Full-scale Activated Sludge Plant Research Articles

Benchmarking procedure for full-scale activated sludge plants

To enhance development and acceptance of new control strategies, a standard simulation benchmarking methodology to evaluate the performance of wastewater treatment plants has recently been proposed. The proposed methodology is, however, for a typical plant and that works under typical loading and environmental conditions. Thus, benchmarking a full-scale plant working under different situations is still a problem that needs to be solved. This paper proposes a data based approach to benchmark any specific full-scale activated sludge plant used for carbon and nitrogen removal and using real design, operational and performance data. The advantage of the proposed approach over the “one-typical-plant” approach, is that it takes into account the very specific design and operational characteristics of the plant that will be benchmarked. A real-world illustrative example is also presented in this paper.

Testing the effect of selectors in the control of bulking and foaming in full scale activated-sludge plants

Selectors were operated at four full-scale activated sludge plants to control bulking and foaming problems due to filamentous microorganisms. Selector effectiveness was not related to reduction of biodegradable organic matter in the contact zone, but was related to soluble COD levels in selector effluent. Significant reductions in the numbers of filamentous m icroorganisms were reported.

A simple empirical model for activated sludge thickening in secondary clarifiers

A simple empirical model for the thickening function of the activated sludge secondary clarifiers is presented. The proposed approach relies on the integration of previous models and it is based on the phenomenon of dilution of the incoming activated sludge in the feeding well of the settling tanks. The method provides a satisfactory description of sludge stratification within the clarifier. The only requirements are limited to parameters which are readily incorporated into the routine analysis performed in an activated sludge plant, thereby eliminating the need for additional experimental or computational effort. The method was tested in a full-scale activated sludge plant and it was found that it describes fairly well the return sludge concentration, the diluted sludge blanket concentration, the sludge blanket solids concentration and the sludge blanket height of full-scale secondary clarifiers.

A PROCEDURE FOR BENCHMARKING SPECIFIC FULL-SCALE ACTIVATED SLUDGE PLANTS USED FOR CARBON AND NITROGEN REMOVAL

To enhance development and acceptance of new control strategies, a standard simulation benchmarking methodology to evaluate the performance of wastewater treatment plants has recently been proposed. The proposed methodology is, however, for a typical plant and typical loading and environmental conditions. Thus, benchmarking a full-scale plant working under different situations is still a problem that needs to be solved. This paper proposes a realistic approach to benchmark specific full-scale activated sludge plants used for carbon and nitrogen removal, based on real design, operational and performance data. An illustrative example is also presented in this paper.

Role of filamentous microorganisms in activated sludge foaming: relationship of mycolata levels to foaming initiation and stability

The relationship between the levels of mycolic acid-containing actinomycetes (mycolata), Gordonia spp. and Gordonia amarae, and foam initiation and stability was characterized using: (1) batch tests involving addition of G. amarae cells to activated sludge, (2) analysis of a full-scale activated sludge plant that experienced seasonal foaming, and (3) a study of lab-scale activated sludge reactors augmented with G. amarae. Using batch tests, threshold Gordonia levels for foam formation and foam stability were determined to be approximately 2×10 8 μm ml −1 and 1×10 9 μm ml −1, respectively. In the full-scale plant, the levels of Gordonia spp. and G. amarae increased during the course of foaming, and the foam formation threshold of 2×10 8 μm ml −1 corresponded to the onset of foaming. This value was also verified in lab-scale reactor washout experiments, where decreasing mycolata levels were observed during the course of foam dissipation. The foam stability threshold of 1×10 9 μm ml −1 was verified in lab-scale reactor studies. The increase in the levels of Gordonia spp. and G. amarae in the full-scale plant corresponded to an increase in temperature, suggesting that the growth of Gordonia spp. was favored during warmer periods.

The shear sensitivity of activated sludge: Relations to filterability, rheology and surface chemistry

Many solid/liquid separation properties are dependent upon the number of fine (micron sized) particles present in the suspension, which in turn is a consequence of the flocculation/break-up balance of the sludge. The present study revealed strong correlations between the dispersed mass concentration, resistance to filtering (capillary suction time) and rheological parameters for activated sludge suspensions of varying solids content. It seems likely that the restriction of flow caused by development of inter-particle networks causes increased surface erosion of primary particles. The increased dispersed mass concentration in turn causes deterioration of filterability. The shear sensitivity parameter quantifies the degree of dispersion in response to turbulent shear and is thus expected to be of prime importance for solid/liquid separation processes. The parameter reflects changes of the surface chemistry, and is therefore suitable as a means of assessing variations of surface chemistry for full scale activated sludge plants. Differences in interaction energies of different sludges may be estimated from shear sensitivity estimates. The observed changes in interaction energy upon addition of sulphide or detergent, changed pH or lowering of the ionic strength were in qualitatively agreement with DLVO-theory.

A New Method for the Design of Sequencing Batch Reactors (SBR) using the Concept of the Hindered Settling Velocity of the Sludge

The paper presents a new methodology for the design of sequencing batch reactors. The design uses the concept of the hindered settling velocity of the sludge, helping in the definition of the settling and idle times during the cycle. Other design routines presented in the literature use the sludge volume index (30-minute settlement in the test), and the time for sludge settling in the reactor is based purely on experience, regardless of the sludge concentration during the react phase. The hindered settling velocity is calculated based on coefficients obtained for each one of five possible sludge settleability ranges (very good, good, fair, poor and very poor). The settling coefficients have been obtained through the combination of values presented in 16 references, with data from a large number of operating full-scale activated sludge plants in different countries of the world. The design routine introduced in the paper is simple, practical and conceptually justifiable, and is believed to represent a step in advancement in relation to other ones currently described in classic textbooks.

Influence of temperature and sludge loading on activated sludge settling, especially on Microthrix parvicella

During recent years modern full scale wastewater treatment plants with biological nitrification, denitrification and phosphorus removal have had increasing problems with foam formation on the surfaces of aerobic tanks and with bulking activated sludge. The results of a survey in 1995 (Kunst and Knoop, 1996) showed that most often the filamentous bacterium Microthrix parvicella is responsible for these problems. Up to today there is only little knowledge about its selection criteria in activated sludge. Therefore several experiments were done in full scale activated sludge plants and in laboratory systems under defined conditions to investigate the influence of low (< 0.1 kg/(kg·d)) and high (≤ 0.2 kg/(kg·d)) BOD5-sludge loading rates on the growth and morphology of M. parvicella and the settlement of activated sludge. Furthermore the influence of temperatures of 5°C, 12°C and 20°C on the growth of M. parvicella was investigated. It was shown that M. parvicella grows at low BOD5-sludge loading rate and low temperature and is the main causative organism of bulking and foaming sludge in nutrient removal plants. On the basis of this investigation it was concluded that the growth of M. parvicella and the settling problems of the activated sludge resulting from excessive growth of this filament will always appear in modern municipal wastewater treatment plants with BOD5-sludge loading rate ≤ 0.1 kg/(kg·d) especially under low temperature conditions.

The Treatment of a High Strength Pulp and Paper Mill Effluent for Waste Water Re-Use

The International Association on Water Quality (IAWQ) Activated Sludge Model No. 1 was used to optimise a full-scale activated sludge plant, treating a high-strength pulp and paper mill effluent operating at a temperature of 38°C. The model was calibrated by determining the mixing characteristics of the reactor and the oxygenation capactity of the aeration system in situ. In addition batch tests were carried out to determine wastewater COD fractions together with the important kinetic parameters necessary to run the IAWQ Model. Steady-state simulations carried out using the calibrated model were in good agreement with average plant operational data. As a result of major modifications which were undertaken to the paper mill white water systems, the investigation was repeated and the results revealed that both the wastewater characteristics and process kinetic features had changed and this was mirrored by changes in plant performance. Again simulations carried out using the revised parameters showed good agreement with plant operating data. As a result the model was used to provide recommendations for optimising plant operation, namely that by reducing sludge age, there would be a reduced oxygen consumption together with an enhanced COD removal. The model proved to be a valuable tool for predicting the steady-state performance of this activated sludge system treating pulp and paper mill effluent at elevated temperatures.

Influence of temperature and sludge loading on activated sludge settling, especially on Microthrix parvicella

During recent years modern full scale wastewater treatment plants with biological nitrification, denitrification and phosphorus removal have had increasing problems with foam formation on the surfaces of aerobic tanks and with bulking activated sludge. The results of a survey in 1995 (Kunst and Knoop, 1996) showed that most often the filamentous bacterium Microthrix parvicella is responsible for these problems. Up to today there is only little knowledge about its selection criteria in activated sludge. Therefore several expenments were done in full scale activated sludge plants and in laboratory systems under defined conditions to investigate the influence of low (< 0.1 kg/(kg·d)) and high (≤ 0.2 kg/(kg·d)) BOD5-sludge loading rates on the growth and morphology of M. parvicella and the settlement of activated sludge. Furthermore the influence of temperatures of 5°C, 12°C and 20°C on the growth of M. parvicella was investigated. It was shown that M. parvicella grows at low BOD5-sludge loading rate and low temperature and is the main causative organism of bulking and foaming sludge in nutrient removal plants. On the basis of this investigation it was concluded that the growth of M. parvicella and the settling problems of the activated sludge resulting from excessive growth of this filament will always appear in modern municipal wastewater treatment plants with BOD5-sludge loading rate ≤ 0.1 kg/(kg·d) especially under low temperature conditions.

Group-specific small-subunit rRNA hybridization probes to characterize filamentous foaming in activated sludge systems.

Foaming in activated sludge systems is characterized by the formation of a thick, chocolate brown-colored scum that floats on the surface of aeration basins and secondary clarifiers. These viscous foams have been associated with the presence of filamentous mycolic acid-containing actinomycetes. To aid in evaluating the microbial representation in foam, we developed and characterized group-, genus-, and species-specific oligonucleotide probes targeting the small subunit rRNA of the Mycobacterium complex, Gordona spp., and Gordona (Nocardia) amarae, respectively. The use of a universal base analog, 5-nitroindole, in oligonucleotide probe design was evaluated by comparing the characteristics of two different versions of the Mycobacterium complex probe. The temperature of dissociation of each probe was determined. Probe specificity studies with a diverse collection of 67 target and nontarget rRNAs demonstrated the specificity of the probes to the target groups. Whole-cell hybridizations with fluorescein- and rhodamine-labeled probes were performed with pure cultures of various members of the Mycobacterium complex as well as with environmental samples from a full-scale activated sludge plant which experienced foaming. Quantitative membrane hybridizations with activated sludge and anaerobic digester foam showed that 15.0 to 18.3% of the total small-subunit rRNAs could be attributed to members of the Mycobacterium complex, of which a vast majority consisted of Gordona rRNA. Several G. amarae strains made up only a very small percentage of the Gordona strains present. We demonstrated that group-specific rRNA probes are useful tools for the in situ monitoring and identification of filamentous bacteria in activated sludge systems.

One-Dimensional Clarifier Model with Sludge Blanket Heights

A one-dimensional clarifier model employing a concentration and feed velocity dependent dispersion coefficient was coupled with an algorithm for predicting sludge blanket height. The model was successfully calibrated using blanket heights; effluent suspended solids and return activated sludge concentrations; and the Vesilind parameters V0 and b . These data were collected from one of the secondary clarifiers at a full-scale activated sludge plant. Model validity was confirmed by comparing predictions of the calibrated model against separate sets of data collected from the same clarifier. The model outperformed other well-known models in predicting clarifier overloading. Better performance of the present model can be attributed to its ability to portray, through a dependence of dispersion coefficient on feed velocity, the detrimental effect of increasing hydraulic loading (for any given applied solids flux) on clarifier performance.

The effect of chemical bulking control on biological nutrient removal in a full scale activated sludge plant

The use of chemicals for sludge bulking control has a direct effect on the biological nutrient removal processes in activated sludge systems designed for this purpose. Chlorine has been used on full scale but information on the use of ozone and hydrogen peroxide is limited to pilot scale tests. The objective of this study was to investigate the effects of chlorine, ozone and hydrogen peroxide on nutrient removal processes when used on a continuous basis for bulking control in a full scale biological nutrient removal activated sludge plant. The full scale studies were conducted over a period of 39 months at the Daspoort sewage works of the City Council of Pretoria. The results indicate that at low dosages the oxidants have limited effects on the nutrient removal processes but at higher levels chlorine had a detrimental effect resulting in the phosphate limit of 1 mg P.1−1 being exceeded. It is concluded that chlorine is the most effective of the three oxidants for bulking control, but that it should be used with caution in order not to upset the biological phosphate removal processes. Ozone at low levels had a small but consistent positive effect on bulking control as well as on nutrient removal. The effects of hydrogen peroxide were very small except at high dosages.

The effect of chemical bulking control on biological nutrient removal in a full scale activated sludge plant

The use of chemicals for sludge bulking control has a direct effect on the biological nutrient removal processes in activated sludge systems designed for this purpose. Chlorine has been used on full scale but information on the use of ozone and hydrogen peroxide is limited to pilot scale tests.The objective of this study was to investigate the effects of chlorine, ozone and hydrogen peroxide on nutrient removal processes when used on a continuous basis for bulking control in a full scale biological nutrient removal activated sludge plant. The full scale studies were conducted over a period of 39 months at the Daspoort sewage works of the City Council of Pretoria.The results indicate that at low dosages the oxidants have limited effects on the nutrient removal processes but at higher levels chlorine had a detrimental effect resulting in the phosphate limit of 1 mg P.1-1 being exceeded.It is concluded that chlorine is the most effective of the three oxidants for bulking control, but that it should be used with caution in order not to upset the biological phosphate removal processes. Ozone at low levels had a small but consistent positive effect on bulking control as well as on nutrient removal. The effects of hydrogen peroxide were very small except at high dosages.

Phosphate uptake by immobilized Acinetobacter calcoaceticus cells in a full scale activated sludge plant.

An in situ study of the P-uptake ability of Acinetobacter calcoaceticus was carried out using the alginate immobilization technique. Immobilized A. calcoaceticus cells displayed a high P-uptake ability (> 97% P-accumulating cells) when immersed in the aerobic zone of an activated sludge system for 30-240 min. The overall P-accumulation pattern of the anaerobic zone depicted a typical P-release mechanism. However, limited P-accumulation was also observed at this stage. Growth and anaerobiosis were not prerequisites for P-uptake. The immobilized cell retention time in the anaerobic zone did not affect remarkably the inherent P-uptake ability of immobilized A. calcoaceticus when exposed to the aerobic stage. P-uptake and release were reversible and depended on the environmental conditions to which immobilized cells were exposed. Immobilization of A. calcoaceticus using alginate can be regarded as a reliable method of studying pure cultures in the activated sludge process.

Advanced purification of carbonization wastewater by activated sludge treatment

Abstract A full scale activated sludge plant has been developed treating 960 m3 of carbonization wastewater daily. Results and process parameters from the first three years of operation are described. In spite of intense physical‐chemical pretreatment, the carbonization wastewater must still be diluted by 50% prior to biological processing due to the presence of inhibitory organic compounds. The activated sludge plant consists of four serially connected aeration tanks. The influent is distributed following a step load regime. Other specific process characteristics are: pure oxygen aeration, high mixed liquor volatile suspended solids (MLVSS) levels of 10–15 kg MLVSS/m3, and a high sludge age of 100–150 days. The first aeration tank is kept anoxic, making it possible to implement combined nitrification and denitrification. Average reductions in chemical oxygen demand (COD) and total reduced nitrogen (NH4 + and CNS‐) were 90% and 77%, respectively. Average final effluent levels obtained, after a four month ...

The dynamic behaviour of nitrifying Activated sludge systems influenced by inhibiting wastewater compounds

More or less severe nitrification inhibition was observed in several pilot and full-scale activated sludge plants treating industrial wastewaters. In order to control the treatment process under inhibiting conditions, extended nitrification models have been developed on base of the ‘Activated sludge model No. 1’. In the case of temperatures between 25 and 30°C, the nitrification process has been expressed as a two-step reaction with nitrite as intermediate. Model elements for competitive and non-competitive inhibition as well as for biodegradation of the inhibitor were added, if required. The dynamic behaviour of the investigated activated sludge systems indicates that there are biodegradable non-competitive inhibitors. Operational as well as simulation results show that nitrifying activated sludge plants may become acclimatized to inhibitory compounds but have to be protected from peak loads of both nitrogen and inhibitory compounds.

The dynamic behaviour of nitrifying activated sludge systems influenced by inhibiting wastewater compounds

More or less severe nitrification inhibition was observed in several pilot and full-scale activated sludge plants treating industrial wastewaters. In order to control the treatment process under inhibiting conditions, extended nitrification models have been developed on base of the ‘Activated sludge model No. 1’. In the case of temperatures between 25 and 30°C, the nitrification process has been expressed as a two-step reaction with nitrite as intermediate. Model elements for competitive and non-competitive inhibition as well as for biodegradation of the inhibitor were added, if required. The dynamic behaviour of the investigated activated sludge systems indicates that there are biodegradable non-competitive inhibitors. Operational as well as simulation results show that nitrifying activated sludge plants may become acclimatized to inhibitory compounds but have to be protected from peak loads of both nitrogen and inhibitory compounds.

Nitrification inhibition – a method for the estimation of actual maximum autotrophic growth rates in activated sludge systems

Since more or less severe nitrification inhibition was observed in several pilot and full-scale activated sludge plants, a means to quantify the extent of inhibition under operational conditions has been sought. Thus a method for the estimation of the actual maximum autotrophic growth rate μ^A,T was developed. This parameter helps to detect low nitrification capacity, before the effluent ammonia concentration increases. Two case studies show that nitrifying activated sludge plants may become acclimatized to inhibitory compounds but have to be protected from peak loads of both nitrogen and inhibitory compounds.

A theory of the operation of full scale activated sludge plants

The results of experimental trials with full scale, carbonaceous oxidation, activated sludge plants, at Manchester, were reported about 30 years ago. An equation linking the activity of a sludge with its concentration was developed which showed that the active mass was a decreasing proportion of the sludge as its concentration was increased. This approach has now been extended to develop equations which explain why there was an optimum sludge concentration for each plant, which gave the required effluent quality for a minimum power consumption and which varied in direct relation to the square root of the load purified. This explanation has been supported by reference to the previous work, using three different methods for assessing sludge activity and points to the reaction within the floc being second order. Another conclusion has been that variation of active mass may produce a minimum rate of substrate removal, at a low active mass, but that increasing concentrations produce a steady increase in removal. Also in the plants, the rate of purification was limited by lack of oxygen and consideration of the strength of the sewage suggests that this is normal for conventional plants. Evidence has been presented to show that, as predicted by the second-order reaction rate hypothesis, the oxygen concentration necessary to maintain effluent quality, increased as the aeration period decreased.