Your sewer is backing up out of the vent pipe. Obviously its low enough to back up out there instead of the house toilet and sinks.
Why septic tank and drainfield systems clog. Your vent pipe is working fine it's the clog below it or the drainfield that's in trouble and failing Its recommend to pump the tank out every 1 to 3 years to control the solids better
Untreated or poorly maintained septic systems can develop severe Bio-Mat (undigested raw sewage) and grime buildup throughout the entire system including the tank, lines and drainfield. This accumulated sludge builds up gradually causing performance to degrade steadily and eventually results in drainage failure. Septic tank systems that have not been properly treated, or have had a decrease in bacterial populations due to everyday cleaners and other common household chemicals, have a difficult time digesting and degrading the raw septic sewage that enters the tank. Most systems see problems in the drainfield where the field lines progressively clog to a point where degradation and digestion is inhibited. The lines have small perforations that allow liquids to pass into the surrounding gravel bed to further digest and degrade. In trouble systems, the soil surrounding the inner pipes becomes extremely compacted with undigested sewage and the liquids have no where to go, leading to backups, wet spots and odors. As clogging progresses, waste sewage also has no where to go and travels out into your yard, home or nearby area causing serious septic system problems. Toilets that flush slowly, raw sewage smell and wet areas are potentially a sign of imminent backups. A clogged drainfield can either be replaced (can cost upwards of five to thirty thousand dollars), or bombarded with potent sewage eating bacteria that will digest the sludge, re-establish healthy bacterial counts and get the septic tank and field back into proper condition. Naturally occurring bacterial strains found throughout your septic tank system are killed off by the use of many household chemicals such as bleach, paint , anti-bacterial soaps and other bacteria killing chemical products that enter your tank via drain lines. NT-MAX Septic Tank Treatment bombards your tank, lines and drainfield with over 14 Trillion bacteria per gallon, reestablishing much needed bacterial colonies to quickly digest waste and unclog your system.
Hopefully this explains your options and the causes There are a lot of products out there and grease is the biggest probem let it solidify in the dish/pan and the throw out
21 Boat
If I Helped To Answer Your Question Please Rate My Answer

Originally Posted by
;
are there any chemical products I might use to keep the pipe clear?
Sorry I was working on two sewer problems and one had a drainfeild.( the windows "advance is all you see and I didn't scroll enough
My reason for the vent pipe putting matter into yard was correct and it being clogged low enough for the back up liquids to discharge out of the vent pipe first instead of backing into the house. The only difference the end run was to main sewer instead of drainfeild.
You asked for a product to use keep the
Sewer line clean
So here is the answer I was supposed to give sorry again
SKU: 705616
Roebic, 32OZ Main Line Sewer/Septic Cleaner, Concentrated Bacterial Waste Degrading Product For
Any Sewer Line, Clears Fats, Grease, Sludge, & Paper, For Any Household,
Whether Septic System Or Sewer. ROEBIC LABORATORIES INC
PRICE: $7.68
Maybe this is what you are looking for and its in stores.
Signed 21 boat
If I Helped To Answer Your Question Please Rate My Answer