Biosolids are a soil-like material produced as part of the wastewater treatment process. They come from treated wastewater (sewage) and the naturally occurring microorganisms (or ‘good bugs’) that help clean our wastewater. These microorganisms play an important role by breaking down organic matter and nutrients like carbon, nitrogen, and phosphorus.

During treatment, this material goes through a digestion process (like composting). This produces methane gas, which is captured and used to generate energy, which we use to run the wastewater treatment process at Māngere.  The remaining stabilised material is biosolids. Due to the digestion process undertaken, biosolids meet safety and environmental standards before reuse or disposal.

At our largest wastewater treatment plant in Māngere, where three quarters of Tāmaki Makaurau’s wastewater is treated, we are using biosolids to rehabilitate an area on Te Motu a Hiaroa (Puketutu Island) that was quarried from the 1950s until the early 2000s.

It's working well but it won’t last forever and by 2035 the old quarry will be full and the rehabilitation project of the island's former maunga profiles will commence. Auckland will need a new proven and reliable solution in place by 2035 that is capable of handling more than 175,000 tonnes of Auckland's biosolids each year.

Over the next year, we’ll be assessing our options for managing biosolids and we want you to help shape our plan. Each approach has benefits and trade-offs. Community feedback helps us understand what matters most to Aucklanders and informs the ongoing evaluation of future biosolids solutions. Email us at [email protected]

Māngere wastewater treatment plant produces up to 150,000 tonnes annually, which is the equivalent of ~20 truck and trailer loads or 400 tonnes per day. Other Watercare wastewater treatment plants in Auckland produce a further 25,000 tonnes. Auckland produces around 175.000 tonnes of biosolids per year.

Biosolids contain the remains of treated wastewater, treated human waste and the expired microorganisms that complete the digestion process. Because biosolids come from wastewater, they can contain low levels of contaminants that enter the wastewater network from homes, businesses and industry. They can contain small amounts of pathogens, very low concentrations of contaminants - heavy metals, microplastics and forever chemicals like PFAS.

PFAS stands for Per- and Polyfluoroalkyl Substances, a group of chemicals used in many everyday products including non-stick cookware, food packaging, cosmetics, personal and household cleaning products, stain-resistant fabrics and firefighting foams.

PFAS are sometimes referred to as "forever chemicals" because they can persist in the environment for a long time. PFAS are found worldwide and are being studied internationally.

For many years, biosolids from Auckland's Māngere Wastewater Treatment Plant have been used to rehabilitate Te Motu a Hiaroa / Puketutu Island, a former quarry site.

This quarry pit is expected to be full by 2035 with the restoration of the old volcanic cones to then be completed. Auckland will need a new long-term solution in place before 2035 to continue managing the biosolids produced by the city's largest wastewater treatment plant.

Today, biosolids from the Māngere Wastewater Treatment Plant are transported to Te Motu a Hiaroa / Puketutu Island where they are used to rehabilitate land previously affected by quarrying activities.

Biosolids are placed into specially constructed areas and covered with soil. Eventually this area will be full and the previous island contours will be formed using soil. Watch the video on the website What are biosolids?

After investigating a wide range of technologies and approaches, Watercare has identified two proven technology options capable of managing the large volume of biosolids produced by Auckland.

• A purpose-built biosolids landfill

• A purpose-built biosolids incineration facility

We are also investigating a range of supplementary reuse opportunities that could work alongside either of these two primary options.

Auckland produces approximately 150,000 tonnes of biosolids each year from the Māngere Wastewater Treatment Plant.

To meet Auckland's future needs, any solution must be proven, reliable, capable of operating at a large scale and able to be delivered by 2035.

Landfill and incineration are currently the only options that meet these requirements and are widely used internationally to manage large volumes of biosolids.

Watercare already reuses all the biosolids from the Army Bay wastewater treatment plant by sending these to My Noke for vermicomposting at the world’s largest worm farm. We are also experimenting with making a potting mix and using pasteurised biosolids for sports turf application. More reuse opportunities are an important part of our investigations and may play a role in Auckland's future biosolids management.

However, Auckland produces a very large volume of biosolids each year and most reuse options currently operate at a much smaller scale. While reuse may help reduce the volume requiring disposal, a primary solution is still needed to safely and reliably manage all biosolids produced from the Māngere wastewater treatment plant.

Biosolids are transported using specialised trucks designed for the safe handling and movement of materials. Transporting biosolids is a routine activity and is subject to strict operational, health and safety requirements.

No. Watercare is currently evaluating possible options and gathering information to help identify the best long-term solution for Auckland.

No decision has been made between landfill and incineration. Technical assessments, specialist advice, engagement with mana whenua, and community feedback will all help inform future decisions.

Mana whenua are important partners in this programme.

Watercare is engaging with mana whenua throughout the project to better understand cultural values, environmental stewardship and the perspectives they bring to decisions that affect future generations.

Mana whenua input helps inform how options are assessed and refined as the programme progresses.

We are committed to keeping Aucklanders informed and involved throughout the programme. You can sign up for project updates (see our website), attend future engagement activities, review project information and participate in consultation opportunities as they arise.

Community feedback helps us understand what matters most to Aucklanders and informs the ongoing evaluation of future biosolids solutions. Email us at [email protected]