The Role of An Adequate Anaerobic Mass Fraction on RAS Hydrolysis/Fermentation For Sustainable EBPR Process

Relevance
Enhanced biological phosphorus removal (EBPR) is known to achieve sustainable phosphorus removal and synchronic phosphorus-recovery. Yet, attaining reliable EBPR has been challenging, especially at low temperature and high peak flows. Recent research has shown that providing proper anaerobic mass fraction through return activated sludge (RAS) and mixed liquor hydrolysis/fermentation in EBPR not only tackles stability concerns but also results in microbial enrichment, environment control and configuration flexibility (Dongqi Wang, 2019) (Varun N. Srinivasan, 2021) (James L. Barnard, 2017). In a study by (Wang et al.), a full-scale comparison of conventional-EBPR and side-stream RAS-fermentation EBPR showed that the RAS fermentation process enhanced both denitrification and EBPR performance. This finding suggests that (1) sufficient anaerobic solid retention time and (2) extended volatile fatty acids (VFA) feeding/generation period can advantage population dynamics and process performance. Moreover, applying intermittent operation in anaerobic reactor allowed a part of the sludge to settle, creating a thick sludge layer and high anaerobic mass fraction. A proper anaerobic mass fraction when treating typical municipal influent-wastewater attributes to desirable P-removal, yet limited studies have quantitively investigated the effect of proper anaerobic mass fraction in the mainstream on fermentation/hydrolysis and EBPR performance (Dold, 2019). This study will discuss the significance of adequate anaerobic mass fraction to promote and accomplish hydrolysis/fermentation in the anaerobic zone of the mainstream in a properly designed EBPR system.
Methodology A full-scale study is being conducted at Bonnybrook Wastewater Treatment Plant (BBWWTP) Calgary, Alberta in relation to RAS fermentation for carbon source generation under deficient influent-VFA. BBWWTP consists of four secondary treatment areas: Area A, B, C, and D (recently commissioned bioreactors 11 and 12). BBWWTP must comply with a monthly discharge total phosphorus (TP) limit of 1.0 mg P/L. The City of Calgary also has phosphorus loading river objective in place, implying that low concentrations need to be achieved. Secondary treatment A relies on chemical precipitation (liquid alum) dosing to maintain low phosphorus concentrations, while B and C P-removal is occasionally dependent on alum with an average dosing of 33,200 kg/day between the three areas. The results of monitoring shows that efficient P-removal in Secondary C is achieved with external VFA, produced from onsite primary sludge fermentation, for the biological process and periodic chemical treatment, making secondary C dependent on chemical precipitation during specific times. In secondary D, Figure 1, stable EBPR has been achieved at an average influent phosphorus concentration of 4.98 mgTP/L with and without VFA addition, with no chemical treatment. To evaluate the impact of readily available VFA and decreased COD/P ratio below the default stoichiometric values established in the literature, VFA via fermented supernatant (FSU) addition in secondary D was reduced (Bioreactor 12 only, from 100 m3/hr to 0 m3/hr) and the performance was compared with an identical parallel train with 20 m3/hr FSU addition. The two processes utilize separate sludge streams. To check the impact of different operational conditions on the EBPR performance, online phosphorus, ammonia, and nitrates analyzers in the anaerobic cell and effluent with a measurement interval of 20 min were monitored Additionally, daily composite samples are completed for influent COD. In this study different operational strategies, such as intermittent mixing and RAS and influent diversion will be implemented during operation without external-VFA addition. Anaerobic intermittent mixing is carried out with different cycling periods. Sludge-decoupling will also be investigated to reach deep anaerobic zones, and RAS fermentation. The trend of oxidation reduction potential (ORP) in the anaerobic zone shall be monitored during different mixing modes to evaluate the impact of sludge-decoupling on oxidative-reductive anaerobic zone and its correlation with fermentation/hydrolysis. To investigate the kinetics of elevated fermentation rates upon the implementation of sludge-decoupling strategies, Plant D was simulated and calibrated using BioWin® 6.2 with VFA addition and after operational strategies implementation. Results and Conclusion This section represents a part of the extended data acquisition and evaluation. For this study, Bioreactor 12 is operated with no VFA addition, as fermented supernatant (FSU) flow is cutoff. As shown in Figures 2 and 3, the FSU deduction lowered the influent COD:P and COD:N ratio from an average 105 and 10.5 to 78 and 7.6 respectively, corresponding to 27% reduction, while limited increase in effluent-phosphorus concentration was observed. The default value suggested by literature for COD:P is over 80 for reaching high P-removal in EBPR systems (Metcalf & Eddy, 2014). Daily monitoring demonstrates that after FSU cutoff, significantly lower anaerobic P-release was inspected from 23 mgP/L to an average of 9 mgP/L, while effluent-P slightly increased from 0.05 mgP/L to 0.20 mgP/L. This phenomenon may be due to a balanced anaerobic zone which provides the possibility of growth for other PAOs where their behavior may differ from the conventional-EBPR yet achieve high P-removal. Lower carbon-source availability had minimal effect on effluent nitrogen content with an average 8.87 mgN/L. Such an event takes place under specific circumstances where proper anaerobic mass fraction is available. This increases the anaerobic zone VFA availability for phosphorus-removing activity rather than denitrification. Without FSU, the model underestimated the biological nutrient removal using the default hydrolysis/fermentation kinetic values under anaerobic condition. Moreover, based on the stoichiometry, the available VFA after FSU cutoff is not sufficient for achieving high P-removal. The COD mass distribution available in Table 1 further justifies the availability of other pathways of biological P-removal different from the ones highly dependent on influent-VFA or internal hydrolysis/fermentation occurrence in anaerobic zone that is not apparent in the COD mass balance. The mixer in the anaerobic cell is modified with an on-off cycling mode. The adjustable cyclic mixing encourages mixed liquor fermentation and higher VFA supply to reach complete influent rbCOD fermentation and improved P-release. The resulting deep anaerobic zone is likely to increase the anaerobic retention time due to sludge decoupling. Moreover, controlling the mixing that entrains oxygen through surface agitation may favor other PAOs with less need of VFA supplement. Preliminary tests with a 23.5 hour off period have shown an increase in the mass fraction from 11% to 21% in the anaerobic cell. A simulation model on adequate anaerobic mass fraction with proper calibration process is developed to indicate the possibility of higher RAS fermentation and substrate generation which will make an important contribution to P-release. In the modeling procedure, anaerobic zone sizing will be considered to quantify mass fractionation. Providing sufficient volume or proper operational strategies for anaerobic mass fraction (15% to 25%) indicates a reliable mainstream EBPR performance. The full manuscript shows how anaerobic intermittent mixing can achieve low ORP levels and deep anaerobic phase, where increased anaerobic fractionation and VFA availability enhances biological P-removal in this full-scale application for successful EBPR.