Anaerobic digester feed stream conditioning for increasing biogas production (Technical report 4)

Anaerobic digester feed stream conditioning for increasing biogas production (Technical report 4)

In this fourth and yearly report we are presenting an updated literature review on ultrasonication and ozone treatments of WAS.

We are also presenting new results on the combined ultrasonication and enzymatic pre-treatment. We have shown the difference between 55°C and 65°C in terms of SCOD, proteins and carbohydrates and the kinetics of solubilization was investigated at these two temperatures. It was shown that ULS improved the kinetics of proteins and carbohydrates solubilization compared to the thermal treatment alone. At 55°C the concentration increased up to 6 hours of thermal treatment and decreased afterwards due to the consumption of nitrogen and carbohydrates by thermophilic bacteria.

The extent and rates of COD, proteins and carbohydrates solubilization was enhanced at 65°C compared to 55°C. When 100% of WAS was ultrasonicated, less than 1 hr of thermal was required to reach 8 g SCOD/L. Therefore, ULS shortened significantly the enzymatic treatment. It was also found that mixing during the enzymatic treatment resulted in a 20% increase in SCOD presumably due to a better enzymes mass transfer. It was also found that it is better to use ULS prior to the enzymatic treatment. The thermal energy during enzymatic treatment can lyze cells which release soluble materials such as colloids and proteins. Because of these colloids the propagation of ultrasound waves is hindered during the subsequent ultrasonication treatment. In other words energy is wasted on soluble materials and dissipated before it can reach intact cells.

It was found that 27% VSS removal can be obtained with ULS (50%) and thermal (65°C for 24 hrs). Ultrasonication of 100% of WAS did not result in higher TSS and VSS removals.

In the second section, we investigated the characterisation of ULS/ozone and ozone/ULS treated WAS samples. It was found that at high energy inputs (30-45 min ozone and 12,000 kJ/kg TS ULS) the sequence ozone followed by ULS is less efficient. This is due to the release of soluble materials during the ozone treatment that hinder the propagation of waves during the subsequent ULS treatment.

At lower energy levels (1min ozone/3,000 kJ/kg TS) there was no difference between the two sequences. It should be noted that no synergistic effect was observed, i.e. the SCOD of the combined treatment was not higher than the sum of both treatment applied separately.

It was found that ULS resulted in a TSS removal in the range 6-16%, while ozone alone results in TSS removals in the range 3-16%. When ULS and ozone are combined, a synergistic effect was observed and the TSS and VSS removals can be as high as 40%. In order to obtain 27-33% VSS removal at least 6 min ozonation and 9,000 kJ/kg ULS were required, and to obtain 37-43% VSS removal at least 12 min ozonation and 18,000 kJ/kg ULS were required. At such a high energy levels, there was a loss of COD of up to 24% due to CO2 production.

It was also found that the combination of treatment at 2 kWh/m3 WAS did not result in significant solubilization in terms of SCOD. Size Exclusion Chromatography (SEC) was used to shed more light on the mechanisms of ultrasonication and ozone treatment.

Finally, we updated the NaOH and ULS pre-treatment study with TSS and VSS removals. It was found that the combined treatment resulted in a TSS and VSS removals of 24% and 22%, respectively. These percentages were higher than the sum of both pre-treatment individually highlighting the synergistic effect.

Unpublished report.