A nitrification model for the contact stabilization activated sludge process

A kinetic model of the contact stabilization process has been developed and experimentally verified with the aid of bench-scale activated sludge units treating domestic sewage. The model provides information on the relationship between the design parameters (process loading, temperature, residence time distribution) and process performance (sludge, production, oxygen uptake, COD-removal, organic nitrogen conversion, nitrification and effluent suspended solids). An oxygen equivalence mass balance equation, which is applicable to all activated sludge process modifications is proposed and may be used in the design and operation of these processes.     

The activated sludge process—IV: Application of the general kinetic model to anoxic-aerobic digestion of waste activated sludge

This paper discusses the application of the general activated sludge model as set out by Dold et al. (Prog. Wat. Technol.12, 47–77, 1980) and extended by Van Haandel et al. (Wat. Res.15, 1135–1152, 1981), to anoxic-aerobic digestion of waste activated sludge. The laboratory scale experimental investigation comprised a 6 day sludge age activated sludge process, the waste sludge from which was fed to a number of digesters operated as follows: single reactor flow-through digesters at 4 or 10 days sludge age (retention times) under aerobic or anoxic-aerobic conditions (with 1.5 and 4 h cycle times) and 3-in-series flow-through aerobic digesters each with 4 days sludge age; all digesters were fed draw-and-fill wise once per day. The general kinetic model simulated accurately all the experimental data without the need to change the values of the kinetic constants. Both theoretical simulations and experimental data indicate that (i) the rate of volatile solids destruction is not affected by the incorporation of anoxic cycles and (ii) the specific denitrification rate constant in a digester is about two-thirds of that in the secondary anoxic reactor of the single sludge activated sludge system; this allows definition of a fourth denitrification rate constant K₄ for the anoxic-aerobic digester with K₄T = 0.046(1.029)^(T-20) mg(NO₃-N) (mgAVSS d)⁻¹, a constant independent of sludge age. An important consequence of (i) and (ii) above is that the denitrification can be integrated readily into the steady state digester model of Marais and Ekama (Wat. SA2, 163–200, 1976) and used for design purposes.     

Benthal stabilization, a low-cost process for the stabilization of waste activated sludge solids

The benthal process is described and a procedure is developed for the rational design of the process for the stabilization of waste activated sludge solids. Kinetic data from another study are used in the application of the procedure to a design example. A benthal stabilization system is compared with two types of aerobic stabilization systems with respect to construction costs and power usage. The economic advantage of benthal stabilization is shown to be dramatic.     

Benthal stabilization of waste activated sludge

Laboratory studies were performed on the stabilization of waste activated sludge solids in a deposit formed at the bottom of an aerated water column. Conclusions reached were as follows: (1) As long as dissolved oxygen is maintained in the water column, the benthal process is stable and odor free. (2) At temperatures within the range of 20 ± 1°C, an average benthal stabilization rate of 79.4 g m⁻² d⁻¹ (biodegradable solids) can be attained if solids are not limiting. (3) Under conditions stated in conclusion (2), as much as 63% of total carbon stabilization can occur via methane formation. (4) A year-old benthal deposit of waste activated sludge solids can be expected to have a solids percentage of about 2.3%     

The activated sludge process—3 single sludge denitrification

The general aerobic bi-substrate active-site death-regeneration activated sludge model including nitrification of Dold et al. (Prog. Wat. Technol.12, 47–77, 1980) is extended to include the kinetic behaviour of the denitrification process in single sludge systems. The extension requires a change in the value of only one of the kinetic constants (K′_mp) in the expression for the particulate substrate utilization rate when the environment becomes anoxic. The extended model simulates very closely the response of the multi-reactor nitrification-denitrification process configurations under both constant and cyclic flow and load conditions. Under constant flow and load conditions, the denitrification response predicted can be reduced to that approximated by a zero order reaction dN/dt = ?KX_a with two rates in the primary and one in the secondary anoxic reactor respectively.     

Dynamic behavior of the activated sludge process under shock loading: Application of the floc model

An integrated system model is developed for an activated sludge system which not only couples the floc behavior to changes in the bulk liquid characteristics but also takes into account the floc size distribution in terms of mass in the reacting system. Intensive simulation is then carried out based on the comprehensive model developed to assess the impact of shock loading on the plant dynamic behavior and to build an understanding of the importance of the floc effects. Simulation results show that more complex dynamic behavior due to the floc effects is likely to occur under severe shock loads which must be considered in the development of the control policy for a safer and better operating performance.     

The effects of organotin on the activated sludge process

Organotin compounds which find increasing use in marine antifouling paints may be present in the discharge from dry dock operations. This investigation was aimed at determining the effect of such wastewater when discharged to a municipal activated sludge treatment plant. Experiments were conducted using a Warburg respirometer and continuous flow bench-scale activated sludge systems. The results showed that unacclimated biological cultures can be inhibited by tributyl tin oxide (TBTO^™) concentrations as low as 25 μgl⁻¹. However, TBTO doses of over 8000 μgl⁻¹ can be tolerated by a well acclimated culture. Continuous loading of up to 1000 μgl⁻¹ TBTO had no effect on organic removal in activated sludge systems. However, an adverse effect on sludge settleability was noticed at 100 μgl⁻¹ TBTO. Shock loadings of 500 and 1000 μgl⁻¹ TBTO had no effect on soluble organic removal but resulted in impaired settling and higher effluent suspended solids. The LC₅₀ of TBTO to the fathead minnow was estimated at 45–200 μgl⁻¹. The toxicity was reduced considerably by activated sludge treatment.     

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.     

Phosphorus removal in the activated sludge process

The results from this research suggest that both calcium phosphate precipitation and enhanced biological uptake play a role in phosphorus removal in the activated sludge process when a non-nitrifying, anaerobic-aerobic system is used to treat a low calcium wastewater. The primary removal mechanism was found to be biological uptake, as calcium phosphate precipitation accounted for only 15–27% of the total phosphorus removed. Calcium phosphate precipitation in the aerobic unit was enhanced because of the pH increase in that reactor. This was the result of low CO₂ production (indicated by low specific oxygen uptake values) and intense aeration which caused excessive CO₂ stripping in the aerobic unit     

The influence of sludge age on heavy metal removal in the activated sludge process

In a laboratory simulation of the activated sludge process ten heavy metals were added continuously to the system which was allowed to equilibrate at six sludge ages between 3 and 18d. Cobalt, manganese and molybdenum removals were poor and were unaffected by changes in the sludge age. The highest removal efficiencies for the other metals occurred at the 15d sludge age. Chromium (trivalent) and cadmium had the highest removal efficiencies, typically greater than 50%. The behaviour of the majority of the metals which were removed to a significant extent was related to one of the parameters influenced by sludge age, i.e. mixed liquor suspended solids, effluent suspended solids or effluent chemical oxygen demand. The metals which were poorly removed showed little affinity for the activated sludge, while most metals exhibited maximum specific uptake by the mixed liquor at a sludge age of 9–12d. However, the affinity of silver for the mixed liquor continued to increase as the sludge age increased to 18d. An affinity series, based on an arbitrary measure of the specific accumulation of metals by the mixed liquor, indicated that chromium, cadmium and silver were most readily adsorbed by the activated sludge.     

The effect of inorganic salts on the activated sludge process performance

The effect of inorganic salts such as sodium chloride and sodium sulfate on the performance of the activated sludge process was examined. When proper acclimation procedures were followed, the adverse effects of salts on the process were minimized. One of the parameters monitored, effluent suspended solids, had very low values (less than 10 mg l⁻¹) up to an inflow sodium chloride concentration of about less than 35 gl⁻¹. The chemical oxygen demand of the effluent increased steadily with increasing sodium chloride concentrations, but biochemical oxygen demand values remained very low (less than 5 mg l⁻¹) which indicated that the increase in chemical oxygen demand was due to the portion that cannot be degrated biologically. The effect of sodium sulfate on the system was even less profound. In addition to the effluent being very clear and low in suspended solids, the chemical oxygen demand removal efficiency remained high.     

The EAWAG Bio-P module for activated sludge model No. 3

An additional module for the prediction of enhanced biological phosphorus removal is presented on the basis of a calibrated version of ASM3. The module uses modified processes from ASM2d but neglects the fermentation of readily degradable substrate. Biomass decay is modeled in the form of endogenous respiration as in ASM3. Moreover, an additional glycogen pool and biologically induced P-precipitation were not taken into account. The module was systematically calibrated with experimental data from various batch experiments, a full-scale WWTP and a pilot plant treating Swiss municipal waste water. A standard parameter set allowed all data to be simulated.     

Soluble phosphate removal in the activated sludge process

This study is intended to be a full scale process feasibility study which will translate experimental results into a form which can be used by practicing engineers as part of the overall effort to combat eutrophication of our waters. Data collected to date show the process to be a reliable method for precipitating phosphorus from domestic wastewater while at the same time realizing enhanced removal of BOD. Detailed operating procedures and cost data are being developed and will be part of the final report.     

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.     

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.     

A simple, conceptual mathematical model for the activated sludge process and its variants

A simple structured kinetic model is applied to the activated sludge process. The objective is less to predict exact process performance than to illustrate some of the possibilities and difficulties in producing a comprehensive model for all the process variants. The rate equations are chosen so as to reduce to the Monod equation during balanced growth. Because these rate equations are linear, the cell growth and substrate uptake in a stirred tank can be defined exactly in terms of the average composition of the biomass. It is shown that this is not valid for other forms of rate equations. The stored substrate to protoplasm ratio in the flocs is found to decrease with increasing mean cell residence time. If extracellular biopolymers are included in the stored substrate this corresponds qualitatively to observations of poor flocculation in extended aeration. The model is also applied to the contact stabilization process and is found to be in agreement with the essential process variables.     

Redox measurement of treatability by the activated sludge process

The change in redox potential with time following the addition of a substrate to an aerated activated sludge suspension follows a well-defined pattern characteristic of the substrate. The shape of the trace indicates the relative ease or difficulty with which the sludge can purify a substrate or waste material. Successive additions causing a sludge to adapt result in a change in the shape of the trace.     

Equilibrium of bod materials in the activated sludge process

A mathematical model based on the equilibrium of BOD materials in the activated sludge process was developed. When the excess sludge is negligibly small and the BOD concentration of the influent is given, the BOD concentration of the activated sludge or mixed liquor and the efficiency of BOD removal are determined only by the aeration period. Moreover, it was verified mathematically that the performance of the activated sludge process became unstable with decrease of the return sludge ratio and the aeration period.     

应用IAWQ2活性污泥模型管理SBR工艺操作运行

活性污泥数学模型是一个十分有用的污水处理厂运行管理工具,它模拟污水处理工艺,寻找工艺缺陷提出修补意见,达到优化污水处理工艺的目的.以海宁城市污水处理厂已运行的SBR工艺数据,应用STOAT软件进行IAWQ2(活性污泥动力学模型)建模,仿真模拟各种运行参数,提高处理能力,降低能源消耗,优化操作工艺.模型的建立可立即回答各种工艺问题.