The town of Te Aroha in New Zealand’s North Island has installed a modern intensive-treatment sewage system, replacing the facultative or oxidation pond treatment that is used by urban councils.
The new intensive-treatment sewage system uses a combined screening and grit removal package plant, prior to a Membrane Biological Reactor (MBR). The old ponds at Te Aroha have been retained for storm water balancing.
Matamata-Piako District Council (MPDC) commissioned the intensive-treatment sewage system in 2006 to service the municipality of about 4000. Kaimai Valley Services (KVS), the works division of MPDC, undertake operation and maintenance of the plant.
A recent assessment by KVS’s wastewater team concluded that the intensive-treatment sewage system was performing well, with the screening and grit removal performing effectively.
According to KVS, they were looking at increasing the wastewater throughput of the system, so they examined the performance of the screen and grit removal closely to see if it could handle the extra load.
KVS observe that a number of Membrane Biological Reactors were installed in New Zealand. KVS were the third or fourth council to install this type of a plant. One of them had issues with the screen and grit removal process. This affected the working of the membrane, which is delicate and easily blocked or torn by debris if this is not screened and separated out.
An inspection of KVS system revealed a small amount of fibrous material. The wastewater team also carried out an analysis of the sludge at the bottom of the tank and found only fine particles.
KVS examined the sludge closely. The median particle size was 100 microns, which is small. The grit removal plant is designed to take out 95% of particles above 250 microns in size. The CST Wastewater Solutions STU/CR combined inlet screen and grit removal package plant was supplied to Matamata-Piako District Council by Smith and Loveless. Similar technology has been employed by CST Wastewater Solutions in Tasmania, Queensland and NSW, including the Olympic site at Homebush.
The Te Aroha installation comprises a compact, completely enclosed system that:
- screens solids from the raw influent
- lifts the screenings out of the wastewater and simultaneously washes the screenings
- de-waters the screenings, which are discharged down into placed skips
- separates the sand particles or grit
- lifts and de-waters the grit, which is discharged into the same skip or a separate skip
- offers optional grease removal
The STU/CR combined inlet screen and grit removal package plant/system handles flow rates up to 200 litres per second, with a 3 to 5mm circular holes screen, dewatering screenings between 25 and 35% , and removes 95% of grit particles above 200 microns and SG 2.1.
According to CST Wastewater Solutions, the STU/C combined inlet screen and grit removal plant/system offers efficient and cost effective solids separation before the wastewater reaches the MBR plant to make sure the membranes do not block.
The STU/C combined inlet screen removes rags and debris, while the grit removal stage gives better separation of particles from the solution, with no accumulation in the reactor.
Design features of the screening and grit removal plant/system include a stainless steel tank and a shaftless spiral screw made of a micro alloy carbon steel. The plant is prefabricated, so that limited civil work is required.
The STU/C combined inlet screen and grit removal plant/system include accessories such as screenings washing for faecal removal, a continuous bagging unit on the outlet and trace heating for cold regions.