Continuous innovation using aerobic granular sludge for conventional activated sludge systems

Theme
Wastewater Treatment

Summary
Since the discovery of aerobic granular sludge at TU Delft in the late 1990s, worldwide research into applications for wastewater treatment has expanded vastly. The collaboration between TU Delft, Royal HaskoningDHV and many other parties in the Dutch water sector has led to the development of Nereda®, an aerobic granular sludge process that is now widely used internationally. Nereda® combines compactness, cost and energy effectiveness with extensive biological removal of COD, nitrogen and phosphate.
Currently, there are challenges for existing conventional activated sludge regarding energy savings, resource recovery and reduction of the footprint in urban areas. The application of aerobic granular sludge could be an opportunity for such plants. Where Nereda® is a semi-continuous process, the integration of aerobic granular sludge into existing infrastructure can offer a solution if robust continuous operation can be achieved. For this reason, TU Delft, the Water Authority of Delfland, Delfluent Services, Evides Industrial Water, the Water Authority of Rijnland and Royal HaskoningDHV are conducting research into the development and operation of a continuous aerobic granular sludge process. A concrete (re)design will also be worked out for the Harnaschpolder WWTP. The research is conducted under the direction of a committee in which the organizations involved are represented.

Results

Results project phase 2 (2021 – 2022) – >UPDATE 28-11-2022
After improving the pilot set-up, full continuous operation was resumed in April 2021. The new approach had an immediate positive effect on both the sludge settleability and the mass fraction of granular sludge that can be maintained in the system. The operation of the pilot set-up will be further optimized in the near future with the aim of maintaining a comparable sludge settleability and fraction of granular sludge as in Nereda®.
The first successful trial was completed from December 2021 to June 2022, following an inoculation with aerobic granular sludge from a Nereda® installation. Over a period of more than 6 months, a share of >65% granular sludge (>200 micrometer) was achieved with an average granule size of 1.2 millimetres, while the SVI30 averaged 50 ml/g. The image below shows the different fractions of sludge, next to an overview of a mixed sludge sample (figure 1).



Figure 1 - (left) Image of the sludge water from the pilot installation with a stereo zoom microscope, of which the proportion of granular sludge, with a diameter greater than 200 micrometres, is 65%. (right) The sludge broken down into several sieve fractions of increasing grain size. The length of the scale indicator corresponds to 1 millimeter.

Work is currently underway to translate the conditions required to form aerobic granular sludge from pilot scale to full scale. This milestone marks the next step, namely researching the best achievable biological yield at full scale. Based on this, it will become clear how much additional treatment capacity can be achieved in an existing activated sludge plant using aerobic granular sludge. The consortium is currently looking for a suitable WWTP to be able to follow up and demonstrate the research at full scale.

Results project phase 1 (2016 – 2020)
Based on the understanding of granulation in Nereda® and prevention of bulking sludge in conventional activated sludge plants (CAS), the design and construction of a continuous pilot scale set-up was started at the start of the project (see figure 1). This was used to investigate whether granular sludge seeded from a Nereda® could be maintained on the wastewater of the Harnaschpolder WWTP. Additionaly, it was studied which boundary conditions such a process would have to meet on a full scale to sustain aerobic granular sludge. The pilot set-up was commissioned at the end of March 2018 and practical research was carried out until the end of 2019. A year-round better sludge volume index (SVI30) than that of Harnaschpolder WWTP was achieved, but the understanding of aerobic granular sludge and practical tools from the experiences with Nereda® proved insufficient to sustain a significant amount of granular sludge in the continuous process. Therefore, a start was made on developing a model-based sensitivity analysis for aerobic granulation at the beginning of 2020. With this model, the influence of process parameters on the driving mechanisms behind the formation of aerobic granular sludge could be better studied, among other things, fed with results from the pilot study. The new insights warrented the continuation of the pilot study. With the knowledge from the modeling work, technical adjustments to the pilot set-up and a plan for follow-up research were defined.



Figure 2 - Overview picture of the pilot set-up of the HARKOS project when it was first commissioned on March 29, 2018.

Results project phase 2 (2021 – 2022)
After improving the pilot set-up, full continuous operation was resumed in April 2021. The new approach had an immediate positive effect on both the sludge settleability and the mass fraction of granular sludge that can be maintained in the system. The operation of the pilot set-up will be further optimized in the near future with the aim of maintaining a comparable sludge settleability and fraction of granular sludge as in Nereda®. At the same time, work is being done on the translation of the boundary conditions for sustaining aerobic granular sludge from pilot scale to full scale.


Full-scale application at WWTP Harnaschpolder
Parallel to the research at pilot scale, the sludge properties of Harnaschpolder WWTP are also being examined. At the moment there is a lot of attention worldwide for improving the sludge settling capacity through the transition from a flocculent sludge morphology to a more granular sludge morphology (so-called densification). These sludge particles are also referred to as 'baby granules'. Known techniques prevention of bulking sludge are performed in more extreme form (i.e. increasing the sludge loading by improving the plug flow character of anaerobic selectors) in order to obtain a more compact floc morphology. This is also the first stage of aerobic granular sludge formation. Because the floc morphology has a direct relationship with the sedimentation rate of the sludge as a whole, an improved SVI30 can be achieved in a short period of time. This is observed periodically at the Harnaschpolder WWTP, although this still occurs spontaneously (see figure 2).





Figure 3 - Example graph of the development of the SVI30 at Harnaschpolder WWTP over the period of January 2020 till the end of September 2021. Periods with a lower SVI in one or more parallel streets start and end abruptly.

The densifications of the floc morphology will generally lead to a small fraction of granular sludge (>200 micrometers), when designed to perform enhanced biological phosphorus removal. Harnaschpolder WTTP is no exception (see figure 3). However, the width of the sludge age distribution of individual sludge particles is generally too narrow to allow these ‘baby granules’ to reach a size >1mm (as are found in Nereda®-installations) or to become a significant part of the total MLSS. The selective advantage of these 'baby granules' is successfully performed in Nereda® and remains the main challenge for a continuous aerobic granular sludge process.



Figure 4 - Image of a sieved activated sludge sample (fraction >212 micrometers) from the Harnaschpolder WWTP. Individual sludge particles are clearly visible, next to some cellulose fibers. The share in the total mass of sludge content is <5%. The red scale indicator in the upper left corner represents 200 micrometers.

Start of Research
September 2016

End of Research
November 2022

Researcher(s)
Mario Pronk (DelftUniversity of Technology)
- Mark van Loosdrecht (DelftUniversity of Technology)
- Edward van Dijk (Royal HaskoningDHV)
- Mariska Ronteltap (WaterAuthority of Delfland)
- Lobke de Pooter (WaterAuthority of Rijnland)
- Marthe de Graaff (EvidesIndustrial Water)
- Marissa Boleij (Delfluent Services)
- Michel Mulders (Delfluent Services)
- Viktor Haaksman (Delfluent Services)

Organisation(s)

- Delft University of Technology
- Delfland Water Authority
- Rijnland Water Authority
- Evides Industrial Water
- Royal HaskoningDHV
- Delfluent Services B.V.

Contact
Viktor Haaksman
[email protected]


Other publications related to the HARKOS-project: