A novel strategy for improving volatile fatty acid purity, phosphorus removal efficiency, and fermented sludge dewaterability during waste activated sludge fermentation

Application and improvement methods of sludge alkaline fermentation liquid as a carbon source for biological nutrient removal: A review

Alkaline fermentation can reduce the amount of waste activated sludge and prepare sludge alkaline fermentation liquid (SAFL) rich in short-chain fatty acids (SCFAs), which can be used as a high-quality carbon source for the biological nutrient removal (BNR) process. This review compiles the production method of SAFL and the progress of its application as a BNR carbon source. Compared with traditional carbon sources, SAFL has the advantages of higher efficiency and economy, and different operating conditions can influence the yield and structure of SCFAs in SAFL. SAFL can significantly improve the nutrient removal efficiency of the BNR process. Taking SAFL as the internal carbon source of BNR can simultaneously solve the problem of carbon source shortage and sludge treatment difficulties in wastewater treatment plants, and further reduce the operating cost. However, the alkaline fermentation process results in many refractory organics, ammonia and phosphate in SAFL, which reduces the availability of SAFL as a carbon source. Purifying SCFAs by removing nitrogen and phosphorus, directly extracting SCFAs, or increasing the amount of SCFAs in SAFL by co-fermentation or combining with other pretreatment methods, etc., are effective measures to improve the availability of SAFL.

Evaluation of pH regulation in carbohydrate-type municipal waste anaerobic co-fermentation: Roles of pH at acidic, neutral and alkaline conditions

This study investigated and evaluated the roles of acidic (pH 4.0), neutral (pH 7.0) and alkaline (pH 10.0) in anaerobic co-fermentation of sewage sludge and carbohydrate-type municipal waste. CO₂, CH₄ and H₂ are produced in acidic, neutral and alkaline fermentation, respectively. The neutral co-fermentation contained the vast number of aqueous metabolites as total of 22.12 g/L, with the advantage of over 50 % biodegradable components in extracellular polymeric substance and over 80 % hydrolysis rate. Acidic and alkaline pH facilitated ammonia release, with the max concentration of 0.46 g/L and 0.44 g/L, respectively. Microbial analysis indicated that pH is the key parameter to impact microbial activity and drive microbial community transition. The high abundance of Lactobacillus, Bifidobacterium and Clostridium was associated with harvest of ethanol, lactic acid and acetate in acidic, neutral and alkaline fermentation. Meanwhile, the floc feature showed better dewaterability (zeta potential -8.48 mV) and poor nutrient convey (distribution spread index 1.03) in acidic fermentation. In summary, acidic and alkaline fermentation were prioritised for targeted spectrum. Neutral fermentation was prioritised for high production. This study presented an upgraded understanding of the pH role in fermentation performance, microbial structure and sludge behaviour, which benefits the development of fermentation processing unit.

Enhanced dewaterability of anaerobically fermented sludge through acid-driven indigenous enzymatic hydrolysis

The poor dewaterability of fermented sludge is an important factor limiting the development of anaerobic fermentation applications. Herein we reported an efficient strategy, i.e., using acidic regulation to stimulate the release of indigenous enzymes, to enhance the hydrolysis and dewatering of fermented sludge. The results showed that after acidic regulation at pH 4.0 for 1 day, the activity of protease and α-glucosidase were improved by 131.4% and 146.0%, while the capillary suction time and specific resistance to filtration were decreased by 93.8% and 69.5%, respectively. Mechanism study revealed that the method firstly destroyed the slime and bound EPS and cells of fermented sludge, causing the release of indigenous enzymes (i.e., protease and α-glucosidase) contained in. Then, the released enzymes directly accelerated the hydrolysis and acidification of fragmentized extracellular polymeric substances, thereby benefited the release of bound water in sludge particles. Finally, such acidic condition decreased the electrostatic repulsive interactions between destroyed sludge particles, further improving their flocculation. The findings not only deepen the understanding of indigenous enzymes contained in fermented sludge affecting sludge dewatering, but also might guide engineers to develop promising strategies to facilitate fermented sludge dewatering and fermentation liquid recovery in the future.

Removing nutrients from wastewater by constructed wetlands under perfluoroalkyl acids stress

Constructed wetlands (CWs) are often used to treat wastewater discharged from wastewater treatment plants (WWTPs), while emerging contaminants (such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS)) have been commonly discovered in WWTPs. However, no research has examined whether PFOA/OS (i.e. PFOA and PFOS) affects the performance of CW. Therefore, this study compared the nutrient removal efficiencies of four CWs with varied configurations under PFOA/OS and no PFOA/OS stress conditions. We found that CW containing plants or/and iron-carbon had higher removal efficiency for nutrients (except NH₄⁺-N) than conventional CW in stable operation under wastewater without PFOA/OS. Plants or/and iron increased the nutrient removal efficiency by plant uptake, chemical reaction, and co-precipitation of iron hydroxides. In contrast, the iron-carbon inhibited the nitrification of nitrifying bacteria by consuming dissolved oxygen, converting NO₃^--N to NH₄⁺-N. Although the removal efficiencies of nutrients by CWs differed after introducing PFOA/OS, the removal order was consistent with those before adding PFOA/OS. Plants or/and iron-carbon effectively increased CWs' resistance to PFOA/OS loading and toxicity, and the function of iron-carbon was superior to the plants. In addition, PFOA/OS reduced the abundances of microbes Hydrogenophaga, Pseudomonas, Sphingomonas, Nitrospira, and Candidatus_Accumulibacter that contributed to nutrient removal.

Enhanced production of short-chain fatty acids from sludge by thermal hydrolysis and acidogenic fermentation for organic resource recovery

Acidogenic fermentation (FM treatment) converts organics in waste sludge to valuable short-chain fatty acids (SCFAs). To maintain a favorable condition for the production of SCFAs, an alkali is often added continuously to maintain an alkaline pH in the fermenter. However, this chemical adjustment is costly and biotic hydrolysis is slow. In this research, thermal hydrolysis (TH) was introduced as a pretreatment to enhance fermentation and SCFA production. The results were compared with those obtained from the untreated sludge that underwent fermentation with a daily pH 10 adjustment (NT-FM_pH10). The TH pretreatment resulted in rapid abiotic hydrolysis within a short period (1 h), releasing more than 30.5% of organics into the liquid phase of the sludge. These dissolved organics in sludge promoted rapid acidogenesis and SCFA production. TH together with a one-time alkali pretreatment further increased the production of SCFAs during sludge fermentation (TH&Alk-FM): it produced 22.8% more SCFAs than the non-treated NT-FM_pH10 sludge with alkaline pH control during fermentation. Semicontinuous fermentation further showed the advantage of the TH&Alk-FM process, as a rapid and high production of SCFAs was achieved when the fermentation time was shortened from 5 d to 2 d. The microbial community analysis revealed that TH&Alk-FM and NT-FM_pH10 sludge samples had simple but varied microbial communities. The dominant genera in the TH&Alk-FM sludge were unclassified Ruminococcaceae (18.9%) and unclassified Porphyromonadaceae (22.3%), belonging to the classes Clostridia and Bacteroidia, respectively. NT-FM_pH10 was dominated by Tissierella (23.7%) and Proteiniborus (13.5%), which belong to Clostridia. Compared with NT-FM_pH10, the microbial consortia in TH&Alk-FM were supplied with sufficient soluble organics and performed better in fermentation and SCFA production, without the need for the daily alkali addition to control pH.

Short reaction times coupled with alkalization improves the release of phosphorus from Al-waste activated sludge

In this study, the performance and mechanism of P release from Al-waste activated sludge (WAS) via wet-chemical treatment at different reaction times were investigated. The maximum P release (46% of TP) was achieved at 20 min when the pH was maintained at 2 during acidic treatment. During alkali treatment, the maximum P concentration (363.96 mg/L, 46.07%) was achieved at 10 min when pH was initially adjusted to 12. Acidic treatment took twice as long to achieve the same efficiency of released P as the alkali treatment. Furthermore, P release mainly originated from Al-P and Ca-P during acidic treatment and Al-P dissolution during alkali treatment. The cost of chemical consumption was 483.96 USD/ton TS sludge with acidic treatment, which was 8.49 times higher than that of alkali treatment without pH control. Thus, short reaction times (ca. 10 min) coupled with alkalization provide an effective approach for improving P release from Al-WAS.

Comprehensive comparison of acidic and alkaline anaerobic fermentations of waste activated sludge

This study conducted a comprehensive comparison of acidic (R_5.0) and alkaline (R_10.0) anaerobic fermentations of waste activated sludge (WAS). The results showed that alkaline fermentation was able to increase biopolymer release and benefitted the production of volatile fatty acids (VFAs). However, large amounts of the released organic matter in the R_10.0 fermented liquid had low biodegradability unsuitable for the biological nutrient removal (BNR) process, resulting in increased C, nitrogen, and phosphorus loads in BNR effluent. Further, Al was more readily released than other metals and its maximum concentration reached 134.52 mg/L in R_10.0, 2.99 times higher than in R_5.0. The fermented sludge filterability was severely deteriorated at R_10.0, as indicated by the normalized capillary suction time and specific resistance to filtration. Considering these findings, VFAs from WAS via acidic fermentation may represent a suitable carbon source for direct use in the BNR process.