Biosolids are the organic wastewater solids treated to a standard suitable for reuse in the environment.
Each year approximately 3500 dry tonnes of biosolids are produced at the Christchurch Wastewater Treatment Plant.
Sludge from all of the Council's wastewater treatment plants, including those on Banks Peninsula, is taken to the Christchurch Wastewater Treatment Plant for processing into biosolids.
The biosolids produced at the Christchurch wastewater treatment plant go through a number of processes including digestion and drying. This means that the levels of organic material have been reduced and stabilised. The resulting material contains a moisture content of less than 5% and is free from pathogens. The product is able to meet the class Ab classification for biosolids, which means that it is suitable for reuse on land.
With a high organic content, biosolids improves soil moisture retention, plant growth, nutrient profile, soil stability and drainage. When applied to the soil in a controlled manner, biosolids are a beneficial soil conditioner and fertiliser.
The fully dried product has no smell and is produced as a small granule, with the resulting product looking like loose, dried soil.
Christchurch City Council has a relationship with Solid Energy NZ, in which the biosolids produced at the Christchurch plant are beneficially reused to rehabilitate mined areas at the Stockton mine site.
Trials have been conducted with the dried material since late 2010 and the Stockton mine site has been receiving regular deliveries of the material since 2012, with all of biosolids produced at the Christchurch plant now being taken to the Stockton site.
The below pictures taken from operational trials in 2012 demonstrate the effectiveness of the application in improving the growth of vegetation on the site and preventing erosion and acid mine drainage.
In Europe and the United Kingdom biosolids are applied to land, sometimes as composted product. However, in some countries there is a trend away from land application and away from disposal in landfills towards incineration and energy recovery.
In the USA there is a wide variety of end uses for biosolids, including application as a soil conditioner and fertiliser, landfill dumping and composting. Drying is also used to produce a more readily usable product.
In New Zealand most major centres dispose of their biosolids to landfill, although compost and drying is also used.