IMPACT OF THE VARIATION OF STORAGE VOLUMES IN CASCADING MULTI-TANK RAINWATER HARVESTING SYSTEMS

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1 IMPACT OF THE VARIATION OF STORAGE VOLUMES IN CASCADING MULTI-TANK RAINWATER HARVESTING SYSTEMS 1 S. Sedaayake 2 K. J. C. Kumara 1 Departmet of Civil Egieerig, South Asia Istitute of Techology ad Medicie Malabe, Sri Laka 2 Departmet of Mechatroics Egieerig South Asia Istitute of Techology ad Medicie Malabe, Sri Laka ABSTRACT Raiwater harvestig is perceived as a acceptable method of supplemetig the reticulated service water supply, particularly i water stressed locatios. Mostly used as stad-aloe types for the provisio of service water for secodary purposes such as laudry, the biggest drawback i Rai water Harvestig (RWH) systems is its raiwater storage tak occupyig a cosiderable space depedig o the daily demad for water, roof collectio area ad the average raifall depth. Attempts have bee made to optimize the storage tak capacity for a desired Water Savig Efficiecy (WSE) with the aim of reducig the size of the tak which would also allow the tak to be placed just below the roof level to feed the service poits through gravity. Still the weight ad volume of the taks ca be high particularly for high demad, multi-storey situatios. To address the problem, Cascadig Multi Tak Rai Water Harvestig (CMTRWH) systems have bee itroduced with the storage capacity distributed amog floor levels where the upper level feeder tak capacities ca be restricted to as low as 1 m 3 posig a miimum disturbace to the buildig evelop, but the paret tak which collects oly the excess roof ruoff cascadig dow from the feeder taks still occupies a cosiderable space at the groud level. If the storage volume of the paret tak ca be further reduced while havig a margial effect o the overall water savig efficiecy, it could have a sigificat impact o miimizig the system cost. Key words: Raiwater, cascadig, multi-tak, eergy efficiet, paret tak, stad-aloe, optimize INTRODUCTION The storage tak of a Rai Water Harvestig (RWH) system is the compoet that by far costig the most compared with the other two mai compoets, amely, the collectio surface, which is usually the roof of the buildig, ad gutterig which covey the roof collectio to the storage tak. Therefore, optimizig the storage tak capacity (S) for a give aual demad (D), roof collectio area (A) ad a aual average raifall depth (R) is of importace if RWH systems to proliferate. Takig the daily service water demad is user specific ad therefore is costat [3], 56

2 ad that all storage taks idividually ad collectively obey Yield After Spillage (YAS) reservoir behavioral algorithm [4] a set of geeralized curves which are idepedet of the spatial ad temporal fluctuatios of raifall has bee developed for Water Savig Efficiecy (WSE) or η of a geeric RWH system [2] (Fig. 1). Depictig WSE agaist locatio ad collectio area idepedet storage fractios S/AR for a give set of demad fractios D/AR, where 0.25 D/AR 2.0 ad S/AR the curves ca be used effectively to determie the optimum storage capacities i RWH systems. Takig ito cosideratio the requiremet of providig the harvested raiwater to service poits i a eergy efficiet maer ad also with miimum structural ad aesthetic disturbace to the buildig evelop, Cascadig Multi-Tak Rai Water Harvestig (CMTRWH) systems have bee itroduced [6] which are particularly suitable for multi-level buildigs. A CMTRWH system cosists of feeder taks of typically 1 m 3 capacity for each floor level ad a paret tak at the groud level to collect the cascadig excess roof ruoff which is pumped up to the uppermost feeder tak to sustai the cycle, ehacig the overall WSE. The pump is triggered o by a floater switch arragemet moitorig the water level at the lowest level feeder tak. The paret tak capacity (S P ) is usually take to complemet the differece betwee the cumulative volume of feeder taks ad the storage volume of a equivalet covetioal RWH system i order to achieve a comparative WSE. If however, the capacity of the paret tak (S P ) ca be optimized with miimum impact o the performace of a CMTRWH system, it will sigificatly reduce the foot prit of the paret tak while reducig the overall cost. The result will be more importat for sigle ad two storey houses with cascadig two or three tak raiwater harvestig systems. Sice RWH is prolific at household level, the study is focused more o Three Tak ad Two Tak models suitable for two ad sigle storey houses respectively. Figure 1: Geeralized curves for WSE 57

3 OBJECTIVE Figure 2: CMTRWH system for a two storey house The objective of the study is to ivestigate the impact of the variatio of the storage capacity of paret taks (S P ) o the quatities of collected raiwater that ca be pumped up (Q) ad therefore o the overall WSE of cascadig two ad three tak raiwater harvestig systems i sigle ad two storey houses. The study also attempts to determie the threshold values for S P for D AR, D = AR ad D AR scearios for give D, A, ad R values while maitaiig the feeder tak capacities at 1 m 3 for the miimum disturbace o the buildig evelop. CALCULATIONS I CMTRWH systems, for a give set of parameters D, A, R, S P ad feeder tak capacity at the i th level S i, the quatity of collected raiwater that is possible to be pumped up from the paret tak (Q) is give by, Q = η P { i 1 Di - i 1 Di i } (1) For which the effective roof collectio at each level is, (AR)i = AR - Di* i (2) Where D i, ad η i are the demad ad WSE at the feeder tak at the i th level ad η P is the WSE of the paret tak. For a equal demad at each floor level, (1) ad (2) ca be modified as, 58

4 Q = η P D{1 1/ i } (3) (AR) i = AR D/ i (4) For 2 where is the umber of floor levels. For a CMTRWH system with a feeder tak for each floor level ad a paret tak, the demad o the paret tak ca be give by, D P = D - Di* i (5) Whe modified for equal demad loadig at each floor level, (5) ca be give as, D P = D D/ i (6) D P, therefore, is the gross shortfall i the total yield, which requires to be satisfied by the quatity of collected raiwater that ca be pumped up from the paret tak (Q). Therefore, (D P Q) is the effective shortfall i the yield (ESY) ad whe take as a percetage of the total demad, idicates a measure of the overall WSE of the system. A high overall WSE is idicated by a low ESY% ad vice versa. ESY% = (D P Q)/D (7) Aalyzig the variatio of (ESY%) with respect to the reductio of paret tak capacities, ( S P ) as a percetage of the origial capacity S P (i.e. S P / S P %), for scearios of D AR, D = AR ad D AR, threshold values for S P ca be foud for the miimum impact o overall WSE. To ivestigate the optimum values for the paret tak capacity S P with respect to system parameters D, A, R, S P ad S i, hypothetical cases of cascadig Three Tak ad Two Tak RWH systems istalled at two storey ad sigle storey houses located i a tropical settig receivig aual average raifalls of 2000 mm are selected. With a effective roof ruoff area of 50 m 2, feeder tak capacities are take as 1 m 3 each, the paret tak capacities are selected as 8 m 3 for the Three Tak model ad 9 m 3 for the Two Tak model to esure that the total capacity ( S i + S P ) is 10 m 3 satisfyig the coditio S 0.1AR for maximum WSE values for a give D/AR value [2]. For costat daily demads of 200, 300 ad 400 Liters, (D P Q) values are calculated for S P values of 12, 8, 4, 2 ad 1(i m 3 ) for Three Tak model ad 9, 6, 4, 2, 1 ad 0.5 (i m 3 ) for Two 59

5 Tak models. The daily demads are selected to suit the three scearios of D AR, D = AR ad D AR. (D P Q), idetified as the Effective Shortfall i Yield (ESY) is tabulated as a percetage of the total demad D agaist S P / S P % where S P is the variatio itroduced to the paret tak capacity ad S P is the origial capacity of the paret tak (i this case 8 m 3 ) (Tables 1-6). Table 1: Three Tak model D < AR sceario Table 2: Three Tak model D = AR sceario Table 3: Three Tak model D < AR sceario

6 Table 4: Two Tak model D > AR sceario Table 5: Two Tak model D = AR sceario Table 6: Two Tak model D > AR sceario

7 RESULTS AND DISCUSSION Whe the Effective Shortfall i Yield as a percetage of the total demad (ESY%) quatities are plotted agaist the percetage chage i the paret tak capacity ( S P / S P %), i the D = AR sceario, i both Three Tak ad Two Tak cases oly a margial icrease i ESY% ca be observed till S P / S P % reached a value of 50% idicatig a paret tak half the capacity of the origially selected tak of 8 m 3 is sufficiet to maitai the cascadig cycle without sigificatly affectig the WSE of the system (Chart 1.0). Figure 3: Effective Shortfall i Yield versus Variatio i paret tak capacity Three Tak Model Figure 4: Effective Shortfall i Yield versus Variatio i paret tak capacity Two Tak Model 62

8 Comparig the Three Tak ad Two Tak models it is see that the ESY% values correspodig to S P / S P % at the optimal D = AR sceario are lower i the Three Tak model whereas the Two Tak model outperformig the Three Tak model at sub optimal D AR ad D AR scearios. Further, whe the demad is varied, for both D AR ad D AR scearios, the respective curves for Three Tak ad Two Tak models, eve though show a slight icrease i ESY% values for the icrease of S P / S P % values, maitai the same shape characteristics. Comparig the curves for the Three Tak ad Two Tak models, it ca be see that at D = AR, the Three tak model showig lower ESY% values ad i all other scearios, the Two Tak showig margially lower ESY% values. I the D AR sceario, the behavior ca be attributed to the relatively high roof ruoff to the paret tak resultig i high η P, hece Q, resultig i low ESY%. I the D AR sceario comparatively, both D P ad Q drop, lowerig the ESY%. Whe D AR, for both Three Tak ad Two Tak models efficiecies of the idividual feeder taks drop for give S i values, pushig the D P values high. Further, i this sceario, the effective ruoffs to the paret taks (AR P ) are small compared to D AR, D = AR scearios, hece icreasig the D P /AR P ratio resultig i low η P values. As a cosequece therefore, (D P Q) icrease, hece high ESY% values for all correspodig S P / S P %. The rapid icreasig of ESY% values with icreasig S P / S P % ca also be attributed to the behavior of η P decreasig rapidly with icreasig D P /AR P ratio. I all situatios a rapid icrease i ESY% is see for ( S P / S P %) over 80%, i.e. whe S P 1 m 3, where the paret tak capacity is less tha that of feeder tak capacity. I that sceario, sice S P /AR P values gettig positioed i the sesitive regio of the geeralized WSE curves, a rapid drop i η P makes low Q hece resultig i high ESY%. This tred is slightly mitigated i the D AR scearios due to drop i AR P values, keepig the S P /AR P values away from the sesitive regio of the WSE curves. CONCLUSION I both cascadig Three Tak ad Two Tak models the effect o ESY% for the variatio of ( S P / S P %) are less tha 3% ad therefore are margial up to 50% for all three scearios of D AR, D = AR ad D AR. Sice ESY% is a measure of the overall WSE of the system, it ca be cocluded that reductio of paret tak capacity by as much as 50% is possible without a sigificat impact o the system performace. Of the three scearios, the rate of icrease of ESY% for the reductio of S P is highest whe D AR, highlightig the cotiued uderperformace of a uder desiged system. O the other had D AR sceario correspods to a over desiged system while i the optimum D = AR sceario, less tha 10% ESY values i both Three Tak ad Two Tak models, idicatig a small drop i WSE ca be justified by the expected cost savig due to reductio of the paret tak capacity S P by as much as 50%. 63

9 It ca also be recommeded that the reductio of S P should ot be below 1 m 3 for the risk of high iefficiecies (η P ) resultig i high ESY% values ad hece low system performaces. Further, sice the equatios used for the calculatio of ESY% are based o the equatios developed for CMTRWH systems, the above fidigs ca be exteded to multi tak models as well. Comparig the two models it is clear that at the optimum system performace coditio of D = AR for a give AR, Three Tak model is outperformig the Two Tak model. Sice this is a result of a higher fractio of the storage capacity distributed to upper floor levels, it ca be deduced that at D = AR the overall WSE to icrease with the umber of feeder taks. I actual practice, due to collectio iefficiecies, ESD% could margially icrease but will ot pose a impact o the result. System losses ad water retaied i the pipig etwork is ot cosidered for the calculatio due to its egligible scale. REFERENCES [1] Fewkes, A. (1999a), Modelig the performace of raiwater collectio systems towards a geeralized approach, Urba Water, 1, [2] Fewkes, A. (1999b), The use of raiwater for WC flushig: The field test of a collectio system, Buildig & Eviromet, 34, [3] Herma, T., Schmida, U. (1999), Raiwater utilizatio i Germay: efficiecy, dimesioig, hydraulic ad evirometal aspects, Urba water, 1, [4] Jekis, D., Pearso, F., Moore, E., Su, J.K., Valetie, R. (1978), Feasibility of raiwater collectio system i Califoria, Cotributio No. 173, Califoria water resources ceter, Uiversity of Califoria. [5] Sedaayake, S., Migutaa, N.P., Jayasighe, M.T.R. (2014), Validatio of desig methodology for raiwater harvestig for tropical climates, Asia Joural of Water, Eviromet & Pollutio, vol. 11, o. 1, pp [6] Sedaayake, S., Jayasighe, M.T.R. (2016a), Multi tak model for eergy efficiet raiwater harvestig Ideal Joural of Egieerig ad Applied Scieces, Vol 2, No. 2, pp , ISSN:

A statistical method to determine sample size to estimate characteristic value of soil parameters

A statistical method to determie sample size to estimate characteristic value of soil parameters Y. Hojo, B. Setiawa 2 ad M. Suzuki 3 Abstract Sample size is a importat factor to be cosidered i determiig