What is a first flush diverter and how does it work?

Cleanawater on 18 July 2019

What do you do with rainwater if you have an unroofed wash bay? Does all that water have to go through your oil water separator before discharging to the trade waste sewer network? Are there compliance issues with allowing rainwater to run from a wash pad directly to the stormwater sewer?

Read on to find out, or watch our video on how a first flush diverter can help your business

All these questions relate to first flush diverter systems for wash bays. Find out more about Cleanawater first flush diverter systems here.

What is first flush diversion?

The principle of first flush diversion is that the first 10mm of rain falling onto an unroofed wash bay is considered to be contaminated. This water picks up any residue of oil, sludge, soaps and detergents on the ground surface. It must be routed to a water treatment process like an oil water separator before discharging into a trade waste sewer.

All rainwater falling on a wash pad after the first 10 mm is considered clean and can be routed directly to the stormwater network. In this way, no contaminants are released into the environment, which could cause harm to plants and fish. Another benefit relates to wastewater treatment plants. These plants have defined capacities based on the expected wastewater volumes. Rainwater will overload treatment plants causing them to release contaminated water into the environment.

How does first flush diversion work?

first flush how it works

A first flush diversion system has a few key components that enable it to fulfil the diversion function.

A sensor monitors wash gun activation so that the switch to stormwater never happens when the wash bay is operational.

Rain is measured using a rain gauge, which keeps the diverter valve open to the wastewater treatment process until the prescribed volume of rain has fallen. At this point, the diverter valve is shut off and all rainwater overflows into the stormwater system until the valve reactivates.

The diverter valve can be programmed to reactivate based on a number of factors. For example, when a wash gun is activated or when a certain time has lapsed without rain. If the rainfall was insufficient to flush the wash bay, the rain gauge is reset, and the diverter valve remains open.

Regulations dealing with rainwater and wash bays

wash bay first flush diverter

It is important to note that any business washing down vehicles, parts or shipping containers is required to install a wash bay. This serves to contain dirty water, which may contain oil, grease, sludge or other contaminants. All this trade wastewater must be treated before being discharged to a trade waste sewer.

Australian regulations specify that any unroofed wash bay larger than 20sqm must have first flush diversion installed. In certain parts of Australia where high rainfall is experienced, unroofed wash bays are not permitted. The roof ensures that rainwater does not fall onto a contaminated ground surface and all rainwater can therefore be diverted to the stormwater system.

Where is first flush diversion used?

first flush diversion wash bay

First flush diversion systems are generally used on unroofed wash bays with a surface area larger than 20sqm. Industries covered by this general guideline include car washes, car rental companies, mining companies, logistics and transport companies. Some case studies of first flush diversion systems are as follows:

Boom Logistics needed a wash bay for their cranes and equipment at their Blackwater location in Queensland. This site serves the local mining community. A particular requirement for Boom Logistics was the need for a portable solution that could be moved should they move away from the Blackwater region. Cleanawater supplied portable wash bays with first flush diversion and recycling and harvesting systems. This sustainable solution enabled to Boom Logistics to meet the requirements of local water authorities but maintain the flexibility they needed.

KW Doggett is a fine paper manufacturer. They had an existing unroofed wash bay that they needed to upgrade for compliance reasons. Cleanawater supplied a first flush diverter system that routed the first 10 mm of rainwater to wastewater treatment and then allowed excess rainwater to discharge via the stormwater system. This solution saved thousands of dollars that KW Doggett would have spent installing a roof over their wash bay to achieve compliance.

Contact Cleanawater for first flush diversion solutions

Cleanawater is an Australian company that specialises in wastewater treatment equipment and solutions. Our first flush diversion systems are low maintenance and are expected to last more than 20 years. We offer both portable and fixed systems to suit our customer needs.

Contact Cleanawater to find out about first flush diversion or call our expert team on 1800 353 788 today to arrange a consultation for your first flush diversion needs.