I am a new inspector and would like to know the best way to do a dye test for
septic system. Some inspectors use liquid dye and others use tablets. If tablets are used, how long would you have to wait before walking the fields to determine if a leak is present? What’s the correct procedure if you use a liquid dye?
What area do you work?
They are a waste of money and a liability on you. Any septic system leaking bad enough to show dye in the yard should be evident by the smell alone.
Tell them to spend their money and have the tank pumped.
I used dye to confirm what my eyes and nose already knew. That’s about it. For example, if i couldn’t confirm where a sump pump discharge was going.
There are other ways to perform a limited visual-only inspection on a septic system, and we wrote a checklist and step-by-step procedure for that. It’s at How to Inspect Septic Systems Course - InterNACHI®
The free, online training course for inspecting onsite (septic) wastewater treatment systems describes two types of inspections:
- A maintenance inspection to determine the need for pumping and minor repairs; and
- A complete functional inspection typically used during a real estate transfer.
The routine maintenance inspection is well suited for a residential home inspector. A maintenance inspection is used to locate system components, describe how the system functions and how it can be maintained regularly. The functional inspection is used primarily during property transfers, includes a maintenance inspection, and designed primarily for septic contractors.
Too many variables involved. If the system hasn’t been used in a long time, it could take days for the ground to get saturated enough to cause the effluent to surface. If you put the dye in before the tank it will take “allot” of dye to see any color in a 1000 or 1500 gallon tank. (dye is not cheap)
I only use dye to verify there is no septic tank between the house and a direct discharge pipe or lagoon. Some houses direct discharge gray water separate and dye would be good for that.
The picture below is a dye test I performed today to verify no septic tank between the house and lagoon.
Sure is, and it’s a Realtor friendly practice. Helps the sale go through, don’t have to waste money on a septic professional that might find things, and you get to stay on the Realtor’s good side for subsequent business. 
Septic dye tests originated many years ago and is why the dye is still available today. 22 years ago, when I first started out, I did septic dye tests as this was the standard for the time.
Today, as pointed out, dye tests can be effective to determine flow. I’ve used dye in cases where older tanks, which have PVC pipes acting as baffles and are not visible, so you can document the flow through the tank.
Here’s an example. This tank is a small round 500 gallon tank with the outlet baffle submerged. Years ago it was common to have a 90 degree elbow which would extend the PVC pipe into the liquid layer with no tee. In this example I used the dye to make sure there was proper flow through the tank. Unfortunately the drywell that serviced this tank was saturated and effluent backed up and the system had to be replaced.
My point being is there are uses for septic dye (I use the powder) but should be only used as a tracer or means to help with the overall evaluation of the system.
One final note, probably the best use of septic dye is for photographic reasons, it makes your images more presentable in your clients reports 
Dye isn’t used to demonstrate everything is O.K. It’s used to demonstrate something isn’t O.K. And for doing that, dye really works well. Dye proves a connection between two points. If that connection shouldn’t exist, nothing works as well as dye to demonstrate the problem.
BTW: I’m a licensed/insured private wastewater treatment system installer.