Cyclic Activated Sludge Technology Research Articles

Comprehensive carbon footprint analysis of wastewater treatment: A case study of modified cyclic activated sludge technology for low carbon source urban wastewater treatment

To reduce pollution and carbon emissions, a quantitative evaluation of the carbon footprint of the wastewater treatment processes is crucial. However, micro carbon element flow analysis is rarely focused considering treatment efficiency of different technology. In this research, a comprehensive carbon footprint analysis is established under the micro carbon element flow analysis and macro carbon footprint analysis based on life cycle assessment (LCA). Three wastewater treatment processes (i.e., anaerobic anoxic oxic, A2O; cyclic activated sludge technology, CAST; modified cyclic activated sludge technology, M-CAST) for low carbon source urban wastewater are selected. The micro key element flow analysis illustrated that carbon source mainly flows to the assimilation function to promote microorganism growth. The carbon footprint analysis illustrated that M-CAST as the optimal wastewater treatment process had the lowest global warming potential (GWP). The key to reduce carbon emissions is to limit electricity consumption in wastewater treatment processes. Under the comprehensive carbon footprint analysis, M-CAST has the lowest environmental impact with low carbon emissions. The sensitivity analysis results revealed that biotreatment section variables considerably reduced the environmental impact on the LCA and the GWP, followed by the sludge disposal section. With this research, the optimization scheme can guide wastewater treatment plants to optimize relevant treatment sections and reduce pollution and carbon emissions.

A back propagation neural network model for accurately predicting the removal efficiency of ammonia nitrogen in wastewater treatment plants using different biological processes

Accurately predicting the water quality of treated water from a water treatment plant (WWTP) based on the obtained operating database is of great significance. However, it is difficult for common mechanistic models to work well. In this study, a back propagation artificial neural network (BPANN) model with high accuracy was developed to predict the denitrification efficiency based on a 1-year operating database. Standardized principal component analysis (PCA) methods were used to address the data, and the PCA processed data exhibited the best accuracy. In three WWTPs adopting the anaerobic/anoxic/oxic (A2O) process, the ammonia nitrogen removal efficiency of WWTPs was successfully predicted by using five variables: inlet flow rate, pH value, original ammonia nitrogen concentration, Chemical oxygen demand (COD) concentration, and total phosphorus concentration. Importantly, the obtained BPANN model can be effectively used for other widely used treatment processes, such as oxidation ditch (OD), sequencing batch reactor activated sludge process (SBR), membrane bioreactor (MBR), and cyclic activated sludge technology (CAST), by simply optimizing the training data ratios between 50/50 and 90/10. This is the first trial to set up a universal model for predicting the denitrification efficiency of WWTPs adopting common biological processes. The model could be used to choose the optimum treatment process in the new WWTP design or take action in advance to avoid the risk of excessive emissions when the already built WWTPs are subjected to sudden shocks.

Diagnostic Method for Enhancing Nitrogen and Phosphorus Removal in Cyclic Activated Sludge Technology (CAST) Process Wastewater Treatment Plant

Ensuring the stable operation of urban wastewater treatment plants (WWTPs) and achieving energy conservation and emission reduction have become serious problems with the improvement of national requirements for WWTP effluent. Based on a wastewater quality analysis, identification of the contaminant removal, and a simulation and optimization of the wastewater treatment process, a practical engineering diagnosis method for the cyclic activated sludge technology process of WWTPs in China and an optimal control scheme are proposed in this study. Results showed that exceeding the standard of effluent nitrogen and phosphorus due to unreasonable process cycle setting and insufficient influent carbon source is dangerous. The total nitrogen removal rate increased by 9.5% and steadily increased to 67% when agitation was added to the first 40 min of the cycle. Additionally, the total phosphorus (TP) was reduced to 0.27 mg/L after replacing the phosphorus removal agent polyferric sulfate with polyaluminum iron. The corresponding increase in the TP removal rate to 97% resulted in a reduction in the treatment cost by 0.008 CNY/t.

Effects of different treatment processes in four municipal wastewater treatment plants on the transport and fate of microplastics

The behavior of microplastics in wastewater treatment plants has been investigated, but specific effects of treatment process on microplastics' fate are still unclear due to varied analysis methods and regional differences. In this study, four wastewater treatment plants in Ningbo of southeastern China with different treatment processes were selected to investigate transport and fate of microplastics. Based on number of microplastic particles, fibers and fragments were the main microplastics types in wastewater, while synthetic cellulose represented the largest fraction. The dominance of fibers (76.7%–90.0%) and small particle sizes (<2.0 mm, 62.5%–81.5%) in effluents suggested that they escaped easily from the wastewater treatment plants. The abundance of microplastics particles decreased from 78.0 ± 2.9 items/L in influent to 6.0 ± 2.8 items/L in effluent for anaerobic-anoxic-oxic process, 100.0 ± 3.1 items/L to 4.3 ± 3.4 items/L for sequencing batch reactor activated sludge process, 105.0 ± 5.3 items/L to 3.5 ± 2.6 items/L for cyclic activated sludge technology, 65.0 ± 4.3 items/L to 3.0 ± 1.6 items/L for oxidation ditch process. The microplastics removal capacity of primary and secondary treatment processes for four wastewater treatment plants ranged from 83.7% to 96.3%. Application of different tertiary treatment processes (coagulation/flocculation, membrane related technology and disinfection) enhanced microplastics removal to achieve overall removal rate of 92.3%–96.7%. The removed microplastics from the wastewater treatment plants were mainly transferred to sludge (226.1 ± 95.7–896.0 ± 144.0 items/g dry weight). The biological treatment unit played an important role in microplastics removal with rates varying between 86.9%–95.2%, while tertiary treatment reduced daily microplastics emission 1.4 × 108–2.3 × 108 items/day. This study suggests that proper selection of wastewater treatment unit could significantly reduce the emission number of microplastics, which supports an efficient control strategy of microplastics in wastewater treatment plants.

Performance of nitrogen removal in an alternating activated sludge reactor for full-scale applications

ABSTRACTA novel wastewater treatment process, known as an alternating activated sludge reactor (AASR), is proposed to treat wastewater in full-scale operations. The AASR is a technical development based on the sequencing batch reactor (SBR) and cyclic activated sludge technology (CAST). The performance of AASR was evaluated in this study and found to be effective for the removal of pollutants. The average effluent NH4+-N, TN, TP, and COD concentrations were 0.5, 17, 0.8, and 40 mgL−1, respectively. The corresponding average removal efficiencies were 97%, 59%, 83%, and 83%, respectively, indicating that the AASR was also a successful operating system for the removal of organic matter. The AASR has many advantages, such as successive filling, high removal efficiency, high stability and reliability, low area requirement, no sludge circulation reflux, and low construction costs. The operation mode of the alternating anoxic, anaerobic, and aerobic conditions displayed a higher efficiency for nitrification than that of conventional SBR. The effective mode for denitrification was a step-feed. The control program of the AASR is highly flexible and can easily be modified by a plant manager to meet various loading requirements.

Concentrations and fate of parabens and their metabolites in two typical wastewater treatment plants in northeastern China

Parabens are widely used in food, pharmaceuticals, and personal care products because they are excellent preservatives. Recently, the environmental fate of parabens has attracted attention owing to their similarity to some endocrine disrupters. Wastewater treatment plants (WWTPs) are both important sinks of parabens discharged from our daily activities and key pollution sources for the environment if the parabens are not completely removed. However, research in this area is scarce, especially in Asia. In this study, 6 commonly used parabens and 4 metabolites were analyzed in wastewater and sludge samples from two typical WWTPs with different treatment processes (the anaerobic-oxic (A/O) and cyclic activated sludge technology (CAST) treatment processes). The average concentrations of parabens in the A/O and CAST treatment processes were 1510 ng/L and 2180 ng/L, respectively, in the influent, and 70.5 ng/L and 19.7 ng/L, respectively, in the effluent. The paraben removal efficiencies in the A/O treatment process were between 56.8% and 100%, which is lower than the efficiencies for the CAST treatment process (97.7% to 100%). The average concentrations of metabolites in the A/O treatment process, which were much higher than paraben concentrations, were 35,200 ng/L in the influent, 334 ng/L in the effluent, and 146 ng/g in the sludge samples. The removal efficiencies for the 4 metabolites were >92% for the A/O treatment process. In total, for the A/O treatment process, 5.07 kg and 16.8 kg of parabens, and 24.4 kg and 16.0 kg of metabolites, were discharged into the environment annually via effluent and sludge, respectively. Overall, the results of this study indicate that the A/O and CAST treatment processes are both effective at removing parabens and their metabolites.

Study On Sewage Quality From Sewage Treatment Plant At Vashi, Navi Mumbai

The aim of this study is to evaluate the quality of sewage generated from 100 MLD Sewage Treatment Plant (STP) located at Vashi, Navi Mumbai which is based on latest Cyclic Activated Sludge Technology. Study of sewage quality of this plant is an essential as the most of the treated effluent discharged into Vashi Creek and remaining used for Gardening purpose. Water samples were collected from raw inlet and treated outlet and analyzed for the major waste-water quality parameters, such as pH, Biological Oxygen Demand (BOD), Dissolved Oxygen (DO), Chemical Oxygen Demand (COD), Total Suspended Solids (TSS) etc. The overall quality of sewage of 100 MLD Sewage treatment plant will be evaluated by collecting samples. The results of these evaluations also determine whether the effluent discharged into the water body are under limits given by MPCB.

Phosphorus Removal Capacity of Domestic Wastewater Treated by a Modified CAST Process Under Different Operating Modes

A modified cyclic activated sludge technology (CAST) treating domestic wastewater was employed to investigate the effects of different operating modes, such as C/P ratio, reflux ratio, and temperature on phosphorus removal. The results illustrated that at room temperature the phosphorus removal of the system improved significantly when the influent C/P ratio increased from 50 to 100, with the removal efficiency increasing rapidly from 15% to 95.6%. Decreasing the C/P ratio to 75, the phosphorus removal performance declined due to influent carbon source deficiency, and the mean phosphorous removal efficiency decreased to 51.4%. At the same time, the long-term addition of an easily degradable carbon source resulted in sludge bulking and a large amount of sludge loss. With a low C/P ratio, the phosphorus removal performance increased by 2.3 times when the sludge reflux ratio decreased from 25% to 12.5%. However, the phosphorus removal performance declined when reducing it to 0. The temperature experiment results showed that the phosphorous removal efficiency of the system remained stable, above 90%, in the low temperature system (14℃±1℃). However, the phosphorous removal efficiency of a shortcut nitrification system at high temperature (27℃±1℃) was only 14.1%, suggesting that a low temperature was beneficial for removing phosphorous. It was observed from the batch tests that the sludge at room temperature could utilize dissolved oxygen, nitrate, and nitrite as electron acceptors to take up phosphate. The sludge in the low temperature system could use both dissolved oxygen and nitrate as the electron acceptors. However, the sludge in the high temperature system could only use dissolved oxygen as the electron acceptors to take up phosphate. In addition, it was also found that sludge under starvation conditions resulting from short-term idling of the system favored phosphorus removal.

Nitrite Type Denitrifying Phosphorus Removal Capacity of Cycle Activated Sludge Technology Processes Under Different Inducing Patterns

A modified cyclic activated sludge technology (CAST) reactor was utilized to investigate the phosphorus and nitrogen removal performance under different inducing patterns in this experiment. The results show that nitrite addition under anoxic conditions has a more inhibitory effect on the denitrifying phosphorus removal performance of the sludge. The phosphorus removal performance of the system was least effective when nitrite dosage was 5 mg·L-1. Compared to an anoxic addition system, the CAST system is more stable under aerobic addition conditions. The phosphorus removal properties have a slight fluctuation during each initial operating condition when the nitrite concentrations are 5, 10 and 15 mg·L-1, respectively. However, the phosphorus removal rate was observed to recover quickly and remain stable at more than 95% after acclimatizing for 10, 6, and 34 days, respectively. The effluent phosphorus concentration was less than 0.5 mg·L-1 in all cases. It was also found that the phosphorus removal performance deteriorated drastically when the nitrite dosage was 20 mg·L-1. Nevertheless, the nitrite type denitrifying phosphorus uptake capacity of the sludge was 10.4 times greater than that of the sludge before acclimatizing, suggesting that the phosphorus performance deterioration due to nitrite addition could be relieved and long-term addition is beneficial to enriching denitrifying phosphorus accumulating bacteria using NO2- as an electron acceptor. Moreover, the sludge settling performance was found to be effective and the sludge concentration decreased continuously when adding a certain concentration of nitrite under aerobic conditions, which is of significant for sludge reduction.

A dynamic modelling of nutrient metabolism in a cyclic activated sludge technology (CAST) for treating low carbon source wastewater.

A new mathematical model incorporating biopolymer kinetics and the process of the simultaneous storage and growth are established for the treatment of low carbon source wastewater with a high effluent quality and energy efficiency. A set of initial parameter values was assigned as a combination of estimated values, literature-based values, and fitted values to simulate a cyclic activated sludge technology (CAST) system effectively. Compared with experimental data from the CAST system, the calibrated model demonstrated a good performance. Model simulations indicated that the recommended condition for a CAST fed with low carbon source wastewater was a volume ratio of the anoxic zone to the aerobic zone of 7/28. Moreover, using high-throughput 16S rRNA gene sequencing not only characterised the microbial communities in the CAST reactors operated under two feeding ratios but also indirectly validated the model predictions.

Application of Emergy Analysis to the Sustainability Evaluation of Municipal Wastewater Treatment Plants

Municipal wastewater treatment plants consume much energy and manpower, are expensive to run, and generate sludge and treated wastewater whilst removing pollutants through specific treatment regimes. The sustainable development of the wastewater treatment industry is therefore challenging, and a comprehensive evaluation method is needed for assessing the sustainability of different wastewater treatment processes, for identifying the improvement potential of treatment plants, and for directing policymakers, management measures and development strategies. This study established improved evaluation indicators based on Emergy Analysis that place total wastewater, resources, energy, economic input and emission of pollutants on the same scale compared to the traditional indicators. The sustainability of four wastewater treatment plants and their associated Anaerobic-Anoxic-Oxic (A2O), Constant Waterlevel Sequencing Batch Reactor (CWSBR), Cyclic Activated Sludge Technology (CAST) and Biological Aerated Filter (BAF) treatment processes were assessed in a city in northeast China. Results show that the CWSBR process was the most sustainable wastewater treatment process according to its largest calculated value of Improved Emergy Sustainable Index (2.53 × 100), followed by BAF (1.60 × 100), A2O (9.78 × 10−1) and CAST (5.77 × 10−1). Emergy Analysis provided improved indicators that are suitable for comparing different wastewater treatment processes.

Occurrence and fate of pharmaceuticals in WWTPs in India and comparison with a similar study in the United States

The objective of this study was to study the occurrence, fate, and seasonal variations of pharmaceuticals at two urban wastewater treatment plants (WWTPs) in India and compare the results with a similar study conducted in the United States. This is the first study of its kind in comparing occurrence and fate of pharmaceuticals in wastewater of two different countries with the same methodology and analytical techniques. Twelve most relevant pharmaceuticals were selected for seasonal monitoring at two Indian WWTPs based on the comprehensive survey and through literature review. The yearly average influent concentrations of total pharmaceuticals were found to be 537 ± 5 μg/L at WWTP-1 and 353 ± 9 μg/L at WWTP-2. WWTP-2 exhibited comparatively higher removal efficiency of total pharmaceuticals (85% versus 59%, excluding monsoon season), possibly due to the cyclic activated sludge technology followed by chlorination employed at WWTP-2. Comparison with a similar study conducted in the United States revealed that concentration of most of the pharmaceuticals detected in the U.S. WWTPs were, on an average, more than 50% lower. U.S. WWTPs also exhibited 10–30% higher removal efficiencies for total pharmaceuticals. Our study results show that preliminary treatment and biological treatment alone are not adequate for complete removal of pharmaceuticals and polishing step and tertiary treatment is a necessity if higher removal of pharmaceuticals is desired in Indian WWTPs. Information obtained from this study will not only aid the local utilities but will also benefit understanding of global trends in usage of pharmaceuticals and their distribution in the environment.

Nitrogen and phosphate adsorption on biofilms in reclaimed water

Biofilms, abundantly attached on the surfaces of multiple media (such as pebbles and aquatic plants) in rivers or lakes reusing reclaimed water (RW), are of great significance to contaminants’ self-purification of water bodies. In this paper, the nitrogen and phosphate adsorption characteristics of such biofilms were studied by the batch method, using biofilms cultivated under an outdoor simulative process. Two types of RW treated by anaerobic-anoxic-oxic process (A2/O) and cyclic activated sludge technology (CASS), and surface water (NW) were chosen as the experimental water bodies. And the glasses of surface roughness of 0.1, 1.0, and 10 μm, respectively, were used as the substrates, where the biofilms adhere to them. The results indicated that the isothermal adsorption of biofilms for ammonia nitrogen and phosphates was proved to conform to the Langmuir and the Freundlich equation, whereas the adsorption capacity was significantly lower than that of soils and sediments limited by the pore structure and surface area. There were also significant differences between the adsorption behaviors. The distribution coefficients were ranked as K dCASS > K dNW > K dA 2 /O, and the maximum adsorption capacities (S m), under these conditions were ranked as S mA 2 /O > S mNW > S mCASS, while other parameters did not change regularly. Extracellular polymers ( $$R^{ 2}_{\text{N}}$$ = 0.95**, $$R^{ 2}_{\text{p}}$$ = 0.84**), iron oxides ( $$R^{ 2}_{\text{N}}$$ = 0.86**, $$R^{ 2}_{\text{p}}$$ = 0.68*), and aluminum oxides ( $$R^{ 2}_{\text{N}}$$ = 0.95**, $$R^{ 2}_{\text{p}}$$ = 0.81**) in the biofilms had a significant impact on the S m for ammonia and phosphates. The roughness of media surfaces also had a considerable influence on the S m, which was in the order of S m10μm > S m0.1μm > S m1.0μm, whereas other parameters did not change regularly. The research aimed to provide theory reference for in situ decontamination of RW in rivers or lakes.

Enhanced nitrogen removal under low-temperature and high-load conditions by optimization of the operating modes and control parameters in the CAST system for municipal wastewater

Sequencing batch reactor (SBR) technology has become one of the commonly used treatment systems for municipal and industrial wastewater in the past decades. Cyclic activated sludge technology is one of the newly developed variations of SBR process. In this study, because the water quality situation of effluent under low-temperature and high-load conditions was unstable in Wang-hill Wastewater treatment plant, and the phenomena of exceeding discharge standard about ammonia-nitrogen (NH3-N) and total nitrogen (TN) were serious in winter, the different operation modes (operation cycle and aeration time) and control parameters were selected and optimized to improve nitrogen removal performance, respectively. The results indicated that the national first level B criteria of the effluent quality could be reached and the stable discharge could be achieved under the operation conditions of the C mode with 6 h per cycle [influent 2 h, aeration 2.5 h (aeration starting from 1.5 h after influent), settling 1 h, draw/idle 1 h], the average removal rates of chemical oxygen demand (CODcr), NH3-N, and TN in the reactor were 91.8, 75.6, and 62.9%, respectively. Furthermore, the dissolved oxygen (DO) level and mixed liquor suspended solids (MLSS) content in the aeration phase were both optimized on the basis of the preferred C mode. The best ranges of DO and MLSS concentration during the aeration phase as high as 2.0–3.0 mg/L and 5,000–6,000 mg/L are essential for the best nitrogen removal performance. The average removal rates of CODcr, NH3-N, and TN in the reactor were 90.3, 83.2, and 69.1%, respectively. Obviously, alternating operation mode combined with parameter optimization was the optimal nitrogen removal strategy for low-temperature and high-load wastewater. These data are of great practical significance for the scientific control and management of the wastewater treatment plant and for references of water engineering professionals.

Occurrence and fate of phthalate esters in full-scale domestic wastewater treatment plants and their impact on receiving waters along the Songhua River in China

The occurrence and fate of six phthalates: dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-butyl phthalate (DBP), butyl benzyl phthalate (BBP), bis (2-ethylhexyl) phthalate (DEHP) and di-n-octyl phthalate (DOP) were investigated as phthalates passed through three full-scale wastewater treatment plants (WWTPs) with different treatment processes, and ultimately into the recipient Songhua River water in Harbin (China). The six phthalates were detected in the majority of aqueous and solid samples, with DEHP being the most abundant compound. The overall removal efficiency of ΣPAEs in the Cyclic Activated Sludge Technology (CAST) process was over 72%, while both the A/O and A/A/O processes achieved approximately 30% removal. The better performance of the CAST process relative to the Anoxic/Oxic (A/O) and Anaerobic/Anoxic/Oxic (A/A/O) processes was attributed to the indoor-conditions of the CAST plants, which effectively maintained the temperature of the treatment system. The fate of PAEs within two different types of WWTPs (CAST and A/A/O) were assessed qualitatively using mass balances. The results suggested that PAEs removal resulted from both biotransformation and adsorption, of which the former was particularly significant in the CAST process, while the latter was more significant in the A/A/O process. Substantial levels of several PAEs were detected in the Songhua River, especially downstream of the WWTPs, which means that the discharge from WWTPs has a strong impact on the water quality of the Songhua River during cold winter conditions.

Applying real-time control for achieving nitrogen removal via nitrite in a lab-scale CAST system

In this study, a bench-scale Cyclic Activated Sludge Technology (CAST) reactor (72?L), fed with domestic sewage, was operated in alternating anoxic–aerobic operation mode to investigate the feasibility of achieving short-cut nitrification and denitrification with a real-time control strategy. An online system for controlling the length of the aeration and stirring phases was implemented, based on pH and oxidation–reduction potential signals, to switch between aerobic and anoxic sequences. Results show that the real-time control strategy was successful in achieving a stable nitrogen removal performance. Furthermore, short-cut nitrification can be achieved by controlling aeration length under the modified real-time control strategy. Gradually reducing the energy supply for nitrite-oxidizing bacteria caused the limitation of their growth and, finally, their elimination. When short-cut nitrification was obtained, the nitrite pathway became the primary way for nitrogen removal, and aeration duration was reduced by 28.4%.

Achieving and maintaining of short-cut nitrification in a cyclic activated sludge system

A lab-scale Cyclic Activated Sludge Technology (CAST) system was operated more than 5 months to evaluate the effects of the operation mode on nitrogen removal performance and investigate a feasible method for achieving short-cut nitrification in the system. Results showed that nitrogen was removed by conventional biological nitrification and denitrification in traditional operation mode (fill/aeration 2 h, settle 1 h, decant 1 h), whereas short-cut nitrification and denitrification was the main nitrogen removal pathway in modified operation mode and the nitrogen removal performance was enhanced. Short-cut nitrification was successfully achieved in CAST system at 17 ± 1 °C by adjusting operation conditions and the average total nitrogen removal efficiency increased by 11.4% compared to traditional mode. It was assumed that low dissolved oxygen (<1.0 mg/L) limitation combined with free ammonia (0.28-0.34 mg/L) inhibition on nitrite-oxidizing bacteria caused nitrite accumulation in modified mode. During maintaining period of short-cut nitrification, preset aeration time enhanced ammonium-oxidizing bacteria dominance. It was also found that low DO could result in overgrowth of filamentous microorganisms and poor sludge settleability. The pH variation could provide effective information for controlling aeration duration in modified mode. However, no evident breakpoint appeared on pH and DO profiles in traditional mode.

Effect of influent nutrient ratios and temperature on simultaneous phosphorus and nitrogen removal in a step-feed CAST

A step-feed cyclic activated sludge technology (CAST) with a working volume of 72 L treating real municipal wastewater was operated to examine the effect of varying ratios of influent COD/TN and COD/P on the nutrient removal. With the increased COD/P and COD/TN, the phosphorus and nitrogen removals exhibited an upward trend. The TN removals had a positive linear correlation with the phosphorus removal efficiencies, mainly because the presence of nitrate in the anaerobic zone negatively affected the phosphorus release thus the nitrogen removal process took priority over the phosphorus removal process to utilize the limited carbon source in the influent in step-feed CAST where simultaneous removals of nitrogen and phosphorus were achieved. By employing the effective step-feed strategy with alternating anoxic/oxic operation, efficient phosphorus and nitrogen removals of 95.8 and 89.3% were obtained with lower influent COD/P and COD/TN ratios of 61.9 and 5.2, respectively. It was also found that lower temperature, e.g. 13 ~ 16°C, did not deteriorate the phosphorus removal, though the nitrogen removal decreased significantly due to incomplete nitrification. As the temperature increased further, TN removal efficiency increased gradually and nitrogen removal via nitrite pathway was successfully achieved with average nitrite accumulation rate above 90% in the system.

Biological nitrogen removal in a step-feed CAST with real-time control treating municipal wastewater

The performance of a 18 L step-feed cyclic activated sludge technology (CAST) combined with real-time control treating real municipal wastewater was evaluated. The operation strategies employed pH and oxidation reduction potential (ORP) as on-line control parameters, which can control the durations of oxic and anoxic phases flexibly. The obtained results showed that the studied process had achieved advanced and enhanced nitrogen removal by several phases of consecutive oxic/anoxic periods. Total nitrogen in effluent was lower than 2 mg/L and the average TN removal efficiency was higher than 98%, while only requiring small amount of external carbon source. Unexpected characteristic points in pH and ORP profiles denoting the depletion of nitrate were also observed during the last anoxic phase. Denitrification rate was found to be more dependent on the system temperature compared to nitrification rate. Moreover, a stable and efficient phosphorus removal rate above 90% was achieved by using step-feed strategy which enabled the influent carbon source to be fully used and the favourable condition for phosphorus releasing to be created during the anoxic phases.

Impact of operating conditions on nitrogen removal using cyclic activated sludge technology (CAST)

In this study, the nitrogen removal performance of a municipal wastewater treatment plant that employs cyclic activated sludge technology (CAST) was evaluated. The impact of key operational conditions such as temperature, dissolved oxygen (DO) concentrations and operating modes were examined in parallel. During summer, when the operating temperature ranged from 27–30°C, the NH4 +-N and total nitrogen (TN) removal efficiencies were 51 ± 7% and 42 ± 7%, respectively, and simultaneous nitrification and denitrification (SND) was considered to be the major nitrogen removal mechanism. In contrast, at a low operating temperature of 10–15°C, the DO concentration increased from 0.7 to 3.0 mg/L; however, the NH4 +-N and TN removals were both inefficient due to poor biomass activities, which was demonstrated by a lower specific oxygen uptake rate (SOUR) of 1.2 mg O2/g SS· h (15°C). Moreover, a 3h-mode for the CAST process was preferable during winter because the effluent wastewater quality was similar to that obtained when the 4h-mode was used. The extremely low organic loading was the primary reason for the poor bioactivity of the sludge in the CAST system, and this eventually led to deterioration of the nitrogen removal efficiencies.