Changes in microbial community during hydrolyzed sludge reduction

In this study, the effects of different enzymes (lysozyme, α-amylase and neutral protease) on sludge hydrolysis efficiency and microbial community in sequencing batch reactor (SBR) were introduced. The results showed that the hydrolysis efficiencies of the three enzymes were 48.5, 22.5 and 31%, respectively, compared with the accumulated sludge discharge of the blank control group. However, it has varying degrees of impact on the effluent quality, and the denitrification and phosphorus removal effect of the system deteriorates. The lysozyme that achieves the optimal sludge hydrolysis effect of 48.5% has the greatest impact on the chemical oxygen demand (COD), total nitrogen (TN), and nitrate nitrogen (NO3--N) of the effluent. The sludge samples of the control group and the groups supplemented with different enzyme preparations were subjected to high-throughput sequencing. It was found that the number of OTUs (Operational Taxonomic Units) of the samples was lysozyme > α-amylase > blank control > neutral protease. Moreover, the abundance grade curve of the sludge samples supplemented with lysozyme and α-amylase was smoother, and the community richness and diversity were improved by lysozyme and α-amylase. The species diversity of the sludge supplemented with lysozyme and neutral protease was great, and the community succession was obvious. The introduction of enzymes did not change the main microbial communities of the sludge, which were mainly Proteobacteria, Actinobacteria and Bacteroidetes. The effects of three enzyme preparations on sludge reduction and microbial diversity during pilot operation were analyzed, the gap in microbial research was filled, which provided theoretical value for the practical operation of enzymatic sludge reduction.

Effect of Different Acid and Base Potassium Ferrate Pretreatment on Organic Acid Recovery by Anaerobic Digestion of Sludge

Potassium ferrate has strong oxidation in both acid and alkali environments, which has attracted extensive attention. However, the impact of the pH environment on this coupling process with the goal of resource recovery has not received attention. Under the goal of the efficient recovery of organic acid, the changes of solid-liquid characteristics of sludge after acid and alkaline ferrate pretreatment and during anaerobic digestion were discussed. The results showed that compared with blank control groups, after alkaline ferrate pretreatment, the volatile suspended solids (VSSs) decreased the most, reaching 28.19%. After being pretreated with alkaline ferrate, the sludge showed the maximum VFA accumulation (408.21 COD/g VSS) on the third day of digestion, which was 1.34 times higher than that of the acid ferrate pretreatment. Especially in an alkaline environment, there is no need to add additional alkaline substances to adjust the pH value, and the effect of sludge reduction and acid production is the best.

In-Situ Sludge Reduction Performance and Mechanism in Sulfidogenic Anoxic-Oxic-Anoxic Membrane Bioreactors

The excess sludge generated from the activated sludge process remains a big issue. Sustainable approaches that achieve in situ sludge reduction with satisfactory effluent quality deserve attention. This study explored the sludge reduction performance of sulfidogenic anoxic-oxic-anoxic (AOA) membrane bioreactors. The dynamics of the microbial community and metabolic pathways were further analyzed to elucidate the internal mechanism of sludge reduction. Compared with the conventional anoxic-oxic-oxic membrane bioreactor (MBR_control), AOA_S150 (150 mg/L SO₄^2- in the membrane tank) and AOA_S300 (300 mg/L SO₄^2- in the membrane tank) reduced biomass production by 40.39% and 47.45%, respectively. The sulfide reduced from sulfate could enhance the sludge decay rate and decrease sludge production. Extracellular polymeric substances (EPSs) destruction and aerobic lysis contributed to sludge reduction in AOA bioreactors. The relative abundance of Bacteroidetes (phylum), sulfate-reducing bacteria (SRB, genus), and Ignavibacterium (genus) increased in AOA bioreactors compared with MBR_control. Our metagenomic analysis indicated that the total enzyme-encoding genes involved in glycolysis, denitrification, and sulfate-reduction processes decreased over time in AOA_S300 and were lower in AOA_S300 than AOA_S150 at the final stage of operation. The excess accumulation of sulfide in AOA_S300 may inactive the functional bacteria, and sulfide inhibition induced sludge reduction.

Metagenomic Insight of a Full Scale Eco-Friendly Treatment System of Textile Dye Wastewater Using Bioaugmentation of the Composite Culture CES-1

Effects of bioaugmentation of the composite microbial culture CES-1 on a full scale textile dye wastewater treatment process were investigated in terms of water quality, sludge reduction, dynamics of microbial community structures and their functional genes responsible for degradation of azo dye, and other chemicals. The removal efficiencies for Chemical Oxygen Demand (COD), Total Nitrogen (T-N), Total Phosphorus (T-P), Suspended Solids (SS), and color intensity (96.4%, 78.4, 83.1, 84.4, and 92.0, respectively) 300-531 days after the augmentation were generally improved after bioaugmentation. The denitrification linked to T-N removal appeared to contribute to the concomitant COD removal that triggered a reduction of sludge (up to 22%) in the same period of augmentation. Azo dye and aromatic compound degradation and other downstream pathways were highly metabolically interrelated. Augmentation of CES-1 increased microbial diversity in the later stages of augmentation when a strong microbial community selection of Acinetobacterparvus, Acinetobacterjohnsonii, Marinobacter manganoxydans, Verminephrobacter sp., and Arcobacter sp. occurred. Herein, there might be a possibility that the CES-1 augmentation could facilitate the indigenous microbial community successions so that the selected communities made the augmentation successful. The metagenomic analysis turned out to be a reasonable and powerful tool to provide with new insights and useful biomarkers for the complex environmental conditions, such as the full scale dye wastewater treatment system undergoing bioaugmentation.

Pilot-Scale Airlift Bioreactor with Function-Enhanced Microbes for the Reduction of Refinery Excess Sludge

A pilot-scale airlift bioreactor (ALBR) system was built and operated continuously for refinery excess sludge (RES) reduction. Combined ALBR and function-enhanced microbes (composed of photosynthetic bacteria and yeast) were integrated into the system. The pilot-scale ALBR was operated for 62 days, and the start-up time was 7 d. Continuous operation showed that the sludge reduction efficiency was more than 56.22%, and the water quality of the effluent was satisfactory. This study focused on investigating the effects of hydraulic retention time (HRT) on the stability of the system and the effect of sludge reduction. Under different HRT conditions of 40, 26.7, 20, and 16 h, the sludge reduction rates reached 56.22%, 73.24%, 74.09%, and 69.64%, respectively. The removal rates of chemical oxygen demand (COD) and total nitrogen (TN) decreased with decreasing HRT, whereas the removal rate of NH₄⁺-N increased. The removal rate of total phosphorus (TP) was approximately 30%. Results indicate that the ALBR and function-enhanced microbe system can reduce sludge and treat sewage simultaneously, and the effluent is up to the national emission standard. Addition of function-enhanced microbes can promote the degradation of petroleum hydrocarbon substances in the sludge, especially alkanes with low carbon numbers. This study suggests that the optimal HRT for the system is 16 h. The total operation cost of the ALBR combined with the function-enhanced microbe system can be reduced by 50% compared with the cost of direct treatment of the RES system.

Effects of bioporous carriers on the performance and microbial community structure in side-stream anaerobic membrane bioreactors

The aim of this study was to investigate the effects of a volcanic rock porous carrier (VRPC) on sludge reduction, pollutant removal, and microbial community structure in an anaerobic side-stream reactor (ASSR). Three lab-scale membrane bioreactors (MBRs), including an anoxic-oxic MBR, which served as the control (C-MBR), an ASSR-coupled MBR (A-MBR), and an A-MBR filled with VRPC (FA-MBR) were stably and simultaneously operated for 120 days. The effect of the three reactors on the removal of chemical oxygen demand (COD) was almost negligible (all greater than 95%), but the average removal efficiency of ammonium nitrogen, total nitrogen, and total phosphorus was significantly improved by the insertion of an ASSR, especially when the ASSR was filled with VRPC. Finally, A-MBR and FA-MBR achieved 16.2% and 26.4% sludge reduction rates, with observed sludge yields of 0.124 and 0.109 g mixed liquid suspended solids/g COD, respectively. Illumina MiSeq sequencing revealed that microbial diversity and richness were highest in the VRPC, indicating that a large number of microorganisms formed on the carrier surface in the form of a biofilm. Abundant denitrifying bacteria (Azospira, Comamonadaceae_unclassified, and Flavobacterium) were immobilized on the carrier biofilm, which contributed to increased nitrogen removal. The addition of a VRPC to the ASSR successfully immobilized abundant hydrolytic, fermentative, and slow-growing microorganisms, which all contributed to reductions in sludge yield.

Application of real-time nitrogen measurement for intermittent aeration implementation in a biological nitrogen removal system: performances and efficiencies

Several automatic strategies for aeration control have been investigated so far to favor a significant decrement of energy consumptions in biological processes in wastewater treatment plants (WWTPs). Above all, the intermittent aeration process has demonstrated to have several advantages. This work reports the results of a case study made in a small municipal WWTP where an intermittent aeration control strategy based on the online measurement of real-time nitrogen forms has been implemented to optimize the nitrogen removal and the energy consumption in an activated process. High organic removal efficiency was achieved during the whole experimental period. Results indicated that nitrogen removal efficiency has been improved, especially during winter months, by the control strategy adopted that helped to determine the perfect length for the nitrification and denitrification phases. The intermittent aeration process increased the activity of the biomass, further selecting phosphate-accumulating bacteria and improving the growth of bacteria able to internally store carbon source. Excellent effects were achieved on energy and chemical consumption, with a reduction of about 15% and 23%, respectively. Finally, a 12% reduction of sludge production was also detected.

Assessment of the Sludge Reduction of the Metabolic Uncoupler 3,3',4',5-tetrachlorosalicylanilide (TCS) in Activated Sludge Culture

Batch experiments were completed to assess the sludge reduction of the metabolic uncoupler 3,3',4',5-tetrachlorosalicylanilide (TCS). The effects of various TCS concentrations on sludge yield were evaluated and the mechanisms associated with sludge reduction were assessed. We discovered that TCS addition resulted in a reduction in sludge. Furthermore, a low dose of TCS (≤3 mg/L) resulted in a slight reduction in the efficiency of the wastewater treatment system, while >3 mg/L TCS reduced matrix removal efficiency, with an especially remarkable inhibition effect on ammonia removal. An increase in TCS addition was associated with a gradual decrease in both the electron transport system (ETS) activity and the specific cellular ATP (SATP) in the TCS system. It was demonstrated that TCS plays an important role in metabolic uncoupling. However, with the addition of TCS, both contents and compositions were increased, and the protein content increased more than polysaccharide production in extracellular polymeric substances (EPS). At TCS concentrations of ≤3 mg/L, DNA content was stable, but it increased rapidly from 4.97 mg/L to 15.34 mg/L as the TCS concentration was elevated from 6 mg/L to 12 mg/L. This implied that the mechanisms of sludge reduction were different for different TCS concentrations, including uncoupling metabolism, maintenance metabolism and lysis-cryptic growth.

Elucidating the microbial community associated with the protein preference of sludge-degrading worms

Sludge predation by aquatic worms results in an increased sludge reduction rate, which is mainly due to the specific removal of a protein fraction from the sludge. As microorganisms play an essential role in sludge hydrolysis a better understanding of the microbial community involved in the worm predation process will provide more insight into the relations between the aquatic worms, their associated microbiome and the efficient sludge reduction. In this study, the microbial community associated with predation by the Tubifex tubifex was investigated. The microbial diversity in the samples of the worm faeces (WF), predated activated sludge and protein-rich substrates were compared. The results indicated that predation on sludge resulted in a microbial change from Actinobacteria (44%) in the sludge, to Proteobacteria (64%) and Bacteriodites (36%) in the WF. Interestingly, the faecal microbial community was more related to the community in (predated) protein-rich substrates than to the community in predated or endogenously respirated activated sludge samples. This similar microbial community could be due to microbial utilisation of protein hydrolysis products. Alternatively, conditions in the worm gut could facilitate a protein hydrolysing community which assists in protein hydrolysis. The genera Burkholderiales, Chryseobacterium and Flavobacterium were found to be associated with predation by T. tubifex.

[Characteristics of a Combined Heterotrophic and Sulfur Autotrophic Denitrification Technology for Removal of High Nitrate in Water]

A combined heterotrophic and sulfur autotrophic denitrification technology for NO₃^--N wastewater treatment was started up by adding elemental sulfur in the heterotrophic denitrifying reactor, and the characteristics of pH constant and sludge reduction were studied. The results showed that the sulfur autotrophic denitrification bacteria in heterotrophic denitrifying reactor could achieve rapid growth. After running for 65d, TOC/N was controlled between 0.65 and 0.75, and the combined denitrification process did not require external alkalinity supplementation as the alkalinity need of autotrophic denitrifiers was supplemented by the heterotrophic denitrifiers. After running for 116d, the total nitrogen removal rate reached above 85%, and the denitrification efficiency was kept steady at 2.5 kg·(m³·d)^-1. Compared to heterotrophic denitrification, the sludge production was greatly reduced, which was only 60% of that produced in combined heterotrophic and sulfur autotrophic denitrification reactor. NO₂^--N accumulated by using collaborative denitrification treatment of high concentration of NO₃^--N wastewater, the concentration reached 20 mg·L^-1 even at the eventual plateau stage, which would require deep processing.

Unravelling the protein preference of aquatic worms during waste activated sludge degradation

Worm predation (WP) by Tubifex tubifex was investigated using waste activated sludge (WAS) as the substrate. In order to better understand the sludge degradation mechanisms during WP, the activity of five common hydrolytic enzymes was determined and compared among the initial feed activated sludge, endogenous respirated sludge and worm predated sludge. The results showed that the enzymatic activity decreased upon aerobic (worm) treatment of WAS and that this activity was predominantly associated with the removed solids fraction of the sludge. Interestingly, the protease activity showed a smaller decrease in activity when the worms were present. Flow cell cytometry revealed the release of intestinal bacteria from the worms, which are presumed to be largely responsible for the observed protease activity. Additionally, experiments in which T. tubifex were treated with antibiotics showed that the worms are responsible for a maximum of 73% of the observed proteolytic activity. The remaining 27% is attributed to the intestinal bacteria that exhibit a synergistic relationship with T. tubifex towards protein hydrolysis.

[Nitrite Type Denitrifying Phosphorus Removal Capacity of Cycle Activated Sludge Technology Processes Under Different Inducing Patterns]

A modified cyclic activated sludge technology (CAST) reactor was utilized to investigate the phosphorus and nitrogen removal performance under different inducing patterns in this experiment. The results show that nitrite addition under anoxic conditions has a more inhibitory effect on the denitrifying phosphorus removal performance of the sludge. The phosphorus removal performance of the system was least effective when nitrite dosage was 5 mg·L^-1. Compared to an anoxic addition system, the CAST system is more stable under aerobic addition conditions. The phosphorus removal properties have a slight fluctuation during each initial operating condition when the nitrite concentrations are 5, 10 and 15 mg·L^-1, respectively. However, the phosphorus removal rate was observed to recover quickly and remain stable at more than 95% after acclimatizing for 10, 6, and 34 days, respectively. The effluent phosphorus concentration was less than 0.5 mg·L^-1 in all cases. It was also found that the phosphorus removal performance deteriorated drastically when the nitrite dosage was 20 mg·L^-1. Nevertheless, the nitrite type denitrifying phosphorus uptake capacity of the sludge was 10.4 times greater than that of the sludge before acclimatizing, suggesting that the phosphorus performance deterioration due to nitrite addition could be relieved and long-term addition is beneficial to enriching denitrifying phosphorus accumulating bacteria using NO₂^- as an electron acceptor. Moreover, the sludge settling performance was found to be effective and the sludge concentration decreased continuously when adding a certain concentration of nitrite under aerobic conditions, which is of significant for sludge reduction.

Effect of intermittent aeration strategies on treatment performance and microbial community of an IFAS reactor treating municipal waste water

This study investigated the effect of various intermittent aeration (IA) cycles on organics and nutrient removal, and microbial communities in an integrated fixed-film activated sludge (IFAS) reactor treating municipal waste water. Average effluent biological oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids, total nitrogen (TN) and total phosphorus (TP) values were noted as 20, 50, 30, 12 and 1.5 mgL^-1, respectively, in continuous aeration mode. A total of four operational conditions (run 1, continuous aeration; run 2, 150/30 min aeration on/off time; run 3, 120/60 min aeration on/off time and run 4, 90/60 min aeration on/off time) were investigated in IFAS reactor assessment. Among the all examined IA cycles, IA phase 2 gave the maximum COD and BOD removals with values recorded as 97% and 93.8%, respectively. With respect to nutrient removal (TN and TP), IA phase 1 was found to be optimum. Pathogen removal efficiency of present system was recorded as 90-95% during the three phases. With regard to settling characteristics, pilot showed poor settling during IA schedules, which was also evidenced by high sludge volume index values. Overall, IA could be used as a feasible way to improve the overall performance of IFAS system.

Reduction of excess sludge in a sequencing batch reactor by lysis-cryptic growth using quick lime for disintegration under low temperature

In the present study, quick-lime-based thermal-alkaline sludge disintegration (SD) under low temperature was combined with cryptic growth to investigate the excess sludge reduction efficiency in the sequencing batch reactor (SBR). The optimized condition of SD was as follows: T = 80℃, pH = 11, t = 180 min, and the SD rate was about 42.1%. With 65.6% of excess sludge disintegrated and returned to the SBR, the system achieved sludge reduction rate of about 40.1%. The lysis-cryptic growth still obtained satisfactory sludge reduction efficiency despite the comparative low SD rate, which suggested that disintegration rate might not be the decisive factor for cryptic-growth-based sludge reduction. Lysis-cryptic growth did not impact the effluent quality, yet the phosphorus removal performance was enhanced, with effluent total phosphorus concentration decreased by 0.3 mg/L (33%). Crystal compounds of calcium phosphate precipitate were detected in the system by Fourier transform infrared spectroscopy and X-ray diffraction, which indicated the phosphorus removal potential of SD using lime. Moreover, endogenous dehydrogenase activity of activated sludge in the lysis-cryptic system was enhanced, which was beneficial for sludge reduction. SD and cryptic growth in the present study demonstrates an economical and effective approach for sludge reduction.

Physical and biochemical changes in sludge upon Tubifex tubifex predation

Worm predation (WP) on activated sludge leads to increased sludge degradation rates, irrespective of the type of worm used or reactor conditions employed. However, the cause of the increased sludge degradation rates remains unknown. This paper presents a comparative analysis of the physical and biochemical aspects of predated sludge, providing insight into the hydrolytic mechanisms underlying WP. To this end, the sessile worm Tubifex tubifex was used as a model oligochaete and was batch cultivated in an 18-L airlift reactor. Predation on activated sludge showed an average reduction rate of 12 ± 3.8%/d versus 2 ± 1.3%/d for endogenous respirated sludge. Sludge predation resulted in an increased release of inorganic nitrogen, phosphate and soluble chemical oxygen demand (sCOD). The sCOD consisted mainly of polysaccharides; however, fluorescence excitation emission matrix spectroscopy analysis also revealed the presence of Tryptophan-protein-like substances. Results suggest that the released polysaccharides contain a protein-like element. Additionally, soluble iron increased slightly in concentration after WP. The extent of hydrolysis seemed to reach an average plateau of about 40% volatile solids (VS) reduction after 4 days, which is substantially higher than the 29% VS reduction for endogenous decay of activated sludge after 30 days. Furthermore, T. tubifex predominantly consumed the protein fraction of the extracellular polymeric substances. Results suggest that that the worms specifically target a fraction of the sludge that is predominantly biodegradable under aerobic conditions, albeit at significantly higher degradation rates when compared to the endogenous decay of waste activated sludge.

Sludge reduction using aquatic worms under different aeration regimes

Adding aquatic worms to a wastewater treatment system can reduce sludge production through predation. The aeration level is crucial for success. To evaluate aeration impacts on sludge reduction and determine an optimal aeration regime, this study investigated the processes of in-situ sludge reduction, using aquatic worms exposed to different aeration levels. The experiment also compared treatment results between a conventional reactor and an aquatic worm reactor (WR). Results indicated that the recommended concentration of dissolved oxygen (DO) was 2.5 mg L^-1. The removal rate of chemical oxygen demand remained steady at 80% when the DO concentration was higher than 2.5 mg L^-1, while the removal rate of ammonia nitrogen continued to moderately increase. Increasing the DO concentration to 5 mg L^-1 did not improve sludge reduction, and consumed more power. With a DO concentration of 2.5 mg L^-1 and a power of 0.19 kWh t^-1 water, the absolute sludge reduction and relative sludge reduction rates in the WR were 60.0% and 45.7%, respectively, and the daily aquatic worm growth rate was 0.150 d^-1 during the 17-d test. Therefore, at the recommended aeration regime, aquatic worms reduced the sludge without increasing the power consumption or deteriorating the effluent.

SRT contributes significantly to sludge reduction in the OSA-based activated sludge process

Though activated sludge systems have contributed significantly to the control of hygiene of our society, the wastewater treatment generates large amount of excess sludge. The oxic-settling-anaerobic (OSA)-based biological processes have been shown to be promising approaches for sludge reduction during wastewater treatment. However, the sludge reduction mechanism is still unclear. Four conditions were examined to clarify the sludge reduction mechanism in the OSA-based process. Sludge retention time (SRT) was the main 'contributor' to sludge reduction. The sludge reduction percentage of the process with side hydrolysis and acidification was 42%, with the contribution by long SRT of 33%, energy uncoupling of 7.7%, and hydrolysis/acidification of 1.1%. In addition, the sludge reduction in the OSA-based process had no obvious impact on the efficiency of nutrient removal. The clarified mechanism for sludge reduction in the OSA-based process could provide valuable clue for future system optimization.

Phosphorus removal performance and population structure of phosphorus-accumulating organisms in HA-A/A-MCO sludge reduction process

We developed a new sludge reduction HA-A/A-MCO (Hydrolysis-Acidogenosis-Anaerobic/Anoxic -Multistep Continuous Oxic tank) process, which has improved phosphate (P) and nitrogen (N) removal. Its biological treatment unit uses an A²/O P & N removal process with hydrolysis acidification, multistep continuous aeration, and continuous flow, coupled with sidestream P removal by draining out anaerobic P-bearing wastewater. The process has advanced synchronization of P and N removal and sludge reduction. The improved performance is closely associated with the population structure of P-accumulating organisms (PAOs). This study investigated the relationship between P removal performance and the population structure of PAOs. The results show that the average effluent P content of HA-A/A-MCO process was only 0.44 mg/L, when the influent P concentration was 8∼12 mg/L. The effluent met the A standard set by GB18918-2002. PAOs were able to effectively release 1 mg of P and absorb 2.8 mg of P. The system removed P by draining out anaerobic P-rich wastewater, as P had been reduced in the aerobic absorption process. This reduced the need for excess P uptake ability of the PAOs. The bacterial pure culture method was applied to isolate 5 PAOs with typical P absorption and removel features. 16SrDNA amplification and sequence analysis revealed that Acinetobacter sp. and Lampropedia sp played dominant roles in anaerobic P-releasing process. Moreover, Devosia sp. and Bdellovibrio sp were the primary strains in the aerobic tank, and, they were the major stains for P absorption. Uncultured Bacterium and other uncultured strains were detected in the anoxic tank.

Study of the diversity of microbial communities in a sequencing batch reactor oxic-settling-anaerobic process and its modified process

To further reveal the mechanism of sludge reduction in the oxic-settling-anaerobic (OSA) process, the polymerase chain reaction - denaturing gradient gel electrophoresis protocol was used to study the possible difference in the microbial communities between a sequencing batch reactor (SBR)-OSA process and its modified process, by analyzing the change in the diversity of the microbial communities in each reactor of both systems. The results indicated that the structure of the microbial communities in aerobic reactors of the 2 processes was very different, but the predominant microbial populations in anaerobic reactors were similar. The predominant microbial population in the aerobic reactor of the SBR-OSA belonged to Burkholderia cepacia, class Betaproteobacteria, while those of the modified process belonged to the classes Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. These 3 types of microbes had a cryptic growth characteristic, which was the main cause of a greater sludge reduction efficiency achieved by the modified process.

Reduction of the excess sludge production by biological alternating process: real application results and metabolic uncoupling mechanism

The biological solution proposed to reduce the wasted sludge production is based on a process of alternating phases realized in a specific reactor (alternate cycles in sludge line (ACSL)) where a quote of the recycle sludge is treated and sent back to the main activated sludge process. The ACSL process was applied in two urban wastewater treatment plants (WWTPs). The reduction was tested by changing the hydraulic retention time and the conditions of oxidation reduction potential. The main mechanism of the process is recognized in the metabolic uncoupling. In fact, an increase in the specific oxygen uptake rate in the biological reactors was recorded (up to 20 mg/g VSS/h), which was stimulated by the fasting condition in the ACSL. The process is able to reduce the observed sludge yield on average of 25-30% with final average values reaching 0.179 kg VSS/kg chemical oxygen demand (COD) for WWTP1 and 0.117 kg VSS/kg COD for WWTP2.

Enhanced sludge reduction in septic tanks by increasing temperature

Septic tanks in most developing countries are constructed without drainage trenches or leaching fields to treat toilet wastewater and /or grey water. Due to the short hydraulic retention time, effluents of these septic tanks are still highly polluted, and there is usually high accumulation of septic tank sludge or septage containing high levels of organics and pathogens that requires frequent desludging and subsequent treatment. This study aimed to reduce sludge accumulation in septic tanks by increasing temperatures of the septic tank content. An experimental study employing two laboratory-scale septic tanks fed with diluted septage and operating at temperatures of 40 and 30°C was conducted. At steady-state conditions, there were more methanogenic activities occurring in the sludge layer of the septic tank operating at the temperature of 40°C, resulting in less total volatile solids (TVS) or sludge accumulation and more methane (CH4) production than in the unit operating at 30°C. Molecular analysis found more abundance and diversity of methanogenic microorganisms in the septic tank sludge operating at 40°C than at 30°C. The reduced TVS accumulation in the 40°C septic tank would lengthen the period of septage removal, resulting in a cost-saving in desluging and septage treatment. Cost-benefit analysis of increasing temperatures in septic tanks was discussed.

Microbial community structure of wastewater treatment subjected to high mortality rate due to ozonation of return activated sludge

AIMS

This study investigated the effects of return activated sludge (RAS) ozonation, on the bacterial community structure of pilot-scale wastewater treatment systems.

METHODS AND RESULTS

Two parallel activated sludge reactors were operated to treat real municipal wastewater for 98 days. The RAS of one of the reactors was subjected to increasing doses of ozone during the experimental period, which resulted in higher reduction in biosolids waste production and higher bacterial growth rate. The bacterial community structures were investigated by 16S rRNA gene amplicon high-throughput pyrosequencing and fluorescence in situ hybridization (FISH). The structures remained highly similar throughout the experiment despite the ozone treatment. Comparative analyses between pyrosequencing and FISH revealed clear discrepancies in the proportion of some bacterial populations.

CONCLUSIONS

The results suggest that RAS ozonation is not a main environmental factor structuring the community composition. Instead, the parallel drifts and slight convergence of the two community structures indicate that other environmental factors such as influent wastewater composition and temperature may be more important. Care should be exercised in interpreting the proportion of sequence reads as pyrosequencing may be biased as compared to FISH.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides new insights on the importance of indiscriminate high mortality rates brought by external factors (here ozonation) on microbial community structures of activated sludge system.