The activated sludge process part 2. Application of the general kinetic model to the contact stabilization process

The general aerobic activated sludge model including nitrification for processes treating principally municipal wastewater is applied to the contact stabilization process treating municipal wastewaters. The application involves two changes to the model: (i) a change in one of the values of the kinetic constants in the expressions of the substrate utilization rates; (ii) a change in the enmeshment mechanism by accepting that a fraction of the particulate COD which is not adsorbed onto the active organisms, does not become enmeshed in the sludge flocs and escapes with the effluent. Accepting only these changes it was found possible to satisfactorily simulate the behaviour of the contact and stabilization reactors of the process under both constant and cyclic conditions of loading.

For design, the general activated sludge model, as applied to the contact stabilization process, requires the process, configuration to be completely specified. To aid in the initial design of the process, a preliminary design procedure is presented by means of which the volumes, sludge concentrations and retention times of the contact and stabilization reactors may be determined from five independent parameters which are assumed to govern the process. These are the sludge age, recycle ratio, fractional distribution of the sludge mass between contact and stabilization reactors, daily COD mass load and the average sludge concentration in the process.     

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.     

Operational estimation and prediction of nitrification dynamics in the activated sludge process

This paper examines the feasibility and discusses the potential of applications of on-line real-time state estimation and prediction in operational control of the activated sludge process. In particular, the dynamics of nitrification are considered with reference to the activated sludge unit at the Norwich Sewage Works in eastern England. A recursive estimation algorithm, the extended Kalman filter, is applied both for reconstructing operating information on the variations in nitrifying bacterial population concentrations and for making predictions of process performance under assumed scenarios for the short-term future operating conditions of the plant. Time-series field data from the Norwich Works are used for the former analysis. Considerations of uncertainty and the possibility of rapid major perturbations in performance, for example, due to spillages of toxic substances or the loss of solids over the clarifier weir, are of special importance to the discussion. The paper is introduced and concluded with some more general comments on the roles of operator experience and decision-making and man-machine interaction in wastewater treatment plant control.     

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.     

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%     

Effect of copper on nitrification in activated sludge

The concentration of free copper in activated sludge with copper added is strongly influenced by pH. For example, at pH 6.5 with 9.13 × 10⁻⁵ mol Cu l⁻¹, the free copper concentration is 4.0 × 10⁻⁷ mol l⁻¹ (pCu = 6.4) and at pH 8.4 this concentration is 10⁻⁸ mol l⁻¹ (pCu = 8.0). In both cases the activated sludge concentration is 0.7 g MLSS l⁻¹. The free copper concentration is also affected by the concentration of mixed liquor suspended solids (MLSS).In batch experiments with constant pH, the effect of copper on the nitrification rate was not regulated by total copper concentration but by copper/sludge ratio or by free copper concentrations. Experiments at different pH showed a linear correlation between nitrification capacity and free copper concentration, suggesting that the pH effect on nitrification below 8.3 is in fact a copper effect.Activated sludge with copper did not become acclimatized to the copper in a period of three days. Addition of nitrilotriacetic acid (NTA) within one day did cancel the copper inhibition.The results were compared with the effect of copper on acetate removal by heterotrophic micro-organisms. The nitrifiers proved to be no more susceptible to copper than heterotrophic micro-organisms.     

Reduction in chlorine requirements by control of nitrification in an oxygen activated sludge process

The objective of the study was to determine if chlorine could be saved by controlling nitrification in an oxygen activated sludge process so that the effluent would produce an ammonia concentration in the 2–4 mg l⁻¹ range. The hypothesis was that the ammonia would react with the chlorine to produce a stable combined residual at a much lower chlorine dosage than if the effluent was completely nitrified. In the latter case, breakpoint would have to be achieved with, potentially, a substantial amount of chlorine dissipated and lost in side reactions with residual organic materials.The study utilized two portable treatment units in two truck trailers on the site of the proposed 69th Street City of Houston 100 MGD Sewage Treatment Plant. The results indicated that chlorine requirements could be cut by one half or more if nitrification was controlled. The gross savings of chlorine were estimated to be in the one quarter of a million dollar range. Other findings were that there was no apparent effect of pH on chlorine residual in the range of 6.5–7.5; and that there was no apparent effect of operating the nitrification stage with air or high purity oxygen.     

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.     

Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization

The pretreatment of waste activated sludge by ultrasonic disintegration was studied in order to improve the anaerobic sludge stabilization. The ultrasound frequency was varied within a range from 41 to 3217 kHz. The impact of different ultrasound intensities and treatment times was examined. Sludge disintegration was most significant at low frequencies. Low-frequency ultrasound creates large cavitation bubbles which upon collapse initiate powerful jet streams exerting strong shear forces in the liquid. The decreasing sludge disintegration efficiency observed at higher frequencies was attributed to smaller cavitation bubbles which do not allow the initiation of such strong shear forces. Short sonication times resulted in sludge floc deagglomeration without the destruction of bacteria cells. Longer sonication brought about the break-up of cell walls, the sludge solids were distintegrated and dissolved organic compounds were released. The anaerobic digestion of waste activated sludge following ultrasonic pretreatment causing microbial cell lysis was significantly improved. There was an increase in the volatile solids degradation as well as an increase in the biogas production. The increase in digestion efficiency was proportional to the degree of sludge disintegration. To a lesser degree the deagglomeration of sludge flocs also augmented the anaerobic volatile solids degradation.     

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 cost-effective system for the aerobic stabilization and disposal of waste activated sludge solids

Five types of aerobic stabilization systems are compared with respect to construction cost, power usage, and operating features. Kinetic data for the aerobic stabilization of waste activated sludge solids are reviewed and evaluated. Guidelines are established for the rational design and operation of (1) a multicellular aerobic stabilizer, (2) a long-term (12 months) stabilized solids storage lagoon and (3) a dedicated land-disposal activity. Emphasis is placed on systems serving smaller treatment plants.     

Modelling nitrification, heterotrophic growth and predation in activated sludge

A mathematical model describing the interaction between nitrifiers, heterotrophs and predators in wastewater treatment has been developed. The inclusion of a predation mechanism is a new addition to the existing activated sludge models. The developed model considered multi-substrate consumption and multi-species growth, maintenance and decay in a culture where nitrifiers, heterotrophs and predators (protozoa and metazoa) are coexisting. Two laboratory-scale sequenced batch reactors (SBRs) operated at different sludge retention time (SRT) of 30 and 100 days for a period of 4 years were used to calibrate and validate the model. Moreover, to assess the predator activity, a simple procedure was developed, based on measuring the respiration rate with and without the presence of the predators. The model successfully described the performance of two SBRs systems. The fraction of active biomass (ammonia oxidisers, nitrite oxidisers and heterotrophs) predicted by the proposed model was only 33% and 14% at SRT of 30 and 100 days, respectively. The high fraction of inert biomass predicted by the model was in accordance with the microscopic investigations of biomass viability in both reactors. The presented model was used to investigate the effect of increasing sludge age and the role of predators on the biomass composition of the tested SBR system.     

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     

Nitrous oxide emissions for early warning of biological nitrification failure in activated sludge

Experiments were carried out to establish whether nitrous oxide (N₂O) could be used as a non-invasive early warning indicator for nitrification failure. Eight experiments were undertaken; duplicate shocks DO depletion, influent ammonia increases, allylthiourea (ATU) shocks and sodium azide (NaN₃) shocks were conducted on a pilot-scale activated sludge plant which consisted of a 315 L completely mixed aeration tank and 100 L clarifier. The process performed well during pre-shock stable operation; ammonia removals were up to 97.8% and N₂O emissions were of low variability (<0.5 ppm). However, toxic shock loads produced an N₂O response of a rise in off-gas concentrations ranging from 16.5 to 186.3 ppm, followed by a lag-time ranging from 3 to 5 h ((0.43-0.71) × HRT) of increased NH₃-N and/or NO₂⁻ in the effluent ranging from 3.4 to 41.2 mg L⁻¹. It is this lag-time that provides the early warning for process failure, thus mitigating action can be taken to avoid nitrogen contamination of receiving waters.     

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.     

铁盐强化活性污泥硝化作用效果研究

向活性污泥系统投加氢氧化铁并保持连续运行,形成铁盐强化活性污泥系统。从硝化反应过程和活性污泥微生物活性的角度出发,研究铁盐对活性污泥系统的影响。     

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.     

Biooxidation in a continuous activated sludge process

The paper presents a critical examination of the biochemical oxidation rate theory in the activated sludge process based on many years of lab, pilot and full scale experiments. A new critical equation relating the overall biochemical reaction rate on the activated sludge load is introduced. The concept introduced is shown to be valid for conventional activated sludge loading range for single and multistage process.     

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.