At the recent RMIT Biosolids workshop, delegates heard about various projects and innovations taking place in several Australian Water Association and Australian & New Zealand Biosolids Partnership member organisations. One such presentation was from Richard Lovett, Process Engineer in the Infrastructure Planning team at Melbourne Water.
Lovett shared insights from the Eastern Treatment Plant (ETP) sludge digestion capacity upgrade project. Providing details of the project’s context, decision making process and outcomes allowed delegates to gain insights into Melbourne Water’s comprehensive approach to evidence-based decision making.
“We currently feed the digesters around 110 tons per day of thickened primary sludge and 65 tons of thickened waste activated sludge (WAS),” Lovett said.
“We have eight mesophilic anaerobic digesters which typically achieve around 55% volatile solids destruction, which is considered very good for mesophilic digestion. This appears to be because our WAS is particularly degradable.
“The issue we are facing is that sludge production is growing in proportion to sewage loads, so we are running out of hydraulic capacity in the digesters, making it increasingly difficult to maintain our target 15 day Hydraulic Retention Time.”
Knowing that they were running out of capacity, in late 2015 Melbourne Water started looking at upgrade options. On commencement of this project, the organisation was clear on the context that this project was set in.
“The last time we built digesters at ETP was in 1995, so rather than defaulting straight to building more of the same, we wanted to learn about the developments that had been made in digestion and sludge pre-treatment over the last 20 years.
“We set a hurdle criterion that the selected process had to be proven at ETP scale. The options we then looked at included variations on single and multi-phased digestion, various sludge pre-treatments and thickening options. Through multiple rounds of assessment we narrowed down from over 40 options initially to our preferred option.
“We found that certain site-specific factors strongly influenced the option selection process.
“Firstly, Melbourne Water regularly reviews options for biosolids reuse, so when selecting a digestion option we had to consider the impact on final biosolids quality and any implications for reuse. We currently achieve T1 treatment, which is required for our current reuse options, by stockpiling our biosolids on site for three years. If our reuse options change or we had less space to stockpile we may have a driver to try to achieve T1 more quickly via some other option such as thermal hydrolysis pre-treatment, or thermophilic digestion.
“Secondly, the digestion process at ETP is already quite complex; the digesters rotate through a number of different mixing and pumping cycles. We were wary of options that added a lot of extra complexity with multiple process steps in series, so this counted against phased digestion processes for example.
“Thirdly, we had to consider asset condition. Four of the digesters date back to the construction of the original plant in the ‘70s. While there’s nothing to suggest that they have structural problems, we considered that converting them to thermophilic operation was a risky proposition from a structural point of view, because they hadn’t been designed to operate at those temperatures.
“Finally, a specific process risk associated with thermal hydrolysis at ETP, is that it produces recalcitrant coloured organic compounds such as melanoidins. At ETP, these could reduce UV transmissivity and so increase ozone demand in our Tertiary treatment plant. These compounds are not an issue at every plant, particularly those that do not have UV disinfection or a discharge limit on colour.”
Following this exploration of risk factors and a filtering process including comprehensive 25 year Net Present Cost calculation, detailed ADM1 process modelling, and bench scale digestion testing, the team selected recuperative thickening as their preferred upgrade option.
Recuperative thickening is a loop process, which makes it robust. If recuperative thickening goes offline then digesters can continue to operate while it is brought back online. The implementation process can be delivered in stages, which can be attractive financially and gives greater planning flexibility. In the context of Melbourne Water it offers the added benefit of maximising the value of their investment in their existing digesters, by intensifying the digestion process.
Lovett outlined the recuperative thickening concept in greater detail:
“Recuperative thickening works by taking sludge from the digester, thickening it, and returning the thickened sludge to the digester. This multiple stage approach decouples the solids retention time (SRT) from the hydraulic retention time (HRT). The operator can target a particular SRT by adjusting the recycle rate. We plan to target a 15 day SRT, which will give equivalent volatile solids destruction to what we achieve currently.
“We have selected rotary drum thickeners (RDTs) as our preferred thickening technology. RDTs are proven in this application, and generally said to be reliable and to not require too much maintenance. This decision was based on part on our visits to the Sydney Water plants at Liverpool and Warriewood, as well as visits to Melton and Mt Martha to look at their RDTs.
Having selected recuperative thickening, Melbourne Water moved into functional design development. Rather than nominating a specific year as the design horizon, the project team set a goal of implementing recuperative thickening to the maximum extent feasible at ETP. They set this goal knowing that over time, the thickness of the digester sludge would need to increase (from its current level of around 2.5%) to maintain the target 15 day SRT. Lovett noted:
“Increasing the sludge thickness will impact on our existing heating, pumping and mixing systems. So, a key piece of work for us to do was to find at what thickness these systems would hit their limits, and decide whether it was worth investing in upgrading them in order to pursue a higher thickness. This work included detailed pipe network modelling in “Fathom” software, and Computational Fluid Dynamics modelling of digester mixing. To support these assessments, we ran a program of testing to generate data on the rheological (flow) characteristics of ETP sludge, as these vary significantly from site to site.
“As a result of the limitations assessments, we have developed a 2-stage approach to implementing recuperative thickening at ETP.”
For Stage 1:
- Install thickeners to increase digester sludge thickness to 3.5%. At this stage the team will also size civil works, pipework, etc. to meet the requirements for Stage 2.
- Replace three small heat exchangers.
- Upgrade sludge pumping to enable the continuation of pumping sludge to the sludge drying pans, the furthest of which is at a distance of 2.5km from the digesters.
For Stage 2:
- Install additional thickeners to increase digester sludge thickness to 4.0%.
- Upgrade the mixing system on digesters 1-4 (CFD modelling showed that it is less capable than the mixing on digesters 5-8).
- Upgrade one large raw sludge heat exchanger.
“This staged approach gives us considerable flexibility, for example we could opt to bring forward or push back Stage 2 depending on the performance of the recuperative thickening system and the growth in sewage loads, or we could choose not to do Stage 2 at all and instead pursue an alternative option such as thermal hydrolysis,” Lovett said.
“There’s also the possibility of doing sidestream deammonification on the centrate stream – being warm and fairly concentrated in ammonia it is a good candidate for deammonification.”
Melbourne Water’s approach to this project demonstrates the value of careful consideration of site-specific factors and heavily investing in experimental testing and modelling to predict performance and explore system limitations.
“This is a challenging application of recuperative thickening due to the complexity of the existing process and the long pumping distances, but we feel that our staged implementation approach offers us great flexibility.
“For me it’s been great to get involved in the experimental work, and to learn from technical experts in a variety of fields. This has been very much a team effort, and I think that as a group we’ve benefited greatly from looking outward and learning from other utilities. I believe that this will result in an efficient and effective long term solution for the Eastern Treatment Plant.”
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