Wanganui Wastewater Project

History

The Wanganui Wastewater System receives domestic sewage, substantial quantities of industrial wastewater and substantial volumes of infiltration as well as stormwater runoff. Approximate average dry weather quantities are:

The system has developed since early settlement as a single combined pipe system. Since the mid 1970s any new development, whether located in a new or old area, has been required to install separate stormwater and sewage systems on the property. In older areas these separated systems are connected to a combined public sewer.

The existing combined sewers are generally old and in many instances have exceeded their design life. Substantial infiltration is entering the pipes through cracks and deteriorated joints, both in the public system and on private property.

Cases of typhoid in the early 1960s prompted the Medical Officer of Health to urge the Council to very strongly consider a complete investigation of the City’s sewage disposal arrangements. As a result, the Castlecliff interceptor, the Beach Road Pump Station and the sea outfall 1,800 metres off South Beach were constructed. This serviced the designated wet industries and approximately 10,000 people.

1992 Scheme

In 1992, the Council obtained Resource Consent for continuing combined sewage discharge to the Whanganui River. The Resource Consent required future improvements. The Whanganui River Wastewater Working Party was formed as a result and recommended a scheme to the Council. After audit, the Council adopted the Working Party's recommended scheme and obtained Resource Consents for it to be implemented over a 15 year period to 1 July 2007.

The scheme reflected the then current state of technology.

The interceptor system was built from 1993 to 1996. Separation of Springvale and a number of areas along the interceptors has also been done. All dry weather flow is now carried to the Beach Road Pump Station and discharged into the sea via the outfall. Rainfall, however, still results in overflows to the river of a combination of stormwater and sewage.

2002 Scheme

Development of the 2002 Scheme included extensive consultation with key stakeholders including Iwi (Tupoho and Ngati Apa), a consultation group made up of recreational users, DoC, Ministry of Health, Friends of the Shoreline, Putiki Marae, the Whanganui River Maori Trust Board and other interested parties. This included major presentations and hui. Consents were granted in 2002.

The extensive research programmes have shown that the most significant impact of the sea (outfall) disposal which has been occurring is bacterial contamination. This is a public health, rather than environmental, effect with the impact being risk to humans who come into contact with contamination, primarily by eating shellfish from the area. The sea bed and the benthic community have been altered over a small area surrounding the outfall only. This area has an elevated level of organic sediments and a correspondingly different animal population to that found along the coastline generally.

Studies have shown that the river has a very large influence on the bacterial contamination of the beaches. When it rains, the river becomes contaminated and the effect of this is carried down to the beaches. The outfall discharge itself is considered to have a more occasional influence on the beaches.

Treatment of both domestic and industrial waste (2007) will result in a much improved environmental outcome and should enable the removal of the exclusion zone from the PPH classified water. So long as the pathogen contribution from upstream of the city is not excessive, collection of shellfish will be possible without risk to public health.

Features

The basic features of the 2002 scheme are:

• Full separation into separate sewers and stormwater drains, both on and off private property.
• An interceptor system to convey separated sewage and industrial tradewaste (together) to the Beach Road Pump Station for screening.
• In the short term, the screened sewage and tradewaste is pumped to the outfall.
• From July 1 2007, the screened sewage and tradewaste will be pumped to a treatment plant and the treated effluent is disposed of to the outfall.
• Stormwater is discharged to the river.

Treatment Plant

The wastewater treatment plant is being constructed adjacent to Wanganui Airport. The plant will be fully operational by 1 July 2007. The process consists of deep lagoons followed by UV disinfection. This photo shows construction work at the treatment plant (March 2007).

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