Going Beyond Carbon Redirection: A Holistic Evaluation of A-stage Operational Parameters To Control The B-stage Influent

Background
The adsorption/bio-oxidation (A/B) process has been gaining traction to be adopted in wastewater treatment plants (WWTP). It offers several advantages like energy savings due to reduced aeration in both stages, higher carbon redirection to the solids stream through the A-stage, and smaller footprint due to the reduced residence times in the A-stage and higher removal rates in the B-stage [1]. The alternating activated adsorption (AAA) is a novel A-stage system that retrofits the primary clarifiers. AAA integrates biological activity with the primary treatment to capture more carbon [2]. Full scale AAA system has been implemented in Strass WWTP in Austria. In previous work, it was demonstrated that AAA system has the potential to outcompete other A-stage systems in terms of carbon removal, redirection, and capture when treating raw wastewater [3]. Carbon redirection can be considered the key advantage of the A-stage systems [4,5]. Nonetheless, in addition to carbon redirection, A-stage significantly influence the B-stage by determining the B-stage influent's carbon characteristics. This includes carbon concentration, carbon fractionation, carbon to nitrogen ratio, and readily biodegradable carbon to phosphorus ratio. In addition, increased phosphorus assimilation has been reported in A-stage systems which affects the downstream phosphorus process. Such effects cannot be overlooked and shall be balanced with carbon redirection to maintain sustainable wastewater treatment. Unfortunately, previous studies focused on developing new A-stage systems [2,6,7] and studying the effect of solids retention time (SRT) on carbon redirection at multiple scales and for different processes [4,8,9]. However, the influence of other operational parameters (i.e., hydraulic retention time (HRT) and dissolved oxygen (DO) control) was rarely discussed.
This study aims at demonstrating the impact of the SRT, HRT, and DO on the performance of lab-scale AAA system. This is an attempt to evaluate the capacity to control an A-stage system to balance carbon redirection with other objectives including effluent quality in terms of overall removal and effluent fraction, and phosphorus removal.
Methodology
Multiple lab scale reactors (with working volume of 1.5L) were operated and fed with synthetic wastewater with concentrations of 470±46 mgCOD/L. The synthetic influent has different sources of organics and nutrients (i.e., soluble and particulate) to simulate real wastewater complexity [4]. The system was originally seeded with sludge collected from full scale activated sludge system. To evaluate HRT and SRT effect, Four SRTs (0.5, 1, 2, and 4 days) and three HRTs (1, 2, and 4 hours) were applied. Moreover, three DO levels were applied; Low DO (<0.5 mg/L), moderate DO (0.5-2 mg/L), and High DO (above 2 mg/L). Other than those three parameters, all other operational parameters were identical. Samples were periodically collected to monitor and quantify systems' performance. Measurements and calculations were conducted over the time interval between two sludge samples. Over this duration, effluent and wastage sludge were discharged to storage tanks from which samples were collected. Meanwhile, at least, two influent samples were collected to assure its consistency and representation of the influent over the same period.
Findings
At SRT of 0.5 and 1 day, comparable carbon redirection was attained while higher oxidation (Figure 1A) took place at SRT of 1 day which resulted in higher TCOD removal. Further increase in the SRT to 2 and 4 days resulted in higher oxidation which increased from 27±7% of the influent COD at SRT of 1 day to above 40% at 2 and 4 days. This increase was at the expense of the COD redirection which decreased to 22±8% from 45±8% at SRT of 1 and 2 days, respectively. No significant difference in the removal efficiency of the influent filtered flocculated COD (ffCOD) were observed at different SRTs (Figure 1B). Yet, particulate COD (pCOD) removal peaked at SRT of 1 day. It can be seen from Figure 1C that SRT did not affect phosphorus removal which averaged around 60%. Interestingly, such a value is much higher than values reported in the literature [9,12] and correlated with ffCOD removal. Finally, SRT is not reliable to control effluent fractions as, for example, ffCOD was always above 20% (Figure 1D). Increasing HRT from 2 to 4 hours has much lower effect on the redirection of the influent COD, as shown in Figure 2A. The average COD redirection only decreased from 45% to 36%. Meanwhile, such an increase was associated with an obvious increase in ffCOD removal (Figure 2B). Accordingly, HRT better controls the process's effluent fractions. As seen in Figure 2D, ffCOD fraction was decreased from more than 40% to less than 10% by HRT increase. On the other hand, pCOD removal declined at higher HRT which can be referred to the lower organics loading rates negatively affecting sludge settleability [14]. Interestingly, more than 30% increase in phosphorus removal was attained at HRT of 4 hours compared with 2 hours reaching 79±6%. Lastly, DO influence was evaluated at short HRT (1hour) and SRT (1day). It can be seen that the influent COD oxidation and removal decreased in correlation with DO concentration. Whereas, a drop in COD redirection was observed at low DO while similar redirection was attained at moderate and high DO (Figure 3A). As demonstrated in Figure 3B, the ffCOD fraction decreased in the effluent with DO increase while the pCOD fraction increased. The differences are not drastic between different DO levels which can be referred to the limited SRT and HRT which might have hampered COD uptake. Meanwhile, phosphorus removal was found to be the same regardless of DO applied averaging at 60%. Again, this might be referred to the limited retention times which did not allow for DO effect to be pronounced. COD oxidation and redirection are strongly correlated when only SRT is changing (Figure 4A). This can elucidate why SRT control results in higher redirection but the system fails to achieve higher oxidation (mainly due to soluble fraction removal) in the meantime. Yet, the lower association of between oxidation and redirection when HRT and DO (Figures 4B and C) is changing implies that better control on effluent COD fraction and achieving high removal and high redirection is possible.
Research significance
This study is the first to investigate the impact of SRT, HRT, and DO on the AAA system and DO and HRT effect on the A-stage process. It can be concluded that high phosphorous and ffCOD can be attained in the effluent while acceptable levels of redirection are maintained relying on HRT extension and keeping SRT less than 1 day. Yet, higher redirection is attained at lower HRT. It is demonstrated that, unlike SRT, HRT and DO have more flexibility to be changed to control the carbon fractions and P concentration of the B-stage influent.