1.1 What is the difference between sludge and biosolids?

To begin with, it is important to distinguish between sludge and biosolids.

Sludge is a mixture of solids and liquids, mostly consisting of excreta and water, along with sand, grit, metals, trash, and various chemical compounds. There are two types of sludge: faecal sludge and wastewater sludge. Faecal sludge comes from onsite sanitation technologies that do not use sewers. It can be raw or partially digested, slurry or semisolid, and results from the collection and storage/treatment of excreta or blackwater, with or without greywater. Wastewater sludge comes from sewer-based wastewater collection and (semi-) centralised treatment processes. The sludge composition determines the type of treatment and end-use possibilities.

Biosolids are, as the name indicates, semi-solid organic materials that are partially or fully stabilised and sanitised because of wastewater or faecal sludge treatment at a treatment facility. The formal definition, outlined in ISO 19698:2020 (Sludge recovery, recycling, treatment, and disposal — Beneficial use of biosolids — Land application), states that sludge has to have been treated to meet specific standards, guidelines or requirements including pathogen reduction, vector attraction and contaminant criteria.

Not all sludge can be classified as biosolids. Some contain harmful substances like persistent toxic chemicals, radionuclides, and heavy metals, which could contaminate soil and water if applied to land.

In this training, we use the term “biosolid” informally to refer to any (semi-) solid material that is produced during the treatment process and removed during primary, secondary, or advanced treatment stages.

As an example to showcase the difference between sludge that is freshly delivered to a treatment plant and biosolids that were collected at the end of the treatment process, the values in the following table can be reviewed.

The main difference between input sludge and biosolids is the total and volatile solids concentrations, indicating the water content and degree of stabilisation. The biosolids have a lower moisture content, indicated by an increased total solids (TS) content, and stabilisation, shown by a decreased content of volatile solids (VS).

For further information, please click on the Materials tab at the top of the page.