The Ngaruawahia Waste Water Treatment Plant (WWTP) receives domestic wastewater from several towns for treatment. Upgrading of the plant required that the flow of the wastewater into the plant is never disrupted and that the quality of the water leaving the plant has received as much treatment as possible.
The latest stage of the plant upgrade included the removal of sludge and other deposits from the sewage treatment pond. This sludge had to be dewatered and the dried residue permanently deposited in designated areas of the treatment plant’s wetlands. A temporary de-watering pond was commissioned for this part of the project and Permathene supplied 3,250 m2 of Permaliner 0.5mm Flexible Polypropylene for the lining of the excavation. This liner was factory fabricated in three separate panels which were welded together on-site. De-watering bags were placed on this membrane and pumped full of sludge as part of the dewatering process. Five new floating baffle curtains were manufactured by Permathene and installed into the treatment pond. The baffle curtains allowed for the creation of an extended flow path through the pond thus separating the area into one facultative and four maturation zones. Baffles 1, 2, 5, and 6 were fabricated with a range of square openings which allowed for a controlled water flow throughout the four maturation zones.
This work significantly improved the performance of the treatment pond and prepared the pond for future upgrades.
Six baffles total length 730m were fabricated from a combination of standard 30m long units and units of non-standard lengths, interconnected by overlapping and secured connections. Buoyancy element baffle units were connected by strong reinforced 2mm thick HDPE plates. The baffle’s slope parts had secondary skirts installed along the bottom part of the slope section, allowing fixation of the slope parts to the wave bund for prevention of material damage due to friction with wave bund surfaces.
Construction of maturation zones inside an existing pond required baffle connections including 3 “T” connections and one complex Cross-Connection.
All connections were made by welding additional 2m wide flaps to the baffle curtains. Firstly, all connecting baffles were connected via tension members. Then, after adjusting tension of the mooring lines, additional flaps were brought together and interconnected by sliding slotted HDPE pipe along reinforced sides of the flaps from the surface of the water. This allowed creation of watertight connections at all “T” connections and Cross-connections. Later positions of cross-connections were secured by installed cross-anchors (short mooring lines 4m long, connected to heavy concrete anchors on the pond’s floor).
The baffles supporting system consists 8mm (Grade 316) stainless steel flexible wire ropes, on-shore anchors, cross-anchors, and wooden poles.
Baffles 1, 2 and 3 have been laterally stabilised by connection to pre-existing wooden poles. These connections were designed to allow accommodation of the pond’s water level variation.
Permathene engineers provided support during the installation process. Total length of the baffle curtain system is 787m and to date is probably the longest floating geosynthetic baffle curtain system used in wastewater applications in New Zealand.
Nikolai Vakhroushev works with Permathene Ltd. View a profile of Permathene on Geosynthetica and visit the company’s website: http://www.permathene.com/
