Rotorua Lakes Restoration Project

Find reagents to control nutrients in lakewater

Paper presented at the 2012 Water New Zealand Conference ‘Ovecoming Rotorua Lakes Ecology Challenges’

Client: Bay of Plenty Regional Council

Period: 2004 to 2012

Location: Lake Rotorua and Lake Rotoehu, NZ

Nutrient enrichment of water caused mostly by land management practices

requires locking of 10 tonne/annum of phosphorus.

‘Lock’ phosphorus in stream water to make DRP less available to phytoplankton and invasive aquatic plants.
Alum dosing of spring fed streams.
‘Lock’ phosphorus in lake bed sediments to limit re-entry of DRP into the water column during lake stratification events.
Two P locking plants at Lake Rotorua treating water of two streams of 2 to 3 m3/s flow each.
Single P locking plant at Lake Rotoehu treating flow of up to 1.8m3/s.

Since the 1960’s the increasing incidence of nuisance algal blooms in some of the Rotorua Lakes has been the catalyst for studies to identify reasons for the general decline in water quality. These studies cite the combined nutrient inputs from anthropogenic and natural sources strongly contribute to the problems of lake water quality.

Over the last decade there has been a strong focus on setting targets to control nitrogen and phosphorus loads, the main drivers for changes in lake trophic condition. In recognition of the importance of the Rotorua lakes, Bay of Plenty Regional Council has set statutory environmental bottom lines in the form of specific Trophic Level Indices (TLIs) for all the Rotorua lakes in the Water & Land Plan (W&LP). This has lead to investigation and implementation of selected lake restoration methods to overcome the immense challenges of managing and treating large bodies of lake water.

The study of nutrient removal reagents and associated implementation costs caused a change in lake restoration strategy away from removal of phosphorus from lake water and instead moved toward locking of phosphorus within the water column and lake bed sediments. This change has saved tens of millions of dollars and is achieving the same outcome as physical removal of the phosphorus.

Since 2005 Enviromex provided specialist equipment, expertise and resources to study the different reagents having phosphorus locking characteristics to achieve lake restoration objectives. These studies showed alum to be very effective both in terms of phosphorus locking performance and OPEX cost. Implementation costs are a fraction of traditional water treatment facilities.

Enviromex designed phosphorus locking facilities, provided tender documents and technical advisory services to construct and commission the new plants.

For further information call Peter Browne at Enviromex NZ Ph 09 9504463 or 021 576 005.

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