Engineering strategies to enhance resource recovery in eerobic granular sludge systems

Abstract

Aerobic granular sludge (AGS) is an emerging sewage treatment technology that combines the possibility of simultaneous removal of carbon, nitrogen, and phosphorus, with the high capacity to recover resources, many of which are of high industrial interest. Associated with the valorization of the circular economy in wastewater treatment plants (WWTP), the recovery of byproducts from AGS biomass has emerged as an efficient and economically sustainable sludge management strategy, especially considering that extracellular polymeric substances (EPS) can make up to 40% of dry biomass. Alginate-Like Exopolymers (ALE) and tryptophan (TRP), present in the extracellular matrix of AGS, are valuable byproducts with promising applications in industry. However, the literature still needs a comprehensive understanding of the impacts of operational parameters on the biosynthesis of these bioresources in sludge, as well as their interconnection with granulation and the system’s long-term stability. In this context, the present research addressed several operational aspects and stimuli for producing of these resources, evaluating the efficiency of production, granulation, and system stability in five operating systems (OS I, OS II, OS III, OS IV, and OS V). In OS I, the influence of different carbon sources (acetate, propionate, glycerol, glucose, and sucrose) on ALE, EPS and TRP production was investigated. Acetate showed promise, resulting in greater production of biopolymers and TRY, although partial disintegration of the granules was observed during operation. On the other hand, propionate was the substrate that best provided long-term stability to the AGS and stimulated a considerable production of bioresources. In OS II, the impact of salinity on removal efficiencies, granule stability, and ALE, EPS and TRP production was explored. Low saline concentrations (up to 7.5 g/L) favored ALE production and increased biomass, but higher concentrations (10 g/L) led to the system’s collapse in a certain period. The detailed analysis revealed the identification of amino acids, such as tyrosine and tryptophan, in greater quantities in the extracellular polymeric substances of reactors operated with salt. In OS III, a comparison was made between staged and conventional feeding under osmotic pressure. Staggered feeding has shown significant benefits in terms of denitrification and long-term stability. The gradual increase in salt (2.5 to 10 g/L) influenced the production of ALE, EPS and TRP, but the staggered feeding did not significantly impact their production. In OS IV, the effects of intermittent osmotic stress were investigated, comparing it with constant stress and a control system without added salt, both as a strategy to reduce the start-up time of aerobic granulation systems and to evaluate whether intermittent osmotic stress has been shown to promote rapid granulation and maintain stable ALE production. The reactor subjected to intermittent salt stress revealed rapid and complete granulation in just 21 days, contrasting with the 59 days required for the reactor with constant salt addition. Furthermore, salt addition significantly positively impacts granulation compared to the control reactor, corroborating previous findings in OS II and OS III. This finding highlights the importance of variation in saline conditions to promote efficient granule formation. Given this, the highlight goes to the reactor under intermittent saline stress, which is identified as the most efficient in granulation, stability, production of resources, and removal of pollutants. In OS V, research focused on the co-treatment of leachate with domestic sewage, with special attention to controlling sludge retention time or sludge age (SRT). The low proportion of leachate (5%) did not harm the performance and stability of AGS, while increasing the leachate fraction (10%) influenced the removal of organic matter and ALE production. Principal component analysis (PCA) highlighted SRT as crucial for optimizing biopolymer synthesis, indicating optimal production in SRT between 10 and 20 days. In summary, this research provides a detailed understanding of the factors that influence the production of bioresources in aerobic granulation systems. The results achieved significantly contribute to optimizing the performance of these systems in terms of start-up time, stability, resource production, and contaminant removal in saline and non-saline effluents, opening new paths for the sustainable evolution of wastewater treatment practices.