Chapter03.pdf

Chapter 3

RAIN WATER HARVESTING SYSTEM DESIGN 3.1 General Rain water harvesting (RWH) is practiced in many forms throughout the world. While surface run-off is collected for agricultural purposes as well as for mitigating flash floods, roof run-off is used to supplement potable water, mainly to households.

Surveys on water usage prove that per capita consumption can be considered a constant for a given set of users and the water usage patterns are habitual. Hence, untreated harvested rain water can be easily used for activities such as flushing of WC and for vehicle washing. As such, research has improved the collection efficiency and the performance of various components of the RWH system with particular attention given to the storage device.

Various storage designs are introduced with a view to cut down the capital cost since the storage tank is identified as the highest cost component of a RWH system. Hence, algorithms are developed to determine the optimum tank size for a given demand to achieve a desired WSE. One such algorithm, developed and presented in graphical form, was introduced by Fewkes (1999) and validated for Sri Lanka (Sendanayake & Jayasinghe, 2006).

However, the harvested rain water has to be drawn-off and supplied to user points to cater to modern conveniences if RWH systems to proliferate. In order to save energy on pumping, it is desirable if at least part of the harvested rain water can be gravity fed. Since the storage capacity required for a higher WSE is substantial, placing it at a higher elevation raises issues such as the need for costly supporting structures and disturbance to aesthetic appearance of the building envelop. Besides in urban areas, lack of space demands the storage tank located below ground level. The issues get further complicated when RWH systems are integrated to multi storey houses. Hence,

69

there is a need for a conceptually new RWH model, which will fully or partially feed collected rain water by gravity to user points, in multi storey situations.

3.2 Rain water harvesting The needs and benefits of rainwater harvesting along with the global usage of such systems will be discussed in detail in the following paragraphs.

3.2.1 Needs of RWH There exists a growing need for RWH systems worldwide due to a number of factors as summarized as follows: •

Inadequacy of existing water supply systems in the face of rapid population growth, creating frequent water shortages and scarcities.

•

Degradation of water quality in primary sources such as rivers, ground water aquifers and natural lakes as a result of wide spread use of chemicals in agriculture (pesticides, herbicides and fertilizer) and their contamination due to industrial and human waste.

•

Escalating cost of providing water (cost per m3) due to high cost of constructing reservoirs for storing reticulated water, high costs in pumping from centralized locations to end user points, filtering and purification costs, distribution system maintenance costs and financial costs on investments such as opportunity costs.

•

Risk of disruption to mains water supply due to break downs or prolonged draughts. The storage facility of the RWH system can act as the buffer for such an emergency.

•

Non-availability of potable water in isolated areas through conventional methods due to lack of water bodies in the vicinity, difficulty in reaching ground water aquifers due to excessive depths and high capital outlay in drilling through rock, non-availability of power supply inherent to isolated hamlets in arid, semi arid and mountainous areas.

•

Depletion of water levels in underground aquifers thus limiting the draw-offs as a result of minimal ground water recharging and increased use of ground water.

70

3.2.2 Benefits accrued from RWH Apart from the obvious benefits of availability of potable water at virtually no cost excluding pumping cost from the storage tank to end user points, there are a host of direct and indirect benefits from a well designed RWH system that can be described as follows: •

Reduced demand on conventional water supply systems by supplementing rain water for needs which do not require high quality water such as WC flushing, washing, gardening, vehicle washing etc., thus saving on purified, treated drinking quality water. This would facilitate managing demand for water and rationalize new investments.

•

Minimized depletion of ground water by recharging in surface run-off harvesting and preserving it at higher levels and quality, minimizing water stress during draughts and enhancing the vitality of all life forms.

•

Increased decentralized water security and local self reliance whilst encouraging family level operation and maintenance.

•

Facilitating urban home gardening and small-holder food production, supplementing rural irrigation and stimulating income generation.

•

Lowered risk of flash flood situations by taking off a sizable quantity of roof run-off from the drainage system.

•

Reduced national energy consumption and water loss in the treatment and conveyance of reticulated water.

•

Reduced conflictive invasion of rural water sources to cater for urban demand by meeting requirements close to the point of harvesting.

•

Increased domestic water security by reducing the unproductive labor, time and hazards faced mainly by women and children in fetching water from a distance, and improved accessibility to safe water for many marginalized communities.

•

Minimized consequences of increased salinity intrusion due to sea level rise, and the threat caused from pollution to traditional sources of water by planned infiltration.

71

3.2.3 Global use of rain water harvesting

Per capita consumption of water is a relatively elusive figure in practical terms as water usage patterns vary significantly with life style, draw-off source, and geographical location of the end user as well as the climatic conditions prevailing in the area. While per capita water consumption is low in dry and low humid areas, it tends to increase in areas with abundant rain. It is observed that the relative ease of availability of water tends to increase the usage while the biggest variation occurs along with life style differences.

Research in many countries has shown that modern household equipment and amenities such as WC in toilets, washing machines, dish washers as well as car washing has significantly increased water consumption. In this chapter, water usage pattern of a typical household having WC fitted toilet facilities is surveyed, where sizable quantity of service water is used for non-drinking purposes. Apart from WC flushing, vehicle washing and gardening require significant quantity of water, for which harvested rainwater can be used disregarding its quality aspects. Studies carried out on water usage patterns reveal that a sizable quantity is being used for WC flushing, car washing and other external uses which do not require drinking quality water. For example, in Sweden, 20% of household water use is for flushing toilets, 15% for laundry and 10% for car washing and cleaning (Villareal & Dixon, 2004). In the UK, 30% of the potable water supplied to the domestic sector is used for WC flushing and the transportation of foul waste (Fewkes, 1999a). In Australia, studies of water usage in homes located in different climatic regions indicate that on average 15% of supplied water being used in toilets while 30% being used for external purposes (Australian Bureau of Statistics, AustStats, 2000)

In Sri Lanka, an extensive survey was carried out (Sendanayake, 2007) and average usage for WC flushing was found to be about 25% of the total water demand. Importantly, this demand was found to be approximately a constant as the water usage in a household is generally of habitual nature. However, it is important to note that 72

harvested rainwater is to be used as a supplementary source of water taking a sizeable load off the reticulated centralized supply.

3.3 Conventional RWH models and their limitations Conventional RWH models are the ones widely used and are fundamentally classified into different types depending mainly on the method of roof run-off collection.

3.3.1 Fundamental types of RWH systems

Design wise RTRWH systems are classified into two basic types. They are as follows: •

Dry systems

A dry system for rainwater collection involves down pipes leading directly into the storage tanks, so after a rain event, no water remains within the collection pipes as shown in Figure 3.1 •

Wet systems

A wet system usually involves underground pipes with the entry to the storage tank being above ground level thereby trapping water within the pipes after rain as shown in Figure 3.2

Figure 3.1: The Dry RTWHS

Figure 3.2: The Wet RTWHS

73

The dry system is preferred as the wet system can lead to water trapped in the conveying pipes going stale and in some cases breeding mosquitoes if the pipe entrances are not securely sealed. Since this additional volume need to be jettisoned through the first flush device thereby increasing the capacity required by the first flush (FF) device (Hermann & Schmida, 1999)

3.3.2 Global RTRWH systems

Many practical RTRWH systems are in use globally and differ to each other mostly on cost factors and the level of sophistication. While many developing countries use simple systems similar to what used in Sri Lanka, most of the developed countries use RTRWH systems as supplementary water sources for existing mains supply. In these systems the discharge is automated so that when collected rainwater in the storage facility drops to a predetermined level, provision is made for automatic change over to mains supply. In the Caribbean Islands and Central American countries, for example, storage tank is made of steel drums of 200 L capacity, large polyethylene plastic tanks of 1300-2300 L capacity or underground concrete cisterns of 100000 – 150000 L capacity and the respective government regulations have made it mandatory that all developers construct a water tank large enough to store a minimum 400 L of rain water per 10m2 of roof area (Economic & Social Commission for Asia & the Pacific (ESCAP, 1989) 3.3.3 Main types of global RTRWH systems

There are 4 main types of typical Roof Top Rain Water Harvesting (RTRWH) systems in use internationally, distinguished according to their hydraulic properties (Hermann & Schmida, 1999) They are as follows: (a) The Total Flow type (b) The Diverter type (c) The Retention and Throttle type (d) The Infiltration type 74

(a) The Total Flow type The total run-off flow is confined to the storage tank, passing a filter or screen before the tank as shown in Figure 3.3. Overflow to the drainage system only occurs when the storage tank is full. It is important that in the case of a clogged screen or filter, that there is no overflow allowed before the tank.

(b) The Diverter type The diverter type, which contains a branch installed in the vertical rainwater type after the gutter or in the underground drainage pipe as shown in Figure 3.4 The collected fraction is separated from the total flow at this branch and a surplus is diverted to the sewerage system; most of these branches contain a fine-meshed sieve diverting most of particles to the sewer. These devices are a typical invention of the period, when rainwater usage was only looked onto save drinking water and the diversion of storm water to a sewer was the usual and accepted habit. The ratio of efficiency of the diverting devices decreases with increasing flow. So, during heavy rain, most of the run-off is diverted to the sewerage system. At low precipitation rates, a minimum flow is diverted to the sewer and the efficiency decreases to zero (Winkler, 1991, Graf, 1995)

(c) The Retention and Throttle type The storage tank here provides an additional retention volume, which is emptied via a throttle to the sewer as shown in Figure 3.5 (Mall-Beton, 1999)

(d) The Infiltration type Local infiltration of the surplus tank overflow is a possible alternative to the diversion to the sewer as shown in Figure 3.6 Hydraulic impacts for an infiltration site were calculated by Herrmann & Schmida (1999). Hermann, Kaup and Hesse (1999) described performance examples and showed that by the combination of rainwater usage and local infiltration, the natural local water balance can be restored and maintained independent of the infiltration capacity of the soil, and independent of available surface for infiltration facilities.

75

Figure 3.3: The Total Flow type

Figure 3.4: The Diverter type

76

Figure 3.5: The Retention and Throttle type

Figure 3.6: The Infiltration type RWHS

77

3.3.4 RWH systems in Sri Lanka Rain Water Harvesting (RWH) systems in Sri Lanka are mainly classified according to the positioning of their storage tanks. 3.3.4.1 RTRWH system with above ground Ferro-Cement tank

This model is introduced to rural areas by the Ministry of Urban Development and Water Supply of Sri Lanka as shown in Figure 3.7. However, space requirement for the tank hinders use in small dwellings where land area is limited.

Figure 3.7: RTRWHS with above ground Ferro-Cement tank

3.3.4.2 RTRWH system with partial underground tank

This model, as shown in Figure 3.8, is introduced to the rural areas by, the Ministry of Urban Development and Water Supply of Sri Lanka. The ease of draw-off due to lower depth is an advantage. However clearing sediments is the biggest drawback.

78

Figure 3.8: RTRWHS with partial underground Ferro Cement tank

3.3.4.3 RTRWH system with below ground brick tank

In this system the space and aesthetics are saved as shown in Figure 3.9, but cleaning of sediments and ease of draw-off is hampered. Another practical difficulty encountered is the roots of nearby vegetation damaging the brick/cement structure of the underground tank. Therefore, for this particular model plastic tanks are recommended.

Figure 3.9: RTRWHS with below ground tank

79

3.4 Components of rain water harvesting systems An operational RTRWH system consists of five basic components. They are, the collector surface also known as the effective roof area or the catchment area, the conveyance system or the piping to convey rain water to the tank, the storage facility or the tank, various filtering devices and a suitable draw-off device.

A typical RTRWH system, as shown in Figure 3.10(a) and 3.10(b), has its storage tank at ground level, requiring a pump to supply collected rain water to end user points. Such a pump will require either grid connected power supply or can be connected to an alternative power source, such as a photo voltaic module.

Figure 3.10(a): Typical RTRWHS for multi-story house

80

Figure 3.10(b): Typical RTRWHS for multi-story house (schematic drawing) 3.4.1 Collector surface

The collection area in most cases is the roof of a house or a building. The effective roof area and the material used in constructing the roof influence the efficiency of collection and water quality. All catchment surfaces must be made of non-toxic material. Painted surfaces should be avoided if possible, or, if the use of paint is unavoidable, only nontoxic paint should be used. Lead, chromium or zinc based paints are not suitable for catchment surfaces due to presence of heavy metals. Overhanging vegetation should also be avoided. Steep galvanized iron roofs have been found to be relatively efficient rainwater collectors, while flat concrete roofs are very inefficient. (Edwards & Keller, 1984)

Rooftop catchment efficiencies range from 70% - 90%. It has been estimated that 1 cm of rain on 100 m2 of roof yield 10000 L. More commonly, rooftop catchment yield is estimated to be 75% of actual rainfall on the catchment area, after accounting for losses due to evaporation during periods when short, light showers are interspersed with periods of prolonged sunshine (Edwards & Keller, 1984)

81

3.4.2 Conveyance system A conveyance system usually consists of gutters or pipes that deliver rainwater falling on rooftop to tanks or other storage vessels. These should be properly supported and sufficiently strong to carry and keep loaded water during the heaviest rain.

The conveyance system should be constructed of chemically inert materials such as plastic, aluminum, or fiberglass in order to avoid adverse effect on water quality.

3.4.3 Storage facility

storage tank or recharge tank can be stationed above ground, partly underground or fully underground depending on the design and spatial arrangements and can be made of reinforced cement concrete (RCC), Ferro cement, masonry, plastic (polyethylene) or metal (galvanized iron) sheets. All rainwater tank designs should include as a minimum requirement: -

A coarse inlet filter

-

An overflow pipe

-

A manhole, sump and drain to facilitate cleaning

-

An extraction system that does not contaminate the water.(A tap or a pump)

Additional features might include; -

A device to indicate the amount of water in the tank

-

A second sub-surface tank to provide water for livestock etc.

3.4.4 Filtering devices in RWH systems

Filters are used to filter out the debris that comes with the rooftop water and prevent them being added to the storage tank. These are of two broad types:

(a) Mesh Filters A wire mesh fixed at the mouth of or on the down pipe to prevent leaves

and

debris

from entering the system. While preventing larger objects these filters alone are not 82

sufficient to obtain a reasonable quality rain water collection. Also mesh filters tend to corrode over time unless the wires are plastic coated. A typical mesh filter is shown in Figure 3.11

Figure 3.11: A typical mesh filter

(b) First Flush (FF) devices First Flush (FF) device is a valve that ensures the run-off from the earliest rains is flushed out and does not enter the system. The first flush of run-off water that occurs at the beginning of a storm event has been reported to contain a high proportion of the pollutant load (Fewkes, 1999a). The main cause of this phenomenon is the deposition and the accumulation of pollutant material to the roof during dry periods. The longer the dry period, the greater the probability of a higher pollutant load in the first flush. It is relatively straightforward to install a device for diverting the first flush away from the collection system (Forster, 1991)

The sizing of the FF devices can follow a simple equation relating to the collection area and estimated pollution load on the roof. Flush Volume (L) = Roof Area (m2) x Pollution Factor x 100

[3.1]

Pollution factors are 0.0005, for nil to light pollution, and 0.001 to 0.002, for heavily polluted sites. This corresponds to 1 mm to 2 mm of initial rainfall (Zobrist, 2000). As a rule of thumb, the first 1 mm rainfall on a catchment area is to be released through the FF device. 83

FF devices have a slow release valve which allows the captured water to slowly drain to the garden or storm water outlet and thereby empty and reset for the next rain event. The concept is to flush the contaminants from the roof and gutter into the device which then closes mechanically when full, allowing the remaining roof water to flow into the tank. The release of the FF water commences immediately and the study by Miller (2003) showed that this release rate can be significant to the efficiency of the storage system. A typical First Flush device is shown in Figure 3.12

Figure 3.12: A typical first flush device

3.4.5 Draw-off devices used in RWH systems

Draw-off devices are used to deliver stored rainwater from the tanks to end user points and can vary according to the design of the particular RTRWH system. A draw-off device can be: -

A simple outlet to the tank

-

A hand pump which are widely used with underground and partial underground storage devices as shown in Figure 2.17

-

A centrifugal or positive displacement pump which can be used to pump collected rainwater from storage facility on the ground to an over head tank.

84

3.5 Optimization of storage size It has been shown that Fewkes generic curves for water saving efficiencies (WSE) can be used to determine the optimum storage capacities for a given demand and for a desired WSE. The curves are validated for Sri Lanka by Sendanayake & Jayasinghe (2007) These minimum annual rainfall figures defining the boundary of the domain in which Fewkes curves hold true are below the minimum annual rainfall figures in the dry zone of Sri Lanka. As such, the curves given in Chart 2.1 can be used for RWH model system sizing in any region of the country and can be accepted as universal within Sri Lanka. However, as the sizing applications move towards drier regions, unless the capture area is significantly increased D/AR tends to increase thus falling into regions of lower WSE of the curves. To maximize the WSE for the given D/AR value, S/AR values will have to be chosen beyond the 0.15 range, indicating bigger storage tanks. A similar scenario can be seen when the demand (D) for harvested rain water increases, even in the wet zone.

3.5.1 Space and weight restrictions It is observed that the harvested rainwater can be utilized for WC flushing and cleaning purposes the where the amount of water used is approximately 40% of the total water usage. However, such requirements need the delivery of collected rainwater to utility points at a sufficient pressure to be used at any given time. One possible energy efficient arrangement is to position the storage tank at an elevation near the capture area (at roof level) so that the collected water can be fed to utility points through gravity. However, when the tank size increases, the space and strength requirements to support the tank will be beyond the meaningful utilization of harvested rainwater. Further, due to limited availability of ground space in urban multi-story buildings, positioning of a larger storage tank above ground will not be feasible and the entire quantity of harvested rain water will have to be pumped up to utility points. Therefore, typical sizes of storage tanks will have to be studied to make the model more practical.

85

Considering a typical household in the wet zone, where the annual rainfall is the highest (1500 mm to 6000 mm), with a capture area of 50 m2, the daily water usage for four occupants can be taken as 800 L (at per capita demand of 200 L) If harvested rainwater is utilized only for WC flushing and cleaning, then the demand for harvested rain water is 800x40% = 320L/day (116.8 m3/year)

As the minimum annual rainfall in the wet zone, Rmin-wet = 1500 mm The value for D/AR can be calculated as D/AR = 1.56 (It should be noted that the minimum rainfall values are selected as a safety factor for performance reliability)

From the WSE curves (Chart 2.1), the maximum possible WSE that can be achieved is found to be 65% and the corresponding value for S/AR = 0.15 giving an optimum storage size (S) of 11.25 m3. Even when the capture area is doubled (100 m2), it would still give a value of 1.5 m3 as the storage capacity for the same WSE of 65%. If however, a WSE of 95% is desired, then the optimum storage capacity (S) will be 15 m3 .

Therefore, if a reasonably high and economically acceptable WSE is to be

employed (typically over 80%), then a higher value for the optimum tank size (S) to be expected. Moreover, as the minimum annual rainfall figure (Rmin) tends to be smaller for the intermediate and dry zones, higher tank capacities are required if the WSE to be achieved above 80%.

It can be observed that in order to provide running water facility, the storage tank has to be placed at a higher elevation-which is not feasible due to volumes concerned. While such bigger tanks can be accommodated in rural single story houses with abundant ground space, for urban multistory houses with the necessity of running water will need a different model to use rain water harvesting effectively and meaningfully. 3.5.2 Alternative methods of storage tank positioning Various methods of positioning bigger sized storage tanks, which can be used to provide running water to utility points and the corresponding plumbing configurations 86

possible for typical households, having Roof Top Rain Water Harvesting (RTRWH) systems supplementing the service water provided by either mains supply or from a well/bore hole, are presented below. The practical water supply situations in both single and two story household situations are looked at in five scenarios.

a) The storage tank at ground level, and draw-off through pressure operated pump (PP)

Collected rainwater is fed to a separate pipeline, feeding WC end user points, at a higher pressure than the mains. A level sensor operates the pressure pump, to prevent the pump running dry. The system can be used in multi-storey situations, but no energy saving is possible. A 5000 L tank connected to a roof area of a minimum 45 m2 is recommended. A schematic diagram is shown in Figure 3.13

Figure 3.13: Plumbing configuration for RTRWHS – scenario (a)

b) The storage tank mounted on eve of multi-storey house

Rainwater is supplied through gravity, hence no energy consumption occurs. However, supply of water to upper stories is not possible due to lack of head. Since the tank is mounted on the eve, space restrictions could occur. Also, a strength analysis of the eve for its load bearing capacity is required 87

Figure 3.14: Plumbing configuration for RTRWHS – scenario (b) It should be noted that if the capture area is > 200 m2, a smaller tank of 2000 L can be utilized, so that the eve can support the additional weight since the tank size is smaller compared to that for a smaller capture area. A schematic diagram is shown in Figure 3.14

c) Rainwater pumped from storage facility to an Overhead Tank

In this situation an extra energy input is required to pump collected rainwater to the OHT. Therefore, the overall system efficiency could be low. A level sensor to operate the pump P1 fixed in the OHT could improve the efficiency in water saving. This system is suitable for locations, where ground water levels drop seasonally. A 5000 L capacity tank connected to a roof area of minimum 45 m2 and a suitable filtering system in between the rain water Tank and the OHT is recommended. A schematic diagram is shown in Figure 3.15

88

Figure 3.15: Plumbing configuration for RTRWHS – scenario (c)

d) Rainwater collected in split sump

To mitigate the unreliability of mains water supply, many households utilize underground sumps. By partitioning the sump so that one part receives roof collection while the other part receives the mains supply, savings can be made on service water. A 5000 L capacity tank connected to a minimum roof area of 45 m2 is recommended for WC flushing water requirement. A schematic diagram is shown in Figure 3.16

89

Figure 3.16: Plumbing configuration for RTRWHS – scenario (d)

e) Rainwater collected in sump with draw-off through filtration

Employing a series of filters such as Carbon and Sediment filters and a UV sterilizer, drinking quality water can be obtained from the collected rainwater. It can be envisaged that, by selecting suitable storage capacities and collection surfaces, substantial water saving efficiencies can be achieved. A 10000 L tank connected to a minimum roof area of 200 m2 is recommended for this configuration. However, a higher capacity tank will ensure water security even in prolonged draught situations. A schematic diagram is shown in Figure 3.17

90

Figure 3.17: Plumbing configuration for RTRWHS – scenario (e)

Except in scenario (b), in all other scenarios the requirement of a pump to provide the harvested rainwater either to an overhead tank or directly to the utility points can be observed. Such arrangements while preserving water utilizes energy to transfer the entire quantity of collected rainwater and as such cannot be considered as energy efficient or as promoting the principles of sustainable development for built environments.

3.6 Cascading multi tank model In the following paragraphs a rain water harvesting model is introduced with the new concept of decentralizing the storage capacity where the roof collection cascading down through storage tanks located at different floor levels.

3.6.1 Description of concept

In any RWH situation, the storage tank has to be placed at a lower elevation than the collection area, thereby facilitating the flow of collected rain water into the tank under gravity. However, the retention volume required for improved WSE levels pose a 91

problem in space requirements in built up areas beside the bigger problem of pumping back the harvested rain water in to service points for the system to be on par with the centralized systems as far as the user convenience is concerned. Such a system will negate the positive contribution of rain water harvesting on sustainability principles by consuming energy in pumping.

In order to minimize the energy requirement in

transferring collected rainwater, a Cascading Multi Tank Rain Water Harvesting (CMTRWH) model is proposed and analyzed as shown in Figure 3.18.

Figure 3.18: CMTRWH system for a two storey house

In the model, a number of smaller volume tanks are positioned at each floor level, with the top most tank just below the collection area, and a bigger volume tank at ground level. Rain water is fed first to the upper tank, the overflow of which will cascade down to the lower tanks finally ending up in the parent tank at ground level. Supply to each floor is from individual smaller capacity tanks by gravity floor and make-up water is 92

pumped from the parent tank to the top most tank as and when required. Essentially the concept of MTRWH model attempts to distribute the storage capacity of the RWH system at various floor levels so that the requirement for pumping is minimized for the same or marginally improved overall WSE.

3.6.2 Assumptions adopted in system operation

In developing an algorithm for the operation of a CMTRWH system, the following are assumed to be valid: •

The height differences between each floor level are a constant.

•

The water usage at any given floor level remains constant for a given set of operating parameters.

•

No loss of water occurring in system operation. i.e., in cascading down or pumping up of collected rain water.

•

All tanks installed at floor levels other than the ground level are taken as of equal capacity.

3.6.3 Advantages and limitations of CMTRWH systems

3.6.3.1 Advantages are the following: •

Possibility of gravity feeding the total usage to service points

•

If pumping is required for higher demands, the reduced energy utilization.

•

Lower spatial and strength demand on the building structure.

•

Reduced adverse impact on the aesthetic appearance of the building envelops.

3.6.3.2 Limitations are the following: •

Reduced supply pressure at user points

•

Requirement of additional storage tanks for upper floor levels.

93

3.7 System dynamics Development of an algorithm to describe the dynamics of the system is an important step to understand the operational aspects fully. 3.7.1 Development of a system algorithm for CMTRWH systems

It can be shown that both the upper and lower tanks, individually and collectively obey the Yield After Spillage (YAS) algorithm (Jenkins, 1978) used to develop Fewkes (1999) generic curves. Therefore, Fewkes generic curves, which have been validated for Sri Lanka have been used extensively to analyze the system dynamics of CMTRWH model.

In order to analyze the performance of the system, the amount of water that has to be pumped up from the lower tank to the upper tank has to be determined. The model can be considered performing optimally if the demand is met by the upper tank with the minimum amount of water transferred. If the water saving efficiency (WSE) of the upper tanks are ηi and the parent tank is ηp for a given capture area A (m2), annual rainfall R (m) and demand D (m3/year), and the tank capacities are Si and Sp respectively, from YAS algorithm and Fewkes generic curves; ηi

=

f{ Si, D, A, R}

ηp

=

f{ Sp, D, A, R}

This can be used to determine the optimum storage tank capacities. For a given A, R and D, D/AR can be calculated. Then for a desired efficiency (ηp) the optimum tank size, Sp can be found. As space and weight restrictions dictate for the installation of a smaller capacity for the upper tanks, a suitable tank size, Si is selected. (Ideally 1 m3 capacity tank can be selected for Si) Then for (AR)i and Di, ηi can be found from the curves. 94

3.7.2 Effective run-off and pumping requirement

For cascading multi tank situations, the following algorithms are valid. For each floor, If the yield is Yi, for i = 1 to n Pumping requirement Qi ; Qi = Di - Yi = Di(1- ηi)

[3.2]

Then for the ith floor (ith tank), When the demand is Di, supply is (AR)i But, (AR)i = (AR)i+1 – Yi+1 Since Yi+1 = Di+1* ηi+1 (AR)i = (AR)i+1 – Di+1* ηi+1

[3.3]

Further, if the total demand is D, n

D=

∑ Di

[3.4]

i =1

The overall WSE for the system is denoted as ηo Therefore, if the number of floors are n and the ground floor is taken as i = 0, it can be shown that; The amount of water that can be pumped up in CMTRWH system, Q, Q=

n

n

n

i =1

i =1

i =1

∑ Qi - ∑ Qi (1- ηP) = ∑ Qi * ηP

From Equation 3.2, n

n

i =1

i =1

Q = ηP { ∑ Di - ∑ Diηi }

[3.5]

When, n

(AR)i = AR -

∑ Di *ηi

[3.6]

i =i +1

95

3.7.3 System limits

The above equations are true when, (AR)i > 0 and 0.25(AR)i ≤ Di ≤ 2.00(AR)i Therefore, for the model to function effectively, Di/ (AR)i for each floor level should behave within the limits validated for Fewkes WSE curves. Further, in order to obtain the maximum WSE in the multi tank situation, Storage capacity for the parent tank, SP is taken so that, S/AR ≥ 0.1 This will ensure that the ratio S/AR falls in the stable region of the WSE curves developed by Fewkes (Chart 2.1)

3.7.4 System equations for equal loads at each floor level

When the demand at each floor level is taken as Di, and the total system demand is taken as D, for i = 1 to n; Since ∑ Di = D, D1 = D2 =………..= Dn = D/n Therefore, from equations 3.5 and 3.6,

n

n

i =1

i =1

Q = ηP { ∑ Di - ∑ Diηi } n

Q = ηPD{1 – 1/n ∑ηi }

[3.7]

i =i +1

n

(AR)i = AR – D/n ∑ηi

[3.8]

i =i +1

3.8 Determining the validity of CMTRWH algorithm To determine the validity of the algorithm developed for CMTRWH systems, the performance of such should be evaluated under different operating conditions.

96

3.8.1 Methodology

A prototype cascading multi tank model with three tanks are installed in a two storey house located in Colombo, Sri Lanka with a roof collection area of 50 m2. The capacity of the parent tank is taken as 12.5 m3 and the upper tanks at 1 m3 each. System performance is monitored for a daily demand of 200 L per floor with the yield from each upper tank measured and tabulated daily with the pump in operation. The pump is connected to floater switch arrangement to cut-in when the water level in the closest tank to the parent tank drops.

The above methodology makes use of the curves

validated for Sri Lanka, initially presented by Fewkes (1999) given in Chart 2.1.

3.8.2 Calculation

Annual yield from the upper tanks and rain water collection AR, are calculated using 15 day moving average method. The moving average method uses a technique where the average value of a number of consecutive data are averaged and developing a progression of average values so that a vastly higher number of data can be obtained from a limited number of data. If the system follows the algorithm, then the maximum yield possible from the total system, i.e. Dηo should be delivered by the two upper tanks. i =n

Therefore, Yo =

∑ Yi generally and Yo = Y1 + Y2 in this case. i =1

If so, ηo calculated for ηo = ΣY/ΣD from measured (Y1 + Y2) should be equal to ηo obtained from Fewkes WSE curves for a set of given D, S and AR.

3.8.3 Results and discussion

The results are shown in Chart 3.1.

97

120 100

WSE %

80 WSE (Chrt)

60

WSE (Calc)

40 20 0 0

10

20

30

40

50

Period Number

Chart 3.1: Comparative WSE values obtained from prototype CMTRWHS From the results, it can be seen that the calculated WSE, ηo(Cal) and the WSE obtained from Fewkes generalized curves, ηo(WSE) are almost the same with the margin of error attributed to system losses.

3.9 Limiting values of Demand (D) for total gravity feed It will be useful if the limiting values of annual demand (D) can be given in a generalized form for total gravity fed scenario.

3.9.1 Calculation of limiting values For the CMTRWH system to confirm to the validity limits of WSE curves, for any Di; 0.25 ≤ Di/(AR)i ≤ 2 For Di/(AR)i ≥ 0.25 and (AR)i = (AR)i = AR -

n

∑ Di *ηi

i =i +1

Di ≥ 0.25[AR -

n

∑ Di *ηi ]

i =i +1

However, when the load is distributed equally among the floors, Di = D/n

where n is the number of floors. 98

n

Therefore, D/n ≥ 0.25[AR – D/n ∑ηi ] i =i +1

n

D/AR ≥ 0.25n/[1 + 0.25 ∑ηi ]

for n ≥ 2

i =i +1

n

However, for D/AR to be a minimum

∑ηi should be a maximum.

i =i +1

i.e., ηi = 1.00 for all i ≥ n + 1 Therefore, for the minimum D/AR and n ≥ 2 n

∑ηi = (η2 + η3 + η4 + …………..+ ηn ) = (n – 1)

for ηi = 1.00

i =i +1

Therefore, the limiting value for D/AR, D/AR = 0.25n/[1 + 0.25(n – 1)]

[3.9]

Chart 3.2: Lower limiting values for D/AR for different floor levels

However, it can be shown from WSE curves that; η = 1.00 when S/AR ≥ 0.05 for D/AR ≤ 0.5

99

n

In multi storey situations, STotal = Sp +

∑ Si i =1

It is also seen from WSE charts (Chart 2.1) that, η = 1.00 when S/AR ≥ 0.02 for D/AR ≤ 0.25 Therefore, for housing units of 2 storey, for ηo = 1.00 and ηi = 1.00 D/AR ≤ 0.4 for STotal/AR ≥ 0.05 and Si/AR ≥ 0.02 And for housing units of 3 storey, for ηo = 1.00 and ηi = 1.00 D/AR ≤ 0.5 for STotal/AR ≥ 0.05 and Si/AR ≥ 0.02 It implies therefore that a CMTRWH system can be designed with STotal/AR ≥ 0.05 and Si/AR ≥ 0.02 for total supply reliability, without the requirement of pumping, where the total demand can reach 0.5AR in 3 storey situations and 0.4AR in 2 storey situations.

For example, for a two storey house in Colombo, Sri Lanka where R = 2500 mm/year and a roof collection area of 50 m2, when Sp and Si are selected as 6.25 m3 and 2.5 m3 respectively, the total demand can be a maximum of 0.4*AR, i.e. 50 m3 per year at 136.9 L/day. Such a demand will ensure that both floor levels are supplied with collected rain water at 100% WSE. It implies that, by increasing the roof collection area A, the desired demand can be met for a CMTRWH system where the pumping requirement is no longer exists.

However, in certain months the rainfall is so low that when converted to annual values, it may be only about 700 mm per year (Jayasinghe, 2001). Therefore, for a foolproof design the month with the lowest average rainfall, the month of February, can be selected to calculate the annual average rainfall though with the disadvantage of having to select the sub-optimum roof collector area.

Similarly, for a CMTRWH system to operate within the validated limits of WSE curves, D/AR ≤ 2.0 and Di/(AR)i ≤ 2.0 It can be shown that n

D/AR ≤ 2n/[1 + 2 ∑ηi ] i =i +1

100

n

Since D/AR is a maximum when

∑ηi is a minimum,

i =i +1

n

It can be shown that, the minimum value for

∑ηi is when

i =i +1 n

∑ηi = 0.5n

i =i +1

Therefore, in CMTRWH situations, for Di/(AR)i ≤ 2.00, The maximum demand that can be sustained by the system is limited by, D/AR = 2n/(1 + n) Therefore, for CMTRWH in multi storey situations, D/AR ≤ 2n/(1 + n)

[3.10]

2

D/AR (limiting value)

1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

2

4

6

8

10

Number of levels, n

Chart 3.3: Upper limiting values for D/AR for different floor levels

3.9.2 Operating phases of a CMTRWH model

From Equation 3.5, the quantity of collected rainwater that can be pumped up for a CMTRWH system with n floors is given by, n

n

i =1

i =1

Q = ηP { ∑ Di - ∑ Diηi }

101

However, when the demand load is distributed equally among the floors, from equation 3.7, n

Q = ηPD{1 – 1/n ∑ηi } i =i +1

If the overall system is equated to a conventional single tank RWH system with overall WSE of ηo having a total storage capacity of S, where S = STotal = Sp +

n

∑ Si , and a total i =1

demand D for the same A and R, Then the quantity of collected rainwater that can be pumped up from such a system is given by QO, where Qo = Dηo Therefore, n

n

i =1

i =1

Q/Qo = ηP/ Dηo { ∑ Di - ∑ Diηi }

[3.11]

However, when the system is having an equally distributed demand load among the floors, it can be shown that, n

Q/Qo = ηP/ηo{1 – 1/n ∑ηi } i =i +1

It is also shown in equations 3.9 and 3.10 that for ‘n’ floor levels and when SP > Si, the lower and upper Limiting values of D/AR are 0.25n/[1 + 0.25(n – 1)] and 2n/(1 + n) respectively for equally distributed demand loads.

Therefore, the performance of a CMTRWH system integrated to a building of ‘n’ floor levels can be evaluated in 4 phases.

i.e. when

D/AR ≤ 0.25n/[1 + 0.25(n – 1)] 0.25n/[1 + 0.25(n – 1)] ≤ D/AR ≤ 1 1 ≤ D/AR ≤ 2n/(1 + n) D/AR ≥ 2n/(1 + n) 102

When D/AR ≤ 0.25n/[1 + 0.25(n – 1)], the system is operating at 100% WSE through gravity with no pumping required from the parent tank. Hence Q = 0. When D/AR increases from the lower limit to 1.00, ηi as well as ηP decrease. However, the rate of decreasing of ηP can be seen as less than that of ηi (from WSE Chart) though with the combined effect of increasing Q with respect to increase of D (Chart 3.4). Hence, dQ > 0 dD Therefore, D = AR is identified as the limiting demand at which the maximum amount of collected rain water can be extracted (the yield, Y) from the system for given storage capacities (Chart 3.4). In other words, the maximum quantity of water that can be pumped up occurs at D = AR for CMTRWH system. Therefore, it is important to focus the attention on supplying the pumping energy required using the most cost effective and efficient pumping system.

Further, in this phase, when the number of floor levels ‘n’ increases for the same demand, it can be seen that the quantity of water that can be pumped up reducing for all D. In other words, the yield is more at D = AR when the number of floor levels, ‘n’ increase. When D/AR increases beyond 1.00 and reaching towards the upper limit, 2n/(n+1), the amount of collected rainwater that can be pumped up decreases as both ηi and ηP decrease. However, from the WSE Chart it can be seen that the rate of decreasing of ηP is much higher compared to that of ηi. Therefore, the yield Y drops as a result of reduced Q and thus, dQ < 0 dD

103

When the number of floor levels increase for a given demand D, the quantity of collected rainwater that can be pumped up Q decreases thus indicating the increased yield from the system.

Beyond the upper limit for D/AR, the behavior of the system with regard to continuity is unpredictable. Mostly, the feed tanks at lower floor levels will not receive the cascading effect and hence η for lower floor levels will be zero, discontinuing the operation.

This process can be visualized by using Fewkes generalized WSE charts with regard to dropping of WSE values for a given set of storage capacities (S) and AR values when the demand (D) varies. Also, it should be emphasized that the above behavior is true only when SP > Si and Si/AR ≥ 0.01 hence beyond the un-defined area of the critical zone of the curves. Further, the above explained behavior is more pronounced when SP/AR ≥ 0.05 where the D/AR lines are in the stable area of WSE curves (Chart 2.1).

3.10 Energy requirement in CMTRWH systems Pumping of a certain percentage of collected rain water in any CMTRWH system is necessary in order to supply the demand in par with the reticulated centralized systems. Therefore, it is important to ascertain the requirement of energy in such situations to select and supply from a reliable source to satisfy the needs of the user. 3.10.1 Energy required in pumping harvested rainwater

Energy required in pumping collected rain water in two types of houses, namely single story and two story houses, are analyzed for daily demands of 200 L, 300 L, 400 L and 600 L. In the single story house, two tanks are employed with the upper tank of 1 m3 capacity located at the eve level, just below the roof collection area. Three tanks are employed in the 2 story house with the upper tanks located at eve and first floor levels and the parent tank at ground level. In the two story house, the demand is taken as 104

equally divided between the two floors. It should be noted that a single pump is used to lift the collected rain water at ground level parent tank to the top most upper level tank, making the static head the same for both multi-tank and the comparative single ground level tank situation. Hence, energy requirement percentage can be calculated using the quantities of rain water that can be pumped up in multi-story situation Q (Equation 3.7) and that of single tank situation QO. Therefore, E% in this case can be represented by Q/ QO. The energy required is calculated using equations 3.7 and 3.8 and is shown as a percentage of energy required to pump collected rain water from a single tank at ground level against D/AR, where A is the collector area in m2 and R is the annual average rainfall in m for a particular geographical region. Use of the parameter D/AR will give more flexibility to use any combination of A and R, for a given constant AR value. Fewkes (1999) generalized curves validated for Sri Lanka (Sendanayake & Jayasinghe, 2007), is used to determine WSE for a given demand and storage volume. All storage tanks located at upper levels are of 1 m3 capacity. The roof collection area is taken as 50 m2 in the wet climatic region of Sri Lanka, where the annual average rainfall is 2500 mm (Meteorological Department of Sri Lanka). Therefore, AR is calculated as 125 m3 and for maximum WSE, Sp is taken as 0.1 AR, i.e. 12.5 m3. As the generic curves for WSE is valid for 0.25(AR)i ≤ Di ≤ 2.00(AR)i, the maximum possible demand is calculated as 600 L/day. The amount of rain water that can be pumped up when only the parent tank is employed is denoted as Qo. The value Q/Qo is representative of the energy requirement in pumping as a percentage (Equation 3.11). Q/ Qo values are plotted against D/AR to determine the operating characteristics of CMTRWH systems, where D is the total daily demand.

This will effectively

compare the CMTRWH situations for two and three tank models with conventional single tank RWH systems under the same A, R and D. Tables 3.1 and 3.2 give the energy requirement percentages Q/Qo for 2 and 3 story houses respectively. η1 and η2 are WSE values for 1st and 2nd floors. Chart 3.4 graphically present the values obtained in Tables 3.1 and 3.2.

105

It is important to note that for an annual rainfall of a lesser value to select a larger roof collection area thereby obtaining a AR value which could satisfy the operating conditions for a given demand D.

A sample calculation is presented in Appendix 1.3 indicating the method of calculation of Q/ Qo for D=300 L/day, A=50 m2 and R=2500 mm/yr for a 3 Tank CMTRWH model.

Table 3.1: Energy requirement % vs. Demand in Two Tank model D L/day 200 300 400 600

D m3/yr 73 109.5 146 219

η0 100 90 65 35

D/AR 0.58 0.87 1.17 1.74

η1 67.5 50 45 32

Q (m3) 23.73 49.28 52.2 44

Qo(m3) 73 101.28 116.8 122.64

Q/Qo% 33 48 44 36

Table 3.2: Energy requirement % vs. Demand in Three Tank model D L/day 200 300 400 600

D m3/yr 73 109.5 146 219

D/AR 0.58 0.87 1.17 1.74

η0 100 92.5 80 56

η1 92.5 77.5 67.5 50

η2 77.5 55 52.5 42.5

Q (m3) 10.95 36.96 35.04 28.06

Qo(m3) 73 101.29 116.8 122.64

Q/Qo% 15 36 26 23

Chart 3.4: Energy requirement % vs. Demand in Two and Three Tank models

106

3.10.2 Energy required in pumping rainwater with make-up water

When make-up water is available, the pumping energy required in CMTRWH situations is compared with the energy required in pumping the total demand as a percentage of the latter.

Table 3.3: Energy requirement % vs. Demand in 2 Tank model – with make-up water D L/day 200 300 400 600

D m3/yr 73 109.5 146 219

D/AR 0.58 0.87 1.17 1.74

η0 100 92.5 80 56

η1 67.5 50 45 32

Q 23.73 49.28 52.2 44

M 0 5.48 28.11 104.24

Q+M 23.73 54.76 80.31 148.24

(Q + M)/D 32.5 50 55 68

Table 3.4: Energy requirement % vs. Demand in 3 Tank model – with make-up water D L/day 200 300 400 600

D m3/yr 73 109.5 146 219

D/AR 0.58 0.87 1.17 1.74

η0 100 92.5 80 56

η1 92.5 77.5 67.5 50

η2 77.5 55 52.5 42.5

Q 10.95 36.96 35.04 28.06

M 0 3.67 20.44 72.5

Q+M 10.95 40.63 55.48 100.56

(Q + M)/D 15 37 40 49

In the above case energy requirement percentage E% is given by (Q + M)/D where Q is the amount of collected rain water that can be pumped up and M is the amount of makeup water required for the overall system and D is the annual demand. A sample calculation is presented in Appendix 1.3 indicating the method of calculating M for given S, D, A and R.

107

Chart 3.5: Energy requirement (E) % vs. Demand in 2 & 3 Tank models With make-up water

3.11 Energy required in pumping rainwater with unbalanced load

The impact on the energy requirement in pumping rainwater when the load is unequally distributed is investigated in a two story house with a cascading three tank RWH system. When the demand in the ground floor is D1 and the upper floor is D2, load distribution of D1/ D2 = 0.5, 1 and 2 are studied for total demands of 300 L/day and 600 L/day.

Table 3.5: Energy requirement for rain water pumping-unbalanced load Q/Qo %

D=D1+D2 D1/D2=0.5

300 L 600 L

38 58

D1/D2=1.0

34 54

E% D1/D2=2.0

31 51

D1/D2=0.5

32 25

D1/D2=1.0

28 17

D1/D2=2.0

25 12

In the Table 3.5, Q/QO% indicates percentage energy required with the system operating when only the collected rain water and E% indicates the total pumping energy required as percentage when the system is operating with make-up water. Make-up water is introduced to the system to meet the inadequacy of collected rain water.

108

Chart 3.6: Percentage pumping energy required for unbalanced load

Chart 3.7: Percentage total energy required for unbalanced load

Table 3.6: WSE for 3 Tank unbalanced load model D1/D2 0.5 1 2

300 L/day

600 L/day

η1

η2

η0

η1

η2

η0

82.5 55 50

62.5 77.5 87.5

92.5 92.5 92.5

52.5 42.5 35

47.5 50 67.5

56 56 56

109

Table 3.6 gives the WSE values for 3 Tank CMTRWH systems in 2 story buildings with the demand distributed unequally as indicated. WSE values are calculated for daily demands of 300 L and 600 L respectively. It is noted that for both supply tanks to operate at same WSE, η1 = η2 = η* Since, Q = ηP{D – D1η1 - D2η2} = ηP{D – (D1 + D2) η*} = ηPD{1 – η*} Since (D1 + D2) = D However, Q0 = D η0 Therefore,

η* = η0/ ηP (1- Q/Q0) for a given D, A, R, Sp and Si

As Q/Q0 is maximum at D/AR = 1.00, it is seen that η* is minimum when D=AR Therefore, it implies that when D=AR, the CMTRWH system operates at minimum WSE though the overall yield is maximized as discussed in paragraph 3.10

3.12 Impact of variation of the storage volume in CMTRWH systems Since the maximum energy requirement in rain water pumping occurs when D/AR is 1.00, the impact of change in the capacity of parent tank at D = AR is investigated. A is selected as 50 m2 and R as 2500 mm/year and the capacity of the parent tank is taken as 12.5 m3.

For a single story, cascading two tank model, the volume of the parent tank is reduced by 20%, 50% and 90% of the original volume of 12.5 m3 for D = 342.5 L/day (at D = AR) Therefore, SP is selected as 10, 6.25 and 1.25 m3 and Q/QO% and E% values are calculated as given in Table 3.7

110

Table 3.7: Energy requirement in pumping with variation in parent tank volume (Two Tank model) S/AR Q Q/Q0 % E% η0 η1 0.08 0.05 0.01

82.5 77.5 45

45 45 45

46.88 40.63 0

45 42 0

55 55 55

50 45 40

Q/Qo %

35 30

Two Tank model

25 20 15 10 5 0 0

0.05

0.1

Sp/AR

Chart 3.8: Energy requirements in pumping with variation in parent tank volume When D=AR

Chart 3.8 shows the variation of Q/QO% with SP/AR. It can be seen that when Q/QO% equals zero SP/AR = 0.01. i.e. when both the upper and lower tanks in the CMTRWH system are of same capacities.

Therefore, when the capacity of the parent tank, Sp is varied at D = AR, i.e. at the maximum Q/Q0 percentage point, the impact is pronounced only when SP/AR < 0.05. It is noted that, while the total quantity of rain water required to pump up remain the same as indicated by same E% value of 55%, the quantity of make-up water required is increased. In fact, it can be deduced that this is true for any D/AR, 0.25 ≤ D/AR ≤ 2, since the energy percentage curve (Q/QO) follows the characteristic of the WSE curve. Therefore, the capital cost of the system can be significantly reduced, with minimum impact on the energy cost, while the cost of make-up water is marginally increased. For

111

example, SP can be reduced from 10 m3 to 6.25 m3 with Q/Q0% dropping only by 3% from 45% to 42% while the quantity of make-up water required M is increased only by 6.25 m3.

3.13 Performance of a Two Tank cascading model – case study Most of the housing units are of either single or two storey type. Though ideally a three tank model is suitable for a two storey house, a two tank model will adequately perform while cutting down the capital outlay by eliminating the eve level tank. Hence it is appropriate to analyze the performance characteristics of the model along with the pumping requirements for different scenarios.

In the proposed TTCRWH model, two storage tanks are utilized. A smaller capacity tank is positioned at a higher elevation (possibly at the eve level) into which the captured rain water be directed. This upper tank (SU) will supply the utility points and feed a bigger tank (SL) at ground level via the overflow. As such when a rain event occurs, captured rainwater will flow into the upper tank and then cascade down into the lower tank and any excess water to be disposed through the overflow of the lower tank. The total storage capacity of the system consists of the combined capacities of the two tanks and a pump is utilized to transfer collected rainwater from the lower tank to the upper tank when the water level in the latter drops. TTCRWH model is shown in Figure 3.19.

112

A schematic diagram of a

Figure 3.19: Schematic drawing of a CTTRWH model

3.13.1 System dynamics – Two Tank Model

As

SL> SU

for the same A, R and D

ηL > ηU

Since for a given demand D, The shortfall in the upper tank (SU) is given by D(1 - ηU) and The shortfall in the lower tank (SL) is given by D(1 - ηL) The amount of water that can be pumped up is given by Q; Q = D(1 - ηU) - D(1 - ηL)

which simplifies to

Q = D(ηL - ηU)

[3.12]

Similarly, if the total demand for water is DT, then the amount of water required from the mains is given by M; M = D(1 - ηL) + (DT – D)

which simplifies to

M = DT - D ηL

[3.13]

113

3.13.2 System performance The performance of the TTCRWH model can be studied using the equations 3.12, 3.13 and Fewkes generic curves varying the parameters A, R, D and SU

3.13.2.1 System performance with change in capture area (A)

It can be observed that by increasing the capture area A, for a given R, D and SU that the dimensionless ratio, D/AR, decrease and as a result achieving higher values for ηL. However since S/AR decrease with the increase of A, the difference between the water saving efficiencies of lower and upper tanks, (ηL - ηU), tends to rise, increasing the quantity of water that has to be pumped up.

3.13.2.2 System performance with change in demand (D)

If the demand is reduced by, for example, using water saving devices the water saving efficiencies ηL and ηU increases rapidly for D/AR > 1 and slightly for D/AR < 1 This is due to the under-performing of the system for D/AR > 1

3.13.2.3 System performance with change in rainfall (R)

It can be noted that moving from wet to dry climatic zones, where the minimum annual rainfall (Rmin) drops, both ηL and ηU dropping and as a result, the dropping of pumping requirement due to lower value for (ηL - ηU) 3.13.2.4 System performance with change in upper tank capacity (SU) By increasing the size of SU for a given set of parameters A, R and D, ηU increases reducing the quantity of water required to be pumped up Q, and as a result negating the purpose of two tank system. It also implies that greater the difference in capacity of the two tanks the higher the pumping requirement.

114

The operating domain of the Fewkes generic curves dictates that a performing TTCRWH model can be designed only for 0.25 ≤ D/AR ≥ 2. For values of D/AR beyond this range the behavior of the curves are found to be unreliable, particularly in the critical zone of S/AR ≤ 0.05 Further, it is noted that for the system to achieve a water saving efficiency (WSE) of over 80% (i.e. ηL ≥ 80%), D/AR < 1 Therefore it can be deduced that, for ηL ≥ 80%

D < AR

[3.14]

It can also be observed that when the system parameters are selected so that D/AR > 1, when either A or R is increased or the demand D reduced, ηL increases rapidly while the increase in ηU moderate due to the fixed nature of the upper tank capacity (SU) The implications of the above behavior becomes apparent when R > Rmin, which is a usual occurrence since for reliability of delivery, the minimum annual rainfall, Rmin is selected in design calculations. It can be shown that when R > Rmin, due to the increase in (ηL - ηU), the quantity of water to be pumped up Q increases which in turn will increase the demand on the power source. The effect will be more profound if a stand alone power source is employed to operate the pump. However when D/AR < 1, for R > Rmin the value (ηL - ηU) actually reduces, preventing excess loading on the power source.

It can be shown that for tank capacities SU, SL and annual demand D, the maximum number of days the system can supply without rain water input is given by, ddry

=

(SU + SL)365 D

[3.15]

From historical data, the average maximum number of non-rainy days (rainfall ≤ 0.5 mm) can be taken as, 10, 24 and 45 days for the wet, intermediate and dry zones

115

respectively (Meteorological Department of Sri Lana). Hence, when selecting a value for SL, it should satisfy Equation 3.15 for system reliability. Therefore, from Equation 3.15, ddry ≥ 10, 24 and 45,

For the wet, intermediate and dry zones.

3.13.2.5 Pumping requirements for water security

Considering the upper tank SU, the maximum number of days for which it can supply without an input from pumping is given by dU(max), dU(max)

=

365SU D

From Equation 3.12, Q = D(ηL - ηU) If the pumping frequency is taken as NP per year, then the number of days between consecutive pumping events is given by 365/ NP It can be deduced therefore, for supply reliability, dU(max) > 365/ NP i.e.

365SU > 365 D NP

Hence, NP > D/SU To compensate for sudden demand loadings, a safety factor K1 can be used, where K1 > 1.5, thus, NP = K1D SU

[3.16]

For a pumping frequency of NP, the pumping volume required at a time is Q/ NP Substituting Equation 3.16 in 3.12 gives, Q = SU(ηL - ηU) [3.17] NP K1 Therefore, when the water level in the upper tank SU drops by a quantity equivalent to 116

Q/ NP, a floater switch arrangement can be made to cut-in to activate the pump.

3.13.2.6 Make-up water requirement for water security From the Equation 3.13, mains water requirement, when the total demand is DT is given by, M = DT - D ηL However, the mains water requirement for the RWH system, ML (i.e. to the lower tank, SL) is ML = D(1 - ηL) If the number of days the system can supply the demand without mains water is dsup Then,

dsup = 365(SL + SU) D

If the frequency of supplying mains water is NM, then the number of days between consecutive supply events is given by; 365/ NM Since, for system supply reliability, 365 < 365(SL + SU) NM D NM >

D (SL + SU)

To compensate for demand surges, a safety factor K2 can be used, where K2 > 1.5. Thus, NM =

K2 D (SL + SU)

[3.18]

Since the quantity of mains water supply required at a time is given by, ML ML = D(1 - ηL) NM Substituting in Equation 3.18 ML = (SL + SU) (1 - ηL) K2

[3.19]

117

3.14 Control of overflow quantities Controlling of overflow quantities from RWH systems is an area which need attention as it is directly linked to the Water Saving Efficiency (WSE) of the system as well as the discharge volumes on the local drainage systems, particularly in built up areas. 3.14.1 Objective It is noted that a substantial quantity of the roof collection is lost as overflow in the RWH systems. This is more so in certain months of the year, such as April/May and October/November, as the established monsoon rainfall peaks for Sri Lanka.

If a

percentage of lost over-flow can be retained, it will not only improve the WSE of the system, but will provide a means of controlling peak loads on the drainage system. This is particularly useful in built-up areas, where the drainage system can be designed for a reduced peak flow, whilst improving the annual quantities of harvested rain water. With high annual rainfall figure in the South-West of the country where most of the built-up areas are concentrated (Figure 3.20), it is useful to ascertain the overflow quantities occurring to address the above issues.

Figure 3.20: Annual rainfall distribution in Sri Lanka (in mm) 118

By establishing a relationship between the overflow quantities and the storage volume of the RWH system for a given demand, a graphical representation can be made to determine the additional storage volume required for a particular percentage of overflow. The graph can be generalized if all the relevant parameters are divided by the roof area (A), used for harvesting of rainwater, thereby allowing provision to relate to any given roof area. Once divided by the projected roof area (A), the storage volume becomes specific storage in L/m2 and the daily demand becomes specific demand in mm/d for overflow as a percentage of roof collection.

3.14.2 Methodology A series of CTTRWH systems with the combined storage capacities of 1, 1.5 and 2.5 m3 in Colombo, in the wet climatic region of Sri Lanka (annual average rainfall 2500 mm) and another set of tanks with similar storage capacities in Anuradhapura in the dry climatic region (annual average rainfall 1500 mm), were set up and daily yields were recorded for calendar year (2008). The daily demand was taken as 200 L/d and 100 L/d, representing a household of 4 and 2 people with daily per capita consumption of 200 L of water, of which 25% is used for WC (Sendanayake & Jayasinghe 2006). The collection area is taken as 25 m2 of clay tiled roof with an inclination of 150 to the horizontal plane. Daily rainfall data are recorded for the entire period of the research at both locations, and verified with data collected at the National Meteorological Laboratories located at close proximity to test sites.

3.14.3 Calculations

The collection coefficient (Cf) of the roof area was calculated as 85%. From daily rainfall data, the daily roof collection was calculated and the overflow quantities determined by deducting the daily yield.

The annual overflow quantities were

calculated as a percentage of the total roof runoff collection and plotted against specific storage volume for two specific demands of 8 mm/d and 4 mm/d.

119

Data collected on daily rainfall, yield and calculated values of over-flow quantities are given in Appendix 2.

Calculated values of annual over-flow quantities for given

specific storage (SS) in L/m2 and specific demand (SD) in mm/day are given in Table 3.8

Table 3.8: Annual OF quantities for given SD and SS SD (mm/d) 4

2

S (m3) 1 1.5 2 2.5 1 1.5 2 2.5

SS (L/m2) 40 60 80 100 40 60 80 100

OF/yr 25.4 19.2 18 17.6 46.1 40.2 38.9 -

Chart 3.9 represents overflow as a percentage of roof collection versus specific storage volume in L/m2 Colombo in the wet region of Sri Lanka. In the legend, d/100 and d/200 represent the daily water demand of 100 and 200 L for WC flushing.

Chart 3.9: Overflow % for different specific storage volumes for Colombo 120

3.15 Summary Any rain water harvesting system integrated in to a building should be capable of providing collected rain water to user points at optimum energy efficiency.

By

introducing a multi tank rain water harvesting model of cascading roof collection type, henceforth termed as the cascading multi tank rain water harvesting (CMTRWH) model, it is found that not only the energy issues, but the spatial, structural and aesthetic issues can also be overcome. With the underlying concept of effectively distributing the storage capacity among different floor levels to feed the bulk of the harvested rain water through gravity, it is found that the energy efficiency of the system can be substantially improved compared to RWH systems of conventional type, particularly in multi storey situations. Considering the importance of generalizing the model for any multi story situation, an algorithm is developed where the system behaves within the limits of validity defined for Fewkes generalized curves introduced by Fewkes for water saving efficiency (WSE). Analyzing the developed algorithm, it is found that for the system to supply water to user points without the requirement of pumping is defined by an equation relating D/AR to the number of floor levels for any multi story situation.

Further, a

similar equation is developed for system continuity, i.e., to prevent the system running dry. It is seen from the curves developed, that as the number of floors increase, so does the maximum possible load D, indicating the inherent energy efficiency of a two story (3 Tank) housing unit over a single story (2 Tank) one for the same total demand. However, on the other hand, a single story house with a two tank model is less vulnerable to system running dry and hence more suitable for dry regions where the maximum available roof collection can be extracted without system failure.

The requirement of pumping is a major issue in any RWH system. Using the algorithm developed for CMTRWH system, the pumping quantities that is possible in the model is compared with that of a single tank model.

It is found that the pumping energy

requirement in a multi story situation, as a percentage of that required in a single tank model, is a function of WSE for each floor level and the number of floor levels (n), when the load distribution is equal for all n floors. It is also found that the optimum 121

performance with regard to the quantity of rain water pumped and the associated utilization of energy occurs when D = AR. When D > AR, the system tends to drop WSE at all floor levels resulting in requiring a higher quantity of make-up water, hence requiring more energy for pumping as demonstrated by the total energy requirement against D/AR graph.

In many practical situations using CMTRWH models, the occurrence of different water usages at different floor levels can be expected. Analyzing the algorithm, it can be seen that the energy requirement is less when the load is biased towards the upper floors. For a two storey house for example, when D1 and D2 are the demands for the upper and lower floors, respectively, the percentage difference in energy usage between D1/ D2 = 0.5 and D1/ D2 = 2.0 is only 15%, indicating the impact is not substantial. However, it also highlights the importance of demand distribution in designing multi storey houses so that the energy utilization in pumping can be minimized.

If the maximum demand is to be extracted from a given combination of A and R with the assistance of pumping, it was shown that D should be equal to AR. Considering the possibility of varying the storage capacity at the optimum conditions when D = AR, it is observed that the capacity of the parent tank can be reduced so that S/AR = 0.05 without affecting the system efficiency. Hence, the capital outlay required for the rainwater harvesting model can be cut down by minimizing the tank size for the optimum conditions.

Overflow quantities in RWH situations

In developing the percentage overflow against specific storage volume chart, if data from a longer time series is taken, more accurate overflow quantities could be possible. However, it can be shown that for most tropical climates, the rainfall is seasonal and the heaviest precipitations occur due to annual monsoons. Studying historical rainfall data for Sri Lanka, it can be seen that although the annual average rainfall vary by as much as 20% in the wet region and by approximately 30% in the dry region, the maximum number of storm events and hence the highest rainfall occurs during April/May and 122

October/November in a calendar year. Further, the average rainfall during the above periods show close similarity in precipitation amounts (in mm) over a 10 year period from 1999 to 2009 (National Meteorological Department of Sri Lanka).

Since the maximum overflow occurs during periods of maximum rainfall, it can be safely assumed that the results obtained from measuring and calculating overflow quantities in a single year closely resembles a similar data set collected over a longer period of time. It is clear from historical data, that the average rainfall during peak rainy months is approximately same with a maximum variation of 15%.

It can be seen from the graph, that for a significant percentage drop in overflow, the specific storage volume has to be largely enhanced. In any case, practically, overflow percentage cannot reach zero due to unpredictability of the strength and intensity of rain events in any particular period of time. However, if a minimum of 50 years of rainfall data are collected for a particular region and simulated to calculate overflow percentages, the maximum additional retention volume required for maximum rainfall occurred as well as average additional retention volume required for annual average rainfall during peak rainy period can be calculated. Whilst the former can be useful in flash flood control situations the latter is useful in RWH situations. Further, it can be seen from the graph that a more pronounced impact can be affected on the overflow percentages by increasing the specific rain water consumption. Therefore, if harvested rain water can be used further to WC flushing, a steeper reduction in overflow quantities can be achieved.

Developing of an overflow percentage chart was not attempted for the dry region of Sri Lanka due to low overflow quantities as well as the availability of non-built-up land for natural seepage for excess roof collection. In conclusion, the graph developed can be used as a design tool in combination with Fewkes generic curves for WSE, in determining the ideal volume for rain water storage, maximizing the WSE whilst minimizing the overflow quantities.

123