DENITRIFICATION PROBLEMS AFTER UPGRADE OF THE OLD BAR STPwioa.org.au/conference_papers/09_nsw/documents/LindasyWalsh.pdf · DENITRIFICATION PROBLEMS AFTER UPGRADE OF THE OLD BAR STP

3rd Annual WIOA NSW Water Industry Engineers & Operators Conference Page No. 103 Tamworth Regional Entertainment & Conference Centre, 31 Mar to 2 April, 2009

DENITRIFICATION PROBLEMS AFTER UPGRADE OF THE OLD BAR STP

Paper Presented by:

Lindsay Walsh

Author:

Lindsay Walsh, Process Operator,

MidCoast Water

3rd Annual WIOA NSW Water Industry Engineers & Operators Conference

Tamworth Regional Entertainment & Conference Centre 31 March to 2 April, 2009


DENITRIFICATION PROBLEMS AFTER UPGRADE OF THE OLD BAR STP

Lindsay Walsh, Process Operator, MidCoast Water. ABSTRACT The Old Bar Sewerage Treatment Plant was commissioned in 1985 and upgraded 2004. The original plant had two 2000 EP intermittent aeration Pasveer channels with a catch pond of 2ML. Effluent held in two ex-filtration ponds and passed through dunes into the ocean. There were also two sludge lagoons of 700kL and two sludge holding beds. With the expected increase in population over the next 20 years, MidCoast Water decided the plant was in need of an upgrade to cope with this increase. The upgraded plant has a design capacity of around double the original plant to 7,800 EP with scope for a further stage 3 upgrade to 18,000 EP. Following commissioning we were unable to achieve satisfactory denitrification in the bioreactor resulting in high nitrate levels in the effluent and loss of alkalinity. 1.0 INTRODUCTION

MidCoast Water operates 14 sewerage treatment plants across the Manning and Great Lakes regions of NSW. These plants range in size from big plants such as the Forster Sewerage Treatment Plant (30,800 EP) and Dawson Sewerage Treatment Plant, to smaller facilities for communities such as Stroud (500 EP), Manning Point and Harrington. All MidCoast Water’s plants treat water to a tertiary level. Over recent years many of MidCoast Water’s sewerage treatment plants have been upgraded to provide a higher level of treatment and to be able to cope with growing populations – particularly in our coastal areas. Old Bar STP receives sewage from the villages of Old Bar and Wallabi Point, which have a combined population of approximately 4500. Both villages are on the coast about 16km east of Taree.


Figure 1: Location of Old Bar

The upgrade of the Old Bar STP consisted of adding an inlet works with a step screen, grit removal and a four cell selector tank. The Pasveer channels were converted into continual aeration bio-reactors with the addition of three blowers and diffused air membranes. A 1.8ML clarifier was built and the existing catch pond was converted into an emergency clarifier. RAS is returned via three pumps working in sequence returning approximately 120% of inflow. An alum dosing system was installed to assist with phosphorous removal and two banks of eight UV lamps were installed for disinfection. A 20ML storm detention dam was constructed and the two existing ex-filtration ponds were upgraded and increased to four. The plant receives 800kL to 900kL ADWF of mostly household waste up to 9ML PWWF during storm conditions. The plant was kept operational throughout the construction period.


Figure 2: Old Bar STP before the upgrade

Figure 3: Old Bar STP after the upgrade 2.0 DISCUSSION

After commissioning and handover of the plant we found we were having difficulty in achieving a satisfactory result in nitrate removal while maintaining low readings of ammonia in our final effluent. After experimenting with MLSS concentrations and aeration cycle parameters we were still no closer to getting the results we required. Both surface aerators were still in use to compliment the diffused air system. We decided to turn one off to try to decrease air introduction into the anoxic zone between the surface aerators but the MLSS settled out in the anoxic zone and hindered the effective mixing of the liquor and denitrification was minimal.


Figure 4: Surface aerators We believed that the selector tank may have been be too large for the load coming into the plant and that as the RAS was entering in the first cell of the selector tank, the carbon source may have been consumed by the RAS before it had a chance to enter the bioreactor. We redirected the RAS to the last cell of the Selector Tank and waited a few days to test our theory. One problem with having one large clarifier as opposed to two smaller ones is the detention time before the effluent reached the UV disinfection channels where it can be tested. This meant that any changes we made in the process could not be tested for about two days. This redirection made a small difference, but our results were still too high. We then decided to lower surface aerators three and four to their lowest level to try and mix the tank without introducing too much oxygen to our anoxic zone. This, combined with a MLSS of around 4000 (a sludge age of about 20 days) gave us a considerable drop in nitrate readings while still keeping the ammonium at reasonable levels. Unfortunately the increased load on the aerators caused the gearboxes to fail so our gain in nutrient removal was negated by the cost in renewal of aerator parts. After much investigation and brainstorming we found what we thought could be a solution. We had used Flygt jet aerators at other plants as temporary aerators/mixers during upgrades. They work by drawing air down through a snorkel into the water, introducing oxygen while mixing. The top of the snorkel has holes in it so you can regulate air introduction. We closed several holes and this limited the air and mixed the oxidation ditch effectively. Finally we replaced the snorkel top with a blank flange plate that stopped the air altogether.


Figure 5: Jet aerator As shown in Fig 6, before installation of jetter in May 2007 we could not balance an acceptable level of nitrates with low ammonium. Since installation and experimentation with the snorkel of the jetter, results, although erratic, began to stabilise. Since we began using the device exclusively as a mixer our nitrate readings have dropped without compromising our ammonium removal.

INDEPENDENT TEST RESULTS 2007

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Figure 6: Independent test results 2007, before installation of the jetter


INDEPENDENT TEST RESULTS 2008

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* Plant inflow April 2 increased to 7.8 ML due to extreme wet weather and flood conditions

Figure 7: Independent test results 2008, after installation of the jetter

Constant testing by ourselves and independent laboratories has regularly returned acceptable results of nitrates and ammonium since the installation. As shown in the second graph since August 2008 results have been more than acceptable and we are confident of improving even more with our next initiative.

3.0 CONCLUSION By using plant and equipment we already had, we did not have to purchase expensive plant for our tests which may not have worked. The utilisation of equipment which was primarily designed for different purposes shows that with a bit of brainstorming within a group, solutions not immediately evident can be discovered.

We are now in the process of removing surface aerators three and four and using the structure to support two Flygt 5.5kw mixers to replace the 15kw jet / aerator. We plan to work the mixers in sequence using the existing electrical switch gear in place for the surface aerators. Although the jet / aerator performed its task well we envisage a saving of approximately 60 per cent on power using the mixers. The lower nutrient levels give us a saving on our EPA licence fees as well as keeping our environment in balance. The jetter will be removed and stored, waiting for another chance to prove its worth in some other capacity.

4.0 ACKNOWLEDGEMENTS

I would like to thank Process co-ordinator Pat Welsh for his encouragement and assistance and MidCoast Water management for encouraging its staff and giving them the freedom to use their initiative to improve plant performances.

DENITRIFICATION PROBLEMS AFTER UPGRADE OF THE OLD BAR STPwioa.org.au/conference_papers/09_nsw/documents/LindasyWalsh.pdf · DENITRIFICATION PROBLEMS AFTER UPGRADE OF THE OLD BAR STP

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