Fate of pathogens in thermophilic aerobic sludge digestion

The effect of autoheated aerobic thermophilic digestion on the pathogen content of sewage sludges was studied and compared to that of conventional mesophilic anaerobic digestion. Both systems were full scale, continuously-fed facilities operated in parallel and utilized a feed sludge of thickened primary and waste-activated sludge.The relative populations of viruses, Salmonella sp., total and fecal coliforms, fecal streptococci and parasites found before and after digestion were compared. The full scale mesophilic anaerobic digesters were operated at relatively constant conditions, i.e. digester temperature constant at 35°C, and loading rates constant, etc., while the full scale autoheated aerobic digester was operated under a wide range of loading conditions. At all of the conditions studied, the autoheated digester temperature exceeded 45 C. Virus and Salmonella sp. concentrations in the effluent from the aerobic unit were below detectable limits in 10 of 11 samples and 6 of 6 samples, respectively, whereas the anaerobic digester effluent contained detectable numbers of viruses and Salmonella sp. Bacterial indicator counts and parasite concentrations were less in the autoheated digester effluent than in the effluent from the anaerobic digester. It was concluded that the simple autoheated aerobic digestion process could be used to produce a virtually pathogen-free sludge at a cost comparable to that of conventional, mesophilic anaerobic digestion.     

Isolation, identification and utilization of thermophilic strains in aerobic digestion of sewage sludge

Two representative thermophilic bacterial strains (T1 and T2) were isolated from a one-stage autothermal thermophilic aerobic digestion pilot-scale reactor. 16S rRNA gene analysis indicated that they were Hydrogenophilaceae and Xanthomonodaceae. These isolated strains were inoculated separately and/or jointly in sewage sludge, to investigate their effects on sludge stabilization under thermophilic aerobic digestion condition. Four digestion conditions were tested for 480 h. Digestion without inoculation and inoculation with strain T2, as well as joint- inoculation with strains T1 and T2, achieved 32.6%, 43.0%, and 38.2% volatile solids (VS) removal, respectively. Removal in a digester inoculated with stain T1 only reached 27.2%. For the first 144 h, the three inoculated digesters all experienced higher VS removal than the digester without inoculations. Both specific thermophilic strains and micro-environment significantly affected the VS removal. DGGE profiles revealed that the isolated strains T1 and T2 can successfully establish in the thermophilic digesters. Other viable bacteria (including anaerobic or facultative microbes) also appeared in the digestion system, enhancing the microbial activity.     

自热式高温好氧消化技术用于污泥处理

系统介绍了自热式高温好氧消化(ATAD)的原理及特点，并详细介绍了ATAD在国外(特别是欧洲和北美)的应用现状及在我国的应用前景。自热式高温好氧消化技术具有反应速度快、停留时间短、病原菌灭活效果好等优点，特别适合于小规模污水处理厂污泥的处理，在我国还没有应用实例。     

Morphological characterisation of ATAD thermophilic sludge; sludge ecology and settleability

Autothermal thermophilic aerobic digestion (ATAD) is a biological wastewater treatment process used for stabilisation of domestic, animal, food and pharmaceutical sludges, and wastewater. It produces a high-quality effluent due to thermophilic processing conditions, however the stabilised sludge has poor settling characteristics, a high water content, low compaction capacity and is difficult to dewater by mechanical processes alone. These factors impact transport and disposal of processed ATAD sludge. We have carried out a detailed morphological characterisation of ATAD sludge at all stages of the ATAD process in an attempt to determine key characteristics of the sludge that might be responsible for its poor dewatering and settleability. A number of microscopic techniques including electron, optical, wide field and laser scanning confocal microscopy were applied to fresh, fixed or embedded sludge taken at various stages during a full scale ATAD process treating domestic sludge. The spatial distributions of structural sludge matrix components were determined and suggested a highly dynamic sludge morphology during the overall process. Large amounts of fibres were observed in the feed sludge, whereas thermophilic sludge liquor with low settleability was shown to have a lower protein to polysaccharide ratio (1:0.9) compared to the easily settled fraction where ratio values were in the range of (1:1.14-1:1.7) with a prevalence of protein constituents. ATAD sludge was also shown to contain colloids, slime, cellulose micro-particles and multiple hydrophobic droplets in the bulk liquor, factors that may markedly impact on sludge dewaterability characteristics. Laser scanning confocal microscopy demonstrated a superior ability to identify composition and spatial localisation of structural constituents in such a dispersed, high water content sludge.     

Evaluation and Development of a Thermophilic Aerobic Digester at Castle Donington Sewage-Treatment Works

An investigation into the practicality of thermophilic aerobic digestion has been in progress at Castle Donington since 1989. The work has been carried out in a glassed steel insulated digester of 123 m³ capacity. Both auto-entraining and blower-assisted venturi systems have been evaluated.
Both aeration systems were able, under favourable conditions, to remove at least 50% of the sludge COD. It was found that the additional flexibility of the blower venturi combination was necessary to accommodate crude sludges varying from 2% to 12% dry solids.
Originally the plant was intended to operate at above 55C with a 10-day retention period. In practice retention periods at this temperature have been at least 15 days. The process has proved relatively susceptible to inhibition and requires the air supply to be closely matched to the load.
The revenue costs have been, at best, around 47.3/tDS. This is competitive with small-scale anaerobic digestion, but not with the large digestion centre approach widely used in the UK. Unless pasteurization becomes a requirement for agricultural disposal, thermophilic aerobic digestion is unlikely to find wide use in this country.     

Phylogenetic analysis of the bacterial community in a full scale autothermal thermophilic aerobic digester (ATAD) treating mixed domestic wastewater sludge for land spread

The bacterial community associated with a full scale autothermal thermophilic aerobic digester (ATAD) treating sludge, originating from domestic wastewater and destined for land spread, was analysed using a number of molecular approaches optimised specifically for this high temperature environment. 16S rDNA genes were amplified directly from sludge with universally conserved and Bacteria-specific rDNA gene primers and a clone library constructed that corresponded to the late thermophilic stage (t = 23 h) of the ATAD process. Sequence analyses revealed various 16S rDNA gene sequence types reflective of high bacterial community diversity. Members of the bacterial community included α- and β-Proteobacteria, Actinobacteria with High G + C content and Gram-Positive bacteria with a prevalence of the Firmicutes (Low G + C) division (class Clostridia and Bacillus). Most of the ATAD clones showed affiliation with bacterial species previously isolated or detected in other elevated temperature environments, at alkaline pH, or in cellulose rich environments. Several phylotypes associated with Fe(III)- and Mn(IV)-reducing anaerobes were also detected. The presence of anaerobes was of interest in such large scale systems where sub-optimal aeration and mixing is often the norm while the presence of large amounts of capnophiles suggest the possibility of limited convection and entrapment of CO₂ within the sludge matrix during digestion. Comparative analysis with organism identified in other ATAD systems revealed significant differences based on optimised techniques. The abundance of thermophilic, alkalophilic and cellulose-degrading phylotypes suggests that these organisms are responsible for maintaining the elevated temperature at the later stages of the ATAD process.     

Sequential polymer dosing for effective dewatering of ATAD sludges

Dewatering problems associated with the sludge from autothermal thermophilic aerobic digestion (ATAD) of sludge, result in large chemical conditioning costs for effective dewatering. A variety of chemical conditioners were used to improve dewatering, but none of them were able to dewater the sludge as desired at acceptable conditioning doses. It was found that during the digestion process, chemical precipitation of divalent cations occurred. Some of the ATAD sludge colloids were found to have a positive zeta potential and these were thought to be the precipitated divalent cations. Sequential polymer dosing using either iron or cationic polymer, followed by anionic polymer, was found to improve dewatering. The use of anionic polymer is essential and allows the use of smaller amounts of iron or cationic polymer for effective dewatering. The use of the less expensive anionic polymer along with cationic polymers has the potential to make the use of the ATAD process more economical.     

Combined anaerobic and aerobic digestion for increased solids reduction and nitrogen removal

A unique sludge digestion system consisting of anaerobic digestion followed by aerobic digestion and then a recycle step where thickened sludge from the aerobic digester was recirculated back to the anaerobic unit was studied to determine the impact on volatile solids (VS) reduction and nitrogen removal. It was found that the combined anaerobic/aerobic/anaerobic (ANA/AER/ANA) system provided 70% VS reduction compared to 50% for conventional mesophilic anaerobic digestion with a 20 day SRT and 62% for combined anaerobic/aerobic (ANA/AER) digestion with a 15 day anaerobic and a 5 day aerobic SRT. Total Kjeldahl nitrogen (TKN) removal for the ANA/AER/ANA system was 70% for sludge wasted from the aerobic unit and 43.7% when wasted from the anaerobic unit. TKN removal was 64.5% for the ANA/AER system.     

Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

The performance of thermophilic and mesophilic temperature co-phase anaerobic digestions for sewage sludge, using the exchange process of the digesting sludge between spatially separated mesophilic and thermophilic digesters, was examined, and compared to single-stage mesophilic and thermophilic anaerobic digestions. The reduction of volatile solids from the temperature co-phase anaerobic digestion system was dependent on the sludge exchange rate, but was 50.7-58.8%, which was much higher than 46.8% of single-stage thermophilic digestion, as well as 43.5% of the mesophilic digestion. The specific methane yield was 424-468 mL CH(4) per gram volatile solids removed, which was as good as that of single-stage mesophilic anaerobic digestion. The process stability and the effluent quality in terms of volatile fatty acids and soluble chemical oxygen demand of the temperature co-phase anaerobic digestion system were considerably better than those of the single-stage mesophilic anaerobic processes. The destruction of total coliform in the temperature co-phase system was 98.5-99.6%, which was similar to the single-stage thermophilic digestion. The higher performances on the volatile solid and pathogen reduction, and stable operation of the temperature co-phase anaerobic system might be attributable to the well-functioned thermophilic digester, sharing nutrients and intermediates for anaerobic microorganisms, and selection of higher substrate affinity anaerobic microorganisms in the co-phase system, as a result of the sludge exchange between the mesophilic and thermophilic digesters.     

The thermophilic anaerobic digestion process

The paper presents a review of the thermophilic anaerobic digestion process, which has been successfully employed in Los Angeles, U.S.A., and Moscow, U.S.S.R. The chief advantages of operating at higher, thermophilic temperatures are lower detention times, improved dewaterability of the sludge, and increased destruction of pathogenic organism. It is anticipated that operation of the thermophilic process will require closer control than is conventionally practised. A dynamic process model is proposed, and the expected response of a thermophilic digester to changes in feed rate, concentration and temperature is illustrated.     

Biological oxidation and disinfection of sludge

Aerobic digestion, which is biological oxidation of sludge, produces heat. Enough heat is liberated to raise the temperature of the wet sludge up to the pasteurizing range (60–70°C). The major heat loss from an aerobic digester is the latent heat of vaporization of the water vapor which is carried of in the waste air. The use of concentrated oxygen to replace air minimizes this heat loss and theoretically permits thermophilic operation if the initial sludge solids are greater than about 4 per cent. Pilot scale studies have shown that thermophilic aerobic digestion is practical and can be carried out in less than 24 h. Capital and operating costs are reduced and the sludge is pasteurized without the use of external heat. The process is a form of high-rate wet composting and produces a sludge ideally suited for disposal on agricultural lands.Recent and current research data from U.S. EPA-sponsored studies on aerobic digestion of primary sludge solids at Hollywood, Florida; mixed primary and waste activated sludge solids at Hamilton, Ohio; and waste activated sludge solids at Denver, Colorado, are presented and critiqued in light of present design methodology and constraints. These data were obtained under a variety of operating conditions. Data and discussion are also given on the use of pure oxygen in digestion of thickened waste activated sludges.     

Investigation of the sludge reduction mechanism in the anaerobic side-stream reactor process using several control biological wastewater treatment processes

To investigate the mechanism of sludge reduction in the anaerobic side-stream reactor (SSR) process, activated sludge with five different sludge reduction schemes were studied side-by-side in the laboratory. These are activated sludge with: 1) aerobic SSR, 2) anaerobic SSR, 3) aerobic digester, 4) anaerobic digester, and 5) no sludge wastage. The system with anaerobic SSR (system #2) was the focus of this study and four other systems served as control processes with different functions and purposes. Both mathematical and experimental approaches were made to determine solids retention time (SRT) and sludge yield for the anaerobic SSR process. The results showed that the anaerobic SSR process produced the lowest solids generation, indicating that sludge organic fractions degraded in this system are larger than other systems that possess only aerobic or anaerobic mode. Among three systems that involved long SRT (system #1, #2, and #5), it was only system #2 that showed stable sludge settling and effluent quality, indicating that efficient sludge reduction in this process occurred along with continuous generation of normal sludge flocs. This observation was further supported by batch anaerobic and aerobic digestion data. Batch digestion on sludges collected after 109 days of operation clearly demonstrated that both anaerobically and aerobically digestible materials were removed in activated sludge with anaerobic SSR. In contrast, sludge reduction in the aerobic SSR process or no wastage system was achieved by removal of mainly aerobically digestible materials. All these results led us to conclude that repeating sludge under both feast/fasting and anaerobic/aerobic conditions (i.e., activated sludge with anaerobic SSR) is necessary to achieve the highest biological solids reduction with normal wastewater treatment performance.     

Effect of ultrasound pretreatment in mesophilic and thermophilic anaerobic digestion with emphasis on naphthalene and pyrene removal

In many anaerobic digestion processes for the treatment of the sludge produced in wastewater treatment plants, the hydrolysis of the organic matter has been identified as the rate limiting step. This study is focused on the effect of ultrasonic pretreatment of raw sewage sludge before being fed to the mesophilic and the thermophilic anaerobic digestion. From particle size reduction, COD disintegration degree and biodegradability test, 11,000kJ/kg TS was estimated as the optimal specific energy in ultrasonic pretreatment. Moreover, the use of pretreated sludge improved significantly the COD removal efficiency and biogas production in lab-scale anaerobic digesters when compared with the performance without pretreatment, specially under mesophilic conditions. During ultrasonic pretreatment, the diffusion of polycyclic aromatic hydrocarbons (PAH) compounds to the aqueous phase was stated by a reduction in the pretreated sludge micropollutants content. With sonication, naphthalene was better removed than without this pretreatment, particularly in the mesophilic digester. However, pyrene removal remained at same efficiency level with and without ultrasonic pretreatment.     

An ASM/ADM model interface for dynamic plant-wide simulation

Mathematical modelling has proven to be very useful in process design, operation and optimisation. A recent trend in WWTP modelling is to include the different subunits in so-called plant-wide models rather than focusing on parts of the entire process. One example of a typical plant-wide model is the coupling of an upstream activated sludge plant (including primary settler, and secondary clarifier) to an anaerobic digester for sludge digestion. One of the key challenges when coupling these processes has been the definition of an interface between the well accepted activated sludge model (ASM1) and anaerobic digestion model (ADM1). Current characterisation and interface models have key limitations, the most critical of which is the over-use of X_c (or lumped complex) variable as a main input to the ADM1. Over-use of X_c does not allow for variation of degradability, carbon oxidation state or nitrogen content. In addition, achieving a target influent pH through the proper definition of the ionic system can be difficult. In this paper, we define an interface and characterisation model that maps degradable components directly to carbohydrates, proteins and lipids (and their soluble analogues), as well as organic acids, rather than using X_c. While this interface has been designed for use with the Benchmark Simulation Model No. 2 (BSM2), it is widely applicable to ADM1 input characterisation in general. We have demonstrated the model both hypothetically (BSM2), and practically on a full-scale anaerobic digester treating sewage sludge.     

Aerobic waste activated sludge (WAS) for start-up seed of mesophilic and thermophilic anaerobic digestion

Since there are very limited numbers of thermophilic anaerobic digesters being operated, it is often difficult to start up a new one using sludge from an existing reactor as a seed. However, for obvious reasons it seems few attempts have been made to compare the start-up performance of thermophilic anaerobic digestion using different sources of seed sludges. The purpose of this study was to evaluate the start-up performance of anaerobic digestion using aerobic waste activated sludge (WAS) from a plant which has no anaerobic digesters and mesophilic anaerobic digested sludge (ADS) as the seed source at both mesophilic (35 degrees C) and thermophilic (55 degrees C) temperatures. In this study, two experiments were conducted. First, thermophilic anaerobic reactors were seeded with WAS (VSS = 4400 mg/L) and ADS (VSS = 14,500 mg/L) to investigate start-up performance with a feed of acetate as well as propionate. The results show that WAS started to produce CH4 soon after acetate feeding without a lag time, while ADS had a lag time of 10 days. When the feed was changed to propionate, WAS removed propionate down to below the detection limit of 10 mg/L, while ADS removed little propionate and produced little CH4. Second, in order to further compare the methanogenic activity of WAS and ADS, both mesophilic and thermophilic reactors were operated. WAS acclimated to anaerobic conditions shortly (< 5 days at both mesophilic and thermophilic) and after acclimating it produced more CH4 per unit amount of seeded VSS than ADS. WAS at mesophilic temperature biodegraded acetate at the same rate as for thermophilic. However WAS at mesophilic temperature biodegraded propionate at a much faster rate than at thermophilic. WAS as the seed source of anaerobic digestion resulted in much better performance than ADS at both mesophilic and thermophilic temperatures for both acetate and propionate metabolism.     

A new plant-wide modelling methodology for WWTPs

This paper presents a new plant-wide modelling methodology for describing the dynamic behaviour of water and sludge lines in WWTPs. The methodology is based on selecting the set of process transformations needed for each specific WWTP to model all unit-process elements in the entire plant. This "transformation-based" approach, in comparison with the conventional "process-based" approach, does not require the development of specific transformers to interface the resulting unit-process models, facilitates the mass and charge continuity throughout the whole plant and is flexible enough to construct models tailored for each plant under study. As an illustrative example, a plant-wide model for a WWTP that includes carbon removal and anaerobic digestion has been constructed, and the main advantages of the proposed methodology for integrated modelling have been demonstrated. As a final consequence, this paper proposes a rewriting of the existing unit-process models according to the new standard transformation-based approach for integrated modelling purposes.     

Mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. Effect of pre-treatment at elevated temperature

Anaerobic digestion is an appropriate technique for the treatment of sludge before final disposal and it is employed worldwide as the oldest and most important process for sludge stabilization. In general, mesophilic anaerobic digestion of sewage sludge is more widely used compared to thermophilic digestion. Furthermore, thermal pre-treatment is suitable for the improvement of stabilization, enhancement of dewatering of the sludge, reduction of the numbers of pathogens and could be realized at relatively low cost especially at low temperatures. The present study investigates (a) the differences between mesophilic and thermophilic anaerobic digestion of sludge and (b) the effect of the pre-treatment at 70 degrees C on mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. The pre-treatment step showed very positive effect on the methane potential and production rate upon subsequent thermophilic digestion of primary sludge. The methane production rate was mostly influenced by the pre-treatment of secondary sludge followed by mesophilic and thermophilic digestion whereas the methane potential only was positively influenced when mesophilic digestion followed. Our results suggest that the selection of the pre-treatment duration as well as the temperature of the subsequent anaerobic step for sludge stabilization should depend on the ratio of primary to secondary sludge.     

Mesophilic and thermophilic anaerobic digestion of faecal sludge in a pilot plant digester

In many developing countries, the sewage consisting of faecal sludge is discharged untreated into rivers, lakes and coastal areas. This poses a health hazard and a risk to the ecosystem, and wastes a resource which could produce sustainable energy. This paper reports results from an anaerobic digester of 1000L used for digestion of faecal waste at mesophilic and thermophilic conditions. The specific biogas production rate from faecal sludge was in the range of 0.06–0.12 m³/(kg DM.d) at mesophilic conditions at NTP (Normal Temperature & Pressure i.e. 25 °C and 1 atm. Pressure) and 0.1–0.21 m³/(kg DM.d) at thermophilic conditions calculated at NTP. The number of toilet users affects the biogas production with changes in the organic loading rate. The results showed 97% reductionin chemical oxygen demand and 90% reduction in biological oxygen demand of anaerobic digester discharge water as compared to inlet substrate values.     

超声破解促进污泥高温厌氧消化研究

城市污水厂的剩余污泥经槽式超声波反应器预处理后,被投加到小型高温厌氧反应器中进行消化处理,通过改变投配率来控制厌氧消化时间,研究超声破解对高温厌氧消化反应速率和效率的影响。试验结果表明,与未经预处理的污泥相比,超声破解能够明显提高污泥高温厌氧消化的生物气产量及对有机物的去除率。控制组在停留时间为20d时对TCOD的去除率为37.29%,而破解污泥在第8天时的去除率就达到了39.60%。这表明污泥经超声破解后其厌氧消化性能得到改善,超声破解不但可以提高厌氧消化对有机物的去除率,而且可以缩短反应时间,在不影响厌氧消化反应正常进行的条件下,还实现了污泥的减量化。     

Operating Experiences of Sludge Disinfection and Stabilization at Colburn Sewage Treatment Works, Yorkshire

A NEW integrated sludge treatment plant was installed at Colburn sewage treatment works (STW) in 1986. The plant comprises a number of novel features including a continuous gravity thickener features including a continuous gravity thickener, a pasteurization unit using submerged combustion of digester gas for sludge heating, a peat-bed odour control system, and a 'pump-our/pump-in'prefabricated anaerobic digester. Plant operation is controlled automatically by a computer system The performance of the thickener has been in accordance with predictions and has achieved at least a halving of raw sludge volume. The submerged combustion plant has consistently heated the sludge to 70°C with high efficiency of gas utilization before the anaerobic digestion stage. Some problems were experienced with the continuity of supply of the raw sludge but overall the plant has performed very satisfactorily with the production of a high quality disinfected and stabilized sludge for use by local farmers.