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The Green Island Wastewater Treatment plant is to the south of Green Island and borders the Green Island Landfill and the Kaikorai Estuary.

How the Green Island plant works

Grit and plastics etc within the wastewater are removed by grit traps and 1 mm milli-screens. The wastewater then goes through several processes before it reaches the secondary treatment process, uv disinfection.

'A' process

The screened liquid enters the first of three aeration tanks. Rotary lobe air compressors supply process air through a series of pipes and diffusers. The diffusers emit air as tiny bubbles into the wastewater from the base of the aeration tank. A surface aerator is also used in the 'A' process tank.

A bacterial mass or "floc" is formed containing large numbers of very active bacteria, together with ciliates, rotifers and protozoa. The bacteria grow rapidly under these ideal conditions, consuming the organic material in the wastewater.

After leaving the activated sludge tanks, the mixed liquor suspended solids flow to a large circular tank known as a clarifier. The sludge, which settles to the floor of Clarifier No 1 is removed and a proportion returned to the inlet end of the activated sludge reactor where it "seeds" the incoming influent.

B process

The process is repeated again with the liquid from Clarifier No 1 being pumped into aeration tanks 2 & 3. After passing through these two tanks, the liquid flows to Clarifier No 2.

Activated sludge

A proportion of the sludge produced in the A & B processes is returned to the aeration tank it came from. This is known as returned activated sludge.

The other portion of sludge, called waste activated sludge, is sent to the thickener. The thickener concentrates the sludge by removing excess water. This sludge is then transferred to the thermophylic digester and then to the mesophylic digester.

Sludge from the mesophylic digester is transferred to one of two centrifuges.

After passing through the centrifuge the sludge has a dry solids content of 22.

Disposal and discharge

Centrate is returned to the wastewater stream for processing. Sludge is mixed with lime and disposed of at the Green Island Landfill.

The final treated effluent from clarifier No 2 is UV disinfected and discharged via diffusers at the end of an 850m long ocean outfall.