Rain water harvesting and ...

Rain water harvesting and re-use is common, the question you need to ask is if you need to mix the typically higher quality rain water with a lower grade wastewater effluent - you may have specific needs without mixing the two and can optimise the distribution and re-use thereof accordingly. If not, yes it can be blended, bear in mind final water use quality requirements, this will drive what upstream treatment processes need to be applied

It is generally not recommended or acceptable to allow stormwater into the sewer for combined treatment, the treatment capacity is greatly increased when rain water is discharged into the sewer, requiring more capital as more infrastructure will be required.