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.

 

 

 

Some dramatic benefits are possible, even in relatively short term projects. We were called in to a large plastics manufacturing factory for a single day survey to look at the efficiency of their air systems, driven by a large centrifugal compressor. 

The main issue that they had was that the power readings did not match with the assumed air flow through the system, showing us that there was a significant inefficiency. The compressor was also consistently running at the surge limit. 

We conducted a series of tests using Air Technology’s flow metering equipment and power meters, linking into the compressor control panel. We found that the compressor was running fully turned down all the time, so efficiency was very poor. In essence, the compressor was too large for the operation. That was later confirmed by performance testing the compressor.

Based on this survey and our detailed recommendations, the company commissioned a completely new compressor house, featuring the latest variable speed drive compressors and dryers with heat recovery. The compressors were specified with help from Air Technology to provide efficient operation now and the scope for future expansion later generating excellent savings compared to the old, inefficient system.

We have worked on a number of Water Treatment sites recently, identifying and solving problems that were creating staggering extra costs and energy usage. 

The work we did on three sites alone has resulted in savings of nearly 1.5 MILLION kWhr per year.

At one site, we spotted that compressed air was being being vented to atmosphere because the unloading valve was faulty on two of the site compressors. In addition, on the same site the wrong pulleys were fitted and there was an issue with the control system that stopped the support blower shutting down in low demand periods. The wrong motor mounting plates had been fitted so there was excessive torsional vibration.

These repairs and changes directly led to savings of nearly 400,000kWhr per year

At another site, we discovered that a control DO valve was faulty on one of the site MBBR tanks, reading much lower than the actual DO, resulting in a flow of over 2,500Nm3/hr more than the other tanks. Once repaired:

The resulting saving was over 600,000kWhr/yr

At a third site, we identified and reduced the leakage from the passing drain trap on the aeration pipework. While that directly produced savings of around 10,600kWhr/yr, the investigation identified problems with the control systemm. Once that was changed to turn one of the blowers off at periods of low demand:

Savings were 450,000kWhr/yr

Three surveys, resulting in major energy savings for these sites. None of the changes required capital expenditure, all were repairs or relatively straightforward changes in approach.

A customer reported that they were having to run more compressors that they had previously needed, despite not having increased production. Factory pressure had been lost twice, causing lost production, despite apparently having plenty of compressor power available.

Air Technology investigated the situation and found anomalies in the stage temperatures and, on testing, the screw compressor was found to be producing no air and would only make pressure of 2.6barg. Further investigation revealed the cause of the problem - the first stage dump valve was stuck open and venting to the atmosphere, because of a corroded bolt. Once rectified the compressor operated as expected.

The compressor control panel, and the system controller, showed that the compressor was fully loaded. A simple fix, taking an hour, solved the problem. The site estimated that the issue had been present for five months, costing the company around £12,500.

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