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Upgrade programme The current wastewater treatment plant upgrading programme began in 1989. It is being progressively modified as new technology and methods of treating wastewater are addressed and standards required to obtain resource consents are defined. We're planning carefully to ensure that money is invested in the most effective way. The purpose of the wastewater network is to remove, treat and dispose of wastewater while ensuring that the community has the best, most cost-effective services available. Further upgrades are envisaged for the treatment plant to ensure capacity to cope with the growth of our region. Initially four stages were planned, followed by a further three stages. Stage 1 upgrade (completed 1995)
The reduction of Biochemical Oxygen Demand (BOD) and ammoniacal nitrogen load on the ponds was necessary to minimise the potential for odour production. Various treatment plant processes were investigated and the activated sludge technology was selected as the best practical option for North Shore City. Turbine blower units pump vast quantities of air through a series of pipes and diffusers in the Activated Sludge Reactor (ASR). A complex biological process results in the removal of organics. Sludge is produced in the process and extracted and treated in the digesters. The resulting "mixed liquor" from the ASR gravitates to the 48-metre diameter secondary clarifier. Residual sludge settles to the floor of the clarifier and is swept to a central hopper and then returned to the inlet end of the ASR where it seeds the incoming effluent. Dissolved Air Flotation (DAF) Tank 1 Sludge wasted from the ASR is very watery, and is thickened in the DAF tank prior to it being pumped across to the digesters. This involves injecting air into the liquid under pressure causing the air to be absorbed by the liquid. When returned to atmospheric pressure in the tank, the air immediately effervesces. The completed DAF unit is located with the blower building to the rear. The tiny air bubbles attach themselves to sludge particles in the liquid and lifts them to the surface where the sludge is skimmed off. The final effluent in the tank flows over a V notch peripheral weir to the outlet pipe and then onto the final stage of treatment, the oxidation ponds. Blower Switch Room Building This building has two turbine blower units installed with space for two more to serve the future activated sludge reactors. Stage 2 upgrade (completed 1999)
Inlet Screenings Building ($2.9 million) The inlet screenings facility is primarily designed for removing inorganic material that would affect other treatment processes and to protect pumps and other equipment from blockages or damage. This area of the plant also measures the incoming flow. The screen design is a New Zealand made rotating cylindrical stainless steel milli-screen, used in conjunction with screw conveyors. These fine screens capture small plastics and other rubbish, which previously passed through into the final dewatered sludge (one reason for restricting possible beneficial re-use such as compost), and this also assists downstream biological processes. (See the Treating Solids section). Digesters 3 and 4 ($5.5 million) The existing two heated sludge digesters were heavily overloaded. Two new digesters were built of a similar proven design but 50 per cent larger. They more than doubled the capacity of the plant's sludge digestion allowing digesters 1 and 2 to reduce loads and allow for future growth. They were built over two years. Sludge Dewatering Building and Lime Stabilisation Works The plant had relied on open air drying lagoons. After the digestion phase, sludge was allowed to age in the lagoons for two or more years before being excavated out in summer. This process was a significant source of odour. The new mechanical sludge dewatering plant thickens and dewaters sludge in a building where all odours can be contained and treated. The dewatered sludge is of a consistency suitable to be trucked off site or disposed of on site without the lagooning phase. The decommissioning of the lagoons resulted in a significant reduction in the level of odour.
North Shore City’s population is expected to grow from 185,000 in 2001 to 226,000 by 2016. It is essential that any upgrades to the plant to increase capacity also meet more stringent environmental controls as set out in the resource consent. The major components in Stage 3 included: Activated Sludge Reactor Two new activated sludge reactors, were built to be similar in size and design to the existing reactor but capable of nitrogen reduction. They and the refurbished reactor 1 use a form of aeration with anoxic, aeration and reaeration zones. Two additional blowers were installed to deliver the greater volumes of air required. Clarifiers Two new clarifiers were built, individually the biggest components of the Stage 3 upgrade. These settle out activated sludge, which falls to the bottom, and the now clear treated effluent is drained off the top, much like the process of clarifying and decanting wine. Each of the clarifiers holds the equivalent of 5.5 Olympic pools, and more than triples the previous clarifying capacity. The effluent is then piped to the oxidation ponds, where it spends an average of 30 days. There the effects of continuing bacterial action and ultra violet (sunlight) further polish the effluent before discharge out to sea from Campbells Bay. New gas and diesel engines A 900-kilowatt generator takes methane gas from the digester process and burns it to generate electricity to run the plant and produce hot water. When running at full capacity, the generator produces power equivalent to that used by a small town. The generator is able to produce 70 per cent of the plant’s required electricity. A 500-kilowatt diesel generator was also installed at the plant as emergency backup. The total cost for power generation was $2.5 million. Stage 3 works The Stage 3 works encompassed the design, construction and an extended commissioning and maintenance period of secondary and tertiary treatment processes and associated equipment at the wastewater treatment plant. This has expanded capacity of processing and includes:
The operational capacities of the activated sludge reactors (ASRs), clarifiers and dissolved air flotation (DAF) units were similar to the Stage 1 works and continued the liquid treatment processes established in Stages 1 and 2. Primary sedimentation The primary sedimentation tanks were upgraded with scum skimmers and associated equipment. A new primary sludge pump transfer system was installed with improved flow and density control systems. All four primary tanks were then covered and a negative pressure air extraction and odour control system installed. Activated sludge process The activated sludge capacity was increased by 200 per cent (7000m³) with the construction of two new reactors and two secondary clarifiers. The reactors were configured as MLE (Modified Ludzack Ettinger) tanks. Under this configuration the treatment process provides nitrification and denitrification to achieve lower nitrogen, in the secondary effluent. This second stage of the activated sludge process increased the overall capacity of the plant and allowed the load to the four existing trickling filters to be reduced from 15 ML/d to around 5 ML/d. At these flows the trickling filters provide a nitrified effluent while during periods of high wet weather flows they will continue to provide secondary treatment of the flow bypassed around the activated sludge system.
Sludge digesters Two new sludge digesters were constructed under the Stage 2 upgrade to provide additional digester capacity. Under Stage 3 the two older digesters were upgraded and retrofitted to a similar standard as the new Stage 2 units. This increased their efficiency resulting in a more robust sludge digestion process and an increase in gas production. Sludge dewatering The capacity of the sludge dewatering facility, constructed at the beginning of the Stage 2 upgrade, was increased by installing a third centrifuge. This unit had a greater treatment capacity than the original units due to technical developments since Stage 2. Improvements to the sludge handling systems in the sludge dewatering building were also made to improve sludge load out and daily operations and maintenance. Power generation The Stage 3 upgrade project implemented the key recommendations of a study to evaluate the options for on-site power generation. These recommendations were to provide a purpose built generation facility to include two new engine generators.
The other engine is a diesel standby unit sized to provide the balance of the essential electrical power demand of the treatment plant. The total eventual load has been determined as 1013 kW. The original gas engine-generator, with a maximum output of 200 kW, was relocated and the control and heat exchanger systems upgraded to provide emergency power generation for the solids dewatering building. The solids dewatering building is served by a power supply that is independent of the main treatment plant and cannot readily be connected to the main plant electrical reticulation system. Stage 4 improvements are expected to take three years and cost $33 million. Work started in 2002. The main components of the Stage 4 upgrade are:
Reducing odours The four trickling filters, which have been in service since the 1960s, were decommissioned by end of July 2003 at a cost of $200,000. Their treatment capacity is replaced by the extensions to the activated sludge plant and the addition of the peak flow capacity for the treatment plant. Demolition of two of the trickling filters will make way for a large biofilter, also called an odour bed, to be completed late 2003. This is a concrete tank filled with bark that neutralizes gases from the gravity thickeners and biosolids dewatering building nearby. Increasing capacity The fourth clarifier that we began building early in 2002 was commissioned early 2003. This is part of the treatment process that separates biological solids from liquid wastewater to produce a clean, clear effluent. So that we can cope with peak flows, e.g. after heavy rain, we have increased the capacity of a number of pipelines around the plant, built peak flow storage chambers and provided additional storage by deepening parts of the ponds. The clay was used to fill the sludge lagoons on site that are no longer required for the treatment process.
Even though our modern plant treats the effluent to a high quality an ultra-violet (UV) disinfection plant at the pond on the eastern side of SH1 (pond two) was commissioned in 2003. Work began in October 2002 on the $3 million, two-story complex. The UV plant will enhance the final disinfection that occurs naturally in the ponds. The water is raised from pond two, flows through an inlet, underneath a bed of UV lamps and then out to the ocean outfall. Flows into and out of the UV plant will be able to be controlled and measured. We have also built gravity thickeners that increase the capacity of the anerobic digesters by further thickening the primary settled sludge and therefore decrease the volume. Effluent irrigation trials are underway and the concept is to implement a permanent system on the treatment plant site, focussing on irrigation of planted areas and monitoring performance of the trials. These improvements are all part of the long term plan to cater for our growing city and to meet the conditions of our resource consent granted by the Auckland Regional Council, in September 2002, for the ongoing operation of the treatment plant. |
The
first stage was targeted to immediately take the load off the oxidation
ponds and enable the plant to be operated in a manner that reduced
odours. It consisted of activated sludge reactor 1 and the associated
blower switch room building, secondary clarifier 1, Dissolved Air
Flotation (DAF) tank 1 and other associated works and cost over $11
million.
The
primary objective of the stage 2 upgrade was to provide desperately
needed additional sludge treatment capacity. This allowed for more
appropriate dewatering than the old open air lagooning with its
associated odours, as well as providing a sustainable dewatered sludge
disposal option. Apart from the mechanical sludge dewatering facility it
also included a major inlet screenings facility and sludge digestion
capacity increase with digesters 3 and 4. Other works included the
secondary bypass, sludge storage and pond outlet control improvements,
and in total exceeded $16.3 million by the end of 1998. These works were
the result of the sludge management plan initiated by the council in
1994. 
In
June 2001 the major part of the Stage 3 upgrade of the treatment plant
was completed. These improvements provide additional capacity to cope
with a population increase of 40,000 people, and to allow the trickling
filters (the original form of secondary treatment) to be decommissioned
and thereby remove a major odour source.
The
sludge thickening plant, comprising a DAF unit, was duplicated to
provide additional capacity and plant standby capability while the waste
activated sludge pumps were fitted with variable speed drives to provide
greater control and compatible flow rates for the sludge thickening
plant. The return activated sludge pump station was upgraded to handle
the flows from clarifiers 1 and 2 with the third clarifier being served
by a new pump station. 
One
of the two engines is a gas engine sized to consume gas up to the
predicted gas production levels in 2012, approximately 7,000 m³/day,
with an allowance for a variation of up to 500 m³/day. The 900 RVA
generator takes methane gas from the treatment process and burns it to
generate electricity to run the plant and produce hot water. The
generator is able to produce 70 per cent of the plant’s required
electricity.
Three
main categories of improvements are underway at Rosedale as part of the
Stage 4 upgrade:
Improving
the quality of effluent and biosolids
Water
introduction