British Standard for Rainwater Harvesting Systems

The recent publication of British Standard 8515 for rainwater harvesting systems, available via http://www.bsigroup.com/, establishes for the first time a yardstick against which systems being supplied in the UK can be measured.

Harvesting rainwater to supply household needs is one of the oldest and most durable forms of supplying water worldwide, although until recently practiced in the UK mainly in the context of garden irrigation, following the introduction by the Victorians of mains-supplied water.

Due to modern lifestyles and population growth, however, UK mains water supplies are now coming under varying degrees of stress – particularly in England south of the Humber where the stress is generally severe (see Environment Agency map).

This is reflected in the mains water consumption requirements set out in the Code for Sustainable homes, and commercial equivalents such as BREEAM assessments. Up to a point, these requirements can be met by economising on water use, but if lifestyle (baths and showers etc) is not to be compromised then substituting recycled water for mains water becomes necessary.

The easiest and most cost-effective way of doing this is to use harvested rainwater instead of mains water for non-potable uses such as toilet-flushing, clothes washing machines, car washing and irrigation. This reduces mains water consumption by around 50% in the home and by more than 80% in any building that combines a large roof with a high demand for non-potable water.

BS 8515 now sets the standard for how systems as a whole (see schematic diagram) and components within the system should function. Very simply, the aim is to route the rainwater falling on the roof to a storage tank, filtering it beforehand to remove any solid impurities. When needed, the water is then pumped (directly or via a header tank) to its point of use via pipework dedicated to non-potable water.

From the users perspective, using rainwater in this way is indistinguishable from using the normal mains supply, with the associated controls automatically controlling all functions and ensuring, using a mains water backup when needed during prolonged dry spell, continuity of supply.

Water Charges set for inflation busting rises

Introduction

Previously agreed water charge increases come in to affect from 1st April 2009. This was agreed sometime ago and it is 1.1% above the latest inflation rate, currently 3%. Ofwat have suggested that this should increase the average water bill by £13 per year. Some water companies such as South West Water are increasing their prices by as little as 0.9%, but they do currently have one of the highest tariffs.

More on the way

Ofwat are currently developing the pricing strategy for the years 2010 to 2015. During uncertain economic times, clearly this is a very difficult balancing act.

Reasons for the rise

There are a number of factors which have been considered in determining the level of money needed to improve or maintain the level of service. We have been very fortunate in the UK that we developed a world leading water network, but this pioneering period has left the UK with an aging network of both water supplies and sewerage.
There is also increased demand in certain already over-stretched such as the South East of England. Several articles have suggested that this area has less water per head than parts of Africa and this highlights the need to maintain the network. For some years now, we have heard of massive leak rates directly resulting from old pipes.

What else can be done?

Clearly investment is needed and will always be, but this must go hand in hand with other water measures. The Code for Sustainable Homes tackles this issue for new-build domestic homes, but there is little incentive to make water saving measures for the current housing stock. New-build accounts for maybe 1%.
Individuals can take steps to reduce their expenditure on water, some of these are listed below:

1) Aerated taps
2) Aerated shower heads
3) Smaller baths or filled to a minimum
4) Rainwater harvesting (suitable for times of renovation or a simple garden system)
5) Displacement of WC cistern volume with a brick or bottle filled with stones.

There are plenty more ways to reduce water in the home. But obviously, should any of the above measures be taken, the property should be metered and not on a rates or fixed charging structure

Rainwater harvesting tanks: Do they need cleaning?

Introduction

Rainwater harvesting systems are becoming increasingly popular, and the fact the Freerain's early customers have had their tanks installed since 1999. Many people are considering the long term effects and issues concerning older rainwater harvesting systems.

Cleaning?

The general water quality and effectiveness of a rainwater harvesting system is largely dependent on the size of the storage tank installed. Too large and it may never overflow and turn the water over enough. Too small and it doesn't store enough water to be effective and the dwelling/project is reliant on mains water top-up.

Assuming the storage tank has been sized correctly, then it should overflow a few times a year, assuming the system includes a calming inlet and has a suitable pre-tank filter, then the tanks really shouldn't need cleaning. The heavier particles should be removed by the leaf and grit pre-tank filter, the finest particles should float and be skimmed off during overflow. The particles in suspension eventually get sucked up by the submersible pump and are then removed by the inline strainer/carbon filter or whatever has been included on the system.

So a well designed system is very unlikely to need cleaning out, but there is one big assumption, this assumes that only roof water has been sent to the system. Water from hard-standing should not be sent to the rainwater tank, nor should water from a green roof system. Although newer green roof systems do filter the water very well. The overflow from the storage tank should also be protected from backing up. In other words, a well designed system, being used in a normal way, should NEVER need cleaning.

Germany

In Germany rainwater harvesting is about twenty years ahead of the UK and their experience is exactly as described above. Only poorly designed or mis-used systems require major maintenance.

Roof top rainwater storage, does it work?

If you think of a traditional rainwater harvesting system, in the UK and most of Europe. Then you would probably think of an underground storage tank, a physical filter before storage and a pumping system. We have previously looked at the options for controls and this is broadly two types. The direct pressure system and the header tank system, being fed from the underground tank.

We at Freerain Ltd, are asked quite often about the option of eliminating the underground storage tank and diverting roof water directly into a header tank. On first thought this seems like a good idea, no excavation, no underground tank and maybe even no pump!! All sounds pretty good?

Domestic system, we need storage of around 800-1,000 litres per person to make the rainwater harvesting system effective. So a 4,700 litre system when full would weight around 5 MT!! and take up a considerable amount of space. In order to strengthen the floor, it could be very costly and indeed more than a non-concrete underground tank installation. (as per Freerain tanks). We have also previously discussed the fact that the water should remain dark, cold and not subjected to thermo-cycling (that is rapid heating and cooling). Whereas inside a building it might be. Finally, the overflow from storage would need to be designed to never flood the building at times of heavy rainfall.

Commercial systems are more likely to be specifically designed to take the weight, if underground or above ground outside is not possible. But internal sectional storage tanks do tend to me more expensive and take up valuable space.

Filtering the water prior to the storage tank can sometimes be problematic. A traditional underground system utilises gravity to move the water from the roof, through/across a filter and finally in to a storage tank. The rainwater filter is normally either housed in the tank neck (as with Freerain domestic tanks) or externally for larger commercial systems. So if you want to have a high-level storage tank it must be designed to be able to accommodate any invert drops across the filters and the overflow.

In summary, for domestic projects we wouldn't consider it as a viable option, too much weight or too small amount of water stored. For commercial projects it is always an option, but only if underground or above ground external has been considered and rejected first.

Rainwater harvesting and water restrictions

Question: I've got my rainwater harvesting system installed, should I worry about hosepipe bans and other water restrictions?





Answer: You need to be aware that most rainwater harvesting systems are in fact ultimately connected to the mains water supply. Therefore, you can use water from a hose, providing the local water company (imposing/enforcing, the restriction) haven't supplied the water.


We at Freerain, routinely supply systems without a mains top-up specially for external/garden rainwater harvesting This ultimately protects the customer from any grey areas or mis-understandings. Quite simply, if the hose/sprinkler has water, its rainwater, if not the tank is empty!!!

For the latest information about hosepipe bans and general water restrictions, I would recommend visiting Hosepipeban.org.uk There you will find links to the water authority sites, water saving tips and a forum.

We may not have had any major water restrictions in the UK for a few years, but with a population increase and the effects of climate change, we are certain to have more water stress.

Another solution

Where rainwater harvesting systems are used for both internal and external demands, a mains top-up is essential. Direct pressure systems (whereby the water is supplied a pump pressure) mains water must pass through a class AA air-gap (see WRAS) to avoid any cross-contamination between potable and non-potable water. The easiest way is to top-up the external (underground) storage tank with mains water at time of low level. This then raises the possibility of a hosepipe supplying mains water on to the garden. Which during a hosepipe ban would make the system fall under the ban. One water to remove this is to install a header tank (break tank) in the property. This way, pump pressure can be sent to the outside tap directly. But all the mains water topping up occurs in the the header tank. So, the only water to pass through a hosepipe would be the locally collected rainwater.

Suitability of rainwater harvesting systems for existing dwellings

Domestic rainwater harvesting systems are generally aimed at new-build.
There are several reasons for this and these are:-

• The drainage can be designed specifically with the rainwater system in mind.
• The way the internal plumbing is arranged differs slightly to separate out the potable and non-potable pipe work
• It is often easier to move the tank around the site before the building is completed.
• Excavation machinery is already available.

The costs associated with installing a system are reduced by the above factors, making them more affordable.

When considering an existing property is can be very much more expensive, starting with the fact that VAT is added to the system price. This is currently 15% (recently 17.5%). But moreover, re-routing pipe work can be very expensive and can involved fairly major cosmetic works. The external drainage is often difficult to access, either due to hidden pipe runs, unworkable invert depths and/or unknown modifications.

It can also be difficult to get the tank to the desirable location around the property. For example a semi-detached house, has only access around one side. Often this is limited. So a crane cost must be added to the project.

Space in the garden can also be an issue, as digging close to foundations can be risky and require a professionally appointed structural engineer to advise on the works.

So, with the extra costs and the fact that water is actually still relatively cheap in the UK, there isn't the financial incentive to harvested the rainwater.

At Freerain, we advise customers with existing properties to consider a system for external/gardening use. This is because the internal plumbing is changed and perhaps not all of the drainage is needed to be centralised. This reduces the costs significantly, but of course not everyone is satisfied with external water only.

For commercial projects, these are reviewed on an individual basis, this is because the very nature of some of of the sites. External drainage can sometimes be very easily accessed, above ground tanks are more acceptable and work technically better.

In summary, rainwater harvesting systems are best suited to new-build projects for domestic dwellings, but perhaps commercial sites can with certain modifications incorporate a system.

Is Legionella problem for rainwater harvesting systems?

Introduction

Legionnaires' disease, or Legionellosis is an infectious disease caused by a genus of bacteria. These bacteria thrive in warm conditions typically between 25-45oC. There are several types of Legionellosis, but for the point of this blog we are only going to cover the most common type. The condition is much like severe pneumonia and can be fatal. The first and still the most famous outbreak was in 1976 in Philadelphia, where it was given the name Legionaries disease. A gathering a war veterans were together for an annual event, see here. In the UK cases are rare, mainly due to the strict regulations now in place, but in 2002 there was a case where 7 people died and about 170 contracted the condition from a nearby cooling tower.

It is estimated that around 10,000 people die from the condition per year in the USA.


Legionella bacteria: Conditions for growth


The bacteria thrives in a range of temperatures between 25-45oC, but does indeed survive at lower temperature, but is dormant at around 20oC. As with most bacteria it requires a moist, if not aqueous environment. So the focus for the regulations has been centered around hot water systems, such as showers, heating and hot tubes. But also ventilation systems, whereby water is used as the indirect heat source.


Pathway and transmission


Infections normally occur through the respiration system in the form of fine spray (aerosols) and so it not the water body itself. Therefore spraying, minor leaks in ventilation systems and bathing or the areas of special concern.


Rainwater harvesting systems, are they a potential risk?


A well designed rainwater harvesting system is designed with the quality of water in mind and this is usually dominated by the tank size. Sizing the storage correctly ensures that the water is not stored for too long (around 20 days) , but balancing the need to supply water. Generally speaking in the UK, we don't have very long dry periods, but you might be surprised to learn that on average we receive significant rainfall about every two and half weeks. Therefore, storing very small quantities (relative to demand) is not efficient and in many ways pointless.

In the UK the rainwater tanks are normally underground and include a pre-tank filter. This helps in several ways. Firstly, being underground the temperature is no more than about 10oC even during mid-summer. Secondly, the pre-tank filter removes much of the organic matter and this prevents the build-up of bacteria generally. It is also dark. So the water is kept cold and dark and in many respects clean. Many systems use calming inlets to the tanks. This helps to retain higher than normal oxygen levels, promoting healthly aerobic bacteria. See the Environment Agency for more water quality discussions.

In certain commercial applications such as schools, we at Freerain advise the use of ultra-violet sterilisation unit. Whilst it is not part of any regulations, we feel that it is a belt and braces approach to rainwater harvesting. Especially where the project utilises a header tank type system. That is a secondary storage tank at high level. This is because of the usage pattern of a school year. There could be several weeks during the year, where water could be sat in the header tank. We are not saying that there is any risk of legionella at all, but many consultants and specifiers are not prepared to include a system without a UV unit.

Another application for adding a ultra-violet unit is for irrigation systems, where there is the chance that rainwater could be sprayed as an aerosol. Also, the water is often kept longer as water quality is less important. Finally, some are above ground tanks. Adding the ultra-violet unit again is not part of regulations, but it is a cheap option, sometimes costing as little as £400.


Summary


Rainwater harvesting systems are not a source for legionella bacteria to thrive, due to the conditions the water is kept. That is much colder than the lower limit for growth as researched by the Reliance Worldwide organisation. The water is generally clean before it enters storage and used quickly before it deteriorates.
The focus is quite rightly on hot water systems and not cold rainwater systems. Care should be taken, but a well designed system from a well respected supplier/designer remains important.