Bells Island Sewage Treatment Plant

Occupying 53ha of Bells Island on the Waimea Inlet, the Bells Island Sewage Treatment Plant includes a pre-treatment system consisting of a step screen and a grit chamber. A high rate activated sludge basin was installed in 2005 at the same time the existing clarifier underwent rehabilitation and improvements.

Wastewater that passes through these stages then moves onto two-stage facultative ponds before being discharged to the Waimea Estuary basin on the outgoing tide. The ponds are three 10ha facultative ponds in parallel and two 10ha maturation ponds in series.

Waste activated sludge is pumped to a Dissolved Air Flotation (DAF) unit before being treated in an Autothermal Thermophillic Aerobic Digestion (ATAD) plant. This produces class A biosolids that is suitable to be pumped across the estuary to Rabbit Island where it is sprayed onto forestry blocks as a fertiliser. This system typically disposes of 50-100m³ per day of biosolids (2 – 3 tonnes of dry solids) and at a cost estimated to be about a quarter the cost of drying and disposing to landfill.

The treatment plant treats sewage equivalent to that generated by a domestic population of around 133,000.

  • ADWF: 15,000m³/day
  • Peak Flowrate: 50,000m³/day
  • Design BOD load: 8,000kg/day
  • BOD of treated effluent: <50g/m³
  • Design Suspended Solids load: 10,000kg/day
  • Suspended solids: <150g/m³
  • Total Nitrogen: <600 kg/day

Bells Island Treatment Plant Schematic

Note: the primary clarifier is currently under construction.

A schematic of the functions of the sewerage system.



The Bells Island Sewage Treatment Plant (BISTP) was commissioned in 1983.The original design population for the BISTP was 33,000 and the plant consisted of a fully mixed aeration basin, three facultative oxidation ponds (in parallel), two maturation ponds (in series), and a tidal discharge.  The original concept allowed for expansion by the addition of one extra aeration basin (alongside the existing aeration basin), and extra facultative ponds as required.

The BISTP operated successfully until overloading of the facultative oxidation ponds (FOPs) was noticed in the late 1980’s.  A major result of overloading was the generation of malodour.  Investigations were undertaken and it was concluded that the cause of the overloading was a combination of stratification and organic load build-up in the ponds considerably in excess of treatment capacities.  As a consequence of the high organic load all oxidation pond oxygen was quickly assimilated, causing anaerobic and putrefactive conditions and noticeable malodour production.

A review of the BISTP in 1992 confirmed that sludge build-up was a primary factor causing the overloading and it was recommended that desludging of the oxidation ponds should be commenced.  This was undertaken over the period 1993-1995.  Other recommendations from the 1992 review that were implemented were:

  • Install mechanical aeration mixing on the three facultative oxidation ponds
  • Install a clarifier and sludge processing plant (Autothermal Thermophilic Aerobic Digestion - ATAD) to remove the sludge, thereby significantly reducing the build-up of waste aerobic sludge

These upgrades were completed in 1996.  Following commissioning of the 1996 upgrade, several operational issues became apparent:

  • Overloading of the aeration basin caused malodours
  • A fungal parasite had infected the ponds, reducing the algal population for short periods with consequential generation of malodours
  • Improved solids capture through recycling of sludge was desirable in the clarifier to reduce solids loadings on the FOPs
  • High nitrogen levels in the biosolids processed by the ATAD plant led to a requirement for additional land to maintain biosolids application rates within consent limits for nitrogen
  • The operation of the ATAD and sludge processing plant needed improvements to the aeration and mixing equipment
  • There were reported high hydrogen sulphide levels around the inlet basin which needed to be addressed

In 2003 NSRBU tendered the design, construction and operation of a retrofit at the Bells Island Waste Water Treatment Plant that included the installation of a Dissolved Air Flotation System (DAF).  This upgrade was implemented during 2004 and 2005. 

After the acceptance of the tender, but prior to the construction, it became apparent that the influent parameters to the Bells Island Facility could, at times, exceed the design parameters used for the upgrade.  However, NRSBU decided to continue with the tender and to review the situation after the installation of the 2004-2005 upgrade.

Treatment Plant Details

Essential statistics of Bells Island Treatment Plant
Essential ComponentsInstall DateResidual Life (years) at 2007Capacity* (Indication only)
Step screen 1996 4 950Lts/sec
Aeration basin 1982 - 2005 5 - 55 9,600kg BOD5/day
Clarifier 1996 - 2004 0 - 39 425Lts/sec
Dissolved Air Flotation System (DAF) 2005 0 - 47 551kg/hr or 13230kg/day
Autothermal Thermophilic Aerobic Digester (ATAD) 1996 - 2006 0 - 40 8,200kg TSS/Day
Three 10ha facultative oxidation ponds 1982 - 2005 6 - 79 50 – 60Kg.BOD5/ha.d in winter 120Kg.BOD5/ha.d in summer
Two 10ha maturation ponds 1982 - 2005 6 - 79  
Outfall to the Waimea Inlet 1982 - 2001 0 - 51 16,000m3 per day

*Details of individual asset capacity is detailed in the Bells Island WWTP Treatment Capacity Report dated November 2006. (766KB PDF)


The clarifier was added to the process in 1996 with the aim of reducing the solids and BOD load on the oxidation ponds.

The clarifier removes the settleable solids from the wastewater.  The resultant sludge is pumped from the base of the clarifier to the digesters.  Effluent from the top of the clarifier is decanted into the facultative ponds.

Dissolved Air Flotation System (DAF)

In 2005 the DAF process was added as a gravity separation system that uses air bubbles in the wastewater holding tank to help float insoluble materials to the surface so they can be removed.  Those materials that are heavier than water are removed by dosing with polyelectrolytes.  The resulting flocculants cause these materials to join together in clusters that are lighter than water and therefore float.

Autothermal Thermophilic Aerobic Digestion (ATAD)

The sludge is stabilised in the two stage ATAD process, also added in 1996, and uses heat released by microbial activity to achieve and sustain minimum operating temperatures of 35°C and 55°C respectively to produce pasturised biosolids (Class A Biosolids).

The resultant “biosolids” are transferred to a storage tank before being pumped across the Waimea Inlet to holding tanks on Rabbit Island.  Tankers transport them to the forest where they are sprayed under the trees as fertiliser.

Facultative Oxidation Ponds (FOPs)

Effluent from the clarifier is split between the three 10ha FOPs.  Bacteria and nutrient in the waste promote vigorous growth of algae.  During the day, near the surface, the algae generate oxygen by photosynthesis, further stabilising the wastes.  The remaining solids settle to the bottom of the ponds and are treated by anaerobic processes.

Maturation Ponds

The two 10ha maturation ponds, in series, complete the stabilisation process and reduce bacteria numbers.  They also provide storage capacity for intermittent release of the effluent.

Outfall to Waimea Inlet

After an average retention time of about 30 days, the treated wastewater is discharged through a gravity driven outfall, into the waters of the Waimea Inlet, on the first three hours of each outgoing tide.

Biosolids Application Facility

Biosolids are disposted of within the forested area of Rabbit and Bells Islands. Total afforested area available for biosolids disposal is approximately 750ha. The forest on Rabbit Island is owned by TDC and a licence to dispose of biosolids is held by NRSBU.

The resultant “biosolids” from the ATAD are transferred to a storage tank before being pumped across the Waimea Inlet to holding tanks on Rabbit Island.  Tankers transport biosolids to the forest where they are sprayed under the trees as fertiliser.  Resource consent details are contained in the supplementary section.