Ultra-violet sterilisation as an upgrade for a rainwater harvesting system

Background

A rainwater harvesting system (sometimes wrongly called a grey water system) can be defined as a system which collects roof water for use as non-potable water. Much is written on the Internet and talked about with rainwater harvesting (recycling) systems about the use of ultra-violet sterilisation as a means of sterilising the water. Typically a rainwater harvesting system will have a physical "leaf and grit" type filter or filters. This is perfectly good enough to clean the water for uses such as WC flushing (accounts for 25% of daily water consumption), washing machines and outside taps. It is possible to upgrade the water to fully drinking (potable) standard. A good upgrade should also include a fine filter prior to the UV unit. The filter needs to be reducing particles smaller than 5 microns. This is because of the "shadowing" effect. This means that should the particles be greater than this, then bacteria and pathogens can be shielded from the UV light source. This finer filtration is often in the form of carbon filtering, and usually in sealed purpose made cartridges.

Potable upgrade or not?

So, it is possible to upgrade the collected water from a rainwater harvesting system, but is it actually worth it? From the paragraph above, you can extra equipment is needed to the rainwater recycling system. This can cost from around eight hundred pounds. With consumble carbon cartridges and a mains power operated ultra violet sterilisation unit, these have a running cost of around two hundred pounds per annum. Which in some circumstances is perfectly reasonable. For example, if a dwelling is off-grid by the remoteness of the site, then a private water supply is very desirable and a running cost like this is very affordable. But where mains water is freely available, and the householder is looking for savings or to be more environmentally responsible, then perhaps this type of upgrade is not worthwhile. As the manufacture of the UV bulbs, equipment and carbon filters contributes to industrial environmental damage. The extra power to use the system and the fact that mains water is already available, makes for a strong environmental case against upgrade, but using a rainwater harvesting system to supplement mains water and in most cases halve the amount used is a much more compelling case.

Quick calculation

Let's take a 150M2 roof in Surrey, 3 bedroom house with 4 people living in it.
The property might collect around 95M3 of water per year. The demand that could be fufilled by rainwater would be around 66M3 per year. If this project was to be upgraded to fully potable (drinking) standard, then the property would be using around 132M3. So, in this case there wouldn't be enough water to meet the demand. So, depending on which type of rainwater recycling system was installed mains water might be re-treated. This is because a direct pressure system tops up the external storage tank with mains water at times of low rainfall (or high demand). Making the idea of installing the potable upgrade not worthwhile.

Commercial rainwater harvesting system

Many commercial specifications include UV sterilisation units, this is not due regulations, but more because the client "decides" it's the thing to do. Often encouraged by industry professionals, using their experience of hot water systems. Legionnaires' disease is often mentioned, but this is only a problem where the water is held at temperatures over 25 oC. Rainwater stored underground is very unlikely to ever reach those levels. So it's not a problem, but in a £30M project a few hundred pounds is a very easy belt and braces approach for specifiers.

Rainwater recycling: Types of control systems

Background and introduction

Rainwater recycling or rainwater harvesting as it is more commonly known is a fairly well mature technology that was developed during the last thirty years. Essentially the storage and filters quite simple. But the control systems are more complex driven by the need for reliability. In the UK, we have mostly thanks the Victorians enjoyed a safe and reliable water supply for the last hundred years or so. It's really in the last fifteen years where terms such as water stress has started to be used. The Environment Agency suggests that everywhere south of the Humber estuary is under some form water stress and of course the drier south and south east is continuing to see an increasing population.

Types of controls

There are three main catagories of controls with most rainwater harvesting systems.

1. Direct pressure systems
2. Header/break tank systems
3. External use only (Gardening) systems

Direct pressure

Direct pressure systems work by supplying water at pump pressure directly to the point of use via a control panel. This panel also controls the mains water back-up arrangements, of which there are a number of different configurations between different manufacturers. But all systems must comply with WRAS regulations and this put simply here is concerned with an air gap (type AA) to eliminate direct contact between potable and non-potable water. There are also regulations regarding pipework identification.

These systems are normally installed in lower demand situations such as domestic single dwellings. Where the demand is much higher, more than one pump can be used as duty standby assist type arrangements.

Header/break tank

These systems differ slightly from the direct pressure systems as the main pump or pumps send the water to a header tank (gravity system) or to a plant room based break tank. Where required additional pumps are used to boost the water supply around the building. These types of systems tend to be used in more commercial/industrial projects. Similar water regulations also apply to these systems and a slot/ cut-away (AB type airgap) is also required.

Garden rainwater harvesting

These systems were developed in response to the hose-pipe ban/water restrictions that were in place arouns 2006 (UK). They tend to have a lower specification generally, and most importantly the mains water top-up arrangements are usually excluded to protect the end user from falling foul of the water restrictions.

At Freerain Ltd, we have developed numerous various on the themes above to suit the project requirements and indeed are contuning to develop new ways to improve the performance and interface with BMS systems and other systems. More information about commercial rainwater harvesting system can be found on our website.

Rainwater harvesting: Millennium Green case study

Gusto Construction Ltd, completed a development site of 24 houses in 1999. Each property on the Millennium Green site was built to a standard far beyond the building regulations and achieved the status of Eco-Excellent. Each property was fitted with a Freerain rainwater harvesting system to meet the water reduction targets. Most of the plots were fitted with a 3,500 litre or a 4,700 litre storage tank, depending on the size of the plot. The system uses a submersible pump to deliver the stored rainwater directly in to the property, without the need for a header tank. The resulting water is suitable for WC flusing, washing machines and outside tap functions. The water is not suitable for drinking or bathing, as this water is non-potable (sometimes called reclaimed or even greywater).

During the year 2001, the Environment Agency and Severn Trent Water carried out a study on one of the plots at Millennium Green to see the progress of the rainwater harvesting system (sometimes called rainwater recycling). They looked at various aspects, including reliability, water quality and amount of mains water was displaced. The weather pattern for the year was not unusal and certainly could be considered an average year.



The above image (available on the main title link in a larger form) shows the level of water in the storage tank day by day. This showed that on a few ocasions a few litres of mains water was added to the storage tank. This is very beneficial as mains water has certain chemicals in it, and the free chlorine does help to maintain the freshness of the water.

Also several times during the year the tank overflowed, which a well designed rainwater harvesting system should do. This allows for any scum layer and suspended particles to be washed. With the Freerain system, the pre-tank filter is housed in the neck of the tank, this allows for the overflowing water to back-wash the filter. Thus reducing maintenance and maintaining water quality.


The study also monitored the amount of water used in the property and the above figure shows the daily percentage split between mains water (in blue) and the rainwater (in grey). The split was almost 50% of each. So when considering the Code for Sustainable Homes you can see that a reduction in mainswater consumption of 50% would mean that per person per day the average usage would drop from 150 litres to 75 litres. An article written on behalf on the Good Homes Alliance suggests that the resulting homeowners might no wish to purchase houses that have the perception of lowering living standards, with water saving device and smaller baths. But the Gusto experience shows that with a typical house and a typical family using water quite freely, a rainwater harvesting system can meet very high standards. All this without comprimising on the lifestyle.

Brief history for Freerain and the Gusto Group.

Gusto Construction is an award winning private developer based in the East Midlands of England. Gusto Construction has built various environmentally friendly housing developments, whereby the houses are rated eco-excellent and some of these were built as long ago as 1999, when this standard of build was truely pioneering. With such a small market for environmental building products back then, the supply chain for the technologies was also difficult.

The most difficult was a rainwater harvesting system or a rainwater recycling system as it was known as. Freerain Ltd (formerally known as Gusto Products) was formed to meet the initial requirements of Gusto Construction Ltd. With other areas of the Gusto also forming, the Gusto Group of companies has been operating since around 2003.

Freerain Ltd was also a founder member of the UK rainwater harvesting association, which was formed to tackle the issue of standards and compliance within the industry.

Freerain initially began supplying Gusto Construction for its Millennium Green Development, but the high quality and ease of installation systems soon attracted attention from other developers and self-builders. Freerain quickly established itself as one of the market leaders in the UK.

Following on from several water saving related awards, larger projects began to consider rainwater harvesting. Commercial rainwater harvesting systems come in all shapes and sizes and with that we bespoke the design as necessary.

Types of rainwater harvesting systems

Introduction



The term rainwater harvesting is used to mean the collection of roof water (typically) from a building to provide that building with water to displace mains water. Rainwater in its untreated form can be used instead of mainswater (potable). This means that you can use rainwater (non-potable) in WC's, washing machines and all outside tap functions. Our studies have shown that this accounts for approximately 50% of all water used. So from an environmental perspective, we reduce our mains water consumption by 50%. (subject to the amount collectable from the roof). In commercial projects the amount could be much higher. As the vast majority of water used in an office is for WC's.

Rainwater harvesting systems are have a essentially three main components, these being:

• Storage tanks


• Filters


• Control systems

But there are of course variations on the above.

1. Storage tanks, these can be underground, above ground single peice, above ground sectional. Made from GRP, concrete, plastic or metal.


2. Filters, course leaf and grit, self-cleaning, backwash and inline strainers.


3. Controls can either be direct pump pressure, header tank (gravity) with or without mains top-up. They have a single pump or multiple pumps. These pump can either be submersible (in the tank) or suction pumps (sited with the control panels).
   

In addition to the main components, all items are sometimes added such as ultra-violet sterilisation or monitoring panels as shown above. These add-ons are not required in order for a system to function, but they make the systems more pleasant work with.

When using a rainwater harvesting system it is important to ensure that it complies with the water regulations (WRAS). This is mainly concerned with the cross-contamination of potable and non-potable water and also correct pipe markings inside and outside the building.

Rainwater harvesting Case Study: National Centre for Autism Education

Freerain has recently supplied a rainwater havesting system for installation by Working Environments Ltd (Mechanical & Electrical contractor). The National Centre Centre for Autism Education has been built for the Treehouse Trust.

Source: Treehouse Trust Website, accessed 15 Oct 2008.

Sustainability
The building has been designed to be a low energy and low carbon building. The building is predicted to use 67% less energy than the energy benchmarks set out in the Greater London Authority's guidance for planners. 10% of the energy will be derived from renewable energy sources (ground coupled air pipes).

TreeHouse’s ‘Green Features’ include:

• 
  The roof will be planted with environmentally friendly sedum, creating a habitat for native flora and fauna; softening the visual impact of the building; and providing insulation to reduce the need for additional heating.
• 
  Rooflights on the first floor will ensure maximum daylight levels. Light shafts will provide daylight from the roof down to the ground floor classrooms to minimise the amount of artificial light needed.
• 
  Artificial lighting will be controlled by photocells linked to dimmers to minimise the artificial lighting load.
• 
  Energy efficient boilers will minimise the gas load for heating and hot water. The efficient fabric, form and air tightness of the building means that the heating load is low.
• 
  Rainwater recycling system will supply the WC flushes, reducing energy used in treating and supplying water to the building and treating surface water drained from the building.
• 
  High performance glazing will minimise solar transmission in summer and heat loss in winter, while still maintaining high light transmission to retain daylight levels.
• 
  A ground coupled air system will use the thermal mass of the ground as a heat source in winter (warming the incoming air) and as a heatsink in summer to cool the incoming air. This will result in a carbon saving of 10% or 1056kg/year.

The rainwater system supplied by Freerain included:

• 51,000L underground storage tank (including 27,000L of attenuation volume)
• 4no. Hydro-filters, these are a relatively new type of filter, originally designed for metal reduction from road run-off pollutants.
• Header tank type control system
• Ultra-violet sterilisation system
• Various pumping controls
• LED rainwater monitoring panel

The combination storage tank has allowed the surface water discharge to be controlled, but also cost savings on the total installation.

Rainwater harvesting systems: More popular

Rainwater harvesting systems are becoming more popular in the UK has this for a number of reasons.

1) Water shortages (mainly in the South East)
2) Storm water attenuation
3) Environmental awareness
4) Gadget factor amongst self-builders
5) Government pressure in the schools and hospitals projects.
6) The code for sustainable homes

This last point is very important as we at Freerain are seeing an increase in the number of developers seeking to gain level three accreditation. In terms of water this means a reduction from 150 litres per person per day to 105 litres per person per day. This can be quite hard to achieve particularly during the harder times in the construction industry. It is possible to reduce mains water demand, but using certain conservation messures such as aerate taps, A-rate washing machines and low-dual flush WC's. But in order to meet level three and certainly the higher levels, then rainwater harvesting systems are the way forward. This is because that broadly speaking and assuming that there is enough roof area and rainfall. Then a rainwater harvesting systems can reduce the overall demand of mains water by around 50% or 75 litres per person per day. This means that a design team can be confident that with a rainwater harvesting system there water aspects of the code performance are taken care of.

Freerain has prepared a short introductory guide to explain the rainwater harvesting systems.