Information Update August 2011
Picton Sewerage Upgrade - Comprehensive Community Information Update - August 2011
The Existing Picton Sewerage Scheme
Picton and Waikawa is currently served by a network of sewers, pump stations, a sewage treatment plant and an outfall at Kaipupu Point. Sewage that is collected from houses and businesses in Picton and Waikawa, flows downhill in sewer pipes to a series of pump stations. The sewage is then pumped from one pump station to the next from Waikawa to Picton along the trunk sewers. The sewage ends up at the Picton Sewage Treatment Plant (STP) where it is treated and disinfected to a high standard before discharge to Picton Harbour at Kaipupu Point. Refer to Figure 1.
The Picton sewer network is separate from the stormwater system, but groundwater and rain water enter the sewers through cracks in sewer pipe joints and unauthorised connections on private properties eg; roof downpipes.
Under capacity in the sewer network can be resolved by increasing the size of the sewer pipes and pump stations, or by reducing the amount of rainwater and groundwater that enters the sewers, or storage, or a combination of the three. During storm events, the rainwater and groundwater component of the flow in the sewers is several times that of the sewage that comes from all of the houses and businesses. Water entering the sewer network during storm events governs the design of the capacity of the system.
The main issues facing Marlborough District Council with regard to the Picton sewerage scheme are:
- Groundwater penetrates the sewer network through cracks in sewer pipe joints and this increases when groundwater level rises.
- Some rainwater enters the sewers from unauthorised connections.
- Sewage overflows to the environment occur during storm events.
- Much of the system is old and in poor condition.
- Existing trunk sewers and pump stations are under capacity.
- The existing Kaipupu Point outfall pipeline is failing and under capacity.
The Proposed Solution in Brief
Work is ongoing to repair and replace sewers that are failing and leaking groundwater into the network. Notwithstanding this programme to improve the sewers, the trunk sewers and pump stations are significantly under capacity. Storage has been considered as an option to reduce overflows, however the storage tanks would have to be so large they are impractical to construct.
It is proposed to:
- Replace the Kaipupu Point outfall with a new underwater outfall near Careys Boatyard, refer to Figure 2, (there has been considerable consultation on the new outfall and resource consents have now been granted).
- Remove the existing Kaipupu Point outfall pipeline and concrete supports from the western shoreline of Picton Harbour.
- Replace sewage pump stations at Surrey Street, Fishermans Wharf and Dublin Street.
- Construct a treatment system at Dublin Street (Bypass Treatment Facility) to treat flows in excess of what can be treated at the Picton STP.
- Upgrade sewage pump stations at Waikawa Wharf and Beach Road.
- Replace the trunk sewers (ie; the backbone' of the sewerage network).
- Construct overflow structures to control and minimise the risk from overflows from the sewer network.
- Obtain resource consents for the construction works and the ongoing network operation and to designate pump station sites.
Main Outcomes and Benefits of the Project
The main outcomes and benefits of the upgrade project will be:
- A reduction in the frequency, duration and volume of sewage overflows and the treatment of overflows to the Waitohi Stream for all but the largest storms.
- Reducing risk to public health from contact with contaminated water
- Improving harbour water quality and the marine environment.
- Increase in capacity of the system.
- Provision for the future growth of Picton and Waikawa.
- Removal of the existing visible outfall pipeline to Kaipupu Point.
- Elimination of what many see as an eyesore to visitors and harbour users.
Environmental and Health Effects
In 2010 Cawthron Institute surveyed the coast that is adjacent to the six main areas where sewage overflows contact the foreshore. Cawthron concluded that while there have been regular overflow events in recent years, all of the coastal areas that were subject to these overflows are modified, have no particular ecological value, and are generally already subject to significant stormwater runoff. Cawthron noted that differentiating ecological effects between sewage overflows and stormwater is almost impossible, especially given that most sewage overflows occur during significant rainfall when there are high stormwater flows.
Notwithstanding Cawthron's findings in regard the ecological values of the foreshore, Council is working to improve the situation by greatly increasing capacity of the trunk sewers and pump stations to reduce the frequency, duration and volume of sewage overflows. It is impractical to build a sewer system that never overflows, but it is possible to increase capacity to give an acceptable standard of performance.
Reducing sewage overflows will lessen public health risks as well as improving the environment.
Design Capacity and Treatment
Council has considered the Picton and Waikawa situation and compared design standards with those of other towns and cities in New Zealand to determine an appropriate basis of design for the upgrade. The capacity of the system is described in terms of annual recurrence interval (ARI). An ARI of two years means that an overflow would occur, on average, every two years. The proposed upgrade uses different design capacities (different ARI) for various parts of the system for reasons discussed below.
The new harbour outfall, Dublin Street Pump Station and Bypass Treatment Facility will be designed to achieve the following (refer to Figure 3):
- Up to 130 litres per second (L/s) of flow will be pumped from Dublin Street to the Picton Sewage Treatment Plant for full treatment.
- For flows in excess of 130 L/s and up to the 2 year ARI storm event (390 L/s) the flow will be pumped to the Bypass Treatment Facility and passed through a fine screen before being disinfected by ultraviolet radiation (UV). The flow will then be discharged to the new harbour outfall along with the treated effluent from the Picton STP.
- Flows in excess of the 2 year ARI event and up to the 20 year ARI event (570 L/s) will be pumped to the Bypass Treatment Facility and passed through a fine screen and disinfected by ultra violet radiation (UV). The treated flow will be discharged to the harbour outfall along with the treated effluent from the Picton STP, up to the outfall's 2 year ARI capacity, with the additional flow being discharged to the Waitohi Stream.
For the balance of the proposed trunk sewer and pump station upgrade, the main design criteria are as follows:
- Surrey Street pump station, Fishermans Reserve pump station and Waikawa Wharf pump station will be designed for 20 year ARI event capacity
- Beach Road pump station will be designed for 5 year ARI event capacity
Refer to Figure 4 for an overview of the design capacity.
The design capacity and treatment strategy has to consider many factors. For example, flows to the Picton STP have been limited to avoid overloading the biological treatment process. The Picton STP utilises a biological treatment system and it is not practicable to upgrade it to cope with high wet weather flows. In comparison, the proposed excess flow treatment system (the Bypass Treatment Facility) at Dublin Street does not rely on biological processes and is therefore better able to cope with high, short duration flows.
The Surrey Street and Fishermans Reserve pump stations will be designed for the 20 year ARI event which is high by the standards of other councils in NZ. This higher standard is considered necessary because the overflows enter the head of Picton Marina. Beach Road pump station is to be designed for substantial capacity but not as high as Surrey Street, Fishermans Reserve or Waikawa Wharf because the overflow discharge point is not as sensitive as these other sites.
Dublin Street Pump Station is currently located in Waitohi Domain on Dublin Street. It is proposed to locate the replacement pump station in the south eastern corner of Waitohi Domain, adjacent to the Waitohi Stream and Dublin Street. The new structures will be built on an earth platform at about the same level as the Waitohi Stream stopbank so as to be above flood level. This new site will provide sufficient land for the upgraded layout of the pump station, which will include a new treatment facility for flows in excess of the Picton STP capacity.
Surrey Street pump station will be replaced with a new station adjacent to the existing. Fishermans Reserve pump station will be replaced next to the existing station. Beach Road pump station will be upgraded as will the relatively small pump station at Waikawa Wharf.
Constructing the new outfall, laying new trunk sewers and constructing new and upgraded pump stations will cause some temporary disruption. Planning and consultation is ongoing to minimise the impacts.
Bypass Treatment Facility at Dublin Street Pump Station
The UV disinfection unit at the Dublin Street Bypass Treatment Facility will be designed for a high level of effectiveness for sewage flows up to the 5 year ARI event. Above that flow, from the 5 year ARI event up to the 20 year ARI event, the UV will be less effective but will still improve microbiological effluent quality. The fine screening will be designed for the capacity to treat up to the 20 year ARI event.
Consent applications for any necessary resource consents, such as the discharge of treated effluent from the Dublin Street Pump Station Bypass Treatment Facility, will be prepared when the preliminary design of the sewerage upgrading is complete. Council will also be applying for designations over land occupied by pump stations which will include the new Bypass Treatment Facility at Dublin Street and the main influent sewer to the Surrey Street pump station at Memorial Park. Designations allow Council to give a clear indication to the public in the relevant district plan, of the intended long term use of the land.
Council will be consulting with key stakeholders. This will include discussions with the Picton Sewerage Consultative Working Group (CWG) which worked with Council in the planning of the proposed harbour outfall. The CWG includes Te Atiawa, Department of Conservation, District Health Board, environmental groups, a residents' association, business groups, Forest and Bird, Port Marlborough and fishing interest groups.
Council will require non-statutory approvals to construct pipelines along public roads and in Port Marlborough land (eg; from organisations such as Marlborough Roads, New Zealand Transport Agency, Kiwi Rail and Port Marlborough). Organisations will also be consulted with that have regulatory and commercial interests in activities within the harbour, such as the Harbour Master, Strait Shipping and the Interislander.
Significant upgrades are planned for water, sewer and stormwater infrastructure in Picton and Waikawa. There is inevitably a rating impact of the associated additional capital and operating expenditure. Council has recognised this and $16.9M of subsidy is being provided from reserves to reduce the effect on rates. A $190,000 land value property is forecast to have a $338 per annum rates increase in five years time and of that $159 is a result of the sewer upgrading. These figures exclude increase due to inflation.
It is proposed to construct the upgrades over the next 5 years, starting with the Picton harbour outfall in 2012. Design is currently underway. It is expected that applications for resource consents and designations will be lodged late 2011 or early in 2012.