A review of thermal sludge pre-treatment processes to improve dewaterability

As a result of the wide application and utilization of the waste activated sludge process, excess sludge presents a serious disposal problem. Many efforts have been devoted to reduce the excess sludge by treatments such as digestion and dewatering. It has been known for many years that a thermal pre-treatment gives an improvement in the dewaterability of sludges. This paper provides a literature review concerning the optimum treatment conditions to obtain enhanced dewaterability and digestibility of sludge. The main commercial hydrolysis processes (Cambi, Porteous and Zimpro) are discussed. The literature findings concerning the optimum treatment conditions of thermal or thermochemical pre-treatments are reviewed. The second part of this paper deals with the fundamentals of improving sludge dewatering. The influence of extracellular polymer (ECP) on settling and dewatering characteristics is discussed, together with the importance of cations and ECP-hydrophobicity in the flocculation and dewatering process. Finally, the effect on exocellular polymer, dewaterability, settleability and colloidal stability of activated sludge by treatment with sulfuric acid was studied.     

Pilot-scale peroxidation (H2O2) of sewage sludge

Municipal and industrial wastewater treatment plants produce large amounts of sludge. This excess sludge is an inevitable drawback inherent to the waste activated sludge process. Both the reduction of the amount of sludge produced and improving its dewaterability are of paramount importance. Novel pre-treatment processes have been developed in order to improve sludge dewatering, handling and disposal. This paper discusses the oxidation process utilising the catalytic activation of H(2)O(2) by iron salts, referred to as Fenton's reagent. In previous work, the authors described the experimental laboratory results of H(2)O(2)-oxidation of thickened sludge. Based upon the optimum conditions obtained in these laboratory tests, pilot-scale experiments are conducted. Peroxidation under its optimum conditions, i.e. (i) through addition of 25 g H(2)O(2) kg(-1) DS (dry solids content), (ii) in the presence of 1.67 g Fe(2+)-ions kg(-1) DS, (iii) at pH 3, and (iv) at ambient temperature and pressure, significantly reduces the amounts of sludge and improves the product quality: the amount DS per equivalent inhabitant per day (DS/IE.d) was reduced from 60 to 33.1 g DS/IE.d and the percentage DS of the sludge cake was 47%, which is high compared with the 20-25% achieved in a traditional sludge dewatering facility. An economic assessment for a wastewater treatment plant of 300,000 IE confirms the benefits. Considering the fixed and variable costs and the savings obtained when the sludge is incinerated after dewatering, a net saving of approx. 950,000 Euro per year or 140 Euro per ton DS can be expected.     

Alkaline thermal sludge hydrolysis

The waste activated sludge (WAS) treatment of wastewater produces excess sludge which needs further treatment prior to disposal or incineration. A reduction in the amount of excess sludge produced, and the increased dewaterability of the sludge are, therefore, subject of renewed attention and research. A lot of research covers the nature of the sludge solids and associated water. An improved dewaterability requires the disruption of the sludge cell structure. Previous investigations are reviewed in the paper. Thermal hydrolysis is recognized as having the best potential to meet the objectives and acid thermal hydrolysis is most frequently used, despite its serious drawbacks (corrosion, required post-neutralization, solubilization of heavy metals and phosphates, etc.). Alkaline thermal hydrolysis has been studied to a lesser extent, and is the subject of the detailed laboratory-scale research reported in this paper. After assessing the effect of monovalent/divalent cations (respectively, K(+)/Na(+) and Ca(2+)/Mg(2+)) on the sludge dewaterability, only the use of Ca(2+) appears to offer the best solution. The lesser effects of K(+), Na(+) and Mg(2+) confirm previous experimental findings.As a result of the experimental investigations, it can be concluded that alkaline thermal hydrolysis using Ca(OH)(2) is efficient in reducing the residual sludge amounts and in improving the dewaterability. The objectives are fully met at a temperature of 100 degrees C; at a pH approximately 10 and for a 60-min reaction time, where all pathogens are moreover killed. Under these optimum conditions, the rate of mechanical dewatering increases (the capillary suction time (CST) value is decreased from approximately 34s for the initial untreated sample to approximately 22s for the hydrolyzed sludge sample) and the amount of DS to be dewatered is reduced to approximately 60% of the initial untreated amount. The DS-content of the dewatered cake will be increased from 28 (untreated) to 46%.Finally, the mass and energy balances of a wastewater treatment plant with/without advanced sludge treatment (AST) are compared. The data clearly illustrate the benefits of using an alkaline AST-step in the system.     

OPTIMUM SELECTION AND OPERATION OF SLUDGE DEWATERING PROCESSES

The paper presents a life cycle costing model and a mathematical programming model to identify the least cost sludge dewatering process and the optimum polymer dose to be used for sludge dewatering, respectively. The life cycle costing model is based on an infinite horizon and it allows for incremental expansion in the capacities of the sludge dewatering process as demand rises. A new nonlinear formulation for the minimization of the combined costs of polymer dose and transportation is presented. These models were used for the selection of the least cost sludge dewatering process and the determination of the optimum polymer dose for two wastewater treatment plants in Kuwait. These formulations are generic and hence can be used for different types of sludge dewatering processes or polymers. Relationships have also been developed to assess the impact of distances to disposal or reuse site(s) from the wastewater treatment plant location on the optimum polymer dose.     

Structural characterization of metabolites of the X-ray contrast agent iopromide in activated sludge using ion trap mass spectrometry

Identification of degradation products of environmental contaminants is a challenging task because not only are they present in very low concentrations but they are also mixed with complex matrixes that interfere with detection. This work illustrates a simple approach using ion trap mass spectrometry combined with H/D-exchange experiments to elucidate the structures of iopromide metabolites formed during biodegradation in activated sludge. Iopromide is an X-ray contrast agent that has been detected frequently in effluents of wastewater treatment plants and in surface waters due to its persistence and high usage. Three metabolites produced by oxidation of the primary alcohols (forming carboxylates) on the side chains of iopromide were identified in a batch reactor with mixed liquor from a conventional activated sludge. Derivatization of the carboxylic acid to form a methyl ester and interpretation of the MS2 data of this derivative aided in the confirmation of the identities of these metabolites. Furthermore, one metabolite formed by dehydroxylation at the two side chains was identified in a batch reactor with mixed liquor from a nitrifying activated sludge. The MS2 fragmentation pattern of iopromide and its metabolites revealed that the iodinated ring remains intact and that minor transformations in the structure occur during biodegradation of iopromide in biological wastewater treatment plants.     

Treatment of sewage sludge using electrokinetic geosynthetics

The treatment and disposal of sewage sludge is one of the most problematical issues affecting wastewater treatment in the developed world. The traditional outlets for sewage sludge are to spread it on agricultural land, or to form a cake for deposit to landfill or incineration. In order to create a sludge cake, water must be removed. Existing dewatering technology based on pressure can only remove a very limited amount of this water because of the way in which water is bound to the sludge particles or flocs. Several researchers have shown that electrokinetic dewatering of sludge is more efficient than conventional hydraulically driven methods. This involves the application of a dc voltage across the sludge, driving water under an electrical gradient from positive (anode) electrode to negative (cathode) electrode. However, there have been several reasons why this technique has not been adopted in practice, not least because the, normally metallic, anode rapidly dissolves due to the acidic environment created by the electrolysis of water. This paper will describe experimentation using electrokinetic geosynthetics (EKG): polymer-based materials containing conducting elements. These have been used to minimise the problem of electrode corrosion and create a sludge treatment system that can produce dry solids contents in excess of 30%. It will suggest different options for the treatment of sludges both in situ in sludge lagoons and windrows, and ex situ as a treatment process.     

Hot acid hydrolysis as a potential treatment of thickened sewage sludge

Municipal and industrial wastewater treatment plants produce large amounts of sludge, containing organic and mineral components and being mechanically dewatered to e.g. 20-25% DS in centrifuges. Both the reduction of the amount of sludge produced and improving its dewaterability are hence of paramount importance. Hot acid hydrolysis can meet these objectives. The current paper describes the results of detailed investigations with respect to acid hydrolysis of thickened sludge (5-6% DS content). A comparison with traditional thermal hydrolysis is also included. As a result of the experimental investigations, it can be concluded that hot acid hydrolysis is efficient in both reducing the residual sludge amounts and improving the dewaterability. Under the proposed optimum conditions it is found that (i) the amount of hydrolysed DS is approximately 70% lower than the initial untreated amount, (ii) the DS-solid content of the dewatered cake is increased from 22.5% (initial untreated) to at least twice this value, (iii) the rate of mechanical dewatering is not significantly affected. The preferential release of ODS into the water phase, and the increased BOD/COD-ratio through hydrolysis, turn this recycle water phase into a possible carbon-source for nitrification/denitrification. Heavy metals and phosphates are also released in the water phase, and can be subsequently precipitated.     

Enhancement of waste activated sludge dewaterability by electro-chemical pretreatment

The potential effects of electro-chemical conditioning on sludge dewatering treatments and its mechanism were investigated in this study. Capillary suction time (CST) and specific resistance to filtration (SRF) were used to evaluate sludge dewaterability. Extracellular polymeric substance (EPS) content and sludge disintegration degree (DD(SCOD)) were also determined in an attempt to explain the observed changes in sludge dewaterability. The results indicated that application of considered low electrolysis voltages (<20 V) enhanced sludge dewaterability, while it exceeded 30 V, the dewaterability of sludge was significantly deteriorated. Also, electrolysis pretreatment slightly enhanced sludge dewaterability with short electrolysis time (<20 min), while it significantly deteriorated sludge dewaterability with long electrolysis time (>30 min). The optimal electrolysis voltage and electrolysis time to give preferable dewaterability characteristics were found to be 15-20 V, and 15-20 min, respectively, which generated sludge with optimal EPS content (15-20mg/L) and DD(SCOD) (1.3-2.0%).     

剩余活性污泥的超声脱水及破解

文章从静态和动态研究了剩余活性污泥的超声脱水及污泥的破解。静态下,污泥的脱水应在低功率(以50W为例)短时间(7min)内进行,最高的脱水率(相对于未超声而言)可增加近16%;污泥破解应在高功率长时间下进行,本实验在250W的功率下连续处理20min,COD的增加值达到近1600mg/L;污泥的破解满足一级反应动力学。动态下,以低功率(50W)为例,脱水及破解曲线都比较平坦,污泥的总脱水率相对于未超声而言增加可达近23%;COD的增加值近似达1800mg/L。通过比较低功率下实验,动态实验效果明显优于静态实验。     

Adsorption of heavy metals from wastewater with waste activated sludge

Abstract

The purpose of this study is the investigation of the adsorption of heavy metals from wastewater with waste activated sludge (WAS) so as to increase the operational flexibility of activated sludge processes for the shock loading problems of heavy metals. By contacting the raw wastewater with WAS, before it entered the activated sludge process unit, some heavy metals in the wastewater were removed, and the pH value of the acidic wastewater rose. Five metals, Cu, Zn, Ni, Cd and Pb, were studied, some factors which affected this biosorption phenomena were investigated, and expressions of the adsorption isotherms were discussed. A semi‐empirical mass transfer‐adsorption model was developed to describe the kinetic experimental data.     

Fluidized-Bed Bioreactor Applications for Biological Wastewater Treatment: A Review of Research and Developments

Wastewater treatment is a process that is vital to protecting both the environment and human health. At present, the most cost-effective way of treating wastewater is with biological treatment processes such as the activated sludge process, despite their long operating times. However, population increases have created a demand for more efficient means of wastewater treatment. Fluidization has been demonstrated to increase the efficiency of many processes in chemical and biochemical engineering, but it has not been widely used in large-scale wastewater treatment. At the University of Western Ontario, the circulating fluidized-bed bioreactor (CFBBR) was developed for treating wastewater. In this process, carrier particles develop a biofilm composed of bacteria and other microbes. The excellent mixing and mass transfer characteristics inherent to fluidization make this process very effective at treating both municipal and industrial wastewater. Studies of lab- and pilot-scale systems showed that the CFBBR can remove over 90% of the influent organic matter and 80% of the nitrogen, and produces less than one-third as much biological sludge as the activated sludge process. Due to its high efficiency, the CFBBR can also be used to treat wastewaters with high organic solid concentrations, which are more difficult to treat with conventional methods because they require longer residence times; the CFBBR can also be used to reduce the system size and footprint. In addition, it is much better at handling and recovering from dynamic loadings (i.e., varying influent volume and concentrations) than current systems. Overall, the CFBBR has been shown to be a very effective means of treating wastewater, and to be capable of treating larger volumes of wastewater using a smaller reactor volume and a shorter residence time. In addition, its compact design holds potential for more geographically localized and isolated wastewater treatment systems.     

Anaerobic stabilisation of sludge produced during municipal wastewater treatment by electrocoagulation

Anaerobic digestion of sludge from small electrocoagulation wastewater treatment plant (SEWWTP) is described. The sludge for digestion (SEWWTP sludge) was taken from pilot-scale SEWWTP with the capacity of about 200-population equivalent (25 m3 of municipal wastewater per day). Due to the technology of wastewater treatment, the characteristics of SEWWTP sludge was different from sludge produced in conventional mechanical-biological wastewater treatment plant. Therefore, experiments were focused on possibilities of anaerobic sludge digestion and determination of conditions and parameters (amount and quality of the sludge, biogas production, etc.). Average COD removal efficiency in the pilot-scale SEWWTP exceeded 80%. Organic content of excess sludge (volatile suspended solids (VSS)) was in the range of 52.1-59.2% (these values are much lower compared to VSS content in raw sludge from conventional municipal wastewater treatment plant, where VSS is about 75%). Biogas production from anaerobic digestion of SEWWTP sludge was approximately three times lower compared to standard production in conventional municipal wastewater treatment plant. Low pH (6.5-6.7), high concentration of iron (up to 1400 mg/L) and aluminium (up to 1300 mg/L) and very low (almost zero) concentration of dissolved phosphorus in sludge water were the main factors limiting the rate of anaerobic processes. Based on these results, anaerobic digestion of SEWWTP sludge was not recommended as an appropriate stabilisation method.     

Elimination of Cu(II) toxicity by powdered waste sludge (PWS) addition to an activated sludge unit treating Cu(II) containing synthetic wastewater

Copper(II) ion toxicity onto activated sludge organisms was eliminated by addition of powdered waste sludge (PWS) to the feed wastewater for removal of Cu(II) ions by biosorption before biological treatment. The synthetic feed wastewater containing 14 or 22 mgl(-1) Cu(II) was mixed with PWS in a mixing tank where Cu(II) ions were adsorbed onto PWS and the mixture was fed to a sedimentation tank to separate Cu(II) containing PWS from the feed wastewater. The activated sludge unit fed with the effluent of the sedimentation tank was operated at a hydraulic residence time (HRT) of 10h and sludge age (SRT) of 10 days. To investigate Cu(II), COD and toxicity removal performance of the activated sludge unit at different PWS loadings, the system was operated at different PWS loading rates (0.1-1 gPWSh(-1)) while the Cu(II) loading rate was constant throughout the operation. Percent copper, COD and toxicity removals increased with increasing PWS loading rate due to increased adsorption of Cu(II) onto PWS yielding low Cu(II) contents in the feed. Biomass concentration in the aeration tank increased and the sludge volume index (SVI) decreased with increasing PWS loading rate due to elimination of Cu(II) from the feed wastewater by PWS addition. PWS addition to the Cu(II) containing wastewater was proven to be effective for removal of Cu(II) by biosorption before biological treatment. Approximately, 1 gPWSh(-1) should be added for 28 mgCuh(-1) loading rate for complete removal of Cu(II) from the feed wastewater to obtain high COD removals in the activated sludge unit.     

Effect of C/N ratio on extracellular polymeric substances (EPS) and physicochemical properties of activated sludge flocs

The influences of C/N ratio on the extracellular polymeric substances (EPS) and physicochemical properties of the activated sludge flocs were investigated using laboratory-scale sequencing batch reactors (SBRs). Flocs sizes decreased when C/N ratio increased from 20 to 100 and decreased from 20 to 4. The amount of total EPS, TB-EPS, and the carbohydrate and protein contents in TB-EPS were independent of the C/N ratio. In LB-EPS, the protein content increased and the carbohydrate content decreased at decreased C/N ratio, whereas the protein content decreased and the carbohydrate content increased at increased C/N ratio. Effluent suspended solids (ESS) content, turbidity, sludge volume index (SVI), capillary suction time (CST), and specific resistance to filtration (SRF) increased when the C/N ratio decreased, indicating poor flocculation, settleability and dewaterability of the flocs. However, when the C/N ratio increased, only ESS content, SVI and CST value increased. These properties of the flocs were deteriorated greatly under decreased C/N ratio as compared to increased C/N ratio. The characteristics of the flocs could be recovered when C/N ratio returned to the original value. Only the content of protein in LB-EPS was positively correlated with the flocculation, settleability and dewaterability of the flocs.     

Coagulation/flocculation process and sludge conditioning in beverage industrial wastewater treatment

Attempts were made in this study to examine the effectiveness of coagulation and flocculation process using ferric chloride and polyelectrolyte (non-ionic polyacrylamide) for the treatment of beverage industrial wastewater. Removal of organic matter (expressed as chemical oxygen demand, COD), total phosphorus (TP) and total suspended solid (TSS) using ferric chloride and organic polyelectrolyte during coagulation/flocculation process were investigated. Also, the optimum conditions for coagulation/flocculation process, such as coagulant dosage, polyelectrolyte dosage, and pH of solution were investigated using jar-test experiment. The effect of different dosages of polyelectrolyte in combination with coagulant was also studied. The results revealed that in the range of pH tested, the optimal operating pH was 9. Percentage removals of 73, 95 and 97 for COD, TP and TSS, respectively, were achieved by the addition of 300mg/L FeCl(3).6H(2)O, whereas 91, 99 and 97% removal of COD, TP and TSS, respectively, were achieved with the addition of 25mg/L polyelectrolyte to 100mg/L ferric chloride. The volume of sludge produced, when ferric chloride was used solely, was higher compared to the use of combination of polyelectrolyte and FeCl(3).6H(2)0. The combined use of coagulant and polyelectrolyte resulted in the production of sludge volume with reduction of 60% of the amount produced, when coagulant was solely used for the treatment. It can be concluded from this study that coagulation/flocculation may be a useful pre-treatment process for beverage industrial wastewater prior to biological treatment.     

Fenton peroxidation improves the drying performance of waste activated sludge

Advanced sludge treatment processes (AST) reduce the amount of sludge produced and improve the dewaterability, thus probably also affecting the heat transfer properties and the drying characteristics of the sludge. This paper studies the influence of the Fenton peroxidation on the thermal conductivity of the sludge. Results demonstrate that the Fenton's peroxidation positively influences the sludge cake consistency and hence enhances the mechanical dewaterability and the drying characteristics of the dewatered sludge. For the two sludges used in this study, i.e. obtained from the wastewater treatment plants (WWTP) of Tienen and Sint-Niklaas--the dry solids content of the mechanically dewatered sludge increased from 22.5% to 40.3% and from 18.7% to 35.2%, respectively. The effective thermal conductivity k(e) of the untreated and the peroxidized sludges is measured and used to determine the heat transfer coefficient h(s). An average improvement for k(e) of 16.7% (Tienen) and 5.8% (Sint-Niklaas) was observed. Consequently the value of h(s) increased with 15.6% (Tienen) and 5.0% (Sint-Niklaas). This increased heat transfer coefficient in combination with the increased dewaterability has direct implications on the design of sludge dryers. A plate-to-plate calculation of a multiple hearth dryer illustrates that the number of plates required to dry the peroxidized sludge to 90% DS is less than half the number of plates needed to dry untreated sludge. This results in reduced dryer dimensions or a higher capacity for an existing dryer of given dimensions.     

Effect of chemical composition on the flocculation dynamics of latex-based synthetic activated sludge

This study investigates the effect of calcium, alginate, fibrous cellulose, and pH on the flocculation dynamics and final properties of synthetic activated sludges. A laboratory-scale batch reactor, fed with standard synthetic sludges was used. The effects of varying calcium concentration (5-25 mM), alginate concentration (25-125 mg/L), fibrous cellulose concentration (0.2-0.8 g/L) and pH (3-9) on the sludge characteristics were studied by varying one parameter whilst keeping the others constant. The results from experiments indicated that the calcium, alginate, fibrous cellulose, and pH had the critical effect on the aggregation rate, flocs size, and made the improvement of the final properties of sludge. Dynamic measurements have established the optimum conditions for floc formation and can accurately reflect the state of formation of the synthetic activated sludge flocs. These correlate well with measurements of settleability and turbidity of the synthetic activated sludge. The results of this study support the bonding theory and indicate that formation of cations-polymer complexes and polymer gelation are important means of flocculation. The development of synthetic activated sludges is suggested also to be a possible surrogate for studying the final properties of activated sludge.     

Electrode surface treatments in sludge electro-osmosis dewatering

The sewage sludge dewatering produced by wastewater treatment plants (WWTP) is a multifaceted process due to the presence of colloid fractions. Electro-osmosis could be a suitable technique to reduce the water content of the final sludge. Electric fields of 10, 15, and 20?V/cm have been studied for electro-osmosis tests under the pressure of a static or rotating piston, obtaining a dry solids content up to 40–45%, with respect to 25–30% obtained by mechanical methods. In order to optimize the process, the corrosion behavior and the wear of the anodic material appear to be the main critical aspects, due to the high circulating current density and the use of a rotating electrode. We compared the efficiency and the corrosion resistance of dimensionally stable anodes (DSA) with respect to bare stainless steel (AISI 304) and stainless steel coated by PVD technique with TiN, AlTiN, and DLC. Characterization of the anode surfaces by SEM and potentiodynamic tests show that DSA is the most suitable material for our application. However, efficiencies of the electro-osmosis processes have been found comparable, in terms of developed current densities and total energy consumptions, for short-test duration.     

Mechanism for sludge acidification in aerobic treatment of coking wastewater

This work was undertaken to investigate the cause of sludge acidification that led to disruption of the activated sludge process treating coking wastewater from a steel-making plant in Taiwan. An activated sludge reactor (ASR) with a working volume of 80 L was used as a model system to simulate the behavior of the real wastewater treatment process. Parameters that may cause acidification or inactivation of the sludge (NH(3), SCN(-), S(2)O(3)(2-) and CN(-)) were studied individually to examine for their effects on the performance of the ASR. The results show that high loading of NH(3), SCN(-) and CN(-) did not lead to pH decrease, while the ASR attained 85% COD removal and nearly 100% SCN degradation. In contrast, when the wastewater was supplemented with ca. 1,000 mg/L of S(2)O(3)(2-), the pH dropped to nearly 4.0 in 2 days and the COD and SCN removal yields were significantly lower (at 50 and 0-20%, respectively). Thus, overloading of S(2)O(3)(2-) was apparently a key factor causing sludge acidification. The results suggest that to ensure a normal functioning of the activated sludge, the influent S(2)O(3)(2-) concentration should be closely monitored and that the pH control of the ASR is indispensable when the S(2)O(3)(2-) loading is in excess.