur focusing on utilising rainwater for non-potable household use such as toilet flushing, washing clothes and general cleaning around the house including car and motorcycle washing.

 

Sizing The Tank

 

For those who are interested in installing a similar rainwater harvesting system for their homes (Kuala Lumpur area only). Tangki NAHRIM is available for download to size the rainwater storage tank based on daily water usage and daily rainfall.

 

User's Guide:

Step by step guide for using Tangki NAHRIM

1. The software maybe run from the diskette or hard disk. Wherever the software is run from, the command line is "drive name : Tangki NAHRIM 1.0.exe". File "vbrun100.dll" is needed to run Tangki NAHRIM 1.0.exe and both file should be in one folder.

2. The starting screen (Figure 1) is where the user needs to specify the following parameters:

  1. The roof characteristics such as the length and width of roof, the run-off coefficient which depends on the material of the roof and the first flush volume which accounts for the amount of water needed to wash the roof with each rainfall before the remaining rain water can be used.
  2. The water usage characteristics which include the number of persons using the water, the volume used per person and the size of storage tank used for the rain water.
  3. Rainfall areas: at present, the software only includes data of rainfall for the Kuala Lumpur area as recorded at the DID Ampang station. Rainfall data for other selected areas will be included in future.

Figure 1: Start-up screen for the simulation model

 

3. In the starting screen, the user have the following options: 

  1. For simulating with the monthly data, the user may proceed directly to the simulation menu by clicking the "Continue" button or may choose to look at the rainfall data ("View R.F. Data"). The rainfall data is given in a tabulated form for the years 1983-1997.
  2. For simulating with the daily data, the "Simulate Daily" button will bring the user to the screen which displays the daily rainfall data. The "Show Data" button will display the data for each month in the table. Repetitively clicking this button will display the data for the following months. Alternatively, the user may specify the particular year, then the data for that year will be displayed in a consecutive manner. Choosing "Continue" will bring the user to the Simulation Menu.
  3. The "Display Updates" button is for displaying the latest values specified by the user for each parameter when returning from the Simulation Menu after changes are made to these values which are different to the default values.
  4. The "Return" button is for the user to go directly to the Simulation Menu after making changes to the values of the parameters.
  5. The "Print Form" button will print this form using the default printer and the "End" button will end the program.

4. In the Simulation Menu (Figure 2), the user have the following options:

 

  1. To run the software with the monthly data, either choose "Fast Simulation" or "Slow Simulation". Fast simulation is for getting the results quickly while slow simulation is for observing the simulation progress at a slower phase. While the simulation is run, the user is shown the conditions of water in the storage tank plus the following information:
    1. Total rain water volume delivered.
    2. Number of days the demand or usage volume not fully met.
    3. Number of days with no rain at all.
    4. Number of days when there is no rain and the storage tank empty.

 

Figure 2: The simulation screen

 

Figure 2: The simulation screen

 


At the end of each simulation run, the following information are provided:

 

  1.  
    1. The reliability ratio.
    2. The percentage of the total time that the storage tank became empty.
    3. The total volume of rainwater collected and delivered to the user.
    4. The average volume of rainwater delivered per day.

 


The user is also provided with the following analysis results by clicking the respective buttons:

  1. The "Analysis Results" button will show the total rainwater captured by the system, the coefficient of rainwater utilization and the storage efficiency.
  2. The "Monthly Results" button will display the reliability values for every month from January to December.
  3. The "Tank Condition" button will show the percentage of the time that the water level in the tank at the different levels i.e. full, three quarters, half, one quarter and empty.

 

  1.  To run the software with the daily data, the user should choose "Fast Simulation" or "Slow Simulation" button in the "Daily Simulation" frame. Again, fast simulation is for getting the results quickly while slow simulation is for observing the simulation progress at a slower mode. 
  2. The "Print Results" button is for printing the daily output for the rain water volume delivered, water level in the tank and ratio of water demand met in a month. For this, the user needs to specify the month and year for which the results are required.

5. When the "Graphic Display" button is chosen, the user is shown the Graphic Display Screen. In this screen, the user can observe the daily fluctuations of the water level in the storage tank for one month at a time for the 15 years period. The following options are available:

  1. "Fast Simulation" is for getting quick results with the monthly data.
  2. "Slow simulation" is for observing the simulation at a slower phase.
  3. "Daily Data" simulation will run the model using the daily rainfall data.
  4. "Specific Month" enables the user to specify to view which particular month.
  5. "Step Simulation" displays the results for the following month.

6. The "Change Parameter" button is for going back to the Starting screen where the user may change the values of the parameters for subsequent simulations.

 

7. Every form in the software is provided with the "Print" or "Print Form" buttons that enable printing of individual forms to be done. Also, "End" buttons are provided for quick exit from the program.

 

 

 

System Design

 

Various subsystem of the rainwater harvesting system is shown below:

 

(i) Catchment’s Subsystem

 

The rooftop is the catchments subsystem. For non-potable use, any roofing material can be used as the catchments subsystem. As for potable use, the best roof materials are metal, clay, and cement materials although all roof material types have been used (except asbestos). The roof material used in the study is of cement tiles.

 

Catchments subsystem depends on the roof configuration of the house. In this study, due to the typical roof design, only the front section of the house is considered. The total roof area is 60 m.

 

(ii) Conveyance Subsystem

The function of this subsystem is to convey the rainwater from the catchments sub-system (rooftop) to the storage tanks. Basically this subsystem consists of 3 main components. These components are the gutter, downpipes and the conveyance pipes. Rainwater from the rooftop is channeled to the gutters (6" or 150mm) and then through the downpipes and finally through the conveyance pipes to the storage tanks.

Aluminum and galvanized iron are usually the best materials for the gutter due to its good corrosion resistance property. For the conveyance pipes, usually UPVC pipes are used. However in this study, UPVC type materials were used as gutter and conveyance pipe due to their flexibility during installation. The gutter slopes at 6 mm. per meter and gutter hangars are provided at 1 m center to center.

At the downpipe inlet a net should be placed. The net is to trap rubbish, leaves and other debris usually found on rooftops. Sharp bends along the run of the gutters should be avoided. This is because, rubbish usually gets stuck at corners and therefore this would affect the flow of the rainwater.

 

(iii) Filtration

FiltrationRainwater from the rooftop (the first 1 mm of rain) is usually contaminated with dirt, bird droppings, leaves and other materials. The first flush (60 gal. or 276 litres) of rainwater from the roof surface is directed into the first flush tank of 200 litres (see figure below) to filter out these materials from the rainwater before it is stored in the storage tanks.

 

 

 

 

 

 

 

 

(iv) Storage Tank

Storage TankIn this study, two number of PVC tanks (see figure below) with a 2500 litres capacity each were installed as storage tanks.

 

 

 

 

 

 

 

 

(v) Booster pump

Booster pumpA 1.0 horsepower pump (see figure below) with minimum head of 12 m was installed to

 

 

 

 

 

 

 

(vi) Roof tank

Additional roof tank (250 litres) were installed on the roof in addition to the existing domestic roof tank. The additional tank provide separate rainwater supply for non-potable household use.

 

(vii) Plumbing to toilets, washing machine and general use

The study house also has a separate plumbing system to cater for rainwater usage. Since rainwater was for non-potable use, the plumbing system was installed in such a way that there will be a bypass connection for each flushing cistern. In case of water shortage or non-availability of rainwater, the public water supply can be always be switched on. The rainwater plumbing system also connects to the washing machine pipe and pipe for general cleaning.

 

Rainwater pipe connection to toilet   Rainwater pipe for washing machine   Rainwater pipe for general cleaning
Rainwater pipe connection to toilet
 
Rainwater pipe for washing machine
 
Rainwater pipe for general cleaning
         
Rainwater Harvesting System

 

Water Quality Aspects

Samples of the quality of rainwater collected in the storage tank are analysed and compared with World Health Organisation (WHO) Drinking Water Quality Guidelines (see Table 2). The results show that the pH which ranges from 6.26 to 6.62 is reasonable. The hardness at between 8.6 to 32.6 is low. The manganese and iron contents are low as well. Toxic metals such as cadmium at less than 0.001 mg/l is below the WHO Guideline while lead at < 0.05 mg/l is above the WHO Guideline. The Faecal Coliform at 20 counts per 100 ml shows contamination from some animals.

Table 1 shows average water use for facilities using rainwater based on twelve months data. Since there was no treatment to the rainwater collected, therefore it is strictly used for toilet flushing, general cleaning and washing clothes.

 

 

Test Parameters
Units
Storage tank
WHO Drinking Water Guidelines
pH
-
6.26-6.62
6.5-8.5
Magnesium
mg/l
0.2-0.27
-
Calcium
mg/l
3.2 – 12.6
-
Sulfate
mg/l
1.4 – 6.8
250
Chloride
mg/l
< 1
250
Silica as SiO2
mg/l
1.2 - 4.2
-
Iron
mg/l
0.01 - 0.02
0.3
Manganese
mg/l
0.01 - 0.04
0.1 - 0.5
Sodium
mg/l
0.1 – 1.1
-
Hardness as CaCO3
mg/l
8.6 – 32.6
-
Turbidity
NTU
0.44 - 2.56
-
Bicarbonate
mg/l
4 – 10.6
-
Nitrate as N
mg/l
0.36 – 1.52
50
Cadmium
mg/l
< 0.001
< 0.003
Lead
mg/l
< 0.05
< 0.01
Potassium
mg/l
0.3 – 2.6
-
Total Dissolved Solids
mg/l
14 - 126
1000
Dissolved Oxygen
mg/l
2.1 - 4.2
-
Ammonia as N
mg/l
0.02 – 0.18
-
Total Alkalinity as CaCO3
mg/l
4 – 18
-
ColiformCount
cfu/100 ml
0 – 230
0
E. Coli Count
cfu/100 ml
0 – 50
-
Faecal ColiformCount
cfu/100 ml
0 – 20
0
Table 2 : Rainwater Quality in Storage Tank
 
The rainwater collected proved to be of very good quality. It can therefore be used for, washing clothes, car washing, plant watering, and general cleaning around the house. With low bacterial contamination the rainwater can even be used for bathing.

 

Water Usage

The house consist of a family of two adults and four school going children. The house have three (3) bathroom. The amount of rainwater used for facilities were monitored manually using mechanical water meter installed on each facility. Reading were taken and recorded manually. The water use figures obtained is compared with water use figures from literature and specifications of the respective products.

Table 1 shows average water use for facilities using rainwater based on twelve months data. Since there was no treatment to the rainwater collected, therefore it is strictly used for toilet flushing, general cleaning and washing clothes.

 
  Average Daily Use (litres) Average Monthly Use (litres) %
Washing clothes 300 9000 66
Toilet Flushing (3 W.Cs) 90 2700 20
General Cleaning (including car and motorcycle washing) 65 1950 14
TOTAL 455 13650 100
 
Monthly Rainwater Use : 13650 litres
Monthly Water Use from public water supply) : 27000 litres
Total Monthly Household Water Use : 40650 litres
Table 1 : Rainwater Use for Various Facilities
 
From the above, household use for non-potable purpose using rainwater constitutes 34% of the total monthly household water use.

 

Cost

 
Various subsystem of the rainwater harvesting system is shown below:
 
 
(i) Installation Cost For Rainwater Harvesting System at a Double Story Terrace (year 2001)

System cost include supply and installation and varied accordingly depending to the type of materials used.

 
   
Amount (RM)
Gutter (uPVC)  
150.00
Conveyance System  
200.00
Plumbing works  
400.00
Water tank (top) 1 No
200.00
Water tank(ground) 2500 litres capacity 2 Nos
1000.00
Water pump(electrical)  
750.00
TOTAL  
RM 2700.00
 

(ii) Maintenance of the system

Maintenance included cleaning of the rainwater collection system, electricity bills and chemicals to prevent mosquito breeding in the storage tank. The maintenance cost is believed to be quite small and therefore not included in the subsequent calculations.

 
(iii) Payback Period
 
Old Tariff    
System Cost : RM 2700.00
Water Saving/month : 13650 litres x RM 1.05/m³ = RM 14.33
Payback Period : RM 2700.00 / (RM 14.33 x 12)
    15.7 Years
 
New Tariff    
System Cost : RM 2700.00
Water Saving/month : 13650 litres x RM 1.70/m³ = RM 23.21
Payback Period : RM 2700.00 / (RM 15.20 x 12)
    9.7 Years
 
* An increase of 60% water rate (>35 m3 / month)
 

(iv) Unit Cost of Water by Rainwater Harvesting

Unit Cost of water by rainwater harvesting system can be calculated using simple formula below:

Cost per 1000 litres = Cost of the system x 0.10
Yield in litres per day x 0.365
Allowing about 10% of the capital cost to cover interest and depreciation each year
 
For the Double Storey Terrace
 
Average yield per day : 298 litres
Cost per 1000 litres = RM 2700 x 0.10
455 x 0.365
  = RM 1.63 compared to Water Supply Department (JBA) rate of RM1.05* (Old Tariff) and RM1.70** (New Tariff)
    * > 35 m³ and above per month
    ** > 40 m³ and above per month
 
From the above it can be seen that the unit cost of water from the rainwater harvesting system at RM 2.48/m³ is still more costly compared with the piped water cost of RM 1.70/ m³. As practiced in Japan and elsewhere the Government may need to provide subsidies to encourage the public to install rainwater harvesting systems.

 

 

 

Benefits

What are the benefits of Rainwater Harvesting?

 

The multifarious benefits of rainwater harvesting are many and some of them are discussed below:

 

(i) Environmental Benefits

 

Rainwater harvesting promotes independence and self sufficiency and also help to develop an appreciation for this precious Allah given resource. This activity conserves water and also energy in the form of the energy input required to treat and pump water over a vast service area for a centralized water system.

Local erosion and flooding due to runoff from impervious areas such as pavement and roofs will be reduced by rainwater harvesting as some rain is instead captured and stored. Consequently, stormwater run-off, which picks up contaminants and degrades our drains, streams, rivers and seas, is tamed and harnessed for human uses.

Promoting underground infiltration of rainwater can prevent cities from being plagued with thermal pollution and water shortage, and help improve the urban environment, and also contribute to groundwater recharge enabling the consumption of good tasting groundwater. Rainwater utilization leads to the holistic solution of water resources and environmental problems in urban areas.

 

(ii) Domestic and Industrial Benefits

The exceptional quality of rainwater have prompted many people to choose rainwater as their primary water source, or for other non-potable uses such as toilet flushing, washing clothes and garden watering. Rainwater quality often exceeds that of ground or surface water as it does not come into contact with soil, dissolving salts and minerals in the process. It is not subject to the myriad of pollutants found in rivers that can also contaminate aquifers.

Rainwater has hardness approaching zero thereby reducing significantly the quantity of detergents and soaps needed for cleaning, as compared to typical piped water. Soap scum and hardness deposits disappear, and the need for a water softener, often an expensive requirement for well water systems, is eliminated. Water heaters and pipes will be free of deposits caused by hard water and should last longer. The purity of rainwater also makes it attractive for certain industries for which pure water is a requirement such as computer microchip manufacturing and photographic processing.

 

 

 

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