Sulphur removal

We experience problems with deposits of elementary sulfur in our waste water treatment.

For years we have used sulfuric acid for pH balancing of our influent water. We never had serious problems. Now we start noticing heavy deposits in our mixing tanks, both before and after de anaerobic digester itself.

We have no view on any deposits in the reactor since it is a closed tank, but it is possible that there are deposits there as well.

Although in the past this sulfur was largely removed from the water stream via our biogas, now this seems to accumulate more in the water, not in the gas.

This is to the point where metal parts are corroded away, although our pH measurement is still relatively neutral (average 6.91 with standard deviation 0.27).

Can anyone help me find the root cause of this phenomenon? Any idea's where to start?

Thank you for any advice or knowledge.