Energy Savings: 919,800kWhr a year

Cost: £60,000

Payback: Less than one year

BACKGROUND

This is a large works with a population equivalent of around 440,000.  The current works treats an average inlet flow of around 98,500m3/day, and comprises of screening and degritting, primary settlement, secondary treatment and clarification in the final settlement tanks.

Secondary treatment takes place in three activated sludge plants ASP1, 2 & 3.   ASP1 and 2 were built in 2001 with ASP 3 in 2003. The ASPs all have the same design of three pass lanes, with ASP1 and 2 having four lanes and ASP3 just two lanes.   The ASPs use fine bubble diffused aeration, with aeration provided via ABS Nopon disc diffusers.

 Air for the ASPs is generated by eight blowers installed in a subterranean blower house adjacent to the aeration lanes on ASP2.    All blowers are single stage air cooled centrifugal blowers installed within fan cooled acoustic enclosures.  The blowers are driven by 132kW motors.

Although located centrally and delivering into a common manifold, the air supplies are split by means of isolation valves.  Two blowers are dedicated to each ASP with a further central two able to be switched between ASP1 and ASP2 and ASP2 and ASP3 respectively. 

On each ASP, the blowers run duty/assist/standby.     With two blowers running the control system matches the output of both and ramps them up and down together to maintain the required DO level.

AIR TECHNOLOGY SURVEY

The main part of the survey was carrying out performance tests on all eight blowers.  These tests identified that the blowers were reasonably efficient at maximum output, but had poor turndown and very poor part load efficiency.

Due to the demand profile and lack of system control, usually five and occasionally six blowers were running, though at times the total air demand could be met by four blowers running at higher outputs, which is where they are most efficient. 

The survey also identified that DO control was very poor with DO levels frequently above 4mg/l, especially on ASP3 due to the limited turndown of the blower and the poor configuration of the PID loops. Under normal loading treatment was complete by half way along the aeration lanes.

Though ammonia control was available, it was not used as it was found that the system was susceptible to ammonia spikes.

Air Technology recommended that the isolation valves are opened so the blowers supply a common manifold.  This involved major control system changes so that the correct pressure set point was being used based on which ASP needed the most air. 

It was also recommended that the control valve and blower PID loops were optimised to avoid excessive DO levels whilst improving response to shock loads and reinstating ammonia control.

The power savings from carrying out this work were estimated 105kW, along with maintenance savings from running fewer blowers.

IMPLEMENTATION

Air Technology managed, commissioned and optimised the whole project, with the control system modifications being sub-contracted to an approved systems integrator.

The project started in March 2013 with the preparation of the new software and installation and commissioning took place in July 2013.

The 105kW savings were quickly achieved with three/four blowers running during the day and two/three blowers overnight. 

DO control has been more stable and the system has been able to respond to ammonia spikes during heavy rainfall which is a particular problem within the catchment.

Energy Savings:692,040kWhr a year

Cost: £85,000

Payback: 1.3 years

BACKGROUND

This case study is for a large water recycling centre with a of 164,000.

Up until 2015, treatment was split between three parallel streams, each taking approximately one third of the incoming flow. “A” and “B” Works comprised rectangular trickling filters and the “C” Works built in 1989 comprised an activated sludge plant with fine bubble diffused aeration.

The poor performance of the filter works meant that “C” Works was run to very low ammonia levels to dilute the poorer quality effluent coming off the filter works.

To keep “C” Works discharge ammonia very low, an upgrade was carried out by others in 2007, including the replacement of the diffusers, installation of new anoxic mixer, replacement of the DO meters, upgrading of the blower and system controls and the replacement of the blower inlet filters. 

Since then, the diffusers have been replaced in 2013 due to high back pressures causing the blowers to surge.  It is believed that the increase in diffuser back pressure is due biological fouling, not contaminants such as grit and rag.

In 2015 a new activated sludge plant, known as “D” Works was brought into service taking 80% of the incoming flow, with 20% going to “C” Works.  The filter works were decommissioned once “D” Works was fully operational. 

The reduction in load in “C” Works means that is now treating a load equivalent to a PE of approximately 33,000 compared to around 50,000 previously.  

AIR TECHNOLOGY LIMITED SURVEY

Several aeration surveys carried out on “C” Works at the higher loads prior to “D” Works coming on line showed that treatment was complete by two thirds of the way along the lane, even during high load periods, with the exit ammonia levels being less than 0.05mg/l, which is less than 1% of the consent limit. 

The aeration surveys also showed very high nitrate levels in the aeration lanes, exceeding 40mg/l on numerous occasions despite there being an anoxic zone in the front of the lane.  Subsequent surveys identified that the anoxic zones were ineffective due to back mixing from the aerobic zone.

When “D” Works was commissioned the works was over treating, with DO levels approaching saturation at the end of the lanes.  This was due to the poor control system and difficult to maintain DO meters due to the mounting arrangement and local of the controllers.

With the reduced load on “C” Works it was also identified that the aerobic volume does not need to be as great.  A trial was carried out which showed that the anoxic zones can be extended, which will improve denitrification and save energy. 

IMPLEMENTATION

The final scope of work was agreed as:

 Extend anoxic zone by fitting auto valves to the Zone 1 air supply

  • Modify the control system so that the Zone 1 valves will open on Low DO
  • Modify control system to provide burst mixing
  • Replace the DO meters and install on chain mounts
  • Install new controllers adjacent to the DO meters
  • Control system modifications to run on average DO on a selectable number of meters
  • Acid clean the diffusers

 

VALIDATION

The savings have been immediate as can be seen from the blower power consumption trends below:

Validated Aeration Savings

CONCLUSION

Overall energy savings of 692,040kWhr a year have been found as well as improved process and reduced stress on the diffuser membranes.

The project ended up with a payback of just 1.3 years.

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.

Energy Savings:        1,442,845kWhr a year

Cost:                           £570,000

Payback:                    4.7 years

BACKGROUND

This is a medium sized works with a population equivalent of 33,169.  The quoted DWF is 8,630m3/day, with a FFT of 25,920m3/day.  In addition, the works treats the sludge from a nearby site, which is pumped in via a pipeline.

Treatment comprises an inlet works followed by primary settlement in two circular primary settlement tanks.  The settled sewage then gravitates to secondary treatment in Activated Sludge Plant, which comprises a four-cell aeration tank, with the cells or pockets arranged in series.

The aeration was originally provided by four 30kW vertical cone aerators, which were replaced in 2003 with 12 off 21kW submersible jet aerators due to noise complaints.

Following secondary treatment, the mixed liquors are settled in the two settlement tanks from where the clarified effluent is routed to the lagoon for further settlement before being discharged into the local river.

The power consumption for the submersible aerators was over 220kW which is very high for a carbonaceous ASP.

AIR TECHNOLOGY SURVEY

Air Technology Limited had identified this works as a high-power use works three years previously, but due to concerns regarding noise from the blowers, proposals to replace the submersible aerators with diffused aeration were originally rejected.  Air Technology continued to monitor the works power consumption and process performance, which showed that the power was progressively increasing and the FE quality was deteriorating.  The amount of solid in the lagoon was getting to a point that it would require emptying at a cost of over £1 million.

 A full aeration survey was carried out in 2011, which involved DO surveys, pollutant abatement mapping and power measurements.  The survey identified that treatment was very poor, with low DO levels throughout the aeration tank despite the power consumption being over 240kW on occasions.  The poor levels of treatment and low DO levels resulted in high levels of foaming on the tank surface as can be seen in the picture.  The low DO levels resulted in high SSVI and consequently poor settlement in the FST’s.   The amount of solids from the FST’s were steadily filling up the lagoon and there were concerns that it would have to be emptied.

The survey also established that the reliability of the submersible aerators was poor and typically four units a year needed to be changed at considerable expense as a crane was required to remove and install a unit as well as the repair costs.

 Air Technology recommended replacing the submersible aerators with fine bubble diffused aeration with the blowers housed in acoustic enclosures in the old press building.  This would save over £120,000 on energy alone, with the maintenance savings being estimated at over £30,000.

IMPLEMENTATION

The installation was managed by a Framework Partner, with the design and specification provided by Air Technology Limited who also designed and carried out the control system upgrade and commissioned the system.

The site work was started in November 2012 and completed in December 2012.  The improvements were immediate, with much reduced levels of moussing and a vast improvement in FE quality, especially solids carryover from the FST’s.  The lagoon has not had to be cleaned.

VALIDATION

The energy savings were almost 4,000kWhr a day as can be seen below:

 Validation Chart

Another benefit was a reduction in reactive power charges as the blower motors have a far better power factor than the submersible aerator motors. Maintenance costs have been much reduced as the blowers and DO meters have been reliable and require far less maintenance.

The quality of the final effluent has improved and at the solids carryover from the clarifiers has reduced dramatically the lagoon has not had to be cleaned out, which has saved over £1million in cleaning costs.

 

 

 

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