Evaluation of Rainwater Harvesting Methods and Structures Using Analytical Hierarchy Process for a Large Scale Industrial Area ()
1. Introduction
The increasing growth in population, industrialization and urbanization is causing severe impact over the water resources. The overexploitation of natural water resources has already created environmental problems all over the world. In India, conflicts on river water sharing between the states have already started. One of the major solutions to meet ever increasing water demands would be storing the available rainwater through rainwater harvesting techniques (RWH) [1]. The term RWH implies conservation of rainwater where it falls [2] which was also an age old tradition in India [1]. The recorded evidence of water harvesting is found in Harappan and pre Harappan civilizations dating back to 4000 to 6000 years [3]. However with the changing world and modernization, with construction of large scale reservoirs and water supply schemes, concept of RWH has lost its presence in middle era. Recently the increasing water demand, nonavailability of space for large reservoirs, and its subsequent problems have forced to revive the concept of RWH.
An overall review on RWH can be seen in Boers and Ben-Asher [4]. Throughout the world many Governmental and Non-Governmental organizations have prepared and issued guidelines regarding RWH [5–8]. Fairly a good number of site specific and case studies based RWH literature is available [9–23]. Most of the above studies concluded that RWH is one of the best methods to solve the serious problem (situation) of catering the increasing water demand, also for Governments it is the drought relief programs. Various types of RWH methods and structures are available in India (Table 1), however the choice of any RWH structure is very site specific and depends on topography, rainfall, runoff, demand, land use pattern and land availability. Almost all RWH studies are aimed at providing good source of water or augmenting irrigation supply or improving the watershed development. Until now in most of the studies, selection of particular type was purely based on hydrological and economic criteria rather than other satisfying criteria. In India, RWH has high potential in large scale industrial sector, where large area is available for RWH. Besides various advantages, the major benefits of RWH in an industrial area are: the end use of harvested water is located close to the source, eliminating the need for complex and costly distribution systems. Rainwater has zero hardness eliminating the need for a sophisticated water treatment process. It can reduce the dependency of the industry on irrigation reservoirs, it is also hoped that this RWH will improve the micro environment inside the industry and contribute to self sufficiency of the industry in its BLUE ENERGY (water power) leading to sustainable development.
In the present study, the main aim is to identify an appropriate RWH structure for a large scale automobile industry in India. The first step in designing any RWH structure is to determine the volume of water to be stored. In this case it was achieved using four methods namely; mass balance, Ripple diagram, analytical and sequent peak algorithm methods. The most appropriate RWH method and number of RWH structures for the given volume of water from various alternatives has been determined using analytical hierarchy process (AHP). The alternatives are evaluated against 16 (quantitative and qualitative) attributes to select an appropriate method and number of RWH structures.
2. Materials and Methods
2.1. Study Area
A large scale automobile industry situated near Nasik (Igatpuri), Maharashtra, India is considered for the study. Presently the industry is purchasing water from an irrigation reservoir (Talegaon dam, situated approximately 2 km on the south of the factory) owned by Maharashtra Jeevan Pradhikaran, Government of Maharashtra. The factory is situated in a tropical wet climatic region and receives an average annual rainfall of 2983 mm. Hence there is scope for reducing the expenditure on water through RWH, and also chances of reducing the dependency on the irrigation reservoir. The industry has a total plot area of 253,000 m2 (25.3 ha) with a total built up area of 46,500 m2 (the main factory alone). The area is moderately undulating with hard rock sub-surface overlaid by a soil cover ranging from 2 to 3 m. The present water consumption of the industry is 6,616 m3/month leading to an annual demand of 79,392 m3. The present annual expenditure (based on slab rates) on water is Rs. 3,652,032 (1USD = Rs. 45).
2.2. Rainfall Analysis over the Study Area
For the present study, 34 years (1971-2004) of daily rainfall data pertaining to Igatpuri rain-gauge station has been obtained from India Meteorological Department (IMD), Pune. The summary of the rainfall analysis is depicted in Table 2. The region receives an average annual rainfall of 2,983 mm occurring over 103 rainy days. The highest observed rainfall over 34 years is 4,205 mm during the year 1994 and minimum is 2,083 mm during the year 2000. 95% of the annual rainfall occurs during the South West monsoon (June to September). Since the rainy days are more during the monsoon months they show high spread and low peak. From Table 2 it can be seen that the month of July receives highest rainfall in a year, 1061 mm and with no rainfall during March. The rainfall has very low spread and high peakedness during the low rainfall months, (November to May) the reason being less number of rainy days. All the rainfall in the low rainfall month occurs in just 2 to 3 day leading to low spread and high peakedness.
The average daily rainfall at the study area for the past 34 years is shown in Figure 1, indicating the variation of the daily rainfall within a year. Figure 2 shows annual rainfall over Igatpuri region along with number of rainy days in a year. Over the past 34 years the area has seen a maximum of 124 rainy days in 1993 and minimum of 78 rainy days in the year 1972. This daily and monthly rainfall data has been used in estimating the volume of water to be stored in order to meet the daily water demand throughout the year.
2.3. Volume of Water to Be Stored through RWH
With basic calculations, the volume of average annual rainwater available from the roof top area of 46,500 m2 with a runoff coefficient of 0.9 (average rainfall of 2,983 mm) is 124,838 m3, whereas the annual demand is 79,392 m3 only. This shows that the runoff available from the single roof top of the industry alone is sufficient to meet the annual water demand. In this case, the supply is more than the demand, thus it is necessary to find the
Table 1. Classification of RWH structures.
Table 2. Statistical properties of the rainfall data.
techno-economical size of the RWH structure. With this as the first objective, the four different methods namely mass balance method, Ripple diagram method, analytical method, and sequent peak algorithm method were em-
Figure 1. Average daily rainfall at Igatpuri station.
Figure 2. Annual rainfall and number of rainy days in a year at Igatpuri.
ployed to determine the volume of water to be stored.
2.4. Choice of RWH Structure
Once the volume of water to be stored is determined, the next step is to select the appropriate RWH structure, Analytical Hierarchy Process (AHP) is used for this purpose. The selected RWH structure should have two important characteristics: first one is assured quantity of water at any given time, second is good quality of water. Based on the topography and economics of the study area, three broad RWH structures are considered for detailed AHP analysis, they are:
• RCC water tanks: these are the closed structures with no seepage and less evaporation losses, least interference with atmosphere. They can provide reliable water supply with good quality with appropriate amount of treatment. But usually the construction costs are very high.
• Surface storage: these can be useful to store surface runoff effectively. They have lower construction cost but prone to seepage and evaporation losses. Also as they are open to surrounding environment and prone to various contaminations and biological activities.
• Ground water recharging: these are effective if sufficient good aquifers available. These have least cost, but the storage capacity depends on many external factors.
The above alternatives have their own advantages and disadvantages over others. The other points to be considered are reliable supply, water quality etc, instead of just going with cost benefit analysis, for this purpose one of the multi criteria decision making processes AHP is used.
2.5. Analytical Hierarchy Process (AHP)
AHP is a general theory of measurement used to derive ratio scales from both discrete and continuous paired comparisons [24]. It is used to determine the relative importance of a set of activities or criteria. The novel aspect and major distinction of this approach is that it structures any complex, multi-person, multi-criterion and multi-period problem hierarchically. Using a method for scaling the weights of the element in each level of the hierarchy with respect to an element (e.g., criterion) of the next higher level, a matrix of pair wise comparisons of the activities can be constructed where the entries indicate the strength with which one element dominates another with respect to a given criterion. This scaling formulation is translated into a principal eigen value problem which results in a normalized and unique vector of weights for each level of the hierarchy (always with respect to the criterion in the next level), which in turn results in a single composite vector of weights for the entire hierarchy. This vector measures the relative priority of all entities at the lowest level that enables the accomplishment of the highest objective of the hierarchy.
3. Results and Discussions
As indicated earlier the primary objective of the study is to select an appropriate RWH method and number of RWH structures for the industry which satisfies the hydrological, technical, economical and satisfaction criteria along with the implementable or amenable solution by the industry. For this purpose first the volume of water to be stored is assessed based on the prevailing hydrologic (rainfall-runoff) condition and demand in the industrial area. Then the appropriate method is selected using AHP based on satisfying criteria, the results are as follows:
Figure 3. Mass balance representation of storage volume.
3.1. Mass Balance Method
In this method the basic assumption is that the demand in rainy (wet) months is met by supply (runoff) during same months. To meet dry months demand, water has to be stored during the rainy months, thus the storage capacity should be at least equal to the total water demand during dry months. Figure 3 shows the average daily rainfall and runoff from rooftop. Assuming the runoff coefficient as 0.9; the runoff from rooftop was estimated using rational method, with appropriate units the equation used for rational method is as follows:
Q = CiA(1)
where, Q – runoff, C - runoff coefficient, i - rainfall intensity and A - rooftop area The shaded portion in Figure 3 is the deficit volume in meeting the demand, this much of volume needs to be stored in the water rich period. Table 3 elaborates the monthly mass balance method. Since 95% of runoff occurs in four months (June, July, August, and September) the demand in these four months is met by the rainfall in these months. However the demand of remaining eight months should be met by the stored water in these four months. From Table 3 it is seen that the demand for eight months is 52,928 m3, hence the size of the reservoir should be 52,928 m3 or atleast 50,761 m3 (as the expected runoff during dry months is 2167 m3).
3.2. Ripple Diagram Method
This method considers the difference between the demand and supply over the period of time. To find out this difference, cumulative runoff is plotted against time. Cumulative demand is plotted and then superimposed on this graph starting from the peak of the dry period. If more peaks are available, the cumulative demand line may be started from each peak. Maximum difference
Table 3. Result of mass balance method.
between the supply and demand over the period of time is the capacity of RWH structure. This method considers two main assumptions:
1) if N years of data is available, the inflow and demands are assumed to repeat in cyclic progression of N year cycles;
2) the reservoir is assumed to be full at the beginning of dry season.
Figure 4 elaborates the procedure of Ripple diagram method. The maximum deficit works out to be 53,409 m3, and is the volume of water to be stored.