Sewage overflows from a weir in the main sewer to a sewer well (balance tank), from which it is pumped to a rotary screen that stops and starts with the raw sewage pump. The screen contains a set of spiral guides that direct the screened material to the front of the screen and down a chute to the surge tank. Solids separated during the treatment process are returned to the main sewer for treatment at LMWQCC.
The screened sewage flows out of the screen into a flash-mixing tank where lime slurry is dosed as required to maintain the pH in the reaction tank at 9.5.
The screened sewage/lime mix flows into a solids separator, which contains a set of downward sloping, corrugated baffles placed parallel to the flow direction. Suspended and colloidal solids settle out as the flow is reduced and gravitate to the bottom of the separator.
The settled solids flow from the separator to the surge tank and then back to the main sewer. Any scum present in the primary treated wastewater rises to the surface of the separator where it can be manually skimmed off to the surge tank.
The outflow from the separator gravitates to a balance tank where it is stored prior to secondary treatment.
The secondary treatment step is comprised of two Biofor reactors. These are upflow biological filtration units (tanks containing granulated media suspended on an internal platform). A constant stream of air flows through the media and this enables bacteria to grow on the media.
The bacteria in Biofor 1 remove biochemical oxygen demand (BOD) from the primary effluent, and some degree of filtration of suspended matter also occurs. The outflow from Biofor 1 is transferred to a balance tank prior to Biofor 2.
The granulated media in Biofor 2 is slightly finer that that in Biofor 1, and the bacteria growing on it nitrify the primary effluent (removing ammonia). Some degree of filtration also occurs. The secondary treated effluent flows to a balance tank prior to tertiary treatment.
After a period of operation, bacteriological growth on the media is sufficient to restrict flow of primary effluent through the Biofors and they require backwashing daily.
Secondary treated effluent is pumped to the Memtec Continuous Microfiltration (CMF) Unit for final filtration down to 0.2 micron through hollow fibre membranes (compared to about 20 to 50 micron for a sand filter).
As the CMF unit run time increases, the hollow fibre membranes begin to block up with fine solid particles. At a predetermined time setting, an automatic backwash is initiated. This backwashing occurs about every 15 to 30 minutes and involves an air and water back flush to dislodge and wash away the solid particles to the surge tank.
As the membranes become clogged with solids over time the CMF unit goes through a self-cleaning cycle at which time a chemical cleaning solution is recirculated through the membrane modules.
The filtered water (permeate) flows from the CMF unit to a contact tank where it is disinfected (using liquid sodium hypochlorite) to eliminate any remaining bacteria and then transferred to the final Treated Water Tank, ready for irrigation use.
The various steps in the treatment process release foul gases into the plant environment, which can be hazardous to operations personnel and may be a source of odour complaints from neighbouring residents.
Foul air is controlled by a wet scrubber tower, which uses a solution of caustic and hypochlorite for odour control.