Kinetic model of excess activated sludge thermohydrolysis

Thermal hydrolysis of excess activated sludge suspensions was carried at temperatures ranging from 423 K to 523 K and under pressure 0.2-4.0 MPa. Changes of total organic carbon (TOC) concentration in a solid and liquid phase were measured during these studies. At the temperature 423 K, after 2 h of the process, TOC concentration in the reaction mixture decreased by 15-18% of the initial value. At 473 K total organic carbon removal from activated sludge suspension increased to 30%.It was also found that the solubilisation of particulate organic matter strongly depended on the process temperature. At 423 K the transfer of TOC from solid particles into liquid phase after 1 h of the process reached 25% of the initial value, however, at the temperature of 523 K the conversion degree of ‘solid’ TOC attained 50% just after 15 min of the process.In the article a lumped kinetic model of the process of activated sludge thermohydrolysis has been proposed. It was assumed that during heating of the activated sludge suspension to a temperature in the range of 423-523 K two parallel reactions occurred. One, connected with thermal destruction of activated sludge particles, caused solubilisation of organic carbon and an increase of dissolved organic carbon concentration in the liquid phase (hydrolysate). The parallel reaction led to a new kind of unsolvable solid phase, which was further decomposed into gaseous products (CO₂). The collected experimental data were used to identify unknown parameters of the model, i.e. activation energies and pre-exponential factors of elementary reactions. The mathematical model of activated sludge thermohydrolysis appropriately describes the kinetics of reactions occurring in the studied system.     

Screening of antimycotics in Swedish sewage treatment plants - Waters and sludge

Concentrations of six pharmaceutical antimycotics were determined in the sewage water, final effluent and sludge of five Swedish sewage treatment plants (STPs) by solid phase extraction, liquid/solid extraction, and liquid chromatography-electrospray tandem mass spectrometry. The antimycotics were quantified by internal standard calibration. The results were used to estimate national flows that were compared to predictions based on sales figures. Fluconazole was the only one of the six investigated antimycotics that was detected (at concentrations ranging from 90 to 140 ng L⁻¹) in both raw sewage water and final effluent. Negligible amounts of this substance were removed from the aqueous phase, and its levels were below the limit of quantification in all of the analyzed sludge samples. In contrast, clotrimazole, ketoconazole and econazole were present in all of the sludge samples, at concentrations ranging between 200 and 1000 μg kg⁻¹, dry weight. There were close correlations between the national measured and predicted antimycotic mass flows. Antimycotic fate analysis, based on sales figures, indicated that 53% of the total amount of fluconazole sold appeared in the final effluents of the STPs, while 1, 155, 35, 209 and 41% of the terbinafine, clotrimazole, ketoconazole, econazole and miconazole sold appeared in the digested dewatered sludge.     

Thermochemical Treatment of Sewage Sludge

Technologies for the thermochemical treatment of sewage sludge are appraised with reference to their efficacy in terms of (a) operational parameters, (b) pre-and post-treatment requirements, and (c) the extent of their use for the application. Particular attention is given to the characteristics of the solid and gaseous products and how such characteristics affect post-treatment, reuse or disposal – this being dictated by environmental legislation.
Of specific interest is the off-gas treatment, which can contribute significantly to operating and capital costs of incineration. On the other hand, gasification and pyrolysis generate a combustible product for use in heat and/or power generation. Available information, although limited, suggests that gasification offers significant advantages over the conventional incineration process, yielding a combustible gaseous product (which can be directly utilised on site) and a solid char product (which is less prone to leaching of toxic metals).     

厌氧—生物接触氧化—滤布滤池深度处理校园污水

西北干旱地区某校园污水先采用水解酸化阶段-产甲烷阶段分离的厌氧生物处理及两级生物接触氧化处理,出水再进行滤布滤池深度处理。结果表明,该工艺能有效地将高有机物浓度、高氨氮的校园生活污水处理后达到城市污水再生利用杂用水的水质标准。该工程利用厌氧污泥回流到产甲烷阶段前,腐殖污泥回流到水解酸化池并加大污泥回流率的方法,基本无剩余污泥排放。     

Effect of linear alkylbenzene sulphonates (LAS) on the anaerobic digestion of sewage sludge

Batch anaerobic biodegradation tests with different alkylbenzene sulphonates (LAS) at increasing concentrations were performed in order to investigate the effect of LAS homologues on the anaerobic digestion process of sewage sludge. Addition of LAS homologues to the anaerobic digesters increased the biogas production at surfactant concentrations 5-10 g/kg dry sludge and gave rise to a partial or total inhibition of the methanogenic activity at higher surfactant loads. Therefore, at the usual LAS concentration range in sewage sludge, no adverse effects on the anaerobic digesters functioning of a wastewater treatment plant (WWTP) can be expected. The increase of biogas production at low surfactant concentrations was attributed to an increase of the bioavailability and subsequent biodegradation of organic pollutants associated with the sludge, promoted by the surfactant adsorption at the solid/liquid interface. When the available surfactant fraction in the aqueous phase instead of the nominal surfactant concentration was used to evaluate the toxicity of LAS homologues, a highly significant relationship between toxicity and alkyl chain length was obtained. Taking into account the homologue distribution of commercial LAS in the liquid phase of the anaerobic digesters of a WWTP, an EC(50) value of 14 mg/L can be considered for LAS toxicity on the anaerobic microorganisms.     

Kinetic modeling of a mixed culture of Pseudomonas denitrificans and Bacillus subtilis under aerobic and anoxic operating conditions

The kinetics of biological denitrification have been studied and several models, with varying degree of complexity, to be used for design purposes have been presented in the recent years. However, most of these kinetic studies were performed with mixed (and not well defined) microbial systems, such as activated sludge. In the present work, kinetic experiments were carried out in order to study the dynamic characteristics of a defined mixed culture of the denitrifiers Pseudomonas denitrificans and Bacillus subtilis under anoxic and aerobic conditions in a defined synthetic medium involving a mixture of organic substrates, in the presence of nitrates and/or nitrites. Denitrification was assumed to occur by the consecutive reduction of nitrates to nitrites and then to nitrogen gas without accumulation of intermediate gaseous products. The behavior of these defined mixed cultures was predicted using a kinetic model based on the kinetic models that have already been developed for each bacterium separately and the predictions were compared with the results from mixed culture experiments. The overall mathematical model that was developed and validated in the present work is capable of describing the behavior of the mixed culture in the above conditions, i.e. the nitrates and nitrites reduction kinetics, the cell growth, and the organic carbon utilization rates.     

Ozonation kinetics of cork-processing water in a bubble column reactor

The oxidative degradation of organic pollutants present in cork-processing water at natural pH (6.45) was studied in a bubble column ozonation reactor. A steady reduction in both chemical oxygen demand (COD) and total organic carbon (TOC) was observed under the action of ozone alone and the feasibility of deep mineralisation (organic matter removal more than 90% in 120 min under the following experimental conditions: liquid volume 9L; superficial gas velocity 6.8x10(-3) m s(-1); ozone partial pressure 1.31 kPa; initial COD 328 mg L(-1); initial TOC 127 mg L(-1)) was demonstrated. The monitoring of pH, redox potential (ORP) and the mean oxidation number of carbon (MOC) was correlated with the oxidation and mineralisation of the organic species in the water. The ozonation of cork-processing water in the bubble column was analysed in terms of a mole balance coupled with ozonation kinetics modelled by the two-film theory of mass transfer and chemical reaction. Under the experimental conditions used, and in contrast with the literature, it was determined that the reaction follows a fast kinetic regime at the beginning of the oxidation process, shifting to the moderate and the slow kinetic regimes at later stages of the oxidation reaction. The dynamic change of the rate coefficient estimated by the model was correlated to changes in the water composition.     

Modelling the dynamics of the activated sludge wastewater treatment process in terms of the carbon variable

Carbon as a process variable was used to characterize adequately the dynamic nature of the activated sludge process. In this study an unsteady state mass balance model in terms of the soluble organic and suspended solid organic carbons has been posed. This model considers the concentration of organic substrate, the concentration of cellular material and the concentration of products stocked in the cells for the aeration unit of the process. Predictions of the models for the substrate and biomass concentrations in the aerator are compared to the observed results of the earlier work. The proposed model is also compared to simpler models based uniquely on the growth kinetic concept using Monod or two-phase expressions. The inadequacy of the simple models in predicting some important aspects of the unsteady state operation is discussed.     

Volatile fatty acid impacts on nitrite oxidation and carbon dioxide fixation in activated sludge

Batch test were performed to assess nitrite removal, nitrate formation, CO2 fixation, gaseous nitrogen production and microbial density in activated sludge exposed to volatile fatty acid (VFA) mixtures. Nitrite removal and nitrate formation were both affected by the presence of VFAs, but to different degrees. Nitrate formation rates were reduced to a greater extent (79%) than nitrite removal rates (36%) resulting in an apparent unbalanced nitrite oxidation reaction. Since the total bacterial density and the nitrite oxidizing bacteria (NOB, Nitrospira) concentration remained essentially constant under all test conditions, the reduction in rates was not due to heterotrophic uptake of nitrogen or to a decrease in the NOB population. In contrast to the nitrogen results, VFAs were not found to impact CO2 fixation efficiency. It appeared that nitrite oxidation occurred when VFAs were present since the oxidation of nitrite provides energy for CO2 fixation. However, nitrate produced from the oxidation of nitrite was reduced to gaseous nitrogen products. N2O gas was detected in the presence of VFAs which was a clear indication that VFAs stimulated an alternative pathway, such as aerobic denitrification, during biotransformation of nitrogen in activated sludge.     

A nitrification model for the contact stabilization activated sludge process

A kinetic model of the contact stabilization process has been developed and experimentally verified with the aid of bench-scale activated sludge units treating domestic sewage. The model provides information on the relationship between the design parameters (process loading, temperature, residence time distribution) and process performance (sludge, production, oxygen uptake, COD-removal, organic nitrogen conversion, nitrification and effluent suspended solids). An oxygen equivalence mass balance equation, which is applicable to all activated sludge process modifications is proposed and may be used in the design and operation of these processes.     

Simultaneous nitrification and p-cresol oxidation in a nitrifying sequencing batch reactor

The tolerance, kinetic behavior and oxidizing ability of a nitrifying sludge exposed to different initial concentrations of p-cresol (25-150mg/l) were evaluated in a sequencing batch reactor (SBR) fed with 200mg NH(4)(+)-N/ld. The nitrifying SBR operated up to 300mg/ld of p-cresol, achieving simultaneously the complete ammonium oxidation to nitrate and the total consumption of p-cresol and its transitory intermediates from the culture. p-Cresol induced a significant decrease in the values for specific rates of ammonium consumption, showing that the ammonium oxidation pathway was mainly inhibited. After 7 months of operation in SBR, the specific rates of NH(4)(+)-N oxidation, NO(3)(-)-N formation, and total organic carbon consumption were 0.6g NH(4)(+)-N/g microbial protein-Nh, 0.3g NO(3)(-)-N/g microbial protein-Nh, and 0.24g total organic carbon/g microbial protein h, respectively. The microbial growth rate was always low (maximum value of 12.2+/-0.4mg protein-N/ld) and settleability of the sludge was good with sludge volume index values lower than 21ml/g. The oxidation of p-cresol and its intermediates was carried out faster throughout the cycles and nitrification inhibition decreased with the number of cycles.     

The decomposition of sewage sludge in seawater

Laboratory experiments were conducted over a 12 week period to follow the decomposition of sewage sludge in seawater and in sediment-seawater mixtures under aerobic and anaerobic conditions at 4° and 21°C. Results showed that the sewage sludge decomposed more rapidly in the presence of oxygen. Dissolved organic carbon, a major carbon source in sewage sludge, abruptly decreased to very low concentrations in 3–4 weeks in aerobic systems; concentrations of particulate carbon decreased only gradually during the 12 week period. Aerobic conditions at the sewage sludge dump site in the New York Bight apex can promote rapid decomposition of sewage sludge if it is distributed evenly over an area of at least 5.2 km² (2 miles).     

反硝化脱氮碳源研究现状

针对目前我国污水处理厂存在的脱氮碳源不足的问题,提出了以纤维素为主的固体碳源、污泥水解产生的挥发性脂肪酸以及有机工业废水等新型碳源,并探讨了这几种新型碳源的研究现状,为提高脱氮效率提供了依据。     

Phosphorus recovery from sewage sludge with a hybrid process of low pressure wet oxidation and nanofiltration

Phosphorus recovery from sewage sludge will become increasingly important within the next decades due to depletion of mineral phosphorus resources. In this work a new process concept was investigated, which aims at realising phosphorus recovery in a synergistic way with the overall sewage sludge treatment scheme. This process combines a low pressure wet oxidation for sewage sludge decomposition as well as phosphorus dissolution and a nanofiltration process to separate phosphorus from heavy metals and obtain a clean diluted phosphoric acid, from which phosphorus can be recovered as clean fertiliser.It was shown that this process concept is feasible for sewage sludge for wastewater treatment plants that apply enhanced biological removal or precipitation with alumina salts for phosphorus removal. The critical parameter for phosphorus dissolution in the low pressure wet oxidation process is the iron concentration, while in the nanofiltration multi-valent cations play a predominant role.In total, a phosphorus recovery of 54% was obtained for an exemplary wastewater treatment plant. Costs of the entire process are in the same range as conventional sewage sludge disposal, with the benefit being phosphorus recovery and reduced emission of greenhouse gases due to avoidance of sludge incineration.     

城市污水处理厂泥区废液水质特性分析

采用常规水质分析方法、三维荧光光谱分析方法、超滤分子质量分级分析方法以及色谱法,对3座典型城市污水处理厂的泥区废液水质特性进行了调查分析。结果表明,泥区废液污染物的负荷较高,其水质状况与污水、污泥处理工艺及其运行工况有关。泥区废液中的主要有机污染物种类有类蛋白质、类富里酸及类腐殖酸,各类物质的含量与污泥稳定程度有关,浓缩池上清液和未经消化的污泥脱水滤液以类蛋白质为主,污泥消化后的脱水滤液以类富里酸或类腐殖酸为主。泥区混合废液中表观分子质量〉6ku的有机物所占比例〉70％,泥区废液中内分泌干扰物质壬基酚及其前体物质壬基酚聚氧乙烯醚含量约为1．35～23．01μg／L。     

湿式氧化处理城市污水厂污泥的研究

在２Ｌ高压釜中用氧气进行了湿式氧化处理城市污水厂活性污泥的研究,并对处理后的污泥上清液中氮、磷、重金属变化规律和剩余污泥各挥发分、热量、有机物含量的变化规律进行了分析。试验结果表明,处理后污泥的沉降脱水性能极佳,对污泥中固体的去除结果也较好。     

太原循环经济环卫产业园固体废物污水处理厂工艺设计

为解决太原循环经济环卫产业园内生活垃圾焚烧处理处置、餐厨垃圾处理处置及其他固废处理处置过程中产生的高浓度废水处理问题,拟新建一座污水处理厂。设计处理规模为1 200 t/d,主要包括750 t/d的焚烧厂垃圾渗滤液和450 t/d的餐厨沼液。渗滤液处理采用"气浮+调节池+内循环厌氧反应器+两级A/O-MBR"的核心工艺,餐厨沼液处理采用"气浮+调节池+两级A/O-MBR"的核心工艺。污泥处理采用"离心脱水+热干化"工艺,处理后污泥含水率≤30%,干化污泥采用密封车辆送至焚烧厂焚烧处理。试运行结果表明,出水水质稳定达到设计标准。污水处理厂总投资为1.3亿元,污水处理直接成本为23.5元/t。     

Effluent treatment and algae recovery by ozone-induced flotation

Separation of algae from oxidation pond effluents was tested in a bench scale flotation column using ozone-enriched oxygen. Experimental results indicated that the process could produce a clear, colorless liquid, over 98% removal of suspended solids, and up to three orders of magnitude reduction in fecal coliforms. Ozone dosage ranged from 15 to 50 mgl⁻¹. The collapsed froth thickens to produce a pure algal mass of up to 8.5% solids. It was hypothesized and supported by experimental results that ozone reaction with algal cells results in the formation of a hydrophobic surface amenable to separation by rising bubbles. The high quality products, both liquid and solid, make the combination of stabilization ponds with ozone flotation very attractive for wastewater treatment under certain conditions.     

Effluent quality variation from multicomponent substrates in the activated sludge process

The objectives of this investigation were: (1) to examine the overall substrate removal of a three component mixture by an acclimated seed using total organic carbon measurements, (2) to observe the rates of removal of each of the three components in the mixture at four different food-to-microorganism ratios (F/M), (3) to show the effects of the composition of the substrate with varying degradability on effluent quality, and (4) to observe the effects of sludge age on total substrate removal.To accomplish these objectives, sludge from a municipal sewage treatment plant was acclimated as seed in a continuous flow, completely mixed laboratory reactor. A multicomponent substrate consisting of a mixture of glucose, phenol and sulfanilic acid, was fed at different F/M ratios. The removal rate was determined as total organic carbon and specific analytical measurements for each substrate in the influent and effluent samples were also made. Various combinations of glucose, phenol and sulfanilic acid were used as a substrate in three runs. This made it possible to observe the removal rate of each substrate in the mixture as related to their individual biodegradability as well as their influent concentrations.The results showed that for the three substrates employed in this study, the overall removal rate, in terms of total organic carbon in a mixed acclimated culture, is the sum of the individual specific removal rates. The effluent quality in terms of soluble TOC was related to the F/M or to the sludge age using Eckenfelder's kinetic model.     

Tracers of sewage sludge in the marine environment: a review

A review of the available and potential tracers of the movement and distribution of sewage sludge ('sludge') in and on marine sediments is presented. A brief review of 'active' sludge tracers is followed by a more extensive review of 'passive' sludge tracers. These can be broken down into five categories: Natural organic components of sludge--organic carbon, faecal steroids, carbohydrate/TOC ratio, tomato seeds. Synthetic organic compounds--silicones, chlorinated hydrocarbons, non-ionic surfactants and their degradation products, linear alkylbenzenes. Stable isotopes of carbon, nitrogen, hydrogen and sulphur. Inorganic substances and their properties--minerals, paint pigments, trace elements, magnetic properties. Biological/Microbiological--faecal bacteria, Clostridium welchii spores, amoebae, viruses, nitrogen-fixing enterobacteriaceae. A wide variety of sludge tracers are available with the choice of suitable tracers in an individual situation governed by the available equipment and expertise, cost considerations, ease of sampling requirements and determination, and the nature of other pollutant inputs in the vicinity of the input of interest. Ideally, the sludge tracers chosen should be unique to the input of interest, but this will rarely be achievable in practice. In areas of gross sludge contamination, a marine geophysical survey technique can be used to delineate the area affected and this provides a rapid cost-effective method for large areas.