Pre-design drainage issues with Commercial rainwater harvesting systems

Often pre-design or design errors make RWH difficult

In our technical department, they often complain that the drainage doesn't work when it comes to the rainwater harvesting system.  They bemoan the fact that a fall or any additional access chambers haven't been drawn on the drawings.  The also complain that the rainwater harvesting system has been ordered too late and much of the drainage has been already installed, making the rainwater harvesting very difficult to function.

An order arrives

As soon as Freerain Ltd accepts an order for a commercial rainwater harvesting system, we request a copy of the latest drainage drawings to check for the following:

• Water is running downhill (we've seen many drawings showing it going up!)
• A pre-tank filter has been drawn with appropriate falls
• The storage tank location has been marked
• Only traditional roof areas are being drainage and not open gullies, drains etc

Below is a generic drawing for what a typical underground rainwater harvesting system may look like on a drainage drawing. In this case no invert level has been specified and you can see the inlet invert is 0.  Of course on site that this is anywhere from 500 to 3,000mm down.

Problems

As mentioned above there can be a number of issues and problems with getting a rainwater harvesting system to work within a commercial application.

• Drainage built with a fall for the rainwater harvesting system
• Fall in the pipework not accounted for
• Not all the intended roof is sent to the tank
• Open gullies and non-roof areas are drained into the tank

 Much of the above is really because of two reasons.  Firstly, the rainwater harvesting system is ordered/considered too late and secondly, the consultant for the drainage hasn't appreciated the requirements of the rainwater harvesting system.

Summary

Rainwater harvesting for commercial buildings and projects should be relatively easy to install, but all too often not all parties in the planning stages come together.  Therefore, on many occasions the late consideration or the lack of planning leads to costly work-arounds on site and on a few occasions this has led to already built drainage being ripped up and re-laid as required to incorporate the rainwater harvesting system.

Housing Association opts for a communal rainwater harvesting system

Intro

Freerain Ltd has recently supplied a communal rainwater harvesting system to Two Rivers Housing Association at their Turley Court site in Cinderford, near Gloucester.
The system supplies ten dwellings with collected rainwater from the roofs to supply WC flushing and an outside tap.

The System

13,000 litre underground storage tank
Freerain DUO204 control set
2 submersible pumps for duty/assist
Full mains top-up

It is estimated that the system will provide 265M3 of water per year, providing a financial saving and helping to reduce the overal environmental impact of the site.

The site also has an extenive range of other environmental measures, which enabled it to reach the Code for Sustainable Homes Level required.

This site is of particular interest as due to the design of the houses a header tank system for rainwater harvesting control system wasn't possible. Normally where dwellings share a collection tank, each dwelling uses its own secondary tank.

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 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.