Saturday, May 28, 2011

Best Combination? Rainwater Harvesting + Desalination

Note: Information provided are examples for discussion. Though it is based on a real and factual example, in the interest of privacy more details are withheld.

There is a development that needs to supply all of it's own water. It presently has a system that provides 220,000 litres of water per day, or equivalent to supply 1,000 people 220 litres of water per day. During the more rainy months they have a problem that the resort floods, producing problems in terms of loss of revenue.

The resort was looking to increase the water supply to 350 litres per person per day, this or what represents an increase of 130,000 litres per day. The first option that they had considered was to install another desalination plant or increase the desalination plant's capacity. We will analyse the costs involved in this. The options considered and why.

We were approached was because as there was abundant rain, so much so that there was flooding, that water harvesting would be an option.

The chart below shows the average rainfall pattern for this location.

The Different Options
As we were looking at different technologies, these mainly break down in three simple groups. An exclusive desalination option, an exclusive water harvesting option and a combined harvesting and desalination option. We will analyse each situation individually. I also provide a link to the spreadsheet where the calculations and graphs are obtained from.

Exclusive Desalination Option
We started with our analysis. The base information that we had of the initial expenditure for the desalination plant would permit us to extrapolate the further costs. The initial cost of the desalination was $1.2 million, so the increase of about 50% in capacity would imply an additional cost of  $600,000 approximately. The operating cost of the the plant was $5.5 per 1,000 litres. This would imply that apart from the Capital Expenditure for the plant to supply the complete requirement of 350,000 litres a day to make the system work would mean an Operating Expenditure of more the $700,000. This would be our base for comparison.

Exclusive Rainwater/Stormwater Harvesting Option
We were approached as we are know for the effectiveness and flexibility of our water harvesting option, which apart from being very flexible are also scalable, which allows them to be used in large civil infrastructure works as a viable and cost effective option.

Considering the rain data, even though there was a fairly abundant amount of water, more than 2,000 mm of rainfall a year, we immediately found that the specific demand of water would exceed even that amount of copious water, in addition to that, the catchment area placed serious limitations to what was possible. In any case the analysis had to be done. It was determined that the required catchment area would be in the vicinity of 160,000 square metres. To get an idea of what we are talking about in size, a football field has an area of about 10,000 square metres this implies, that we are talking about 16 full size football fields, which is a size that is not really available for catchment in this case. In addition to this it would need a rather large tank, a minimum size of 10,000,000 litres would be required. At an installed cost of $0.40 per litre, this would represent a total Capital Expenditure of $4,000,000. Though with this option the Operating Expenditure is reduced to practically nothing as all that is required are small pumps, to lift the water from the bottom of the tank at 1 metre of depth.

In any case it is interesting to make an analysis and see over a long period of time, if it were possible to have enough catchment area, what would be the savings. Below is the cost comparison that shows, the savings over a 10 year period.

Cost Analysis comparison of only Desalination vs. only Harvesting.

Project Cost Yearly Forecast
Only desalination
Only harvest
CAPEX $600,000.00
CAPEX $4,000,000.00
OPEX $702,625.00
OPEX $0.00

OPEX Saving $702,625.00
Current OPEX $441,650.00
Install cost per litre $0.40
OPEX Increase $260,975.00
Recovery time (years) 4.84
10 year comparison

CAPEX + OPEX $7,626,250.00
CAPEX + OPEX $4,000,000.00
Total saving $3,626,250.00

Saving Ratio 52.45%

Combined Harvesting and Desalination Option
The previous analysis is a bit of a futile exercise, even though it serves to show the long term benefits. What should be done is an analysis that is in fact feasible. The presence of the existing desalination plant is an advantage in terms of supply water during the drier months, in this case from July to November as the initial rain fall chart shows.

Whenever the storage tank falls bellow a certain level then the desalination units enters operation supply the necessary demand of water.

The model to be used follows the flow chart shown below.

Cost Analysis comparison of combined Harvesting with Desalination.

Project Cost Yearly Forecast
Only desal
Combined harvest + desal
CAPEX $600,000.00
CAPEX $1,600,000.00
OPEX $702,625.00
OPEX $156,266.00

OPEX Saving $546,359.00
Current OPEX $441,650.00
Install cost per litre $0.40
OPEX Increase $260,975.00
Recovery time (years) 1.83
10 year comparison

CAPEX + OPEX $7,626,250.00
CAPEX + OPEX $3,162,660.00
Total saving $4,463,590.00

Ratio 41.47%

The graph of how this system behaves and the amount of desalinated water that is provided is shown in the graph below. 
The spreadsheets, data and graphs for the above situation can be seen here:

From the above cost analysis it can easily be seen that the best solution is a combination of Rain/Stormwater Harvesting used together with a desalination unit order to provide water for the drier months. The higher capital expenditure that the installation of a large water storage tank implies is quickly recovered in less than 2 years and the saving over a period of 10 years are quite considerable amounting to just under $4.5 million, or $450 thousand per year. In addition to this as there is a large amount of water that is being captured and managed underground, the significant yearly flooding problems that the resort has would be averted.

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