'Free' Water reality check

zeupater

MFW_ASAP wrote: »

Thanks for the interest.

I'm still digging out the channels and laying pipework I'm afraid, so no update as yet.

I did look at larger tanks but read that a lot of rainwater harvesting companies recommend tanks that are oversized in order to increase their profit (larger tank = more money), but that it increased waterborne disease risks due to water stagnation.

According to many sites I researched, the safe way to determine the size of the storage tank was to calculate the amount of rainwater you receive (using rainwater tables, the size of roof collecting the rain and the type of roofing material) against the water useage in the home (using info on number of people in the house, number of toilets, washing machines, size of garden) and using the smaller figure to calculate your tank size.

It seemed a logical suggestion as too large a tank will never be filled if you have a small roof and even if it is filled, it'll stagnate if you only use a few litres a day to flush a toilet.

I used an online form that asked the above questions (and included a safe amount of extra water to cover a drought) and it suggested a 5000l tank, so that's what I'll go with. I dare say that during extreme periods of heavy rain I'll be kicking myself for all that 'lost' water (though I wouldnt be able to use all this water anyway!) or during periods of drought I'll be lamenting using tap water for flushing toilets, but you can't spec a system (and have the additional costs) for cases that may only occur very rarely, perhaps only once or twice in the system's lifetime.

Hi

I was a little concerned with the size of the tank related to the usage for 6 people ... our ~200sqm roof would collect around 5tonnes per 1" of rainfall ... last week there were a number of occasions where that would have been well inside 1hour, but that's rare ... prior to that it had been pretty dry for well over a month, which at your 2000l/week usage (probably mostly toilet-water) would likely have resulted in an empty system for a week-or-so ...

Eric's posts seem to backup a rule-of-thumb (which I heard when looking into RWH a number of years ago) that planning around 1tonne storage per person would cover average domestic household usage, but this needs to be a little flexible depending on the level of daytime occupancy, leaving very little leeway for garden/car washing etc for which Eric's solution is to have 5tonnes of secondary storage whilst our's is a mere 1 tonne (in butts) which regularly stands empty in the summer ...

Legionella really shouldn't be a worry in the underground storage due to the conditions ... I'd be more worried about having standing water in a header tank in a warm loft space if you're going for a gravity-fed solution, makes me wonder how much of that information is down to a particular supplier having a swipe at the competition as opposed to being useful information ...

If it's plastic drains then you really need to ensure that the works are backfilled before the winter .... if it was me, I'd have started by having the mini-digger needed to excavate for the tank do all of the backbreaking trench-work too ... have the tank delivered, install & connect, back fill everything, do something with upwards of 20 tonnes of excavated earth (garden feature/skip), then send the digger back (possibly hire/contract on two occasions) ... plumb it in, job done, start moneysaving - as it is it looks as if you're currently spending money and saving nothing ...

HTH
Z

zeupater

MFW_ASAP wrote: »

Using the calculation above, the Annual Rainwater Yield (ARY)for our house comes to 6400 litres and the Annual Rainfall Demand (ARD) is 4800L.

Taking the lower of the two, I should go with a 4800 litre tank, bu I'm assuming an extra 200l is neither here nor there, so I'll go with 5000L.

I'm guessing from the calculation that the British Standards Institute has assumed an annual water useage of 16,000 litres (non-potable water so no bathing or drinking, just flushing a toilet and washing clothes). That's 43 litres a day. According to the 'tinternet, the average toilet flush uses 4 litres of water, so I can go to the loo 10.96 times per day!

According to the same site, an average washing machine uses 50 litres per wash. At 4 washes per week multiplied by two washing machines, that's 8 washes totalling 400 litres. Divide that by 6 people, it comes to 66 litres per week or 9.52 litres per day.

Therefore with my 43 litres per day reduced by 9.52 (call it 10) litres for the washing machine, that leaves 33 litres for flushing the toilet - or 8.25 individual flushes.

If these figures are correct then I should be OK except during periods where a dodgy curry is consumed! However, this doesn't take into consideration that 4 of the six of us are out all day at school or work, and will therefore be poo or wee flushing with non MSE_ASAP stored water.

5000 Litres should be OK for us I think.

Hi

Before going any further check your sources .... if we emptied 2x2litre fizzy-drink bottles of water into our cistern it'd not be anywhere near the level for the syphon to work .... try it out, it's likely closer to 4 gallons !

HTH
Z

EricMears

Our rainwater is used only for WC flushing and I tend to pump 100 litres from basement to attic every day when there are just two of us in the house. If MFW_ASAP's 43 litres per day is a per person figure then it's in the same parish.

Our 4 tonnes of indoor storage was severely tested last week when I got down to 800 litres of 'stock' but a wet weekend & another thunderstorm yesterday brought that back to 3.7 tonnes in stock by last night (when I stopped collecting because I didn't want to be woken up by the nearly full alarm). Not sure that I'm a case to support Z's 1 tonne per person theory. We have been a 3 person household for last few weeks but even so a 3 tonne storage capacity would have left us using a few hundred litres of tap water and that of course is without any extra allowance for washing etc.

BTW, I don't wash the cars very often (not a great deal of point when you live half a mile off tarmac) but do occasionally clean their windows when I use mains water.

MFW_ASAP

The websites I looked at for cistern flushes mentioned the figures of 12 litres of old fashioned toilets, 6 litres for full flush and 3 litres for half flush for dual flush modern toilets. I therefore averaged the flushes to 4 litres. We have water saving toilets, taps, showers as I believe that conservation should always be the first step before any other solution is put in place. If you google 'how many litres to flush a toilet' you'll see the results. If your toilet won't flush at all with 4 litres of water, then perhaps you have an older toilet?

MFW_ASAP

You might be right about the legionella, but the tank sizing calculation comes from the British Standards Institute and seems to be widely used on a lot of rainwater harvesting sites for calculating their tank sizes. I tried a few sites and all seemed to suggest the 4800/5000 litres for our usage.

i understand your concerns about running out of water, but I don't want to spend a lot of extra money to store water I won't use except during extreme conditions. We live in the Pennines, so rain int a rare commodity

MFW_ASAP

Regarding the comment about just getting on with the work and start saving money... I wish I could afford to, but with the other work on the house, mortgage overpayments and other major demands on my pay (aka children), I can't afford to do if all at once. I'm digging by hand and doing bits as I go along. The costs are really minimal - free labour (I'm enjoying the exercise) and plastic drainage is cheap as chips).

Bringing the rainwater pipes together is the cheap, but time consuming part. The expensive bits will be the header tank and the underground tank, I'll sort these out once I've saved up for them.

zeupater

EricMears wrote: »

Our rainwater is used only for WC flushing and I tend to pump 100 litres from basement to attic every day when there are just two of us in the house. If MFW_ASAP's 43 litres per day is a per person figure then it's in the same parish.

Our 4 tonnes of indoor storage was severely tested last week when I got down to 800 litres of 'stock' but a wet weekend & another thunderstorm yesterday brought that back to 3.7 tonnes in stock by last night (when I stopped collecting because I didn't want to be woken up by the nearly full alarm). Not sure that I'm a case to support Z's 1 tonne per person theory. We have been a 3 person household for last few weeks but even so a 3 tonne storage capacity would have left us using a few hundred litres of tap water and that of course is without any extra allowance for washing etc.

BTW, I don't wash the cars very often (not a great deal of point when you live half a mile off tarmac) but do occasionally clean their windows when I use mains water.

Hi Eric

The information referenced by MFW_ASAP multiplies the number of residents by 16000 so it's per person. Although modern low capacity dual flush eco-cisterns can use around 4litres for a short flush, they'll use considerably more for a full-flush, however, the majority of cisterns are still likely to be between 3 & 4 gallons, so say around 16litres, conveniently representing 1000 flushes/person/year, or ~3/day .... that's probably the basis of how the figure was originally arrived at ....

Now, even when allowing for more efficient toilets, flushing 3x/day per person is probably a little on the low side even if you're not at home during the day whilst double that is probably more likely if you're at home all day .... at Christmas we have elderly relatives stay and, believe me, six flushes would just about cover the daylight hours, hence the qualifier "1tonne storage per person would cover average domestic household usage, but this needs to be a little flexible depending on the level of daytime occupancy" ....

To calculate the storage volume based on usage simply takes 5% of the 16000l/person/year. What this really means is that the storage is based on a buffer of 18days (365x0.05), so about 2.5weeks .... of course, depending on type of toilet and the number of flushed, this could easily be much closer to 1 week ....

On average, the calculations would suggest that somewhere around 150Tonnes of water would fall on our roof/year but we should only collect 20% of it and be restricted on flushing the toilet incase we run-out ?? ... I'd certainly take the guidance as what it is - "guidance" and look to see what my own toilets would flush, maybe replace them with eco-units, maybe oversize the storage ... after all, as long as the roof isn't the controlling factor, you can always spray the garden ... :cool::D

HTH
Z

zeupater

MFW_ASAP wrote: »

Regarding the comment about just getting on with the work and start saving money... I wish I could afford to, but with the other work on the house, mortgage overpayments and other major demands on my pay (aka children), I can't afford to do if all at once. I'm digging by hand and doing bits as I go along. The costs are really minimal - free labour (I'm enjoying the exercise) and plastic drainage is cheap as chips).

Bringing the rainwater pipes together is the cheap, but time consuming part. The expensive bits will be the header tank and the underground tank, I'll sort these out once I've saved up for them.

Hi

I follow the reasoning, but really question the sanity....

To dig a hole to bury a 5 cubic metre tank, you'll likely need to excavate somewhere around 12 to 15 cubic metres of soil and that's only allowing for a 500mm clearance around the tank, that could total around 40 tonnes .... the issue stands that if you're going to have to pay for the equipment to open-up the hole, then why not put the pipe trenches in at the same time ?? ... it'll save your back, get the job done sooner and likely cost no more as the equipment will be there anyway ....

HTH
Z

MFW_ASAP

zeupater wrote: »

Hi

I follow the reasoning, but really question the sanity....

To dig a hole to bury a 5 cubic metre tank, you'll likely need to excavate somewhere around 12 to 15 cubic metres of soil and that's only allowing for a 500mm clearance around the tank, that could total around 40 tonnes .... the issue stands that if you're going to have to pay for the equipment to open-up the hole, then why not put the pipe trenches in at the same time ?? ... it'll save your back, get the job done sooner and likely cost no more as the equipment will be there anyway ....

HTH
Z

Ah, I understand your confusion - you're not aware of the full story (I didn't go into it because I didn't think it was relevant to rainwater harvesting or indeed! interesting to anyone). However, as you're now questioning my sanity (!) , I'll explain...

We have a solid walled house with a slight damp problem. I'm installing internal wall insulation and so needed to remedy the damp issue first. The property has land drains around the base of our house but they were covered up with soil that has a high clay content, which impacts their effectiveness.

To remedy this, I'm digging them out and filling with large sized gravel for drainage (and edging the gravel with paving). This is called a French Drain and is a well known solution to damp in older houses.

As I work from one end of the house to the other, I do remedial work on the drains as I come across them . So far I have fitted an external stop tap to the main supply of the house and changed the kitchen sink waste so that it enters the trap below the grid rather than on the grid (stopping our usual autumn floods when leaves clog the drains, and I'm meeting current building regs).

During this remedial work, I had the idea of disconnecting the rainwater down pipes from the sewer and routing them together and off into our stream. As a 'green' person I hated the idea of relatively clean rainwater going into a sewer to be processed when it could go unprocessed into a stream.

I then researched rainwater harvesting and decided that it would be even 'greener' to use this relatively clean water to flush my toilets and wash my clothes, rather than use potable, processed drinking water.

I'm not in the financial position to install the rainwater recycling kit just now, but I do need to sort out my damp problem so I can complete my internal insulation. So I decided to continue with the French drain and just re-route the rainwater to a central point to await the eventual installation of a tank.

If I waited until I had the money to do the full harvesting system, I'd have to dig out some of the French drain and lift up paving as well, creating work for myself.

Hope that makes more sense now and you'll question my sanity no more

MFW_ASAP

Phase 2 of the project will involve installing a header tank (connected just to the mains for now) and plumbing this to the toilets and washing machines. The reason this is a separate phase from installing the underground tank is because I'm fitting internal wall insulation one room at a time and so when I get to the bathrooms I have access to the pipe work and do a 'first fit' of the rainwater pipes (i.e. Installing the pipe work but not connecting it). Once all the bathrooms and utility rooms have internal insulation and pipework, I can fit the loft tank and connect everything up.

Once that is complete, I can then look at installing the underground tank, connecting it to the header tank in the loft and having a completed system. This might be a while off (perhaps even a couple of years because installing internal insulation is neither cheap nor quick!) but in the meantime, I will benefit from having the rainwater run into the stream because I will receive a rebate from United utilities for surface drainage. Two years or so of this rebate will pay for the underground pipe work that I'm installing.

It's all planned out, and planned around other work that I'm doing, so please don't worry about the sanity of my approach!