Biodegradability of activated sludge organics under anaerobic conditions

From an experimental and theoretical investigation of the continuity of activated sludge organic (COD) compounds along the link between the fully aerobic or N removal activated sludge and anaerobic digestion unit operations, it was found that the unbiodegradable particulate organics (i) originating from the influent wastewater and (ii) generated by the activated sludge endogenous process, as determined from response of the activated sludge system, are also unbiodegradable under anaerobic digestion conditions. This means that the activated sludge biodegradable organics that can be anaerobically digested can be calculated from the active fraction of the waste activated sludge based on the widely accepted ordinary heterotrophic organism (OHO) endogenous respiration/death regeneration rates and unbiodegradable fraction. This research shows that the mass balances based steady state and dynamic simulation activated sludge, aerobic digestion and anaerobic digestion models provide internally consistent and externally compatible elements that can be coupled to produce plant wide steady state and dynamic simulation WWTP models.     

A new plant-wide modelling methodology for WWTPs

This paper presents a new plant-wide modelling methodology for describing the dynamic behaviour of water and sludge lines in WWTPs. The methodology is based on selecting the set of process transformations needed for each specific WWTP to model all unit-process elements in the entire plant. This "transformation-based" approach, in comparison with the conventional "process-based" approach, does not require the development of specific transformers to interface the resulting unit-process models, facilitates the mass and charge continuity throughout the whole plant and is flexible enough to construct models tailored for each plant under study. As an illustrative example, a plant-wide model for a WWTP that includes carbon removal and anaerobic digestion has been constructed, and the main advantages of the proposed methodology for integrated modelling have been demonstrated. As a final consequence, this paper proposes a rewriting of the existing unit-process models according to the new standard transformation-based approach for integrated modelling purposes.     

An ASM/ADM model interface for dynamic plant-wide simulation

Mathematical modelling has proven to be very useful in process design, operation and optimisation. A recent trend in WWTP modelling is to include the different subunits in so-called plant-wide models rather than focusing on parts of the entire process. One example of a typical plant-wide model is the coupling of an upstream activated sludge plant (including primary settler, and secondary clarifier) to an anaerobic digester for sludge digestion. One of the key challenges when coupling these processes has been the definition of an interface between the well accepted activated sludge model (ASM1) and anaerobic digestion model (ADM1). Current characterisation and interface models have key limitations, the most critical of which is the over-use of X_c (or lumped complex) variable as a main input to the ADM1. Over-use of X_c does not allow for variation of degradability, carbon oxidation state or nitrogen content. In addition, achieving a target influent pH through the proper definition of the ionic system can be difficult. In this paper, we define an interface and characterisation model that maps degradable components directly to carbohydrates, proteins and lipids (and their soluble analogues), as well as organic acids, rather than using X_c. While this interface has been designed for use with the Benchmark Simulation Model No. 2 (BSM2), it is widely applicable to ADM1 input characterisation in general. We have demonstrated the model both hypothetically (BSM2), and practically on a full-scale anaerobic digester treating sewage sludge.     

Potential nanosilver impact on anaerobic digestion at moderate silver concentrations

Silver nanoparticles (AgNPs, nanosilver) entering the sewers and wastewater treatment plants (WWTPs) are mostly accumulated in the sludge. In this study, we determined the impact of AgNPs on anaerobic glucose degradation, sludge digestion and methanogenic assemblages. At ambient (22 °C) and mesophilic temperatures (37 °C), there was no significant difference in biogas and methane production between the sludge treated with AgNPs at the concentrations up to 40 mg Ag/L (13.2 g silver/Kg biomass COD) and the control. In these anaerobic digestion samples, acetate and propionic acid were the only detectable volatile fatty acids (VFAs) and they were depleted in 3 days. On the other hand, more than 90% of AgNPs was removed from the liquid phase and associated with the sludge while almost no silver ions were released from AgNPs under anaerobic conditions. Quantitative PCR results indicated that Methanosaeta and Methanomicrobiales were the dominant methanogens, and the methanogenic diversity and population remained largely unchanged after nanosilver exposure and anaerobic digestion. The results suggest that AgNPs at moderate concentrations (e.g., ≤40 mg/L) have negligible impact on anaerobic digestion and methanogenic assemblages because of little to no silver ion release.     

The effect of primary sedimentation on full-scale WWTP nutrient removal performance

Traditionally, the performance of full-scale wastewater treatment plants (WWTPs) is measured based on influent and/or effluent and waste sludge flows and concentrations. Full-scale WWTP data typically have a high variance which often contains (large) measurement errors. A good process engineering evaluation of the WWTP performance is therefore difficult. This also makes it usually difficult to evaluate effect of process changes in a plant or compare plants to each other. In this paper we used a case study of a full-scale nutrient removing WWTP. The plant normally uses presettled wastewater, as a means to increase the nutrient removal the plant was operated for a period by-passing raw wastewater (27% of the influent flow). The effect of raw wastewater addition has been evaluated by different approaches: (i) influent characteristics, (ii) design retrofit, (iii) effluent quality, (iv) removal efficiencies, (v) activated sludge characteristics, (vi) microbial activity tests and FISH analysis and, (vii) performance assessment based on mass balance evaluation. This paper demonstrates that mass balance evaluation approach helps the WWTP engineers to distinguish and quantify between different strategies, where others could not. In the studied case, by-passing raw wastewater (27% of the influent flow) directly to the biological reactor did not improve the effluent quality and the nutrient removal efficiency of the WWTP. The increase of the influent C/N and C/P ratios was associated to particulate compounds with low COD/VSS ratio and a high non-biodegradable COD fraction.     

Operational window of a deammonifying sludge for mainstream application in a municipal wastewater treatment plant

The present work aimed to study the mainstream feasibility of the deammonifying sludge of side stream of municipal wastewater treatment plant (MWWTP) in Kaster, Germany. For this purpose, the deammonifying sludge available at the side stream was investigated for nitrogen (N) removal with respect to the operational factors temperature (15–30°C), pH value (6.0–8.0) and chemical oxygen demand (COD)/N ratio (≤1.5–6.0). The highest and lowest N-removal rates of 0.13 and 0.045 kg/(m³ d) are achieved at 30 and 15°C, respectively. Different conditions of pH and COD/N ratios in the SBRs of Partial nitritation/anammox (PN/A) significantly influenced both the metabolic processes and associated N-removal rates. The scientific insights gained from the current work signifies the possibility of mainstream PN/A at WWTPs. The current study forms a solid basis of operational window for the upcoming semi-technical trails to be conducted prior to the full-scale mainstream PN/A at WWTP Kaster and WWTPs globally.     

基于生物营养盐去除工艺所产污泥的处理对策

青岛市两座采用生物营养盐去除工艺的污水处理厂的运行实践表明,初沉污泥和剩余污泥应尽可能在好氧状态下处理,以避免污泥反硝化上浮、磷酸盐和氨氮的厌氧释放以及由此带来的鸟粪石结垢问题.在生物营养盐去除技术背景下,采用厌氧消化进行污泥稳定的污水处理厂,可以利用"有控制的鸟粪石结晶析出技术"来解决鸟粪石结垢问题;如污泥可以土地利用,宜采用好氧堆肥技术对污泥进行稳定.传统活性污泥法污水厂升级时不但要增加营养盐去除措施,还要考虑到对污泥处理系统进行相应的改造.     

Tracking the inorganic suspended solids through biological treatment units of wastewater treatment plants

From an experimental and theoretical investigation of the continuity of influent inorganic suspended solids (ISS) along the links connecting the primary settling tank (PST), fully aerobic or N removal activated sludge (AS) and anaerobic and aerobic digestion (AerD) unit operations, it was found that (i) the influent wastewater (fixed) ISS concentration is conserved through primary sludge anaerobic digestion, and AS and AerD unit operations. However, the measured ISS flux at different stages through a series of WWTP unit operations is not equal to the influent ISS flux because the ordinary heterotrophic organisms (OHO) biomass contributes to the ISS flux by differing amounts depending on the OHO (active) fraction of the VSS solids at that stage.     

Continuity-based model interfacing for plant-wide simulation: a general approach

In plant-wide simulation studies of wastewater treatment facilities, often existing models from different origin need to be coupled. However, as these submodels are likely to contain different state variables, their coupling is not straightforward. The continuity-based interfacing method (CBIM) provides a general framework to construct model interfaces for models of wastewater systems, taking into account conservation principles. In this contribution, the CBIM approach is applied to study the effect of sludge digestion reject water treatment with a SHARON-Anammox process on a plant-wide scale. Separate models were available for the SHARON process and for the Anammox process. The Benchmark simulation model no. 2 (BSM2) is used to simulate the behaviour of the complete WWTP including sludge digestion. The CBIM approach is followed to develop three different model interfaces. At the same time, the generally applicable CBIM approach was further refined and particular issues when coupling models in which pH is considered as a state variable, are pointed out.     

白龙港污水处理厂污泥厌氧消化系统的设计和调试

白龙港污泥处理工程主要处理白龙港污水处理厂200×104 m3/d污水处理产生的污泥,采用浓缩、中温厌氧消化、污泥脱水和部分脱水污泥干化处理工艺,消化产生的沼气用于加热消化池污泥,多余的沼气作为能源干化脱水污泥。干化处理后冷凝水的余热回收辅助用于污泥消化系统供热。介绍了污泥厌氧消化系统的设计参数、工艺特点及调试情况,可供设计人员参考。     

Effects of soluble and particulate substrate on the carbon and energy footprint of wastewater treatment processes

Most wastewater treatment plants monitor routinely carbonaceous and nitrogenous load parameters in influent and effluent streams, and often in the intermediate steps. COD fractionation discriminates the selective removal of VSS components in different operations, allowing accurate quantification of the energy requirements and mass flows for secondary treatment, sludge digestion, and sedimentation. We analysed the different effects of COD fractions on carbon and energy footprint in a wastewater treatment plant with activated sludge in nutrient removal mode and anaerobic digestion of the sludge with biogas energy recovery. After presenting a simple rational procedure for COD and solids fractions quantification, we use our carbon and energy footprint models to quantify the effects of varying fractions on carbon equivalent flows, process energy demand and recovery. A full-scale real process was modelled with this procedure and the results are reported in terms of energy and carbon footprint. For a given process, the increase of the ratio sCOD/COD increases the energy demand on the aeration reactors, the associated CO₂ direct emission from respiration, and the indirect emission for power generation. Even though it appears as if enhanced primary sedimentation is a carbon and energy footprint mitigation practice, care must be used since the nutrient removal process downstream may suffer from an excessive bCOD removal and an increased mean cell retention time for nutrient removal may be required.     

Biological hydrolysis and acidification of sludge under anaerobic conditions: the effect of sludge type and origin on the production and composition of volatile fatty acids

New wastewater treatment processes resulting in considerably reduced sludge production and more effective treatment are needed. This is due to the more stringent legislations controlling discharges of wastewater treatment plants (WWTPs) and existing problems such as high sludge production. In this study, the feasibility of implementing biological hydrolysis and acidification process on different types of municipal sludge was investigated by batch and semi-continuous experiments. The municipal sludge originated from six major treatment plants located in Denmark were used. The results showed that fermentation of primary sludge produced the highest amount of volatile fatty acids (VFAs) and generated significantly higher COD- and VFA-yields compared to the other sludge types regardless of which WWTP the sludge originated from. Fermentation of activated and primary sludge resulted in 1.9-5.6% and 8.1-12.6% COD-yields, soluble COD (SCOD)/total COD (TCOD), in batch experiments, respectively. The COD-yields for primary, activated and mixed sludge were 19.1%, 6.5% and 21.37%, respectively, in semi-continuous experiments operated at solids retention time (SRT) of 5d and temperature of 37 degrees C. The benefit of fermentation for full-scale application was roughly estimated based on the experiments performed in semi-continuous reactors. The results revealed that even though the VFA production of primary sludge was higher compared to activated sludge, substantial amounts of VFA could be produced by fermentation of activated sludge due to the substantially higher production of activated sludge in WWTPs.     

Fate of beta blockers and psycho-active drugs in conventional wastewater treatment

The removal of beta blockers and psycho-active drugs was investigated in a representative conventional German WWTP by long-term measurement campaigns along different biological treatment processes. The activated sludge treatment with an elevated SRT of 18 d was the only process which led to a significant removal of certain beta blockers and psycho-active drugs. The removal efficiency was below 60% for all compounds except for the natural opium alkaloids codeine and morphine being removed by more than 80%. Primary biological transformation and sorption onto sludge as the main removal mechanisms were examined in lab-scale batch experiments. Sorption onto activated sludge was found to be negligible (<3%). The biological transformation could be described by pseudo-first order kinetics and the transformation constants k_biol were used to predict the removal of beta blockers and psycho-active drugs in an activated sludge unit with a model. For most compounds the removal efficiencies measured on the full-scale WWTP were within the 95% confidence intervals predicted by the model. The results from full-scale measurements and modeling indicate that biological transformation in the nitrification tank together with parameters such as the sludge retention time and the temperature is crucial regarding the biological transformation of beta blockers and psycho-active drugs in conventional WWTPs.     

Nitrous oxide emission during wastewater treatment

Nitrous oxide (N₂O), a potent greenhouse gas, can be emitted during wastewater treatment, significantly contributing to the greenhouse gas footprint. Measurements at lab-scale and full-scale wastewater treatment plants (WWTPs) have demonstrated that N₂O can be emitted in substantial amounts during nitrogen removal in WWTPs, however, a large variation in reported emission values exists. Analysis of literature data enabled the identification of the most important operational parameters leading to N₂O emission in WWTPs: (i) low dissolved oxygen concentration in the nitrification and denitrification stages, (ii) increased nitrite concentrations in both nitrification and denitrification stages, and (iii) low COD/N ratio in the denitrification stage. From the literature it remains unclear whether nitrifying or denitrifying microorganisms are the main source of N₂O emissions. Operational strategies to prevent N₂O emission from WWTPs are discussed and areas in which further research is urgently required are identified.     

Fossil organic carbon in wastewater and its fate in treatment plants

This study reports the presence of fossil organic carbon in wastewater and its fate in wastewater treatment plants. The findings pinpoint the inaccuracy of current greenhouse gas accounting guidelines which defines all organic carbon in wastewater to be of biogenic origin. Stable and radiocarbon isotopes (¹³C and ¹⁴C) were measured throughout the process train in four municipal wastewater treatment plants equipped with secondary activated sludge treatment. Isotopic mass balance analyses indicate that 4–14% of influent total organic carbon (TOC) is of fossil origin with concentrations between 6 and 35 mg/L; 88–98% of this is removed from the wastewater. The TOC mass balance analysis suggests that 39–65% of the fossil organic carbon from the influent is incorporated into the activated sludge through adsorption or from cell assimilation while 29–50% is likely transformed to carbon dioxide (CO₂) during secondary treatment. The fossil organic carbon fraction in the sludge undergoes further biodegradation during anaerobic digestion with a 12% decrease in mass. 1.4–6.3% of the influent TOC consists of both biogenic and fossil carbon is estimated to be emitted as fossil CO₂ from activated sludge treatment alone. The results suggest that current greenhouse gas accounting guidelines, which assume that all CO₂ emission from wastewater is biogenic may lead to underestimation of emissions.     

Biodegradation of the endogenous residue of activated sludge

This study evaluated the potential biodegradability of the endogenous residue in activated sludge subjected to batch digestion under either non-aerated or alternating aerated and non-aerated conditions. Mixed liquor for the tests was generated in a 200 L pilot-scale aerobic membrane bioreactor (MBR) operated at a 5.2 days SRT. The MBR system was fed a soluble and completely biodegradable synthetic influent composed of sodium acetate as the sole carbon source. This influent, which contained no influent unbiodegradable organic or inorganic materials, allowed to generate sludge composed of essentially two fractions: a heterotrophic biomass X_H and an endogenous residue X_E, the nitrifying biomass being negligible (less than 2%). The endogenous decay rate and the active biomass fraction of the MBR sludge were determined in 21-day aerobic digestion batch tests by monitoring the VSS and OUR responses. Fractions of X_H and X_E: 68% and 32% were obtained, respectively, at a 5.2 days SRT. To assess the biodegradability of X_E, two batch digestion units operated at 35 °C were run for 90 days using thickened sludge from the MBR system. In the first unit, anaerobic conditions were maintained while in the second unit, alternating aerated and non-aerated conditions were applied. Data for both units showed apparent partial biodegradation of the endogenous residue. Modeling the batch tests indicated endogenous residue decay rates of 0.005 d⁻¹ and 0.012 d⁻¹ for the anaerobic unit and the alternating aerated and non-aerated conditions, respectively.     

Evaluation of the fate of perfluoroalkyl compounds in wastewater treatment plants

Recent studies have shown that the wastewater treatment plant (WWTP) is a significant source of perfluoroalkyl compounds (PFCs) in natural water. In this study, 10 PFCs were analyzed in influent and effluent wastewater and sludge samples in 15 municipal, 4 livestock and 3 industrial WWTPs in Korea. The observed distribution pattern of PFCs differed between the wastewater and sludge samples. Perfluorooctane sulfonate (PFOS) was dominant in the sludge samples with a concentration ranging from 3.3 to 54.1 ng/g, whereas perfluorooctanoic acid (PFOA) was dominant in wastewater and ranged from 2.3 to 615 ng/L and 3.4 to 591 ng/L in influent and effluent wastewater, respectively. Principal component analysis (PCA) results provided an explanation for this variation in PFC distribution patterns in the aqueous and sludge samples. The fates of PFCs in the WWTPs were related with the functional groups. The PFOS concentrations tended to decrease after treatment in most WWTPs, whereas PFOA increased. The different fates of PFOA and PFOS in WWTPs were attributed to the higher organic carbon-normalized distribution coefficient of perfluoroalkylsulfonate (PFASs) than that of the carboxylate analog, indicating the preference of PFASs to partition to sludge. Although industrial WWTPs contained high concentration of PFCs, they are not the main source of PFCs in Korean water environment because of their small release amount. WWTPs located in big cities discharged more PFCs, suggesting household sewage is one of the significant sources of PFCs contamination in the environment.     

集中式污水处理系统的最佳规模研究

综合分析污水处理厂的吨水投资、管网投资和运行费用有助于弄清城镇污水末端处理的规模效应。研究表明,中、小规模污水处理厂的吨水投资、运行费用在不同处理工艺间均存在规模效应,且相关性显著;大型和特大型污水处理厂的吨水投资和运行费用均保持在较低的范围内,且随着处理规模的增加而逐步稳定;当污水管网的收集规模<6×104m3/d时,其吨水投资的规模效应突出;随着污水管网规模的增大,吨水管网投资趋于较低的水平。当污水处理厂仅作为排水系统的末端环节时,综合考虑污水处理厂和收集管网的单位投资及运行费用等因素,最佳的处理规模为10×104m3/d左右;当污水处理厂具有污染治理和污水再生回用的双重功能而成为水资源循环的中间环节时,适宜的污水处理规模受多种因素的影响,区域性的半集中式处理模式将成为一种可行的选择。     

Treatment of high strength acidic wastewater with a completely mixed anaerobic filter

A completely mixed anaerobic filter, in which the influent organic matter concentration is diluted with recirculated effluent, was found to effectively remove organic matter concentrations in high strength acidic wastewater, at a range of organic loadings and shockloads. The methane in the generated gas accounted for 93% of the COD removal in the unit, while a solids balance indicated that only 0.012 g VSS was produced per g COD removed. Due to the low solids production and the initial seeding of the unit with digested sludge no nutrient additions were required during the 518 day period even though the COD:P and COD:N ratios were as high as 4360:1 and 39:1, respectively. Although a possible heavy metal toxicity was observed in the unit, this was eliminated after sulfide addition.     

Thickening of waste activated sludge by biological flotation

Waste activated sludge was thickened by biological flotation without polymer flocculant dosage. The BIOFLOT® process utilizes the denitrifying ability of activated sludge bacteria. Gaseous products of anaerobic nitrate reduction cause spontaneous flotation of the sludge suspended solids. Laboratory tests confirmed the dependence of sludge thickening efficiency on available nitrate concentration, flotation time and temperature. Full-scale experiments were performed in a fully automatized unit for discontinuous sludge thickening from wastewater treatment plants with a capacity of up to 5000 I.E. Waste activated sludge from wastewater treatment plants at Pisek. Milevsko and Björnlunda was thickened from 6.2, 10.7 and 3.5 g/l MLSS to 59.4, 59.7 and 66.7 g/t MLSS, respectively. Concentrations of COD, ammonium and phosphate ions were decreased in sludge water. The average nitrate consumption for bioflotation was 21.2 mg NO₁⁻ per 1 g of MLSS of activated sludge. Flotation time ranged from 4 to 48 h.