Full-scale vacuum degassing of activated sludge – A case study over 2-years of operation

https://doi.org/10.1016/j.jwpe.2021.101992Get rights and content

Highlights

  • sludge vacuum degassing improves sludge settling.

  • very good sludge settling allows increasing the amount of biomass in the bioreactor.

  • the use of sludge degassing results in better wastewater treatment efficiency.

  • sludge production from gram of COD is 20 % lower when using vacuum degassing.

Abstract

The effect of vacuum degassing of activated sludge on biological wastewater treatment over a total period of 4 years was examined. In half of the period studied, the treatment system was classic, where wastewater, after a biological reactor with nitrification-denitrification and biological dephosphatation, was directed to secondary settlers. In the second period, the treatment system was modified so that the sludge from the biological reactor was directed to the vacuum degasser, followed by the secondary settling tanks (vacuum degassing activated sludge system; VDAS). As a result of using the VDAS, a significant improvement in sludge settling ability was recorded, which allowed the plant operators to increase the amount of biomass in the biological reactor. Differences in the quantity and quality of the wastewater influent were found during both periods. However, the quality of treated wastewater and the efficiencies of nutrient removal and removal rates were usually better in the system with vacuum degassing. An improvement was achieved not only for the elimination of the different forms of nitrogen, which by definition were to be removed as ammonia and gaseous nitrogen, but also for organic compounds expressed as COD and phosphorus. As a result of the lower sludge loading, sludge production expressed in g SS per g COD removed was significantly lower (20 %) with the VDAS.

Introduction

The settling tank is a critical element in a traditional wastewater treatment plant. The amounts of suspended solids in the effluent and the range of control of the biological phase of wastewater treatment depend on the proper operation of the plant. The range of control is based on the need to maintain the highest amount of biomass in the bioreactor and to separate the sludge from the treated wastewater. The main goals of its operation are therefore the separation of activated sludge from treated wastewater and, the recycling of biomass to the biological reactor. The effectiveness of activated sludge sedimentation depends on several parameters, such as solid retention time, concentration of suspended solids (biomass) [1], degree of turbulence [2], dissolved oxygen concentration [3], and surface properties of activated sludge flocs [4,5].

The presence of filamentous or zoogleal bacteria, which can cause sludge bulking and/or foaming [6] or toxic substances, causing the formation of so-called “pin-floc“, is incidental and periodic [7]. The saturation of wastewater and sludge with various gases, as a result of biological processes or because of turbulence in the aeration chamber, is a constant and integral element of the treatment process. The concentration of dissolved nitrogen, which reaches the secondary settling tank, is close to the level of full saturation [8]. Therefore, the gas accumulating as a result of denitrification in the secondary settling tank is not able to dissolve in the water phase, and gas bubbles are formed. Larger bubbles can cause the sludge to float to the surface of the secondary settling tank, whereas smaller gas bubbles result in a higher sludge volume and an impeded sludge thickening [9]. Therefore, sludge degassing prior to the introduction into the secondary settling tank is a viable approach.

In the 1990a, the ​​continuous vacuum degassing technology has been created [10]. The degassing of a mixture of treated wastewater and activated sludge is as a result of pressure lowering while transporting the mixture from a biological chamber to a secondary settling tank. According to Henry's law, the reduced pressure results in a reduced solubility of the gases contained in the wastewater and the activated sludge floc' matrix, allowing the gases to be discharged into the atmosphere. The Biogradex® technology has been patented in Poland and other countries, both in Europe and elsewhere [11]. Currently, there are several dozen treatment plants that use this technology (e.g., in Poland, Ireland, Estonia, Canada, and China) [[12], [13], [14]]. The operation of this technology is shown in Fig. 1. Due to the vacuum, the activated sludge from the bioreactor is sucked into one of the arms of the U-shaped system. The pressure drop causes the removal of gases from the sludge which are discharged in the upper part of the system. The degassed sludge falls from the second arm and is directed to the secondary settling tank. As a result of degassing, the saturation of the mixture of sludge and wastewater with nitrogen gas drops by approximately 25–50 %.

Despite the numerous applications of this technology, there is little scientific information about the effectiveness of its long-term use in full scale. Table 1 presents several studies (mainly conference reports) describing the effects obtained in three wastewater treatment plants. In two cases, analysis time was only a few weeks after system installation. Besides, the presented results are mainly based on the quality of the treated wastewater, which is the most crucial information from a technologist’s point of view. As a result of the degassing of activated sludge due to the improvement of the sedimentation index, the concentration of suspended solids in the biological chamber could be increased up to 8600 mg/L [15,16]. A significant improvement of the effluent quality was observed in terms of the concentrations of both total and ammonium nitrogen.

However, there is no information on how the changed conditions and intermittent stress influence sludge properties other than SVI, such as sludge activity and removal efficiencies. Vacuum degassing is a continuous process; however, from the point of view of the activated sludge and the bacteria inhabiting it, this process is intermittent. Activated sludge flocs together with resident bacteria circulate in the system and, from time to time, reach the degassing system. Therefore, they are subjected to lower pressure at time intervals equal to the time of sludge retention in the reactor and the settling tank. In this sense, from time to time, the floc-surrounding environment changes considerably as there is not only a change in pressure but also enhanced turbulence caused by sucking the sludge up and the movement of gas bubbles. Literature reports indicate that when using a vacuum, activated sludge flocs and bacteria are subjected to stress, potentially causing the breakdown and morphology changes of activated sludge flocs [14], the breakdown of bacterial cells [17,18], and changes in bacterial activity [[19], [20], [21]] or biocenosis structure [21,22]. Most key changes in the environment and in biological systems occur over prolonged periods; thus, a long period of wastewater treatment has been taken into account [23,24]. In this context, we estimated whether the use of vacuum degassing has an impact on the effectiveness of wastewater treatment based on 4-year data collected as part of the ongoing control of wastewater treatment plants.

Section snippets

Plant configuration and wastewater composition

The WWTP of interest treats municipal wastewater from around 150,000 population equivalents via biological nutrient removal (BNR) (the identification of the treatment plant is concealed at the request of the management of the treatment plant). The system of biological wastewater treatment consists of the following zones: anaerobic (dephosphatation), anoxic (denitrification), anoxic-aerobic (denitrification/nitrification) and aerobic (nitrification). Around half of the plant capacity is used for

Wastewater characteristics

The control of the biological stage of the wastewater treatment plant was based on, inter alia, COD and BOD analyses and the concentrations of nitrogen and phosphorus forms in the influent and effluent. Fig. 3 shows the monthly average results obtained. The amount of organic compounds, including those that are readily biodegradable, in the influent was 314 ± 78 mg O2/L for the period A, with higher levels in the period after the introduction of the VDAS, i.e. 365 ± 72 mg O2/L (Fig. 3A and B).

Conclusions

The main conclusions that can be drawn from this study are that the sludge degassing improves the sludge volume index by about 30 %. As a consequence, a higher biomass concentration may be sustained in the bioreactor. The treatment system can thus run with higher sludge retention times. Higher average percentage removals and removal rates and lower sludge production per organic compound removed were obtained in a vacuum degassing-based system than without this technology. The obtained

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

The research was financed by the National Science Centre (Poland) [2013/11/D/NZ9/02608]. Publication was supported under the Rector's grant in the area of research and development works of the Silesian University of Technology [08/080/RGJ21/008].

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