In an effort to advance India’s wastewater and sludge treatment in alignment with impressive national sustainability goals, a multi-location study called SARASWATI 2.0 tested various wastewater technologies across the country. One of these studies compared thermophilic anaerobic digestion with thermal hydrolysis (THP) combined with mesophilic anaerobic digestion to understand the potential of these sludge treatment methods.
Project SARASWATI 2.0 is a comprehensive study aimed at discovering which technologies for wastewater treatment and resource recovery would be best suited to the country. The study began in 2019 and ran for four years, sponsored by the European Union through the Horizon 2020 funding program and the Indian government. It stands on the legs of the original Saraswati study which took place from 2012-2017.
The project has 10 pilot study locations spread out over seven Indian states. These pilots fall into one of these technology buckets:
One of the pilot studies, headed by the Indian Institute of Technology in Roorkee, India, aimed to compare three different digestion strategies, one of which involved Cambi’s thermal hydrolysis process.
There are several treatment methods for sludge, but anaerobic digestion has proven to be a cost-effective and sustainable approach for many medium to large wastewater treatment facilities. The study chose to compare three digestion strategies:
The three methods were applied to two different types of sludge: one obtained from a conventional activated sludge process (CAS) and one from a sequence or sequencing batch reactor process (SBR), both from wastewater treatment plants in Haridwar, India. Cambi provided a mini THP set-up for the analysis.
The study's added focus on SBR sludge is significant for India and many other countries that use the technology.
The study methodology included:
This pilot study on thermal hydrolysis bore several papers which showed the benefits and challenges of using THP over the two standalone digestion methods. Key findings from these papers are listed below. They focus on methane generation, net energy balance, pathogen removal and biosolids quality, as well as the presence of heavy metals and organic micropollutants (OMPs).
Of all the types of sludge treatments in the study, mesophilic and thermophilic digestion on SBR sludge yielded the least methane, while THP with mesophilic digestion on CAS and SBR sludge yielded the most. MAD with thermal hydrolysis yielded 1.9 to 2.3 times more methane than TAD, as seen in the table below. This can mean promising energy outputs for wastewater treatment plants in India with similar sludge.
In the same study, the researchers computed the net energy balance for the six scenarios. According to their methodology and as seen above, the THP+MAD combination provides the highest net energy balance of all the methods, both for sludge retrieved from sequence batch reactor and conventional activated sludge facilities. Thermophilic anaerobic digestion, on the other hand, resulted in the lowest energy net balance, specifically for SBR sludge.
The study tested for the presence of two types of bacteria: Salmonella and faecal coliform, which is an indicator of the presence of other disease-causing organisms. The low temperature of mesophilic digestion, though capable of lessening the concentrations of these organisms in sludge, was not able to sufficiently reduce their concentration down to Class A biosolids levels.
Thermophilic digestion and THP both achieved high-quality biosolids by US EPA standards, but only thermal hydrolysis resulted in non-detectable levels of the two.
Though they did not include comparisons with thermophilic digestion, some other findings in the pilot study shed light on the reaction of certain important contents of sludge when treated with thermal hydrolysis.
The presence of organic micropollutants and emerging contaminants such as PFAS in biosolids is a topic of current importance in the public health sphere. Selected organic micropollutants (hormones, antibiotics, and other pharmaceutical drugs) were tested in SBR sludge, particularly raw sludge, digested sludge (MAD), and THP-treated sludge of four varying temperatures (ranging from 120ºC to 180 ºC) combined with MAD.
The findings show that THP resulted in significantly lower concentrations for most of the tested pollutants:
Heavy metal concentration was also tested in the same study, though it was not done for MAD/TAD/THP+MAD comparison but rather for raw and THP-treated sludge. Samples of raw and hydrolysed sludge from both the SBR and CAS facilities were analysed for the following heavy metals:
Heavy metals tested:
The findings are consistent with other studies done on THP-treated sludge. THP, similar to other thermal pretreatment methods, were found to release heavy metals bound that were trapped in sludge flocs. The concentration of most of the heavy metals listed increased in both sludge types, while few had insignificant changes or decreased.
Though the thermal hydrolysis process resulted in this release of heavy metals, the concentrations of these were below US EPA limits.
The combination of thermal hydrolysis and mesophilic digestion has historically proven to be a beneficial energy investment for wastewater treatment plants, as evidenced by the increasing number of facilities investing in this form of advanced digestion.
This latest study by the Project Saraswati 2.0 team in India adds to this body of research and also highlights the effectivity of thermal hydrolysis on sludge from the Sequencing Batch Reactor process, which is used by many smaller wastewater treatment facilities.
Wish to see more scientific research on thermal hydrolysis? Head over to our Literature section.
All graphs and tables are sourced from studies published by Proj. Saraswati 2.0 researchers.