Power economy studies on activated sludge treatment

This study was focused on power economy of the activated sludge process which is the most energy consuming stage. The optimisation of oxygen requirements by using a Box complex algorithm method was investigated for different inflow rates, BOD loadings and related operating parameters. Linear regression analysis was used to obtain relationships between oxygen requirement, BOD loading and BOD removal under various operating conditions. The effect of such operating parameters as the MLSS concentration, hydraulic retention time and BOD loading on the power economy was investigated. Organic load and oxygen requirement relations for typical municipal wastewaters are presented in an attempt to obtain power economy in the aeration tank.     

An improved method using respirography for the design of activated sludge aeration systems

For the computation of the oxygen uptake for the aeration tank of the biological treatment plant several methods are available such as the OC/L method, Emde's method with fixed coefficients, Eckenfelder's method, and the method of levels of dissolved oxygen. The method for the determination of oxygen uptake for the treatment of industrial wastewaters was developed. The advantage of this method is a simple calculation of oxygen uptake for industrial biological treatment plant due to independent measurement of oxygen uptake for individual phases. The method is rapid and allows easy determination of individual phases of oxygen uptake.     

A simplified kinetic model to simulate soluble organic substances removal in an activated sludge aeration tank

A simple kinetic model for the removal of soluble organic substances, SOS, in the activated sludge aeration tank was proposed. The model consists of the instantaneous biosorption of SOS in the influent wastewater and the consecutive biooxidation of the remaining SOS in contact with the activated sludge under the aerated condition. By using samples taken from a municipal wastewater treatment plant, model parameters such as the amount of instantaneous biosorption and the rate of biooxidation, respectively, of SOS were empirically determined, and given as a function of soluble COD concentration. By combining the obtained kinetic data of SOS removal with the quantitative information of the liquid mixing characteristics in a multi-staged aeration tank, a mathematical model to simulate the distributions of SOS concentration in the aeration tank was presented. The simulation calculation was illustratively carried out and the results were shown in comparison with the experimental data in the same plant.     

Optimal aeration control in a nitrifying activated sludge process

An important tool to minimise energy consumption in activated sludge processes is to control the aeration system. Aeration is a costly process and the dissolved oxygen level will determine the efficiency of the operation as well as the treatment results. What aeration control should achieve is closely linked to how the effluent criteria are defined. This paper explores how the aeration process should be controlled to meet the effluent discharge limits in an energy efficient manner in countries where the effluent nitrogen criterion is defined as average values over long time frames, such as months or years. Simulations have been performed using a simplified Benchmark Simulation Model No. 1 to investigate the effect of different levels of suppressing the variations of the effluent ammonium concentration. Optimisation is performed where the manipulated variable for aeration (the oxygen transfer coefficient, K_La) is minimised with the constraint that the average daily flow-proportional ammonium concentration in the effluent should reach a desired level. The optimisation results are compared with constant dissolved oxygen concentrations and supervisory ammonium control with different controller settings. The results demonstrate and explain how and why energy consumption can be optimised by tolerating the ammonium concentration to vary around a given average value. In these simulations, the optimal oxygen peak-to-peak amplitude range between 0.7 and 1.8 mg/l depending on the influent variation and ammonium level in the effluent. These variations can be achieved with a slow ammonium feedback controller. The air flow requirements can be reduced by 1-4% compared to constant dissolved oxygen set-points. Optimal control of aeration requires up to 14% less energy than needed for fast feedback control of effluent ammonium.     

Promotion and inhibition of oxygen transfer under fine bubble aeration by activated sludge

The promotion and inhibition of inactivated and activated sludge on oxygen mass transfer (OMT) were studied using lab‐scale experiments. The results showed that the α‐values and oxygen transfer efficiency (OTE) decreased with increasing mixed liquor suspended solids (MLSS) concentration (1–10 g/L). Although OMT promotion rate by microbial respiration in activated sludge system increased from 39.8–97.5% for the α‐values and OTE, the two parameters were found to fall sharply when MLSS concentration was over 5 g/L. This indicated that the sludge concentration is a major influence factor on OMT in activated sludge system. Such results provide valuable knowledge for the operating optimization of the aeration system in wastewater treatment process.     

Control of the aeration volume in an activated sludge process using supervisory control strategies

In this paper, a simulation benchmark of a pre-denitrifying activated sludge process is utilized in order to evaluate a supervisory aeration volume control strategy. The aeration volume control strategy has also been evaluated in a pilot plant at Hammarby Sj?stad in Stockholm, Sweden. The main idea has been to let the dissolved oxygen (DO) concentration in some of the aerated compartments be determined by a higher level controller driven by the DO concentration in other compartments. In this way, only sensors for measuring the DO concentrations are needed for the decision of time varying DO set-points. The high reliability of such sensors implies robust input values for the proposed control strategy. Moreover, it is known that the respiration rate is affected by the content of substrate and nitrogen in the compartments; therefore, the suggested manipulations of the DO set-points are indirectly determined by the current load into the plant. Compared to constant DO control and a supervisory DO set-point control strategy based on ammonium measurements in the last aerobic compartment, the suggested aeration volume control strategy could reduce the effluent nitrate and ammonium concentrations significantly without increasing the aeration energy.     

Studies on dissolved oxygen concentration and sludge retention time affecting the full-scale activated sludge process

We have made a series of experiments on dissolved oxygen (DO) concentration and sludge retention time (SRT) to affect the full-scale activated sludge process.

For DO control, a better effluent water quality and a reduced current consumption have been achieved as compared with those in a constant air flow operation. The best control point for the DO control operation is at the outlet side of the aeration tank.

For SRT control, effluent COD seems to fluctuate slightly, and is favorably stable when SRT is between 10 and 20 days. It takes about twice as many days as SRT value set up for the total mass of sludge in the process (M) to be stabilized in case of making set value change as a result of simulation using mathematical models.     

Correlations of performance for activated sludge using multiple regression with autocorrelation

A method is developed to correlate activated sludge response variables, such as SVI, effluent volatile suspended solids or effluent BOD, with state variables such as F/M ratio, influent BOD, dynamic sludge age, etc. The method is based on multiple linear regression with autocorrelated errors. The method was applied to three data sets from two full-scale activated sludge plants; one a regional municipal utility, the other an industrial facility. Highly statistically significant models were found which could explain 65–82% of the variability in effluent total oxygen demand, 22–60% in effluent volatile suspended solids and 48–88% in sludge volume index. It was only necessary to model up to second order lags. The models were tested for bias using different data sets and produced correlation coefficients between predicted and observed values as high as 0.96. These results show that useful predictive relationships can be developed for full-scale activated sludge processes. The methods could be used to develop a range of automatic process control schemes.     

Evolution of microbial communities in the activated sludge process

In a series of bench scale activated sludge experiments, the evolution of sludge microbial communities was studied. The different communities required 2–3 months to reach functional optimum as measured by parameters such as substrate removal efficiency, effluent suspended solids and sludge volume index. Nevertheless, a period of at least 4 months, corresponding to 10 mean cell residence times, was necessary before full nitrification and minimum endogenous respiration were reached. Inoculation with wastewater sludge enhanced the evolvement of the microbial community, but was not essential. Activity parameters such as invertase and ATP-content, as well as the behaviour of the numerically dominant species, suggest that a microbial community evolves to a climax pattern rather than to a distinct type of climax state.     

The influence of reactor mixing characteristics on the rate of nitrification in the activated sludge process

The effect of longitudinal mixing on nitrification was evaluated in two bench scale activated sludge reactors of equal volume, one approximating complete mixing (∂ = 0.62) and one approximating plug-flow mixing (∂ = 0.07). The onset of nitrification was more rapid under plug-flow conditions and a higher rate constant for nitrification was observed. Both the numbers and species of nitrifying bacteria were the same in both reactors and thus this did not contribute to the observed differences. Lower reaction rates in the complete mix reactor were shown to result from a high concentration of free ammonia in the mixed liquor, which gave rise to inhibition of nitrifying bacteria. Over an extended operating period, the plug flow reactor produced a sludge which demonstrated superior settling properties to that of the complete mix reactor. In addition incidences of sludge bulking were absent, whereas they were a regular feature of the complete mix system.     

A model of localised oxygen sinks around bacterial colonies within activated sludge

A new mathematical analysis of diffusional resistances of oxygen in activated sludge is developed. It assumes that the distribution of bacteria within activated sludges flocs are in colonies rather than the usual assumption of a homogeneous distribution of bacteria throughout the sludge. The solutions are for steady-state. The bacterial colony model results in a completely different shape of dissolved oxygen gradient in the floc when compared to the homogeneous bacterial distribution model. The new “bacterial colony” model predicts highly localised oxygen demands around the colony with maximum dissolved oxygen deficits in a 20–40 μm dia floc of 2–3 mg l⁻¹.     

改良型氧化沟污泥膨胀原因的分析与控制

为了有效控制低负荷改良型氧化沟工艺的污泥膨胀现象,对该工艺的运行参数进行了系统的分析。结果表明,工艺发生污泥膨胀的主要原因是由于氧化沟循环廊道内的溶解氧浓度分级不明显。针对污泥膨胀原因及相关的膨胀机理,采用在循环廊道内加设插板来保证缺氧区、好氧区溶解氧浓度的合理分布。确定了试验装置的最佳运行条件为：好氧区溶解氧含量为1．5—2．0mg／L,缺氧区溶解氧含量小于0．5mg／L,污泥指数为120～150mL／g,污泥质量浓度为3．5-4．0g／L。     

利用COD指标进行活性污泥法系统的设计

阐述了利用ＣＯＤ指标进行活性污泥法系统设计的主要思想和过程,并建立一套用于硝化和反硝化的活性污泥法ＣＯＤ设计方法。大量实际运行结果表明,利用该法对系统剩余污泥量和耗氧量以及活性污泥的组成计算所得的结果要较传统的ＢＯＤ５方法更为精确。     

Biological treatment of low temperature carbonization wastewater by activated sludge process—A case study

This paper outlines the compositional characteristics of wastewater from a low temperature carbonization (LTC), plant manufacturing domestic coke, generating tar and light oil. Wastewater characteristics from this plant show the presence of a variety of pollutants like phenols, ammonia, cyanide, sulphide and thiocyanate in appreciable concentration owing to the absence of byproduct recovery operations. Under suitable conditions, biological treatment of LTC wastewater in a two stage activated sludge process (ASP) mainly results in good removal of BOD (95%) and COD (78%). Concentrations of different phenols and their fate in these treatment units show that the phenols except pyrogallol can be removed efficiently. Ammonia cannot be stabilized to nitrite or nitrate even after maintaining a high sludge retention time (SRT) in the bioreactors. Cyanide removal in these units is very poor. Microbiological status of these units reveals that most of the active biomass is comprised of phenol-utilizing organisms. The system constants for biological unit operations for ASP, like oxygenation capacity of LTC wastewater (a = 0.50 and B = 0.36) and biokinetic constants (Y = 0.13, k_d = 0.12 d⁻¹, μ_max = 0.59 d⁻¹ and k_s = 88.25 mg 1⁻¹), have been evaluated.     

Possible cause of excess sludge reduction in an oxic-settling-anaerobic activated sludge process (OSA process)

Modification of a conventional activated sludge process by inserting a sludge holding tank in a sludge return line forms an oxic-settling-anaerobic (OSA) process that may provide a cost-effective way to reduce excess sludge production in activated sludge processes. In this paper we systematically evaluate the following possible scenarios that may explain the reduction of excess sludge in the OSA process: (i). energy uncoupling, (ii). domination of slow growers, (iii). soluble microbial products (SMPs) effect and (iv). sludge decay in the sludge holding tank under a low oxidation-reduction potential (ORP) condition. Results show that only the final scenario may reasonably explain this reduction. It has also been found that the sludge decay process in the sludge holding tank may involve the reduction of the cell mass.     

Nitrous oxide emissions from the oxidation tank of a pilot activated sludge plant

This study discusses the results of the continuous monitoring of nitrous oxide emissions from the oxidation tank of a pilot conventional wastewater treatment plant. Nitrous oxide emissions from biological processes for nitrogen removal in wastewater treatment plants have drawn great attention over the last years, due to the high greenhouse effect. However, even if several studies have been carried out to quantify nitrous oxide emission rates from different types of treatment, quite wide ranges have been reported. Only grab samples or continuous measurements over limited periods were considered in previous studies, which can account for the wide variability of the obtained results. Through continuous monitoring over several months, our work tries to fill this gap of knowledge and get a deeper insight into nitrous oxide daily and weekly emission dynamics. Moreover, the influence of some operating conditions (sludge age, dissolved oxygen concentration in the oxidation tank, nitrogen load) was studied to determine good practices for wastewater treatment plant operation aiming at the reduction of nitrous oxide emissions. The dissolved oxygen set-point is shown to play a major role in nitrous oxide emissions. Low sludge ages and high nitrogen loads are responsible for higher emissions as well. An interesting pattern has been observed, with quite negligible emissions during most of the day and a peak with a bell-like shape in the morning in the hours of maximum nitrogen load in the plant, correlated to the ammonia and nitrite peaks in the tank.     

Decolorization of an azo dye by unacclimated activated sludge under anaerobic conditions

Studies on the reductive decolorization of a complex azo dye, Reactive Red 3.1, were made as part of the development of a practical approach to better exploit the metabolic potential of biomass in wastewater treatment. Decolorization was achieved at low and variable rates by mixed microbial cultures under various environmental conditions, including low pH and high salt concentration. It was caused by reductive cleavage of the azo bond to yield two aromatic amines. More reliable and effective decolorization rates, of up to 20–30 mg l⁻¹ h⁻¹, were given by unadapted activated sludge, (6 g l⁻¹) incubated with 400 mg l⁻¹ of Reactive Red 3.1 under anaerobic conditions. Decolorization also occurred best in static conditions.     

Dynamic response of nitrifying activated sludge batch culture to increased chloride concentration

Dynamic response of nitrifying activated sludge batch cultures to increased chloride concentration was studied in this paper, which focused upon the changes in the specific nitrification rate (SNR) and nitrifier population when the chloride level was gradually or stepwise increased to 30,000 mg Cl L-1. The dominant species of ammonia-oxidizers and nitriteoxidizers in the population were examined by Fluorescent in situ hybridization technique with 16S rRNA-targeted oligonucleotide probes. It was found that neither chloride increasing approaches affected the SNR of the batch cultures before the chloride concentration exceeded 10,000 mg Cl L-1, after which the stepwise increase approach reduced the SNR more significantly than the gradual increase approach. From 10,000 to 18,000 mg Cl L-1 a down-and-up pattern of the SNR variation appeared in both approaches, which was associated with the change in the dominant species of ammonia-oxidizers from non-saline-resistant species such as Nitrosomonas europaea-lineage and Nitrosomonas eutropha to saline-resistant species, such as the Nitrosococcus mobilis-lineage. Nitrobacter was the only dominant species when the chloride concentration was below 10,000 mg Cl L-1, where no nitrite-oxidizers survived. Therefore, the 10,000 mg Cl L-1 chloride level is a critical level for the shift of the nitrifier population in the nitrifying activated sludge batch cultures.     

Control of activated sludge filamentous bulking—VI. Formulation of basic principles

Overgrowing of filamentous micro-organisms in activated sludge is affected by the following factors: (a) composition of treated waste water; (b) actual concentration of dissolved oxygen in an aeration tank; (c) actual concentration of soluble substrate under which micro-organisms grow; (d) technological parameters of the process (sludge loading and age). All these factors were analysed and discussed in the light of the formulated basic principles for control of filamentous bulking.     

Parameter and state estimation of the activated sludge process-I. Model development

A new approach is introduced for combined state and parameter estimation, named Linearized Maximum Likelihood (LML). The LML method is motivated by the uncertain and nonlinear activated sludge dynamics. In this article, the theoretical background leading to the development of the LML method is presented, along with the resulting numerical algorithm. In a companion article (Kabouris and Georgakakos, 1996) the LML method is applied to the parameter and state estimation of a nitrifying activated sludge process, modelled by the IAWPRC Activated Sludge Model 1.