Clifton plant

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 and the sewage is treated to tertiary effluent standard before being discharged. Treatment and disposal of Invercargill's and Otatara's sewerage and wastewater:

Step 1

Pre-treatment (screening, solids removal)
Sewage enters the plant and flows through a 6mm filtered screen. Solids that accumulate on the screen (such as tampons and condoms) are removed and washed before being collected in a large bin. The solids are then taken to a landfill. 

Step 2

Pre-treatment (further solid separation)
The screened sewage is pumped to pre-aeration tanks for further separating and cleaning. In the pre-aeration tanks the sewage is agitated by a continuous flow of air from inside the tank and this allows the solids (grit) to separate and settle. The settled solids are pumped into a bin and any water present is removed. The solids are then taken to a landfill. 

Step 3

Primary treatment (sludge and scum removal)
From the pre-aeration tanks the sewage flows into three sediment tanks, where settling takes place. The sewage can remain here for a number of hours or days depending on the weather. Sludge settles to the bottom of the tanks, and scum (a foam-like substance) that settles on the top of the liquid, are mechanically removed from the tank for further treatment. The remaining liquid sewage in the tank flows over weirs  at the end of the sediment tanks, and is pumped to another tank containing cone-shaped plastic filters. 

Step 4

Secondary treatment (trickling filtration)
As the liquid trickles down the filters, the remaining solids and suspended pollants are slowly dissolved by a large number of helpful micro-organisms which feed on the sewage. The purified sewage and slime flows to another tank where the slime settles. The settled slime is then transferred to digesters for further treatment and the remaining liquid over-flows on to clarifying weirs before discharge into holding ponds.  

Steps 3a and 4a

Sludge and scum treatment
The collected sludge and scum from the primary and secondary sewage treatments, as well as industrial wool scour waste, are collected and pumped to a separate tank which contains special bacteria which feed on and digest the sludge and scum.

This tank does not contain any air and is kept at 35degC, which is vital for the bacteria to survive and breakdown the sludge and scum. Biogas is collected from the tanks (a mixture of methane and carbon dioxide), which is given off by the bacteria. This biogas is used for the heating the tanks, and can also be used as a fuel for a standby electric alternator. Any excess biogas is burned.

Sludge is retained and mixed in the tanks for 30 days, and is then pumped into holding ponds. Water is removed over time and the remaining solids are then taken to a landfill.

Step 5

Tertiary treatment
The effluent from the secondary treatment process (step 3) is transferred to four holding ponds, and then flows into two wetland areas. Naturally occurring algae in the ponds and wetlands break down the liquid and slime further and the bacteria present is killed by sunlight. This whole process takes about 14 days. 

Step 6

Disposal of treated wastewater
The treated wastewater is finally discharged to the New River Estuary. Water from the Estuary is frequently sampled and tested to ensure it complies with the standards for recreational and shellfish gathering waters.