Thermophilic digestion vs THP+MAD: Project SARASWATI 2.0 results

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. 

A comprehensive study to modernise India’s wastewater treatment

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:

• Decentralised wastewater treatment technologies
• Black water treatment technologies
• Sludge treatment technologies
• Post-treatment technologies

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.

A selection of sludge treatment technologies for India

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:

• Mesophilic anaerobic digestion or MAD, which occurs at the temperature range of 30-40°C. MAD is the most commonly form of digestion due to its operational stability and affordability.

• Thermophilic anaerobic digestion or TAD, which typically operates between 50 to 60°C. TAD has been proven to generate more methane and remove more pathogens in sludge compared to mesophilic digestion.

• Mesophilic anaerobic digestion with thermal hydrolysis as pretreatment or THP+MAD, a form of advanced anaerobic digestion. THP can be used with MAD in a handful of configurations but this is the most common scheme, where THP precedes digestion. THP breaks down sludge using high temperature and pressure, which increases the biodegradability of sludge for digestion.

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:

• Operation of Digesters: 35-liter digesters operated at 15 days hydraulic retention time (HRT)
• Operation of THP: fed at 14-16% dry solids (DS) at 160ºC, 6 bar, for 30 minutes. The team found this temperature-time combination to be the most effective by testing various possibilities. This is also in line with how full Cambi’s THP plants are operated (140-165ºC at 6 bar for 20-30 minutes HRT)

Thermophilic digestion vs thermal hydrolysis and mesophilic digestion: The results

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).

Result 1: Sludge treated with thermal hydrolysis yielded the most biogas

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.

Result 2: Both thermal hydrolysis + MAD and thermophilic digestion produced Class A biosolids

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.     

Organic micropollutants and heavy metals: The effect of thermal hydrolysis

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.

Result 3: THP reduced the presence of most organic micropollutants

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:

• HORMONES – THP was most effective at removing the female hormone 17β-estradiol from sludge. Removal of the female hormone estrone (a type of estrogen) was above 50% at the highest temperature. The removal of progesterone and 17α -estradiol at the highest temperature in the study was below 50%.

• ANTIBIOTICS – Mesophilic digestion alone was effective at antibiotics removal for two of the tested drugs, and the effectivity of removal only increased with thermal hydrolysis. Moreover, THP had an important effect on the reduction of Ciprofloxacin.

• OTHER PHARMACEUTICAL DRUGS - Thermal hydrolysis resulted in the significant removal of Dicoflenac (an anti-inflammatory drug) and Bezafibrate ( a lipid-lowering drug).

Result 4: Thermal hydrolysis resulted in heavy metals release

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:

• Arsenic (As)
• Cadmium (Cd)
• Chromium (Cr)
• Copper (Cu)
• Lead (Pb)
• Mercury (Hg)
• Molybdenum (Mo)
• Nickel (Ni)
• Selenium (Se)
• Zinc (Zn)

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