INTRODUCTION Sustainability is increasingly more important for the operation of wastewater treatment plants. To meet sustainability goals and better energy recovery, the current business as usual approach to anerobic digester operation is not ideal. Improving digester operation is crucial for effective energy recovery. Having a reactive approach to digester foaming prevents optimal digester performance and causes other problems like digester failure, poor biogas recovery, uncontrolled release of contaminated waste to the environment and odour. Figure 1 shows some of these impacts. The best way to mitigate digester foaming is through robust design, but if this is not possible there are several operating conditions that can be improved to control foaming. Sydney Water Corporation (SWC) recognised the importance of preventing anaerobic digester foaming. Digester foaming at SWC plants has been caused by multiple different issues and foam management can therefore be largely reactive. There are operational methods and digester design features in place at some plants that aim to reduce the occurrence of the impacts of foam events, however this is not widespread or consistent. Investigation into 5 plants was undertaken to determine the likely causes of digester foaming and gas entrainment and the most effective ways to manage this and where possible through design changes as upgrades occur. For plants where design changes are not possible in the near future, key operating conditions were reviewed, early warning signs for digester foaming events determined and management practices recommended to mitigate digester foaming. METHODOLOGY Digester foaming was investigated at 5 SWC Water Resource Recovery Plants (WRRP), including Cronulla, Malabar, Warriewood, West Hornsby and Hornsby Heights. Digester operation data was analysed to determine the likely causes of digester foaming issues and recommendations on proactive and reactive measures for foam management including how to prevent or mitigate foaming at these plants and across broader SWC plants were established and operator training provided. Digester operation parameters and recommended monitoring were also outlined. Best practice digester design to minimise foaming and maximise energy production was investigated, drawing on international experience. These design components were presented to SWC designers, along with recommended implementation methodologies. RESULTS The main issues causing digester foaming at the SWC plants investigated were seasonal foaming related to activated sludge processes, lack of effective redundancy or contingency measures associated with the digester process, and illegal industrial discharges to the incoming sewer. There are other fundamental operating issues that cause digester foaming, such as running with one digester offline for extended periods (for example, at 2 plants digesters were offline due to issues with digester lids, which resulted in excessive volatile solids load in remaining digester/s particularly when food waste was accepted), turning off mixing, high volatile solids loading rates in digesters, poor temperature control, unstable feed to the digesters, and large step changes in temperature and feed. Figure 2 shows the volatile solids (VS) loading rate and specific volatile solids loading rate (SVSLR) in the primary digester at one of the plants where primary and secondary digesters are currently operated in series. At this plant, there is inadequate solids retention time (SRT) in the digesters, as well as a high VS loading to the primary digester. The average VS load to the primary digester was 3.74 kgVS/m3.d and a peak loading rate of 4.5 kgVS/m3.d, where the recommended range is 1.6-3.2 kgVS/m3.d . Therefore, the anaerobic digesters are operated beyond the typical recommended VS loading rates. The high VS loading rate is also exacerbated by the unloading/loading sequence of flow, which results in intermittent VS loading, rather than constant loading across the day. The high VS load impacts the stability of the sludge and increases risk of foaming in the digesters. The SVSLR is a variation of VS loading rate that considers the VS in the digester as a function of active biomass. However, SVSLT may not account for the feedstock other than the wastewater sludge such as high strength organic matter. The maximum recommended SVSLR for mesophilic digestion without any pre-treatment is 0.16 kg VS/kg VS in digester day. As shown in Figure 1, along with the VS loading rate, the SVSLR for the primary digester is consistently above the recommended limit, making it vulnerable to the digester foaming and failure with fluctuation in digester loading rate. Best practice digester design to avoid foaming issues and maximise energy recovery include the use of standpipes for surface overflow (see Figure 2) and emergency withdrawal via p-traps (see Figure 3), minimisation of the top water surface area, installation of fixed covers and external gas storage, the use of pumped mixing to reduce short-circuiting and having adequate redundancy to ensure that there is no single point of failure. Digesters should have a conical shape to facilitate grit removal and more efficient mixing. Wasting foam from the surface in secondary activated sludge processes will also minimise the amount of trapped foam, improving conditions in the downstream digesters. Rolling out design changes in a systematic way as digesters are taken offline for planned maintenance is an effective way to facilitate continuous removal of foaming issues from upgraded digesters. In addition to benefits of maximising energy generation, achieving maximum hydraulic capacity within existing digesters by having external gas holders and running digesters full may allow capital upgrades of digesters to be delayed for a number of years if not decades, further improving sustainability of WRRPs. Where design changes cannot be implemented, there are operational improvements that can be made to manage digester foaming and increase energy production. Generally, if there are filamentous bacteria in the activated sludge bioreactor at a plant, there is a high likelihood of foaming in the digester. These foaming incidents are manageable through adjusting the operating parameters in the activated sludge plant. Digesters should be brought online slowly with small step increases in feed and temperature over time. A common misconception is that digester mixing should be turned off to control a foam event, however mixing should be turned down. Digesters should be operated with a top water level high into the lid, with sprays used to suppress foam and move foam to wasting points. Digested sludge should be withdrawn from the top and bottom of the digester, and consideration given to alkalinity correction when the pH in the digester starts to drop. Running the digesters without any safety factor and running some process equipment at loading rates higher than designed for will increase the risk of process upsets that could also contribute to foaming. Allowing a safety factor in terms of solids retention time and volatile solids loading provides plants with the ability to cope with under upset conditions and avoids the need to detune the digesters and the plant to minimise the risk of foaming. CONCLUSION By successfully managing digester foaming and operating digesters in a manner than maximises power generation, WRRPs can achieve more effective energy recovery and meeting energy sustainability and neutrality goals. Some fundamental issues that generally arose to cause digester foaming were: -Running with one digester offline for long periods resulting in much higher loadings and shorter SRT than originally designed -High VS loading rates in the digesters -Reduced effective SRT due to mixing being turned off and accumulation of grit and screening -Mixing generally not designed for current solids loading due to intensification -Poor temperature control for the digesters -Unstable feed to the digesters -Large step changes in temperature or feed due to heater failure or storm conditions Foaming subsequently resulted in lower biogas production, high costs for digester reinstatement, and poor run times on biogas engines due to poor biogas quality. Robust digester design which minimises digester foaming and gas entrainment is the most preferable management mechanism, and changes to digester components can be implemented over time as maintenance is undertaken. Where design changes are not possible, operational improvements can be made to greatly reduce digester foaming. This paper outlines some best practice measures that can save millions of dollars by avoiding catastrophic foaming events and will help maximise biogas production and energy generation.
This paper was presented at the WEF/IWA Residuals and Biosolids Conference, May 16-19, 2023.
Author(s)A. McDonald1, J. Cesca2, G. Bharambe3, D. Parry4, J. Gonzalez5,
Author affiliation(s)Jacobs1; Sydney Water Corporation2
SourceProceedings of the Water Environment Federation
Document typeConference Paper
Print publication date May 2023
Volume / Issue
Content sourceResiduals and Biosolids