Martinborough Wastewater Treatment Plant – Stage 2 Improvements

In 2012, Council adopted a wastewater management strategy to gradually upgrade district facilities, and to shift from discharge to river to discharge to land.  This strategy underpins the Martinborough Wastewater Treatment Plant’s 2016 resource consent which requires all wastewater to be discharged to land by 2035 (except in exceptional circumstances).   

The planned work will help ensure that the Martinborough Wastewater Treatment Plant can accommodate residential development and growth to at least 2050, and it will remove one source of pollution from the Ruamāhanga River. 

The 2016 resource consents authorised irrigation of treated wastewater to Pain Farm.  However, this is no longer a viable option (see below) and now the Council is investigating irrigation of treated wastewater to land to the north of Pain Farm.  This will require a change to the 2016 resource consents, which specifically refer to Pain Farm, and new consents to authorise irrigation at the new site will be needed.

1. The existing wastewater treatment plant receiving wastewater from Martinborough will be upgraded so that the treatment in the plant includes inlet screens for large object removal, bioreactor tanks for nitrogen removal, and a more powerful UV unit for improved disinfection.
2. A pressurised pipe will be laid from the plant, and the tertiary treated effluent will be pumped along this pipeline to a lined storage pond on the new site. The liner is likely to be ultraviolet resistant high-density polyethylene over geotextile cushioning layer, with welded and tested seams to avoid leaks and be suitable for varying water level.
3. The effluent will be irrigated to the surrounding land when the soil is sufficiently dry for the plants on the land to take it up.
4. The whole system, including the transfer pumps, will be electronically controlled and monitored to ensure automatic shut down under high wind, low pond level or high soil moisture circumstances.

For a number of reasons, including community concern about the intent of the original bequest of Pain Farm, the Council decided to pursue an alternative option and has been investigating irrigation of treated wastewater to another site nearby. 

Yes.  It is a slightly closer to the Martinborough Wastewater Treatment Plant than Pain Farm. Importantly, it also has features that make it better suited to accommodate treated wastewater. For example, as compared to Pain Farm, Kellys Stream does not run through the property (it is located on the eastern boundary), a large part of the land proposed to be irrigated drains westwards, away from Kellys Stream and the presence of tile drains as well as a clay pan in this area prevent treated wastewater getting into the underlying groundwater aquifer.  

Additionally, the new site is sufficiently elevated to be above the floodplain. 

The new site is the closest land to the Martinborough Wastewater Treatment Plant that is located outside of the floodplain, suitably distant from and down-hill of the Martinborough urban area (approximately 1km away) and does not entail a crossing of the Ruamāhanga River.  The land was available to the council and is suitable for treated wastewater irrigation. 

• Irrigation at a rate where the water is generally held in the soil and nutrients taken up by grass. 

• Any excess water moves down through the soil (being cleaned as it goes) and will be collected by drains under the turf and carried to soakage basins. These basins further clean the treated water and allow evaporation and soakage of remaining cleaned water to ground. 

• To ensure that the tile drains work well, they will be checked and repaired as necessary. A new tile drain will also be installed in the northern section of the site. 

• No treated wastewater will enter groundwater – the existing underlying clay pan in the soil profile prevents this from happening. 

• Site and design include a storage pond to buffer supply of treated wastewater from the oxidation ponds and irrigation.  There is space for additional storage to be developed now or in the future. This means that discharging treated wastewater to the Ruamāhanga River will eventually be able to stop (except, if necessary, in high water / flood events). 

• The operation at the new site can be scaled to meet requirements, which means it will be suitable for Martinborough’s projected growth to 2050. 

We will monitor water volume and quality collected by the drains and the performance of the soakage basins. This data will allow us to modify how much and when we discharge treated wastewater to optimise land treatment at the site, and to make sure things are working as they should be.  This monitoring work informs any need to improve performance or, overtime, capacity considerations. This can be done because current calculations show that we need 38 hectares to irrigate treated wastewater to land and the site is 74 hectares in size. Alternatively, or as well as, we can build bigger storage ponds, and/or larger soakage basins.  

We will use the following methods to manage potential residential amenity (odour and spray drift) effect: 

• No discharges within 25 metres of any boundary (in practice we will be set back much further from the southern and eastern boundaries) 

• Irrigation nozzle heights will be a maximum of 1.52 metres above ground level and will not incorporate an ‘end gun’ 

• Irrigate at low pressure, and medium water droplet size 

• Irrigation will stop when wind speeds exceed 12 metres per second (or 7.5 metres per second sustained for a period of 15 minutes) in a direction towards a residential property within 300 metres of the site 

• The treated wastewater must have median E.coli concentration of less than 100cfu/100ml 

• Monitoring of odour at the boundary of the site 

• Shelter trees will be planted along the south-west site boundary. 

Our expectations are: 

• No odour at or beyond the boundary. This is a condition of the current 2016 consent and there are no records of this not having been met. There will be an Odour Management Plan. 

• No spray drift (because of wind speed cut-offs and droplet size) beyond the boundary. 

• The irrigation method could be by a central pivot irrigator or another method such as solid set irrigators – this is yet to be decided. 

We will have clear odour and spray drift monitoring and complaint processes.  In the unlikely event they were to become a problem, the system will be flexible to enable changes, including adjusting where irrigation occurs, wind speeds and direction, and planting. 

In 2025, the landowner approached the Council and asked if there was interest in acquiring a part of the farm.  The Council undertook a due diligence to determine whether the land could accommodate treated wastewater in the same way planned for Pain Farm.  As these investigations were positive, the Council continued with the land purchase and in parallel undertook further site investigations.  

Subsequently, in August 2025, the owner’s property was subdivided into 8 lots, with Lots 1 (72.6ha) and 12 (1.42ha) offered to the Council. In early February 2026, title for the new irrigation site was issued and the land purchase was completed. 

Our investigations at the site have considered climate, groundwater and soil as well as flooding and ecology. The groundwater and soil profile and can be summarised, as follows:  

• Soil samples have been taken and analysed in laboratories to determine the volume of treated wastewater that can be held in the soil (this determines the potential volume able to be discharged). 

• The amount of treated wastewater that can be discharged at any point in time will depend on the soil moisture content. This is because the soil can only hold so much moisture before it will percolate through and we want to discharge treated wastewater at a rate that means that it is held in the soil so that plant roots can take up the nutrients. This is land treatment. 

• We have modelled rainfall at 2050 allowing for climate change so that we can estimate how often and how much we can discharge to the soil. Calculations show that based on what the soil can accommodate (per square metre), to meet the estimated volume of treated wastewater at 2050, we need an irrigation area of 38 hectares. 

• Soil profiles show that there is an underlying impermeable clay pan at depths of between 350mm – 700mm below ground level.  Water that percolates through the overlying layers is prevented from going deeper to the underlying groundwater aquifer by this pan. Tile drains were installed in the 1960s to drain any excess water and they will be upgraded to drain into planted soakage basins (that we will construct), where water will soak into ground to be taken up by plants and / or evaporate. 

Applications to authorise the discharge by means of irrigation of treated wastewater to land is planned to be lodged later this year.  These consent applications are in addition to the held Martinborough Wastewater Treatment Plant consents (WAR120258 [31707, 32044, 32045, 33045]).