Elimination of contamination in household water supply
Sanson is a small settlement located southern half of New Zealand’s North Island. Adjacent to Sanson is the Royal New Zealand Air Force (RNZAF) base Ōhakea.
Rural Ōhakea and Sanson residents had previously been drinking bottled water for four years due to contamination from the nearby air force base. The water had been contaminated by toxic firefighting foam used during training. Through use of the harmful chemicals, PFAS and PFOS, had ceased in 2002, the build-up had run off into nearby groundwater, contaminating the water supplies of neighbouring properties.
The Manawatu District Council required upgrades to the Ōhakea water scheme to achieve elimination of contamination in household water supply. This included a new bore, reservoir, water treatment plant, and pipe network as well as connections to properties. FILTEC were awarded the contract for the design and build of the new water treatment plant. The bore and treated water reservoir were completed by other contractors.
What We Did
FILTEC were responsible for the design and construction of a new 1,500 m3/d water treatment plant required for the Ōhakea rural water scheme. The new plant needed to supply drinking water to residences and farms in the Ōhakea rural areas, Sanson and the RNZAF Base.
Iron and Manganese Removal
A new deep bore was constructed to replace the existing contaminated source and replace the current reliance on tanker supplied drinking water. Three greensand filters were installed for iron and manganese removal. Prior to the greensand filters pre-chlorine dosing is occurs in the bore water to oxidise dissolved iron and manganese.
Water enters through the top of the greensand filters, passes down through the media and out the bottom of the vessel. As the water passes through the media, any solids including oxidised iron and manganese are caught and do not pass through the filters. The water flow through the filters is controlled with a flowmeter and control valve. Each filter has the ability to backwash individually while the other filters stay in operation. The filtered water is passed through to a 3M cartridge filtration system which filters the remaining solids from the water.
The filtered water passes through a Trojan UV Swift SC disinfection system. The UV reactor emits UV radiation that will inactivate micro-organisms in the water. The water flowrate through the UV reactor is measured and communicated to the UV control panel. This ensures the UV reactor provides a consistent UV dose of at least 40 W/cm2.
Following UV disinfection, the water enters the treated water reservoir with a capacity for 1 million litres of water. Treated water is pumped from the treated water reservoir to the network.
Post-chlorine dosing was added as the final stage of the water purification process. This eradicates unwanted biological organisms (pathogens) by preserving a low level of chlorine in the piping network or plumbing system.
FILTEC also constructed the treatment building and supplied the associated water treatment plant pipework, electrical work and backup generator.
How we were challenged
FILTEC overcame several obstacles during this project. The project needed to run on tight timeframes due to the urgency of delivering safe, uncontaminated water to residents. This was inconvenienced further due to supply issues and alert level restrictions brought on by the COVID-19 pandemic. Another major obstacle was the process of adding iron and manganese removal to the water treatment plant design. FILTEC were able to adapt to the design change and carry out raw water quality tests during the detailed design phase.
The new water scheme was rolled out in August 2022. The scheme delivers a new bore, reservoir, water treatment plant, and pipe network as well as connections to 80 houses and farms and to the Ōhakea air force base.
The upgrades to the water scheme have reduced the community’s fears around their personal health and achieve elimination of contamination in household water supply. Residents now have access to a reliable drinking water supply. The project reached everyone in the community eligible to receive water under the planned scheme.
The water treatment plant delivered by FILTEC produces up to 1500m3/day of safe compliant drinking water. The plant has been designed to accommodate population growth with the ability with the ability to expand to 4000m3/d through the installation of additional greensand filters. FILTEC attended the opening of the new Ōhakea rural water scheme where the scheme was switched on by the Manawatū District mayor and Environment Minister.