Pre-treatment of Sewage
Pre-treatment, which includes screening and grit removal, is carried out at the start of the treatment process. Pre-treatment is designed to remove solid objects, along with grease and oil, which impede efficient wastewater treatment and are undesirable in the end product biosolids.
Removal of solid objects is also undertaken to protect machinery (especially pumping equipment) and to prevent blockages in
Smaller pipes and channels, which transport the wastewater around the treatment plant.
Pre-treatment also reduces the biochemical oxygen demand (BOD) of the wastewater. BOD is a measure of the strength or pollution potential of the wastewater.
Pre-treatment occurs when wastewater from Auckland's wastewater
Interceptors enters a mixing chamber at the start of processing. The interceptors - Western, Eastern, Southwestern and Southern interceptors - are Auckland's main sewers (the Southern
interceptor combines with the Eastern before it enters the treatment plant.) Odorous air and gases are extracted at this point and at numerous stages throughout the treatment process and passed through odour control biofilters. After the mixing chamber, the wastewater flows into six channels, each capable of taking 2,700 litres per second.