Year: 2020 | Volume: 1 | Issue: 1 | Page No.: 33-37
Recieved: April 02, 2020 Accepted: April 09, 2020 Published: April 15, 2020
Rain Water Harvesting Capacity for Buildings to Reduce over Extraction of Ground Water in Chittagong City
Noor A. Md. Amanat Ullah, Mahamudul Hasan and Sourav Das
Shortage of water for residential use is a concern all over the Bangladesh, especially in major cities like Chittagong. The current decline in groundwater availability in Chittagong necessitates the formulation of alternative water management plan through proper assessment of the available resources. In this research, rainwater harvesting has been considered as an option for reducing groundwater extraction. Analysis of the catchment availability and size of storage tanks are needed to find the potential of rainwater harvesting in the building and the capacity of buildings to store rainwater and use it. Four selected buildings of Chittagong area are taken for case study. To check the technical feasibility and potentiality of rainwater as alternative water source, ARC method is taken to measure the maximum capacity of storable water and size of storage tanks are determined by both Sizing Formula and AC-VC relation method. With respect to available storable water, demand satisfaction is also measured to check feasibility of the system.
Groundwater, Aquifer, Rainwater Harvesting, Technical Feasibility, Demand Satisfaction
TO CITE THIS ARTICLE
Noor A. Md. Amanat Ullah , Mahamudul Hasan and Sourav Das
2020. Rain Water Harvesting Capacity for Buildings to Reduce over Extraction of
Ground Water in Chittagong City. Journal of Applied Sciences and Research, 1: 33-37
The groundwater table in Chittagong City is at present in a position from where it is difficult to pump groundwater by shallow tube well. Profound tube well is required practically in each place to locate the fresh water from the ground. For the most part, the area, profundity, size and piece of aquifers are dictated by seasonal rainfall intensity. Some areas in Chittagong experience a great depletion of groundwater level which is shown in Fig. 1 .
Being a tropical nation, which gets overwhelming precipitation due to north-easterly breezes amid the stormy season, water can be a potential wellspring of option water supply in numerous zones of Bangladesh . Institute of Water Modeling (IWM) as of late evaluated that with the present measure of precipitation, around 149,160 million liters of water can be reaped amid monsoon. The average yearly precipitation in Bangladesh fluctuates from 2200 to 2800 mm, 75% of which happens amongst June and October. The high precipitation power gives a great chance to rainwater harvesting . Despite the fact that Bangladesh has six seasons (each season comprises of two months) eventually those are covered with each other. As a rule, the normal most extreme temperature in the mid-year months is in the mid 30°C. Bangladesh gets substantial precipitation amid the stormy season, which stretches out from May to September, with the pinnacle of precipitation occurring amid June, July, and August. Rain more often than not falls as showers that can keep going for couple of minutes to a few hours.
|Fig. 1.||Seasonal fluctuation groundwater table|
|Fig. 2.||Average yearly rainfall data in Chittagong district from 1989 to 2008|
The normal yearly precipitation under the typical climatic conditions is around 2320 mm in Bangladesh while that in Chittagong locale is around 1800 mm. Precipitation information for a time of 20 years (from 1989 to 2008) are introduced in Fig. 2. Such information demonstrates that there is a lot of water that can be harvested in the rainy season. On the premise of this precipitation RWH framework can be viably actualized for family unit use .
The main objectives of the study were to determine the maximum storable volume of water and optimum volume size of tank for various size of rooftop catchment area and to determine the feasibility of rainwater harvesting system for selected buildings.
2. MATERIALS AND METHOD
2.1. Measuring Water Demand
Water demand per household is very important to measure the storage tank capacity. The water demand for specific purposes like cooking, drinking, showering, flushing of the households have been collected by a direct questionnaire survey.
2.2. Maximum Storable Volume of Water
The quantity of water that runs off a roof into gutter system in this study is calculated by using Eq. (1) according to the study “Roof water Harvesting: A Handbook for Practitioners” done by Thomas and Martinson known as ARC method .
Where, Q is the quantity of water that runs off, C is the runoff coefficient, R is the total rainfall (mm/year), and A is the roof area or the catchment area (m2).
A runoff coefficient of 0.8 was embraced in this study for the count of potential water collected from the catchment range which are made of tiles.
2.3. Size of Storage Tanks
2.3.1. Sizing Formula
These variables were considered and calculated to determine the best tank sizes; D: (m3) Daily water demand for a household, T: (m3) Tank volume, St: (m3) Daily storage, calculated by adding the initial stored water and the rain volume and then subtracting the daily supply, St-1: (m3) Initial stored water, from previous day, the first valued was assumed as 0. For the next days, St-1 is equal to the previous daily storage (St), Sp: (m3) Supply. For each day, the value of supply will be the smallest value from comparing the demand and the available rainwater volume that can be stored. (Smaller of D and St-1) (Eq. 2).
2.3.2. Area Consumed (AC) – Volume Consumed (VC) Relation
According to Schiller and Latham  the volume of storage tank can be determined using the area consumed (AC) - volume consumed (VC) relation method. In this method critical catchment area per person (m2/capita) and minimum storage volume provided per person serviced (m3/capita) is determined based on the following equations which has been multiplied by the total number inhabitants of a building to find the optimum volume (Eq. 3).
Where, Amin is the minimum Catchment Area (m2/capita), C is the monthly demand per capita (litres/month), f is the runoff coefficient (0.08), Rmin is the lowest annual rainfall over the observed period (mm/year).
In Eq. 4, Vmin is the minimumvolume ofstorage tank and Dmin is the minimum depth of the storage tank.
Where, RAvg is the average depth of storage tank and ∑Ri is the rainfall in the period.
2.4. Demand Satisfaction
Demand satisfaction is calculated by the total water demand of a building with respect of its maximum available water to storage in the calculated volume of storage tank and presented in Fig. 3.
3. RESULTS AND DISCUSSION
Rainwater harvesting is emerging as an intelligent option for water supply in residential  and it could not come in a better time since major water source of Chittagong city, groundwater is declining day by day . Nevertheless, most of the studies in literature focused on the potential of rainwater harvesting in reducing water consumption, whereas few examples examined their efficiency in the off demand and satisfaction which varies with size and stories of residential buildings.
3.1. Building No.1 in High Level Road (Two Storied Building with Single Unit in a Floor)
Catchment area of 18 m2 makes the available storable water 24.18 m3. According to sizing formula the size of storage tank is 7.952 m3 and according to AC-VC relation the size of storage tank is 10.02 m3. Monthly demand satisfaction with respect to supply has been shown in Fig. 3.
3.2. Building No.2 in WestKhulshi R/A (Six Storied Building with Dual Units in a Floor)
Catchment area of 61.6 m2 makes the available storable water 82.75 m3. According to sizing formula the size of storage tank is 18.744 m3 and according to AC-VC relation the size of storage tank is 20.53 m3. Monthly demand satisfaction with respect to supply has been shown in Fig. 4.
3.3. Building No.3 in Amirbag R/A (Four Storied Building with Four Units in a Floor)
Catchment area of 324 m2 makes the available storable water 435.12 m3. According to sizing formula the size of storage tank is 45.44 m3 and according to AC-VC relation the size of storage tank is 49.657 m3. Monthly demand satisfaction with respect to supply has been shown in Fig. 5.
|Fig. 3.||Monthly demand satisfaction of two storied building with single unit in a floor|
|Fig. 4.||Monthly demand satisfaction of six storied building with dual units in a floor|
|Fig. 5.||Monthly demand satisfaction of four storied building with four units in a floor|
|Fig. 6.||Monthly demand satisfaction of one storied building with four units in a floor|
3.4. Building No.4 in Dewanhat (One Storied Semi-Pucca Building with Four Units in a Floor)
Catchment area of 144 m2 makes the available storable water 217.6m3. According to sizing formula the size of storage tank is 11.36 m3 and according to AC-VC relation the size of storage tank is 15.97 m3. Monthly demand satisfaction with respect to supply has been shown in (Fig. 6).
From results and discussion, it is observed that during rainy seasons (Jun-Jul) due to heavy rainfall by harvesting rainwater maximum percentage of demand satisfaction can be gained. Though the system cannot serve well in other seasons but as it will be beneficial to the inhabitants during rainy season to cope with the existing water crisis it can be a very good alternative to have. It served exceptionally well for the one storied with four units building and two storied single unit building as they had comparatively large catchment area with respect of number of inhabitants of building than others. But according to the analysis rainwater harvesting system can only serve well in the month Jun-Jul. For the rest of the months still Chittagong has to be dependent on groundwater. But over extraction of groundwater resulting in depletion of ground water table. This has become so severe that in some areas deep tube wells going down over 300 ft under the ground are unable to extract water. There remains no alternative of groundwater recharging to solve this problem.
At first, all praises belong to the Almighty Allah, the most clement, most generous and bounteous to all living creatures and their actions. We express our profound gratitude and indebtedness to our thesis supervisor Saurav Das, Lecturer, Department of Urban and Regional Planning (URP), Chittagong University of Engineering and Technology for his cordial encouragement, constant guidance, inspiration and valuable suggestion to prepare this paper.
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