uk; rainwater harvesting systems for communal buildings - bradford university

8/3/2019 UK; Rainwater Harvesting Systems for Communal Buildings - Bradford University

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A Whole Life Costing Approach for Rainwater Harvesting SystemsRichard Roebuck PhD, Bradford University

Rainwater harvesting software from:www.SUDSolutions.com

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7.0 Rainwater Harvesting Systems for Communal Buildings

7.1 Introduction

This chapter investigates the water saving reliability and financial performance

of a potential RWH system for a proposed school building. This building type

was selected as it has significantly different characteristics from those in the

domestic studies in terms of occupancy, water usage patterns and roof area.

The previous chapter demonstrated that domestic rainwater systems are not

cost effective. It was considered possible that installations in larger buildings

may perform better financially but this required further investigation.

The building in question was a proposed mixed junior and infants school with an

estimated population of 680 pupils and staff (340 male, 340 female). The facility

was been designed by a Local Authority (LA) who provided details of the

building upon request. Discussion with the LA revealed that, although no RWH

system was planned in this instance, they were not opposed in principal to the

installation of such water saving technology. However, it was stated that the

health and safety of the pupils was paramount and that if such a system was to

be installed then uses would be limited to low-risk applications such as toilet

flushing and that the inclusion of a UV unit would be preferred. For these

reasons the selected applications were limited to WC and urinal flushing and

the use of a UV unit was assumed.

A RWH system supplier was contacted1 and asked for information and advice

on a suitable approach. The use of an indirect system with an in-building header


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tank was recommended (see chapter two, section 2.4.1). Other relevant

information is shown in tables 7.1-7.3. All prices are for the 2007 period and

include VAT. Note that the water demand figures shown in table 7.1 were based

on the system suppliers own estimation methods.

Table 7.1 Estimation of term time daily water demand

Parameter Item Value

Occupancy details Number of males 340Number of females 340

W.C. flushing Volume per flush (litres) 6.0% of males who visit W.C. per day 30%Number of visits per day for above 1% of females who visit W.C. per day 100%Number of visits per day for above 3

Urinal flushing Volume per flush (litres) 7.5Number of urinal ranges 10Operating time (hrs/day) 8.0Number of hours between flushes 0.5

This gave an estimated total daily water demand for WC and urinal flushing of

7.9m3/day. This figure was used for the term-time period, Mondays to Fridays. It

was assumed that no water use occurred during weekends or school holidays.

This gave an annual water demand of 1,547m3.


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Table 7.2 Key RWH system and building components

Component Value(s) Comments

Rainfall Daily rainfall data forEmley Moor weatherstation

Adapted for UKCIP (2000b) medium-high emissions climate change scenario

Catchmentsurface

Roof plan area: 1,845m2

Runoff coefficient: 0.90

Initial losses: 0.25mm

Roof material: pitched roof tiles

See table 3.2. High value used becauseinitial losses also taken into account

See table 3.3 and also Fewkes (1999a)First flushdevice

No first flush device Use of first flush devices is limited in theUK

Coarse filter Coarse filter coefficient:0.90

Commonly applied value. See table 3.5

Storage tank Initial degree of filling:

100%

Top-up location: in-building header tank

During commissioning and testing the

tank is filled to capacity

Assumes indirect RWH system used,see chapter 2, section 2.4.1

Pump Power rating: 1.0kWPumping capacity: 55l/min

See table 3.6

UV unit Power rating: 55W KMC required UV unit in order tominimise perceived health and safetyrisks. See chapter 2, sections 2.5.6 and2.6

Water and

seweragecharges

Supply charges:

1.09/m3

for 2007-08period, increasing yearly

Sewerage charges:1.17/m3 for 2007-08period, increasing yearly

Annual increase estimated using

regression analysis of historic YorkshireWater price data, see chapter 4, section4.7.5 and appendix two

See chapter 4, section 4.7.5 andappendix two

Electricitycharges

Unit charge: 8.7p/kWhrfor 2007, increasingyearly

Annual increase estimated usingregression analysis of average historicdata, see chapter 4, section 4.7.5 andappendix two

Decom. costs Assumed zero System assumed to be decommissioned

at same time as rest of building.Associated costs likely to small part oftotal decommissioning expenses


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Table 7.3 Maintenance activities and costs for school RWH system

MaintenanceItem

Value (inc. VAT) Comments

Annual

maintenancecontractoffered bysystemsupplier

250/year for contract

UV unit: supply and installreplacement bulb once peryear, 60

UV unit: supply and installcartridge filters every 6months, 60

Annual site visit by engineer who will:

check and clean filters, check pumpand repair/replace as required at noextra charge, replace othercomponents at no extra charge ifmade available

Componentreplacement

Storage tank: assumed willremain functional for selectedanalysis time horizon

Coarse filter: replace every15 years, 2,450 + 50installation fee = 2,500

Electronic controls, replaceevery 15 years, 140 + 50installation fee = 190

Plastic pipes (internal),replace every 35 years, totalcost (parts + labour) = 500

Replace mains top-upsolenoid valve every 7.5years, 175 + 50 installationfee = 225

Replace mains top-up levelswitch in header tank every12.5 years, 20 + 50installation fee = 70

UV unit replacement every

10 years, 840 + 50installation fee = 890

For all items in this section: seechapter 4, section 4.7.4 and appendixtwo. Note that complete componentreplacement was assumed, not

repair, as cannot be sure repair willbe possible (e.g. specific componentsmay not be manufactured in thefuture)

KMC QS estimate

Component purchase and delivery to site cost data for fifteen different tank

sizes were obtained from a number of RWH system suppliers. Installation costs

were estimated by the LA. Further information on how these were derived is

available in appendix four. Estimated total capital costs for each tank size are

presented in table 7.4.


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Table 7.4 Capital cost estimates for school RWH systems

Tank size (m3) Capital cost () Tank size (m3) Capital cost ()3.0 9,792 15.0 12,0485.0 10,285 18.0 11,695

6.0 10,074 22.5 12,3887.0 10,544 27.5 13,0469.0 10,873 36.0 14,36211.0 11,213 45.0 15,67812.0 10,967 54.0 16,97113.0 11,601

The LA predicted that the operational life of the building would be at least 50

years and so this was used as the discount period. Discount rates of 3.5%, 5%,

10% and 15% were investigated. LAs would normally employ discounts rates

towards the lower end of this spectrum, however the higher rates were included

in order to maintain consistency with the domestic RWH system studies from

the previous chapter.

7.2 Analysis stage 1: determine optimum tank size

Analysis of the system was conducted using the thesis model described in

chapter five and was conducted in two stages. First, the coarse hydrological

and financial performance of each of the fifteen tanks shown in table 7.4 was

assessed. The results from this were used to select the most appropriate tank

size which was then taken forward for a more detailed study. Results from the

preliminary analysis are shown in figures 7.1 and 7.2.


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Figure 7.1 Predicted hydrological performance of various tank sizes

0

25

50

75

100

0.01 0.1 1 10 100 1000

Tank size (m3)

%demandmet

= tank sizes included

in school case study

Figure 7.2 Predicted financial performance of school RWH systems over

50 year period

-20,000

-15,000

-10,000

-5,000

0

5,000

10,000

15,000

20,000

25,000

30,000

0 10 20 30 40 50 60

Tank size (m3)

RWHsystemsavings()

r = 3.5%

r = 5%

r = 10%

r = 15%

3.5%

5%

10%

15%


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The hydrological results showed that the maximum percentage of demand that

could be met was approximately 71% although this would have required a very

large tank size. For the capacities assessed the volumetric reliability ranged

between 12% (3m3) and 57% (54m3). The discount rate was found to

significantly influence the financial savings achievable. For higher rates (10%

and 15%) most systems showed a financial loss with a limited number showing

a small financial gain. For the lower discount rates (3.5% and 5%) the smaller

systems (3-11m3) showed a loss whilst the larger ones (12-54m3) showed net

benefits. At the lower discount rates, increasing system capacity was correlated

with increasing savings, although the rate of improvement showed diminishing

returns as tank size increased.

A decision had to be made as to which tank size to take forward for the more

detailed analysis. The 54m3 tank was selected for the following reasons:

It showed the best water saving reliability in terms of the percentage of

non-potable demand met (57%).

With the lower discount rates (3.5% and 5%) it was the best financially

performing tank of those for which data was available.

Larger tank sizes may have benefits in terms of peak flow reduction in

the downstream drainage network. Although this is not explicitly

considered here it is a possible benefit.

Further, institutions such as LAs are likely to use lower discount rates since

they are required to consider the wider impacts of their actions both spatially

and temporally, e.g. due to policies such as Local Agenda 21 initiatives. For


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public sector works Central Government recommends a declining discount rate

of 3.5% (HM Treasury, 2003). When society is the stakeholder then discount

rates in the range of 2-4% are commonly used (Ashley et al, 2004; Herrington,

2006). On this basis the selection of the 54m3 tank was the logical choice since

it showed the best overall performance when lower discount rates were

employed.

7.3 Analysis stage 2: detailed analysis of selected tank

Details of the 54m3 tank were input into the detailed analysis module of the

thesis model. The simulation was run using discount rates of 3.5%, 5%, 10%

and 15%. WLC, AIC and financial savings results are shown in tables 7.5-7.6

and figure 7.3. Note that in figure 7.3 the RWH system savings at n = 5, 10 and

25 years have been also been marked on the graph.

Table 7.5 WLC results for school case study (n=50 years)

Discount rate (%) RWH WLC () Mains-only WLC () RWH savings ()3.5 80,606 104,621 24,0155.0 65,095 79,535 14,440

10.0 42,142 42,438 29615.0 33,639 28,776 -4,917

Table 7.6 AIC results for school case study (n=50 years)

Discount rate RWH AICs (/m3) Mains AICs (/m3) Ratio RWH/Mains1

3.5% 1.04 1.35 0.775% 0.84 1.03 0.82

10% 0.54 0.55 0.9915% 0.44 0.37 1.17

1Ratios < 1 show that the RWH system was more cost effective than the equivalentmains-only system


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Figure 7.3 RWH system savings for school case study (n=50 years)

-20,000

-15,000

-10,000

-5,000

0

5,000

10,000

15,000

20,000

25,000

0 5 10 15 20 25 30 35 40 45 50

Year

RWHsystemsavings()

r = 3.5% r = 5% r = 10% r = 15%

15%

10%

5%

3.5%24,015

14,440

296

-4,917

-9,004

-9,235

-9,916

-10,485

-3,998

-4,674

-6,532

-7,932

9,889

6,245

-1,374

-5,314

Figure 7.3 shows a general trend of increasing year-by-year savings although

this pattern was occasionally reversed due to maintenance requirements.

However, the overall magnitude of the savings was greater than the associated

operating/maintenance costs. Despite this, for the high discount rate of 15% the

system still incurred a loss because the present value of future savings were

relatively quickly attenuated to very low values. For a rate of 10% the system

just reached the breakeven point before the gradient of the graph effectively

became zero. For the lower rates of 3.5% and 5% the attenuation effect was

much less pronounced and in these instances payback was achieved in 15 and

16 years respectively. These results emphasise that in this particular instance

the choice of discount rate was of paramount importance in terms of the

perceived financial performance.


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From the information presented it can be seen that, for the school system:

The selected discount period had a large influence on the perceived

RWH system savings. Shorter periods would have resulted in diminished

savings or even net losses for potentially profitable systems.

The variation in predicted savings between different discount rates

increased as the discount period increased.

Lower discount rates resulted in greater system savings.

At lower discount rates the system is profitable, unlike the domestic

systems in chapter six which all showed a financial loss.

7.4 Detailed breakdown of costs

The relative contribution of a number of key cost items to RWH system WLCs

were determined. The cost elements consisted of capital, maintenance, UV

operation, pump operation and mains top-up costs. Discount rates of 3.5%, 5%,

10% and 15% were assessed. All other parameters were as shown in tables

7.1-7.3. Figures 7.4 and 7.5 show the comparative results from the four

simulations. Table 7.7 presents the same data as figure 7.5 in numerical form.


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Figure 7.4 Percentage contribution to school RWH system WLCs of

various key cost elements

0%

20%

40%

60%

80%

100%

1 2 3 4

Discount rate (%)

%contributiontoRWHWLC

Capital

Maintenance

UV op.

Pump op.

Mains top-up

3.5% 5% 10% 15%

Figure 7.5 Percentage contribution to total maintenance costs of

individual maintenance items

0%

20%

40%

60%

80%

100%

1 2 3 4

Discount rate (%)

%contributiontomaintenancecosts Item 1

Item 2

Item 3

Item 4

Item 5Item 6

Item 7

Item 8

Item 9

3.5% 5% 10% 15%


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Table 7.7 Tabulated maintenance data from figure 7.5

ItemNo. Description

Low cost(@NPV)

High cost(@NPV)

Low cost(%)

High cost(%)

1 Annual maintenance contract 1,915 6,124 36.9 45.4

2 Replace UV unit bulb 460 1,470 8.9 10.93 Replace UV unit cartridge filters 919 2,939 17.7 21.8

4 Replace coarse filter unit 402 3,055 9.5 18.4

5 Replace electronic control unit 31 232 0.7 1.4

6 Replace internal plastic pipes 4 158 0.1 1.0

7 Replace mains top-up solenoid valve 132 623 3.1 3.8

8 Replace mains top-up level switch 16 111 0.4 0.7

9 Replace UV unit 336 1,868 8.0 11.3

Figure 7.4 shows that the capital, maintenance and mains top-up costs

accounted for the majority of the WLCs in all cases. The proportional

contribution of each varied as the discount rate was changed. Higher discount

rates placed more emphasis on the capital costs, which was not unexpected

since higher rates have the effect of increasing the relative contribution of near-

term expenditures. Capital costs accounted for between 21-50% of the WLCs.

Maintenance, UV operating, pump operating and mains top-up costs displayed

the opposite trend and had greater effect on system WLCs with lower discount

rates. This was also not unexpected since lower rates increase the relative

contribution of expenditures that occur in the future. Maintenance activities

accounted for between 13-21% of the WLCs and mains top-up for between 35-

55%. UV operating and pump operating (electricity) costs accounted for only a

minor proportion at between 1.2-2.1% and 0.7-1.2% of the WLCs respectively.

Figure 7.5 and table 7.7 show the relative contribution of each maintenance

activity to the total maintenance cost. The annual maintenance contract was the

most expensive single item at between 37-45% of total costs. Replacement of

the UV unit cartridge filters was second highest with a cost contribution in the


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range of 18-22%. Replacement of the coarse filter unit was next at between 10-

18%, then UV bulb (9-11%) and UV unit itself (8-11%). The remaining items

accounted for a relatively minor proportion of the costs. These consisted of the

replacement of the mains top-up solenoid valve (3-4%), electronic control unit

(0.7-1.4%), internal plastic pipes (0.1-1.0%) and mains top-up level switch (0.4-

0.7%).

Variations in the discount rate had some degree of influence on the relative

contribution of each maintenance item but in most cases this was not

particularly large. It was noticed that higher rates tended to give more weight to

maintenance items that occurred on a regular basis (e.g. yearly maintenance

contract, replacement of UV bulb and filters) and less weight to those that

occurred irregularly, for instance replacement of the coarse filter and UV units.

7.5 Sensitivity analysis of financial results

A sensitivity analysis of the proposed RWH system was conducted. Sensitivity

to changes in eight key parameters were investigated. The selected parameters

were daily rainfall depth, capital cost, maintenance costs, mains water supply

and sewerage charges, roof area, discount period, water demand and the

storage operating parameter . Variations in each batch of key parameters were

analysed in conjunction with discount rates of 3.5%, 5%, 10% and 15%.

Percentage changes to RWH system savings (the difference between mains-

only WLCs and RWH WLCs) were selected as the method for assessing

sensitivity as this was deemed to be the key parameter in judging the financial

success (or otherwise) of the modelled system.


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Changes to parameter values in the range of 100% were investigated where

possible. Exceptions were changes to the storage operating parameter , which

was varied between 0-1 in 0.1 increments, and changes to the discount period

which for obvious reasons could not be reduced by as much as 100%. To

enable the percentage changes in RWH system savings to be placed in context,

the original savings associated with each discount rate were as follows (from

section 7.3):

r = 3.5%, savings = 24,015.

r = 5%, savings = 14,440.

r = 10%, savings = 296.

r = 15%, savings = -4,917.

Figures 7.6-7.10 show the sensitivity analysis results for the four discount rate

scenarios. Note that on the graphs the label SupSew Charges refers to mains

supply and sewage volumetric charges, not mains standing charges.


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Figure 7.6 School sensitivity analysis results, r = 3.5%

-300%

-200%

-100%

0%

100%

200%

300%

-100% -50% 0% 50% 100%

% change in parameter

%

changeinsavings()

Rainfall

CapCosts

Maintenance

SupSewCharges

RoofArea

DiscountPeriod

WaterDemand

Rainfall

CapCosts

Maintenance

SupSew Charges

RoofArea

Discount Period

Water Demand

Figure 7.7 School sensitivity analysis results, r = 5%

-400%

-300%

-200%

-100%

0%

100%

200%

300%

400%

-100% -50% 0% 50% 100%


%changeinsav

ings()

Rainfall

CapCosts

Maintenance

SupSewCharges

RoofArea

DiscountPeriod

WaterDemand

Rainfall

CapCosts

Maintenance

SupSew Charges

RoofArea

Discount Period

Water Demand


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-10000%

-8000%

-6000%

-4000%

-2000%

0%

2000%

4000%

6000%

8000%

10000%

-100% -50% 0% 50% 100%


%

changeinsavings()

Rainfall

CapCosts

Maintenance

SupSewCharges

RoofArea

DiscountPeriod

WaterDemand

Rainfall

CapCosts

Maintenance

SupSew Charges

RoofArea

Discount Period

Water

Demand


-400%

-300%

-200%

-100%

0%

100%

200%

300%

400%

-100% -50% 0% 50% 100%


%changeinsav

ings()

Rainfall

CapCosts

Maintenance

SupSewCharges

RoofArea

DiscountPeriod

WaterDemand

Rainfall

CapCosts

Maintenance

SupSew Charges

RoofArea

Discount Period

Water

Demand


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Figure 7.10 School sensitivity analysis results for changes in storage

operating parameter (YAS/YBS algorithm)

0%

3%

6%

9%

0.0 0.2 0.4 0.6 0.8 1.0

value

%changein

savings()from

=0

0%

50%

100%

150%

200%

250%

r = 15%

r = 5%

r = 3.5%

r = 10%

3.5%

5%

10%

15%

= 0, YAS

= 1, YBS

Note: secondary y-axis (right hand side)

refers to r = 10%. Primary y-axis (left hand

side) refers to r = 3.5%, 5% and 15%

%changeinsavings

()from

=0

In figure 7.8 the percentage changes in RWH system savings were much larger

than for the other discount rates. This was due to the fact that the savings were

relatively small at 296 over 50 years. Therefore even small absolutechanges

in this figure resulted in large percentagechanges. This needs to be borne in

mind when comparing the results from this graph to those in figures 7.6, 7.7 and

7.9.

Examination of sensitivity analysis results

Investigation of the results showed that not all of the relationships were linear in

nature. The sensitivity of RWH system savings to changes in rainfall depth, roof

area, discount period, water demand and value displayed an exponential


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decay type relationship (increasing form). For the discount rate and water

demand the rate of change was relatively small for variations in the range of

-50% to +100%, suggesting relative insensitivity to these two parameters for all

but large negative changes. Variation in RWH system savings showed a linear

relationship with changes in capital and maintenance costs. The slope of the

corresponding lines was negative, showing that as capital and maintenance

costs decreased the RWH system savings increased. Sensitivity to changes in

mains supply and sewerage charges was also linear although in this instance

the gradient of the line was positive, indicating that the RWH system became

more cost effective relative to the mains-only system as the cost of mains water

increased.

With one exception, in figures 7.6-7.9 the slopes of the lines were similar

between the different discount rate scenarios. This indicated that the relative

sensitivity of RWH system savings to changes in the selected parameters was

generally similar despite variations in the discount rate.

The exception to this general trend was the capital costs, which displayed an

increasingly negative gradient the more the discount rate increased. This

indicated an increasing sensitivity (in terms of RWH system savings) to changes

in capital costs at higher discount rates. This can be explained by the fact that,

since they occurred in financial year zero, the capital costs had the same

present value regardless of the selected discount rate. By comparison, the other

parameters all had financial impacts that occurred at various stages during the

operational phase. Increasing the discount rate led to a reduction in the present


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value of these parameters, or reduced their financial impacts in the case of the

hydrological variants. This in turn led to reduced sensitivity in RWH system

savings to changes in these parameters. This manifested itself in the results as

increased sensitivity to capital costs, the only parameter not affected by

changing the discount rate.

Table 7.8 shows the gradients associated with each of the key parameters,

except for which is discussed in the following subsection. Note that for the

non-linear curves these results represent the average gradients. The steeper

the gradient the more sensitive RWH systems savings were to changes in that

particular parameter. Positive values show that the system savings increased

as the value of the corresponding parameter increased, whilst negative values

showed the opposite relationship (savings decreased as parameter value

increased). The maximum gradients (most sensitive) have been highlighted in

red.

Table 7.8 Sensitivity analysis results: parameter gradients

Discount rates and corresponding gradientsParameter 3.5% 5% 10% 15%

Rainfall depth 1.66 2.11 55.52 2.28Capital cost -0.71 -1.18 -57.42 -3.44Maint. costs -0.69 -0.87 -21.98 -0.86

Mains charges1 2.51 3.18 83.71 3.45Roof area 1.61 2.05 53.87 2.21

Discount period 0.97 0.93 14.99 0.45Water demand 1.15 1.46 38.58 1.59

1Mains supply and sewerage unit charges, not standing charges

The data shows that the RWH system savings were most sensitive to changes

in mains water supply and sewerage charges. The slope of the results was


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constant and positive, i.e. as mains charges increased there was a

corresponding linear increase in system savings. The results here are in sharp

contrast to those for domestic systems. For these, system savings were

typically very sensitive to changes in capital costs and relatively insensitive to

changes in the other parameters (with the exception of the LA/Government

stakeholder perspective). The results for the school case study show a higher

sensitivity to more parameters and there was no parameter to which system

savings were not sensitive (m0). Sensitivity to changes in capital costs were

still apparent, particularly at higher discount rates, but were less important than

in the domestic studies.

Increasing the discount period beyond 50 years made little difference in most

cases (slightly more for lower discount rates than for higher ones). Compare

this to the results from section 7.3 in which it was apparent that system savings

were highly sensitive to discount period variations in the range of 1-50 years,

especially at lower discount rates. With regards to the sensitivity analysis

results, the relative insensitivity for discount periods greater than 50 years

indicated that system savings were approaching the maximum that could be

achieved, particularly for the higher discount rates.

Given the high sensitivity of the system savings it can stated that accurately

predicting the long-term financial performance of RWH systems such as the one

modelled for the school case study is difficult. Even at low discount rates, such

as might be employed by public institutions, variations were large. For example,

at a discount rate of 3.5% changes in capital and maintenance costs of 50%


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resulted in variations in system savings of about 30%, or approximately

7,000. Changes in mains water costs resulted in even bigger swings. For

instance, with a 20% reduction in expected future mains prices the system

savings reduced from 24,015 to 11,963, a decrease of about 12,000.

Conversely, increasing future mains charges by 20% resulted in an increase in

the predicted savings of about 12,000. From data presented in chapter three it

can be seen that the price of mains water, although generally becoming more

expensive since privatisation of the industry in 1989, has tended to fluctuate in

price. The results presented here have shown that even relatively small

fluctuations can make significant differences to the perceived financial

performance of systems such as the one modelled for the school case study.

This needs to be borne in mind when using the predicted results to aid the

decision making process (i.e. whether or not to install the system).

Justification for using the YAS algorithm

From examination of figure 7.10 it can be seen that the percentage difference in

system savings predicted by the YAS and YBS algorithms were moderately

small for three of the selected discount rates and large for one. The smaller

differences corresponded to discount rates of 3.5%, 5% and 15%, which

showed percentage variations between YAS/YBS of 6.3%, 7.9% and 8.4%

respectively. The large difference corresponded to the use of a 10% discount

rate and the variation in system savings between the YAS/YBS algorithms was

204%. This latter result was primarily due to the fact that the predicted savings

were small. YAS predicted 296 over 50 years whilst YBS predicted 897 over

50 years, 601 more than in the former case.


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It is concluded that, whilst the school case study showed more variation

between YAS/YBS algorithms than did the domestic studies, the use of YAS

was still justified since the variations between the two approaches was

moderately small in monetary terms. This was especially true considering the

discount period employed. The noted differences in savings would be small on

an average per-year basis when distributed across 50 years.

7.6 Comparison of thesis results with RWH system suppliers method

Regarding the private sector company who supplied the water demand

estimation given previously in table 7.1, details were obtained of their in-house

design and assessment method for commercial systems such as that proposed

for the school building. A comparative study was then conducted between the

predicted RWH system performance using the suppliers method and the thesis

model.

The suppliers method was similar to that described in chapter six and appendix

two for the approach labelled Company 1 methodology. Hydrological

components explicitly considered consisted of the roof (plan) area, runoff

coefficient, coarse filter coefficient, average monthly rainfall for the region in

question and average monthly water demand (assumed same for all months).

Financial aspects consisted of savings on mains water bills and an assumption

that each cubic metre of harvested water supplied incurred an operating cost of

0.03/m3. No maintenance costs were included and no discounting techniques

were used (discount rate effectively zero).


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In order to maintain consistency between the system supplier and thesis

methods, the data for the hydrological elements was based on the information

collected for the thesis. Where necessary, the data was scaled and adjusted so

that it matched the characteristics of that required for the suppliers method, e.g.

daily data scaled to monthly data.

A spreadsheet was created that mimicked the suppliers method as outlined

previously. Further information and a statement of the assumptions used are

given in appendix four. The suppliers tank sizing method was based on

providing 6 days of average daily demand and this gave a recommended

capacity of 25.4m3. No tank size of this specific size was available on the

market and so the next largest was selected for which data was available. This

was a tank with a capacity of 27.5m3 and capital cost of 13,046. The

performance of this tank was then simulated using both the suppliers method

and the thesis model, using a zero discount rate in the former case and rates of

3.5%, 5%, 10% and 15% in the latter. Further, the suppliers approach assumed

no maintenance requirements. For the thesis simulations the maintenance

requirements where as given in table 7.3. Comparative results from the

modelling exercise are shown in figure 7.11 and table 7.9.


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Figure 7.11 Comparison of performance predicted by system suppliers

assessment method and thesis model

-15,000

-10,000

-5,000

0

5,000

10,000

15,000

20,000

0 10 20 30 40 50

Year

RW

Hsystemsavings()

r = 0% r = 15% r = 10% r = 5% r = 3.5% CapCost

Capital

expenditure

= 13,046

15%

10%

5%

3.5%

0% (supplier

method)

Savings =

119,304

17,652

10,515

1

-3,866

Table 7.9 Summary of results produced by system suppliers

assessment method and thesis model

MethodologyTank size

(m3)% demand

metAnnual

savings ()1Payback(years)

Total savingsover 50yrs2

System supplier 27.5 77.6 2,647 5 119,304Thesis, r = 3.5% 27.5 47.4 353 14 17,652Thesis, r = 5% 27.5 47.4 210 17 10,515

Thesis, r = 10% 27.5 47.4 0.02 50 1Thesis, r = 15% 27.5 47.4 -77.31 Never -3,8661Annual savings do not take into account repayment of capital cost expenditure, i.e.they are simply the difference between the annual running costs of the mains-onlysystem and the proposed RWH system2Calculation of total savings over 50 years included repayment of capital expenditure

From figure 7.11 it can be seen that the comparative financial results displayed

large differences. The suppliers method calculated the annual savings based


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on the volume of mains water substituted (minus 0.03/m3 for assumed

operating expenses) multiplied by the discount period of 50 years, then

subtracted the initial capital expenditure. Since no discount rate was applied,

the present value of the 2,647 annual savings were the same for all years.

Application of discounting techniques in situations such as this is correct since

consideration must be given to the opportunity cost of capital and it is a

shortcoming of the suppliers method that this was not done. Using the same

basic methodology but applying discount rates of 3.5%, 5%, 10% and 15% to

the cumulative 2,647/yr savings was found to give NPVs of 65,361, 50,970,

28,892 and 20,277. All of these are considerably lower than the result

obtained when no discount rate was applied (119,304).

For the thesis predictions, the predicted total savings over 50 years were also

significantly different. For discount rates of 3.5%, 5%, 10% and 15% the

predicted savings were 17,652, 10,515, 1 and -3,866. Payback periods

were given as 14, 17 and 50 years for discount rates of 3.5%, 5% and 10%

respectively, and no payback at all was achievable for the 15% rate. This

highlights the sensitivity of the results to the selected discount rate and the

importance in selecting an appropriate value.

The effect of assuming no maintenance requirements is also evident in figure

7.11. The lines corresponding to the thesis results display a general increasing

trend punctuated by irregular dips in system savings. These dips were due to

the cost of maintenance, a consideration that was excluded entirely from the

suppliers methodology.


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Table 7.9 shows that the suppliers assessment produced very different

hydrological results compared to the thesis model, predicting a volumetric

reliability of 77.6%. However, this was based on the implicit assumption that all

of the collected water could be used, i.e. that the tank capacity was effectively

infinite with no overflows. The more realistic finite storage method employed by

the thesis model predicted a lower volumetric reliability of 47.4%.

It is acknowledged that these results were based on the consideration of only

one case study, and that an investigation into a wider range of circumstances

would be beneficial, e.g. a larger range of tank sizes, different schools and other

commercial/institutional buildings. However, from this brief study it can be

concluded that the RWH system suppliers method:

Did not realistically model tank behaviour and overestimated the volume

of harvested rainwater that could theoretically be supplied.

Did not adequately account for likely operation and maintenance costs.

Significantly overestimated the magnitude of financial savings that were

ultimately achievable because no discounting techniques were applied.

Based on these limitations it is advised that predictions of water saving reliability

and financial performance be treated with caution if they have been generated

using similar methods to the one investigated here. It is strongly recommended

that a more thorough and realistic approach be used whenever possible.


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7.7 Summary and discussion of results

7.7.1 Overview

The aims of this chapter were to investigate the water saving reliability and

financial performance of an indirect RWH system for a proposed school building

with 680 pupils and staff. Water uses were to be WC and urinal flushing with a

combined yearly demand of 1,547m3. The system was to include a UV unit in

order to minimise any health risks. The predicted operational life of the school

building was fifty years and so this was used as the discount period when

assessing the RWH system. Discount rates of 3.5%, 5%, 10% and 15% were

used. A maintenance schedule was created that included regular activities

(yearly maintenance contract and replacement of UV filters and bulb) and

irregular activities (component replacement).

Analysis stage 1: determine optimum tank size

Capital costs were obtained for commercial rainwater tanks in the range of

3-54m3. The first stage of the analysis involved using the assess savings

module of the thesis model. The aim here was to determine the most cost

effective tank size from the range available, and then to take the selected tank

forward for a more detailed analysis. Out of the fifteen commercial tanks for

which data was available, it was deemed that the 54m3 capacity was the best

selection (also the largest tank of those assessed). This tank was selected

primarily because it a) showed the best hydrological performance with a

predicted volumetric reliability of 71%, and b) was the best performing financial


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tank under discount rates of 3.5% and 5%, saving an estimated 24,015 and

14,440 respectively.

Analysis stage 2: detailed analysis of selected tank

Details of the selected 54m3tank were input into the detailed analysis module

of the thesis model. For discount rates of 3.5%, 5%, 10% and 15% RWH

systems savings were predicted to be 24,015, 14,440, 296 and -4,917

respectively. AICs and AIC ratios were also calculated for the same discount

rates. For the same discount rates listed previously, the AICs in /m3 were 1.04,

0.84, 0.54 and 0.44. AIC ratios were 0.77, 0.82, 0.99 and 1.17. This showed

that the RWH system was more cost effective than the equivalent mains-only

system for all discount rates except 15%. AIC ratios were more favourable

towards the RWH system at lower discount rates. The range of figures also

demonstrated the sensitivity of the results to the selected discount rate.

The discount period was found to strongly influence the perceived system

savings. The results at n = 50 years were compared to those at n = 5, 10 and

25 years. It was concluded that:

The selected discount period had a large influence on the perceived

RWH system savings. Shorter periods resulted in diminished savings or

even net losses for potentially profitable systems.

The variation in predicted savings between different discount rates

increased as the discount period increased.

Lower discount rates resulted in greater system savings.


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378

Stakeholders that take a shorter term view are likely to perceive less

financial benefit than those who take a longer term view.

7.7.2 Detailed breakdown of costs

The relative contribution to system WLCs of the capital, maintenance, UV unit

operating, pump operating and mains top-up costs were determined. It was

found that, for the discount rates assessed the capital, maintenance and mains

top-costs accounted for the majority of the WLCs. Capital costs account for

between 21-50%, maintenance 13-21% and mains top-up 35-55%. UV unit and

pump operating costs were only a minor proportion of the total costs at between

1.2-2.1% and 0.7-1.2% respectively. The relative contribution of each cost

element was found to vary with different discount rates. Higher rates placed

more emphasis on the capital costs whilst lower rates gave more emphasis to

the other cost components. This result was not unexpected since higher

discount rates had the effect of reducing the present value of future

expenditures, thus increasing the percentage contribution to system WLCs of

the capital costs borne in financial year zero. Lower discount rates had the

opposite effect, that of increasing the present value of future expenditures and

thus increasing their percentage share of the WLCs.

The percentage contribution of each maintenance item to the total maintenance

cost was determined. The annual maintenance contract was found to form the

largest fraction at between 37-45%. Replacement of the UV filters was next

highest at between 18-22%, followed by the coarse filter unit (10-18%). Next

highest were the UV bulb (9-11%) and UV unit itself (8-11%). The remaining


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items accounted for a relatively minor proportion of the costs. These consisted

of the replacement of the mains top-up solenoid valve (3-4%), electronic control

unit (0.7-1.4%), internal plastic pipes (0.1-1.0%) and mains top-up level switch

(0.4-0.7%). It was also observed that higher discount rates tended to give more

weight to maintenance items that occurred on a regular basis, e.g. yearly

maintenance contract, replacement of UV filters and bulb.

7.7.3 Sensitivity analysis of results

A sensitivity analysis of the financial model was conducted in order to determine

the robustness of the school case study results. Percentage changes in RWH

systems savings were selected as the metric for judging model sensitivity.

Variations of up to 100% in a number of key parameters were assessed.

These were daily rainfall depth, capital cost, maintenance costs, mains water

supply and sewerage charges, roof area, discount period and water demand.

The effect of varying the storage operating parameter between 0 (YAS) and 1

(YBS) was also investigated. Variations in each batch of key parameters were

analysed in conjunction with discount rates of 3.5%, 5%, 10% and 15%.

Not all relationships were linear. Sensitivity of RWH systems to changes in

rainfall depth, roof area, discount period, water demand and displayed an

exponential decay type relationship (increasing form). For the discount rate and

period, results were relatively insensitive to changes in the region of -50% and

+100%. Linear relationships were apparent in changes to capital costs,

maintenance costs and mains supply and sewerage charges. Increasing capital

and maintenance costs was found to decrease RWH system savings, and visa


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versa. Increasing mains supply and sewerage charges was found to also

increase RWH system savings, and visa versa.

RWH system savings were most sensitive to changes in mains supply and

sewerage charges. Contrast this to the domestic studies in which the sensitivity

analysis results were dominated by the capital costs. Results for the other

parameters varied but system savings showed moderate to high sensitivity in

most cases. Exceptions were the water demand and discount period to which

system savings were relatively insensitive for changes in the 0 to +100% range

(but was still nevertheless sensitive to changes in the 0 to -100% range).

Sensitivity to the storage operating parameter was also relatively low in

relation to variations in monetary savings.

Capital costs showed greater sensitivity at higher discount rates. This occurred

because higher discount rates reduced the present value of other ongoing costs

but did not affect the present value of the capital costs borne in financial year

zero. Therefore, at the higher discount rates the capital costs constituted a

greater proportion of RWH system WLCs and it follows that the savings

(difference between RWH WLCs and mains-only WLCs) would be more

sensitive to any change in capital cost value.

Given these results it can be stated that accurately predicting the long-term

financial performance of RWH systems such as the one used in the school case

study example is problematic. However, as will be discussed in the next

subsection, existing alternative assessment methods, such as those commonly


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381

employed by private sector systems suppliers, would appear to be even less

useful than the thesis model for predicting future financial performance.

Therefore it is argued that, whilst the approach taken in the thesis may be

sensitive to assumptions regarding future conditions, the thesis model is still

more realistic and more useful as a decision support tool than many of the

existing methodologies.

7.7.4 Comparison of thesis model results with RHW system suppliers

method

Details were obtained of a RWH system suppliers in-house method for

assessing RWH systems such as the one considered for the school case study.

The hydrological and financial performance predicted by the suppliers

methodology was then compared to that predicted by the thesis model.

Hydrological components explicitly considered by the supplier consisted of the

roof (plan) area, runoff coefficient, coarse filter coefficient, average monthly

rainfall and average monthly demand (assumed same for all months). Financial

aspects consisted of savings on mains water bills and an assumption that each

cubic metre of harvested water supplied incurred an operating cost of 0.03. No

maintenance costs were included and no discounting techniques were applied

(discount rate effectively zero).

A spreadsheet was created that mimicked the system suppliers approach. The

recommended tank size was based on providing 6 days of average daily

demand which in this case was 25.4m3

. The nearest actual tank size was


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27.5m3 (capital cost = 13,046) and so this was selected. The performance of

this tank was then simulated using both the system suppliers method and the

thesis model, using a zero discount rate in the former case and rates of 3.5%,

5%, 10% and 15% in the latter. No maintenance requirements were assumed

for the supplier approach whilst the thesis simulations used the maintenance

schedule given previously in table 7.3

There was a marked difference between the two approaches in the predicted

water saving reliability. Supplier methodology estimated a reliability of 77.6%

whilst the thesis model estimated 47.4%. The difference was primarily due to

the different ways in which storage tank behaviour was taken into account. In

the suppliers approach it was simply assumed that all harvested rainwater

would be available for use, essentially an infinite reservoir approach. The thesis

model employed a finite storage algorithm in which overflow occurred if the

capacity of the tank was exceeded. This more realistic approach explains why

the thesis model predicted a lower volumetric reliability.

The predicted ultimate financial savings were significantly different. Over a 50

years period the suppliers methodology predicted constant savings of 2,647

per year, or 119,304 over 50 years including repayment of the capital costs.

The thesis model predicted savings of 17,652, 10,515 and 1 for discount

rates of 3.5%, 5% and 10% respectively. For the 15% discount rate an overall

loss of 3,866 was predicted. It was apparent that the differences in predicted

savings between the two approaches was large. The closest result from the

thesis simulations was for the 3.5% discount rate with a saving of 17,652. The


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system suppliers results were nearly eight times this amount. A primary reason

for this disparity was found to be due to the exclusion of any discounting

methodology. The yearly savings of 2,647 forecast by the supplier were simply

multiplied unadjusted by the analysis time period to arrive at savings figure of

119,304. Using the same assessment method but applying discounting rates

of 3.5%, 5%, 10% and 15% resulted in savings of 65,361, 50,970, 28,892

and 20,277 at NPV. All of these were considerably less than the original

estimation.

It was acknowledged that this comparative investigation was based on the

consideration of only one RWH system, and that an investigation into a wider

range of circumstances would have been beneficial. For example other school

facilities and other types of buildings such as offices. However, from this brief

study it was concluded that the method used by the system supplier:

Did not realistically model tank behaviour and overestimated the volume

of harvested rainwater that could theoretically be supplied.

Did not adequately account for likely operation and maintenance costs.

Significantly overestimated the magnitude of financial savings that were

ultimately achievable because no discounting techniques were applied.

Based on these limitations it was advised that the water saving reliability and

financial performance results generated by such methods should be treated

with caution. It was strongly recommended that a more thorough and realistic

approach be used whenever possible.

uk; rainwater harvesting systems for communal buildings - bradford university

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