EVALUATION OF POTENTIAL ENGINEERING OPTIONS FOR REDUCTION OF NITROGEN INPUTS TO LAKE ROTORUA R4181 EVALUATION OF POTENTIAL ENGINEERING OPTIONS FOR REDUCTION OF NITROGEN INPUTS TO LAKE ROTORUA Contract Report No. 4181 August 2017 Project Team: Jo McQueen-Watton - Report author William Shaw - Report author, peer review Prepared for: Toi Moana Bay of Plenty Regional Council P.O. Box 364 Whakatāne 99 SALA STREET, WHAKAREWAREWA, 3010, P.O. BOX 7137, TE NGAE, ROTORUA 3042 Ph 07-343-9017; Fax 07-343-9018, email
[email protected], www.wildlands.co.nz CONTENTS 1. INTRODUCTION 1 2. OVERVIEW OF THE LAKE CATCHMENT 1 2.1 Ecological context 1 2.2 Geology 2 2.3 Lake Rotorua 2 2.4 Sub-catchments 4 3. KEY FACTORS AFFECTING NITROGEN REMOVAL 5 4. ASSESSMENT OF POTENTIAL TECHNOLOGIES 6 5. REJECTED TECHNOLOGIES 7 5.1 Overview 7 5.2 Nanobubbles 7 5.3 Flocculent application 8 5.4 Aeration-driven destratification 11 5.5 Lake bottom oxygenation 12 5.6 Wave barriers 13 5.7 Dredging 13 5.8 Grass carp 15 5.9 Hamurana diversion wall (with and without gates) 16 5.10 Wave barrier at Waiohewa 17 5.11 Floating wetlands 17 5.12 Removal of sewage from the catchment 19 6. POTENTIALLY FEASIBLE TECHNOLOGIES 20 6.1 Overview 20 6.2 Denitrification plant (same as Tikitere) 20 6.3 Lake weed harvesting 21 6.4 Natural wetlands: protection, maintenance and enhancement 22 6.5 Constructed wetlands 24 6.6 Denitrification beds/carbon walls 26 6.7 Watercress beds 28 6.8 Seepage wetlands and grass hedges 30 6.9 Anionic PAM blocks 32 6.10 Removal of N-fixing plant species from the catchment 33 6.11 Other considerations 35 7.