OXLEY CREEK WASTEWATER TREATMENT PLANT

By Louis Jones

Introduction

Wastewater Treatment Process

Sludge Treatment by Anaerobic Digestion

Future Project

Plant Operation

Plant Testing and Licence Requirements

The Future

Oxley Creek WWTP Process Flowsheet (54 ML/d)

Technical Data

Footnotes

INTRODUCTION

The Oxley Creek Wastewater Treatment Plant is the second largest sewage treatment plant in Queensland. Owned by Brisbane City Council, the Plant is operated by Brisbane Water, a business unit of Brisbane City Council. The plant is located in Donaldson Road at Rocklea and serves the southern and western suburbs of Brisbane. These suburbs include Oxley, Acacia Ridge, Salisbury, Indooroopilly, Chapel Hill and Graceville. With Chapel Hill to the north, Algester to the south and Jindalee to the west.

The plant became operational in 1969 and several new process units have been added since then to meet the demand, from the increasing daily flows of wastewater.

WASTEWATER TREATMENT PROCESS

Treatment is aimed at converting domestic sewage and industrial wastewater into a clear effluent by removing organic and other solid material. The average dry weather flow treated at the plant is around 54 megalitres per day, with up to 300 megalitres after rain, in the catchment areas.

Primary Treatment

Wastewater entering the plant flows into the main pump well where it is raised 14 metres to the rotary drum screens. The rotary drum screens remove paper, rags and other large objects which could cause blockages or damage equipment. The wastewater then flows by gravity through the other stages of treatment.

After screening, the wastewater flows into grit removal channels where heavy particles such as sand and stones settle out and are removed daily, using a flow torque pump and cyclone unit.

The sewage then flows into primary sedimentation tanks which allow the organic solids to settle to the bottom. The lighter materials, such as oils and fats float to the surface and are skimmed of into the raw sludge pits.

The settled material (raw sludge) is regularly removed from the bottom of the primary tanks and pumped to the digesters, along with the floating materials.

Secondary Treatment By Activated Sludge Process

Settled sewage from the primary sedimentation tanks still contains suspended organic material which is removed by secondary treatment, using the activated sludge process.

The activated sludge process is a biological wastewater treatment process in which microorganisms, referred to as Amicrobes@ or Abugs@ (in other words - bacteria), accelerate the decomposition of wastes in an aerobic environment (the term Aaerobic@ refers to the presence of dissolved oxygen). This oxygen is supplied by large capacity blowers, which also suppy air to the grit and pre-aeration tanks. These Abugs@ grow and multiply in the aeration tanks while consuming the organic waste materials.

As more organic waste material is consumed, the wastewater becomes much more clearer and the microorganisms develop into dense and more readily settleable, floculant particles.

After the floculation is formed, these are settled out and seperated from the treated wastewater in the final settling tanks.

The mixture of effluent and activated sludge, or mixed liquor, flows to eight final settling tanks where the activated sludge settles out, leaving a clear liquid. This final effluent is disinfected using chlorine and then channelled from the plant into the Brisbane River through a submerged outfall pipe.

The sludge which settles to the bottom of the final settling tanks is returned to the inlets of the aeration tanks, so that the biomass is retained, within the process.

As there is a continuous growth of microorganisms, a portion of the activated sludge is wasted. This surplus sludge is thickened in a dissolved air floatation (DAF) system, and then pumped into the anaerobic digestion tanks, together with the raw sludge.

SLUDGE TREATMENT BY ANAEROBIC DIGESTION

The raw sludge from the primary settling tanks and the thickened waste activated sludge is pumped to four anaerobic digestion tanks, where the organic material is broken down by anaerobic microorganisms (the term Aanaerobic@ refers to the absence of elemental oxygen). The stabilised material which results from the digestion process, is known as digested sludge.

The digested sludge is stored in two unheated secondary digesters and subsequently dewatered in a centrifuge. The dewatered sludge is disposed of to a secure landfill. The centrate from the centrifuge is run to the drying beds, the leachate from the beds returns to the inlet of the plant.

Biogas production

During the anaerobic digestion process, sludge gas containing 55 to 65 % methane is produced by the continuous mixing of the contents, which also assists in the heating process. This biogas is used in boilers and the energy is used to heat the digesters to a favourable operating temperature of 350C by using heat exchangers. Excess biogas is burnt in the waste gas flares.

FUTURE PROJECT

A project is currently underway to investigate the economics of constructing a co-generation facility at Oxley Creek WWTP that could convert the energy contained in the biogas to electricity. Such a unit already exists at the Brisbane City Council Luggage Point wastewater treatment plant.

Biogas from the anaerobic digestion process is often referred to as green gas because it can reduce greenhouse gas emissions when it replaces fossil fuels for energy generation.

PLANT OPERATION

Computer monitoring with trending facilities, monitors all flows throughont the plant, also allows some major equipment to be remotely operated, with some automatically controlling for some of that equipment. This control is set by the operator, to allow efficient operation of the treatment processes. The computer equipment also stores important daily data, and is equipped with alarms, for every aspect of plant equipment and general plant operation.

PLANT TESTING AND LICENCE REQUIREMENTS

To ensure high standards in the treatment process and to meet the effluent licence parameters set by the Environment Protection Authority (EPA), twenty-four hour composite samples are taken weekly for analysis by the Brisbane Water Scientific and Analytical Services Laboratory.

The plant holds an Environmental Authority Licence which requires the Plant to meet less than 20 mg/L BOD5 (Biochemical Oxygen Demand) and less than 30 mg/L suspended solids concentration in the effluent. In the future, a total nitrogen limit on the effluent may also be imposed.

THE FUTURE

Brisbane City Council faces the continuing challenge of improving and enlarging its sewage collection and treatment systems to provide an essential service to people living in a progressive, growing city.

This will require the upgrading of wastewater treatment plants to satisfy higher public expectations and the more stringent requirements of the EPA.

The Brisbane River and Moreton Bay Wastewater Management Study has determined that the ecological and recreational uses of our water system are under threat. Current nitrogen inputs from wastewater treatment plants and catchment runoff are not sustainable and steps to reduce this nitrogen input are required to maintain and enhance the long-term health of the Brisbane River and Moreton Bay system.

To ensure the long term protection of the Brisbane River and Moreton Bay, Brisbane Water is committed to upgrading the Oxley Creek Waste Water Treatment Plant to achieve significantly lower nitrogen concentrations in the effluent.

Oxley Creek WWTP Process Flowsheet (54 ML/d)

(Click on image for enlargement)

Technical Data

Typical Sewage Characteristics

Raw sewage BOD: ~ 180 mg/L

Raw sewage TSS: ~ 210 mg/L

Primary effluent TKN: ~ 40 mg/L

Final effluent TKN: ~ 10 mg/l

Raw Sewage Pumping Station

Normal dry weather flow (ADWF): ~ 54 ML/day

Total capacity of six raw sewage pumps: ~ 300 ML/day

Fine Screening Complex

Three rotary drum screens

Flow capacity of each: ~ 1750 l/sec

Primary Settling Tanks

Four rectangular primary settling tanks

Normal detention time: ~ 2.5 hours at ADWF

Total raw sludge drawn: ~ 300 - 400 kL/day

Aeration Tanks

Stage 1&2 (7 separate tanks) volume: ~ 6694 m3

Stage 3&4 (2 separate tanks) volume: ~ 7702 m3

Tank depths: ~ 2.7 m for Stage 1&2 & 2.8 m for Stage 3&4

Normal detention time: ~ 6 hours at ADWF

Mixed Liquor Suspended Solids MLSS: ~ 1200 - 1800 mg/L

Stirred settleability (30 min): ~ 20 - 40 %

Calculated Sludge age: ~ 4 - 8 days

Oxygen level DO in aerators (setpoint): ~ 1.5 mg/L

Final Settling Tanks

Eight final settling tanks, cone type floor slope: ~ 20 degrees

Return sludge flows for each stage: ~ 10 - 15 ML/day

Total waste activated sludge flow: ~ 10 - 20 L/s

Waste Activated Sludge Thickening

Dissolved Air Floatation unit (DAF)

Total thickened WAS: ~ 150 - 200 kL/day

Thickened sludge solids content: ~ 3 - 5%

Subnatant suspended solids: ~ 10 - 20 mg/l

Anaerobic Sludge Digestion

Four heated primary digesters

Two unheated secondary digesters for sludge storage

Operating temperature of heated digesters: ~ 350C

Total biogas production: ~ 7700 m3/day (approx.)

Total detention time in primary digesters: ~ 20 days (approx.)

Digested Sludge Dewatering

One centrifuge

Centrifuge sludge flow capacity: ~ 15 L/s

Dewatered sludge solids content: ~ 13 - 16%

Centrate suspended solids: ~ 1000 - 1400 mg/l

Foot Note:-

Back in the earlier days of this Oxley Creek Treatment Plant, we had more staff - including two full-time gardeners who were able to maintain the perimeter of our section of the Oxley Creek.

To the extent - that it was so much more attractive, to anyone that was priviledge to visit that area and to those of us - that were working, next to the "creek".

This property borders several kilometres of Oxley Creek, so has a very close association - with the "creek".

I well remember back, when there was a large tree that had fallen across the creek near where the effluent flow from the Plant entered the creek and this tree during tidal flows in the creek, often held back the hyacinth that infested the waterway at that time.

It was at that time - I only wished that I could have remove this obstacle from the "creek", but it was completely - out of our hands.

This tree was probably swept away in the 1974 floods. But I do know, that this Treatment Plant - "went" completely "under-water" during that unfortunate time for so many people, who lived in this area and the problems it would have caused.

My experience with this Treatment Plant, goes back to 1969 when I was employed as a Shift Operator, working in partnership with a Shift Electrician. There was and still is, continous shift work maintained on the Plant. He was responsible for the pumping of the raw sewage to the Plant and I was responsible for the treatment of that raw sewage.

But things have changed drastically over the thirty odd years I've been here. I am the only original Operator here now and the Electricians have been made redundant, so as an Operator, I'm fully responsible for the whole operation of the Plant and the performance of the treatment process.

The overall staff, has been reduced to just a hand full, over the last few years, with only one full time gardener, who has his hands full - just looking after the essential grounds work.

As to the "creek", we have no possible way of taking care of the general area bordering the "creek", that we did in the past - any more.

So this section of the "creek", has gone back to the sorry state - that it is in, today.

But as far as I'm concerned - the "creek" has great potential and there should be some way, that it could be made more "people friendly".

The "creek" had much more flow in it when we were discharging the Plant's effluent into it, but once that flow was diverted to the Brisbane River - the "creek" took on a much poorer appearance. That's my own opinion - but it's not to say, that the "creek", doesn't have great potential. It really has!!!

There is still plenty of wild life along the "creek", as it often wanders onto our site. Especially the Water Hens (or Dab-chicks) - they are quite often feeding around the main buildings during the early mornings. It's a pleasant sight to see them wandering onto the property and sometimes we even have foxes roaming around the general area.

I don't live in this part of town, but having worked in this area - for so long, do consider that the "creek" SHOULD be made, a more friendly place to visit.

I fully enjoyed your Web-site, because of the interesting stories about it's past and just how things were - in the good old days and have it listed among my favourites on my home computer.

The Treatment Plant has been here since before 1969, when there was a lesser density of population living across the "creek" - but over the recent years, many more people are now living in that area. And they were generally unaware, that there is a Sewerage Treatment Plant - just across the "creek" from them.

I do feel sorry for these folks! - but there isn't very much that I can do about their situation. I only wish there was!!!

We have been getting a lot of complaints from folks in that area - about noises and odours, but this is what it is - and having to handle and process, much greater flows of raw sewage, as each year goes by.

An enormous amount of money has been spent on this Plant, over the last five to six years - mainly to handle the extra flows into the plant , but very little to increase the capacity of the plant - so at the moment - it is generally over-loaded!!

A certain aspect ot the treatment process has been brought out into the open - which should have been better considered - before construction was commenced.

But we have to work in this environment and we could see, that this exposure of the raw sewage - was likely to cause some odour problems - and we weren't wrong!!!

Large sums of money are now being spent to eliminate this problem and it could well be - that the problem will be lessened, or eliminated, in the very near future.

There have been numerous suggestions - that we should re-locate, but that is un-thinkable and surely impossible. So we will continue to maintain the best possible environment friendly, treatment of raw sewage - of large quantity, to the very best of our ability. With as little inconvenience to our neighbours - as possible!!!

What more can I say!

By the way - our operation is not closed-off to the public, we invite ANY interested people to visit our Plant - all they have to do is give us a ring and arrange a day and time they would like to visit the site. We will be only too happy to show them over the site. We do this for many schools and other interested people - and have done for many years.

So getting back to the potential there is with the "creek", - it's there and every effort should be made to improve it's foreshores and the natural waterway it's self, to encourage more folks to enjoy the basic beauty of one of our largest and very important creeks, which is so close to our beautiful city.

I would just love to be able to see during my lifetime, the area along the "creek", fully maintained with walkways and picnic areas - all along the waterway and ensuring that the natural flora and wild life is preserved - for everybody. Especially our following generations.

I've been working in the Brisbane City Council for just on forty - three years now, so feel honoured to be able to share - just a portion of my involvement, to other interested people. I have been very proud - to have been a part of this large organization, but the time is now very close - when I am seriously considering my retirement.

But before I do that - I feel that I may be able to contribute something to people, with concerns and if I am able, will willingly assist them, in whatever way that I can!

Trusting that my personal in-sight and concerns for the "creek", might be of interest to the local community - I extend my very best wishes to you all - for the future.

Yours sincerely,

Louis Jones.