Sunday, May 15, 2011

Reiterative Reuse Water Harvesting Model

In my experience in working with water reuse one of the most common questions asked is how to appropriately size a tank system for water harvesting. Related to this there is a fair amount of information, and the costing is a simple consequence of the required volume. However paradoxically this modelling only considers only the one time use of the water, which is then "wasted" after being consumed.

A more efficient and cost effective possibility is available:
Recently I have been working on a rather large project, there is a "significant" requirement of water considering the source is just harvested rainwater and stormwater, that is 5 million litres of water a day. To propose a traditional system that considers only the one time use of water would not be feasible. For the economic and technical proposal a reiterative reuse of harvested water is necessary, and as such a new model had to be developed.

The above flow chart diagram shows a much more efficient model where water once consumed is reused reiteratively. The processing for reuse of the water use technology that is passive, practically does not require external energy and has very low maintenance, if any. The quality of the influent is one that can be considered as a grey water, and the resultant purified water quality is sufficiently high for all uses except potability, however this can also be achieved with relatively simple polishing of the water.

Source data webpage
The above graph shows the result of this modelling. However to arrive to a graph that has any meaning it is necessary to have to have previously done the relevant calculations. The spreadsheet for this can be seen  here: Reiterative Reuse Water Harvesting Model. This spreadsheet is modelled taking into account the following aspects.

Design Considerations
A prime and original consideration in terms of the modelling is made assuming the following situations:

Dual type tank system
The tank system is composed of 2 main types of tanks, a harvesting tank, that captures directly the rainfall, and a reuse tank, that has the double purpose of collecting harvested water as well the system that recaptures the water that has been consumed.

Reuse percentage
For practical effects the percentage of reuse is estimated at approximately about 80%, this can be defined to whatever is desired.

Tank Use Prioritization
The water from the reuse tank, for the effects of the model, is given priority in terms of consumption, once the capacity of the reuse tank is consumed, then the water from the harvest tank begins to be consumed. The prioritization of the reuse tank, is recommendable because it is desirable that that the water that is being reused is of less quality that the virgin harvested rainwater. Once water has received some degree of contamination it is recommendable that it be in movement and aerated as much as possible, in this case it receives this action in the course of its normal use. To leave contaminated water to sit a rest, will cause it to deteriorate in quality, entering a cycle of stagnation.

Modelling criteria
As the daily consumption is to be a considerable amount and maximum efficiency of use has to be obtained from the system. As such the modelling of the water level in the tank system has to be done on a daily basis. For this not only the data of the monthly rainfall has to be taken in consideration but also the frequency of that rainfall. When the system contains water to supply the full capacity that is required it will do so, however once the contents of the system are depleted, so as not to be able to provide the complete required capacity, what is available will be consumed, and also recycled in the percentage amount that has been defined, and will re-enter the system, making it available again for use the following day.

Modelling Resolution
As the modelling has a resolution to individually discrete days, the average frequency of rainy days is rounded up and then the amount of monthly rainfall is evenly distributed and averaged to evenly spaced days in the calendar month. The rounding up of frequency does not affect the total quantity of rainfall. This way an average quantity of rainfall that is captured per individual precipitation events can be estimated and then considered to be available for consumption.

Stabilization Period
The model is also performed for a period of 2 years to allow it stabilize. This is especially relevant in locations where rainfall is very seasonal, such as a monsoonal environment. It may however be the case when requirements are such that it is not possible to arrive to a stabilization, considering that complete provision of water is made available, such is what seen in 2 of the 3 examples of the modelling. In such case the recycled water will serve the purpose of being a complementary source. As such, as is normal available budget has much to do with what can be provided.

Controllable variables
Variables which are defined or controlled by the design engineer are the capacity of the tanks, as the water harvested or reused cannot physically exceed this volume.

Consumption defined
The consumption also can be defined by the design engineer. In relation to this to simplify the modelling a single daily consumption figure was chosen, however as the modelling has a daily resolution this figure can also be adjusted to increase consumption in periods of more abundance of rainfall and then reduced for periods of more scarcity. For demonstration purposes a suitable figure has been chosen that gives a balance of a reasonably high consumption, as much as possible while, while maintaining a minimum quantity of water in the system at all times.

The most interesting situation with this reiterative reuse model, that we have specifically studied, is that on a yearly basis that amount of water that is consumed through the recycling of harvested water is 3 times the amount of water that is used if the harvested water was consumed only once. This means it is possible to provide a client with a system which costs, practically a quarter, considering the initial use of water, of what the system would cost with out reuse. 

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