Energy Savings: Up to 3,066,000kWhr a year

Cost:  ~£1,400,000

Payback: ~ 4 years

BACKGROUND:

This project was undertaken at a large treatment works in the midlands. The works has an actual population equivalent of over 160,000, with most of the load comes from the numerous food processing factories in the area.

Secondary treatment comprised two aeration systems arranged in series. 

The first stage, known as the High Rate system used a MTS jet aeration system, which comprises a 30m diameter circular tank in which three radial jet aeration manifolds are installed. The mixed liquors are circulated by three 55kW pumps through the centre of nozzles and the air generated by three 0ff 150kW blowers is introduced in a secondary nozzle where it mixes with the liquors and creates a fine bubble stream.

Following treatment in the High Rate system the mixed liquors were settled in the three intermediate settlement tanks before being treated in the two Low Rate aeration ditches. 

Each of the aeration ditches are fitted with eight horizontal surface rotors with 45kW motors.  The rotors were to be controlled by DO levels with rotors starting and stopping to maintain the DO set point.  However, the system was unreliable and the rotors were in hand control.

AIR TECHNOLOGY SURVEY

The surveys were carried out over a number of years due to concerns about making major changes on a works which was subject to high loads. Air Technology identified that the aeration power consumption was excessive for a 160,000PE works, at an average of 884kW, compared to a benchmark of 320kW.

Despite the high power use, there were issues with odour around the High Rate tank due to low DO levels.  Air Technology identified that lack of aeration air was the problem and recommended the installation of a larger and considerable more efficient blower from a nearby site, as well as bypassing a percentage of the flow direct to the Low Rate ditches.

The installation of the new blower took place in 2012 and along with other measures reduced the odour, whilst the more efficient blower reduced the total aeration power to 746kW.

Air Technology conducted a trial after the new blower was installed by diverting a percentage of the flow to the Low Rate system, which reduces the load on the High Rate system, but increases the load on the Low Rate system.  This trial was partially successful but the rotor immersion depth was too low which resulted in lack of oxygen capacity for the higher loads. 

The lack of immersion depth was traced to the actuated valves controlling the weirs having failed.  Air Technology assisted is raising the weir height and subsequent trials showed that a percentage of the flow could be treated in the Low Rate ditches, but they would struggle during shock loads.  The situation was made worse by not being able to run all the rotors as this caused settlement issues in the clarifiers.

To enable the Low Rate system to treat more load on the Low Rate, Air Technology recommended that additional capacity needed to be installed.

This additional capacity would involve the installation of liftable diffused aeration grids in part of the ditch.  As diffused aeration is more efficient than both the existing systems, the majority of the treatment will be carried out by the diffused aeration, with the rotors used to provide ditch circulation and base load treatment. 

In 2012, Air Technology highlighted concerns regarding the existing Low Rate controls, and identified modifications, which were not carried out due to other options being proposed by the major capital Framework Partners.

The options proposed by the Framework Partners were unsuccessful and the project was passed back to Air Technology.

IMPLEMENTATION

The scope of work for the first phase was agreed as:

  • Install diffused aeration between 5th and 7th rotors in each ditch
  • Install two off 160kW rotary lobe blowers with VSD’s
  • Install six new DO meters
  • Install ammonia meters in the effluent pump chamber on the ditch exit
  • Install a new PLC based control system for the diffusers and rotors
  • Implement a partial flow split so that 60% of the flow goes to the ditche

 

The work started in April 2016 and was completed in August 2016.  The performance of the diffused aeration system was excellent, achieving immediate savings and allowing more crude flow to be routed directly to the ditches.

The flow of crude direct to the ditches was increased to such an extent that in late September 2016, the High Rate tank was fully bypassed and all the treatment was carried out in the Low Rate ditches.    The High Rate system, which is in poor condition was mothballed so it can be used if required

As the Low Rate upgrade did not provide spare blowers and does not have sufficient capacity for summer peak loads, further work is required to ensure that the mothballed High Rate system is not required.

 VALIDATION

The savings have been gradual from when commissioning started in August 2016 due to the progressively increasing the percentage of the crude going directly to the ditches as can be seen from the site power consumption below:

validated aeration savings

Once the system is fully optimised the savings will be 3,066,000kWhr a year, though savings equating to 2,190,000kWhr a year have already been achieved.

 Other benefits include being able to take the High Rate system off line and carry out repairs as the system is suffering from severe corrosion.  A further benefit is reduction in H2S levels around the intermediate settlement tanks, which had caused the corrosion.

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