- The Client:Go Transit/Metrolinx
- The Asks:LEED Gold Certification, Stormwater Management
The Problem:
Automated bus wash systems typically contribute to a large amount of potable water usage and therefore potable water use reduction was a logical opportunity. In addition, the large site area needed to control stormwater runoff to meet pre-development water balance requirements for stormwater management (SWM) meant quantity reduction was an absolute must.
The Answer:
Net Zero Water planned and provided the Go Transit Bus Storage and Maintenance Facility with a complete rainwater harvesting system that included a pre-treatment filter, cistern, cistern accessories, pre-assembled pump/treatment/controls skid, day tank and day tank accessories.
The sequence of operation was demonstrated and tested to run during low flows with demand from one or two toilets to full capacity where the bus wash, bus wash recycle water pit, and toilets flushing are running at the same time.
The rainwater harvesting system collects rainwater from the roof and pre-treats it with Vortex Filters prior to storage in a 22000 litre (6000 us gallon) fiberglass cistern buried below grade. The pre-treatment filter was installed to remove any large material from entering the cistern and damaging the pumps or degrading water quality, and reducing the sediment load in the cistern.
From the cistern, submersible pumps transfer water to the rainwater skid located inside the building. At the rainwater skid, cistern water is filtered through two stages of filtration and disinfected with UV before storage in the day tank. A booster pump draws from the day tank and delivers treated non-potable water to the toilets and bus wash system on demand. Potable make-up water is provided to top up the day tank or bypass the rainwater harvesting system during maintenance. The footprint of the skid mounted treatment package was optimized to fit the allocated space while also making all equipment easily accessible for maintenance.
The treatment selection of the system was specific to its end use. Due to the potential for inhalation of non-potable water from the bus wash system, UV disinfection was selected with a design objective of 30mj/cm2. The dose, however, is dependent on the ultraviolet transmittance (UVT) of the water. The UVT can be decreased with increased total dissolved solids in the water. Roofing and conveyance material can contribute to the degradation of the water quality. Since the UVT of the water was unknown, Net Zero Water assumed a low UVT of 75% for the new white roof. Potable water is typically over 90% UVT.
The HMI on the rainwater controller allows the operator to easily see the status of the equipment, the amount of water in the cistern and day tank, the total amount of potable and non-potable water consumed and simple monitoring of the filters and UV for maintenance.
The Design:
After a year of operation, Go Transit’s Net Zero Water rainwater harvesting system has already re-used and saved over 513,135 litres (135,556 usgal) of water, while constantly meeting the specified water quality deliverables. To view a sample design of this application, download the spec below.
