A friend, who's recently bought a house on tank water (or rather, off main supply) asked this on Blue Sky

This is something we've wrestled with over the past 11 years we've been on Waiheke, so rather than leaving it as a very long thread, I thought I'd put it in a post.

First of all, there are three things you need to care about if you're off main supply

• Water In - ie, capture rain water, use it for drinking etc
• Water Out - septic, usually
• Water... um, Across - where does your stormwater and overflow go

Water in was a problem when we bought in 2014. We had 8K L capacity. Water Out has only been a problem recently due to the intense rain we get now, tho I think I've mostly solved it. Water Across caused the Water Out problem.

Context

I live on Waiheke, and we have no reticulated water supply here - input or output. It's all either from the sky, or from a bore - on your property, or via a tanker. The usual cost for 10k L water is around $440.

We have a 20K L tank which feeds the house, and a 4K L tank which is feed off the roof and pumps up to the big tank, mostly due to how the house was setup when we bought it. We also have 2x 1K L caged tanks off the studio which is a totally seperate supply.

We've done a bunch of work around water over 11 years, tho some I'd have done earlier if we'd known about it.

Water In

To be able to turn your tap on, you need to have captured some water some how, and it needs to be in a state to drink.

Our setup is fairly common:

• Roof captures into a tank (4K L, then into the 20K L when I turn the pump on)
• House pump pumps from the 20K L into the house via 2x particle filters (5 and 20 micron) and a UV filter.

Things to do when you buy

• Look at how much capacity you have (20K L in our case) and work out how long that will last. If you don't have a tank meter, get one (cheap, doesn't need to be high tech tho there are good ones now which do WIFI - or just tap on the side of the tank if it's plastic). 20K L lasts 2 of us around 3-4 months of normal use, often a bit longer in summer and if we are careful. YMMV with kids, dogs, etc. You don't need to know you have 15,645 L. You need to know you are around 75% full, and last week you were about 85% full.
• If the house has no filters, get them. Right away. Drink bottled until you get them. Expect to change the cartridges every 3-6 months (DIY, its easy, takes 10 mins to change them out). You want a dual-cartridge one - a corse one (20 micron) for particles and a fine one (5 micron) for bacteria.
• If the house has no UV filter, get one. You're likely fine to use the water until you get one, but at the end of summer, you're also likely to, um, get the squirts. Cost was around $800 installed, from memory. If you need both, it's one labour job for all of it. Bulb lasts 12 months. You can self install but... I don't. Usually around $350 installed for the bulb. It'll beep/scream at you when the timer runs out.
• If you have the tank people out, get them to look if it needs a clean. It's fairly cheap and easy to get it cleaned.
• Do some Googling for the local water delivery companies, and see if you can find out from their website what the lead time is. Most will do online bookings, so it's fairly easy. Keep looking especially over summer (Jan/Feb/March in NZ). Knowing what a tanker load will cost is also handy.

Things to do over time

• Once you have an idea of how much you use, look at your capacity, and work out if you need either a new additional tank, or replace the one(s) you have. We started with 2x 4k L (seriously). It was clear fairly quickly that was fine for a bach, but not for permanent living. So we got the 20K L and kept one of the 4K due to the pipes / guttering. We are thinking about replacing the 4K with a low-profile one as it blocks the view from the bedroom.
• Look at your roof and guttering, esp in the crazy downpours we've had in the past 3 years. Ours couldn't handle it all, and it was just overflowing, so we got the "Typhoon" guttering installed. It's about 2x the capacity and it never overflows now.
• Find low-flow shower heads. Oddly, Aliexpress is excellent for this. Unless you're doing washing/dishes all the time, you don't need to get super-efficient ones, tho if you have to replace a washing machine / dishwasher, get a water-frugal one.
• Consider your garden if you have one. Watering plants takes a HUGE amount of water - we can drain a 1K L tank in 2-3 days with a light watering. You need to weigh up the cost of water vrs the cost of plants. That $300 palm is likely worth watering in a hot summer if the 10K of water costs $150.
• Install a tank on any roof space you have. Never let a roof's water go to waste :) If it's not convenient to get it into the main tank, get some smaller 1K L caged water tanks (black plastic if you can, inhibits growth in the water). You can also fill them from winter overflow in the main tanks, and then use them for the garden over summer. We have 2, and I'm looking to replace one (white) with 3 more (black).
• If you install a new tank, get a float on it. This makes your water supply draw from the TOP of the water level, not the bottom. So the sediment sinks, and you get the clean water at the top. Can be retrofitted, but it's easy when a new tank goes in.

So now you know roughly how much you use, and how long it'll take to get more if you are running out. We usually think about getting a refill around 25%, depending on the lead times. (We have access to the neighbours bore now, so it's less of a problem)

Water Out

At yes. That end. Most people don't think about it at all.

There isn't a lot you can do with septic systems, they are often regulated, and very messy to deal with. However:

• Know where the tank lid is, as it's often buried. If it's buried, consider a riser (the tank pumping place will often be able to install one for you)
• Know what kind of system you have. Tank + deep sink holes? Tank + pump + dripper lines? The former is no longer legal in Auckland (you can't install, do major repairs, replace them like for like), the latter is "current".
• If you have sink holes, know where they are, and check the caps to make sure they are at least somewhat sealed (need to let a little air in/out, but need to far enough above ground so rainwater doesn't get in). They legally need maintenance/ emptying / inspection every 12-36 months depending on your volume. You're local septic pump place will tell you. Make sure they get vacuumed when the main tank gets cleaned.
• If you have dripper lines, know where they are, and if they work. They legally need maintenance / inspection every 6 months (same with the pump).
• Be paranoid about what goes in. Esp things which kill bacteria. Don't have a waste disposal. Avoid putting fat down the sink (some is fine, but don't empty the fat from a fry-up down there). Wet wipes, even the ones which say "can be flushed", cannot be flushed.

Once you have an idea of how water flows across your property, you can make sure that it's not sitting/standing in the septic field, as this can cause back-flow into the tank (for sinkhole systems). Adding French Drains to move water to where you want is fairly easy, if a bit labour intensive.

Behind that tree is our septic field. Under about 5cm of moving water during the 2023 floods/cyclones. You can't see it in that, but that water is MOVING, its more a river than a flood.

Water Across

This is basically stormwater. In theory, you are capturing everything off your roof into a tank (or tanks), but...

• What happens if the tank is full? Where does it go then? It might be fine just overflowing... or not. Get an overflow pipe if it's not. Ours when right over the top of the septic tank... which was underground, and flooded. Bad times (and fixed now)
• Where does the neighbours / road overflow go? It's illegal to dump your overflow over the boundary, but water will go where it wants, and you might need to add drains etc to handle it. This is hard to know until you have a LOT of rain, and in our case, only if the neighbours tank is ALSO full, and possibly the neighbour on the other side of them.

This is harder to work out until it rains a lot, so be prepared to put a raincoat on during really heavy rain and go look.