Improving nutrients ratio in class A biosolids through vivianite recovery: Insights from a wastewater resource recovery facility.

Class A biosolids from water resource recovery facilities (WRRFs) are increasingly used as sustainable alternatives to synthetic fertilizers. However, the high phosphorus to nitrogen ratio in biosolids leads to a potential accumulation of phosphorus after repeated land applications. Extracting vivianite, an FeP mineral, prior to the final dewatering step in the biosolids treatment can reduce the P content in the resulting class A biosolids and achieve a P:N ratio closer to the 1:2 of synthetic fertilizers. Using ICP-MS, IC, UV-Vis colorimetric methods, Mössbauer spectroscopy, and SEM-EDX, a full-scale characterization of vivianite at the Blue Plains Advanced Wastewater Treatment Plant (AWTTP) was surveyed throughout the biosolids treatment train. Results showed that the vivianite-bound phosphorus in primary sludge thickening, before pre-dewatering, after thermal hydrolysis, and after anaerobic digestion corresponded to 8 %, 52 %, 40 %, and 49 % of the total phosphorus in the treatment influent. Similarly, the vivianite-bound iron concentration also corresponded to 8 %, 52 %, 40 %, and 49 % of the total iron present (from FeCl3 dosing), because the molar ratio between total iron and total incoming phosphorus was 1.5:1, which is the same stoichiometry of vivianite. Based on current P:N levels in the Class A biosolids at Blue Plains, a vivianite recovery target of 40 % to ideally 70 % is required in locations with high vivianite content to reach a P:N ratio in the resulting class A biosolid that matches synthetic fertilizers of 1:1.3 to 1:2, respectively. A financial analysis on recycling iron from the recovered vivianite had estimated that 14-25 % of Blue Plain's annual FeCl3 demand can potentially be met. Additionally, model simulations with Visual Minteq were used to evaluate the pre-treatment options that maximize vivianite recovery at different solids treatment train locations.

Introducing bioflocculation boundaries in process control to enhance effluent quality of high-rate contact-stabilization systems.

High-rate activated sludge (HRAS) systems suffer from high variability of effluent quality, clarifier performance, and carbon capture. This study proposed a novel control approach using bioflocculation boundaries for wasting control strategy to enhance effluent quality and stability while still meeting carbon capture goals. The bioflocculation boundaries were developed based on the oxygen uptake rate (OUR) ratio between contactor and stabilizer (feast/famine) in a high-rate contact stabilization (CS) system and this OUR ratio was used to manipulate the wasting setpoint. Increased oxidation of carbon or decreased wasting was applied when OUR ratio was <0.52 or >0.95 to overcome bioflocculation limitation and maintain effluent quality. When no bioflocculation limitations (OUR ratio within 0.52-0.95) were detected, carbon capture was maximized. The proposed control concept was shown for a fully automated OUR-based control system as well as for a simplified version based on direct waste flow control. For both cases, significant improvements in effluent suspended solids level and stability (<50-mg TSS/L), solids capture over the clarifier (>90%), and COD capture (median of 32%) were achieved. This study shows how one can overcome the process instability of current HRAS systems and provide a path to achieve more reliable outcomes. PRACTITIONER POINTS: Online bioflocculation boundaries (upper and lower limit) were defined by the OUR ratio between contactor and stabilizer (feast/famine). To maintain effluent quality, carbon oxidation was minimized when bioflocculation was not limited (0.52-0.95 OUR ratio) and increased otherwise. A fully automated control concept was piloted, also a more simplified semiautomated option was proposed. Wasting control strategies with bioflocculation boundaries improved effluent quality while meeting carbon capture goals. Bioflocculation boundaries are easily applied to current wasting control schemes applied to HRAS systems (i.e., MLSS, SRT, and OUR controls).

Towards more predictive clarification models via experimental determination of flocculent settling coefficient value.

Improved settleability has become an essential feature of new wastewater treatment innovations. To accelerate adoption of such new technologies, improved clarifier models are needed to help with designing and predicting improvement in settleability. In general, the level of mathematics of settling clarifier models has gone far beyond the level of existing experimental methods available to support these models. To date, even for simple one-dimensional (1D) clarifier models, no experimental method has been described for flocculent settling coefficient (rp). As a consequence, rp cannot be considered as a sludge characteristic and is used as a calibration parameter to achieve observed effluent quality. In this study, we focused on the development of an empirical function based on a simple and practical experimental approach for the calculation of the rp value from sludge characteristics. This approach provided a similar approach as currently taken for hindered settling coefficient calculations (Veslind equation) and allowed for the model to predict effluent quality, thus increasing the power of the 1D model. The threshold of flocculation (TOF), which describes the collision efficiency of particles, directly correlated with the effluent quality of the five tested activated sludge systems and was selected as experimental method. The proposed empirical function between TOF and rp was validated for four years of validating data with five different sludge types operated under different operational conditions and configurations. The good effluent quality prediction with this approach brings us one step closer in making the clarification models more predictive towards effluent quality and clarifier performance.