Official Information Request - Geothermal discharge

3. For (2) what are the main chemical components of the geothermal discharge.

Geothermal fluids vary in their composition and can range from acidic to alkaline in nature. We do not have enough knowledge to be able to accurately answer your question, as the results we have on file have been collected for specific purposes, not to be able to characterize geothermal discharge.

4. For (3), what are the effects of these geothermal water components on the primary, secondary and tertiary treatment phases (including spray irrigation).

The pH and Sulphur compounds in geothermal fluids impact network infrastructure, decrease the life of the assets and increase maintenance costs.

Water entering the treatment plant with negligible nitrogen, leaves the treatment plant with a low concentration of nitrogen, which increases the mass load discharged to the catchment.

Geothermal muds and water in the wastewater has the potential to inhibit the biological process if discharged in high concentrations or high volumes (reports provided below). Restrictions and management requirements are in place to reduce the risk of any impact to the network and treatment processes.

Water Solutions Ltd, 2005 (Jurgen H Thiele).Temperature Phased Anaerobic Digestion of RDC WWTP Biosolids - Final Test Report (Attachment 1)

A report commissioned to determine whether the sludge from the Rotorua wastewater treatment plant was suitable for a modern anaerobic digestion technology determined among other things:

• RDC biosolids do not contain significant amounts of substances that preclude thermophilic anaerobic digestion of soluble substrates because otherwise an increasing content of RDC sludge should have suppressed the methane production.
• Contrary to expectations, the H2S content in the biogas was only moderate (1300 +/- 100 ppm) and was comparable to the biogas composition of food waste digestion systems.
  The Wastewater Specialists 2023 (Hugh Ratsey). Geothermal Mud Inhibition Testing (Attachment 2)
• Test results of two of four samples of slurries of local geothermal muds indicate that they would inhibit secondary treatment biological process if present at high concentrations.
  Note: The names and identifying information for the specific sites tested is redacted as per section 7(2)(a) of the Act ‘to protect the privacy of natural persons’.
  The Wastewater Specialists 2022 (Hugh Ratsey). Toxicity Testing of Geothermal Samples (Attachment 3)
• Initial test results of local geothermal fluid indicated that one of six samples, that appeared to be anomalous, would inhibit secondary treatment biological process if undiluted. A repeat test found the anomalous sample was not inhibitory “This re-run suggested the sample was benign”.

While geothermal discharges contain heavy metals which could increase the risk of the concentrations exceeding the guidelines for the discharge of biosolids to land, the levels are reduced through the vermicomposting process to ensure it is safe to discharge to land.

The level of heavy metals in the discharge to land have no impact on the spray irrigation site.

Scion (Beets, Gielen, Oliver, Pearce, Graham) 2013. Determination of the level of soil N and P storage and soil health at the Rotorua Land Treatment site (Attachment 4)

• “Soil heavy metal levels in irrigated soils were well below heavy metal limits (NZWWA, 2003) and only Cu, Zn and Cr were higher in the irrigated than the non-irrigated soils. Nevertheless, more than 250 years of irrigation at current rate would not cause the irrigated soils to reach the current heavy metal limits for these elements”.

5. Have cultural interests in geothermal waters been received for the above (summary).

Cultural feedback we have on file tends to be more feedback on the project or solution that is being investigated. There have been no reports done specifically on geothermal waters as the subject. BOPRC may have further information, as it is responsible for the geothermal field.

You have the right to seek an investigation and review by the Ombudsman of this response/decision. The Ombudsman’s Office can be contacted by calling 0800 802 602, emailing info@ombudsman.parliament.nz, online at www.ombudsman.parliament.nz, or by post to The Ombudsman, PO Box 10152, Wellington 6143.

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Regards