Thermophilic anaerobic digestion for sewage sludge stabilization

The microbially mediated biochemical reactions that occur during anaerobic digestion processes for methane production from soluble carbon energy substrates are well known, but in spite of this, the interactions within the multi-species cultures responsible for the overall process require more detailed elucidation. When the process feed comprises mixed, solid, carbon energy substrates, as in the case of waste sewage sludge stabilization, many aspects of both the process biochemistry and microbiology are unresolved. This mini-review seeks to identify some of these unresolved questions, particularly with respect to operation at thermophilic temperatures.     

Inactivation of Salmonella typhi by high levels of volatile fatty acids during anaerobic digestion

Survival of Salmonella typhi was investigated in an anaerobic digester for cattle dung with volatile fatty acid (VFA) levels of 5000 mg l⁻¹ and pH 6·0. The organism was added to the digester only once in the first experiment and daily in the other. Survival was monitored on alternate days. In the single dose experiment, the counts of Salm. typhi declined rapidly and the pathogen was completely eliminated within 12 d in the experimental digester (VFA ca 5000 mg l⁻¹ and pH 6·0), whereas 26 d were required in the control digester (VFA ca 100 mg l⁻¹ and pH 6·8). T ₉₀ values for the experimental and control digesters were 2·44 d and 4·80 d, respectively. In the daily dose experiment, a four log reduction in the pathogen count was observed in the experimental digester, but only a two log reduction in the control digester at the end of the experimental period. The mean T ₉₀ values for the experimental and the control digester were 4·22 d and 18·63 d, respectively. In both the experiments, statistical analysis of the data showed significant differences in the survival pattern of Salm. typhi in the two digesters.     

Inactivation of Salmonella typhi by high levels of volatile fatty acids during anaerobic digestion

Survival of Salmonella typhi was investigated in an anaerobic digester for cattle dung with volatile fatty acid (VFA) levels of 5000 mg l(-1) and pH 6.0. The organism was added to the digester only once in the first experiment and daily in the other. Survival was monitored on alternate days. In the single dose experiment, the counts of Salm. typhi declined rapidly and the pathogen was completely eliminated within 12 d in the experimental digester (VFA ca 5000 mg l(-1) and pH 6.0), whereas 26 d were required in the control digester (VFA ca 100 mg l(-1) and pH 6.8). T90 values for the experimental and control digesters were 2.44 d and 4.80 d, respectively. In the daily dose experiment, a four log reduction in the pathogen count was observed in the experimental digester, but only a two log reduction in the control digester at the end of the experimental period. The mean T90 values for the experimental and the control digester were 4.22 d and 18.63 d, respectively. In both the experiments, statistical analysis of the data showed significant differences in the survival pattern of Salm. typhi in the two digesters.     

Inactivation of animal viruses during sewage sludge treatment

Using a previously developed filter adsorption technique, the inactivation of a human rotavirus, a coxsackievirus B5, and a bovine parvovirus was monitored during sludge treatment processes. During conventional anaerobic mesophilic digestion at 35 to 36 degrees C, only minor inactivation of all three viruses occurred. The k' values measured were 0.314 log10 unit/day for rotavirus, 0.475 log10 unit/day for coxsackievirus B5, and 0.944 log10 unit/day for parvovirus. However, anaerobic thermophilic digestion at 54 to 56 degrees C led to rapid inactivation of rotavirus (k' greater than 8.5 log10 units/h) and of coxsackievirus B5 (k' greater than 0.93 log10 unit/min). Similarly, aerobic thermophilic fermentation at 60 to 61 degrees C rapidly inactivated rotavirus (k' = 0.75 log10 unit/min) and coxsackievirus B5 (k' greater than 1.67 log10 units/min). Infectivity of parvovirus, however, was only reduced by 0.213 log10 unit/h during anaerobic thermophilic digestion and by 0.353 log10 unit/h during aerobic thermophilic fermentation. Furthermore, pasteurization at 70 degrees C for 30 min inactivated the parvovirus by 0.72 log10 unit/30 min. In all experiments the contribution of temperature to the total inactivation was determined separately and was found to be predominant at process temperatures above 54 degrees C. In conclusion, the most favorable treatment to render sludge hygienically safe from the virological point of view would be a thermal treatment (60 degrees C) to inactivate thermolabile viruses, followed by an anaerobic mesophilic digestion to eliminate thermostable viruses that are more sensitive to chemical and microbial inactivations.     

Developments in pre-treatment methods to improve anaerobic digestion of sewage sludge

During wastewater treatment, most organic matter is transferred to a solid phase commonly known as sludge or biosolids. The high cost of sludge management and the growing interest in alternative energy sources have prompted proposals for different strategies to optimize biogas production during anaerobic sludge treatment. Because of the high solid content and complex structure of sludge-derived organic matter, methane production during digestion is limited at the hydrolysis step. Therefore, large digester volume and long retention times of over 20 days are necessary to achieve adequate stabilization. Pre-treatments can be used to hydrolyze sludge and consequently improve biogas production, solids removal and sludge quality after digestion. This paper reviews the main pre-treatment processes, with emphasis on the most recent developments. An overview of the different technologies is presented, discussing their effects on sludge properties and anaerobic digestion. Future challenges and concerns related to pre-treatment assessment and implementation are also addressed.     

污泥厌氧消化产酸发酵过程中乙酸累积机制

[目的]研究污泥厌氧消化产挥发性脂肪酸(VFA)过程中的有机物碳流的转化机制,阐明乙酸累积机理。[方法]研究溴乙烷磺酸盐(BES)和氯仿(CHCl3)抑制模型下中间代谢产物和气体的累积,检测各产乙酸功能菌群数量,推断污泥产酸发酵过程中的有机物碳流方向和乙酸累积机理。[结果]BES模型乙酸浓度达27 mmol/L,fhs基因拷贝数比对照组高2-3倍,产氢产乙酸菌略有下降。CHCl3模型乙酸浓度达22 mmol/L,fhs基因拷贝数比BES组低一个数量级,产氢产乙酸菌下降明显。[结论]BES特异性较高,除产甲烷菌外对其他厌氧产酸细菌没有影响,乙酸浓度增加并且其主要来源于水解发酵产酸以及同型产乙酸过程。氯仿除抑制产甲烷菌外,对同型乙酸菌和产氢产乙酸菌也有强烈的抑制作用。     

Inactivation of animal viruses during sewage sludge treatment.

Using a previously developed filter adsorption technique, the inactivation of a human rotavirus, a coxsackievirus B5, and a bovine parvovirus was monitored during sludge treatment processes. During conventional anaerobic mesophilic digestion at 35 to 36 degrees C, only minor inactivation of all three viruses occurred. The k' values measured were 0.314 log10 unit/day for rotavirus, 0.475 log10 unit/day for coxsackievirus B5, and 0.944 log10 unit/day for parvovirus. However, anaerobic thermophilic digestion at 54 to 56 degrees C led to rapid inactivation of rotavirus (k' greater than 8.5 log10 units/h) and of coxsackievirus B5 (k' greater than 0.93 log10 unit/min). Similarly, aerobic thermophilic fermentation at 60 to 61 degrees C rapidly inactivated rotavirus (k' = 0.75 log10 unit/min) and coxsackievirus B5 (k' greater than 1.67 log10 units/min). Infectivity of parvovirus, however, was only reduced by 0.213 log10 unit/h during anaerobic thermophilic digestion and by 0.353 log10 unit/h during aerobic thermophilic fermentation. Furthermore, pasteurization at 70 degrees C for 30 min inactivated the parvovirus by 0.72 log10 unit/30 min. In all experiments the contribution of temperature to the total inactivation was determined separately and was found to be predominant at process temperatures above 54 degrees C. In conclusion, the most favorable treatment to render sludge hygienically safe from the virological point of view would be a thermal treatment (60 degrees C) to inactivate thermolabile viruses, followed by an anaerobic mesophilic digestion to eliminate thermostable viruses that are more sensitive to chemical and microbial inactivations.     

污泥厌氧消化中同型产乙酸菌富集研究

在市政污泥厌氧消化过程中,采用三阶段选择性富集同型产乙酸菌,通过监测培养过程中pH值、挥发性脂肪酸、关键酶的活性以及不同碳源利用率等,研究同型产乙酸菌在系统中的累积情况.实验数据表明:经过40 d的富集培养,pH值稳定在8.0,乙酸含量逐渐趋于稳定,占COD的比例为46.4％,同型产乙酸菌对底物的利用率达到了87％,乙酸激酶和乙酰磷酸转移酶呈现明显上升的趋势.结果显示,通过本方法可在市政污泥中有效地富集同型产乙酸菌.     

Survival of Salmonella spp. in a simulated acid-phase anaerobic digester treating sewage sludge

The presence of pathogenic microorganisms in municipal waste sludge may create a serious outbreak of water borne diseases if the sludge is used for agricultural purpose. An attempt to decrease the number of pathogenic microorganisms, Salmonella spp. using a simulated acid-phase anaerobic digester was tested in a laboratory-scale batch experiment. Reduction of Salmonella spp. was demonstrated in a mixture of sludge and organic acid, simulating an acid digester of a two-phase anaerobic digestion process. A high concentration of organic acid at a pH value of 5.5-6.0 prevents a decrease in Salmonella spp. concentration. Almost complete destruction of Salmonella spp. is observed within two days if the pH value is maintained below 5.5.     

Survival of Salmonella spp. in a simulated acid-phase anaerobic digester treating sewage sludge

The presence of pathogenic microorganisms in municipal waste sludge may create a serious outbreak of water borne diseases if the sludge is used for agricultural purpose. An attempt to decrease the number of pathogenic microorganisms, Salmonella spp. using a simulated acid-phase anaerobic digester was tested in a laboratory-scale batch experiment. Reduction of Salmonella spp. was demonstrated in a mixture of sludge and organic acid, simulating an acid digester of a two-phase anaerobic digestion process. A high concentration of organic acid at a pH value of 5.5-6.0 prevents a decrease in Salmonella spp. concentration. Almost complete destruction of Salmonella spp. is observed within two days if the pH value is maintained below 5.5.     

Inactivation by ionizing radiation of Salmonella enteritidis serotype montevideo grown in composed sewage sludge.

S. enteritidis ser. montevideo were grown in composted sewage sludge to levels of approximately 10(9)/g. These bacteria were found to be inactivated by ionizing radiation at approximately the same rate (30 krads/log) as Salmonella species in liquid digested sludge.     

Inactivation of virus during anaerobic digestion of manure in laboratory scale biogas reactors

Reduction of porcine parvovirus, bovine enterovirus and faecal enterococci were measured in biogas reactors continuously run on manure and manure supplemented with household waste at 35°C and 55°C and in batch test run at 70°C. The aim of the experiments was to study the sanitation effect of anaerobic digestion and to evaluate the use of faecal enterococci as an indicator of sanitation. Parallel studies on the reduction of virus and faecal enterococci were done in physiological saline solution. Heat ínactivation was found to be an important factor in thermophilic biogas plants and the overall dominant factor at 70°C. However, other environmental factors with a substantial virucidal and bactericidal effect were involved in inactivation. The death rates for faecal enterococci were generally higher than for porcine parvovirus and lower than for bovine enterovirus. For faecal enterococci, a logarithmic reduction of 4 (corresponding to the recommended minimum guaranteed retention time) was obtained after 300 hours at 35°C and after 1–2 hours at 55°C. In continuously-fed reactors, a high reduction rate was initially seen for the virus tested, followed by a reduction in the rate. For porcine parvovirus, a minimum guaranteed retention time of 11–12 hours is necessary at 55°C in the initial phase (0–4 hours) and 54 hours hereafter (4–48 h). Correspondingly, for bovine enterovirus a MGRT of 23 hours was necessary at 35°C and < 0.5 hours at 55°C. The data indicate that faecal enterococci measurements give a good indication of inactivation of enterovirus and other more heat sensitive virus, especially under thermophilic conditions. Parvovirus is very suitable for comparative investigations on inactivation in the temperature range of 50–80°C, due to the extreme thermal resistance of this virus. However, in stipulating sanitation demands for biogas reactors it seems more reasonable to use less resistant virus, such as a reovirus or picornavirus, which better represents the pathogenic animal virus.Abbreviations BEV bovine enterovirus - CFU colony forming unit - FE faecal enterococci - HRT hydraulic retention time - MGRT minimum guaranteed retention time - ND not detected - PPV porcine parvovirus - TCID₅₀ tissue cell infective dose 50 % - VFA volatile fatty acids - VS volatile solids     

Autohydrolysis pretreatment of secondary sludge for anaerobic digestion

The increase in the number of wastewater treatment plants and the quality required for the residue produced makes it necessary to improve the efficiency of anaerobic digestion of sludge. Pretreatments of secondary sludge have shown important advantages in the elimination of volatile solids and pathogenic microorganisms from the sludge, and they have also had a positive effect on biogas production. However, such methods are associated with high operating costs. This paper shows the behavior of a autohydrolysis pretreatment, which consists of subjecting the secondary sludge to a temperature of 55 °C for 12–24 h with a limited amount of oxygen under batch operation. The pretreatment results in a high solubilization of organic matter, increasing the fluidity of the sludge and improving the biogas production. This study focuses on the evaluation of the influence of oxygen and the initial sludge concentration on the pretreatment behavior. The main results obtained showed that when autohydrolysis pretreatment was carried out for 12 h, with a high solid concentration and microaerobic conditions, the solubilization of organic matter was increased by 40%, the methane productivity was improved by 23%, and there was an overall improvement in sludge fluidity. Moreover, the energy assessment of the autohydrolysis pretreatment and anaerobic digestion system showed the energetic feasibility of this treatment method, since the increase in energy production compensates for the extra energy required to carry out the pretreatment.     

Thermophilic anaerobic digestion of sewage sludge: focus on the influence of the start-up. A review

The thermophilic anaerobic digestion (TAD) of sewage sludge has often been found to be less stable than mesophilic treatment. In comparison to mesophilic digesters, thermophilic reactors treating sludge are generally characterized by relatively high concentrations of volatile fatty acids (VFA) in the effluent along with poor effluent quality, indicating a lower level of process stability. However, reviewing the literature related to the procedure for obtaining a thermophilic inoculum, it seems that most of the problems associated with the instability and the accumulation of organic intermediates are the result of the manner in which the thermophilic sludge has been obtained. In this paper, the different options available for obtaining an anaerobic digester operating at thermophilic temperature (55°C) have been reviewed. In this light, rapid heating to the target temperature followed by the development of thermophilic microorganisms, which can be determined by VFA dropping to ≤500?mg acetic acid L^?1 before increasing the organic loading rate (OLR), has been determined the most suitable means of establishing TAD.     

Microbial populations and sludge characteristics in thermophilic aerobic sewage sludge digestion

Summary Estimates of bacterial numbers from raw sewage sludge and sludge treated by thermophilic aerobic digestion were compared with simple indicators of sludge quality and concentrations of potential substrates. Significant differences were found between sludge types for all but one of the variables examined (frequency of dividing cells). During a stable period of digestor operation, the average number of viable obligate thermophiles present in digested sludge (1.63 × 10⁶ ml^–1) was approximately 10²-fold greater than in feed sludge (1.10 × 10⁴ ml^–1). Total numbers of bacteria were slightly greater in digested sludge (3.24 × 10¹⁰ ml^–1) than in feed sludge (2.39 × 10 ml^–10), as were viable counts of bacteria at incubation temperatures of 37°C and 55°C. Significant correlation was found between viable counts of bacteria at 37°C and 55°C for digested sludge, and 65°C and 55°C for feed sludge. The numbers of obligate thermophiles present and the total of bacteria present were related to the temperature and pH of the digested sludge and inversely related to the numbers ofEscherichia coli and coliforms present, which were not detected at temperatures greater than 50°C.     

Biological hydrogen production by anaerobic digestion of food waste and sewage sludge treated using various pretreatment technologies

The purpose of this study was to enhance the efficiency of anaerobic co-digestion with sewage sludge using pretreatment technologies and food waste. We studied the effects of various pretreatment methods (thermal, chemical, ultrasonic, and their combination) on hydrogen production and the characteristics of volatile fatty acids (VFAs) using sewage sludge alone and a mixture of sewage sludge and food waste. The pretreatment combination of alkalization and ultrasonication performed best, effecting a high solubilization rate and high hydrogen production (13.8 mL H₂/g VSS_consumed). At a food waste:pretreated sewage sludge ratio of 2:1 in the mixture, the peak hydrogen production value was 5.0 L H₂/L/d. As the production of hydrogen increased, propionate levels fell but butyrate concentrations rose gradually.     

Survival rates of parasite eggs in sludge during aerobic and anaerobic digestion.

The effects of mesothermic anaerobic or aerobic sludge digestion on survival of eggs from the roundworms Ascaris suum, toxocara canis, Trichuris vulpis, and Trichuris suis and from the rat tapeworm Hymenolepis diminuta were studied. Destruction of eggs throughout a 15-day treatment period, as well as their viabilities after reisolation, was analyzed. The laboratory model digesters used in this study were maintained at a 15-day retention schedule, partially simulating a continuously operating system. Ascaris eggs were destroyed in the anaerobic (23%) or aerobic (38%) digesters, and 11% Trichuris eggs were destroyed in the aerobic digesters. Trichuris eggs in anaerobic digesters and Toxocara eggs in either anaerobic or aerobic digesters were not destroyed. Destruction of eggs in digesters was correlated with the state of the eggs before subjection to the treatment processes; i.e., some Ascaris and Trichuris eggs were already embryonated in host intestinal contents or feces and hence past their most resistant stage. The viabilities of Ascaris and Toxocara eggs that survived the digestion processes were greater in anaerobically treated than in aerobically treated material. Eggs from Hymenolepis were nonviable before use in the experiments. However, they were more effectively destroyed in aerobic digesters than in anaerobic digesters.     

Experiences with the dual digestion of municipal sewage sludge

The dual digestion process was investigated using sludge samples collected at the WWTP of Tomaszow Mazowiecki (Poland). Mixed sludge was treated in a laboratory setup under batch and semi-continuous conditions. Dual digestion with a 1d SRT aerobic thermophilic pretreatment followed by an anaerobic step with 20 d of SRT turned out to be optimal, since a 44-46% VS reduction and a biogas yield of 480 dm(3)/kg VS fed were achieved. In the course of the process, the concentration of nitrogen in supernatant increased over 5 times and its major portion was converted into ammonia. Phosphorus also entered the supernatant, reaching over 200 g/m(3). The dual digestion noticeably deteriorated the sludge dewaterability. Following completion of the process, capillary suction time measurements averaged 64 s for the raw sludge, 400 s for aerobically pretreated sludge and 310-360 for the anaerobically digested sludge. Aerobic pretreatment consistently reduced Enterobacteriaceae content to below detectable limits.