101: How the sludge minimisation facility will work

Current system 

To break down the new plant, we need to paint a picture of the current system. Right now, Wellington’s waste is sent to the existing four stage wastewater treatment plant (WWTP) in Moa Point.

 After it’s been treated, the liquid side (effluent) of the waste goes out the long outfall into the ocean, while the sludge side gets pumped 9km to Carey’s Gully dewatering WWTP at the Southern Landfill location. The sludge has the consistency of a milkshake.

There, the dewatering equipment separates the liquids and solids further creating a sludge cake, which is the biosolid part of wastewater that goes to landfill. The milkshake liquid gets pumped into a centrifuge system that turns the milkshake into a cow pat- like cake that is 25 percent dry solids. That means you're going from the original product of 2 percent dry solid and 98 percent water to 25 percent solids – essentially drying it out to reduce the liquid.

This cow pat cake is then transferred from Carey’s Gully into trucks and trailers, which then gets taken up the hill and mixed in with general household waste at the landfill. This is done because the waste is a wet unstable product that is full of pathogens and to stabilise it, it needs to be mixed with household waste. 

Sludge minimisation facility method

At the new facility, the aim is to reduce as much liquid as possible and remove the need to pump sludge 9km through the city. The same thin milkshake-like sludge is taken and dewatered to produce a thicker cake.

Then that product is pumped into the Thermal Hydrolysis Plant (THP), the first of its kind in New Zealand. This acts like a pressure cooker which is filled with steam, and the sludge is held in a controlled environment of high heat and pressure for over 20 minutes which gives a more stabilised and sterile Grade A sludge.

What this does is breaks down the product, making it more digestible for the next stage in the process. The THP product gets pumped into one of the two digesters, which are two large vessels maintained at 40 degrees. While the THP was at 165 degrees, the drop in temperature breaks down the product even more. This mimics the natural decomposition process of waste, breaking down biodegradable matter in the sludge.

There are microscopic bugs within the digesters that then eat and utilise the energy before it is converted into biomethane. Digesters act in a similar way to how a human stomach digests food and breaks it down.

The methane captured is good quality bio gas, and is used through CHP engines (combined heat and power engines) which convert the gas into electric and heat. The biogas production not only is utilised through the CHP engines to create electricity, the actual biogas created is also used to fuel the steam and hot water boilers. Therefore, we are not using natural gas off the city network during standard operation. 

The sludge that comes out of the digesters seems more like a thick shake at this stage, and treated through another centrifuge which makes the sludge more like a dry compost this time, in readiness to feed the thermal dryer. 

With the sludge product going through the THP and the digesters, the minimisation facility reduces the sludge solid volumes by up to 60 percent more than standard. 

The Grade A biosolid is then put through a thermal dryer which does one last dry of the product, making it change to 90 percent dry solids and only 10 percent water, which makes a very dry coffee granule-type finish. 

This process massively reduces the volume of the mass of sludge. The new facility will reduce the mass of treated sludge produced by up to 80 percent. This process creates what’s known as a Grade A biosolids product, which is pasteurised (so safe from a germ, disease and vermin attraction perspective) and is significantly less odorous.

The biosolids can be utilised as a fertiliser, soil enhancer, and other positive land applications, as done for decades in Australia, Europe, and USA.

Key words to know

• Sludge – Sludge is a by-product that comes from wastewater treatment. 
• THP – Thermal Hydrolysis Plant - Thermal hydrolysis exposes sewage sludge to high temperature and pressure.
• CHP – Combined heat and power engines.
• Digester – Process which mimics the natural decomposition process of waste, breaking down biodegradable matter in the sludge and producing biogas.

Find out more about the sludge minimisation facility on our website.