Sludge – a hazard and a resource

A semi-solid slurry produced from water treatment, wastewater treatment and on-site sanitation systems, sludge can be both a dangerous source of pollution and disease, and a valuable by-product.

Sludge from on-site sanitation (faecal sludge) is collected from pit latrines, on-site digesters and septic tank systems. With wastewater treatment plants, there is sludge in the settling tanks (primary sludge), then there is sludge from the biological wastewater treatment (secondary sludge or activated sludge) and sometimes there is even tertiary sludge from a further wastewater treatment step like adding a flocculation agent,” explains Santiago Septien, research engineer: Water, Sanitation & Hygiene Research & Development (WASH R&D) Centre, University of KwaZulu-Natal.

Pollution All sludges must be treated or disposed of in a way that will not be harmful to the environment or a threat to public health. Bacterial microorganisms within the sludges can potentially contaminate groundwater and other water resources.

“An example of the harmful effects of sludge contamination is cholera. Sewage and faecal sludge need to be treated and managed carefully to stop the spread of this disease. Open defecation, release of untreated sewage, or dumping of faecal sludge from pit latrines or septic tanks into the environment need to be prevented. In many cholera-affected zones, there is a corresponding low degree of sludge management,” adds Septien.

Sludges have high nutrient content that can cause excessive growth of algae and hyacinth when released in an uncontrolled manner into rivers and dams. High amounts of algae and hyacinth can cause other aquatic flora as well as fauna to die by depleting oxygen reserves in the water.

“Some sludges also contain heavy metals, microplastics, pesticides and/or pharmaceuticals that can be highly toxic and may cause the contamination of soil, surface water, groundwater and crops, as well as have a negative impact on living organisms, including animals and humans. Certain chemicals that can be found in sludges do not biodegrade but accumulate, and are endocrine disruptors,” states Septien.

Valuable resource Due to the potential hazard that sludges can pose to humans and the environment, the processes used to treat, reuse and dispose of sludge are very important.

“If managed correctly, sludges are a valuable resource, and have a multitude of uses,” states Septien.

Some of the benefits include: • Nutrients found in sludges can be used as agricultural products to grow crops.

• The high organic content in sludges can allow it to be used as biofuel – creating heat or renewable energy.

• Sludges can be carbonised and manufactured into briquettes – their high net calorific value makes them an ideal alternative fuel source.

• Sludges can also be carbonised into biochar or activated carbon that can be used as a soil container, holding water and nutrients, releasing them slowly for plants to absorb. The activated carbon can also be used to absorb contaminants for wastewater treatment.

• Sludge can be used as a building material for the production of bricks and cement.

• The biochar used from sludge carbonisation can be used as electrode material for highend capacitors.

• The sludge can serve as substrate to feed insects, such as the black soldier fly that is rich in protein and is in turn used to feed chickens or fish.

Barriers Septien argues that while there are a multitude of possible uses for sludge, a number of barriers exist.

“Currently, there is a lack of regulatory framework that provides guidance on how to assess and treat sludges, particularly in the case of faecal sludge from onsite sanitation facilities. There are no regulations that provide parameters about how to deal with the different types of product derived from sludge. There is also a need for funding and investments in order to fully develop products from sludge (like bricks) that can be brought to market. Public acceptance is also a barrier; there needs to be more education to deal with the ‘yuck factor’ – often people do not trust the treatment process of sludge.”

As landfill space becomes more limited and the related fees increase, the disposal of sludge can no longer be seen by the majority of local authorities as the default option. The current increasing population growth and levels of urbanisation may result in more sludge being produced. It is thus becoming increasingly unsustainable to dispose of sludge.

“There is a need to stimulate business interest in sewage and faecal sludge. We must unlock and maximise the potential of sludge as an alternative resource for use,” concludes Septien.