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.

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