Treatment of activated sludge to improve the biological wastewater treatment process Ultrasound versus plasma treatments

Worldwide an increasing amount of wastewater in generated due to an increasing population and continuous industrial growth. Biological wastewater treatment is by far the most used technique to treat these large wastewater quantities. Before discharge into the environment, the wastewater is treated by activated sludge. Due to the large microbial diversity of the activated sludge, various substrates can be treated. The increasing wastewater quantities and stricter discharge regulations, emphasize the need for the continuous improvement of the biological wastewater treatment process. Two main focusses can be identified, i.e., (i) optimizing the process parameters  such as pH and dissolved oxygen level and (ii) improving the microbial activity of the activated sludge. The main objective of this research lies within the second approach to improve the activity of the activated sludge.

In aerobic conditions, heterotrophic bacteria that are present in the activated sludge will degrade carbon containing organic substances, via intermediate components, to the stable end products CO2 and H2O. Hereby dissolved oxygen is used as electron acceptor. The rate at which the dissolved oxygen concentration decreases, is a measure for the rate at which the activated sludge removes the organic substances present in the wastewater and is, therefore, a measure for the microbial activity. Via respiration measurements the oxygen consumption and hence the (specific) oxygen uptake rate, can be determined.

When focusing on the improvement of the microbial activity of the activated sludge, it is important that an applied technique does not negatively influence other activated sludge characteristics. Along with the activity, these characteristics determine the overall efficiency of the biological wastewater treatment. A deteriorated flocculation and settleability of the activated sludge, for example,  cause a deficient separation between the activated sludge and the purified water before discharge to the environment.

This work assessed the impact of three stress-inducing techniques on activated sludge with as main objective an improved and faster wastewater treatment without negatively affecting important activated sludge characteristics. As a first technique, activated sludge is treated with ultrasonic waves. In the literature, it is already observed that, when using appropriate energy levels, ultrasound can improve the microbial activity of activated sludge. Therefore, ultrasound was chosen as a benchmark technique to compare with other, innovative treatments. However, a large disadvantage of an ultrasound treatment is the frequently observed negative effect on the activated sludge flocculation and settling. Apart from ultrasound, two plasma techniques, i.e., Glow Discharge Plasma (GDP) and Cold Atmospheric Plasma (CAP) were examined as an alternative for their effect on the activated sludge system.

This research first studied the effect of plasma generation in the presence of activated sludge. It was observed that with a conductivity up to 2.25 mS/cm, GDP could be generated at lower energies when compared to plasma generation in water with identical conductivity. As an explanation, it was hypothesized that the presence of charged groups in the activated sludge facilitated the electric current passing through. Also, the presence of particulate matter can act as a site for charge accumulation, enhancing the localized electric field.

It was observed that mainly the mechanical stress, induced by the ultrasound treatments had a large impact on the release of intracellular material and negatively affected the activated sludge characteristics. Unlike a GDP treatment, which induced a much lower release of intracellular material. An improvement of the activated sludge settleability was observed. No release of intracellular material could be observed after exposure of the sludge to CAP. Additionally, CAP induced only a small decrease in activated sludge settling. When focusing on the activated sludge characteristics, it could be concluded that both plasma techniques are an interesting alternative for ultrasound treatment.

Via respirometry, the effect of the three techniques on the microbial activity was determined. On the one hand, an effect on the carbon removal rate was observed. On the other hand, the influence on the nitrification activity was studied. Results using an ultrasound treatment confirmed the results already observed in the literature, i.e., carbon removal rate could be improved by exposure to ultrasound by using low specific energy inputs. Also GDP and CAP improved the activity at specific energy levels. However, ultrasound tended to be much more energy efficient, but CAP was the only technique that induced a positive effect that lasted more than 24 hours at low energy inputs. In contrast to carbon removal, nitrification activity was never positively influenced by ultrasound or GDP, whereas CAP did improve the nitrification activity.

Finally, an energetic evaluation was made for the treatment of activated sludge with the three proposed techniques on a full scale. Herewith a first indication could be made of the economic feasibility of these techniques.