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.     

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.     

Influence of operational parameters on nitrogen removal efficiency and microbial communities in a full-scale activated sludge process

To improve the efficiency of total nitrogen (TN) removal, solid retention time (SRT) and internal recycling ratio controls were selected as operating parameters in a full-scale activated sludge process treating high strength industrial wastewater. Increased biomass concentration via SRT control enhanced TN removal. Also, decreasing the internal recycling ratio restored the nitrification process, which had been inhibited by phenol shock loading. Therefore, physiological alteration of the bacterial populations by application of specific operational strategies may stabilize the activated sludge process. Additionally, two dominant ammonia oxidizing bacteria (AOB) populations, Nitrosomonas europaea and Nitrosomonas nitrosa, were observed in all samples with no change in the community composition of AOB. In a nitrification tank, it was observed that the Nitrobacter populations consistently exceeded those of the Nitrospira within the nitrite oxidizing bacteria (NOB) community. Through using quantitative real-time PCR (qPCR), nirS, the nitrite reducing functional gene, was observed to predominate in the activated sludge of an anoxic tank, whereas there was the least amount of the narG gene, the nitrate reducing functional gene.     

Effect of low ORP in anoxic sludge zone on excess sludge production in oxic-settling-anoxic activated sludge process

This paper studied the effect of oxidation-reduction potential (ORP) in the anoxic sludge zone on the excess sludge production in the oxic-settling-anoxic process (OSA process), a modified activated sludge process. Two pilot-scale activated sludge systems were employed in this study: (1) an OSA process that was modified from a conventional activated sludge process by inserting a sludge holding tank or namely the "anoxic" tank in the sludge return line; and (2) a conventional process used as the reference system. Each was composed of a membrane bioreactor to serve the aeration tank and solid/liquid separator. Both systems were operated with synthetic wastewater for 9 months. During the operation, the OSA system was operated with different ORP levels (+100 to -250 mV) in its anoxic tank. It has been confirmed that the OSA system produced much less excess sludge than the reference system. A lower ORP level than +100 mV in the anoxic tank is in favor of the excess sludge reduction. When the ORP level decreased from +100 to -250 mV the sludge reduction efficiency was increased from 23% to 58%. It has also been found that the OSA system performed better than the reference system with respect to the chemical oxygen demand removal efficiency and sludge settleability. The OSA process may present a potential low-cost solution to the excess sludge problem in an activated sludge process because addition of a sludge holding tank is only needed.     

Denitrification potential and rates of complex carbon source from dairy effluents in activated sludge system

A predictive model for the denitrification performance of complex carbon sources was proposed based on compositional data. Potential and rates of denitrification of single dairy components (lactose, lactate, proteins, fat), as well as binary and complex (modelled "process water") mixtures were assessed using test for nitrogen uptake rate (NUR). In all experiments, denitrification potential of mixtures was found to be significantly higher than the sum of individual potentials and denitrification rate with the readily biodegradable moiety of the mixtures was similar to the highest rate obtained with individual components (lactose or lactate). This work shows that activated sludge acclimated to dairy components can be modelled as a single biomass where the maximal anoxic growth rate of the biomass limits the denitrification rate with dairy components. As a consequence, lactose or lactate determine the maximal denitrification rate possible using dairy effluents.     

Performance comparison of a pilot-scale UASB and DHS system and activated sludge process for the treatment of municipal wastewater

This study compares the performance of a pilot-scale combination of UASB and DHS system to that of activated sludge process (ASP) for the treatment of municipal sewage. Both systems were operated in parallel with the same sewage as influent. The study was conducted for more than 300 days, which revealed that organic removal efficiency of UASB+DHS system was comparable to that of ASP. Unfiltered BOD removal by both systems was more than 90%. However, UASB+DHS system outperformed ASP for pathogen removal. In addition, volume of excess sludge production from UASB+DHS was 15 times smaller than that from ASP. Moreover, unlike ASP, there is no requirement of aeration for the operation of UASB+DHS system, which makes it an economical treatment system. Considering the above observations, it was concluded that UASB+DHS system can be a cost-effective and viable option for the treatment of municipal sewage over ASP, especially for low-income countries.     

Comparison of control strategies for nitrogen removal in an activated sludge process in terms of operating costs: a simulation study

In this paper several control strategies for nitrogen removal are proposed and evaluated in a benchmark simulation model of an activated sludge process. The goal is to determine which control strategy delivers better performance with respect to plant operating costs. In the study, constant manipulated variables and various PI and feedforward control strategies are tested and compared with predictive control, which uses an ideal process model. The control strategies differ in the information used about the process (number of sensors and sensor location) and in the complexity of the control algorithms. To determine the set-points that yield optimal operating costs, an operational map is constructed for each control strategy. Results of the simulation show that with PI and feedforward controllers almost the same optimal operating costs can be achieved as with more advanced MPC algorithms under various plant operating conditions. More advanced control algorithms are advantageous only in cases where the plant is highly loaded and if stringent effluent fines are imposed by legislation.     

Influence of alternating oxic/anoxic conditions on growth of denitrifying bacteria

Denitrifying bacteria that are switched from oxic to anoxic conditions can experience diauxic lag, which is the time required for re-synthesis of nitrate reductase and other denitrifying enzymes. Pseudomonas denitrificans were exposed to alternating oxic/anoxic phases in a continuous flow reactor with either 4-h or 8-h anoxic phase lengths, in comparison to a measured diauxic lag of 9.5 h following steady-state oxic conditions. The P. denitrificans were unable to sustain anoxic growth at either of the anoxic phase lengths tested. Diauxic lag observed after several cycles of alternating oxic/anoxic phases was significantly longer than the diauxic lag measured after steady-state oxic conditions. This may be attributed to increase of cell maintenance energy requirements due to substrate accumulation during anoxic phases and concomitant high specific growth rates during oxic phases.     

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.     

A method for characterizing the complete settling process of activated sludge

Based on batch settling tests, a model describing the compression settling process of activated sludge was developed and validated by experiments. Furthermore, a theoretical equation for determining the critical point when the compression settling stage commences, and a new velocity function for the compression settling were deduced from the model. By combining the new model and the conservation of mass, it was proved that the Vesilind function was also capable of describing the compression settling velocity on condition that the appropriate parameters were estimated. Dividing the complete settling process of activated sludge into the zone settling and compression settling stages, and describing them by the Vesilind function with different parameter sets was more reasonable for characterizing the complete settling process of activated sludge. The method was applied to predict the sludge blanket height during batch settling tests, and the results showed that the settling processes could be simulated well.     

A thermodynamic analysis on adsorption of estrogens in activated sludge process

The adsorption behaviors of estrone (E1), 17beta-estradiol (E2), estriol (E3), 17 alpha-ethinylestradiol (EE2), and equol were studied with a deactivated sludge subjected to heat treatment at 80 degrees C for 30 min. The heat-treatment hardly changed the adsorption features of activated sludge (AS). The adsorption equilibrium of all estrogens was approached within 10 min at 20 degrees C, and a high removal of estrogens was achieved simultaneously. The equilibrium data were well fitted by a Freundlich isotherm. The adsorption behaviors of E1, E2, E3 and EE2 in the AS system were independent of their Kow values. Thermodynamic analysis revealed that the adsorption behaviors of E1, E2, E3 and EE2 could be considered as an exothermic, physical and reversible process, resulting in their higher adsorption capacities at lower temperature. Regarding equol, its adsorption was an endothermic, chemical and irreversible process.     

Parameter and state estimation of the activated sludge process: On-line algorithm

The Linearized Maximum Likelihood (LML) method for the simultaneous estimation of activated sludge states and parameters from noisy process measurements (Kabouris and Georgakakos, 1996a, Wat. Res., 30, 2853–2865) is simplified, in terms of its memory storage and computational requirements, for efficient on-line implementation. This is achieved by processing only the four most recent sets of 5-min on-line measurements at each estimation instance, along with the utilization of simplified estimation equations for tracking state and parameter variations, following the initial convergence period. The algorithm is tested in a computational case study involving a nitrifying activated sludge process, modelled by the IAWQ Activated Sludge Model 1 and incorporating a dynamic settling and clarification model. The on-line LML algorithm is capable of tracking the process states and parameters under dynamic conditions of process inputs and model parameters.     

Influence of operational conditions on the performance of a mesh filter activated sludge process

Recently, a new type of wastewater treatment system became the focus of scientific research, the mesh filter activated sludge system. It is a modification of the membrane bioreactor where a membrane filtration process serves to separate the sludge from the purified effluent. The difference is that a mesh filter is used instead of the membrane. Due to the much larger pore size of the mesh, the effluent is not of the same excellent quality as with membrane bioreactors. Nevertheless, it still resembles the quality of the now most widely used standard treatment system, where settling tanks are used to retain the activated sludge. At the same time, the new system features all the other advantages of membrane bioreactors including elevated sludge concentrations resulting in decreased volumina of basins and complete substitution of the settling tank. Therefore, this process presents a potential future alternative where a small footprint of the plant is required. However, so far only a few preliminary studies on this innovative process type have been done. In this paper, the effects of suspended solids concentration, flux rate as well as aeration rate on the effluent quality are discussed. Furthermore, the characteristic of the sludge floc was identified as a factor of vital importance. Therefore, another influencing parameter, the food to microorganism (F/M) ratio, which is known to have a significant effect on floc characteristics, was studied. The main result demonstrated that the process was very effective under most of the operation conditions. The suspended solids concentration in the effluent was below 12 mg l(-1), the average COD in the effluent was between 24 and 45 mg l(-1) and the BOD(5) was lower than 5 mg l(-1). High flux rates of up to 150 l m(-2)h(-1) were also achieved.     

Operational strategies for an activated sludge process in conjunction with ozone oxidation for zero excess sludge production during winter season

A pilot-scale activated sludge system coupled with sludge ozonation process was operated for 112 days of a winter season without excess sludge wasting. The concept of this process is that the excess sludge produced is first disintegrated by ozone oxidation and then recirculated to a bioreactor in order to mineralize the particulate and soluble organic compounds. The basis of operation was to determine either the optimal amount of sludge in kg SS ozonated each day (SO) or the optimal ozonation frequency under the variable influent chemical oxygen demand (COD) loading and temperature conditions, since the ozone supply consumes costly energy. The optimal SO was obtained using the theoretically estimated sludge production rate (SP) and experimentally obtained ozonation frequency (n). While the SP was mainly subject to the COD loadings, sludge concentration was affected by the temperature changes in winter season. The optimal n was observed between 2.5 and 2.7 at around 15 degrees C, but it was doubled at 10 degrees C. Mixed liquor suspended solids (MLSS) concentration was leveled off at around 5000 mg/L in bioreactor at 15 degrees C, but the volatile fraction of MLSS was fixed around 0.7 indicating that there was no significant inorganic accumulation. Suspended solids (SS) and soluble COD in effluents kept always a satisfactory level of 10 and 15 mg/L with sufficient biodegradation. It was recommended to apply a dynamic SO under variable influent COD loadings and temperature conditions to the activated sludge system without excess sludge production for saving energy as well as system stabilization.     

Optimization of the activated sludge anoxic reactor configuration as a means to control nutrient removal kinetically

Factors influencing the determination of optimum reactor configuration for activated sludge denitrification are investigated in this paper. A kinetic optimization method is presented to evaluate optimal pre- and post-denitrification bioreactor stages. Applying the method developed, simulation studies were carried out to investigate the impacts of the ratio of the influent readily biodegradable and slowly biodegradable substrates and the oxygen entering the denitrification zones on the optimal anoxic reactor configuration. In addition, the paper describes the effects of the slowly biodegradable substrate on the denitrification efficiency using external substrate dosing, and it demonstrates kinetic considerations concerning the hydrolysis process. It has been shown that as a function of the biodegradable substrate composition, the stage system design with three optimized reactor compartments can effectively increase reaction rates in the denitrification zones, and can provide flexibility for varying operation conditions.     

Comparative study of laboratory-scale thermophilic and mesophilic activated sludge processes

Laboratory-scale mesophilic (20-35 degrees C) and thermophilic (55 degrees C) activated sludge processes (ASPs) treating diluted molasses wastewater were compared in effluent quality, removal of different COD fractions, sludge yield, floc size, and sludge settleability. The effect of polyaluminium chloride (PAC) with high cationic charge on sludge settleability and effluent quality was also studied. In the ASPs, the hydraulic retention time was 12h in both processes, corresponding to a volumetric loading rate of 3.2+/-1.0 kg COD(filt) m(-3)d(-1). The mesophilic ASP gave 79+/-18% and 90+/-2% and the thermophilic ASP gave 50+/-6% and 67+/-11% total COD (COD(tot)) and GF/A-filtered COD (COD(filt)) removals, respectively. The higher COD values in the thermophilic effluent were due to dispersed particles, such as free bacteria, measured as colloidal COD (COD(col)=difference between soluble COD (COD(sol)) and COD(filt)). Both ASPs gave high (90%) COD(sol) removals. Aeration (24-h) of the thermophilic ASP effluent at 35 degrees C reduced the density of the free bacteria by half, which was also measured as 90% COD(col) removal. Post-aeration of thermophilic effluent at 55 degrees C had no influence either on the density of free bacteria or on the COD(col) values. The use of PAC did not increase COD(tot) or COD(filt) removals in either process, but reduced the mesophilic and increased the thermophilic sludge volume index values.     

Kinetic model of excess activated sludge thermohydrolysis

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

Ammonium adsorption in aerobic granular sludge, activated sludge and anammox granules

The ammonium adsorption properties of aerobic granular sludge, activated sludge and anammox granules have been investigated. During operation of a pilot-scale aerobic granular sludge reactor, a positive relation between the influent ammonium concentration and the ammonium adsorbed was observed. Aerobic granular sludge exhibited much higher adsorption capacity compared to activated sludge and anammox granules. At an equilibrium ammonium concentration of 30 mg N/L, adsorption obtained with activated sludge and anammox granules was around 0.2 mg NH₄-N/g VSS, while aerobic granular sludge from lab- and pilot-scale exhibited an adsorption of 1.7 and 0.9 mg NH₄-N/g VSS, respectively. No difference in the ammonium adsorption was observed in lab-scale reactors operated at different temperatures (20 and 30 °C). In a lab-scale reactor fed with saline wastewater, we observed that the amount of ammonium adsorbed considerably decreased when the salt concentration increased. The results indicate that adsorption or better ion exchange of ammonium should be incorporated into models for nitrification/denitrification, certainly when aerobic granular sludge is used.     

Suspended biofilm carrier and activated sludge removal of acidic pharmaceuticals

Removal of seven active pharmaceutical substances (ibuprofen, ketoprofen, naproxen, diclofenac, clofibric acid, mefenamic acid, and gemfibrozil) was assessed by batch experiments, with suspended biofilm carriers and activated sludge from several full-scale wastewater treatment plants. A distinct difference between nitrifying activated sludge and suspended biofilm carrier removal of several pharmaceuticals was demonstrated. Biofilm carriers from full-scale nitrifying wastewater treatment plants, demonstrated considerably higher removal rates per unit biomass (i.e. suspended solids for the sludges and attached solids for the carriers) of diclofenac, ketoprofen, gemfibrozil, clofibric acid and mefenamic acid compared to the sludges. Among the target pharmaceuticals, only ibuprofen and naproxen showed similar removal rates per unit biomass for the sludges and biofilm carriers. In contrast to the pharmaceutical removal, the nitrification capacity per unit biomass was lower for the carriers than the sludges, which suggests that neither the nitrite nor the ammonia oxidizing bacteria are primarily responsible for the observed differences in pharmaceutical removal. The low ability of ammonia oxidizing bacteria to degrade or transform the target pharmaceuticals was further demonstrated by the limited pharmaceutical removal in an experiment with continuous nitritation and biofilm carriers from a partial nitritation/anammox sludge liquor treatment process.     

Fermentation of agro-food wastewaters by activated sludge

This study investigates the potential of agro-food wastewater fermentation by activated sludge. Experimental results shown that all of the agro-food wastewaters studied initially had considerable concentrations of fermentable substrates, ranging from 19% to 68% of the total COD, but not all of the substrates contained in this COD fraction were fermented after an anaerobic stage of 10h. The percentage of fermentable substrates fermented by the activated sludge during the anaerobic stage ranged from 14% to 68%, presenting the potato processing wastewater the highest concentration of fermented substrates. A mathematical model was proposed to describe the fermentation process. By using this model, the maximum fermentation rates were obtained, ranging their values from 0.6 to 3.4 (gCODgCOD(-1)d(-1)). The fermentation potentials of the agro-food wastewaters were also determined, their values ranged from 0.17 to 0.20 (gCODgCOD(-1)) except for the cheese and the winery wastewater which presented fermentation potentials of 0.12 and 0.14 (gCODgCOD(-1)), respectively.