All of Invercargill’s and Otatara’s residential and industrial sewage and wastewater is piped to the Clifton Wastewater Treatment Plant for treatment.

The plant treats an average of 25,000 cubic metres of waste each day to tertiary effluent standard before discharging to the New River Estuary. The treatment process is:

Part 1:  Pre-treatment

Invercargill Waste Water Plant Part 1 from Invercargill City Council

Screening: Sewage enters the plant and flows through 6mm rotating screens. Solids collected on the screens are lifted into bins by a screw conveyor which also washes and squeezes water from them.

Pre-aeration:  From the screens, sewage flows through a pre-aeration tank. Air is continually bubbled into this tank to add oxygen so that odour is avoided, and to keep organic material in suspension while heavier grit is allowed to settle, and then  pumped to a grit bin.

Screening and grit bins are periodically emptied, and the contents taken to landfill.

Part 2:  Primary Treatment

Invercargill Waste Water Plant Part 2 from Invercargill City Council

Sedimentation:  From the pre-aeration tank, sewage flows into three sedimentation tanks where it remains for at least four hours. Sludge settles to the floor of the tanks, and is scraped to hoppers, while scum rises to the water surface, and is driven by water to scum collectors. Both sludge and scum are pumped to digesters for further treatment. The remaining liquid flows over weirs and on to the secondary treatment stage.

Part 3: Secondary Treatment

Invercargill Waste Water Plant Part 3 from Invercargill City Council

Trickling filters:  After primary treatment, remaining solids are suspended or dissolved in the liquid, and the trickling filters convert these into solids which settle more easily. The trickling filters are filled with randomly packed plastic "media" on which slime can grow. The primary effluent is sprayed on the top of the filters, and trickles through, and the slime feeds on and removes solids from the liquid.  Eventually the slime grows too thick to remain on the media, and is washed of, and through to the clarifier.

Clarifer:  Effluent from the trickling filters flows into a clarifier where secondary solids (slime from the trickling filters) settles to the floor and is collected by a rotating boom and pumped to the digesters. Secondary effluent flows over weirs and on to the tertiary treatment stage.

Part 4: Tertiary Treatment

Invercargill Waste Water Plant Part 4 from Invercargill City Council

The tertiary treatment stage consists of four facultative ponds and two wetlands covering 26ha in total. Secondary effluent flows through this system over a period of about 14 days, and during this time is disinfected (bacteria killed) by UV radiation from sunlight.

Naturally occurring algae in the ponds feed on the small remaining amount of solids, and  become a food source for aquatic life in the estuary after discharge. The final treatment stage, the wetlands, further filters the effluent through wetland plants, and acts as a storage area, until effluent is discharged to the estuary over a three hour period following each high tide.

Part 5: Sludge processing to biosolids and application to Land

Anaerobic Digesters: Sludge and scum from the primary and secondary treatment stages is fed to anaerobic digesters, and held there for about 30 days. Anaerobic bacteria feed on and stabilise the sludge, producing biogas (methane and carbon dioxide), and reducing sludge solids volume by about 50%. 

It is important to keep the digesters absolutely free of oxygen, and at a constant temperature of 35C for the bacteria to thrive. Floating lids help to maintain temperature, keep air out, and collect the biogas, which is used to heat digesters. Excess biogas is flared.