Kokiri Centre, Whāingaroa

Using wetlands to treat marae greywater.

Wastewater challenges

Marae perform a range of important roles for both Māori and the wider community, acting not only as community meeting places, but also variously as homes, offices, early childhood teaching facilities (kohanga reo), health clinics, and sometimes local civil defence centres. They may host small events such as meetings (hui) for a few people, or less frequent gatherings (wānanga), funerals (tangi) or weddings where several hundred people may be present for 2-3 days.

In rural situations, where wastewater treatment is frequently via on-site systems, the range in event size can place considerable strain on the performance of existing facilities which can be exacerbated where this infrastructure is outdated, undersized or in poor condition.

The design and implementation of new marae infrastructure must not only reduce human health risks and risks to the local environment, but also address tikanga (cultural design criteria) which commonly favours treatment of human wastes via the cleansing capacity of soil (Papatūānuku) and the avoidance of water.

Marae wastewater treatment and dispersal systems typically consist of septic tanks draining to soil infiltration fields. However, these systems may be undersized for modern wastewater loads and can struggle when exposed to shock-loads which exceed normal flows by many-fold. Marae communities often feel they have limited options available to help them adapt and cope with such situations, particularly when they arise in the middle of large events.

Solutions

 At Tainui Awhiro’s Kokiri Centre near the mouth of Whāingaroa Harbour (Raglan), the existing wastewater system (septic tank and infiltration field) was discovered to be undersized, particularly for the larger events held at the site. Tainui Awhiro did not want to connect to the nearby town wastewater treatment plant because, against their wishes, it discharges effluent directly into the harbour adjacent to their ancestral lands. Instead, they chose to reduce the hydraulic loading to their existing wastewater facilities with low/dual flush toilets and by diverting greywater (showers and hand basins only) to a newly constructed separate land-based waste treatment system (Figure 1).

Low flow shower heads were also retrofitted as part of their commitment to reduce water usage and to reduce the load to the greywater system.

The new secondary greywater waste treatment system, comprises a sub-surface flow gravel-bed wetland followed by an infiltration swale.

In the wetland, plants, microbes and invertebrates (worms and insects) help break-down the waste to purify the water slowly percolating through the gravel media.

The wetland was sized to accommodate the anticipated greywater volumes for events commonly held at this facility which typically extend over 2-3 days.

Incoming wastewater gradually displaces treated wastewater from previous events which may have been present anywhere between several days or weeks previously. Thus hydraulic residence times and associated treatment levels were anticipated to be high.

The marae and local community were involved in and contributed significantly to all stages of the system construction from excavation to planting.

The wetland was planted with a mixture of native vegetation (Carex secta, C. virgata and Cyperus ustulatus). Plant species were selected by local kaumātua (James “Tex” Rickard), who chose species that had previously been common in the Whāingaroa area but are now much less so due to urban and rural development. The native harakeke (Phormium tenax) which has many cultural uses was also planted along the infiltration swale.

Benefits

The reduced loading on the existing septic tank system (now treating mainly blackwater) means that it is now able to cope with wastewater flows and loads even during large events.

Use of a sub-surface flow design throughout reduces the likelihood of direct human contact with wastewaters.

The new greywater wetland is being monitored for removal efficacy of key contaminants including:

  • BOD (Biochemical Oxygen Demand, a measure of decomposable organic matter).
  • SS (Suspended Solids, a measure of particles in the water).
  • Nitrogen and phosphorus (nutrients that can cause excessive growth of algae).
  • Faecal microbes (indicators of potential pathogen risk).

Preliminary performance monitoring data from one summer and one winter event showed that on its own the wetland significantly improves water quality, notably faecal microbes (almost 99.9% removal), nitrogen (98% removal), phosphorus (>90% removal) and SS (90% removal) (Table 1 below).

The infiltration swale has been able to readily accommodate the discharge from the wetland, with minimal measureable effect on groundwater concentrations.

On-going performance monitoring of the constructed wetland and infiltration swale will continue to improve our understanding of the ability of such technologies to help fulfil the wastewater management needs of marae communities.

Constructed wetlands appeal to marae communities due to their utilisation of natural processes, low maintenance requirements, capacity to cope with fluctuating loads and ability to be built and maintained by communities themselves.

The collaborative process facilitated the incorporation of design features to improve the cultural acceptability of the technology.

This project continues to provide an important opportunity to physically demonstrate these technologies “in action” and it is hoped that this will promote greater engagement and support of such approaches in other marae communities around New Zealand.

Table 1: Constructed wetland performance monitoring results measured over one summer and one winter event.

Building of the Kokiri Centre Greywater treatment wetland, community involvement, and the ethos behind treatment of wastes in a land-based system were the subject of Episode 11 of the first Project Mātauranga Series on Māori TV. The episode can be viewed online – follow link below.

Maori TV Mātauranga - Episode 11

Related articles have been published in the Land Treatment Collective Newsletter Issue No. 43, Nov. 2013 and more recently in the “Wet & Wild” – The National Wetland Trust Newsletter

Land Treatment Collective Newsletter Issue No. 43 article

The National Wetland Trust Newsletter article

Conceptual schematic of a greywater treatment wetland.
Established constructed wetland plantings.
Raglan Harbour.
Discharge from the outlet of the greywater treatment wetland before it goes through the harakeke infiltration trench.