Bell Island Wastewater Treatment Plant

Occupying 53ha of Bell Island on the Waimea Inlet, the Bell Island Wastewater 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 passes through these stages, then moves on to 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), at a cost estimated to be about a quarter of the cost of drying and disposing to landfill.

The treatment plant has the capacity to treat sewage equivalent to that generated by a domestic population of around 133,000 people.

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

The clarifier was added to the process in 1996 with the aim of reducing the solids and BOD (Biochemical Oxidation Demand) 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.

Clarifier

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.

Dissolved Air Flotation System (DAF)

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.

Autothermal Thermophilic Aerobic Digestion (ATAD)

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.

Facultative Oxidation Ponds (FOPs)

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.

Maturation Ponds

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.

Outfall to Waimea Inlet