Master66... sorry, I forgot to check back in on this thread.
What is the difference between the two types of wwtp that you mentioned?
Anaerobic plants have "digesters" that break down the sludge in an enclosed space where there is very little oxygen present and as a by-product produce a fairly large amount of hydrogen sulfide and methane gases. Normally you can tell this type of design because of the digesters, which are large (usually circular) buildings with floating roofs. Because of the expected levels of gas there are a large number of locations at the plant that will be classified.
Aerobic plants can have classified areas, but, many few locations at the plant will be a concern. This style plant has large mixing tanks and lagoons that "churn" up the water and use that action to break down the sewage. Because the actual digestion process takes place in the open and in the presence of oxygen there are very few classified areas.
Let me know if you are interested and I'll have Bill post a photo of each design.
