Sorry, this might be a long post, but I think it's important to include all the information in the hope of tracking down an answer.
In mid 2025 we moved into a house (built in 2009) that has a concrete, underground, 4500-litre dual-purpose rainwater reuse and stormwater detention tank. The bottom 2000L is rainwater reuse and the top 2500L is stormwater detention, which slowly bleeds off into the municipal stormwater system. The tank supplies our toilets, one garden tap, the laundry tub, and (unfortunately) our washing machine. When it runs low, it automatically tops up from the mains, triggered by a float valve, just enough to keep the supply continuous. Until recently, we never had any problems with the tank water beyond a bit of a yellow tint. We installed a whole-house sediment and tannin filter to protect the workings of our brand new washing machine and stop our clothes discolouring.
In July this year we got the gutters cleaned and the roof moss treated. They used Envirochem Moss & Mould Killer (75% concentration of Quaternary Ammonium Chloride) that is marketed as odourless. We were explicit that we wanted the downpipes disconnected so no product got into the tank, mainly because we were worried it would contain something that ruined our clothes. The company covered every downpipe below the leaf guard, but unfortunately didn't notice that two of them had no collars where they went into the ground, but were instead hanging down into a larger, open-topped pipe. The product didn't appear to get into these holes on application, but after the first rainfall (while the downpipes were still disconnected) the runoff likely poured down the outsides of the downpipes and into the holes below. We noticed within hours of the rain that the water in our toilets had turned milky white and a little foamy. That dispersed within a couple of days as we used the tank water and it was replaced by the auto top-up.
However, within the next two weeks our tank water (which continued to be supplied only by the automatic mains top-up) began to stink. It was a cross between wet dog and rotten cabbage. A kind of dank, swampy smell. We thought maybe it was just because the water level had gotten super low with the downpipes disconnected, although it would surely have reached similar low levels in the previous year, and had never smelled like that. We asked to have the downpipes reconnected in the third week because we wanted to get some rain in there to hopefully wash the smelly water out. We also had a water tank cleaning company come and drain the tank completely and scrub it out (no chemicals used), and they left the auto-top up to refill it to the minimum level. It turned out that they'd cleaned the tank two years previously for the last owner, and had been told it had never been cleaned prior to that. Once the tank was clean, we also replaced our sediment filter and washed the tannin filter.
For four weeks, all was well. No smell. And then it came back, and it got worse and worse and worse. A decent rainfall would almost completely resolve the issue, and then it would return as soon as the tank level dropped. The odour turned sulphuric. Our washing smelled disgusting, but was fine as soon as it dried. Our toilets stank when we flushed them. We tested the water with aquarium tests. 1ppm ammonia, 0.25ppm nitrite, 18ppm nitrate. Our water contains none of these. It suggests there is a nitrogen cycle occurring within the tank, similar to what happens in an aquarium. However, none of my cycling aquariums have ever made that smell. But it is indicative of decaying organic matter.
We got the tank cleaners back to have a look. They said the water looked dirty again. We have no first flush diverters (only leaf screens) and an asphalt shingle roof, so all the accumulating dust and pollen is probably going straight in there. They said it needed cleaning again, assuming that it had been recontaminated with product still running off the roof. However, we don't want to do that until we know what's causing the smell, and can be sure it won't just return four weeks later.
We got the gutter cleaners back to sniff our water - they agreed it was putrid. They said that they'd never had an instance of their product in a rainwater tank producing that smell. They have no idea how to remediate it. They did however take immediate full ownership of the issue and offer to pay for the tank to be re-cleaned. But again, we don't want to do that until we know why it keeps re-stinking. The guy looked inside the tank and noted that the walls were stained black to about where the 2000L rainwater reuse retention level would be. It didn't scrape off when he tried. We pulled up some water and confirmed it was stinky, so the issue is definitely coming from within the tank. He also thought he might have seen a bubble or two rise from the concrete floor, but we had just disturbed the water.
At this point we are stumped. We've downloaded Auckland Council rainfall data from a nearby weather station and we've had 283mm since the roof was treated. The gutter company says that the product would have washed off the roof within the first few weeks. And the tank is currently being regularly topped up with fresh water from the mains because there's been so little rain these last few weeks. Yet it continues to stink whenever the level is low. We are wondering if the product killed the tank's natural microbiome, and the smell is somehow related to that. But why does it continue to smell after being scrubbed clean? Could there be residual product in the concrete that is interacting with pollen washing in from the roof?
We have just worked out we can manage the smell by poking our mains-supplied garden hose into one of the open downpipe drains and filling the tank up beyond the meagre auto-top-up level. That's given us a couple of days of no smell, but it does seem to be creeping back. Obviously we'd like to work out what is creating the stink so we can get rid of it, get the tank re-cleaned, and go on with our lives.
Both companies have been really responsive and tried to troubleshoot, but they are at a loss and so are we. So if anyone out there has any ideas about what has happened here and how to fix it, please chime in.