Hiblow Aerobic Septic Air Diffuser

Is the end opposite the fitting plugged or open?

The end of the diffuser is plugged.

what is the length of this diffuser?

The length is 12" total and has 1/2" npt threads. 

how many diffusers should be installed with an HP80 pump?

You can use one diffuser but it shortens the life of the septic air pump by a year to to years depending on what region of the United States you live in. In the southern part 2 years in the northern 1 year. It also depends on the diffuser a corse air bubble diffuser would have no effect on life time. The Clearstream diffusers are very restrictive and need changing yearly and we only recommend them in clearstream septic systems. The Hiblow diffuser is the the longest lasting poly bar type diffuser and normally can be cleared every 5 years using some muradic acid that can be bought at any hardware store. 

what size pipe thread? What are the dimensions? Thank you

This is a 1/2 npt pipe thread for this diffuser. 

How many diffusers should I use with the Hiblow 120 LL pump do avoid back pressure? diffusers would sit in about 6 feet of water

They are rated for 80 lpm each so 2 would work. That being said most systems running an 80 LPM pump are using two. 3 would not hurt and would be recommended. 

Where is the diffuser located ?

The diffuser is located in the aerobic chamber. The aerobic tank is either the first tank or the second tank if you have a trash tank.

Checking to see if this type diffuser is rigid or flexible ?

This a ridged diffusers it does not have any type of flexibility.