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HEALTH AND SANITATION
Rainwater harvesting:
Financial Assessment
This is the final section in our series on Rainwater harvesting.
By Water Research Commission
8.1 INTRODUCTION
For the financial assessment, the most appropriate
approach is the whole life costing (Roebuck et
al., 2011) which provides a robust rationale for
asset management as it considers the costs and
performance over extended periods of time. It is also
able to take operation and maintenance costs into
account at appropriate times. The main components
of the financial model developed by Roebuck and
Ashley (2007) are presented in Figure 8.1 and
Figure 8.2, both for the mains-only water system
and the RWH system.
The financial variables included in the whole life
costing methodology are provided in Table 8.1.
8.2 CAPITAL AND DECOMMISSIONING EXPENSES
Although significant, the capital cost of the mains
water supply is the same whether there is an
RWH system or not. It can therefore be ignored.
The cost of purchasing and installing the RWH
equipment can also be significant, especially in the
case of dual systems. This cost cannot be ignored.
Decommissioning of the system only occurs at the
end of its lifespan.
8.3 MAINTENANCE EXPENSES
Figure 8.1: Schematic representation of mains-only financial model (Roebuck and Ashley, 2007).
Maintenance costs comprises of costs associated
with the scheduled cleaning of system components,
the maintenance of the different components of
the system and the replacements of components
that have reached their service lives (such as UV
units, valves, taps, and pipework). One can argue
that the cost of cleaning the catchment surface by
hosing or sweeping and, if applicable, trimming
overhanging tree branches/foliage, fixing roof,
gutters or downspout should not be added to the
RWH system.
8.4 OPERATING EXPENSES
The operating cost of the UV units and the pump
is related to the energy usage, which in turn is
dependent on their operating time and power rating.
The replacement cost of the various filters is part of
the maintenance expense.
8.5 WATER AND SEWERAGE CHARGES
Figure 8.2: Schematic representation of the RWH financial model (Roebuck and Ashley, 2007).
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Municipal charges have a step-wise structure
called block tariffs. The charge is set per unit (for
example cubic meters of kilo watts) of water or
electricity consumed and remains constant for a
certain quantity of consumption (first block). As the
water or electricity use increases, the tariff shifts to
the next block of consumption and so on for each
block of consumption until the highest one. Cross
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September 2019 Volume 25 I Number 7