Hunting for harmful bacteria in UK rivers

Universities and citizen scientists alike are on the lookout for indicators of faecal matter...
25 July 2022

Interview with 

Susannah Salter, Cambridge University, Anne Miller, Cam Valley Forum


River sewage


Harry Lewis and James Tytko have been thinking about their early morning swim. Guests along their summer trip down the River Cam have left them worried about the cleanliness of the water and whether it was safe to swim in. Susannah Salter is a research assistant in the Cambridge Department of Veterinary Medicine, she's been looking for the presence of E.coli in the local river. Whilst Susannah is testing the same site daily, Anne Miller from Cam Valley Forum and her army of volunteer heroes have been looking for traces of E.coli and enterococci bacteria across a range of different sites...

James - And here we are with two people who've been monitoring the health of the ecosystem very closely. Can I ask you both to introduce yourselves please?

Anne - I'm Anne Miller from Cam Valley Forum, and we've been taking water samples and getting them analysed to look at the water quality.

Susannah - And I'm Susannah Salter. I'm a research assistant at the Department of Veterinary Medicine, and I've also been monitoring E. Coli in the river.

James - So Harry and I had a dip in the river earlier, did some wild swimming with a local swimming club. And it strikes me now that it probably would've been a good idea to speak to you guys before we jumped in. Should we be worried?

Anne - Well, the key thing is, where were you swimming? If you were swimming a couple of miles downstream of Haslingfield sewage works, I really wouldn't have recommended it. But if you were in the popular swimming spots in Cambridge, like Newnham riverbank club and Sheep's Green, I think it's probably okay, but just so long as you didn't drink it.

Susannah - So I've been measuring a site that's a couple of miles upstream of Cambridge at Byron's pool, which is in a nature reserve between Grantchester and Trumpington, and I've been doing frequent sampling, weekly sampling of the river and measuring E. coli levels. So E. coli is a faecal indicator organism and, week by week, it changes quite a lot actually. And the season depends on what the levels are like in the river.

James - Why are you looking for indicators of faecal matter?

Anne - It's an indication of whether there's poo in the water. And from our point of view as a community group, we are looking for quite a simple way of seeing is this water safe. Personally, I don't want to swim in poo and the bathing water directive sets thresholds at which the levels should be. And so we're trying to see whether the water would be classified as good.

James - And you're a community driven organisation. How do you cooperate with perhaps a more scientific outlet?

Anne - We've got lots of volunteers, but we haven't got very much money. We've done about six batches of samples over the last year at typically 20 sites going as far upstream as about 10 to 15 miles upstream of Cambridge and a few miles downstream of Cambridge. And then we send our samples, water samples that we collect, off to a professionally accredited lab to be analysed. So that's the real sort of science bit. And then we put them on the website so anybody can see what they are, but then Susannah does the more detailed work on a particular site.

Susannah - I started looking at the river because, a few years ago, I was involved in a detailed survey called punt seq, the punt seq project, where we were sampling from different places along the Cam, and then sequencing the DNA in the samples to have a look at what bacteria were present in the river. Lots of interesting results came out of that, but one that piqued my interest was that there was a gut associated pattern in some places, not all the time, though. And one of the places that we saw that was at Byron's pool. And so I wondered whether it was something that was a really rare event or whether it was actually something that happened regularly. And so we thought if we did this pilot study to look in much more detail with much more frequent sampling, that we'd be able to see whether that was something that happened often.

James - And can I ask the both of you to just summarise your sampling techniques?

Anne - We take a clean sample bottle that we get from the labs and we attach it with elastic bands to the end of a rake that go about three metres long, and we dip it into a particular depth in the water, take our samples, and then they all have to get to the lab by, I think, one o'clock in order to be analysed. So it takes us probably a few weeks sometimes before we've got the results back.

Susannah - My sampling technique, on the other hand, the collection of the samples is very similar, but instead of looking at a wide range of places, I'm looking at one place but at a really high frequency. I do the testing myself in my laboratory, in the department of veterinary medicine. And some of the assays that I do are very similar to what's done in the commercial labs, but I'm also comparing that with other kinds of methods. I collect my samples on my way to work in the morning. So they go straight from the river into the lab.

James - Can you talk us through the results of that sampling?

Anne - So the main focus of our work was to try to find out where the sources were, or at least where the major sources were of the pollution that we know is coming into the Cam. And what our work seems to be showing is that the Cambridge sewage works isn't too bad - usually. Sometimes, just once, it was absolutely dreadful, but the main offender seems to be Haslingfield sewage works, which is a few miles upstream of Cambridge and possibly Melbourn sewage works. So we've only done two readings there, which is a little bit further still. And sometimes we've seen leaks coming from the pipes going to the sewage works and so that's completely raw sewage that's flowing into the river. That hopefully doesn't happen for very long because they go and fix it. But we've literally seen raw sewage running down the bank into the river, which is really shocking and shouldn't be happening.

James - Is this sewage in the water that you're describing enough to be harmful to the environment, to people?

Anne - Sometimes. So the readings we got in June at Haslingfield were 17 times higher than the level that the environment agency would call poor quality. So that must be harmful. But on the other hand, a day later, we think it was probably okay. So one of the things that we are really seeing is the way it varies. Everybody goes and has a poo in the morning and a few hours later, it gets through the sewage works and the levels are then higher. By the afternoon, it's not so bad. So what we really, really want and probably Susannah does too, is an instrument that would allow us to measure in real time continuously what's going on.

James - Do you agree? Do your results concur, Susannah?

Susannah - Yeah. So the results that I've had have shown that in winter, the levels are quite high. My sampling site is a couple of kilometres downstream from Haslingfield, and some of the highest levels that I've recorded have coincided with known recorded sewage spills during very bad weather. So this is a storm overflow. This is something that happens when you have really heavy rain and very bad weather, and to stop the system from being overloaded, they release partially treated effluent. The highest levels that I've seen were last year, sort of towards the end of October, which were in excess of 10,000 CFU (colony forming unit) per hundred ml, which is quite a lot. So we were talking about acceptable levels that you would have in bathing waters. Normally that would be significantly below 1000 CFU. So this is very high. And 10,000 is like, if you were to take a gram of fresh faeces and mix it in about 10 litres of water, you'd get a similar result from that. So it is quite a high level to be having a significant distance downstream of the sewage plant, but there are other times where the levels have been quite persistently, unacceptably high, but it doesn't map with one of these storm events or extremely heavy rain. And the water company are not reporting that they have had a spill on those dates. So I don't think that the only explanation of that - of course animals can also be a source of waste material getting into the rivers - but also it's possible that where you have silty stretches along the river, that when you have very heavy rain, for example, it may not be enough to cause a problem at the sewage plant, but it does kick up all of the silt that's at the bottom of the river. The river becomes very turbid. And if you have some of this material sort of surviving in the silt at the bottom of the river, that's normally settled down when it all gets re-suspended in the water, you may see your levels temporarily rise up. And, certainly in terms of how long the levels stay high, that very high level back in October, it stayed above 10,000 CFU for two weeks. So it's not quite as simple as just waste going into the water and then being washed through and then it not being a problem anymore.

James - Those results kind of alarm me when I think about how frivolously we dived into the river earlier. I mean, maybe I'd have kept my mouth more tightly pursed than I did.

Susannha - I collected a sample two days ago and I got my most recent results this morning. And actually this week, the levels are fairly low. So they're around about sort of 5 to 600 CFU. So that's not quite so bad. And I swim in the river too, although not in the winter.

Anne - And we took a reading a couple of weeks ago from pretty much where you were swimming. And I seemed to remember that the reading was about 400, which is okay, but I would keep my mouth shut. And I too swim in the river, but I keep my mouth shut.

James - And moving forward with the data you've collected, what can we do with it? What are the next steps?

Anne - We're using it to apply pressure to the people who are causing the pollution to clean it up. First, we want to find out where it is in a proper scientific way, as well as we can. And so clearly some of it is the responsibility of Anglian Water, who own Haslingfield sewage works, and they actually seem to really be listening to us and we hope are being responsive. We are starting to explore whether we can get hold of an instrument to let us monitor continuously, because we also strongly suspect that it's not just the sewage works that's the problem. It's the farmers emptying out their slurry and things like that. Earlier this year, we were doing some tests outside Cambridge sewage works, and one of our heroes was taking samples of the effluent coming out of the sewage works and there were brown bits in it. And yes, it was what we feared, it was poo. And we discovered that this was because somebody had fly tipped some slurry liquid poo, basically, at the inlet of the sewage works and it had flown through completely overwhelming the system. And that's why there was bits of poo at that instant going into the river

James - For someone who doesn't know a lot about these things. What you've said to me so far today sounds quite alarming. Susannah, is it something we should be more worried about?

Susannah - When I started sampling, I was surprised at how high the levels were too. And clearly this is something that we are looking at locally, but it is a really significant issue all around the country in rivers everywhere. This is a nationwide issue. And because people enjoy our rivers and also our coastlines as well for swimming and for recreation, it's really important that people know that the water's going to be clean. And in some places there is monitoring in place so that people know when there's been an event, that means they can't swim there, but in other places, that's not the case. And for a lot of inland rivers, that's not the case. So yeah, it is something that people really should be concerned about.

Anne - And on top of that, the water companies need to have the power to tell developers that they can't build houses because they haven't got the sewage capacity because that's part of the problem. And the other part of the problem is that, particularly in a dry place like Cambridge, we haven't really got much water in the aquifer. And so what they're doing is, they pump water, clean water out of the chalk aquifer, to put into the river, to flow down the river, to delete the sewage to acceptable levels. That is completely and utterly nuts. So we need to stop the problem at source, have good quality sewage works, and not put excess development where it can't sustainably be.

James - And that's what Cam Valley Forum are all about. That's what they're doing?

Anne - That's right. And if, and if people want to be involved with what we're doing or check out our results, go and have a look on our website, which is


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