Saturday 10 February 2024

Rainwater harvesting: dammed by regulation

For the whole of the East of England water supply is an increasing problem. Here in Cambridge we need to reduce the amount we take from our aquifers by about half in order to protect the chalk streams. This will be achieved with a combination of reducing leakage, reducing demand and increasing supply (by building large reservoirs). One way to reduce demand is to harvest rainwater for use where it does not have to be drinking quality, such as flushing toilets. 

Eddington in North West Cambridge has a rainwater harvesting system for 3000 homes, the largest in the country. It is often cited as an exemplar for water saving, with every home having two supplies: one from rainwater (non-potable) for use in the garden, in the washing machine and flushing the loo alongside the one from the mains for everything else. As I recall, the pipework has different colours. The rainwater is stored in a rather beautiful lake and is fairly clean but not as clean as the water we get from the chalk aquifer; there is equipment for basic treatment on site. The problem is, the rainwater harvesting supply has never been commissioned due to concerns from the Drinking Water Inspectorate (DWI). 


Brook Leys Lake at Eddington – stores rainwater for supply to homes, but not commissioned for use.

According to the latest Water Resource Management Plan (revised draft 2024): 

Eddington – We are working closely with Defra and the Drinking Water Inspectorate to progress legislative changes that will enable this scheme, and future schemes, to be able to operate at its full potential. [1]

 The DWI requires that the water is treated to potable standard. In the water industry, water quality is the number one critical priority, for which we are all grateful. However, if a home has a rainwater supply as well, this is a potential source of contamination. As far as I can tell, at least under current regulations, nothing will satisfy the DWI except that the rainwater is treated to potable standard, which means additional cost both for infrastructure and operations.

Rainwater can run out in a drought, but harvesting is still worthwhile.

Some say that rainwater harvesting is pointless because when you have a bad enough drought you will use up all your stores and then you need to use the mains water anyway. This means that the mains supply has to have enough capacity (treatment and pumping) to supply our total water needs. However, it is easy enough to build more infrastructure for supply; the problem is finding enough water. Since we do have droughts, this means we need water storage. The chalk aquifer that Cambridge Water uses has plenty of space (since we have been taking so much, levels are much lower than they used to be). If we used less when we are not in drought there would be more in reserve for the critical periods.

We need to reduce abstraction by half.

This chart from the Water Resources Management Plan [1] compared predicted demand with allowed supply on the baseline (business as usual) scenario. The blue line shows decreasing levels of abstraction as the Environment agency caps come in, limiting the amount of water we can abstract in order to repair damage to rivers and streams. The target headroom is breached in 2029/30 and things get very bad in 2040 when the cap is about half the current level. However, if there was more water in the aquifer, then licences for the drought periods could be increased.


Baseline critical period supply/demand balance and components of demand. Figure 12 from [1]. The blue line is the level of abstraction allowed.

Rainwater harvesting can cause contamination due to dodgy plumbing.

Many rainwater harvesting systems have a connection to the mains as well so that the store can be topped up when necessary. According to WaterRegsUK (a water industry representative body), any connection from the mains supply to a rainwater harvesting system must have a physical air gap to prevents any risk of contamination entering drinking water supplies [3].

Anglian Water found regulations were breached in 70% of existing installations.

The local water supply company is responsible for ensuring that rainwater harvesting systems conform to regulations. This means inspections of the installation and at regular intervals afterwards. Quite often, something goes wrong with the plumbing and someone fixes it – not necessarily a qualified plumber. During 2015-2020 Anglian Water inspected installs every two years (1200 properties) and found bodges that did not comply with the regulations in 70% [3].

Actual contamination on an Eco-Housing development.

Breaches of regulations are one thing, actual contamination is another. Back in 2010, at the Upton Eco-housing development (550 houses) one house complained about a sewage smell from tap water. To cut a long story very short, the problem was a cross connection from the rainwater supply to the mains and E-coli bacteria were found in water from the tap (though not in the first sample taken). In all, 88 homes on the estate were found to have a rainwater harvesting system with a cross connection however in all but three cases the valve in the connection was closed. More samples were taken from those houses but only the first house had E.coli [4].

Is contamination a serious risk?

I understand that there is a risk but the dearth of documented cases of contamination on the internet suggests this is not a high risk. Exactly how low or high is not well understood, but the water industry takes it very seriously. Here are some quotes from the WaterRegsUK response to a consultation about incentivising developers to build houses to save water [5]. 

… There is also a lack of basic research on the risks to human health associated with the use of non-drinking water for toilet flushing

i.e. we are not sure it is safe to use rainwater even for flushing the toilet.

 Water companies do not appear to have the right to provide unwholesome water that is not a potential danger to health for the purposes of toilet flushing. This may have implications for water companies who may be willing to own and operate community water re-use systems. 

That means that even if rainwater is OK to use for flushing the toilet, water companies are not allowed to provide it. (However they do mention that these regulations are due to be reviewed).

Perhaps 3rd parties are allowed, but personally I would be rather wary of a 3rd party rainwater supplier; whereas whatever else I may think of my water company I do know they are quite reliable at supplying water.

Thinking from the point of view of a water company.

(This is my interpretation, of what I have read, not an actual response from the water company.) 

  • Rainwater harvesting does not reduce demand during a drought.
    • However, especially in places like Cambridge taking water from an aquifer, reducing consumption in normal times does mean there is more available when there is a drought.
  • Rainwater harvesting systems in individual homes require regular inspections because bodged plumbing fixes can lead to contamination with the mains. This is a cost (which ultimately the customer will pay).
  • Doing it at large scale under water company control could be a less risky approach but water companies are not (currently) allowed to supply water that is not treated to drinking water standard. 
  • In any home with a rainwater supply there is a theoretical danger of someone doing something silly. A person ignorant of the plumbing system and its colour code could tap into a rainwater pipe for a new shower. Or someone might try drinking from an outside tap meant only for watering the garden.

How much risk is acceptable?

It is impossible to allow rainwater harvesting systems with zero risk to public health. There is always a risk of either contamination of a mains supply or inadvertent drinking of the untreated rainwater. How much risk is reasonable? For that matter, who should decide?

[1] Revised Draft Water Resources Management Plan 2024 (Cambridge Water) 

[2] Brook Leys parkland opens (Eddington Cambidge) 2018 

[3] Rainwater Harvesting Case Study (WaterRegsUK) 

[4] Drinking Water Quality Event (Upton Eco-Housing Development) DWI, 2019

[5] OFWAT Consultation on environmental incentives to support sustainable new homes – consultation response from WaterRegsUK (OFWAT) August 2023


  1. I am just getting quotes (with difficulty) for a retro-fit rainwater harvesting system in Dorset. So far no-one has mentioned water inspections other than the installation inspection by Building Control. Does it vary by region, or have I missed something?

  2. Can you comment on the net benefit of rainwater harvesting particularly in a water-rich area? I know that overall it will cost me money, so I would be basing the decision on environmental impact only. I have seen studies that show that the carbon cost of the tank, plumbing and extra pumping exceeds the carbon cost of mains water.

    1. Well using rainwater avoids the need for treating and pumping mains water to you. Your water bills will be less. Also, from your point of view, if you are in a hard water area you may benefit from less scale because rainwater is soft. Using rainwater in the garden is especially nice for your plants.

  3. I don't know exactly. I suspect although water companies have responsibility to check there are no requirements for any particular frequency for doing so. Also I don't know if you have to pay for this check - if not then it is likely no-one would mention it.


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