Rainwater or greywater?

In the second in a series of articles, Stormsaver's Michael Farnsworth discusses the relative benefits of rainwater harvesting and greywater recycling, and explains the advantages and disadvantages of each method.

There are two main methods of capturing and reusing water that are often confused with each other – rainwater harvesting and greywater recycling.

Rainwater harvesting is the process of collecting and filtering rainfall from the roof of a building. Greywater recycling, on the other hand, recycles wastewater from domestic appliances such as washing machines, baths, showers and sinks, but not usually the kitchen sink, and never the toilet.

Whilst the aim of both technologies is to reduce the reliance on mains water, the methods, results and applications can differ greatly.

With Stormsaver specialising in rainwater harvesting systems, I am understandably biased towards the capture and reuse of rainwater. However, there are some situations where greywater recycling could offer an effective alternative, and I will endeavour to offer an impartial view.

I will focus on the three main factors to be considered when assessing the suitability of either system: the method of water collection and processing; the end result; and the possible applications.

Rainwater harvesting is all about rainfall. This can be collected from any open space on or around a building, typically the roof or car park. The water is then filtered into a storage tank, where it can be pumped on demand through filters, and possibly disinfection, to the point of use. Greywater recycling involves reusing domestic wastewater that is usually sourced from washing machines, baths and sinks, but not the kitchen sink. The wastewater undergoes a series of filtration stages, as well as chemical disinfectant to ensure it is fit for use.

Water contamination

The water can be stored for up to 24 hours before being pumped to the point of use. After this time the water must be sent to the drain, otherwise it can experience a proliferation of bacterial growth that could be detrimental to public health.

The evaporation process of the water cycle naturally purifies rainfall, so normally contaminants within it are picked up during the fall, or when landing on the collection area. Because of this, the type of contaminants found in rainwater harvested from a roof can usually be readily identified and normally fall into a number of given categories.

Although it is possible to collect water at ground level from a drive, car park or roadway, additional filtration is required and sometimes disinfection too. From ground collection, the likelihood is that water will contain petrol, oils, chemicals, animal faeces, road salts, chemical sprays such as pesticides, cleaning materials and minerals from the ground.

Because of the variability of water quality and the difficulty in being able to readily identify those contaminants, I would not recommend collecting from the ground. Having an initially clean water source and being able to identify contaminants reduces the need for filtration and disinfection.

The Stormsaver system provides three stages of particle filtration and some of our systems use UV disinfection as a non-chemical treatment for the water, which provides a way of ensuring the best quality of water is being used.

Clear of the mains

In a properly designed and installed system, the rainwater is clean, clear and odourless, and therefore it is often impossible to identify its source as any different from mains water.

It may, however, contain bacteria, which is why we do not recommend it to be used where the water could be ingested.

Whether it is a rainwater or greywater system, it is critical to ensure the reclaimed water cannot cross-contaminate the mains water supply – the Water Supply Fitting Regulations (1999) and WRAS guidelines are very clear on defining this aspect of a system.

Greywater collected from sinks, baths and showers is generally grey in appearance and is not a source of water that is initially clean. A greywater system is topped up by water that can vary greatly in quality. It is not uncommon for the water to contain detergents, soap, shampoos toothpaste and bleach.

This soup of chemicals is generally mixed with more solid material, such as skin deposits, body fats and hair, the latter of which require a great deal of particle separation as well as chemical disinfection before it’s ready to use.

The most positive attribute for use of greywater is the consistency of the greywater supply. Greywater can provide a regular supply topped up throughout the day, and the greater the use of water the more greywater is generated and therefore saved.

The consistency of rainwater is less predictable. The effects of seasonal variation are mainly overcome in rainwater harvesting systems by increasing the size of the storage reservoir, which can be designed to retain enough water to service a building for up to 21 days.

Systems will be linked to a mains water source that will be automatically activated in periods of low rainfall. Looking at historic rainfall data in the UK, it is common to receive at least one downfall within a 14-day period and therefore a correctly sized system can provide a continuous supply of water, but this does entail storing much greater volumes of water than are required of a greywater system.

Following the installation and commissioning of a rainwater harvesting system, users benefit from comparatively low maintenance, clear water and reduced utility bills.

Those who reuse greywater also benefit from reduced water bills. However, extensive disinfection and more contaminants mean increased maintenance costs and water discolouration. Sometimes, the cost of maintenance can outweigh the financial benefits.

For buildings with a consistent supply of wastewater and limited roof areas such as high-rises and buildings in densely populated areas, greywater recycling is generally the best solution, as a rainwater recovery system requires roof area and a space to store the water.

Topography hurdles

Germany, Australia and Japan are perfect examples of the relative benefits of rainwater harvesting and greywater recycling. Japan’s rainfall generally falls within a very limited timeframe and due to the topography of the landscape discharges quickly.

The dense urban centres mean greywater recycling predominates offering a continuous source of water not dependant on rainfall.

In Japan, the technologies have been developed using biological treatment to treat water in a similar manner to that of the water utilities. Germany’s climate and less densely packed urban centres mean rainwater harvesting features in the majority of both domestic and commercial buildings.

Possibly the most important consideration when specifying a water conservation system is the application: greywater recycling systems can be used mainly in toilet flushing. Greywater is considered by some as not suitable for use for irrigation due to the damage the chemicals can cause to the growth of plants and the potential to release harmful bacteria – greywater for irrigation is outlawed in some US states.

Harvested rainwater can be used for most non-potable functions, including toilet and urinal flushing. Rainwater is also soft and therefore not subject to problems associated with limescale, making it particularly suitable for use in washing machines. The lack of chemicals and contaminants in rainwater make it a great choice for uses such as watering gardens and vehicle washing.

When it comes to rainwater or greywater, just remember they can be confused easily, so when choosing between them consider the method, applications and benefits to make the right decision.

Michael Farnsworth, managing director, Stormsaver, is on 08448 440 015

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