In this article, we explore how to manage water quality in school rainwater harvesting systems during the holidays and in gravity fed systems where the header tank water is dormant for longer periods
Many education institutes in the UK have installed gravity type rainwater harvesting systems to flush their vast amounts of toilets. It makes sense. Schools, Colleges and Universities have a lot of roof space to enable them to collect free rainwater and have many toilets to flush. Why flush expensively cleaned mains water down the toilet, when rainwater will do the job just as well?
Simply put, using rainwater to flush toilets saves money and, on these scales, rainwater harvesting systems can payback in less than 5 years.
Public buildings of this type experience several high water demand events throughout the day during which high peak flow rates and rapid water consumption are expected. Large gravity systems have therefore become the preferred solution in educational buildings. The associated concerns of water stagnation and water quality become a factor that needs to be addressed by design, particularly in high profile public sector environments.
During normal use, the high turnover of water in the header tank, often daily, reduces such a risk to an absolute minimum, but periods of inactivity particularly in summer holiday months can result in a full header tank of considerable size (20 to 30m3 are common) sitting dormant with warm water for several weeks.
To address potential water quality issues we were contacted by an M&E contractor keen to provide a bespoke solution for their client who was to retrofit several schools with upgraded controls. 3P Technik’s standard commercial header tank controller already provides a drain-down facility activated at the push of a button, however, this did not meet the exact mechanical specification provided by the specifier and so an alternative was required.
The end-user of the College’s Rainwater Harvesting System (the Permanent Caretaker) required a system that was easy to set for three different water usage requirements; during term-times, holidays and to easily drain down the system and turn it off entirely for maintenance purposes.
During term time, the system needed to fill the header tank to the maximum level with rainwater, failing over to mains water at a lower level. In the holidays the 2nd set of different and lower fill levels were required (to be set by the system installers on-site) at the turn of a switch, and an OFF mode in which the tank could be manually drained.
The college’s Rainwater Harvesting Systems controller specification was easily achieved with a TC340 versatile level controller (instead of the usual dedicated commercial header tank controller – the Rainforce H Series).
The TC340 is a non-dedicated advanced tank level controller whose outputs and inputs can be set up as required for conditionally filling or draining between programmable levels. Four outputs were set for rain and mains fill levels of both level control sets, with the output pairs joined into 2 single outputs within the unit, one for rainwater and one for mains. A clearly labeled rotary switch was then added to the front face of each controller and it’s switch outputs connected to 2 of TC340’s digital inputs which conditionally allow or disallow each level control set for “Normal Operation”, “Holiday” and “Off” modes. The units were then programmed with provisional level settings and despatched to the contractor for installation.
The most advanced features of TC340 were not required for this job and the controller was easily able to cope with the client’s requirement, with the ability to be reconfigured to accommodate different scenarios in the future if required.
As a general overview, when installing automatic Rainwater Harvesting Systems we recommend;
- Installing a Pressure Vessel of an appropriate size, after the pump (for direct pressure systems).
- Controlling header tanks with failsafe sensor based systems instead of float switches.
- Leak detection on either the pump or the electronic controller.
- Installing an easily serviceable pump.
To talk through your bespoke electronic controller or rainwater harvesting system requirements, please email and overview of your needs in the first instance to email@example.com or call us during office hours on 01239 623506.