Rainwater Harvesting Is The Collection Of Rainwater Environmental Sciences Essay

Rainwater harvesting is becoming an increasingly important factor in urban water management strategies in the UK. It is highlighted in numerous government documents, such as the Building Research Establishment’s Environmental Assessment Method (2) and is stated to have an important part to play in Future Water (3) which is the government’s water strategy document (4). The Environment Agency advocates a ‘twin track’ approach to manage water demand, of which the installation of a RWH system is an option as a substitute for mains water (1). In October 2012, Schedule 3 of the Flood and Water Management Act 2010 was instated. This requires sustainable drainage of surface water to be included in developments that require planning approval or have drainage implications. Taking these facts into account the progress of RWH is going to be significantly more important in the near future.

Types of System

There are three main types of rainwater harvesting system:

1.”water collected in storage tank(s) and pumped directly to points of use [Figure 1 (5) shows a typical layout];

2.water collected in storage tank(s) and fed by gravity to points of use; and

3.water collected in storage tank(s), pumped to an elevated cistern and fed by gravity to the points of use” (1).

Figure 1 – Schematic of a typical contemporary domestic RWH system (5) adapted from (6).

The first option is viable in a domestic building as the water does not have to be pumped a large distance wherever the tank or outlet are situated. For a large office building the second or third options would be more efficient as on average there will be a greater vertical distance to pump the water than in a domestic dwelling if the storage tank is located at ground level. With these options a header tank may be used in the roof area of the building in conjunction with a storage tank and the water is allowed to move under gravity to the points of use (7). More than one storage tank may be used if the building is large enough to necessitate it.

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Calculation for domestic use

The Environment Agency suggests a shorthand method for domestic dwellings, shown below, to determine the storage tank size:

Size of storage tank (m3) = Annual rainfall (mm) x effective collection area (m2) x drainage coefficient (%) x filter efficiency (%) x 0.05

Where;

Annual rainfall (mm) is the average yearly rainfall;

Effective collection area is the area of the roof;

Drainage coefficient depends on the roof type, see below;

Filter efficiency is specified by the manufacturer, commonly 90%.

Drainage Coefficients:

Pitched roof – 0.9

Pitched roof with riles – 0.8

Flat roof with gravel layer – 0.8

For non-domestic buildings a larger RWH system is required, so it is suggested to consult the British Standard Code of Practice 8515:2009 – Rainwater Harvesting Systems (8) for a more accurate design. The code of practice gives a more detailed approach to include more accurate demand data where demand is variable throughout the year, as well as in-depth rainfall data, such as the daily rainfall in the area for the past 5 years (9). These values are then used in a computer model which calculates the systems behaviour over time. Results attained via this method are much more accurate than the ‘rule of thumb’ based approach suggested by the Environment Agency for domestic RWH systems.

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Cost-Benefits

Considerations

The main consideration for installing a rainwater harvesting system will be the savings that can be achieved. Any potential savings need to be assessed using a number of factors: demand for non-potable water, the amount of rainwater it is possible to collect, whether or not the property is on a water meter, how much the system costs to maintain and the energy cost associated with the pumping of harvested rainwater. It is advisable to have a meter, which the vast majority of commercial and industrial customers do, because without one there will be no savings using a RWH system. Savings are likely to be higher in large commercial buildings because they tend to have a larger roof catchment area, as well as a greater demand for the non-potable water than private property (1). A method has been developed by (Ward, et al., 2010b) (4) to estimate energy costs associated with rainwater pumping within the system. It assumes the RWH system used has a header tank and uses volumes of harvested rainwater with pump parameters to estimate the energy used by the pumps. Ultimately, the financial feasibility of a RWH system will be assessed by calculating the basic payback period, after first determining the financial benefit-cost ratio as these are well-established methods of assessing RWH systems (5). The financial benefit-cost ratio is calculated using the costs and benefits associated with a given system. The payback period is then determined by summing the costs and benefits over the system’s expected lifespan and then dividing the total costs over total benefits.

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UK rainwater harvesting system suppliers commonly use short-hand methods when designing storage tanks for their systems (10). There are, however, more detailed models better suited to the design of RWH systems and their performance and these are researched in Rainwater Harvesting: Model based design-evaluation (Ward, et al., 2010).

A more comprehensive detailing of modelling methods has been produced by (Roebuck, 2008b) (5), Roebuck’s method is similar to the formula given above by the Environment Agency. Roebuck shows that previous reports showing the cost-benefits of rainwater harvesting systems are unreliable and have not been extensively investigated. To properly determine the feasibility of a rainwater harvesting system all possible costs must all be incorporated into the financial analysis. The author also postulates that mains water prices have been increasing (11) and so should be estimated as continuing to increase in the foreseeable future.

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Calculation for non-domestic use

The full list of financial variables that should be considered when designing a rainwater harvesting system are as follows:

Investment Capital for the full system;

Pump replacement;

Electricity (for pump);

Maintenance;

Mains water supply charge.

To estimate the costs associated with RWH systems there is ample information supplied by best practice manuals and manufacturers. However, some of this information can be vague or subject to large ranges, for example the discount rates vary from 3.5% (local authorities) to 15% (homeowners) (12). The storage tank prices range from ~£2000 for a 1.5m3 tank to ~£5000 for a 15m3 tank (5). Future water and electricity prices can be extrapolated to estimate future costs, as they have been steadily rising since the water industry was privatised in 1989 (11). Maintenance prices are also given by manufacturers (5).

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Conclusion

The results in (Roebuck,2008b) (5) suggest that rainwater harvesting systems are not cost-effective as the long term difference tended to be a deficit equal to the initial capital invested in the RWH system. This means that an RWH system would only be worthwhile if the initial stakeholder did not have to pay the cost of installing the system themselves. RWH may still be worthwhile if the funds for installation are not met by the stakeholder but rather are provided through government investment. However, the report only takes into account RWH systems on a single-building scale; therefore on a large scale with a number of buildings the overall savings may be sufficient to return the investment. It could be shown that on a large scale site a RWH system may be worthwhile even if the funds for initial investment have to be met by the stakeholder.