Performance of a Water and Energy Efficient House Over the Last Decade

Written by peter@uwcs.com.au

November 9, 2013

Key messages

IMG_0649-featRainwater tanks used to supply all water uses and a front loading clothes washer were installed at an inner city house to demonstrate the potential of local water cycle management solutions.

The local water cycle solution was subject to monitoring over the last decade. The rainwater tanks have produced 62% (128 kL) annual reduction in mains water use. In addition, the front loading clothes washer and pressure reduction has produced a 16% (32 kL) annual reduction in mains water use. The Carrington house has experienced a long term average reduction in mains water use of 78%.

Monitoring results and subsequent economic analysis has revealed that the costs of the local solution were $0.12/kL of rainwater supply and demand reduction. The cost of the rainwater supply was $0.58/kL and the front loading clothes washer provided a benefit of $0.80/kL.

The long term annual electricity use of the household has reduced by 12% since the installation of the rainwater tanks and front loading clothes washer. Inclusion of solar panels during a recent renovation has reduced the net electricity use of the house by 61% to 5 kWh/day.

Monitoring at the site has also revealed that the quality of rainwater improves substantially in the rainwater treatment train from the roof to the tank to the hot water system to the tap. The quality of rainwater at the tap is consistent with Australian Drinking Water Guidelines.

A recent renovation of the Carrington house has included modernisation of the rainwater harvesting system that includes a 5,000 Litre slimline tank, a submerged pump, an under sink water filter and solar panels.

Introduction

An integrated water cycle strategy was established at an old miners cottage in Carrington, located within the inner city of Newcastle in New South Wales, to test and showcase the performance of local scale solutions. The ongoing monitoring program into the performance of local scale or source control solutions continues with the original installation of two rainwater tanks to supply all household uses and a water efficient clothes washer in 2002. Previous studies by the author analysed the performance of rainwater tanks at Figtree Place and the Maryville house.

Figtree Place is a water sensitive urban redevelopment consisting of 27 residential units located in Hamilton, an inner suburb of Newcastle. The site uses underground rainwater tanks to supply hot water and toilet uses in the units. The prototype water sensitive urban design (WSUD) redevelopment was subject to many design and construction faults. Nevertheless the microbial and chemical quality of rainfall runoff from roofs was found to improve in the rainwater tanks and tank water used in storage hot water systems set at temperatures ranging from 50°C to 65°C was compliant with Australian Drinking Water Guidelines. In addition, mains water use was reduced by around 45% and small cost savings were experienced.

The Maryville house project in the inner city of Newcastle was established in 1999 using the lessons learnt from the Figtree Place redevelopment. An old house was fitted with a large above ground 9,060 Litre rainwater tank to supply hot water, toilet and outdoor uses. The improved rainwater tank design cost $1,851 to install and resulted in mains water savings of 52% (62 kL/annum) in the three person household. Rainwater supply at the Maryville house was estimated to cost $0.30/kL. Similar to the Figtree Place redevelopment, the microbial and chemical quality of rainfall runoff from roofs was found to improve in the rainwater tanks and tank water passing through the instantaneous hot water service set at 55°C was compliant with Australian drinking water guidelines

A monitoring program was established in 2003 to analyse water use, energy consumption and rainwater quality in the rainwater tank and at various end uses in the house. In addition, the performance of the water and energy systems in the house was also demonstrated by the billing records provided by the various utilities. This paper discusses the design, monitoring results and economic performance of the dual water supply system. During 2012, the Carrington house was subject to renovation that also incorporated an upgrade to the rainwater harvesting system, installation of solar panel and inclusion of an under sink water filter.

Original design of the dual water supply system

The cottage has a roof area 95 m2 and it situated on an allotment with an area of 178 m2. Two small rainwater tanks with capacities of 2,200 Litres were installed during July of 2003 and a water efficient (4A rated) washing machine was installed during January of 2004. Rainwater is used to supply all household water uses. Prior to installation of the rainwater tank the average annual water use in the two person household was 204 kL. A monitoring program has been installed to analyse water use and rainwater quality in the rainwater tank and at various end uses in the house.

 Carrington-House-Two

The Carrington House with rainwater tanks (circa 2003)

The dual water supply system and locations of the water meters used to monitor water use are shown below. Rainfall from 84% of the roof area (80 m2) discharges to the two rainwater tanks and supplied to the household via a water filter using a Davy XP350 pump.

 Carrington-House-Three

Plan view of the original dual water supply solution at Carrington

The configuration of the rainwater tanks in the dual water supply system is shown below. A mechanical system was used to top up the rainwater tanks when tank water levels are drawn down to a depth below 0.4 metres. In the event of a pump or power failure the rainwater supply system can be bypassed. Design of the rainwater supply scheme provides a minimum storage volume of 0.72 m3, a rainwater storage volume, airspace between the mains water inlet and the tank overflow to prevent backflow of rainwater into the mains water system, and rainwater supply drawn from above the anaerobic or sludge zone.  A dual check valve for backflow prevention was placed at the water meters.

Carrington-House-Four

If the volume of stored water in the rainwater tanks falls below 0.72 m3, the shortfall is overcome by the mains water trickle top up system. The mechanical trickle top up system includes a diaphragm valve connected to a stiff arm and a float. Vertical movement of the float allows trickle top up at a variable rate governed by the depth of water in the tanks. A low water depth will trigger a greater top up flow rate than a small drawn down of the storage volume.

The two tanks were interconnected to the pump using a three way plumbing connection. This allows the water levels in the tanks to equalise and for water supply to be drawn from both tanks. Rainwater is drawn from the tanks at a point 0.1 m above the base to avoid entraining sediment into the water supply from the base of the tank.

The Approval Process (circa 2002)

State Environmental Planning Policy 4 (SEPP4) in New South Wales allows the installation of above ground rainwater tanks that have storage capacities of 10 kL or less without a development approval from local government. However the use of a mains water top up scheme requires approval from the local water authority. A sketch of the dual water supply scheme was provided to Hunter Water Corporation.

The original costs

There has been considerable debate about the cost of rainwater supply to a house. The cost and performance of the rainwater supply system at Carrington were monitored closely in an attempt to understand the true costs and benefits of the system.  The total cost to install the rainwater supply system was $2,350 and the itemised costs are shown below.

Costs to install the rainwater supply system in 2002 (Australian $)

Item Cost ($)
Aquaplate tanks

756

Pump

389

Plumber + electrician

600

Fittings

455

Concrete slab

150

Total

2,350

One of the common assumptions is that rainwater tanks occupy a large area and therefore must be installed underground at considerable cost. However the two 2,200 Litre capacity rainwater tanks at the Carrington house were chosen to occupy a minimum area of about 2 m2. This is approximately 2% of the small land area occupied by the allotment.

Performance of the Carrington house

Additional water meters were installed in the rainwater supply pipe on the downstream side of the pump and in the mains water top up pipe. In combination with the existing water meter at the property boundary, the meters were used to determine mains and rainwater use at the house. Daily climate data from the nearby Maryville weather station operated by the Hunter Valley Research Foundation was incorporated with the meter readings, water billing records from the Hunter Water Corporation and a diary study of household water use to understand the water balance at the house.

A manual monitoring programme to collect and analyse water samples from the rainwater tank and household taps commenced in July 2003. The results for total water use and rainwater use from the initial 260 day period of operation in 2003 and 2004 are shown in the following Figure.

 Carrington-House-Five

Initial water balance at the Carrington house during 2003 and 2004.

The above initial monitoring of the dual water supply scheme at Carrington was conducted during a period of drought that has affected most of NSW. Little or no rainfall has been experienced in the majority of water supply catchments whilst rainfall depths have decreased in the urbanised coastal regions. Rainfall records from the Maryville weather station show that 620 mm of rainfall has been experienced during the 260 day monitoring period that represents a 24% reduction in rainfall at that site. In addition, 30% of the entire rainfall depth during the monitoring period occurred during two rainfall events.

During the monitoring period, analysis of readings from the water meters revealed that the total water use at the site was 98 kL. This water use was supplied by 56 kL of mains water and 42 kL of rainwater. The use of the rainwater tanks resulted in a 43% reduction in mains water use during the monitoring period. Comparison to the water billing records for the previous year from the Hunter Water Corporation revealed that a 53% reduction in mains water use was experienced.

The house was supplied with rainwater during and after rainfall events. After the stored rainwater was used, household water demand is met with mains water from the trickle top scheme. Average daily water demand in the house during the monitoring period was 0.376 kL/day indicating that the full rainwater tanks can supply total water demand for about 11 days. Analysis of rainfall also revealed that 42 kL of the 50 kL of roof runoff during the monitoring period was utilised for household water use. This is a yield of about 85% from the rainwater catchment. Only 15% (7 kL) of roof runoff from the rainwater supply catchment has overflowed from the rainwater tanks.

The combination of small rainwater tanks and a mains water trickle top up system has created an efficient dual water supply scheme. This is due to the drawdown of the rainwater storages between rainfall events.

A design philosophy for rainwater tanks used in a dual water supply scheme is highlighted here: the designer should aim to maximise drawdown of the rainwater tank between storm events whilst maximising yield from the roof catchment.

Replacement of the 4.5 kg capacity top loading washing machine with the 4A rated front loading washing machine with a 7 kg capacity has resulted in reducing water use per washing load from 114 litres to 58 litres. This was a 14% reduction in total water use in the house. However the 4A rated washing machine was installed in January 2004 therefore the impact of the washing machine on the reduction in average water use during the monitoring period is a 4% reduction in water use. The remaining 6% additional reduction in water use is due to the pump supplying water at a lower flow rate and pressure to the house.

The 4A rated washing machine cost $940 to purchase. In addition to reducing water use, replacing the old top loading washing with the 4A rated washing machine reduced the frequency of clothes washing and minimised use of washing powder. This reduced the expense of purchasing washing powders from $7.45/week to $3.66/week.

A diary study of water use was combined with the meter readings to determine the water use categories in the household for the period following the installation of the 4A rated washing machine. The results are shown below.

Carrington-House-Six

Categorisation of water use at the Carrington house

The highest water use category in the house is for showering (41%) and the lowest water use is at the kitchen sink (5%). The combined water use of laundry, toilet, garden, cleaning and shower is 91% of total water use in the inner city house.

Quality of rainwater

Carrington is an inner city suburb located in a dockland area within the industrial area of Newcastle. The house is situated close to roads with high traffic loads and trees containing birds overhang the house roof. Roof runoff at the site was expected to contain a variety of contaminants. A first flush device has not been installed and a small 125 litre capacity storage hot water service that is set at a temperature of 60°C operates in the house.

Previous studies conducted by the author have established that the rainwater treatment train, including the roof, tanks, pump and hot water service can remove the majority of contaminants from rainwater. A number of processes act to improve the quality of stored rainwater including flocculation, settlement and bio-reaction. In addition both instantaneous and storage hot water services have observed to remove bacteria from rainwater.

The water quality monitoring programme at the Carrington house includes sampling and analysis of water from three different locations in the tank water column (top, middle and bottom), at the cold water tap and at the hot water tap in the kitchen. Preliminary average water quality results are compared to the results from the roof and within the rainwater tank are presented in the following Table.

Cold water quality at the tap exceeded the guidelines for Faecal and Total Coliforms. Water quality in the rainwater tank, cold and hot water taps was also found to be compliant will the drinking water guidelines for metal, physical and chemical parameters including lead, iron, zinc, ammonia, nitrates, dissolved and suspended solids.

The significance of exceeding drinking water guidelines for Coliform bacteria in rainwater supplies is unknown. Total Coliforms are no longer recognised as being a suitable indicator of faecal contamination of water or as having any relevance to health. Even though many studies have found Coliform bacteria in rainwater tanks and 3 million Australians rely on rainwater for drinking water supply there are no health epidemics attributed to rainwater tanks. Indeed only a small number of reported health concerns have been attributed to rainwater tanks in Australia. Notably the epidemiological study by Heyworth found that drinking rainwater posed a lesser health risk than drinking mains water in Adelaide.

Microbial water quality results at various locations

Location

E. Coli

(CFU/100 ml)

Total Coliform

(CFU/100 ml)

Pseudomonas Sp.

(CFU/100 ml)

Heterotrophic Plate Count

(CFU/ml)

Roof runoff

135

359

59,604

1,362

Water surface in tank

108

1,050

3,100

1,050

Medium depth in the tank

34

900

780

427

Bottom of the tank

55

862

4,060

1,252

Cold water tap

<1

200

412

76

Hot water tap

0

2

<1

<1

Australian Drinking Water Guidelines

0

0

American Drinking Water Guidelines

0

0

200

Energy

Electricity use at the Carrington house was extracted from quarterly billing records provided by Energy Australia to determine the impact of using rainwater supplied by a pump. The cyclic nature of electricity use at the Carrington house includes higher use in the winter for heating and lower use in the summer. An increased use of a clothes dryer, installation of an air conditioner and the 4A rated washing machine has partially obscured the impact of the rainwater pump on electricity use. The installation of the air conditioner has significantly increased the use of electricity during the summer period.

Using information provided by the pump manufacturer it is estimated that 0.78 kWh of electricity is used to supply 1 kL of rainwater to the house. The expected annual electricity use of the pump is 123 kWh at an annual cost of $27. However, long term monitoring of electricity billing records for the house show a 12% reduction of use of electricity following the installation of the rainwater tanks and the front loading clothes washer. This is a reduction in electricity use from 15 kWh/day to 13 kWh/day.

Long term performance of the rainwater tanks and the front loading clothes washer

The performance of the rainwater tanks and the front loading clothes washer at the Carrington house was confirmed by a decade of water billing records from Hunter Water Corporation from July 2003 to July 2013 as shown in the following Table.

Water balance results from long term monitoring of the Carrington dual water supply scheme

Category

Total

Mains water

Rainwater

Demand management

Water supply (kL/annum)

204

46

126

32

Proportion of water demand (%)

100

23

62

16

 

The rainwater tanks produced a 62% reduction (126 kL/annum) in mains water demand and demand management processes resulted in a 16% reduction in mains water demand. The demand management response was achieved by the low pressure/flow rate delivered by the pump and the front loading washing machine that delivered mains water savings of 4% (8 kL/annum) and 12% (24 kL/annum) respectively. The long term performance of the rainwater tanks and the front loading clothes washer revealed a 78% reduction in mains water demand (158 kL/annum).

Economic results

The long term performance of the rainwater tanks and the front loading clothes washer were included in an economic analysis to determine the lifecycle costs of the integrated water cycle management system. The analysis used the following assumptions

  • A real discount rate of 5%.
  • The installation costs of the rainwater supply scheme and the front loading clothes washer were $2,350 and $940 respectively.
  • The expected useful life of the rainwater tanks, pump and the clothes wahser was 25 years, 10 years and 10 years respectively
  • The expected replacement costs of the rainwater tanks, pump and clothes washer were $756, $389 and $940 respectively
  • The operating cost of the rainwater tank scheme was assumed to be $27/annum
  •  Use of the front loading clothes washer created a saving in use of detergents of $3.66/week
    • The rainwater supply scheme and the front loading clothes washer produced mains water savings of 126 kL/annum and 32 kL/annum respectively

The present values and cost of water supply (in year 2003 dollars) from the rainwater supply scheme, the front loading clothes washer and the combined strategy was derived using a 50 year planning horizon and is shown below.

Economic analysis of the dual water supply scenarios

Scenario

Present value ($)

Cost of supply ($/kL)

Rainwater tanks

3,673 (cost)

0.58 (cost)

4A washing machine

1,282 (benefit)

0.80 (benefit)

Combined system

1,027 (cost)

0.13 (cost)

The analysis reveals that the costs of rainwater supply and demand management are substantially less that the costs of mains water supply. Note that the avoided cost of not having to purchase mains water has not been counted and the full costs of the clothes washer have been counted – thus the economic results are conservative.

Note that the three strategies considered in the analysis were less expensive than purchasing mains water. Another planning consideration is also obvious: one may be tempted to select the 4A rated washing machine in preference to the rainwater strategy because of the higher lifecycle benefits and lower installation costs.

The resulting small annual mains water saving of only 32 kL/annum from that choice highlights the need to mindful of primary objectives such as reducing mains water consumption in selection of strategies. The most important finding is that the combination of the rainwater tanks and the front loading clothes washer creates considerable lifecycle benefits, mains water savings and stormwater management benefits in spite of the installation costs.

Note that this economic analysis has under-estimated the benefits of installing the rainwater tanks and the front loading clothes washer by not considering the benefits derived from reducing water demand on water supply and security infrastructure, and reducing the requirement for stormwater infrastructure. 

New rainwater and energy systems

The Carrington house was renovated during 2012. This activity provided the opportunity to modernise the rainwater supply system and to include solar panels to supplement electricity supply. A new slimline rainwater tank with a capacity of 5,000 Litres, a submerged pump and an under sink water filter was installed (cost of $5,940). Note that the new pump system includes a mains water bypass that operates when rainwater depths in the tank are low. In addition, eight 190 Watt solar panels were installed to reduce the net electricity use at the house (cost of $2,171). The new slimline rainwater tank is shown below.

Carrington-House-Seven

The new slimline rainwater tank

Inclusion of the solar panels has reduced electricity balance of the house by 2,978 kWh/year (8 kWh/day). This is a 61% reduction in the net electricity use of the house from 4,961 kWh/year (13 kWh/day) to 5 kWh/day. Water use at the Carrington house has reduced by 78% to 0.13 kL/day.

Carrington-House-One

The Carrington House with solar panels and rainwater tanks (circa 2013)

About
Dr Peter Coombes

Dr Coombes has spent more than 30 years dedicated to the development of systems understanding of the urban, rural and natural water cycles with a view to finding optimum solutions for the sustainable use of ecosystem services, provision of infrastructure and urban planning.

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