Three general levels of wastewater treatment have been defined: primary, secondary, and tertiary. The three Wastewater Treatment Plants (WTP’s) to be built as part of the Harbour Solutions Project will employ a hybrid of primary treatment known as advanced-primary treatment. Primary treatment involves mechanical processes which remove a certain amount of the floating and suspended solid material in the wastewater. Advanced primary fits between primary and secondary treatment, and removes up to 70 per cent of the suspended solids in the wastewater.
Advanced primary treatment (click on diagram below for full-size version) improves upon the basic level, through the addition of chemicals known as flocculants to the settling process. These are the same type of flocculants used in the treatment of drinking water. Essentially, the effect of these chemicals is to pull together some of the smaller particles that would otherwise remain suspended in the water column, and cause them to settle to the bottom of the settling tanks, where they and the flocculant will be removed as part of the biosolids sludge.
The final stage of the sewage treatment process is the disinfection of the effluent before it is released into the harbour. Many sewage treatment plants today use a chlorination process to disinfect effluent, but this chemical is not particularly good for the natural environment. The Harbour Solutions Project will utilize a more environmentally beneficial process known as UV disinfection. In this process, no chemicals are added, but rather high-intensity ultra-violet lights are used to kill bacteria in the effluent before it is released into the harbour.
The addition to this ‘‘advanced" stage and the use of ultra-violet disinfection to the process significantly improves upon the basic primary process and results in cleaner effluent being discharged into the harbour.
Should future regulations or public opinion demand a higher level of treatment, the proposed design of the three WTP’s will provide for the future addition of secondary treatment processes. The plant’s capacity can also be expanded to handle increased volumes.
Finally, the by-product of sewage treatment is sludge, which accumulates at the bottom of the settling tanks and must be removed. In other municipalities sludge has often been incinerated or land filled, but these options will not be utilized by the Harbour Solutions Project. Instead, the Request for Proposals presented to the bidders outlines clearly that the sludge must be used for an environmentally beneficial purpose.
