Ventilation systems at wastewater treatment works, South Wales

Requirements and challenges

The site was unusual for a treatment works because virtually the whole site was underground.
The original concept was to put a roof over the treatment works and backfill with topsoil to provide a ‘no nuisance’ type of works.

This necessitated a robust odour control system which principally keeps the building under a slight negative pressure; thus preventing the uncontrolled release of odour to the outside.

All the extracted airflow from within the building is scrubbed clean before exhausting to the atmosphere.

First, we duly carried out the testing and compared the findings with the original design data provided when the building was first put into service, more than 20 years previous.

Our testing revealed only minor deterioration in the performance of the ventilation systems and, following discussions with the water board staff, it transpired that the main reason for carrying out the checks was due to reports of high levels of hydrogen sulphide being detected within certain areas of the building.

Following a long warm dry spell (when hydrogen sulphide can evolve quite quickly), the water board supplemented the originally installed ventilation by hiring additional fans and carbon filters to increase the number of air-changes in the problem areas.

R&B Industrial were asked what could be done to resolve the issues. We identified that there were two main areas where the hydrogen sulphide was most prominent. We then looked at how and where the gas was able to escape.

Unlike most waste water treatment works, the covered skips that hold the solids separated from the raw sewage are stored indoors. These are filled via a screw feed and pipework from the centrifuges and then taken off site to be emptied each day. The site manager commented that on a Monday morning when, up to 6 of the skips could be full, the H2S levels could well exceed the workplace exposure limit, preventing operators from going into the area.

Often, the roller doors would be left open to naturally ventilate the area which then compromised the negative pressurisation of the building and allowed nuisance odours to escape.

We quickly identified that the breather ports on the skips were allowing an uncontrolled release of H2S gas into the area and that general ventilation was not the most appropriate engineered control of the hazard.

Solutions

We then decided that the best approach was to capture the gas release at the source by fitting small hoods over the breather ports which then exhausted the gas stream into the scrubbing system.

We used the same principles as we do for the Fire Service with the diesel fume extraction system on their fire engines. H2S gas is corrosive to many materials, so we needed to consider this in the selection of the components that made up the Local Exhaust Ventilation system.

Through years of wear, tear and repairs where 10 pieces are taken off and 9 pieces go back on, the belt thickener area was seeing increasing levels of H2S, Applying HSG258 principles, we fitted polycarbonate sheets to the open weld mesh guarding and added flexible PVC curtaining to the existing canopy to make the machine as close to airtight as possible.