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.     

Micro-profiles of activated sludge floc determined using microelectrodes

The microbial activity within activated sludge floc is a key factor in the performance of the activated sludge process. In this study, the microenvironment of activated sludge flocs from two wastewater treatment plants (Mill Creek Wastewater Treatment Plant and Muddy Creek Wastewater Treatment Plant, with aeration tank influent CODs of 60-120 and 15-35 mg/L, respectively) were studied by using microelectrodes. Due to microbial oxygen utilization, the aerobic region in the activated sludge floc was limited to the surface layer (0.1-0.2mm) of the sludge aggregate at the Mill Creek plant. The presence of an anoxic zone inside the sludge floc under aerobic conditions was confirmed in this study. When the dissolved oxygen (DO) in the bulk liquid was higher than 4.0mg/L, the anoxic zone inside the activated sludge floc disappeared, which is helpful for biodegradation. At the Muddy Creek plant, with its lower wastewater pollutant concentrations, the redox potential and DO inside the sludge aggregates were higher than those at the Mill Creek plant. The contaminant concentration in the bulk wastewater correlates with the oxygen utilization rate, which directly influences the oxygen penetration inside the activated sludge floc, and results in redox potential changes within the floc. The measured microprofiles revealed the continuous decrease of nitrate concentration inside the activated sludge floc, even though significant nitrification was observed in the bulk wastewater. The oxygen consumption and nitrification rate analyses reveal that the increase of ammonia flux under aerobic conditions correlates with nitrification. Due to the metabolic mechanisms of the microorganisms in activated sludge floc, which varies from one treatment plant to another, the oxygen flux inside the sludge floc changes accordingly.     

Bio-augmentation by nitrification with return sludge

Bio-augmentation can be used to obtain nitrification in activated sludge processes that operate at sub-optimal solid retention times. In this study, we evaluated the potential of augmenting the endogenous nitrifying bacteria, by implementing a nitrification reactor in the sludge return line. This reactor can be fed with an internal N-rich flow (e.g. effluent from the sludge treatment) or with an external ammonium source. A mathematical model based on ASM1 was developed and used to evaluate the potential of this technique. The bio-augmentation studied here aimed to enhance the nitrification process of highly loaded activated sludge systems. A calibrated simulation model of a high loaded wastewater treatment plant in The Netherlands was used for this study. A side stream process (the named BABE process) was included in the simulation. This process was fed with the ammonia-rich water generated by sludge digestion and subsequent thickening by centrifugation (the so-called rejectwater). An external source (artificial) of ammonium was also considered to evaluate the differences between the two origins of ammonium. The results showed that with the augmentation process, high loaded activated sludge systems can achieve nitrification even at low winter temperatures. The best effect is obtained for systems operating at approximately 50% of the minimal SRT without augmentation. The use of an internal ammonia source is more effective than an external source. The results of this study give a quantitative basis for the design of process internal bio-augmentation processes and the effect on the N-removal capacity of the treatment plant.     

Nitrification efficiency and nitrifying bacteria abundance in combined AS-RBC and A2O systems

This study makes a comparison between the nitrification performance of TNCU-I (a combined activated sludge-rotating biological contactor process) and A2O systems by the use of a pilot plant and batch experiments. The nitrifier abundance in both systems was determined, using cloning-denaturing gradient gel electrophoresis (DGGE) and fluorescent in-situ hybridization (FISH), to investigate the role of rotating biological contactor in the TNCU-I process. The stability of the nitrification performance and the specific nitrification rate were found to be greater in TNCU-I system than in the A2O system. RBC biofilm promoted nitrifying activity that contributed to the nitrification performance, especially at a low SRT. By using the cloning-DGGE method, the genera Nitrosospira and Nitrospira were found to be present in all the samples, while the genus Nitrosomonas was observed only in the TNCU-I RBC biofilm. In addition, the proportions of ammonia oxidizer in the TNCU-I RBC biofilm, the TNCU-I activated sludge and the A2O activated sludge were 11.4%, 13.2%, and 4.1%, respectively, higher than the nitrite oxidizer fractions of 3.3%, 5.7% and 2.1%, respectively, according to the cloning-DGGE method. On the other hand, the proportions of ammonia oxidizers in the afore-mention materials were 10.3%, 13.7%, and 5.2%, higher than the nitrite oxidizer fractions of 2.5%, 3.6% and 2.3%, according to the FISH experiments. This implies that the proportion of ammonia oxidizer in the TNCU-I process was 3.2 and 2.6 times that in the A2O process, determined by the cloning-DGGE and FISH methods, respectively. These amounts are also close to the ammonia oxidization rate of 2.9 times. All the data show that RBC added to the aerobic zone of TNCU-I process would increase the nitrifier abundance and enhance the nitrification performance of the system.     

Effect of ZnO particles on activated sludge: role of particle dissolution

The release of nanoparticles (NPs) into the environment, including wastewater treatment plants, is expected to increase in the future. Therefore, it is important to understand the potential effects of these NPs on activated sludge treatment processes. A pulse-flow respirometer was used to study the toxicity of nano-ZnO on activated sludge endogenous respiration, BOD biodegradation, and nitrification. In addition, toxicities of bulk ZnO particles and Zn ion (e.g. soluble Zn) were also studied. All three Zn forms were found to adversely impact the activity of activated sludge, with soluble Zn exhibited the greatest toxicity. The effects of nano-ZnO and bulk ZnO on activated sludge were caused by soluble Zn resulting from ZnO particle dissolution. The IC₅₀ values of soluble Zn on activated sludge endogenous respiration, BOD biodegradation, ammonia oxidation, and nitrite oxidation were 2.2, 1.3, 0.8, and 7.3 mg-Zn/L, respectively. Therefore, the first step of nitrification was most sensitive to Zn.     

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.     

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.     

Bacterial response to a shock load of nanosilver in an activated sludge treatment system

The growing release of nanosilver into sewage systems has increased the concerns on the potential adverse impacts of silver nanoparticles (AgNPs) in wastewater treatment plants. The inhibitory effects of nanosilver on wastewater treatment and the response of activated sludge bacteria to the shock loading of AgNPs were evaluated in a Modified Ludzack-Ettinger (MLE) activated sludge treatment system. Before shock-loading experiments, batch extant respirometric assays determined that at 1 mg/L of total Ag, nitrification inhibitions by AgNPs (average size = 1-29 nm) and Ag⁺ ions were 41.4% and 13.5%, respectively, indicating that nanosilver was more toxic to nitrifying bacteria in activated sludge than silver ions. After a 12-h period of nanosilver shock loading to reach a final peak silver concentration of 0.75 mg/L in the MLE system, the total silver concentration in the mixed liquor decreased exponentially. A continuous flow-through model predicted that the silver in the activated sludge system would be washed out 25 days after the shock loading. Meanwhile, a prolonged period of nitrification inhibition (>1 month, the highest degree of inhibition = 46.5%) and increase of ammonia/nitrite concentration in wastewater effluent were observed. However, nanosilver exposure did not affect the growth of heterotrophs responsible for organic matter removal. Microbial community structure analysis indicated that the ammonium-oxidizing bacteria and nitrite-oxidizing bacteria, Nitrospira, had experienced population decrease while Nitrobacter was washed out after the shock loading.     

通过控制泥龄实现亚硝酸盐型同步硝化反硝化

采用序批式活性污泥法处理人工配制的城市生活污水，通过控制泥龄成功地实现了亚硝酸盐型同步硝化反硝化，曝气过程中NO2^-N／NOx^-N值始终保持在84．48％以上，曝气结束时大约有80．39％的氨氮通过同步硝化反硝化途径被去除。在适宜的曝气量下，利用排泥的方法控制反应器内适宜的泥龄，可以实现稳定的亚硝酸盐型同步硝化反硝化。     

Nitrification of ammonia-nitrogen in refinery wastewater

Wastewater from integrated petroleum refineries normally contains 20–80 mg/l ammonia-nitrogen which is harmful to the fishes in the receiving water bodies. To remove ammonia-nitrogen completely in the biotreatment system of refinery wastewater, various enrichment and cultivation methods of nitrifying bacteria were tried.

Results indicated that by supplementing glucose nutrient, over 90% ammonia-nitrogen in the refinery wastewater could be converted into nitrite within 7–14 days in either an activated sludge or rotating biological contactor system. Trace nitrate was detected. Further addition of yeast powder to the treatment systems stimulated the growth of Nitrobacter which accelerated the oxidation of nitrite to nitrate. Complete oxidation of the influent ammonia-nitrogen was observed 7 days later after adding 5 mg/l yeast powder to the influent wastewater. It also showed that neither glucose nor yeast powder were required for sustaining ammonia nitrification after the augmentation of Nitrobacter was achieved.     

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.     

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.     

利用原生动物削减剩余活性污泥产量

采用两段式膜生物反应器作为原生动物哺育系统,培养富含原生动物的污泥,然后将其定期接种于普通活性污泥中,利用原生动物对细菌的捕食原理,达到削减剩余污泥量的目的。污泥削减试验中采用了半连续流普通活性污泥系统,通过对比试验,发现接种原生动物以后,污泥产率由０．０２ｋｇ泥／ｋｇＣＯＤ减小至－０．４７ｋｇ泥／ｋｇＣＯＤ,同时污泥絮凝沉降性能得到改善,系统的ＣＯＤ去除率、硝化率得到提高,出水悬浮物浓度得以降低     

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.     

Comparison of various revised k–ε models and LES applied to flow around a high-rise building model with 1:1:2 shape placed within the surface boundary layer

This paper compares computational fluid dynamics (CFD) results using various revised k–ε models and large eddy simulation (LES) applied to flow around a high-rise building model with 1:1:2 shape placed within the surface boundary layer. The first part of the paper examines the accuracy of various revised k–ε models, i.e. LK model, MMK model and Durbin's revised k–ε model, by comparing their results with experimental data. Among the computations using various revised k–ε models compared here, Durbin's revised k–ε model shows the best agreement with the experiment. The reason for the good performance of Durbin's model is discussed on the basis of ‘Realizability’ of predicted results. The second part of the paper describes the computations based on LES with and without inflow turbulence applied to the same flowfield. The results are compared with those of the experiments and Durbin's k–ε model in order to clarify the effect of velocity fluctuations on prediction accuracy of time-averaged velocity fields around the building. Special attention is paid to prediction accuracy for reproducing flow behind a building. The LES results with inflow turbulence show generally good agreement with experimental results in terms of the distributions of velocity and turbulence energy in this region. This improvement is mainly due to the fact that the periodic velocity fluctuation behind the building is well reproduced in LES.     

Microbial degradation of a new detergent builder, carboxymethyltartronate (CMT), in laboratory activated sludge systems

An approach for determining the biodegradability of new detergent builders under activated sludge conditions was investigated using carboxymethyltartronate (CMT) as a representative material. Preliminary biodegradation studies in river water and activated sludge indicated a 4–8 week period before acclimation to and biodegradation of CMT. More detailed studies in continuous flow activated sludge units established that acclimation to CMT degradation was not readily lost upon CMT starvation, could be maintained at low temperatures and under conditions of variable organic loading, and was unaffected by the presence of a variety of metal ions. The laboratory evaluation of CMT, which stressed the use of natural sewage/activated sludge systems in place of tests employing commercially available laboratory media, would predict CMT to be satisfactorily removed in full-scale activated sludge waste treatment plants.     

Assessing the influence of pronounced diurnal temperature variations in nontemperate zones on the denitrification/nitrification rate using the COST Benchmark activated sludge model no. 1 simulation

Activated sludge plants are usually designed using formulae based on fixed wastewater temperatures. In some nontemperate zones, these procedures may need altering, particularly for areas that experience large diurnal and seasonal temperature variation such as in desert or high‐altitude environments. This study investigates these temperature effects on the denitrification/nitrification rate by utilising the COST Benchmark activated sludge model no. 1 (ASM1) simulation model. The COST Benchmark was developed under the European Cooperation in Science and Technology programme Action 624 which specifically focused on the Optimal Management of Wastewater Systems. Various simulations and associated control strategies were carried out using gPROMS software package (Process Systems Enterprise Limited, London, UK) to ascertain the following: any likely nutrient removal implications; use of a nitrate and/or ammonia sensor coupled to a strategy to change the solids loading inventory to minimise falls in nitrification rate during night time hours; and changes to controller set point levels during the day and night to reflect diurnal load, flow and temperature variations using a temperature‐dependent control algorithm. It was found that inclusion of temperature dependency in simulation models may prove beneficial when designing plants located in these regions.     

Comparison of the cost of the pressurized wastewater treatment process with other established treatment processes

A biological wastewater treatment unit under the influence of pressure was operated at pressures of up to 6 bar. It was demonstrated the capability of operating effectively with high carbonaceous oxidation and nitrification efficiencies at loadings of up to 13 g BOD m⁻² day. Another advantage of the pressurized treatment unit was observed to be its low sludge production. The cost of the pressurized treatment unit has, however, become an important factor for the applicability of the pressurized treatment unit. An investigation was carried out to determine the approximate cost of a proposed full-scale pressurized unit. The comparison between the pressurized unit and the established treatment processes were made for three populations of 500, 1000 and 3000 persons. The costs were then compared with those of various other conventional biological treatment processes capable of treating an equivalent wastewater load. The conventional processes selected for comparison were the activated sludge, biological filtration and conventional RBC.

The sludge disposal cost for the pressurized unit was appreciably lower than that for the other processes. The results indicated that the cost of the pressurized unit (present value for a 20-yr period) and the costs of the activated sludge and conventional RBC processes were found to be similar for the smaller populations. However, a substantial saving could be obtained with the pressurized unit for the larger populations. In addition, there was an indication that the land requirement of the pressurized treatment unit decreased appreciably as the flow rate increased.     

Determination of non-ionic surfactants and their biotransformation by-products adsorbed on alive activated sludge

A procedure has been developed for the determination of non-ionic surfactants (NS) adsorbed on particles of alive and dead activated sludge. The procedure also enables the determination of adsorption of major biodegradation by-products: short-chained ethoxylates, long- and short-chained PEG. The basis of measurement is the determination of NS concentration in a slurry of activated sludge and in a solution phase. The difference between these two concentrations represents the NS adsorbed on activated sludge. Separation of NS and their biotransformation by-products from samples and then on narrower fractions was performed by a sequential liquid-liquid extraction and precipitation with modified Dragendorff reagent. The indirect tensammetric technique (ITT) was applied for the final determination. The developed method was checked using the example of the treatment of the surfactant C12E10 (oxyethylated fatty alcohol) (C12E10) in the continuous flow activated sludge facility. No statistically significant accumulation of C12E10 on the alive activated sludge was detected, probably because of faster C12E10 fission than its adsorption. However, significant adsorption of the short-chained ethoxylates (including free alcohol) on the alive activated sludge was found, as well as statistically significant adsorption of long- and short-chained PEG. The adsorption of surfactant C12E10 and its biodegradation by-products on dead activated sludge was found to be higher than the species adsorption on alive activated sludge.     

Survival of a non-flocculating bacterium, Thiobacillus thioparus TK-1, inoculated to activated sludge

The survival of a non-flocculating bacterium inoculated to activated sludge and the development of the bacterial activity were examined. Thiobacillus thioparus TK-1, which degrades O,O-dimethyl phosphorodithioate (DMDTP) contained in the wastewater of pesticide manufacturers, shows no flocculation property. This strain was inoculated to activated sludge at the concentration of 5 × 10⁷, 10⁵ and 10³ cfu/ml, and a wastewater containing DMDTP was treated with this sludge. No degradation was observed for the first 3, 5 and 7 days, respectively. However, after this lag time the activated sludge developed the stable degradation activity. The population of the inoculated strain in the activated sludge decreased at first, but then increased very rapidly up to 10⁸ cfu/ml and remained stable at this level.