Sludge is the by-product from our 650+ wastewater treatment works. It's the organic material that's collected from the bottom of the settlement tanks that are part of the wastewater treatment process.

The sludge is pumped from the settlement tanks into sludge holding tanks.  From there, it's collected by tanker and transported to one of the 29 sludge treatment centres across the region. There it is treated and turned into a bioresource.  

Sludge is full of nasty bacteria (pathogens), so once it arrives at the treatment centre, it will go through one of these two processes that kills these pathogens.

Lime stabilisation - lime is added to the sludge which raises the pH to 11 or more, which deactivates the pathogens.

Anaerobic digestion - the sludge is heated in sealed tanks to around 37 degrees celsius for a number of days to kill the pathogens. This process has the benefit of generating methane which we can use to create electricity to power our sites!

Part of both treatment processes includes a stage called ‘dewatering’.  Raw sludge is typically in a liquid form (1-3% dry solids), but by using technologies such as centrifuges, means that we can remove some of the water content and create a final treated product that is more like soil (normally 20-25% dry solids)

After treatment, sludge has become a safe bioresource. It meets the standards required to be used as a natural fertiliser in agricultural, and helps to recycle valuable nutrients back into the soil. Our bioresources are beneficial to farmland all across Devon and Cornwall.