Sludge as a resource: A look at thermal hydrolysis in Asia

The experience of several modern plants in Asia can spell a change in the horizon for sewage sludge management in the continent. Find out how thermal hydrolysis plants in South Korea, Singapore, China and Australia are helping to change the sentiment towards sewage sludge from bane to boon.

The THP unit at the Xiahongmen Water Reclamation Plant is one of several in China and one of the largest in the world (Photo credit: Cambi).

Sewage sludge in Asia has historically been regarded the way the rest of the world has been the material – a nuisance to be managed and disposed of. Approaches to its management are as diverse as the legislative, environmental, and technological contexts that characterise the continent. On one end of the spectrum, there are developing countries where certain cities still discharge untreated wastewater into rivers and streams. Sludge – the mostly organic solids present in wastewater – is therefore not separated from water that can be reused, and the practice signals hazards for public health and the natural environment. On the other end, the continent has some exemplary plants utilising technologies and strategies that are making sewage sludge treatment an endeavour not just for sanitation but also for renewable energy production and better agriculture.

This might not be new. Many medium and large wastewater treatment plants have been producing biogas and biosolids products for some time. The gas produced by these facilities, however, is often too little, while the biosolids or treated sludge is often a smelly, vector-attracting material that often incites public complaints and lowers the value of nearby neighbourhoods. Such outputs are typical results of using the conventional anaerobic digestion process, which offers limited performance and possibilities for biosolids handling.

To recover resources from sludge more cost-effectively and sustainably, utilities cannot depend on traditional anaerobic digestion alone. This is where Advanced Anaerobic Digestion (AAD) takes precedence in the discussion. AAD refers to either the use of a complementary technology with conventional digestion or a modification of the digestion process to increase biogas output and destroy pathogens in sludge. The processes often make a biosolid product that is higher-quality and with more applications.

There are several technologies and processes that can be used in tandem with digestion to achieve such results, including, but not limited to, pasteurisation, ultrasonic pre-treatment, chemical pre-treatment, enzymatic pre-treatment, and several hydrolysis processes. The pre-treatment technology considered to be most mature at present, however is the Thermal Hydrolysis Process (THP) using steam.

The full article is published in the Water & Wastewater Asia Nov/Dec 2021 issue. To continue reading, click here.