Wastewater Treatment & Processing

Currently, wastewater from the homes, businesses and facilities in the Ōpōtiki township makes its way to the WWTP via an underground piped network to be treated in a three-stage process.

Stage 1 - Imhoff Tank

The primary treatment stage uses milliscreens and the original 1950's Imhoff tank at Whitikau reserve to remove sediment and inorganic solids from the wastewater.

As part of this project council will be putting together a strategy to decommission the Imhoff tank.

Stage 2 - Oxidation Pond

The oxidation pond uses natural elements like wind, sunlight and algae to remove most of the remaining solids from the wastewater stream. Organic solids settle in a sludge layer at the bottom of the pond which break down through natural processes called “anaerobic decomposition” through lack of oxygen. This limits the amount of sludge that is required to be removed off site.

Dissolved nutrients in the sewage (such as nitrogen and phosphorus) feed algae, which are microscopic plants floating and living in the water. The algae also use carbon-dioxide (CO2) and bicarbonate as food and release oxygen during the day as a waste product through the process of photosynthesis.

This oxygen is then used by helpful bacteria to breakdown incoming waste. Pond oxygen levels, and some other characteristics like pH, will therefore change throughout the day, and from day to night-time.

Mechanical Aerators were added to the oxidation ponds in 2022 to help add oxygen into the ponds natural processes, and reduce risks of the natural treatment process being overloaded, which would cause a very bad smell as well affecting the treatment processes.

Excess sludge was removed for the first time in the early 2020's. So far 1,200 dry tonnes of sludge have been removed. Collected sludge is disposed of to a worm farm in Kawerau, which further breaks the material down.

Stage 3 - Disposal Field

The final treated wastewater effluent is discharged from the oxidisation pond on to the sand dunes in the Waioeka estuary.

This land based discharge provides for filtration of the treated wastewater through the soil column, as well as making use of the microbial action on the sandy soils to further remove any unhealthy bacteria or nutrients.