The nature of the carbon source rules the competition between PAO and denitrifiers in systems for simultaneous biological nitrogen and phosphorus removal

The presence of nitrate in the theoretical anaerobic reactor of a municipal WWTP aiming at simultaneous C, N and P removal usually leads to Enhanced Biological Phosphorus Removal (EBPR) failure due to the competition between PAO and denitrifiers for organic substrate. This problem was studied in a continuous anaerobic-anoxic-aerobic (A²/O) pilot plant (146 L) operating with good removal performance and a PAO-enriched sludge (72%). Nitrate presence in the initially anaerobic reactor was studied by switching the operation of the plant to an anoxic-aerobic configuration. When the influent COD composition was a mixture of different carbon sources (acetic acid, propionic acid and sucrose) the system was surprisingly able to maintain EBPR, even with internal recycle ratios up to ten times the influent flow rate and COD limiting conditions. However, the utilisation of sucrose as sole carbon source resulted in a fast EBPR failure. Batch tests with different nitrate concentrations (0-40 mg L⁻¹) were performed in order to gain insight into the competition for the carbon source in terms of P-release or denitrification rates and P-release/C-uptake ratio. Surprisingly, no inhibitory or detrimental effect on EBPR performance due to nitrate was observed. A model based on ASM2d but considering two step nitrification and denitrification was developed and experimentally validated. Simulation studies showed that anaerobic VFA availability is critical to maintain EBPR activity.     

Kinetics of anaerobic orthophosphate release and substrate uptake in enhanced biological phosphorus removal from synthetic wastewater

This paper presents some new results about the kinetics of orthophosphate release and substrate uptake occurring under anaerobic conditions in a lab-scale activated sludge plant consisting of an anaerobic first and an aerobic second step A/O-process. The synthetic wastewater contained acetate, peptone and yeast extract. The mixed culture was enriched from activated sludge samples from a large-scale plant with enhanced biological phosphorus removal. Anaerobic batch experiments were carried out with this enriched mixed culture using acetate as the only source for carbon and energy. In the first experiments, high acetate concentration was only partly consumed by the bacterial culture, resulting in a nearly total emptying of the poly-P-store (poly-P-limitation). In the next experiments, only a relatively low acetate concentration was added, resulting in a total acetate uptake and in partial emptying of the poly-P-store (acetate-limitation). This experimental strategy was successful in studying the complex kinetics of the anaerobic process in enhanced phosphorus removal. Most of the calculated kinetic coefficients depend on temperature; a dependence on pH cannot be postulated with certainty. A comparison with known results shows conformity but also differences.     

Incorporating microbial ecology into the metabolic modelling of polyphosphate accumulating organisms and glycogen accumulating organisms

In the enhanced biological phosphorus removal (EBPR) process, the competition between polyphosphate accumulating organisms (PAO) and glycogen accumulating organisms (GAO) has been studied intensively in recent years by both microbiologists and engineers, due to its important effects on phosphorus removal performance and efficiency. This study addresses the impact of microbial ecology on assessing the PAO-GAO competition through metabolic modelling, focussing on reviewing recent developments, discussion of how the results from molecular studies can impact the way we model the process, and offering perspectives for future research opportunities based on unanswered questions concerning PAO and GAO metabolism. Indeed, numerous findings that are seemingly contradictory could in fact be explained by the metabolic behaviour of different sub-groups of PAOs and/or GAOs exposed to different environmental and operational conditions. Some examples include the glycolysis pathway (i.e. Embden-Meyerhof-Parnas (EMP) vs. Entner-Doudoroff (ED)), denitrification capacity, anaerobic tricarboxylic acid (TCA) cycle activity and PAOs’ ability to adjust their metabolism to e.g. a GAO-like metabolism. Metabolic modelling may further yield far-reaching influences on practical applications as well, and serves as a bridge between molecular/biochemical research studies and the optimisation of wastewater treatment plant operation.     

Seasonal variation of phosphorus release from the sediments of Shallow Lake Balaton (Hungary)

Phosphorus release was low from intact sediment cores of the mesotrophic area of the lake throughout the year, and amounted to 0.3 mg P m⁻² day⁻¹ during autumn in short-term incubations. In the hypertrophic area maximum release (2.8 mg Pm⁻² day⁻¹) was measured during summer.

Phosphorus release showed a rapid increase from long-term incubated intact sediment cores with the increasing pH of the overlying water. At the ecologically real maximum pH the release may amount to 0.8 and 4.0 mg P m⁻² day⁻¹ in the mesotrophic and hypertrophic areas, respectively. A release of 2.0–3.9 mg P m⁻² day⁻¹ was estimated from sediment suspensions of the hypertrophic area within a pH range of 8–9. These values are similar to the external phosphorus loadings of the respective areas.

The most important phosphorus mobilizing factors are pH and the decomposition of the organic matter in the sediments. Redox conditions may play a significant indirect role in the regulation of the internal loading.

A positive feedback is hypothesized between the internal phosphorus loading and primary production both processes being affected by the external loading in different ways.     

The effect of pH on the competition between polyphosphate-accumulating organisms and glycogen-accumulating organisms

In enhanced biological phosphorus removal (EBPR) processes, glycogen-accumulating organisms (GAOs) may compete with polyphosphate-accumulating organisms (PAOs) for the often-limited carbon substrates, potentially resulting in disturbances to phosphorus removal. A detailed investigation of the effect of pH on the competition between PAOs and GAOs is reported in this study. The results show that a high external pH ( approximately 8) provided PAOs with an advantage over GAOs in EBPR systems. The phosphorus removal performance improved due to a population shift favouring PAOs over GAOs, which was shown through both chemical and microbiological methods. Two lab-scale reactors fed with propionate as the carbon source were subjected to an increase in pH from 7 to 8. The phosphorus removal and PAO population (as measured by quantitative fluorescence in situ hybridisation analysis of "Candidatus Accumulibacter phosphatis") increased in each system, where the PAOs appeared to out-compete a group of Alphaproteobacteria GAOs. A considerable improvement in the P removal was also observed in an acetate fed reactor, where the GAO population (primarily "Candidatus Competibacter phosphatis") decreased substantially after a similar increase in the pH. The results from this study suggest that pH could be used as a control parameter to reduce the undesirable proliferation of GAOs and improve phosphorus removal in EBPR systems.     

Data modelling and the application of a neural network approach to the prediction of total construction costs

Neural network cost models have been developed using data collected from nearly 300 building projects. Data were collected from predominantly primary sources using real-life data contained in project files, with some data obtained from the Building Cost Information Service, supplemented with further information, and some from a questionnaire distributed nationwide. The data collected included final account sums and, so that the model could evaluate the total cost to the client, clients' external and internal costs, in addition to construction costs. Models based on linear regression techniques have been used as a benchmark for evaluation of the neural network models. The results showed that the major benefit of the neural network approach was the ability of neural networks to model the nonlinearity in the data. The ‘best’ model obtained so far gives a mean absolute percentage error (MAPE) of 16.6%, which includes a percentage (unknown) for client changes. This compares favourably with traditional estimating where values of MAPE between 20.8% and 27.9% have been reported. However, it is anticipated that further analyses will result in the development of even more reliable models.     

The implications of integrated assessment and modelling studies for the future remediation of chromite ore processing residue disposal sites

Chromite ore processing residue (COPR) waste from a former chromium chemical works (1830-1968) is still contaminating groundwater in Glasgow, Scotland, with carcinogenic hexavalent chromium, Cr(VI). An integrated analytical, experimental and modelling approach has identified and accounted for mineral phases and processes responsible for the retention and release of Cr(VI) under prevailing field conditions. Both the nature of mineral phase retention and the buffered high pH of the sites, however, militate against direct remediative treatment of the source material, for example by the application of generic methods (e.g. FeSO4) that have been successfully employed elsewhere for the reduction of Cr(VI) to Cr(III) in other matrices. The interception and treatment of groundwater to remove Cr(VI) and the capping of sites to reduce human exposure to airborne Cr(VI)-contaminated dust may well be more realistic and effective, at least in the short to medium term.     

Dynamics and dynamic modelling of H2S production in sewer systems

Accurate and reliable predictions of sulfide production in a sewer system greatly benefit formulation of appropriate strategies for optimal sewer management. Sewer systems, rising main systems in particular, are highly dynamic in terms of both flow and wastewater composition. In order to get an insight in sulfide production as a response to the dynamic changes in sewer conditions, several measurement campaigns were conducted in two rising mains in Gold Coast, Australia. The levels of various sulfur species and volatile fatty acids (VFAs) were monitored through hourly sampling for periods ranging from 8 to 29 h. The results of these field studies showed large temporal as well as spatial variations in sulfide generation. A dynamic sewer model taking into account the hydraulics and the biochemical transformation processes was formulated and calibrated and validated using the data collected during the four measurement campaigns at the two sites. The model was demonstrated to reasonably well describe the temporal and spatial variations in sulfide, sulfate and VFA concentrations. Application of the model was illustrated with a case study aimed to optimize oxygen injection to one of the two mains studied, which is being used as a means to control sulfide production on this site. The model predicted that, moving the current oxygen injection point to a location close to the end of the sewer line could achieve the same degree of sulfide control with only 50% of the current oxygen use. This study highlighted that the location at which oxygen is injected plays a major role in sulfide control and a dynamic model could be used to make a proper choice of the location.     

A review of the state of the art in modelling progressive mechanical breakdown and associated fluid flow in intact heterogeneous rocks

The purpose of this review is to present techniques, advances, problems, and new developments in modelling the progressive mechanical breakdown of, and associated fluid flow in, intact heterogeneous rock. In general, the theoretical approach to this physical process can be classified into the three categories of discrete models based on fracture mechanics, the continuum damage mechanics approach, and statistical approaches. This categorisation forms the skeleton of this article.Recognising that intact rock contains ubiquitous cracks and flaws between grains and particles of various shapes, fracture mechanics has been widely used to study the mechanical breakdown process in terms of the growth of these discrete defects. Two types of fracture mechanics models, namely open crack and sliding crack models, are used to simulate the progressive microfracturing of rock upon loading, and the application of these to modelling mechanical breakdown is discussed in Section 2.As an alternative to the explicit treatment of cracking given by fracture mechanics, continuum damage mechanics (CDM) takes a phenomenological route that considers the averaged effect of microstructural changes, and has found widespread use in simulating macroscopic stress–strain responses. Through the introduction of damage variables as state operators that quantify change in mechanical properties such as stiffness and strength with respect to damage, CDM models are capable of reproducing realistic hydro-mechanical responses during rock disintegration. This method is discussed in Section 3.Recognising that intact rock is never strictly an isotropic and homogeneous material, statistical approaches use, in general terms, statistical distributions as a means of describing the variation of material properties in natural rock. These approaches have enjoyed a great deal of attention in past decades. By employing different numerical schemes, three major statistical models have been developed: continuum-based damage mechanics models, particle models, and network/lattice models. The merits and drawbacks of these models are discussed in Section 4.Coupled deformation and pore fluid diffusion can be important in the process of progressive breakdown. This poromechanical effect involves the interaction between the solid constituents of, and the interstitial fluids in, heterogeneous rocks under those circumstances when mechanical perturbation occurs sufficiently rapidly that induced pore pressure changes cannot fully dissipate. The mechanisms and analytical approaches, and their development relevant to progressive mechanical breakdown and fluid-flow modelling, are outlined in Section 5.Finally, based on this review, remarks are made summarising the current progress of, and fundamental problems with, these developments. Ideas for further improvements towards more comprehensive and robust techniques are suggested.     

Transient flows in enclosures: A generalised approach for modelling the effects of geometry,heat gains and wind

Transient flows in a confined ventilated space induced by a buoyancy source of time-varying strength and an external wind are examined. The space considered has varying cross-sectional area with height. A generalised theoretical model is proposed to investigate the flow dynamics following the activation of an external wind and an internal source of buoyancy. To investigate the effect of geometry, we vary the angle of the wall inclination of a particular geometry in which a point source of constant buoyancy is activated in the absence of wind. Counter-intuitively the ventilation is worse and lower airflow rates are established for geometries of increasing cross-sectional areas with height. We investigate the effect of the source buoyancy strength by comparing two cases: (1) when the buoyancy input is constant and (2) when the buoyancy input gradually increases over time so that after a finite time the total buoyancy inputs for (1) and (2) are identical. The rate at which the source heat gains are introduced has a significant role on the flow behaviour as we find that, in case (2), a warmer layer and a more pronounced overshoot are obtained than in case (1). The effect of assisting and opposing wind on the transient ventilation of an enclosure of constant cross-sectional area with height and constant heat gains is examined. A Froude number Fr is used to define the relative strengths of the buoyancy-induced and wind-induced velocities and five different transient states and their associated critical Fr are identified.     

Experimental and modelling study on the uptake and desorption kinetics of 133Ba by suspended estuarine sediments from southern Spain

Dispersion of pollutants in aquatic environments depends on their uptake by suspended solids. This work deals with the uptake kinetics of 133Ba (gamma-emitter and a good analogue of 226Ra) by suspended estuarine sediments (which can be resuspended into the water column under certain conditions). This study presents a wide set of tracing experiments, including second tracing, decantation and desorption processes. The purpose is to characterize 133Ba uptake by sediments and to investigate the use and limitations of box models in order to describe the uptake kinetics. Water and sediment samples were collected in the Huelva estuary (Spain), where environmental 226Ra concentrations have been increased by two phosphate fertilizer industries. Samples were characterized by granulometric, organic carbon content, cation exchange capacity and XRF-EP analyses. Results revealed three-step kinetics, with characteristic times of minutes, hours and days. These results enabled the selection and calibration of a suitable box model and facilitated the testing of its use as a fully predictive tool.     

怎样控制沥青路面裂缝的产生和扩展

以临安市公路建设中的沥青路面裂缝为研究对象,结合沥青路面施工实践经验,就沥青路面裂缝的种类、产生和扩展原因进行了分析,同时提出相应的处理措施和方法,以达到提高沥青路面使用寿命的目的。     

铅锌尾矿重金属释放影响因素的探讨

铅锌尾矿废渣若不经过处理,会通过风蚀、径流对环境造成重金属污染,对尾矿废渣污染的研究一直是大家关注的方向。多年来,pH被证实是影响重金属释放的重要因素。本文通过实验,研究了浸提方法、pH和盐度对铅锌尾矿废渣重金属元素释放的影响。结果表明,Pb、Zn、Cd、As在pH值呈酸性和碱性时浸出率都较高,Cu、Ni则在pH值为碱性时浸出率更高,盐度对Pb的释放影响效果更为明显。     

Climate change induced salinisation of artificial lakes in the Netherlands and consequences for drinking water production

In this paper we present a modelling study to investigate the impacts of climate change on the chloride concentration and salinisation processes in two man-made freshwater lakes in the Netherlands, Lake IJsselmeer and Lake Markermeer. We used a transient compartmental chloride and water balance model to elucidate the salinisation processes occurring under present conditions and assess future salinisation under two climate forcing scenarios. The model results showed that the Rhine River is the dominant determinant for the chloride concentration in both lakes, followed by drainage of brackish groundwater from the surrounding polders. The results further show that especially during dry years, seawater intrusion through the tidal closure dam is an important source of chloride to Lake IJsselmeer. The results from the climatic forcing scenarios show that Lake IJsselmeer is especially vulnerable to climate-induced salinisation whereas effects on Lake Markermeer are relatively small. Peak chloride concentrations at the raw water intake of the Andijk drinking water facility on Lake IJsselmeer are projected to increase to values above 250 mg/l in the most far-reaching climate change scenario W+ in 2050 for dry years. This is well above the maximum allowable concentration of 150 mg/l for chloride in drinking water.Modelling showed that climate change impacts the chloride concentrations in a variety of ways: 1) an increasing occurrence of low river flows from summer to autumn reduces the dilution of the chloride that is emitted to the Rhine with a constant load thereby increasing its concentration; 2) increased open water evaporation and reduced rainfall during summer periods and droughts increases the chloride concentration in the water; and 3) rises in sea level increase seawater intrusion through the tidal closure dam of Lake IJsselmeer. The processes described here are likely to affect many other tidal rivers or lakes and should be considered when planning future raw water intake stations for drinking water production or agricultural water supply.     

Evaluation of sediment capping with active barrier systems (ABS) using calcite/zeolite mixtures to simultaneously manage phosphorus and ammonium release

The efficiency and mechanism of sediment capping with an active barrier system (ABS) using calcite/zeolite mixtures to simultaneously prevent phosphorus (P) and ammonium (NH₄⁺) release from eutrophic lake sediments under anaerobic conditions was investigated through a series of batch and sediment incubation experiments. For this, natural calcite and various zeolites (natural, NaCl-pretreated and CaCl₂-pretreated zeolites) were applied. Batch tests showed that the calcite was efficient for the removal of phosphate in aqueous solution and the zeolite was an efficient adsorbent for the removal of NH₄⁺ from aqueous solution. Sediment incubation experiments showed that the P and NH₄⁺ fluxes from the anaerobic sediments were significantly reduced by the ABS using the mixture of calcite and natural zeolite. Higher calcite dosage was found to be favorable for the prevention of P release from the sediments using the ABS. For controlling the P release from the sediments, the mixture of calcite and CaCl₂-pretreated zeolite as a capping material was more efficient than that of calcite and natural zeolite, whereas the mixture of calcite and NaCl-pretreated zeolite was less efficient than that of calcite and natural zeolite. Batch and sediment incubation experiments proved that the zeolite as a component of the ABS using the mixture of calcite and CaCl₂-pretreated zeolite has a dual function: (i) preventing NH₄⁺ release from the sediments; and (ii) supplying Ca²⁺ through a Ca²⁺/NH₄⁺ exchange to improve the ability of the capping material to immobilize P release from the sediments.     

Performance-based modelling of long-term deterioration to support rehabilitation and investment decisions in drinking water distribution systems

Managing the urban drinking water system in the long term in order to maintain system performance can be challenging due to the difficulty of modelling future deterioration of the networks. This paper establishes a methodology for cohort survival models where historical (empirical) data on decommissioning ages of pipes are used to calibrate survival functions of pipe cohorts according to service level targets. The benefit of the approach is that remaining useful life of pipes, future renewal rates and investment needs can be governed by a required level of service in the network. A case study shows how the methodology can be applied to a cohort of drinking water pipes to create a ‘calibration curve’, which is a survival function calibrated with empirical data.     

浅议冬期商品混凝土生产及施工

在分析冬期混凝土受冻原因的基础上,根据混凝土冬期生产施工经验,介绍了一些有效措施和注意事项,以供广大混凝土行业生产及施工人员借鉴和参考。     

The precipitation of phosphorus from wastewater through pH variation in the presence and absence of coagulants

Phosphorus and calcium present in the aeration tanks of two activated sludge systems were found to form an inorganic precipitate with a stoichiometric molar ratio Ca: P = 2. Variations in the solution pH resulted in the release of P and Ca into the solution (lower pH) or the precipitation of them from solution (higher pH). Lowering of the pH due to the addition of the aluminum coagulants resulted in the release of phosphorus and calcium into the solution and therefore in an apparent lower efficiency of the coagulants. It was also found that after the addition of the coagulants, the nature of the supernatant suspended solids changed from biological to chemical.     

Import-export balance of nitrogen and phosphorus in food, fodder and fertilizers in the Baltic Sea drainage area

Nitrogen (N) and phosphorus (P) are essential elements for life, but in excess they contribute to aquatic eutrophication. The Baltic Sea is a brackish semi-enclosed sea that is heavily influenced by anthropogenic loading of nutrients, resulting in a major environmental problem, eutrophication. In this study, the nutrient balance of the food production and consumption system in seven countries in the Baltic Sea drainage area was quantified for the period 2002-2005. The food production and consumption system accumulates nutrients in the Baltic Sea drainage area, due to extensive imports to the system. The average annual net surplus of nutrients was 1 800 000 tons N and 320 000 tons P in 2002-2005, or annually 28 kg N and 5 kg P per capita. The average total annual import was 2 100 000 tons N and 340 000 tons P during 2002-2005. The largest imports to the system were fertilizers, totaling 1 700 000 tons N and 290 000 tons P. Traded nutrients in food and fodder amounted to a net annual surplus of 180 000 tons N and 25 000 tons P. The nutrient load to the Baltic Sea due to the food consumption and production system was 21% N and 6% P of the respective annual net inputs to the region. This study shows that large amounts of nutrients to Baltic Sea drainage area are inputs from outside the region, eventually contributing to eutrophication. To reduce the nutrient imports, fertilizers should be used more efficiently, nutrients should be recycled more efficiently inside the region, and food system should be guided toward low-nutrient intensive diets.