Case Study: Large Office Building

Clearwater Technology Ltd were asked to investigate the constant presence of Legionella in a large office block in Southampton. The block had numerous cold water storage tanks, which were fed off a ring main. The number of people on site had dropped dramatically from over 2000 to just under 1000 over the past 10 years because several key processes had been mechanised. Unfortunately, no work had been done to reduce the size of the water services.

Due to the age and size of the building, as well as proposed plans to shut the building within 2 years, the remedial works to bring the site up to ACOP(L8)2001 requirements were deemed by the client to be onerous.

A combined strategy of instant disinfection to minimise the risk of the current Legionella problem, backed up by constant disinfection of the building was chosen after consultation with the client.

The buildings tanks were chlorinated using 50ppm chlorine dioxide, using the Sprayball system and then pulled through to the outlets. A Clearwater Technology Ltd X4 Catalytic Chlorine Dioxide unit was installed and commissioned at the same time. The unit dosed 0.3-0.4ppm of chlorine dioxide at the sites ring main.

Samples taken after the main disinfection proved to be negative for the presence of Legionella. For about 3 months after the main disinfection isolated Legionella was still found but in much lower concentrations than before.

After 3 months, no more Legionella was found.

The client requested that the chlorine dioxide system was turned off for a period of 1 week to ascertain cost effectiveness of the system. The positive Legionella samples returned instantly and then went away again after the unit was turned back on.