Comparative economic analysis of the impacts of mean cell retention time and denitrification on aeration systems

Biological nutrient removal is practiced in various modifications of the activated sludge process (ASP) throughout the world. This paper compares conventional, nitrifying-only and combined nitrifying/denitrifying (NDN) processes. The authors performed 113 oxygen transfer efficiency measurements with the off-gas method over 20 years. This dataset was analysed and used to perform an economic analysis for three example scenarios, one for each layout (conventional, nitrifying-only and NDN). Field oxygen transfer efficiency and relevant plant operative costs and credits were considered (i.e., aeration cost, sludge disposal cost, methane production credit). The conclusion is that NDN operations always have lower aeration costs, and generally have the lowest combined operating cost. Reduced aeration costs result because of improved aeration efficiency at higher mean cell retention times and the use of nitrate as an electron acceptor. The improved aeration efficiency overcomes the increased oxygen required at higher cell retention time due to cell decay.     

The optimum operational condition of membrane bioreactor (MBR): cost estimation of aeration and sludge treatment

A methodology to obtain the most economical operational condition of membrane bioreactor (MBR) is developed. In order to achieve the optimum design parameters of MBR with which operational costs are minimized, aeration and sludge treatment costs were estimated for various operational conditions. Generally sludge treatment cost and aeration cost were inversely proportional to each other, which means sludge treatment cost is minimized when aeration cost is maximized and vice versa. Therefore, there might exist an optimum point between the two extreme cases. However, sludge treatment cost turned out to overwhelm the aeration cost over the reasonable operational conditions. Therefore, sludge minimization was considered to be a key for the economical operation of MBR. In the case of typical municipal wastewater of which COD was 400mgL(-1), steady-state MLSS was expected to increase from 11,000 to 15,000mg/L without sludge removal when HRT was decreasing from 16 to 12h. For the range of operational conditions considered in this study, economically optimum HRT and target MLSS were turned out to be 16h and 11,000mg/L, respectively. Under this condition, aeration for the biodegradation of organic matters would be 13.3m(3) air/min when influent was 1000m(3)/day.     

BSM-MBR: a benchmark simulation model to compare control and operational strategies for membrane bioreactors

A benchmark simulation model for membrane bioreactors (BSM-MBR) was developed to evaluate operational and control strategies in terms of effluent quality and operational costs. The configuration of the existing BSM1 for conventional wastewater treatment plants was adapted using reactor volumes, pumped sludge flows and membrane filtration for the water-sludge separation. The BSM1 performance criteria were extended for an MBR taking into account additional pumping requirements for permeate production and aeration requirements for membrane fouling prevention. To incorporate the effects of elevated sludge concentrations on aeration efficiency and costs a dedicated aeration model was adopted. Steady-state and dynamic simulations revealed BSM-MBR, as expected, to out-perform BSM1 for effluent quality, mainly due to complete retention of solids and improved ammonium removal from extensive aeration combined with higher biomass levels. However, this was at the expense of significantly higher operational costs. A comparison with three large-scale MBRs showed BSM-MBR energy costs to be realistic. The membrane aeration costs for the open loop simulations were rather high, attributed to non-optimization of BSM-MBR. As proof of concept two closed loop simulations were run to demonstrate the usefulness of BSM-MBR for identifying control strategies to lower operational costs without compromising effluent quality.     

The cost of a package plant membrane bioreactor

The capital and operating costs associated with a small package plant MBR for small-scale domestic duty has been appraised based on a medium-strength municipal wastewater. The three main membrane configurations were considered, these being multi-tube, hollow fibre and flat sheet, with the most appropriate plant design chosen for each configuration. The analysis proceeded via a consideration of the estimated amortised capital costs of the plant individual components and their installation, coupled with operating costs based largely on energy demand and residuals management. Energy demand was calculated from aeration and pumping costs, with aeration based on a combination of empirical relationships for membrane aeration and mass balance, and the modified Activated Sludge Model version 2 used for estimating tank size and sludge generation. Results indicate that it is possible to produce a single household MBR at a capital cost similar to the current market cost for package treatment plants. Desludging and maintenance of these plants is similar but power requirements for an MBR are around 4 times that associated with more conventional package plants. Economies of scale exist from 6-20 p.e. plants but above 20 p.e. there is little cost difference per head, due to the design assumptions made. CAPEX and OPEX are to some extent interchangeable; reductions in CAPEX are associated with an increase in OPEX and vice versa. Whilst costs are high, the market for package MBRs is significantly influenced by the recycling potential of the effluent produced.     

Feasibility of using a chlorination step to reduce excess sludge in activated sludge process

The ultimate disposal of excess sludge generated from activated sludge processes has been one of the most challenging problems for wastewater treatment utilities. Previous work has shown that excess sludge can be minimized successfully by using sludge ozonation to dissolve it into substrates to be oxidized in the aeration tank. However, this approach is a costly option. Therefore, as an alternative solution, we propose to use chlorination to replace ozonation in excess sludge minimization in the light of operational cost. To investigate the feasibility of this low cost approach, this paper mainly focuses on the effect of chlorination on sludge reduction rate, formation of trihalomethanes, sludge settleability, and effluent quality. Two identical activated sludge membrane bioreactors were continuously operated with synthetic wastewater under the same operation conditions for several months. During this period, one pilot unit was used as the reference system without chlorination of excess sludge, while another served as a testing unit, where excess sludge was taken out for conducting chlorination at a dose of 133mg/g MLSS every day and the chlorinated liquor was then returned to the aeration tank. The sludge production rate and the water quality of both the units were analyzed daily. It was observed that the sludge production could readily be reduced by 65% once the chlorination treatment was involved. However, the chlorination treatment also resulted in poor sludge settleability as well as significant increase of soluble chemical oxygen demand in the effluent, which creates potential difficulties in the operation of a conventional treatment plant with gravity clarifiers. However, it has been demonstrated that by integrating the immersed membrane into the activated sludge process these difficulties can be overcome effectively.     

Economic evaluation of alternative wastewater treatment plant options for pulp and paper industry

Excessive water consumption in pulp and paper industry results in high amount of wastewater. Pollutant characteristics of the wastewater vary depending on the processes used in production and the quality of paper produced. However, in general, high organic material and suspended solid contents are considered as major pollutants of pulp and paper industry effluents. The major pollutant characteristics of pulp and paper industry effluents in Turkey were surveyed and means of major pollutant concentrations, which were grouped in three different pollution grades (low, moderate and high strength effluents), and flow rates within 3000 to 10,000 m³/day range with 1000 m³/day steps were used as design parameters. Ninety-six treatment plants were designed using twelve flow schemes which were combinations of physical treatment, chemical treatment, aerobic and anaerobic biological processes. Detailed comparative cost analysis which includes investment, operation, maintenance and rehabilitation costs was prepared to determine optimum treatment processes for each pollution grade. The most economic and technically optimal treatment processes were found as extended aeration activated sludge process for low strength effluents, extended aeration activated sludge process or UASB followed by an aeration basin for medium strength effluents, and UASB followed by an aeration basin or UASB followed by the conventional activated sludge process for high strength effluents.     

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) in sewage treatment plants

In this study, the concentrations of PFOS and PFOA in the biological units of various full-scale municipal sewage treatment plants were measured. Samples of influent, primary effluent, aeration tank effluent, final effluent and grab samples of primary, activated, secondary and anaerobically digested sludge were collected by 5 sampling events over one year. The two sewage treatment plants (STPs) selected for this study include plant A receiving 95% domestic wastewater and plant B receiving 60% industrial wastewater and 40% domestic wastewater. PFOS and PFOA were observed at higher concentration in aqueous and sludge samples in plant B than that of plant A. Mass flow of PFOS increased significantly (mean 94.6%) in conventional activated sludge process (CAS) of plant B, while it remained consistent after the secondary treatment in plant A. Mass flow of PFOA increased 41.6% (mean) in CAS of plants A and B and 76.6% in membrane biological reactor (MBR), while it remained unchanged after the treatment of liquid treatment module (LTM). Our results suggest that mass flow of these two compounds remains consistent after treatment of activated sludge process operating at short sludge retention time (SRT). Seasonal variations of PFOS in concentrations of raw sewage were found in plant A, while PFOA did not have significant seasonal variation in both plants A and B.     

Experiences of Digested Sludge Thickening by Aercon

Increasing restrictions on sewage sludge disposal routes and the tightening of treatment requirements have led to an escalation of sludge-disposal costs which are out of proportion to other sewage-treatment costs: consequently reducing sludge volume is becoming increasingly cost-effective. Consolidation of anaerobically digested sludge has traditionally been difficult and takes many weeks in secondary digesters or deep storage tanks. Pre-aeration of the sludge using the Aercon process can enable consolidation to take place in days instead of weeks.
This paper reports the results of a pilot-scale plant designed to investigate further the mechanism by which aeration speeds up consolidation. It was concluded that the rate of consolidation depended on aeration intensity and cooling of the sludge. Inhibition of anaerobiosis was more important than gas stripping in enhancing consolidation.     

Surfactant effects on alpha-factors in aeration systems

Aeration in wastewater treatment processes accounts for the largest fraction of plant energy costs. Aeration systems function by shearing the surface (surface aerators) or releasing bubbles at the bottom of the tank (coarse- or fine-bubble aerators). Surfactant accumulation on gas-liquid interfaces reduces mass transfer rates, and this reduction in general is larger for fine-bubble aerators. This study evaluates mass transfer effects on the characterization and specification of aeration systems in clean and process water conditions. Tests at different interfacial turbulence regimes show higher gas transfer depression for lower turbulence regimes. Contamination effects can be offset at the expense of operating efficiency, which is characteristic of surface aerators and coarse-bubble diffusers. Results describe the variability of alpha-factors measured at small scale, due to uncontrolled energy density. Results are also reported in dimensionless empirical correlations describing mass transfer as a function of physiochemical and geometrical characteristics of the aeration process.     

Basic studies on carbon dioxide/air control system for activated sludge processes

Basic facts about the production of carbon dioxide by activated sludge in an aeration tank were gathered with a continuous-flow-type apparatus, and the dynamic characteristics of activated sludge was analyzed with a batch-type aeration tank. In addition, a carbon dioxide/air control system was designed and applied to an activated sludge pilot plant, where the carbon dioxide in exhausted air was continuously measured with an infrared analyzer. The control system maintained the carbon dioxide concentration in exhausted air within ± 0.005 vol%, with a typical diurnal flow rate pattern in sewage, and maintained the oxygen content in mixed liquor within ± 0.4 mg l⁻¹.     

A review of secondary sludge reduction technologies for the pulp and paper industry

The broader application of the activated sludge process in pulp and paper mills, together with increased production, have amplified sludge management problems. With sludge management costs as high as 60% of the total wastewater treatment plant operating costs, and increasingly stringent environmental regulations, it is economically advantageous for pulp and paper mills to reduce their biosolids production. In order to provide a state-of-the-art review of secondary sludge reduction technologies, we have considered the scenarios of lower sludge production through process modifications, and waste-activated sludge reduction through post-treatment. A critical evaluation of all candidate reduction technologies indicates that sludge reduction through treatment process changes appears more appealing than post-treatment alternatives. The former approach offers a clear advantage over the latter in that the treatment process changes reduce sludge production in the first place, thus decreasing sludge management cost. Although it is technically feasible to eliminate the need for sludge disposal, it is unlikely to be economically feasible at this time.     

Using the sewerage system main conduits for biological treatment. Greater Tel-Aviv as a conceptual model

The long-sought idea of using main sewerage conduits as a step-fed plug-flow reactor, where biological sludge is introduced at the head of the main pipelines (hence loop system) and where aeration is provided at various points along the loop, was investigated in this paper in both a bench-scale physical model and using the Greater Tel-Aviv (Dan Region) sewerage with design population of 1.3 million and flow of 300,000 m³ day⁻¹, as a conceptual model.Due to the inherent advantages of a step-fed plug-flow reactor where high substrate concentrations are maintained along most of the length of the reactor, (except for the final section which provides effluent polishing), the viability of the biomass is 3–5.2-fold higher than the one usually encountered in a conventional activated sludge plant, thus substantially increasing its rate and efficiency of organic substrate removal.It has been shown that in the case of Greater Tel-Aviv, replacing the activated sludge conventional treatment plant with the loop system (with only secondary clarifiers to provide the return biological sludge), or alternatively resorting to the loop system instead of doubling an existing overloaded treatment plant, will produce final efluents well below the requirements of 25 mg l⁻¹ BOD₅ and will provide savings of more than 50% in construction costs. An additional advantage of corrosion control along the main conduits thus increasing their longevity is also discussed.     

Hydraulic effects on sludge accumulation on membrane surface in crossflow filtration

Membrane filtration technology for application of wastewater treatment has been developing recently. In the application to wastewater treatment, it is major concern to remove cake layer on membrane surface effectively with crossflow shear stress. Hydraulic effect of sludge accumulation process on membrane surface in bubble and non-bubble driven crossflow filtration was studied. Maximum sludge accumulation. sludge accumulation rate, and lag phase were introduced to describe sludge accumulation process, and the effects of hydraulic conditions were clarified experimentally. Maximum sludge accumulation and sludge accumulation rate were dependent on aeration intensity, and were less depend on flow channel width and MLSS concentration. Their tendencies were explained by shear stress. Shear stress was thought to be the major hydraulic factor that influences them. Lag phase was dependent on aeration intensity, flow channel width, and MLSS concentration. A non-dimensional equation was proposed to explain dependencies of flow channel width based on consideration of hydraulic behavior of MLSS particles and shear stress.     

Effect of cyclic aeration on fouling in submerged membrane bioreactor for wastewater treatment

Due to the inefficiency of aeration measures in preventing fouling by soluble and colloidal particles. The effect of alternating high/low cyclic aeration mode on the membrane fouling in the submerged membrane bioreactor was studied by comparing to fouling in a constant aeration mode. Results indicated a higher overall fouling rate in the cyclic aeration mode than in the constant aeration. However, a higher percentage of reversible fouling was observed for the cyclic aeration mode. The membrane permeability can be more easily recovered from physical cleaning such as backwashing in the cyclic aeration mode. The activated sludge floc size distribution analysis revealed a floc destruction and re-flocculation processes caused by the alternating high/low aeration. The short high aeration period could prevent the destruction of strong strength bonds within activated sludge flocs. Therefore, less soluble and colloidal material was observed in the supernatant due to the preservation of the strong strength bonds. The weak strength bonds damaged in the high aeration period could be recovered in the re-flocculation process in the low aeration period. The floc destruction and re-flocculation processes were suggested to be the main reason for the low irreversible fouling in the cyclic aeration mode.     

Capital efficiency, its measurements and its role in regulatory reviews of utility industries: Lessons from developed and developing countries

Designing regulatory regimes that incentivise the delivery of agreed capex at as low a cost as possible has proven relatively straight-forward when it is possible to establish credible forecasts of the necessary capex and its associated cost. Greater problems arise when there is uncertainty about what capex is needed, when it may be needed and how much it will cost. Yet it is still possible to establish incentive based systems for regulating capex with some or all the characteristics that create uncertainty. This paper explores some of the issues related to incentive based regulation of capex and how flexible approaches can be developed to handle the uncertainty inherent in certain types of capex. Examples of incentive based systems are also provided.     

An aeration energy model for an immersed membrane bioreactor

A simple model for evaluating energy demand arising from aeration of an MBR is presented based on a combination of empirical data for the membrane aeration and biokinetic modelling for the biological aeration. The model assumes that aeration of the membrane provides a proportion of the dissolved oxygen demanded for the biotreatment. The model also assumes, based on literature information sources, a linear relationship between membrane permeability and membrane aeration up to a threshold value, beyond which permeability is unchanged with membrane aeration. The model was benchmarked against two full-scale plant to obtain the most appropriate and conservative value of the slope of the flux:aeration curve and the blower efficiency. Benchmarking in this way produced a match to within 20% of all key process plant operational parameters.The model demonstrated that significant reductions in aeration energy could be obtained through operation at lower flux and reducing the membrane aeration requirement accordingly, so-called “proportional aeration” at lower flows. Similarly, increasing oxygen transfer from membrane aeration would also be expected to decrease energy demand. A sensitivity analysis of some of the key parameters revealed that, of the key operating parameters, loading, SOTE and MLSS concentration remain the most critical in determining energy demand. It is suggested that a key parameter representing membrane aeration in MBRs is the mean in-module air upflow velocity U, since this gives a reasonable representation of the shear applied through membrane aeration. U was found to vary between 0.04 and 0.1 m/s across a number of modern large pilot and full-scale plant.An analysis reveals that significant reductions in energy demand are attained through operating at lower MLSS levels and membrane fluxes. Evidence provided from recent controlled pilot trials implies that halving the flux can reduce the aeration is suggested whereby the number of membrane tanks on line and/or the membrane aeration intensity is adjusted according to the flow, and thus flux, so as to reduce the overall aeration energy demand.     

Powdered activated carbon in an activated sludge treatment plant

Addition of powdered activated carbon (PAC) to the aeration basin of an activated sludge treatment plant fed with dye-works waste waters increases the purifying capacity of the plant: removal efficiency rises from 55.8 to 75.6% (COD) and from 78 to 98.5% (BOD₅) and the nitrification-denitrification capacity of the system also increases.The sludge growth parameters and the kinetic constant of biological oxidation were determined on the plant with and without PAC. The addition of PAC decreased overall sludge growth rate and the auto-oxidation factor, but increased the biological removal rate of the substrate by about one order of magnitude.     

Side-Stream Treatment of Raw-Sludge Liquors using Activated Sludge: Pilot Investigations and Full-Scale Design Rationale

The end of sludge disposal at sea in 1998 will lead to increased volumes of sludge requiring treatment and disposal. Accordingly, there will be further quantities of high-strength liquors arising from sludge thickening and dewatering processes, and additional treatment capacity will therefore be required at treatment works.
This paper describes a three-year project to develop a dedicated side-stream treatment process for the treatment of raw-sludge liquors at Knostrop sewage-treatment works (Leeds) based on the activated-sludge process. Large pilot-scale trials are described which assisted in the construction of simple process design models for full-scale design. Together with additional pilot-scale aeration testing and modelling, a cost model for a full-scale plant was developed, allowing the lowest whole-life cost plant, based on 'net present cost'analysis, to be specified.     

Sludge dewatering in a conventional plant with phosphorus removal-I: Analysis of additional costs

The aim of this research is to assess the differences in quantity and quality of sludges produced in a conventional activated sludge plant, modified for phosphorus chemical removal. Tests were carried out at the Cesenatico sewage treatment plant, where in summer phosphorus is removed simultaneously to the biological process with the aid of aluminium or ferrous sulphate. Characteristics of the sludge produced with or without phosphorus removal were compared. The sludge was then dewatered by a centrifuge and a filter-press on pilot scale and many tests were carried out under different operating conditions.In this part, the plant's most important performances are discussed with regard to phosphorus removal efficiency, sludge production, characteristics and dewaterability. The experiments showed that phosphorus could be removed up to residual concentration of 1.6 mg 1^?1 without detrimental effects on the biological process, with an increase of sludge production of 21 and 36% using aluminium or ferrous sulphate respectively. On the basis of experimental results a costs analysis was carried out to assess the costs for phosphorus removal, including the additional ones for sludge conditioning, dewatering and cake transport to landfill. It results that additional costs vary from 3290 to 6380 Lit per capita per year (29–54%). The higher costs refer to smaller plants (50,000 inhabitants), in which aluminium sulphate is used and sludge dewatered by centrifuge. The use of ferrous rather than aluminium sulphate allows savings of 1300–1600 Lit per capita per year.     

曝气量对膜生物反应器污泥特性和膜污染的影响

设置M1、M2和M3等3个反应器的曝气量分别为150、200和300 L/h,研究了曝气量对膜生物反应器(MBR)的运行效果和污泥特性的影响.结果表明,随着曝气量的增加,对COD和NH4+-N的去除率提高,反应器内溶解氧浓度和污泥的比耗氧速率均增加,表明增加曝气量有助于提高污泥的活性.污泥絮体的粒径变小,溶解性微生物代谢产物(SMP)浓度则由M1的46.8mg/L增加到M3的71.0 mg/L,而且其中的成分也发生了变化,蛋白质与多糖的比值随着曝气量的增加而增加,M1的为1.5,M3的则增至1.9.随着曝气量的增加则膜污染速率加快,表明膜污染以SMP污染为主.