Rainwater harvesting: water quality

Rainwater harvesting: water quality

By Water Research Commission

Guidelines for Human Settlement, Planning and Design placed a highly restrictive guideline on the use of rainwater. They suggested that rainwater should be considered as a supplementary supply for non-potable use since it could pose a health risk.

Numerous studies have shown the potable use of rainwater as long as contamination during collection is minimised and efficient treatment of water post harvesting occurs. The ambiguity with regard to the use of rainwater would be avoided if there was in existence established rainwater harvesting guidelines that ensured harvested rainwater quality for its various uses.

Applicable standards and guidelines

The quality of rainwater is assessed against drinking water guidelines as an interim solution to the lack of clear guidelines that govern the use and quality of rainwater. The South African Water Quality Guidelines developed by the Department of Water and Sanitation (DWAF, 1996) states exactly the water quality requirements required by the four (domestic, recreational, agricultural, and industrial) classes of water use (DWAF, 1996).

Table 1: Definition of water quality terms
Page27 Table

However, these guidelines are unable to address other components of the rainwater harvesting process that also have an effect on the ultimate quality of the water.

It is critical that rainwater that will be used for potable use within the public water distribution system meets the same criteria as that of other sources of water used in the water distribution system. Once source water (in this case rainwater) has been shown to be of a potable standard for use in a public distribution system, there are other inbuilt mechanisms within a distribution system that then ensure that the quality of water that comes out of the tap is not compromised.

One such system is the Blue Drop Certification Programme, which is a nationwide programme designed with a list of regulatory criteria, which when implemented in conjunction with South African Drinking Water Guidelines (DWAF, 1996), ensures that excellent drinking water is produced by a given water distribution system. The Blue Drop Certification Programme is a means to regulation, which was designed and implemented with the core objective of safeguarding tap-water quality management.

Recommended minimum water guidelines for rainwater harvesting

In order to maintain the quality of harvested rainwater, the minimum water guidelines developed should address the six basic components that make up a rainwater harvesting system:

  1. Catchment surface, environment, atmospheric conditions.
  2. Gutters and downspouts through which water moves from the roof into the tank.
  3. First flush diverters, leaf screens and roof washers — remove large pieces of debris and dust before the harvested water goes into the tank.
  4. Storage tank(s).
  5. Controls and pumps — determine the level of water in the tank, minimise air gaps, and prevent backflow.
  6. Filtration, treatment, and disinfection — depends on the end use of the water. For potable use, filtration, treatment, and disinfection need to be used. While for non-potable use, filtration and treatment might be sufficient.

Guidelines for catchment surfaces, gutters, downspouts, storage tanks, and first flush diverters

Improving the roof run-off quality of rainwater minimises the contamination levels that could be subsequently observed in stored rainwater. Removal of vegetation from around the catchment area and the installation of leaf guards prevent large debris from entering the storage tank.

Birds, rodents, and small animals should also be monitored so that they do not wander around the catchment area, thereby introducing microorganisms into the run-off rainwater, primarily through their faecal droppings.

The material the roof or catchment area is made of, has an influence on the quality of the water harvested. A metal roof made out of powder-coated steel is a smooth surface that allows efficient water run-off from the roof and furthermore, it is highly resistant to corrosion. Slate is also ideal as a catchment surface; however, compared to metal, it is costly. Irrespective of the type of roofing material used, testing of the run-off must be conducted to determine the microbial and chemical quality of water obtained from the various roofing materials.

Guttering systems are often designed to maximise the amount of water that can be harvested. Guttering is usually made of polyvinylchloride (PVC), vinyl, and seamless aluminium. These systems can also be critical control points when it comes to the introduction of contamination into stored rainwater.

Some of the designs of guttering systems that minimise contamination include the use of a primary filter such as coarse screen or fine screen. A leaf screen is a type of coarse screen that is able to prevent larger particulate matter from entering the tank. For rainwater harvesting systems installed in areas with dense vegetation, it should be a requirement to have one of these types of screens.

The fine screen, such as a strainer bucket, sits between the delivery pipe and the tank, preventing small insects and possibly rodents from having access to the storage tank. The sizes of these filters can be regulated depending on what type of particulate matter needs to be removed. The first litres of rainwater that are harvested should be drained off through the use of first flush diverters. This reduces the contamination that can arise from a dirty roof or catchment surface area. The first flush of water need not be wasted and can be diverted to an area with vegetation and plant life that would require watering, with the exclusion of small gardens and anything grown for ingestion.

There are various types of first flush diverters that can be used, with the simplest ones being made out of a PVC standpipe. Various factors need to be taken into consideration when determining how much water must be diverted initially. These factors include the duration between rainfall events, the slope and smoothness of the catchment area, as well as the intensity of the rainfall event. In general, first flush diverters should be able to divert at least 38ℓ of water per 92m2 of roof area.

The storage tank is another critical control point in the design of a rainwater harvesting system. Although there are a variety of options when it comes to choosing a storage tank, there are certain characteristics that all tanks must have in order to optimise the quality of rainwater. All tanks must have lids and be covered at all times, and preferably with vents, which hamper mosquito breeding.

To block sunlight, thereby preventing algal growth, tanks must be opaque. If the tank is not opaque at the time of purchase, it should be painted. With continual use of the tank, sediment build up and biofilm formation might occur in the tank. It is necessary to have easy access to the tank in order to clean and perform the necessary maintenance, thereby maintaining the quality of stored rainwater. The location of the tank can have negative impacts on the quality of stored water, especially if the tank is stored underground. Tanks that are stored underground should be located 15 meters away from animal breeding grounds and any wastewater treatment-related activity.

Non-potable use of rainwater (indoor)

Non-potable uses of rainwater include flushing toilets, car washes, and also for laundry purposes. Guidelines governing microbial contamination in non-potable water are not as stringent as compared to that of potable water. However, total coliform and faecal coliform levels are still defined for non-potable water in order to ensure a general microbial quality of the water. Total coliform levels in non-potable water should be <500cfu/100mℓ and faecal coliform levels <100cfu/100mℓ (cfu = colony forming units).

Potable use of rainwater (household level)

General drinking water guidelines apply to the potable use of rainwater as well. It is critical that potable water does not contain any microbiological contaminants after undergoing treatment. There should be zero amounts of total coliforms, faecal coliforms, viruses, and protozoan cysts (Giardia sp. and Cryptosporidium sp.) found in potable rainwater.

The turbidity of water is an indicator of the amount of organic and inorganic material, plankton, clay, and silt in a particular water sample. A high turbidity level of a particular water sample is often linked with a greater chance of microbial contamination. Turbidity often interferes with the water disinfection process and hence it is preferable to have the lowest turbidity as possible for drinking water, preferably <1 Nephelometric Turbidity Units (NTU).

Potable use of rainwater (water distribution systems)

It is critical that rainwater that will be used for potable use within the public water distribution system meets the same criteria as that of other sources of water used in the water distribution system. Once source water (in this case rainwater) has been shown to be of a potable standard for use in a public distribution system, there are other inbuilt mechanisms within a distribution system that then ensure that the quality of water that comes out of the tap is not compromised.

One such system is the Blue Drop Certification Programme, which is a nationwide programme designed with a list of regulatory criteria, which, when implemented in conjunction with South African Drinking Water Guidelines (DWAF, 1996), ensures that excellent drinking water is produced by a given water distribution system.

Table 2: Rainwater use in public water distribution systems: suggested minimum water quality guidelines for potable and non-potable use.
Table 3.4 Rainwater use in Public Water Distribution Systems


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