Sequential anaerobic/aerobic digestion of waste activated sludge: analysis of the process performance and kinetic study

Sequential anaerobic-aerobic digestion was applied to waste activated sludge (WAS) of a full scale wastewater treatment plant. The study was performed with the objective of testing the sequential digestion process on WAS, which is characterized by worse digestibility in comparison with the mixed sludge. Process performance was evaluated in terms of biogas production, volatile solids (VS) and COD reduction, and patterns of biopolymers (proteins and polysaccharides) in the subsequent digestion stages. VS removal efficiency of 40%, in the anaerobic phase, and an additional removal of 26%, in the aerobic one, were observed. For total COD removal efficiencies of 35% and 25% for anaerobic and aerobic stage respectively, were obtained. Kinetics of VS degradation process was analyzed by assuming a first order equation with respect to VS concentration. Evaluated kinetic parameters were 0.44 ± 0.20 d(-1) and 0.25 ± 0.15 d(-1) for the anaerobic stage and aerobic stage, respectively. With regard to biopolymers, in the anaerobic phase the content of proteins and polysaccharides increased to 50% and 69%, respectively, whereas in the subsequent aerobic phase, a decrease of 71% for proteins and 67% for polysaccharides was observed. The average specific biogas production 0.74 m(3)/(kg VS destroyed), was in the range of values reported in the specialized literature for conventional anaerobic mesophilic WAS digestion.     

Application of alkaline treatment for sludge decrement and humic acid recovery

A new method was introduced to reduce waste activated sludge and extract humic acid for liquid fertilizer. Sludge was disintegrated with NaOH (0.4 g/g dry solid, 8 h) and then centrifuged to obtain the supernatant. The residual sludge was then dewatered, while the supernatant was used to extract humic acid with an ultrafiltration membrane. The results showed that the alkaline treatment dissolved more than half of the sludge organic matter, which was composed of 24% humic acid by mass. After the supernatant was concentrated 20 times using a membrane with a molecular weight cutoff of 1000, the retentate contained 94.5% of the dissolved organics and could be used to produce humic acid fertilizer. Additionally, only 26% of the NaOH was consumed and the residual NaOH in the permeate flux could be reused. Due to the removal of water and organics, the dewatered sludge could be reduced by 60% when compared to samples that did not receive the alkaline treatment.     

Dewatering of Extracellular Polymer and Activated Sludges by Thermochemical Treatment

Of the dewatering characteristics of activated sludges in our previous paper, the dewatering rate of sludge decreased in proportion to increasing amounts of extracellular polymer. As extracellular polymer in activated sludge was one of the important factors in the dewatering process, the change of the dewatering characteristics of thermochemically treated sludge (containing extracellular polymer) were compared with that of extracellular polymer extracted from sludge. The ultrafiltration rate of extracellular polymer flocculated by thermochemical treatment was much faster than that without treatment, indicating improved dewatering characteristics. Under the same treatment conditions, the dewatering characteristics of sludge were also much improved. The addition of the extracellular polymer treated thermochemically below pH 3 had no effect on the dewatering characteristics of the sludge. The addition of a flocculant to the thermochemically treated sludge was found to further improve the dewatering characteristics. The thermochemical treatment under low pH condition facilitated the flaking of the cake from the filter.     

Influence analysis for the behavior of dewaterability of excess sludge in a two-stage vermifilter

To improve excess sludge dewaterability, a two-stage vermifilter was developed to qualitatively and quantitatively analyze sludge physico-chemical properties (fractal dimension, zeta potential, extracellular polymeric substances (EPS), particle size distribution, etc.) and to correlate them with sludge dewatering characteristics (specific resistance to filtration (SRF) and capillary suction time (CST)). Results demonstrated that sludge dewatering performance was significantly improved after the primary vermifilter VF₁ and the second-stage vermifilter VF₂. In addition, the further VF₂ treatment exhibited higher effects on sludge dewatering performance. The particle boundary of sludge after VF₂ treatment was clearer and smoother than VF₁ sludge (VF₁S), apart from the fact that sludge morphological structure got denser and more compact. Comparing with VF₁S, the fractal dimension D₁ calculated within 1D topological space was closer to 1 after VF₂ treatment, and the fractal dimension D₂ within 2D topological space closer to 2, indicating a better dewatering performance after VF₂ treatment. Additionally, the changes of sludge floc surface properties (such as zeta potential and EPS) resulted in small particles agglomerating into larger ones and then the increase of particle diameter. In summary, the two-stage vermifilter got a better sludge dewatering performance, and thus beneficial for subsequent processing of sludge.

     

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.     

Anaerobic waste-activated sludge digestion-a bioconversion mechanism and kinetic model

The anaerobic bioconversion of raw and mechanically lysed waste-activated sludge was kinetically investigated. The hydrolysis of the biopolymers, such as protein, which leaked out from the biological sludge with ultrasonic lysis, was a first-order reaction in anaerobic digestion and the rate constant was much higher that the decay rate constant of the raw waste activated sludge. An anaerobic digestion model that is capable of evaluating the effect of the mechanical sludge lysis on digestive performance was developed. The present model includes four major biological processes-the release of intracellular matter with sludge lysis; hydrolysis of biopolymers to volatile acids; the degradation of various volatile acids to acetate; and the conversion of acetate and hydrogen to methane. Each process was assumed to follow first order kinetics. The model suggested that when the lysed waste-activated sludge was fed, the overall digestive performance remarkably increased in the two-phase system consisting of an acid forming process and a methanogenic process, which ensured the symbiotic growth of acetogenic and methanogenic bacteria. (c) 1993 Wiley & Sons, Inc.     

Changes in the composition of extracellular polymeric substances in activated sludge during anaerobic storage

Changes in the chemical composition of organic compounds in total activated sludge, activated sludge extracellular polymeric substances (EPS), and sludge bulk water during anaerobic storage (12 days) were studied. The background for the study was that anaerobic storage of activated sludge, which often takes place at wastewater treatment plants before dewatering, causes a deterioration of the dewaterability. The reasons are not known at present, but may be related to changes in exopolymer composition of the flocs. The results showed that a fast decrease in total sludge protein and carbohydrate took place within 3 days of anaerobic storage as a result of degradation processes, which accounted for approximately 20% of the organic fraction. The amount of uronic acids and humic compounds remained almost constant in the sludge. The EPS were extracted from the floc matrix using a cationexchange resin. In the EPS matrix a similar initial (2–3 days) degradation of proteins and carbohydrate took place, whereas the content of DNA and uronic acids showed minor changes. The extractability of humic compounds increased during the first 3 days of storage. No changes in extractability of the carbohydrate were observed. A fraction of the EPS protein was found to be difficult to extract but was observed to be degraded during the anaerobic storage. The EPS composition was further characterized by high-performance size-exclusion chromatography analysis obtained by on-line UV detection and post-column detection of proteins, carbohydrates, humic compounds and DNA. Four fractions of polysaccharides were found, of which only one was responsible for the decrease in the carbohydrate content observed with storage time. The fraction was presumably of low molecular mass. Humic compounds and volatile fatty acids (acetate and propionate) were released to the bulk water from the flocs during the storage. A possible mechanism to explain the reduced dewaterability developed during anaerobic storage, partly because of the observed changes in EPS, is discussed.     

Dynamic behavior of a complete-mixing activated sludge system

The dynamic behavior of a laboratory-scale activated sludge biological waste treatment process with recycle and wasting of sludge was investigated by subjecting the system to step changes in the influent waste concentration, the recycle flow rate, or the sludge wasting rate. The dynamic behavior of the system was examined by measuring adenosine triphosphate (ATP) in addition to dissolved chemical oxygen demand (COD) and cell dry weight in the aeration tank. Cell dry weight of the recyle flow and effluent COD were also measured. Analysis of the results and estimation of time constants assuming first order responses showed that the time constants characterizing the dynamic responses of the sludge were directly related to the sludge mean residence time. The time constants estimated from dissolved COD measurements were of the same order of magnitude as the fluid residence time in the aeration tank. The ATP transient response was frequently different from that of the cell dry weight in the aeration tank.     

Microbial extracellular polymeric substances: central elements in heavy metal bioremediation

Extracellular polymeric substances (EPS) of microbial origin are a complex mixture of biopolymers comprising polysaccharides, proteins, nucleic acids, uronic acids, humic substances, lipids, etc. Bacterial secretions, shedding of cell surface materials, cell lysates and adsorption of organic constituents from the environment result in EPS formation in a wide variety of free-living bacteria as well as microbial aggregates like biofilms, bioflocs and biogranules. Irrespective of origin, EPS may be loosely attached to the cell surface or bacteria may be embedded in EPS. Compositional variation exists amongst EPS extracted from pure bacterial cultures and heterogeneous microbial communities which are regulated by the organic and inorganic constituents of the microenvironment. Functionally, EPS aid in cell-to-cell aggregation, adhesion to substratum, formation of flocs, protection from dessication and resistance to harmful exogenous materials. In addition, exopolymers serve as biosorbing agents by accumulating nutrients from the surrounding environment and also play a crucial role in biosorption of heavy metals. Being polyanionic in nature, EPS forms complexes with metal cations resulting in metal immobilization within the exopolymeric matrix. These complexes generally result from electrostatic interactions between the metal ligands and negatively charged components of biopolymers. Moreover, enzymatic activities in EPS also assist detoxification of heavy metals by transformation and subsequent precipitation in the polymeric mass. Although the core mechanism for metal binding and / or transformation using microbial exopolymer remains identical, the existence and complexity of EPS from pure bacterial cultures, biofilms, biogranules and activated sludge systems differ significantly, which in turn affects the EPS-metal interactions. This paper presents the features of EPS from various sources with a view to establish their role as central elements in bioremediation of heavy metals.     

Modeling of activated sludge wastewater treatment processes

The performance of an activated sludge wastewater treatment process consisting of an aeration tank and a secondary settler has been studied. A tanks-in-series model with backflow was used for mathematical modeling of the activated sludge wastewater treatment process. Non-linear algebraic equations obtained from the material balances of MLSS (mixed liquor suspended solids or activated sludge), BOD (biological oxygen demand) and DO (dissolved oxygen) for the aeration tank and the settler and from the behavior of the settler were solved simultaneously using the modified Newton-Raphson technique. The concentration profiles of MLSS, BOD and DO in the aeration tank were obtained. The simulation results were examined from the viewpoints of mixing in the aeration tank and flow in the secondary settling tank. The relationships between the overall performance of the activated sludge process and the operating and design parameters such as hydraulic residence time, influent BOD, recycle ratio and waste sludge ratio were obtained.     

The extracellular polysaccharides of the red microalgae: Chemistry and rheology

Cells of red microalgae encapsulated within sulphated polysaccharides, are thought to have a wide range of potential industrial applications. Our group is thus carrying out a comprehensive research program aimed at bringing these biopolymers into industrial use. The program includes physiological studies on polysaccharide production, outdoor cultivation of the microalgae, and characterisation of the polysaccharides. Chemical composition and structure and physicochemical properties were investigated for the polysaccharides of three red microalgae, Porphyridium sp., P. aerugineum and Rhodella reticulata. Differences were found among the three species in the composition of the monosugars, half ester sulphate groups and glucuronic acid content, but a disaccharide isolated was identical in all the species examined. This disaccharide is thought to be the basic building block of these polysaccharides. In addition, monosugar sulphates were isolated and characterised. Fractionation by charge showed the polysaccharides to be heterogenous and composed of at least two fractions that differed in their composition. Although the polysaccharides differed in composition, their rheological characteristics were found to be similar. Aqueous solutions of the biopolymers were stable over a wide range of pH values and temperatures and were compatible with monovalent cations. Mixtures of the algal polysaccharides with locust bean gum exhibited synergism and syneresis. When the gel strength was compared with that of agar gel at the same concentration the polysaccharide gels were found to be weaker.     

The effect of Ni(II) on properties of bulking activated sludge and microbial analysis of sludge using 16S rDNA gene

This study investigated the effect of nickel on properties and microbial community of bulking activated sludge when 60-240 mg/L Ni(II) was dosed continuously in a sequencing batch reactor (SBR) over 350 days. Results showed that 120 mg/L nickel did not significantly inhibited removal of organic pollutant by activated sludge. However, the system was completely upset when 240 mg/L Ni(II) was dosed. Improvement of settling and dewatering ability was also observed with the addition of Ni(II). In addition, investigations by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) of 16S rDNA of bacteria strain demonstrated that Ni(II) significantly affected microbial community of bulking activated sludge, judging from the elimination of original species and emergence of possible new nickel-resistant bacteria. The effect of nickel on shift of microbial community was an important cause resulted in the improvement of sludge properties in this bulking activated sludge system.     

Nutrients release and phosphorus distribution during oligochaetes predation on activated sludge

Release of nitrogen and phosphorus into effluent of activated sludge process was reported in the recent researches of sludge reduction induced by oligochaetes. In this study, batch test and radioisotope ³²P tracer test were therefore carried out to further investigate nutrients release and phosphorus distribution among supernatant, sludge and worm during predation of oligochaetes on sludge. Results showed that more nutrients release into supernatant occurred in the tests of worms with sterilized sludge than that of worms with activated sludge, and release of nitrogen and phosphorus was few in the tests of worms with activated sludge. Statistical analysis showed that no factor was significantly correlated with phosphorus concentration in the supernatant, but time and worm were only two factors significantly affecting total nitrogen (TN) concentration in the supernatant in the test of worms with activated sludge. Through test of radioisotope ³²P tracer, ³²P mainly distributed in activated sludge and supernatant, and more release of ³²P into the supernatant occurred in the test of sludge with worms. After 24 h, ³²P concentration of supernatant in the test of sludge with worms was 9% higher than that in the test of sludge without worms, and ³²P concentration of worm increased by 2.7%. Additionally, the release rate of phosphorus into supernatant caused by worm's predation on activated sludge was 0.1211 mgTP/gWorm (wet weight) h.     

Assessment of characteristics and biological treatment technologies of Jordanian wastewater

Demand for wastewater treatment facilities will increase as Jordan's population grows. In addition, currently available systems of treatment desperately need upgrades in capacity or supplementary systems; especially in the Amman-Zarqa region. Overall; based on the current wastewater flow rates; approximately 85% of the collected sewerage is treated in stabilization ponds, 10% in trickling filters, and 5% in activated sludge systems. This study was carried out to analyze and identify the properties of Jordanian wastewater; compare it to the common characteristics internationally known; and couple that with a proposal of an appropriate treatment technology. Five treatment plants were selected to achieve the objectives of this study; the flow rate of which constitutes approximately 80% of the total treated wastewater in Jordan, based on the design capacity. The study concluded that the wastewater generated in Jordan is classified as strong in terms of total dissolved solids content, total suspended solids content, and chemical and biochemical oxygen demands (COD and BOD). The efficiency of the selected technologies in removing dissolved solids from wastewater was low while it was reasonably high in terms of suspended solids removal. The technology achieving highest percent removals of BOD and solids was that of activated sludge and its modifications. Based on the factors considered in evaluating and selecting unit operations and processes, the activated sludge and its modifications are probably the process technology that should be used in treating Jordanian domestic wastewaters.     

Pre-treatments to enhance the biodegradability of waste activated sludge: Elucidating the rate limiting step

Pre-treatments for waste activated sludge (WAS) are, in most cases, an attempt to increase the biodegradation and/or improve hydrolysis rate of WAS after anaerobic digestion. This review presents an extensive analysis of WAS pre-treatments effectiveness focusing on increasing the biodegradability. In the first part of the review, WAS is considered as a cluster of organic components: proteins, carbohydrates, humic substances and cells. Based on this breakdown into components, the effect of different pre-treatments on each component (and in combination) is described. Also, possible reasons for the contradictory results frequently found among different studies dealing with the same pre-treatment are included. In the second part, the review describes the effects on volatile solids removal by digestion after pre-treatment and on the dewaterability of the final digestate. The energy balance and potential limiting factors for each pre-treatment are also taken into account. From the published works it is concluded that some pre-treatment techniques, such as thermal hydrolysis, thermal phased anaerobic digestion and low-temperature pre-treatment are effective ways to increase energy production and to improve other sludge properties, such as dewatering. However, these techniques are very energy intensive and require a large capital outlay, so research on milder pre-treatment techniques is valuable.     

Stoichiometric analysis of dissolved organic carbon flux into storage and growth in aerobic granules culture

This study attempted to address a fundamental question of whether metabolic behaviors of aerobic granules are different from their counterparts, such as activated sludge and biofilms. A series of respirometric experiments were carried out using mature aerobic granules with mean sizes of 0.75–3.4 mm. Results suggested that metabolism of aerobic granules comprised three consecutive phases: (i) conversion of external dissolved organic carbon to a poly-β-hydroxybutyrate-like substance; (ii) growth of aerobic granules on the stored poly-β-hydroxybutyrate-like substance derived from phase I, and (iii) subsequent endogenous metabolism of aerobic granules. The stoichiometric analysis revealed that the conversion yields of external dissolved organic carbon to the poly-β-hydroxybutyrate-like substance, the growth yields of biomass on storage, and the overall growth yields of biomass on external dissolved organic carbon were not significantly correlated to the sizes of aerobic granules, i.e., the metabolism of aerobic granules would be size independent. The conversion coefficients and growth rates of aerobic granules were found to be comparable with those reported in the activated sludge and biofilms cultures, indicating that there would not be significant difference in the metabolisms of aerobic granules over activated sludge and biofilms. This information will be useful for modeling and designing aerobic granular sludge processes.     

A model for the filterability of activated sludge supported by mixed-liquor biochemical data

A predictive model was developed to estimate the dewatering characteristics of waste-activated sludges. This model utilizes the COD-nitrogen ratio of the wastewater and the organic loading rate of the process to predict sludge filterability in terms of specific resistance. A completely mixed, continuous flow secondary treatment process with solids recycle was used for the cultivation of activated sludges. The sludge wasted from this process was used in Buchner funnel specific resistance determinations. The basic concepts involved in the development of the model were supported by sludge carbohydrate, protein, and surface charge data.     

Monovalent cations and their influence on activated sludge floc chemistry, structure, and physical characteristics

Multivalent cations have been known to be important components of activated sludge floc structure due to their bridging ability of the negatively charged sites on the biopolymer network. Recently in batch systems it was found that excess concentration of monovalent cations led to the deterioration in settleability, dewaterability of sludges and effluent quality of the system. In this study, effect of influent monovalent cations (potassium and sodium) on activated sludge floc structure was investigated in semi-continuous reactors. Results revealed that the increase in concentration of both ions correlated to the general increase in total EPS concentration. The zeta potential values were affected by the cation type and dose in such a way that sludge from sodium reactors had always higher zeta potential values (higher negative charge) than the sludges from potassium reactors. Flocs from sodium reactors were more fragile and weak and the capillary suction time values of these sludges were higher compared to those from potassium reactors. The findings of this research conclude that the floc structure is significantly weakened with the increase of monovalent cations. Even though EPS is produced, it is unable to bind the floc components together. With this, the physical properties of sludge deteriorate for both cations.     

Estimation of Biodegradation Potential of Xenobiotic Organic Chemicals

A method is described to estimate the biodegradation potential of soluble, insoluble, and unknown organic chemicals. The method consists of two stages: (i) generation of a microbial inoculum in a bench scale semicontinuous activated sludge system during which microorganisms are acclimated to test material and the removal of dissolved organic carbon is monitored and (ii) biodegradability testing (CO₂ evolution) in a defined minimal medium containing the test material as the sole carbon and energy source and a dilute bacterial inoculum obtained from the supernatant of homogenized activated sludge generated in the semicontinuous activated sludge system. Removal and biodegradation are measured using nonspecific methods, at initial concentrations of 5 to 10 mg of dissolved organic carbon per liter. Biodegradability data are accurately described by a nonlinear computer model which allows the rate and extent of biodegradation for different compounds to be compared and statistically examined. The evaluation of data generated in the combined removability-biodegradability system allows the biodegradation potential of a variety of xenobiotic organic chemicals to be estimated.     

The investigation of effect of organic carbon sources addition in anaerobic–aerobic (low dissolved oxygen) sequencing batch reactor for nutrients removal from wastewaters

The effect of addition of organic carbon sources (acetic acid and waste activated sludge alkaline fermentation liquid) on anaerobic–aerobic (low dissolved oxygen, 0.15–0.45 mg/L) biological municipal wastewater treatment was investigated. The results showed that carbon source addition affected not only the transformations of polyhydroxyalkanoates (PHA), glycogen, nitrogen and phosphorus, but the net removal of nitrogen and phosphorus. The removal efficiencies of TN and TP were, respectively, 61% and 61% without organic carbon source addition, 81% and 95% with acetic acid addition, and 83% and 97% with waste activated sludge alkaline fermentation liquid addition. It seems that the alkaline fermentation liquid of waste biosolids generated in biological wastewater treatment plant can be used to replace acetic acid as an additional carbon source to improve the anaerobic–aerobic (low dissolved oxygen) municipal wastewater nutrients removal although its use was observed to cause a slight increase of effluent BOD and COD concentrations.