Dewatering and disinfection of aerobic and anaerobic sludge using an electrokinetic (EK) system

The objectives of the study were to upgrade sewage sludge to Class A Exceptional Quality biosolids (as defined by US EPA) using an electrokinetics dewatering system. The pathogens monitored were Salmonella spp, and fecal coliforms (FC). Ten bench-scale electrokinetic cells were set up for the disinfection of the following sludges: primary, secondary (attached growth culture and suspended culture), and anaerobically digested sludge. A conditioning liquid was also added to five cells. Blower system to aid in dewatering and drying was used in in four EK cells. Sludge characteristics such as water content, volatile solids content, sulfate and chloride ions concentrations, FC and Salmonella spp. before and after the tests were monitored. The highest total solids content (98% TS) was achieved in the cell with the low voltage gradient, in the presence of the conditioner and with the blower system. An average reduction by 50% of volatile solids was observed. The highest, 11 log-reduction of Salmonella spp. was observed in a cell with anaerobically digested sludge. No fecal coliforms were observed in any of the cells after EK treatment.     

Evaluating the stabilisation degree of digested sewage sludge: investigations at four municipal wastewater treatment plants.

Further reduction of volatile suspended solids (VSS) during a post-stabilisation step was applied to evaluate the stabilisation degree of digested sewage sludge. For this purpose digested sludge was collected at four municipal wastewater treatment plants (WWTPs) and further stabilised in lab-scale chemostat reactors either under anaerobic or aerobic conditions. Experimental results showed that even in adequately digested sludge a consistent amount of VSS was degraded during aerobic post-stabilisation. It seems that aerobic conditions play a significant role during degradation of residual VSS. Additionally, specific VSS production (gVSS/peCOD110.d) as well as specific oxygen uptake rate were shown to be suitable parameters to assess the degree of sludge stabilisation at WWTPs. Fourier transform infrared spectroscopy was used to reveal changes in the sludge composition. Spectra of treated and untreated sludge samples indicated that the major component of residual VSS in stabilised sludge for instance consisted of biomass, while cellulose was absent.     

Stability and activity of anaerobic sludge from UASB reactors treating sewage in subtropical regions.

The production of small amounts of well-stabilized biological sludge is one of the main advantages of upflow anaerobic sludge bed (UASB) reactors over aerobic wastewater treatment systems. In this work, sludge produced in three pilot-scale UASB reactors used to treat sewage under subtropical conditions was assessed for both stability and specific methanogenic activity. Stability of primary sludge from settling tanks and digested sludge from conventional sludge digesters was also measured for comparison purposes. Kinetic parameters like the hydrolysis rate constant and the decay rate constant were calculated. High stability was observed in sludge from UASB reactors. Methanogenic activity in anaerobic sludges was relatively low, probably due to the low organic matter concentration in influent sewage. Knowledge on sludge growth rate, stability, and activity might be very useful to optimize sludge management activities in full-scale UASB reactors.     

Aeration of anaerobically digested sewage sludge for COD and nitrogen removal: optimization at large-scale.

The paper will report about the experiences at an Austrian large wastewater treatment plant of 720,000 population equivalents, where anaerobically digested sewage sludge is further stabilised under aerobic conditions. Enhanced stabilisation of the anaerobically digested sludge was required at the plant in order to get a permit for landfill disposal of the dewatered stabilized sludge. By implementing a post-aeration treatment (SRT approximately 6d; 36 degrees C) after anaerobic digestion the organic content of the anaerobically well digested sludge can be decreased by 16%. Investigations on site showed that during digested sludge post-aeration anoxic phases for denitrification are needed to provide stable process conditions. In this way the pH value can be kept in a more favourable range for micro-organisms and concrete structures. Additionally, inhibition of the biological process due to nitrite accumulation can be avoided. By optimising the aeration/pause ratio approximately 45% of total nitrogen in digested sludge can be removed. This significantly improves nitrogen removal efficiency at the wastewater treatment plant. NH(4)-removal occurs mainly through nitritation and denitritation with an efficiency of 98%. The costs/benefit analysis shows that post-aeration of digested sludge results in an increase of total annual costs for wastewater treatment of only 0.84%, corresponding to 0.19 Euro/pe/a. Result of molecular biological analyses (DGGE) indicate that all four ammonium-oxidizing bacteria species present in activated sludge can survive anaerobic digestion, but only two of them can adapt in the digested sludge post-aeration tanks. Additionally, in the post-aerated digested sludge a further ammonium-oxidizing bacteria species was identified.     

Enhanced heavy metal bioleaching efficiencies from anaerobically digested sewage sludge with coinoculation of Acidithiobacillus ferrooxidans ANYL-1 and Blastoschizomyces capitatus Y5.

Prolonged bioleaching period was required to remove heavy metals from anaerobically digested sewage sludge in the presence of low molecular weight organic acids. The purpose of the present study was therefore to enhance metal solubilization efficiencies through introducing organic acid-degrading microorganisms into this artificial bioleaching system. An acetic and propionic acid-degrading yeast Blastoschizomyces capitatus Y5 was successfully isolated from a local Yuen Long sewage sludge and it could achieve optimum growth in synthetic liquid media containing 2000 mg l(-1) acetic acid or 1000 mg l(-1) propionic acid. When it was inoculated simultaneously with Acidithiobacillus ferrooxidans ANYL-1 into anaerobically digested sewage sludge, which contained 648 mg l(-1) acetic acid and 731 mg l(-1) propionic acid, both acids were completely decomposed within 24 hours. As a result, ferrous iron oxidation was greatly improved, resulting in enhanced metal solubilization. Compared with the 8, 10 and 12 days required for maximum solubilization of Zn, Cu and Cr for the control sludge, the bioleaching period was significantly shortened to 4, 5 and 8 days respectively for sludge receiving co-inoculation.     

Post-aeration of anaerobically digested sewage sludge for advanced COD and nitrogen removal: results and cost-benefit analysis at large-scale.

At a large Austrian municipal wastewater treatment plant enhanced stabilisation of anaerobically digested sewage sludge was required in order to get a permit for landfill disposal of the dewatered stabilized sludge. By implementing a post-aeration treatment after anaerobic digestion the organic content of the anaerobically well digested sludge can be decreased by 16%. Investigations at this plant showed that during digested sludge post-aeration anoxic phases are needed to provide stable process conditions. In this way the pH value can be kept in a more favourable range for micro-organisms and concrete structures. Additionally, under the process conditions applied nitrite accumulation would inhibit the stabilisation process if denitrification is not adequately applied. By optimising the aeration/pause ratio approximately 45% of total nitrogen in digested sludge can be removed. NH4-removal occurs through nitrification and denitrification with an efficiency of 98%. This significantly improves nitrogen removal efficiency at the wastewater treatment plant. The costs/benefit analysis shows that post-aeration of digested sludge results in an increase of total annual costs for wastewater treatment of only 0.84%, corresponding to 0.19 Euro/pe/a. Specific costs for nitrogen removal (0.32 Euro/kgN) are comparable with other biological processes for N-removal in reject water.     

Energy saving system with high effluent quality for municipal sewage treatment by UASB-DHS

An up-flow anaerobic sludge blanket (UASB) - down-flow hanging sponge (DHS) was applied to Japanese municipal sewage treatment, and its treatability, energy consumption, and sludge production were evaluated. The designed sewage load was 50 m(3)/d. The sewage typically had a chemical oxygen demand (COD) of 402 mg/L, a suspended solids (SS) content of 167 mg/L, and a temperature of 17-29 °C. The UASB and DHS exhibited theoretical hydraulic retention times of 9.7 and 2.5 h, respectively. The entire system was operated without temperature control. Operation was started with mesophilic anaerobic digested sludge for the UASB and various sponge media for the DHS. Continuous operational data suggest that although the cellulose decomposition and methanogenic process in the UASB are temperature sensitive, stable operation can be obtained by maintaining a satisfactory sludge volume index and sludge concentration. For the DHS, the cube-type medium G3-2 offers superior filling rates, biological preservation and operational execution. The SS derived from the DHS contaminated the effluent but could be removed by optional sand filtration. A comparison with conventional activated sludge (CAS) treatment confirmed that this system is adequate for municipal sewage treatment, with an estimated energy requirement and excess sludge production approximately 75 and 85% less than those of CAS, respectively.     

Minimization of greenhouse gas emission by application of anaerobic digestion process with biogas utilization.

To assess the impact on greenhouse gas emission, different process schemes for municipal sludge treatment were evaluated based on the data from pilot-scale experiments and review of annual operation reports. A modified anaerobic digestion process with partial ozonation of digested sludge to improve biological degradability and the conventional anaerobic digestion process were compared with respect to the energy demand in each process schemes. Options for beneficial use of biogas included (1) application of biogas for power production and (2) recovery as an alternative to natural gas utilization. The analysis indicated that the partial ozonation process with power production led to minimal greenhouse gas emission because the extra energy production from this scheme was expected to cover all of the energy demand for the plant operation. Moreover, the final amount of dewatered sludge cake was only 40% of that expected from the conventional process, this significantly minimizes the potential for greenhouse gas emission in the subsequent sludge incineration processes.     

Evaluation of sludge properties in a pilot-scale UASB reactor for sewage treatment in a temperate region

In this study, continuous operation of a pilot-scale upflow anaerobic sludge blanket (UASB) reactor for sewage treatment was conducted for 630 days to investigate the physical and microbial characteristics of the retained sludge. The UASB reactor with a working volume of 20.2 m(3) was operated at ambient temperature (16-29 °C) and seeded with digested sludge. After 180 days of operation, when the sewage temperature had dropped to 20 °C or lower, the removal efficiency of both total suspended solids (TSS) and total biochemical oxygen demand (BOD) deteriorated due to washout of retained sludge. At low temperature, the cellulose concentration of the UASB sludge increased owing to the rate limitation of the hydrolytic reaction of suspended solids in the sewage. However, after an improvement in sludge retention (settleability and concentration) in the UASB reactor, the process performance stabilized and gave sufficient results (68% of TSS removal, 75% of total BOD removal) at an hydraulic retention time (HRT) of 9.7 h. The methanogenic activity of the retained sludge significantly increased after day 246 due to the accumulation of Methanosaeta and Methanobacterium following the improvement in sludge retention in the UASB reactor. Acid-forming bacteria from phylum Bacteroidetes were detected at high frequency; thus, these bacteria may have an important role in suspended solids degradation.     

城市污水处理厂污泥热值及影响因素分析

以重庆市主城区城市污水处理厂污泥为研究对象,测定了污泥的热值并分析了影响污泥热值的主要因素,探讨了水分对热值利用的影响。结果表明:元素分析法和量热仪法测定的污泥热值较吻合;污水水质、排水体制、污水及污泥处理工艺通过影响污泥挥发分的含量而影响污泥热值,建议消化污泥和合流制区域污水处理厂污泥不宜采用焚烧处理。污水处理厂污泥具有自持燃烧特性时的最高含水率为68%;通过元素分析发现,不同污水处理厂污泥挥发分元素的计量组成基本一致,可用计量化学式CH1.72O0.55N0.12S0.03粗略地表示重庆市主城区污水处理厂污泥挥发分的组成。     

Enhanced heavy metals removal without phosphorus loss from anaerobically digested sewage sludge

Heavy metals removal without phosphorus loss from anaerobically digested sewage sludge was investigated by conducting batch experiments using hydrogen peroxide and/or iron sulphate under acidified conditions at pH 3. The addition of hydrogen peroxide to the sludge improved the elution efficiencies of As, Cd, Cu and Zn with phosphorus loss from the sludge. The optimum initial concentrations of hydrogen peroxide were. Respectively. 0.1% for As, Cd, Mn and Zn and 0.5% for Cu and Ni. The combined process of 0.1% hydrogen peroxide and 1 g Fe/L ferric sulphate enhanced the initial elution rate of Cu and Cr compared to the addition of either ferric sulphate or hydrogen peroxide, indicating that oxidants stronger than hydrogen peroxide were produced in the sludge. Furthermore, the combined process immobilised phosphorus in the sludge due to co-precipitation with ferric hydroxide or precipitation as ferric phosphate. It was concluded that there is a possibility that the combined process could remove heavy metals effectively without phosphorus loss from anaerobically digested sewage sludge.     

Sludge production and management processes: case study in China.

In 2010, the sewage sludge production rate will be 178,500 t dried solids (ds) for Beijing and 294,000 t-ds for Shanghai, respectively. Beijing adopts a centralized system to stabilize 78% of her sludge in three rural Stabilization Centres. Aerated composting technique will be used. Shanghai on the contrary decentralizes the management plan to treat the sludge on site. Diverse treatment trains, such as aerobic/anaerobic digestion, drying, incineration, and composting will be applied. Production rate, treatment plan, and the associated costs, energy consumption, carbon dioxide emission, and risk assessment for heavy metals and pathogens on human health were evaluated in this report for sludges yielded in Beijing and Shanghai, China.     

A critical review of experimental and predicted methane generation from anaerobic codigestion

Anaerobic digestion is increasingly being considered as a treatment option for an extensive range of waste biomass, due to the potential for energy recovery, in the form of methane production, and lower sludge volumes relative to aerobic treatment processes. Furthermore, when two substrates are codigested (i.e. digested together), added benefits are foreseeable, such as increased methane production and detoxification of toxic compounds via cometabolic degradation pathways. The objectives of this study were to compare experimental and predicted methane production from codigestion literature studies in order to objectively evaluate digester performance. Two predictive methods were used, both assuming methane yields are additive: literature values for digestion of single substrates and a stoichiometric method using model substrates to represent different substrates. Waste sources included in the analysis were primary sewage sludge, waste activated sludge, cow manure, waste paper, grease trap sludge, fat oil and grease and algal sludge. It was found that methane production could approximately be predicted using both methods, with literature methane yields from the same study being the most accurate predictor. One important finding from this study was that the assumption that methane yields are additive is a reasonable one. Furthermore, both predictive methods may be usefully employed as a screening tool to compare methane yields between different types and blends of substrates.     

Life cycle assessment of an intensive sewage treatment plant in Barcelona (Spain) with focus on energy aspects

Life Cycle Assessment was used to evaluate environmental impacts associated to a full-scale wastewater treatment plant (WWTP) in Barcelona Metropolitan Area, with a treatment capacity of 2 million population equivalent, focussing on energy aspects and resources consumption. The wastewater line includes conventional pre-treatment, primary settler, activated sludge with nitrogen removal, and tertiary treatment; and the sludge line consists of thickening, anaerobic digestion, cogeneration, dewatering and thermal drying. Real site data were preferably included in the inventory. Environmental impacts of the resulting impact categories were determined by the CLM 2 baseline method. According to the results, the combustion of natural gas in the cogeneration engine is responsible for the main impact on Climate Change and Depletion of Abiotic Resources, while the combustion of biogas in the cogeneration unit accounts for a minor part. The results suggest that the environmental performance of the WWTP would be enhanced by increasing biogas production through improved anaerobic digestion of sewage sludge.     

Process performance and change in sludge characteristics during anaerobic digestion of sewage sludge with ozonation.

A new process configuration combining anaerobic digestion with ozonation, and operated at long SRT, was studied with the objective of on-site reduction in sludge quantity and improving biogas recovery. The process performance with respect to solid reduction efficiency and other important process parameters like accumulation of inorganic solids, changes in sludge viscosity and dewatering characteristics were evaluated from the data of long term pilot scale continuous experiments conducted using a mixture of primary and secondary municipal sewage sludge. Due to sludge ozonation and long SRT, high VSS degradation efficiency of approximately 80% was achieved at a reactor solid concentration of 6.5%. A high fraction of inorganic solid (>50%) consisting mainly of acid insoluble and iron compounds was found to accumulate in the reactor. The high inorganic content accumulated in the digested sludge did not, however, contribute to the observed increase in sludge viscosity at high solid concentration. The sludge viscosity was largely found to depend on the organic solid concentration rather than the total solid content. Moreover, higher inorganic content in the digested sludge resulted in better sludge dewaterability. For a quick assessment of the economic feasibility of the new process, an economic index based on the unit cost of digested sludge disposal to unit electric cost is proposed.     

Anaerobic digestion with partial ozonation minimises greenhouse gas emission from sludge treatment and disposal.

A novel anaerobic digestion process combined with partial ozonation on digested sludge was demonstrated for improving sludge digestion and biogas recovery by full-scale testing for 2 years and its performance was compared with a simultaneously operated conventional anaerobic digestion process. The novel process requires two essential modifications, which are ozonation for enhancing the biological degradability of sludge organics and concentrating of solids in the digester through a solid/liquid separation for extension of SRT. These modifications resulted in high VSS degradation efficiency of ca. 88%, as much as 1.3 times of methane production and more than 70% reduction in dewatered sludge cake production. Based on the performance, its energy demands and contribution for minimisation of greenhouse gas emission was evaluated throughout an entire study of sludge treatment and disposal schemes in a municipality for 130,000 p.e. The analysis indicated that the novel process with power generation from biogas would lead to minimal greenhouse gas emission because the extra energy production from the scheme was expected to cover all of the energy demand for the plant operation, and the remarkable reduction in dewatered sludge cake volumes makes it possible to reduce N2O discharge and consumption of fossil fuel in the subsequent sludge incineration processes.     

Development of an ecologically sustainable wastewater treatment system

The present study aimed mainly for the development of a wastewater treatment system incorporating enhanced primary treatment, anaerobic digestion of coagulated organics, biofilm aerobic process for the removal of soluble organics and disinfection of treated water. An attempt was also made to study the reuse potential of treated water for irrigation and use of digested sludge as soil conditioner by growing marigold plants. Ferric chloride dose of 30 mg/l was found to be the optimum dose for enhanced primary treatment with removals of COD and BOD to the extent of 60% and 77%, respectively. Efficient anaerobic digestion of ferric coagulated sludge was performed at 7 days hydraulic retention time (HRT). Upflow aerobic fixed film reactor (UAFFR) was very efficient in removals of COD/BOD in the organic loading rate (OLR) range of 0.25 to 3 kg COD/m(3)/day with COD and BOD removals in the range 65-90 and 82-96, respectively. Photo-oxidation followed by disinfection saved 50% of chlorine dose required for disinfection of treated effluent and treated water was found to be suitable for irrigation. The result also indicated that anaerobically digested sludge may be an excellent soil conditioner. From the results of this study, it is possible to conclude that the developed wastewater treatment system is an attractive ecologically sustainable alternative for sewage treatment from institutional/industrial/residential campuses.     

Behaviour of endocrine disrupting chemicals during the treatment of municipal sewage sludge.

Agricultural application of municipal sewage sludge has been emotionally discussed in the last decades, because the latter contains endocrine disrupting chemicals (EDCs) and other organic micropollutants with unknown fate and risk potential. Bisphenol A (BPA) was chosen as a model substance to investigate the influence of sludge conditioning on the end-concentration of EDCs in sludge. Adsorption studies with radioactive-labelled BPA showed that more than 75% BPA in anaerobically digested sludge is bound to solids (log Kd = 2.09-2.30; log Koc = 2.72-3.11). Sludge conditioning with polymer or iron (III) chloride alone had no influence on the adsorption of BPA. After conditioning with iron (III) chloride and calcium hydroxide desorption of BPA took place. Apparently, it occurred due to the deprotonation of BPA (pKa= 10.3) as the pH-value reached 12.4 during the process. The same behaviour is expected for other phenolic EDCs with similar pKa (nonylphenol, 17beta-estradiol, estron, estriol, 17alpha-ethinylestradiol). This study shows high affinity of BPA to the anaerobically digested sludge and importance of conditioning in the elimination of EDCs during the sludge treatment. Addition of polymer is favourable in the case of sludge incineration. Conditioning with iron (III) chloride and calcium hydroxide shows advantages for the use of sludge as fertiliser.     

The role of sludge conditioning and dewatering in the fate of nonylphenol in sludge-amended soils.

One of the main concerns associated with the recycling of biosolids to arable land is their contamination by organic pollutants, like endocrine disruptors. Conditioning and dewatering are usually the last steps of the sewage sludge treatment, before its further utilization. The choice of the specific conditioning/dewatering method may have an effect, not only on the amount of residues in the biosolids, but also on the fate of these compounds in amended soils. Anaerobically digested wastewater sludge was conditioned at lab-scale by means of physical and chemical methods and subsequently dewatered by centrifugation. The produced biosolids plus non-conditioned and non-dewatered sludges were amended separately to soil and spiked with 14C radiolabelled single isomer of nonylphenol. The persistence and leaching potential of nonylphenol after an incubation period of three months were correlated to the sludge treatment method. In comparison to non-conditioned sludge, 54% and 72% higher amount of pollutant residues were extractable when freeze-thawed and limed sludge, respectively, were used. Conditioning of sludge with cationic polymer decreased the leaching potential of nonylphenol in sludge-amended soils, while liming increased it. Fractions of the model compound recovered as extractable and bound residues were analyzed in order to interpret nonylphenol fate.     

A new approach to improve sludge dewatering using a semi-continuous hydraulic press system.

In future, it will be necessary to incinerate an increasing proportion of the sludge produced by public wastewater treatment. In this context, equipment for more effective dewatering of sewage sludge is required. A hydraulic press system (Bucher HP) which has been used to date in the food industry could represent an alternative to the sludge dewatering systems currently used. The basic suitability of this press system was demonstrated in comprehensive industrial-scale trials at the Glarnerland STP (Switzerland). Dry solids contents of up to 43% were measured in the pressed sludge. Currently, the optimization of individual process parameters is being investigated in a pilot plant at the Schwelm STP operated by Wupperverband (Germany).