Lab Work, Pilot Trials and Brains Trust Delivering Fresh Supply for Thriving Towns

Will Jacobson – Civil Engineer and Project Technical Lead, Stantec NZ Ltd

Improvements to the drinking water supplies are necessary to deliver compliant drinking
water for the growing communities of Alexandra and Clyde in Central Otago, New Zealand.
In addition, Central Otago District Council confirmed their desire to pursue a treatment
solution which provides 4-log inactivation for protozoa, with residual chlorination to achieve
bacterial compliance. The Lake Dunstan Water Supply project delivers on these needs by
combining the benefits of a well selected water treatment technology and supply system,
and a collaborative design and procurement model.

A pilot trial in 2018 showed that treating the raw water via cartridge filtration, UV
disinfection and chlorination was not feasible. This was primarily due to fine suspended
solids (‘glacial rock flour’) causing the cartridge filters to rapidly blind and this technology
is not backwashable. Lindavia diatom algae was also observed in the filters which presents
risks of “lake snow” polysaccharides further blinding filters. Stantec completed a broad
review of available treatment options for the specific challenges observed. Two filtration
treatment options were subsequently pilot trialed in 2020:
• Cartridge filtration combined with Arkal/Amiad pre-treatment system
• Membrane filtration

The pilot trials have yielded the following key results:
• Disc filter plus microfibre filter pre-treatment was not able to remove sufficient
material to protect and prolong the cartridge filter run time. The resulting
requirement for frequent replacement of cartridge filters ruled out this option.
• Membrane filtration performed well at removing rock flour and algae
concentrations.
• Operating flux rates, backwashing frequency and chemical cleaning regime to
inform later stages of design.
• The bores installed within the constructed embankment demonstrate good filtering
of lake snow.
• Raw water sampling confirmed low concentrations of dissolved iron and dissolved
organics. The requirement for coagulant chemical dosing and/or pre-treatment
upstream of the membranes as part of initial works was therefore excluded.
• Further treatment trials are recommended at other localities in Central Otago to
challenge test the pilot plant under high lake snow conditions.

With the treatment technology confirmed, the project team have embarked on a
collaborative design process involving broad technical and contractor input. This is resulting
in an efficient scheme with the different parties challenging each other along the way. This
is delivering the benefits of a lean design and Design and Build Contract whilst ensuring
appropriate risk reduction. To date the project team has delivered significant smart
solutions, some of which are noted below:
• Review of network flow data has allowed for a reduction in the WTP capacity when
combined with storage capacity at Alexandra Northern Reservoir site upsized to
6,000 m³. The resulting required plant capacities of 14 MLD from 2023 and 20
MLD from 2028 are well matched to the design capacities of the membrane skids
(7 MLD each).
• Removal of DAF pre-treatment system anddentification of future upgrades to
respond to potential changes in raw water quality.
• Reservoir material selection assessment. Concrete v bolted steel tank.
• Removal of additional filtrate tank and booster pumps by matching the site layout
and scheme hydraulics to the membrane feed pump hydraulic capacities.
• Reduction of flows to wastewater system by discharging backwash waste to Clutha
River under Permitted Activity rules.

This paper summarises the critical benefits and outcomes from the pilot trials and
collaborative design process.