All systems go at Banks Street wetlands trial

The liquid in the first pond comes straight from the Biological Trickling Filter treatment plant. Paddles keep the water circulating and there is no smell. All pictures supplied.

TRIALS to see whether treated human wastewater can be spread on to native plants in a wetland, rather than into the waters of the bay, are progressing well.

Sited at Banks Street alongside the city’s Wastewater Treatment Plant, the trials are a joint project between Gisborne District Council, NIWA, the Institute for Environmental Science and Research (ESR) and the Centre for Integrated Biowaste Research.

The aim is to assess whether a wetlands complex in Gisborne’s climate would be a viable alternative to ultra-violet disinfection.

The council’s 2007 wastewater consent said UV disinfection had to be added to the city’s Biological Trickling Filter (BTF) treatment plant by the end of 2014.

An amendment to this consent clause enabled a wetlands option to be investigated. The two trials focus on further treating the liquid and fine solids or sludge from the city’s BTF-treated wastewater.

The BTF effluent (or liquid) treatment trialThe trial uses two algal ponds and two artificial wetlands to further treat liquid (or effluent) that has been through the Banks Street treatment plant.

In science talk, the ponds are called High-Rate Algal Ponds; the wetlands are Surface Constructed Wetlands.

How it works:

A pipe takes wastewater treated in the Biological Trickling Filter plant into two tanks to settle.

Fine solids or sludge settle quickly and the continuous flow of treated wastewater into this tank pushes the liquid out the top where it trickles by gravity into an algal pond.

A paddle keeps water circulating and prevents it from settling.

Special algae in the pond help to break down organic matter and reduce nutrients and harmful bacteria. Water is held in the pond for about a week.

Because the ponds are open to the elements, sunlight helps to disinfect the water. From the ponds, water flows into algal harvesters where algae settles.

The resulting liquid then goes into the first constructed wetland where native club rush and jointed twig rush grow. This wetland further removes organic matter and nutrients.

From here, it flows into a woodchip filter to remove remaining nitrates and then to a second constructed wetland for further “polishing” before being discharged.

Water samples are regularly taken from each step to see what is being achieved. Each step has a specific job in removing different contaminants.

The sludge (fine solids) drying wetlands trialTHIS trial aims to see how well a sludge treatment reed bed system works. Sludge is applied to a growing stand of plants. Reed beds have been used in Denmark for the past 30 years but in Gisborne, native species raupo and club rush are being trialled.

The plants absorb moisture and nutrients from the sludge for their growth.

Water drains out of the sludge through the gravel or bark they are growing in and evaporates from the surface.

In time, the system reduces the volume and increases the quality of sludge, reduces pathogens and hazardous organic compounds.

The trial includes assessing which species grow best in Gisborne sludge, whether these grow better in bark or sandy gravel and how much and how often sludge can go on to the plants at one time.

The results will help determine the size of any sludge treatment reed bed system that may be built.

How it works:

Six 1000-litre tanks are filled at the same time with BTF-treated wastewater.

After a few days, the liquid part is drained off and the sludge that settles is pumped into one big tank and mixed well.

A small part of this sludge is used to dose the plants in the barrels. The plants are dosed at different rates and the time between dosing varies.

The idea is for plants to keep growing as the sludge builds up around them.

Five rows of barrels stand on raised platforms. Each has six barrels comprising club rush in bark, and in sand; raupo in bark, and in sand; then two control barrels with no plants.

Tests on the liquid collected at each barrel’s base help show how effectively water, pathogens, heavy metals and organic contaminants are taken out of the sludge.

After about 10 years, built-up sludge will resemble well-matured compost.