Silent Night: A Noise Investigation Case Study

When an upgrade and extension to a wastewater treatment plant was approved near a northern English village, the local residents were hopeful that all would be calm and that they would continue to enjoy a silent night.

Having been approved for planning with a BS4142 assessment, the works owners were surprised to be receiving noise complaints from one of the neighbours, so what was the issue?  That is where Moniteye  were bought in to help!

The Ghost of Wastewater Past

There had been a wastewater treatment works on the development site for many years, but it had recently been expanded to capture all of the waste from the nearby town of roughly 96,000 residents.  All of the sewers associated with the town would now flow into this one location which utilises some cutting edge technologies to handle the human waste.  This includes magnetite drum separators, which adhere magnetite to the particles of effluent in the water and trap them making them easier to separate, hundred-metre long odour extract ducts to remove the smells of the wastewater, and a series of Activated Sludge Processors (ASP’s) which oxygenate the water and allow bacteria in the water to consume the organic waste.  Everything that is not removed biologically is then formed into bricks which can be used as fertiliser in the agricultural industry.  The whole system is control by layers of sophisticated computer technology meaning that pumps and air movement units can be ramped up and down automatically, with the assistance of artificial learning intelligence.

Of course, the modern works doesn’t look like this, but who can resist an ewwww picture?

The new plant has been constructed approximately twenty metres from an existing cottage and a BS4142 assessment predicted that noise levels outside the property would only increase by an insignificant 0.5 dBA.  Inside the cottage, however, the residents could hear an intermittent, low rumbling noise accompanied with vibration that could be felt through the pillow at night.  The owners of the works had been unable to locate the source of the noise, so Moniteye was tasked with measuring both noise and vibration at the property, and conducting an investigation into the cause of the noise.

Magi?  No, Moniteye…

A Moniteye engineer attended the site and first visited the residents to establish what it was they could hear and feel.  It was described as a pulsing sound, at a very low frequency reoccurring roughly every second.  It had not always been audible, but had been off and on now for some six weeks. 

Although unable to distinguish the noise themselves, the engineer had a good description of the sound and an approximation of where the residents thought it was coming from; the prime suspect was a new Primary Sedimentation Tank, or PST.

An example of a PST, this one without a hood

The PST plays a critical role in treating wastewater, as it acts to remove solids and suspended particles in the effluent.  A long arm rotates within the wastewater at a rate of approximately forty-five minutes per rotation to stir the wastewater, and any solids sink to the bottom.  From here, the solids are pumped out from the bottom of the tank using a Primary Desludging Pump which moves them to a separate structure for repurposing into fertiliser.  Finally, and uniquely for this PST, a plastic composite hood had been added to cover the PST in order to trap any odours to stop it being unpleasant for the nearby cottage.  These odours and gases needed to be transported away from the PST, so a large duct of around one-hundred metres fed into the top of the hood.  This odour extraction was then driven by two large extract fans, with one operating as a redundancy for the other.

So there we had it, three potential suspects for the problem noise.

The Ghost of Wastewater Present

Over the course of the day, noise measurements were made of each of the items associated with the PST, in third-octave bands to assess which of the plant items could be causing the issue.

First, the rotating arm in the PST, or the Bridge as it is called, was measured.  The motor of this device caused a higher pitched, almost whining noise, and the rotation of the arm was far too slow for it to be causing the vibration felt at the cottage, so the Bridge was eliminated from our suspect pool.

Next, the Primary Desludging Pump, which does operate intermittently was measured.  The plant itself was not very loud, however, it did use large pistons to pump the solid waste at a rate of around one movement per second, exactly as the residents had described.  With the neighbours listening then, this plant was switched on and off to see if it was the cause of the noise.  Unfortunately, with the PDP switched off, the noise was still audible.

The large fans driving the odour extract from the PST

Finally, the extract fans were measured.  Although these fans were very loud (approximately 82 dBA at one metre), these did not have the tonal qualities associated with the noise and, at a distance of roughly ninety-seven metres from the cottage, were unlikely to be heard within the cottage.

We were stumped with all of our suspects seemingly innocent of the crime.

That evening, the engineer braved the cold and dark of the night to sit outside the cottage and listen out for the noise with the residents.  Now, with the extra noises of the day’s activities removed, the engineer was, for the first time, able to hear what they believed to be the problem noise. Although not particularly tonal the noise was indeed a low rumbling that seemed unobtrusive at first but became more penetrative as you continued to hear it.

Armed with a sound level meter and a torch, the engineer again measured the three main suspects as well as the PST itself, both from the side and from above.  Now, the data suggested that there was indeed a rumble coming from the PST, but how could this be?  All of the plant associated with the PST had been eliminated as the cause of the noise?

Feliz Noisydad

The following day, equipped this time with a vibration monitor, the engineer made another inspection of the PST.  The engineer now observed that the long extract fan ducting was rigidly attached to the PST hood and appeared to be vibrating the hood itself.  This vibration was confirmed with the vibration monitor and, once the fans had been switched off, the neighbours confirmed that the noise had stopped.  We had found our phantom noise!

Although we had located the source of the noise, the extract fans were required to be operational in order to extract gases away from the PST.  So how could the issue be resolved? 

A section of the extract ducting, together with its stanchions.

Moniteye consulted its expert contacts in the anti-vibration industry and together they suggested that the fans could be better isolated from the ducting, the ducting could be isolated from its stanchions, and that an inline attenuator could be introduced into the ducting.

These were potentially expensive solutions, so we discussed our findings with the owners of the site.  It was then that one of their engineers informed us that the extract fans used to be ran at 50Hz, however, it was thought that they were a little noisy, so their duty had been reduced to 45Hz.  Additionally, the damper at the end of the ducting had been set to an arbitrary amount of 60% open.  The Moniteye engineer then asked at what duty the fans could be ran at without it compromising the system as a whole.  They were informed that 25Hz was an acceptable low. 

Now the damper on the end of the ducting had to be tuned.  It was found that when the damper was open past 60%, this caused the ducting and the hood of the PST to vibrate whilst anything below 30% open would cause a whistling noise in the duct.  With the help of the onsite engineers, Moniteye set the damper to 50% and the duty of the fans was dropped to 25Hz.  Now, instead of 82 dBA at one metre, the fans measured 69 dBA at one metre; a significant reduction in source noise.

We then asked the residents if they could hear the noise… the noise had gone!

The residents tried to hear the noise with the windows open, the windows closed, and lying on the pillow, but the problem noise had been eliminated.  The resident said that as soon as the changes had been made, it “felt like heaven!” 

Problem Noises 0 – 1 Moniteye

So that is an abridged account of how Moniteye was able to locate and solve a problem noise for one of our clients and their neighbours.  Of course, in the future if the duty or the damper of the odour extract system needs to be adjusted, then more treatment of the fans and ducting may be required, but for now the problem has been solved solely for the low price of a Moniteye investigation.  A win for everyone!

Troubled by noises you can't pin down? Then you need an expert noise and vibration investigation.