Rainwater harvesting storage tanks: Part 1

By Water Research Commission

The storage container (cistern, tank) is often the most visible or recognisable component of a rainwater harvesting (RWH) system where the captured rainwater is diverted to and stored for later use.

The main purpose of the storage tank is to store water that is safe to use, while preventing its access to children or animals. There are several topics related to storage containers and you should go through each before making a decision on purchasing one.

There are a number of different RWH systems available with a range of features, depending on the manufacturer. These systems can be grouped into three basic types of RWH systems:

  • water collected in storage tank(s) and pumped directly to points of use;
  • water collected in storage tank(s) and fed by gravity to points of use; and
  • water collected in storage tank(s), pumped to an elevated cistern and fed by gravity to the points of use.

All the systems listed above have a reservoir that is used to store rainwater harvested from roof catchments referred to as a rainwater storage tank.

Applicable standards and guidelines for rainwater storage tanks

The applicable standards found in the South African National Standards are listed in Table 5.1.

Table 5.1: Standards and codes applicable to rainwater storage tanks

Table 5.1Tank capacity

As a rule of thumb, the larger the tank, the greater the volume of rainwater that can be collected and stored during rainfall events (collection efficiency). However, this is true only up to a certain point — after which other factors, such as local rainfall patterns, roof catchment area, and rainwater demand, will limit the amount of rainfall that can be collected and utilised by the system.

Thus, for any RWH system with a given roof catchment area, rainwater demands, and local rainfall patterns, the storage capacity of the tank can be described as either:

  • Too small — Much of the collected rainwater overflows during rainfall events. Significant improvements in collection efficiency can be achieved with minor incremental increases in storage volume.
  • Optimum — Rainwater tanks in this range provide the best balance between collection efficiency of the RWH system and minimising its size and cost.
  • Too large — Rainwater tanks in this range rarely fill to capacity. A smaller tank can be used without a significant drop in the collection efficiency of the RWH system. An oversized rainwater storage tank, however, may be desirable if stormwater management is a strong driver for installing an RWH system.

Plastic TanksTank sizing

The correct sizing of a RWH tank is important in order to avoid extra costs incurred when the tank is oversized and for avoiding low efficiency when it is undersized. The effectiveness of a RWH system depends on factors such as the catchment size, rainfall variability, temporal and spatial variation, the water demand, and the tank size.

Rainwater systems can be set up with various functions for time and purpose of use, from consumption to agricultural use. Credit: FBLN Malawi

Tank sizes have an important role since they dictate the maximum amount of water that gets stored. However, there must be other attributes that must be considered:

Catchment area

The size and nature of the catchment area determine the amount of rainfall that can be harvested. The run-off coefficient is defined as a dimensionless value that estimates the portion of rainfall that becomes run-off, taking into account losses due to spillage, leakage, catchment surface wetting, and evaporation. According to the performance of a RWH system, it is sensitive to the run-off coefficient value only for small tank sizes.

Rainfall variability

The efficiency of a RWH system is largely affected by the distribution patterns of rainfall. The optimum size of RWH is likely to differ in South Africa’s five rainfall regions; all year, winter, early summer, mid-summer, late summer, and very late summer regions

Water demand

It is the actual volume of water extracted from the tank for various uses at a given time. When optimising RWH system, scholars usually use a single figure to express the potable water demand. This is not a reflection of the actual water demand, which varies throughout the year. Moreover, no good correlation has been established between the tank capacity of a RWH system and the fixed daily potable water demand. Therefore, a rainwater tank must never be sized according to potable water demand only.


Reliability is the probability that a system can meet the expected demand. It can be divided into two types: time-based reliability and volumetric reliability. The former is the probability that a reservoir will be able to meet a certain demand on a specific time interval, while the latter is the ratio of the amount of water supplied to the total water demand during the simulation period. The reliability of the rainwater tank is very important for domestic water conservation as it indicates the ability of the tank to satisfy the demand of the household on a given day.

Temporal resolution

Time intervals affect the quality of the simulation results. The use of monthly rainfall data to calculate the storage of a RWH tank results in an underestimation of the required storage capacity because it overlooks the temporal distribution of rainfall. Hourly or daily time series are said to provide a more accurate simulation of system performance than monthly time intervals. However, daily time steps have found more preference in sizing RWH tanks because sub-daily time series data are generally unavailable.

Tank sizes

The storage tank is the most expensive component of a RWH system. The storage capacity of the tank dictates the maximum amount of water that can be stored. Most RWH systems in the country are installed using a rule of thumb law that does not involve proper sizing; as a result, the RWH tanks installed are either oversized or undersized tanks.

Storage tank optimisation is an important but often overlooked design step of RWH systems. Before the most cost-effective proportion of the area of a roof to be used for harvesting the water that will be stored in a tank can be determined, the relationship between roof area and storage capacity must be outlined, along with the major parameters such as release rule and reliability, interval used in simulation, record length of rainfall data, and run-off coefficient. The release rule can either be yield before spillage (YBS) or yield after spillage (YAS). In the YBS rule, the water is abstracted for use before the inflow. This leads to an underestimation of the required storage volume. The opposite applies with the YAS rule, which is more conservative and therefore usually preferred.

August 2019

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