Greytown Wastewater Treatment
At the Greytown Wastewater Treatment plant, sewage flows through an aerated facultative pond, a maturation pond and ultraviolet treatment. Discharge of the treated effluent is managed to either an adjacent block of land by irrigation or to the Papawhai Stream, depending on the season, river levels and conditions of the Resource Consents.
The plant was granted new consents on 11 February 2016. These consents will continue for 35 years (expire 11 February 2051).
In general, the consents allow SWDC:
- to discharge treated wastewater to land via an irrigation system where there is a soil moisture deficit that is greater than the depth of discharged wastewater.
- to discharge treated wastewater to the Papawai Stream at an annual average daily flow of up to 750 cubic meters per day and at a maximum daily rate of up to 1,500 cubic meters per day.
- to discharge treated wastewater to land via seepage from the oxidation ponds
- to discharge contaminants and odours from the oxidation ponds within the boundary
- to discharge contaminants and odours from irrigation of treated wastewater to land within the boundary.
Resource consents
Plant performance
Current Status: Compliant, but with the risks identified below
Period: May 2024
Commentary:
In 2023, Greater Wellington Regional Council issued letters requesting explanations of non-compliance. Wellington Water is implementing the required corrective actions where possible within the plant and resource constraints.
Major investment is required, and current approved funding levels do not meet this requirement.
A compliance upgrade project is currently underway however the scope of that does not currently allow for growth, and the plant is already operating beyond its design loading capacity.
The degree of desludging that will be achieved at Greytown is not yet determined, as the priority is to fully desludge Martinborough WWTP. Further desludging funding is likely to be required within a 3-year window (can be incorporated into the regular, ongoing maintenance desludging programme).
Items of significance:
Current plant design and processes are inadequate resulting in a risk of non-compliance (specifically related to Ammonia concentration in the effluent) and possible growth restrictions.
By the end of the irrigation season in mid-May, a total of 71,000m3 of treated effluent has been discharged to land; this achieves the intention of the resource consent. Critically, discharge to Papawai Stream during low flows has been avoided.The dry season was a factor in achieving this result.
Wellington Water is planning a series of tests to determine the reason for a significant discrepancy between the inflow and the outflow measurements.
GWRC has agreed that the 'Stage 1B Efficacy Report' will be updated at the end of the irrigation season. GWRC will then be able to decide what action to take at Greytown based on a full set of information including the results of this successful irrigation season.