I can't quite figure out why, if you're installing a Septic tank why not take the extra step & do it as a biogas generator - yes the generator has a few pipes, lids. dials & taps, and is a 2 chamber unit.... but both use an anaerobic bacterial environment to do the job.

As for the alang alang, I'm not too worried, generations of Balanese have used alang alang for roofing, anyway when (if) it does disintegrate into powder... then the damn roof just goes into the biogas unit.