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History
Introduction
General Plant Description

 Inlet Structure
 Pre-treatment
 Screenings
 Grit Removal
 Foul Air Fans
 Primary Treatment
 Flow Balancing
 Contact/Stabilisation (activated sludge reactor)
 Waste Activated Sludge
 Secondary Clarifiers
 UV Disinfection
 Anaerobic Digestion
 Emergency Generator
 Manpower

History

In 1913 the Bay of Plenty Times published a report by the Borough Engineer, Mr H.W.Climey which gave a schedule of works to be done to build a sewerage system for the residential and commercial parts of Tauranga Borough. The area served by the proposed scheme extended from Brown Street to Second Avenue. The sewage was treated by a large septic tank in The Strand and the effluent discharged to the harbour. In subsequent years because of the excellent draining characteristics of the pumice soils, residential development outside of this area took place on individual septic tanks and soak holes on each property. In the post war year the Borough expanded rapidly and Tauranga was declared a city in 1963. In the 1960’s it became apparent from visible discharges of ground water seepages around the foreshore that with increased density of developments a full reticulation and sewage scheme was required for the City. Tauranga City Council commissioned a firm of consulting engineers to prepare a report on a sewerage scheme for the city and a sewage treatment plant.

The engineers divided the city into thirteen areas based on natural drainage catchments. From these areas, a programme of construction was developed which provided for reticulation to expand roughly west and south from the centrally located treatment plant which was built on reclaimed land adjacent to the Chapel St causeway.

The first stage of the city’s sewage treatment plant in Chapel Street was built at a cost of $1.6 million and commissioned in 1969. The plant had a capacity to treat 5,900m3 of sewage daily, the flow from equivalent population of 20,000. The plan provided primary and secondary treatment using the activated sludge process with effluent disposal via an outfall 1,500m long into the Otumoetai channel of Tauranga Harbour.

In 1978, resulting from a review of the treatment plants capacity related to the reticulation progress, consultants were commissioned to submit plans for Stage 2 extensions. These plans were of major importance as they included proposals to modify the original activated sludge process to the “contact stabilisation” mode of biological treatment.

The stage 2 development provided treatment of sewage for an equivalent population of 48,000. As part of the stage 2 extensions a second primary sedimentation tank equal in size to the original tank was built. The secondary treatment unit was modified to “contact stabilisation” and two final effluent clarifiers were added. A second anaerobic, heated sludge digester was built. The administration building was extended to house a new system of blowers for the activated sludge plant and a larger diesel standby generator was provided for emergency power supply. Pumps were also installed to increase the capacity for pumping raw sewage, to remove the sludge from the primary and secondary sedimentation tanks and to increase the flow rate of treated effluent through the outfall. Stage 2 also included the provision of centrifuges for anaerobic sludge dewatering. By 1987 a third primary sedimentation tank was constructed.

A 1992 report on the existing plant proposed process modifications. These modifications lead to the introduction of fine screens and grit removal facility, biofilter for removal of odours, a flow balancing tank with intermediate pumping between the primary sedimentation tanks and the activated sludge plant and dissolved air flotation to allow waste activated sludge to be separately thickened. Prior to this waste activated sludge was co-settled in the primary sedimentation tanks. An ultra-violet disinfection unit was added after the secondary clarifiers. A pipeline was constructed to take the disinfected effluent across to new wetlands at Te Maunga. En-route, disinfected final effluent can be taken from this pipeline and used on parks and golf courses in the area. The original harbour discharge was eliminated. Ultimately the effluent is pumped, together with the Te Maunga flows into the Pacific Ocean via a 950 metre outfall at Omanu.

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Introduction

The Chapel Street wastewater treatment plant provides treatment for domestic, commercial and industrial communities from the various catchments within the city.

The mean daily flow is 19,007m3/d (12 month average for July 2010 - June 2011) , a population equivalent of 67,200 people. The industrial contribution to the plant is not significant; 4% of the flow to the plant is estimated to be industrial and this contributes approximately 10% of the COD load.

The current design flow for the plant is 20,000 m3/d. 

Current plant influent organic concentrations are:

  Mean
Suspended solids 376 (g/m3)
SS Load 6736(kg/d)
   
BOD 290 (g/m3)
BOD Load 5794 (kg/d)
   
COD 742  (g/m3)
COD Load 13290 (kg/d)

 

Current plant effluent organic concentrations:

 

  Mean
Suspended solids 10 (g/m3)
BOD 12 (g/m3)

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General Plant Description 

Inlet Structure

Sewage arriving at the works flows to the wet well under the administration building. In this inlet pumping station two fixed speed and two variable speed pumps are used in different combinations to cope with the variable incoming flow. From this point the sewage is pumped to the pre-treatment building. The average daily peak flow is 340L/sec.

Pre-treatment

Pre-treatment is generally the first stage of any treatment process once the influent has arrived at the treatment plant. At this point objects such as rags, plastics, tins, wood, stones and grit are removed. Many of these foreign objects are likely to cause blockages, wear within pipes and generally be a nuisance factor within the plant. Once the screenings and grit are removed they go to landfill for disposal.

Screenings

In the pre-treatment building there are two Hydro-Dyne band screens designed to remove large pieces of material such as, plastics, rags, down to 3mm in size.  Each screen has the capacity to handle 500 L/sec of flow.

From the screens, material is deposited into a washer to break up and remove organic material.  Washed screenings move along a conveyor that compacts the material to remove water.  The compacted material is discharged into a 10 m3 skip for transporting to landfill.

The old Sindico step screens have been retained in position in case of emergencies.

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Grit Removal

Grit removal happens in the pre-aeration tank which is part of the pre-treatment building. Through the introduction of air and tank design, heavy grit particles settle to the bottom of the tank while the lighter organic composition of the sewage remains in suspension.

The grit is removed by centrifugal pumps which pump the grit to a grit classifier where the grit settles out, while water and floating organics are returned to the effluent stream. The grit particles are screw conveyed up an incline and discharged into the same bin as the compacted screenings. The slope of the conveyor allows water to flow back.

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Foul Air Fans

There are two foul air extraction fans, a duty and standby. Odorous gases are extracted from within the pre-treatment building, raw sewage inlet pump station, associated influent channels and from under the covered primary tanks where it is force blown to an odour bed consisting of a soil and bark mixture.

The odour bed is approximately 600 mm deep with a 300 mm pipe along its length with lateral pipes at approximately 2 metre centres. The laterals have small holes along their length to allow gas to flow up into the soil and bark mixture before dispersing to atmosphere. Micro-organisms living within the odour bed strip odorous compounds from the gas.

The odour bed media requires replacement approximately every 8 years as over time the bark will break down and rot away. The bark allows the odour bed to be porous and provides a large surface area for micro-organisms to attach.

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Primary Treatment

From the pre-aeration tank the sewage is distributed evenly between the primary sedimentation tanks (PST's). Sewage flows down through a centre stilling ring in each tank and flows outwards. Its velocity reducing as it does so. Suspended solid matter slowly sinks and the resulting settled sewage gradually makes it way to a weir at the edge of the tank. The solids settle to the bottom to form a sludge which is slowly scraped down the sloping floor into a central hopper and pumped to the anaerobic digesters. At this point settled sewage has only about 40% of the suspended solids of the original raw sewage but still contains 60% of the BOD (biological oxygen demand). 

The primaries were covered to reduce the level of odours on site, the odorous air is past through the biofilter.

Chapel Street Primary Treatment

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Flow Balancing

Settled sewage (primary effluent) flows from the PST’s to the 3,000m3 flow balancing tank. This unit evens out the flow and hence the load to the activated sludge reactor. Three pumps are installed in the pump well of this tank and are computer controlled from the plant SCADA system to balance the incoming and outgoing flows, maintaining a flow close to 220 L/sec over the whole day.

In the event of very high flows the flow balancing can be bypassed and the settled sewage passes directly to the activated sludge reactor.

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Contact/Stabilisation (activated sludge reactor)

The Contact/Stabilisation reactor consists of a central stabilisation tank and four external contact tanks in series. Primary effluent from the flow balancing tank is split between contact tank number 1 and 2. Activated sludge flows from the stabilisation tank into number 1 contact tank.

Activated sludge consists of a mass of micro-organisms that lives and feeds on the organic material in the primary effluent which mainly consists of protein, sugars, starches, fats, carbohydrates and fine suspended organic matter. This mass of micro-organisms or activated sludge typically operates at a concentration of 1100 to 1600 g/m3 in the contact tank while the concentration in the stabilisation tank varies between 3800 g/m3 to 4800 g/m3 but at times maybe higher.

In the contact tank the activated sludge plus primary effluent is aerated continuously and mixed together for between one to two hours before the resultant mixed liquor passes to the secondary clarifiers. Within the secondary clarifier the activated sludge settles to the bottom of the clarifier from where it is then returned to the stabilisation zone for further aeration. This additional aeration period allows all the organic matter absorbed by the micro-organisms to be metabolised. This is essential if the micro-organisms returning to the contact zone are to be effective in treating more primary effluent.

The concentration of activated sludge in the stabilisation tank is generally determined by how much waste activated sludge (W.A.S) is removed and/or how well the activated sludge is settling within the secondary clarifier and the rate of return activated sludge (R.A.S) from the secondary clarifier.

To break down the organic material micro-organisms require oxygen which is supplied by three Spencer blowers that operate in a duty, assist and standby mode. Each blower is rated at 100kw and delivers 995L/sec. The concentration of oxygen in each of the activated sludge tanks is closely monitored and controlled to provide the optimum conditions for treatment. Dissolved oxygen meters continuously measure DO while valves automatically open and close to maintain the desire DO level. Typically the dissolved oxygen level in the stabilisation tank is 0.5 to 0.8ppm, in the reaeration tanks it is between 1.8 to 2.2ppm.

Chapel Street Aeration Diffuser

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Waste Activated Sludge

Waste activated sludge (W.A.S) is the surplus amount of activated sludge, or put another way the micro-organisms surplus to the treatment process. The amount of activated sludge to be wasted each day is determined by carrying out a sludge solids inventory, a process of calculating the amount of biomass within the process.

The W.A.S can be pumped back to the head of the plant to be settled out in the primary clarifier with incoming raw sewage. Recently  two new Gravity Belt Thickeners were installed on site. As W.A.S is around 0.4% solids it is too watery to pump directly into the anaerobic digesters. With the addition of a coagulant to the W.A.S it is pumped on to the Gravity Belt. Thickener where water drains through the belt raising the solids content of the W.A.S to anything from 4-6% solids making it ideal for pumping to the digester.


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Secondary Clarifiers

The function of the secondary clarifier is to provide a tank for activated sludge to settle. The activated sludge or micro-organisms, as they settle leave a clear liquid, referred to as final effluent.

At this stage of the treatment process between 90 to 95 % of the suspended solids and BOD have been removed.

A centrally driven scrapper arm moves slowly along the bottom of the clarifier dragging the settle solids to a central cone where the micro-organisms in the mixed liquor are returned to the stabilisation tank. This returned sludge is referred to as returned activated sludge (R.A.S). There are two R.A.S pumps per clarifier, always one pump as a standby. Each pump returns 80L/sec.

Surface scum on the secondary clarifiers is scrapped to scum traps and returned to the head of the plant for further treatment.

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UV Disinfection

Effluent from the secondary clarifiers passes to the disinfection plant where it flows through channels, each containing three banks of ultra violet tubes. Each bank is 8m deep and 1.3m wide. The Trojan 3000 UV system controls a total of 624 lamps. The kill rate of bacteria is 99.99%; this renders the effluent suitable for watering parks and golf courses.

Chapel Street UV Disinfection

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Final Effluent Pump Station

Following UV disinfection the treated effluent is pumped 7 kilometres then it is distributed between two 4 hectare man made wetlands at Te Maunga before being pumped out to sea through a 3 kilometre pipe line which extends 950 metres off shore. This ocean discharge pipe handles both the effluent from Chapel St and Te Maunga wastewater treatment plants.

 Chapel Street Fnal Effluent

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Anaerobic Digestion

The plant operates two anaerobic digesters each with a volume of 1260m3. Primary sludge from the bottom of the primary tanks is pumped to the digesters where it is broken down by a different process. This is called an anaerobic process because air is kept out of the digesters. The bacteria in the digester operate differently from those in an aerobic environment. Through the break down of protein, sugars, starches, fats, carbohydrates and organic matter they produce organic acids such as acetic acid, the constituent of vinegar. Then a very special type of bacteria takes over. These bacteria are called methonic bacteria and they convert acids to methane gas and carbon dioxide.

Approximately 2,300m3 of gas is produced daily from the anaerobic sludge digestion and is utilised to run a generator, producing up to 2500 kW of power, thus helping to supplement the plant's power demand. As a result of the generator burning gas, heat is generated; this is used to maintain the digester temperature between 35 and 37oC.

In a well operated digester bacteria will reduce the dry solids content of the primary sludge from 4% down to 2%. To maintain the ideal conditions in a digester it is important the digester is continuously mixed, this is achieved by pumping sludge back and forth through the digester at various points around the tank.

Chapel Street Anaerobic

Generator running on methane gas produced from anaerobic sludge.

The centrifuges dewater the sludge from the digesters.
There are two centrifuges, one a duty and the other standby. The centrifuge starts and stops automatically depending on the sludge level within the digesters. The rise in digester level triggers valves to open, pumps to start and the centrifuge to start.

As sludge is introduced into the centrifuge and coagulant is added to aid in the separation of sludge and water. The sludge entering the centrifuge is 2% dry solids but after centrifuging at 3500 rpm the sludge or cake finishes up at 20 to 22% dry solids. Dewatering to such high level of dry solids cake makes it possible to transport it off site for disposal.

Chapel Street Centrifuge

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Emergency Generator

A 900 kVa diesel generator is available in the event of loss of power to the treatment plant.

Chapel Street Emergency Generator

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Manpower

Five plant Technicians carry out all the tasks required to maintain the Chapel Street and Te Maunga treatment plants. There is one trade waste officer, an administrator, plant supervisor, and plant manager. The basic working week is Monday to Friday 7.30 am to 4.00 pm. There is always one Technician on call for after hour events. The laboratory is manned by a part time laboratory chemist, one full time, and one part time technician plus a laboratory manager.

A contracting company is employed to carry out electrical and mechanical maintenance of equipment. The same contractor is employed to operate and maintain the city's 148 sewerage pump stations and attend to after hour calls.

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Last Reviewed: 05/05/2014