Land Treatment Optimisation - A Collaborative Journey

The Ōtaki Wastewater Treatment Plant (WWTP) was granted a new, non-notified Resource Consent in 2016 for the discharge of treated effluent to land. The consent required KCDC to engage a specialist to undertake an optimisation study of the Land Disposal Treatment Area (LDTA), in partnership with Ngā Hapū o Ōtaki (NHoŌ), to investigate options to optimise the distribution and infiltrative capacity of the area. This condition was incorporated to collaboratively address operational concerns raised by NHoŌ prior to the consent being renewed.

The liquid treatment process at Ōtaki incorporates preliminary screening, an aerated lagoon, a clarifier, and oxidation lagoons. The secondary treated wastewater is then discharged to 10.9 hectares of land for rapid infiltration via a perforated pipe system, comprising 18 parallel pipes. In 1995, the area had been upgraded from a border dyke system, to a rapid infiltration disposal system with plantings of harakeke. Maintenance was carried out by operators but eventually was limited by the lack of access through the immense vegetative growth (harakeke, grass, weeds, blackberry).

The LDTA lies next to the Ōtaki River, which recharges the aquifer under the LDTA and flows towards the coast. The community’s water supply bore is located 550m down gradient from the closest point of the disposal area. There is also a kai gathering area nearby. Investigations determined there was a less than minor effect on the environment from the disposal activity, at the buffer zone boundary. However, these factors, along with the state of the LDTA in terms of maintainability, drove the inclusion of an optimisation study in the consent conditions.

KCDC, NHoŌ and Cardno worked collaboratively in all aspects of the study, including site testing, concept options development, and the final decisions for the future of the LDTA. Workshops were undertaken to share ideas and work together towards the best outcome for the LDTA. This proved an effective and beneficial approach, introducing alternative perspectives and problem solving strategies that resulted in the best overall outcome for the LDTA, as well as providing an opportunity to strengthen relationships between council, iwi and the consultant. Moving forward, NHoŌ will continue to be involved in monitoring of the performance of the LDTA throughout the term of the consent.

The outcomes from the study will be discussed in this paper, which include use of managed pasture, manuka boundary planting and the future installation of automated valves to control discharge volumes. The final outcome from the collaborative approach between iwi, council and the consultant saw an overall cost effective and culturally acceptable solution.