Studies undertaken prior to construction of the Barrage showed that during periods of sustained high ambient temperatures, low wind and low river flows, dissolved oxygen concentration in the water could fall below the 5mg/l standard.  It was therefore identified that an intervention system would be required to ensure compliance with the standard.

After assessing alternative types of mitigation available, the Harbour Authority decided to install a fixed aeration/mixing system in the Bay.

The system comprises a series of steel reinforced rubber pipelines, laid on the beds of the Bay and Rivers Taff and Ely, connected to approximately 720 diffusers. Compressed air enters the pipeline system from cabinets located at five sites around the Bay.  The compressed air exits the diffusers and bubbles rise to the water surface causing the water to mix, thereby creating an even distribution of dissolved oxygen in the water column.  In the absence of mixing, particularly during summer months when the water temperature is warm, water near the bed of the Bay would become depleted of oxygen due to sediment oxygen demand. The system is therefore pro-active in that it continually mixes the Bay and river water during the summer months of the year,  preventing dissolved oxygen sags occurring.

The system was designed by the Environmental Advice Centre Limited (now part of Amec Earth and Environmental) and installed by Edmund Nuttall Limited.

A diagrammatic plan and cross-section through the Bay are shown below:

The intervention system installed in the Bay is effective in mixing the water in Cardiff Bay.  For the vast majority of the time, this mixing maintains the dissolved oxygen levels above the 5mg/l level in all places.  However, there have been periods when the total quantity of dissolved oxygen in the water has been insufficient for the standard to be maintained in all places even when the water is mixed.  These periods have typically occurred in the summer months when intense rainfall  follows a relatively long dry spell or during sustained periods of high temperatures and low wind.  At such times, the dissolved oxygen failures have been found to occur in the deeper water of the Bay and in the River Ely.

The Environment Agency therefore requested the Harbour Authority to implement methods for maintaining the dissolved oxygen levels above the minimum standard at times when the mixing system is unable to maintain the standard.

Alternative methods of maintaining the dissolved oxygen standard during such periods were investigated.  Two methods of "adding" oxygen into the water were considered:
 

It was concluded from a trial undertaken in the Bay that the use of hydrogen peroxide in Cardiff Bay would not be an effective way of raising dissolved oxygen levels.

A mobile oxygenation barge has been purchased.  The barge was built by McTay Marine Limited in conjunction with the British Oxygen Company.  The barge is deployed using the Harbour Authority's Water Witch vessel. The barge is capable of dissolving up to 5 tonnes of oxygen in 24 hours into the Bay water. Liquid oxygen is stored in a tank on the barge, passed through an electrically heated vapouriser unit and the resulting gaseous oxygen injected into a side stream of water which is returned to the bay.