Rain water harvesting for garden irrigation tank size?

Jibber123

I've just seen they have put the unit price of water up to nearly £4 a m3, we live on a small holding and 95% of our water is used outside for animal and plants. I've spoken to Anglia water about an alternative tariff but they are aimed at agriculture/ industry, we simply don't use enough. We already have grey water reclamation to reuse the bath/shower water to flush the toilet. But I'm looking at putting a large 4000 litre rainwater storage tank I've got in the garden mainly for watering the crops and plants we grow. I've got an irrigation system that comes on at night to water. I read the meter last night and again this morning to find we are using 1300litres a night at the moment (worst case at the moment as its so hot). On that basis the tank will only last about 3 days, and currently were getting rain about once a month if were lucky, so I'm back to paying for the water the other 27 days a month. I could go for a larger tank say 50,000 litres but the cost significantly outweighs the savings and I'm looking at a ROI of 20 years! the online tools for calculating tank size don't seem to take into account we get rain very infrequently now in the summer. The past 2 years have been particularly bad, has anyone got any experience in actual tank sizing and how much they saved?

QrizB

Before getting too carried away, how much roof area do you have that you can divert to the tank?

Norwich, for example, gets an average of 320mm of rain a year. 100 square metres of roof (a larger-than-average property) will collect a total of 32 cubic metres of rain, worth £128.

JSHarris

Have you looked at getting a borehole?

We don't have the luxury of mains water here so I had to put a borehole in.  Legally you can do this without restriction as long as you don't abstract more than 20,000 litres per day.

Although the borehole cost around £6,000 to put in, the running costs are very low.  The pump uses about 620W when delivering around 30 litres/minute, so will provide a 1,200 litre tank top up each day in about 40 minutes.  If electricity costs 40p/kWh then that 1,200 litres would cost under 17p.  That's a fair bit cheaper than the £4.80 it would cost you from mains water, but takes no account of repaying the cost of the borehole.

mmmmikey

I put this in my garden a few years ago - IIRC it's about 2000l. It took me the best part of a month to dig the hole - it quickly became a pick axe job and very soon after that backbreaking hammer and chisel work. I've never been as fit:

The limiting thing is not the size of the tank, it's the amount of rainfall. I live in Lincolnshire and we can go for weeks in the summer months without significant rainfall. I probably save about £5 or £10 per month in water charges - I daresay I could save a bit more, but in practice what I'm doing is more watering now I can do if that makes sense.

I'd suggest you start by looking at the rainfall for wherever you live (good data from the Met Office website) for the months you need the water and use that and the area of your roof to guesstimate the total potential collection. From that, you'll be able to work out the maximum possible savings, and I suspect that might be enough to put you off the idea.

It was a fun project and may just about pay for itself in my lifetime but I'd struggle to justify it on cost grounds. More of a "mad scheme" than a genius money saving idea in my case

MrInvisible1

Does anyone know how to harvest rain water for drinking?

JSHarris

MrInvisible1 said:

Does anyone know how to harvest rain water for drinking?

Not hard to make it safe, but it's debatable as to whether it makes sense, economically.  For all intents and purposes our borehole water may as well be rainwater, in terms of the environmental health requirements.  There's an assumption that ground water will be contaminated in the same way as rainwater (mainly from animal faeces, bird faeces in the case of rainwater). 

The system we have for treating our water is pretty much identical to mains water supplies.  We have a sand filter (backwashed every 4 days, automatically) followed by a 5µ filter, then a disinfection unit.  Instead of chlorination (which is the predominant way mains water is disinfected) we use a UV unit to disinfect the water.  Works very well and leaves no nasty chlorine residue in the water, but does have a significant running cost.

Our UV unit uses a mercury vapour lamp, inside a stainless steel tube, to zap the (non-existent) bugs.  This uses around 30W, and has to run 24/7, so uses a fair bit of energy.  Also, the UV light needs replacing every year, and isn't cheap (the tubes cost around £30 each when bought in bulk - more if bought one-off each year).

A rainwater system would need something similar, a coarse sand filter to take out the inevitable big bits of stuff that get washed in, a finer 5µ filter to take out the stuff that makes the water a bit cloudy (any cloudiness stops the UV disinfection working properly) and the UV system (or an ozone or chlorination plant - both more hassle than needed for a domestic system).

Biggest problem is as already mentioned, the available rainfall in your area.  We think the UK is wet, but in reality we can get very little rain during the summer months.  I have a 500 litre water butt, used for watering the garden, and that very often runs dry in July and August.

Reed_Richards

JSHarris said:

Our UV unit uses a mercury vapour lamp, inside a stainless steel tube, to zap the (non-existent) bugs.  This uses around 30W, and has to run 24/7, so uses a fair bit of energy.  Also, the UV light needs replacing every year, and isn't cheap (the tubes cost around £30 each when bought in bulk - more if bought one-off each year).

Have you looked to see if there is an LED alternative?  If so that should use a lot less energy.

JSHarris

Reed_Richards said:

JSHarris said:

Our UV unit uses a mercury vapour lamp, inside a stainless steel tube, to zap the (non-existent) bugs.  This uses around 30W, and has to run 24/7, so uses a fair bit of energy.  Also, the UV light needs replacing every year, and isn't cheap (the tubes cost around £30 each when bought in bulk - more if bought one-off each year).

Have you looked to see if there is an LED alternative?  If so that should use a lot less energy.

Unfortunately UV-C LEDs are mega expensive, aren't very powerful and don't last long.  I live in hope that they will one day be able to make long-lasting and energy efficient UV-C LEDS, but it seems they tend to self-destruct from the light they produce.  UV-C is lethal stuff, it severely damages a lot of materials.  Our disinfection unit uses a quartz tube to house the UV mercury vapour lamp, running down the centre of a polished stainless steel housing.  It's sealed at either end with Viton O rings, but even those O rings fail every couple of years from the UV exposure.

ispookie666

Here I was thinking my 300L was huge ☺️
@JSHarris
For UV have a look at Troptronic units. I used to use them in my Marine Tank. The company does refurbish the burners, they are not cheap but very very good. Also used to Use Ozone - but you need a dedicated reactor for the ozone to work (unless you can run it through an aerator or skimmer).  Ozone will eat through pretty much everything - maybe PTFE, Viton and high quality Silicone will work better. 

Why not add multi stage RO and then UV.  10micron, 5 and then 1 micron. Along with dual membranes in series and post filter carbon block along with RO pump(Osmotics are a good company). I used to use this when I had my tank. The RO booster pumps are self priming. 

markin

Are you using drip or sprinklers?

Land for a pond?

JSHarris

ispookie666 said:

Here I was thinking my 300L was huge ☺️
@JSHarris
For UV have a look at Troptronic units. I used to use them in my Marine Tank. The company does refurbish the burners, they are not cheap but very very good. Also used to Use Ozone - but you need a dedicated reactor for the ozone to work (unless you can run it through an aerator or skimmer).  Ozone will eat through pretty much everything - maybe PTFE, Viton and high quality Silicone will work better. 

Why not add multi stage RO and then UV.  10micron, 5 and then 1 micron. Along with dual membranes in series and post filter carbon block along with RO pump(Osmotics are a good company). I used to use this when I had my tank. The RO booster pumps are self priming. 

The problem is the volume handling capacity of the disinfection system.  RO is a non-starter, this is a whole house supply and needs to meet the Environmental Health standards at the max flow rate (about 30 litres/minute).  Same goes for any LED system, they are far too puny to be able to kill off all the bugs at that sort of flow rate.

I don't much like the mercury vapour lamps, as much for the fact they become hazardous waste at EOL as anything else, but there's nothing on the market that comes close in terms of being able to treat relatively fast flowing water to the required standard.  The very best UV-C LEDS are only about 10% of the efficiency of a mercury vapour lamp.  We need something that can generate at least 40mJ/cm² at UV-C wavelengths (typically around 254nm) inside the reaction vessel (just a stainless steel tube that the water flows through).  This energy needs to be imparted to the whole water volume flowing through the reaction vessel in less than a second.

Ozone is a nightmare for domestic scale drinking water disinfection for several reasons.  First is the fact that it corrodes anything it gets near, as it's an extremely powerful oxidiser.  That means you can't use most plumbing materials in any part of the system where there might be residual ozone, only stuff like PTFE.  The volume of ozone that needs to be injected is another issue.  Not hard to build a powerful enough ozone generator but you can't turn them up and down easily.  This means having to have a way to safely vent off unwanted ozone when a tap is turned off and neutralising the vented ozone it so it doesn't corrode anything nearby.

I did run an ozone injection system to oxidise the ferrous iron and hydrogen sulphide in our borehole water.  Very effective at doing that, but it wreaked havoc with everything in the water plant room.  The tiniest leak and pipes and cables would rot out in days.  I replaced the ozone injector and reaction vessel with a Aquamandix oxidising filter, that uses injected air to replenish the oxidising media.  That works very well, and has been running for years now with no problems.