Enhancement on biodegradation and anaerobic digestion efficiency of activated sludge using a dual irradiation process

A dual irradiation process involving aerobic thermophilic irradiation pretreatment (ATIP) and intermittent irradiation anaerobic digestion was developed to improve the digestion of waste-activated sludge. First, the effect of ATIP on further anaerobic digestion of activated sludge in batch mode was investigated. When exposed to ATIP for 24 h, the digestion reactor gave the highest methane yield, removed the most dissolved organic carbon (DOC) and showed the most effective reduction of VS compared to other irradiation times. This process was further enhanced by using an anaerobic fluidised-bed reactor packed with carbon felt in semi-continuous mode for digesting the pretreated activated sludge under intermittent irradiation conditions. Dual irradiation for 24 h followed by 60 min of anaerobic irradiation processing per day turned out to be optimal. This resulted in 65.3% of VS reduction, 83.9% of DOC removal ratio and 538 ml/g-VS of methane yield.     

Improvement of sludge digestate biodegradability by thermophilic bioaugmentation

The sludge digestate stabilized by mesophilic anaerobic digestion was further degraded through thermophilic anaerobic digestion using 0–10 % (v/v) of thermophilic, proteolytic Coprothermobacter proteolyticus, and/or methanogenic granular sludge. The results demonstrated that the temperature shift to thermophilic condition promoted abiotic solubilization of proteins and reactivated the fermentative bacteria and methanogens indigenous in the sludge digestate, resulting in a final methane yield of 6.25 mmol-CH₄/g-volatile suspended solid (VSS) digestate. The addition of C. proteolyticus accelerated the hydrolysis and fermentation of proteins and polysaccharides in the digestate during the early stage of thermophilic anaerobic digestion and stimulated methane production by syntrophic cooperation with methanogenic granular sludge. In the treatment with granular sludge and inoculated with 10 % (v/v) of C. proteolyticus, a final methane yield of 7 mmol-CH₄/g-VSS digestate was obtained, and 48.4 % proteins and 27.0 % polysaccharides were degraded. The dissolved proteins were contributed by abiotic factor, C. proteolyticus, and indigenous digestate bacteria, respectively, by around 16, 28, and 56 %.     

Enhanced Methane Production from Anaerobic Digestion of Disintegrated and Deproteinized Excess Sludge

To improve biogas yield and methane content in anaerobic digestion of excess sludge from the wastewater treatment plant, the sludge was disintegrated by using various methods (sonication, alkaline and thermal treatments). Since disintegrated sludge contains a high concentration of soluble proteins, the resulting metabolite, ammonia, may inhibit methane generation. Therefore, the effects of protein removal from disintegrated sludge on methane production were also studied. As a result, an obvious enhancement of biogas generation was observed by digesting disintegrated sludge (biogas yield increased from 15 to 36 ml/g COD_added·day for the raw excess sludge and the sonicated sludge, respectively). The quality of biogas was also improved by removing proteins from the disintegrated sludge. About 50% (w/w) of soluble proteins were removed from the suspension of disintegrated sludge by salting out using 35 g MgCl₂·6H₂O/l and also by isoelectric point precipitation at pH 3.3. For deproteinized sludge, methane production increased by 19%, and its yield increased from 145 ml/g COD_removed to 325 ml/g COD_removed. Therefore, the yield and quality of biogas produced from digestion of excess sludge can be enhanced by disintegrating the sludge and subsequent protein removal. Revisions requested 14 November 2005; Revisions received 13 January 2006     

Ultrasonic sludge disintegration for enhanced methane production in anaerobic digestion: effects of sludge hydrolysis efficiency and hydraulic retention time

Hydrolysis of waste activated sludge (WAS) has been regarded as the rate limiting step of anaerobic sludge digestion. Therefore, in this study, the effect of ultrasound and hydraulic residence time during sludge hydrolysis was investigated with the goal of enhancing methane production from anaerobic digestion (AD). WAS was ultrasonically disintegrated for hydrolysis, and it was semi-continuously fed to an anaerobic digesters at various hydraulic retention times (HRTs). The results of these experiments showed that the solids and chemical oxygen demand (COD) removal efficiencies when using ultrasonically disintegrated sludge were higher during AD than the control sludge. The longer the HRT, the higher the removal efficiencies of solids and COD, while methane production increased with lower HRT. Sludge with 30% hydrolysis produced 7 × more methane production than the control sludge. The highest methane yields were 0.350 m(3)/kg volatile solids (VS)(add) and 0.301 m(3)/kg COD(con) for 16 and 30% hydrolyzed sludge, respectively. In addition, we found that excess ultrasound irradiation may inhibit AD since the 50% hydrolyzed sludge produced lower methane yields than 16 and 30% hydrolyzed sludge.     

Effect of Iron Hydroxide on Phosphate Removal during Anaerobic Digestion of Activated Sludge

The addition of iron (III) hydroxide during methanogenic digestion of activated sludge by anaerobic sludge displaying an iron-reducing activity resulted in a microbial reduction of iron (III) with the formation of iron (II), capable of precipitating phosphates. The feasibility of eliminating 66.6 to 99.6% of the dissolved phosphate at initial concentrations of 1000 to 3500 mg PO^3- ₄/l by adding 6420 mg/l iron (III) hydroxide into a reactor for anaerobic fermentation of activated sludge was analyzed. The optimal ratio of iron (III) added to dissolved phosphate removed (mg) providing a 95% removal amounted to 2 : 1. These results may be used in new technology for anaerobic wastewater treatment with phosphate removal.     

Aerobic and two-stage anaerobic-aerobic sludge digestion with pure oxygen and air aeration

The degradability of excess activated sludge from a wastewater treatment plant was studied. The objective was establishing the degree of degradation using either air or pure oxygen at different temperatures. Sludge treated with pure oxygen was degraded at temperatures from 22 degrees C to 50 degrees C while samples treated with air were degraded between 32 degrees C and 65 degrees C. Using air, sludge is efficiently degraded at 37 degrees C and at 50-55 degrees C. With oxygen, sludge was most effectively degraded at 38 degrees C or at 25-30 degrees C. Two-stage anaerobic-aerobic processes were studied. The first anaerobic stage was always operated for 5 days HRT, and the second stage involved aeration with pure oxygen and an HRT between 5 and 10 days. Under these conditions, there is 53.5% VSS removal and 55.4% COD degradation at 15 days HRT - 5 days anaerobic, 10 days aerobic. Sludge digested with pure oxygen at 25 degrees C in a batch reactor converted 48% of sludge total Kjeldahl nitrogen to nitrate. Addition of an aerobic stage with pure oxygen aeration to the anaerobic digestion enhances ammonium nitrogen removal. In a two-stage anaerobic-aerobic sludge digestion process within 8 days HRT of the aerobic stage, the removal of ammonium nitrogen was 85%.     

The occurrence of intestinal parasites in swine slurry and their removal in activated sludge plants

Thirteen intensive pig farms and two activated sludge treatment plants for pig slurry in north-western Spain were studied from April 2005 to June 2006 in order to evaluate the presence of enteric pathogens (Cryptosporidium, Giardia and helminths) and the efficiency with which they were removed. These parasites were present on 53%, 7% and 38% of the farms studied, respectively, with concentrations of 10(4)-10(5) oocysts per litre (/L) for Cryptosporidium, 10(3)cysts/L for Giardia and 10(2)-10(3) eggs/L for helminths. The overall removal of parasites in the pig slurry treatment plants ranged from 86.7% to over 99.99%. The results revealed a constant reduction at each stage of the treatment system, with activated sludge processes being the most effective treatment in reducing pathogens in pig slurry, 78-81% for Cryptosporidium oocysts and over 99.9% for helminth eggs. A heat drying procedure for sludge removed 4.3 log units of Cryptosporidium oocysts, demonstrating the excellent effectiveness of this treatment for reducing pathogens in sludge intended to be applied to land.     

A hybrid anaerobic membrane bioreactor coupled with online ultrasonic equipment for digestion of waste activated sludge

Anaerobic membrane bioreactor and online ultrasonic equipment used to enhance membrane filtration were coupled to form a hybrid system (US-AnMBR) designed for long-term digestion of waste activated sludge. The US-AnMBR was operated under volatile solids loading rates of 1.1-3.7 gVS/L·d. After comprehensive studies on digestion performance and membrane fouling control in the US-AnMBR, the final loading rate was determined to be 2.7 gVS/L·d with 51.3% volatile solids destruction. In the US-AnMBR, the improved digestion was due to enhanced sludge disintegration, as indicated by soluble matter comparison in the supernatant and particle size distribution in the digested sludge. Maximum specific methanogenic activity revealed that ultrasound application had no negative effect on anaerobic microorganisms. Furthermore, implementing ultrasound effectively controlled membrane fouling and successfully facilitated membrane bioreactor operation. This lab-scale study demonstrates the potential feasibility and effectiveness of setting up a US-AnMBR system for sludge digestion.     

Studies on the effect of inoculation of activated sludge with bacteria actively degrading hydrocarbons on the biodegradation of petroleum products

Eighteen strains of bacteria were isolated from activated sludge purifying petroleum-refining wastewaters. These strains were plated on solidified mineral medium supplemented with oil fraction in concentration 1000 mg/l. Four of the strains that grew best in the presence of oil were selected for further studies. The strains were identified based on Bonde's scheme and microscopic observations. Three of them belonged to the genus Arthrobacter and one to the genus Micrococcus. Stationary cultures of single strains and their mixtures were set up in mineral medium containing oil (sterile and non-sterile) as sole carbon source in concentration 1000 mg/l. The oils were found to be removed the most efficiently by a mixture of the strains. After 14 days of culture the amount of oil was utilized by from 63 to 95%. In the next stage of the studies the bacteria were used to inoculate activated sludge. Stationary cultures of the activated sludge were set up in mineral medium with oil. The utilisation of petroleum products by non-inoculated activated sludge (control), activated sludge inoculated with a single strain or a mixture of all four strains was examined. In both inoculated activated sludge cultures approximately 80% of the oils were removed, compared to 60% in the control activated sludge. Therefore, inoculated activated sludge showed 20% higher effectiveness of removal of petroleum derivatives.     

Stabilization of waste-activated sludge through the anoxic-aerobic digestion process

During the aerobic digestion process, the nitrogen which had been embedded in the activated sludge is solubilized to form ammoniacal and nitric nitrogen which are in turn transferred to the liquor and cause the increase of nitrogen loading in the sewage treatment plant. In this study, the anoxic-aerobic sludge digestion system which is a modified form of the conventional aerobic sludge digestion is made up of aerobic and anoxic tanks and are designed to remove both the volatile suspended solids and the total nitrogen (TN) simultaneously. The removal efficiencies of both VSS and TN were investigated by feeding waste-activated sludge continuously and semicontinuously. The maximum percent reduction of both VSS and TN was achieved at a Q(r)/Q(s) ratio of 2 in the continuous process. The semicontinuous process was used to improve the nitrogen removal efficiency further. In the semicontinuous process, the VSS reduction efficiency as well as the nitrogen removal efficiency increased remarkably under a constant Q(r)/Q(s) ratio of 2. This process also achieved a VSS reduction efficiency higher than the aerobic digestion process (control). It was suggested that the additional anoxic tank enhanced the sludge digestion. Furthermore, the anoxic-aerobic digestion system can be applied to other treatment media like the primary sludge, industrial sludge, animal manure, etc.     

Improved phosphate removal by selective sludge discharge in aerobic granular sludge reactors

Two lab-scale aerobic granular sludge sequencing batch reactors were operated at 20 and 30°C and compared for phosphorus (P) removal efficiency and microbial community composition. P-removal efficiency was higher at 20°C (>90%) than at 30°C (60%) when the sludge retention time (SRT) was controlled at 30 days by removing excess sludge equally throughout the sludge bed. Samples analyzed by fluorescent in situ hybridization (FISH) indicated a segregation of biomass over the sludge bed: in the upper part, Candidatus Competibacter phosphatis (glycogen-accumulating organisms--GAOs) were dominant while in the bottom, Candidatus Accumulibacter phosphatis (polyphosphate-accumulating organisms--PAOs) dominated. In order to favour PAOs over GAOs and hence improve P-removal at 30°C, the SRT was controlled by discharging biomass mainly from the top of the sludge bed (80% of the excess sludge), while bottom granules were removed in minor proportions (20% of the excess sludge). With the selective sludge removal proposed, 100% P-removal efficiency was obtained in the reactor operated at 30°C. In the meantime, the biomass in the 30°C reactor changed in color from brownish-black to white. Big white granules appeared in this system and were completely dominated by PAOs (more than 90% of the microbial population), showing relatively high ash content compared to other granules. In the reactor operated at 20°C, P-removal efficiency remained stable above 90% regardless of the sludge removal procedure for SRT control. The results obtained in this study stress the importance of sludge discharge mainly from the top as well as in minor proportions from the bottom of the sludge bed to control the SRT in order to prevent significant growth of GAOs and remove enough accumulated P from the system, particularly at high temperatures (e.g., 30°C).     

Nitrification of swine waste

Complete oxidation of ammonia nitrogen (approximately 1000 mg/L) to nitrite was observed in stabilized swine waste after 49 days in incubation at 400 rpm and 29 degrees C, only if 10% (v/v) activated sludge from a wastewater treatment unit and 1.5% (w/v) CaCO3, were added. Stabilized swine waste contains less than 0.09 most probable number (MPN) per millilitre of nitrosobacteria and 2.3 MPN/mL of nitrobacteria. In activated sludge, the concentrations of these bacteria were 2.4 MPN/mL for nitrosobacteria and 4.2 x 10(5) MPN/mL for nitrobacteria. In the swine waste where ammonia was oxidized to nitrite, the nitrosobacteria growth increased to 5.5 x 10(5) MPN/mL, while the nitrobacteria growth decreased to 2.3 MPN/mL. Inoculation of a freshly stabilized swine waste with 10% (v/v) of the active nitrifying waste and addition of 1.5% (w/v) CaCO3, accelerated the oxidation of ammonia nitrogen to nitrite; the reaction was completed after only 5 days of incubation. Increasing the incubation period to 10 days resulted in the complete oxidation of the accumulated nitrite to nitrate. In the stabilized swine waste, complete nitrification without accumulation of nitrite was obtained in only 5 days of incubation when the waste was inoculated with both enriched nitrifying populations (10(6)-10(7) MPN/mL).     

Production of biodegradable plastics from activated sludge generated from a food processing industrial wastewater treatment plant

Most of the excess sludge from a wastewater treatment plant (60%) is disposed by landfill. As a resource utilization of excess sludge, the production of biodegradable plastics using the sludge has been proposed. Storage polymers in bacterial cells can be extracted and used as biodegradable plastics. However, widespread applications have been limited by high production cost. In the present study, activated sludge bacteria in a conventional wastewater treatment system were induced, by controlling the carbon: nitrogen ratio to accumulate storage polymers. Polymer yield increased to a maximum 33% of biomass (w/w) when the C/N ratio was increased from 24 to 144, where as specific growth yield decreased with increasing C/N ratio. The conditions which are required for the maximum polymer accumulation were optimized and are discussed.     

Strategy for minimization of excess sludge production from the activated sludge process

Increased attention has been given to minimization of sludge production from activated sludge process since environmental regulations are being more and more stringent in relation to excess sludge disposal. In a biological process, the more organic carbon utilized in carbon dioxide production, the fewer sludge produced, and vice versa. This paper, therefore, reviews strategies developed for minimization of excess sludge production, such as oxic-settling-anaerobic process, high dissolved oxygen process, uncoupler-containing activated sludge process, ozonation-combined activated sludge process, control of sludge retention time and biodegradation of sludge in membrane-assisted reactor. In these modified activated sludge processes, excess sludge production can be reduced by 20-100% without significant effect on process efficiency and stability. It is expected that this paper would be helpful for researchers and engineers to develop novel and efficient operation strategy to minimize sludge production from biological systems.     

Laboratory-scale ultrasound pre-treated digestion of sludge: Heat and energy balance

The objective of this work was to maximize the digestibility of biological sludge to elucidate the feasibility of a new sludge management strategy to recover good quality sludge for agricultural use. The combined effects of organic loading rates (from 0.7 to 2.8 g VS L⁻¹ d⁻¹) and the degree of disintegration by anaerobic digestion of sonicated activated sludge were discussed, and the thermal and energetic balances were evaluated. Despite low sonication inputs, sludge digestion performance improved in terms of solids degradation and biogas production depending on the soluble organic load. The biogas production by sonicated sludge was higher (up to 30%) with respect to the control. Filterability improved during digestion of sonicated sludge at medium OLR due to a significant abatement of the fines. Thermal balances indicated that sonication may be a proper system to guarantee self-sustaining WAS mesophilic digestion. Nevertheless, thickening is a pre-requisite to achieve a positive energy balance.     

厌氧消化污泥微生物组研究方法及应用

污泥厌氧消化是在消化污泥微生物组的协调下将剩余污泥中有机物转化为甲烷的微生物过程。与传统厌氧消化过程不同,污泥厌氧消化系统的进料底物为含有大量微生物细胞及胞外多聚物等复杂大分子有机物的剩余污泥。因此,厌氧消化污泥微生物组的种群组成、功能及种群间互作关系等异常复杂,使厌氧消化污泥微生物组分析成为难点问题。但近年来高通量测序技术及生物信息学分析方法的快速发展为消化污泥微生物组研究提供了契机,并迅速推动了该研究领域的发展。本文从4个方面梳理、总结厌氧消化污泥微生物组的研究及应用现状:剩余活性污泥结构、组成及其厌氧消化;基于16SrRNA基因序列测序的微生物组研究;基于宏基因组及宏转录组分析的微生物组研究;厌氧消化污泥微生物组研究案例分析。最后我们提出了厌氧消化污泥微生物组研究亟待解决的关键科学问题。     

Nutrient removal in an A2O-MBR reactor with sludge reduction

In the present study, an advanced sewage treatment process has been developed by incorporating excess sludge reduction and phosphorous recovery in an A2O-MBR process. The A2O-MBR reactor was operated at a flux of 17 LMH over a period of 210 days. The designed flux was increased stepwise over a period of two weeks. The reactor was operated at two different MLSS range. Thermo chemical digestion of sludge was carried out at a fixed pH (11) and temperature (75 °C) for 25% COD solubilisation. The released phosphorous was recovered by precipitation process and the organics was sent back to anoxic tank. The sludge digestion did not have any impact on COD and TP removal efficiency of the reactor. During the 210 days of reactor operation, the MBR maintained relatively constant transmembrane pressure. The results based on the study indicated that the proposed process configuration has potential to reduce the excess sludge production as well as it didn’t detoriated the treated water quality.     

Investigations on cellulose biodegradation in activated sludge plants

Cellulose is the major carbon substrate entering treatment plants for municipal waste waters. In the present investigation an attempt was made to study its degradation in activated sludge. Cellulolytic micro-organisms were enumerated in different treatment plants and at one plant they were assessed after different steps over a period of about 1 year. The degradation of cellulose contained in Nylon bags suspended in the mixed liquor was also studied and the activities of cellulase components were assayed. Finally, the concentrations of cellulose and lignin in the suspended solids taken from different treatment steps were determined. The results showed that active cellulolysis occurred in activated sludge. The degradation was mainly bacterial, although no significant enrichment of such bacteria was found in the sludge floc. Cellulase activity, however, showed an increase. Experiments with the Nylon bag indicated that 60% of the weight of cotton wool was degraded in 4–5 weeks. It was concluded that about 60% (w/w) of the cellulose entering the system could be degraded by bacteria during aerobic treatment, while 50–60% of that present in the surplus activated sludge was degraded during anaerobic sludge digestion.     

导电碳颗粒促进污泥厌氧消化及微生物种间电子传递的研究进展

添加导电碳颗粒能够促进厌氧消化过程稳定性、底物降解率以及产沼气品质的同步提高。本文总结了以活性炭和生物炭为代表的导电碳颗粒对城市污泥厌氧消化的影响,探讨了导电碳颗粒促进城市污泥厌氧消化的机理,阐述了导电碳颗粒介导的微生物直接种间电子传递(Directinterspecies electrontransfer,DIET)在强化污泥厌氧消化中的作用机制,分析了复杂厌氧消化体系中微生物DIET互营关系的研究现状,同时对导电碳颗粒的物理化学特性及其对污泥厌氧消化产甲烷的影响进行了分析,最后对未来导电碳颗粒促进城市污泥厌氧消化的研究进行了展望。     

Sludge minimisation technologies

The treatment and disposal of excess sludge represents a bottleneck of wastewater treatment plants all over the world, due to environmental, economic, social and legal factors. There is therefore a growing interest in developing technologies to reduce the wastewater sludge generation. The goal of this paper is to present the state-of-the-art of current minimisation techniques for reducing sludge production in biological wastewater treatment processes. An overview of the main technologies is given considering three different strategies: The first option is to reduce the production of sludge by introducing in the wastewater treatment stage additional stages with a lower cellular yield coefficient compared to the one corresponding to the activated sludge process (lysis-cryptic growth, uncoupling and maintenance metabolism, predation on bacteria, anaerobic treatment). The second choice is to act on the sludge stage. As anaerobic digestion is the main process in sewage sludge treatment for reducing and stabilising the organic solids, two possibilities can be considered: introducing a pre-treatment process before the anaerobic reaction (physical, chemical or biological pre-treatments), or modifying the digestion configuration (two-stage and temperature-phased anaerobic digestion, anoxic gas flotation). And, finally, the last minimisation strategy is the removal of the sludge generated in the activated sludge plant (incineration, gasification, pyrolysis, wet air oxidation, supercritical water oxidation).