Wastewater produces a sludge during the precipitation and/or water treatment processes at a treatment plant. This sludge must be conditioned before dewatering, sanitised and stabilised. That is, the sludge must be treated in such a way that it easily releases the water during dewatering, does not present a risk of infection and stabilises to prevent odours.
Lime can be used for two types of treatment. One method is called the Orsa method, where quick lime is added directly. The other method is conditioning where slaked lime or quick lime is added to the raw sludge before dewatering to improve the dewatering properties.
Lime-treated sludge offers many benefits:
- It has high settleability and sought after in agriculture. It supplies the soil with lime, nutrients and structure.
- Lime makes phosphorus more available in iron and aluminium precipitated sludge. This makes the nutrients in the sludge more immediately available, which is important for plants in the establishment phase.
- There is a new focus on phosphorus as a limited resource and the availability of phosphorus in sludge from wastewater.
- The Orsa method entails low operating and investment costs.
- Lime-treated sludge is easy to handle and spread due to its high content of dry solids and its granular consistency.