Nitrogen and Phosphorous Removal in Municipal Wastewater Treatment Plants in China: A Review

Surface water environment in China was degraded rapidly in the last two decades, resulting in increasingly tighten criteria issued for municipal wastewater treatment plants (WWTPs). This paper reviewed the recent advances of process design and operational optimization for nutrients removal. Three major processes, as anaerobic-anoxic-oxic (AAO) process, oxidation ditch (OD), and sequencing batch reactor (SBR) occupied 65% of WWTPs amounts and 54% of treatment volumes of China in 2006. However conservative process designs and operational faults often impaired the process performances and energy efficiency. Therefore, typical processes were modified, combined, and innovated to meet the requirements of the diverse influent characteristics and lower energy consumptions. Furthermore, operational optimization techniques by modeling, simulation, and real-time control were also developed and applied in China to improve the process operation. Although great efforts had been contributed to improve the WWTPs performances in China, attentions should be continuously paid to the introduction, instruction, and implementation of advanced techniques. At last, the technical demands and appropriated techniques of WWTPs in China were briefly discussed.


Introduction
In the last two decades, surface water environment in China was rapidly degraded by the booming industrialization and fast urbanization.The first large scale municipal wastewater treatment plant (WWTP) in China was constructed and operated 25 years ago [1].In recent ten years, the construction of municipal WWTPs was catalyzed by the serious deterioration of surface water around cities, as shown in Figure 1.The reduction of ammonia nitrogen (NH 3 -N) and total phosphorous (TP) are also shown in the figure [2] in the unit of part per million (ppm) and also mg/L in water.The ammonia reduction was only 13.2 to 18.3 mgN/L and the phosphorous reduction was almost constant at 2.5 mgP/L from 2001 to 2007 [1], implying unsatisfied process performances in current WWTPs.
The rapid development of municipal WWTPs and their unsatisfied performances promoted the engineering innovations, as well as the academic studies.In this paper, the recent progresses of nitrogen and phosphorous removal in municipal WWTPs were reviewed in the aspects of process design and operational optimization, the advances in academic studies, and the practices in engineering fields in China.
Figure 2 shows the WWTPs amounts and the treatment wastewater volumes for BNR processes in China in 2006.The statistic data covered 74% and 77% of the total ranges of  WWTPs amounts and treated volumes, respectively.Among the investigated processes, OD, SBR, and AAO were the three major processes, which together occupied 65% of WWTPs amounts and 54% of treated volumes.The different occupations among the investigated processes were related with their distinct features, for example, easiness of operation and maintenance, instrumental requirements, and energy consumptions, and also of great importance, related with the process reformation and upgrade.It could be expected that WWTPs numbers and treated wastewater volumes of AAO process would be continuously increased by the strong potential of the reformation of CAS process.   in China.Typical problems in process design and operation were usually derived from diverse influent characteristics, inevitable design defects, and undeveloped operation levels.

Inadequate Studies on Influent Characteristics.
The design and operation of WWTP processes in China were based on experiences of engineers rather than scientific conclusion.There are very few investigations on the components in the influent of municipal WWTPs, as well as the spatial and seasonal variations of influent concentrations in China, resulting in inadequate knowledge on the diverse influent characteristics.The studies on influent characterization were affected by the greatly varied natural and social conditions all over the country, for example, developing industries and constructing sewer systems may intensively change the influent components in short period.Furthermore, the difficulties of continuous monitoring, the lack of data accumulation, and the negative wills of managers in WWTPs hindered the optimization of BNR processes.

Defects in Process
Designs.Process designers in China often followed the similar influent characteristics to start the design without careful investigations on the actual concentrations of pollutants.Except of the uncertainty of influent characteristics, process designers might face more complicated situations like the absence of sewer system data, inaccurate prediction of population, uncontrolled industrial wastewater discharge, and so forth, limiting their possibility to design the configuration with satisfied flexibility.Therefore big variation coefficients, large volumes of tanks, and very long hydraulic retention times were utilized in the designs to ensure the security of process operation.This situation produced conservative and rigid processes and resulted in operational difficulties, for example, very few designs allowed engineers to change the return flows of mixed liquor along with the influent flows.

2.3.3.
Undeveloped ICA Levels.The WWTPs were almost tripled in the last seven years as shown in Figure 1, requesting not only the huge amount of infrastructural investments but also a number of skilled and experienced operational engineers which could not be cultured in short time.Instrumentation, control, and automation (ICA) techniques could alleviate this human resource crisis, which had been developed and successfully applied in municipal WWTPs in Europe [4].In this field, the researchers in China made rapid progresses in the theoretical studies, control algorithms, optimization strategies, and supervisory systems.However, the application of ICA techniques in actual processes was still very limited due to the expensive online instruments and the current huge amount of employees.

Process Design and Augmentation
3.1.1.Process Design for Municipal WWTPs.After 25 years of practices on large-scale municipal WWTPs, the process design has been gradually improved in China.Table 4 shows the processes and performances of several example  [5].
The scheme diagrams of above processes were shown in Figure 3.The more candidate processes gave designers more possibilities to configure good processes with lower cost and higher efficiency.For example, in order to deal with the distinct conflicts between nitrogen removal and phosphorous removal in AAO process, designers may adapt reverse AAO (r-AAO) or A-AAO process to enhance the nitrogen removal with the price of unsatisfied phosphorous reduction [6], and other designers may prefer simple AO process coupled with chemical precipitation.Furthermore, decision-making tools for complicated multicriteria, such as capital and operation cost, land area, nutrient removal, and sludge disposal, were developed to obtain scientific and reasonable process among municipal wastewater treatment alternatives [7].
Local enterprises in China had integrated or modified the imported processes in their engineering practices as shown in Figure 4, such as CSBR (continuous flow/constant level SBR) modified by THUNIP Company, DF-MBR (dual filtration membrane bioreactor) integrated by NEPRI (national electricity environmental protection institute of China), modified Carrousel OD developed by Anhui Guozhen Corp., and HSCBR (high efficient separation composite biological fluidized reactor) owned by Jiangsu Yihuan Corp [8].

BNR Augmentation Attempts.
Bioaugmentation was often applied to enhance the nutrients removal in conventional processes in China.The most popular solution was to introduce specially designed porous media into the aeration tanks to augment biofilm reactions [9].The total biomass concentrations could be augmented from 3.2 g/L to 4.3-5.8g/L with the carriers in 15%-30% of volumetric portion [10].The media characteristics of surface areas and hydrophobic properties were important for the performance of biofilm and its startup.
In South China, domestic wastewater is always low in strength due to the high water level of groundwater and setting of septic tank at the beginning of wastewater collection system [6].The bioaugment process, which installed polyurethane foams as carriers and inoculated specialized bacteria on the media, was found more easily started up in lower DO concentrations [11].Zeolite was also used to enhance conventional contact stabilization activated sludge process and improved nitrogen removal for 27%, by the mechanism of preferential ion exchange of zeolite to ammonium [12].

Process Innovation for Municipal
WWTPs.Some innovated processes have been demonstrated to achieve good nutrients removal in China, such as (1) natural purification system, for example, various infiltrations and wetlands, (2) anaerobic biological treatment processes, for example, upflow anaerobic sludge blanket reactor, anaerobic filters, and anaerobic baffled reactor, (3) biofilm reactors, for example, three-phase fluidized reactor, and (4) wastewater reclamation techniques with the core of membrane separation [8,13].

Innovation for Critic Environmental Conditions.
Municipal wastewater treatment encountered some critic conditions such as high salinity in coasts, and frozen temperature in chilly zones.An integrated process (SANI, see Figure 4) consisting of sulfate reduction, autotrophic denitrification and nitrification was developed in Hongkong and applied in coastal areas of China to reclaim saline water without any excessive sludge discharge [14].SBR was found also capable of removing nitrogen in saline wastewater by short-cut nitrification [15].In chilly seasons, an internal circulation compound bioreactor worked well by isolation and immobilization of cold-adapted microorganisms onto soft polyurethane foams, with COD removal of 86% in the conditions of 4-10 • C [16].

Innovation for Low Concentration Wastewater Treatment. The TN removal was often limited by the low concentrations of influent organics in South
China.There were several techniques developed to deal with the lack of carbon sources, for example, a primary treatment process enhanced by composite flocculant which combined bioflocculants Pullulan and poly-aluminum-chloride (PAC) could achieve 91% and 15% of TP and NH 3 -N removal, respectively [17].Biological contact oxidation process and biological aerated filter (BAF) process were also capable to enhance the nutrients removal by careful selection of the media [18].The limited carbon source could be effectively utilized by optimized feeding methods, for example, four-stage stepfeeding AO process [19].Another option was to utilize short-chain fatty acids generated during waste activated sludge fermentation in alkaline conditions as internal carbon sources [20].

Innovation for Lower Cost and Energy Consumption.
Some techniques were innovated or optimized to save the land use and investments.Anaerobic-anoxic-nitrification (A2N, see Figure 4) process with two stages of return sludge flow utilizes denitrifying phosphorous removing bacteria (DPB) to removal nitrate and phosphorous simultaneously [21], which is more promising than UCT/VIP processes (see Figure 3) [22].By simple modification of the flow lines, the DPB could be isolated and accumulated in the system to achieve better performance of denitrification [23].Simultaneous nitrification and denitrification (SND) process via nitrite could obviously reduce the aeration and thus save the energy consumption, which could be achieved in SBR [15] and hybrid SBR systems [24].The airlift oxidation ditch (AOD, see Figure 4), using underwater aeration to achieve simultaneous aeration and agitation, was operated in lower cost and higher efficiency than other oxidation ditches.The land use and energy consumption could be saved for 25% and 55%, respectively, according to the pilot-scale experiments [25].After integrating a transverse flow membrane module and a MBR in an innovative configuration, the system gained the advantages of lower energy consumption and more resistance to membrane fouling [26].

Advances in Process Optimization
Nutrients removal was often harmed by inadequate denitrification, inhibition of phosphorous removal by nitrate, over aeration, over dosage of chemicals, and improper operation of settling pretreatment tanks.In the last decade, attempts from both academic and engineering sides in China were contributed to improve the process operation in municipal WWTPs.

Modeling, Simulation, and Optimization of Process.
Modeling and simulation based on activated sludge models (ASMs) were useful to screen optimization strategies for process operation by an effective and economic method.The manipulative variables in actual AAO process, such as dissolve oxygen (DO), volumetric portions for denitrification, return flow rates, and excessive sludge flow rate, could be refined by the systematic procedure of modeling, simulation, and optimization [4].Benchmarking simulation models (BSMs) were intensively studied for real-time control in last decades in China, helping to accumulate knowledge and testify the novel control methods like model predictive control strategies and sophisticated feedback control systems [27].
New models based on ASMs were developed in China for scientific studies and engineering feasibility.An integral model named fully coupled activated sludge model (FCASM) described the interactions of eight functional microorganisms involved in BNR processes [28], while a simplified model based on carbon oxidation steps in ASM1 was successfully applied in Jizhuangzi WWTP at Tianjing City [29].
Sedimentation tanks play important role for guarding the effluent with good quality.By using computational fluid dynamics (CFDs), the distribution of suspended solids in secondary sedimentation tanks could be simulated by the three-dimensional two-fluid model, resulting in useful information for process design [30].Particle image velocimeter was useful to study the settling of activated sludge flocs under turbulent flow conditions, indicating that the settling velocity was correlated to the particle size, turbulence intensity, and solid concentrations [31].Whatever, conventional models, such as Vesilind model and double exponential model, were still feasible for simulation of compression settling process by careful model calibration [32].
CFD technique was systematically applied in the field simulation, optimization of aerators and shapes of oxidation ditches.Particle dynamic analyzer (PDA) and CFD simulation were found useful to optimize the designs of Carrousel OD [33].Nitrogen removal could be improved in Carrousel OD by controlling the return sludge flow, dissolved oxygen (DO), and influent sites according to the influent loadings and temperature [34].The promotion of DO distribution to nitrogen removal was also observed in a fluidized bed reactor, which was affected by the vertical stratification of active sludge populations [35].

Real-Time
Controlling for BNR Process.Operational optimization of process requires high levels of ICA techniques in municipal WWTPs.Unfortunately, the studies of real-time controlling system and relative strategies were very few in China, and mainly focused on SBR systems.In SBR process, even single variant control by DO level could achieve better biological nitrogen removal and sludge population optimization [36].The control strategies utilized pH and oxidation reduction potential (ORP) as on-line control parameters could remove TN as high as 98% with very low influent carbon/nitrogen ratio (C : N = 3.5) [37].By a real-time aeration control strategy based on DO, pH, and ORP, the short-cut nitrogen removal could be achieved by nitrification and denitrification via nitrite [38].Furthermore, the performance and benefits of the real-time controlling system were validated in a pilot-scale SBR [39].Fuzzy control strategies were applied in scientific studies for the real-time control of activated sludge process.For example, AO process could be optimized by using on-line sensors DO, pH, and ORP and fuzzy inference system to control aeration, nitrate recirculation flow, and external carbon dosage [40].

Implementation of ICA Techniques.
Although current ICA techniques are in state-of-art, application of ICA in municipal WWTPs in China was obstructed by the expensive instruments and controllers, rare and ineffective maintenance, easily broken actuators, lack of knowledge, and absence of consultancies.Although automatic control was utilized in most of SBR systems, it was limited in unit operations instead of process optimal operation.The first application of feed-forward control by ICA techniques, in its original meaning, might be the reformation project of Bailonggang WWTP in Shanghai City in 2008, in which a bioprocess intelligent optimization system (BIOS, BioChem Company, USA) [41] was installed to control the aeration based on the online signals of ammonia and nitrate.

Future Perspectives
The appropriated processes or technologies for municipal WWTPs in China have the features of high efficiency and stability, lower energy consumption and operational cost, easy operation and maintenance, and lower specific footprint to reduce the occupied land area and the investment [13].Additionally, suitable technologies for wastewater handling in small communities were necessary for the purpose of scattering treatment instead of centralizing sewer systems [42].
Although a lot of attempts have been contributed for nutrients removal, more attentions should be paid to the introduction, instruction, and implementation of advanced techniques in municipal WWTPs by considering the urgent crisis of surface water contamination and water resource shortage.The Major Projects on Control and Rectification of Water Body Pollution (2005-2020), which are supported by the central and local governments and enterprises, has covered the most topics of municipal wastewater treatment and will greatly promote the development and application of new techniques.

Conclusions
This paper reviewed the advances of nitrogen and phosphorous removal in municipal WWTPs in China.The construction of municipal WWTPs was gradually increased in last decade.AAO, SBR, and OD processes were major candidates for process design, but their performances were unsatisfied according to the more strict discharge criteria.Typical problems in process design and operation were summarized as inadequate knowledge of influent, design defect, and undeveloped ICA techniques.Great efforts had been made by researchers and industries to deal with the problems, including process design innovated for critic conditions, low concentration of influents, and saving energy consumption, and process optimization achieved by modeling and simulation, real-time control, and implementation of ICA techniques.Despite of all those efforts, more attentions should be paid to the implementation of advanced techniques in municipal WWTPs.Finally, the appropriate techniques and future perspectives were briefly discussed.

AAO:
Anaerobic-anoxic-oxic process A-AAO: AAO process with preanoxic zone AO: Anoxic-oxic or anaerobic-oxic process CAS: Conventional activated sludge process ASMs: Activated sludge models A+OD: Carrousel-OD with preanaerobic zones BAF: Biological aeration filter BNR: Biological nutrient removal process BSMs: Benchmarking simulation models Carrousel-OD: Carrousel type oxidation ditch CAST: Cyclic activated sludge technology, a type of SBR CFD: Computational fluid dynamics COD: Chemical oxygen demand DAT-IAT: Demand aeration tank and intermittent aeration tank, a type of SBR DE-OD: Dual channels oxidation ditch DF-MBR: Dual filtration membrane bioreactor DO: Dissolved oxygen DPB: Dionitrification phosphorous bacteria HSCBR: High efficient separation composite biological fluidized reactor ICA: Instrumentation, control and automation ICEAS: Intermittent cycle extended aeration system, a type of SBR NH 3 -N: Ammonia nitrogen MBR: Membrane biological reactor OD: Oxidation ditch process Orbal-OD: Orbal type oxidation ditch ORP: Oxidation and reduction potential PDA: Particle dynamic analyzer r-AAO: Anoxic-anaerobic-oxic process SBR: Sequencing batch reactor SND: Simultaneous nitrification and denitrification T-OD: Triple channels oxidation ditch TP: Total phosphorous TN: Total nitrogen UASB: Upflow anaerobic sludge blanket reactor UCT/VIP: University of Campton Tank/VIP UNITANK: A type of SBR WWTP: Wastewater treatment plant.

Figure 4 :
Figure 4: Innovated or modified processes in WWTPs in China.

Table 1 :
Capacities of WWTP processes in geographic zones of China in 2006 a .
b Conventional activated sludge processes including primary treatment processes.
2.1.2.Spatial Distributions of WWTPs Processes.Table1shows the spatial distributions of the major processes in the seven geographic zones of China.The amounts of wastewater were usually determined by the population densities and economic levels, therefore larger volumes of wastewater were treated in the North, South, and East China than other zones.The process design of WWTPs was often affected by comprehensive factors in China, such as distinct features of different processes, natural conditions like temperature and water resources, social situations like population and economics, and the orientations of process designers.For example, OD process has never been applied in the Northeast China because of the very low ambient temperature there.
However, it was popularly adapted in the East and Middle China because of the less production of excessive sludge and the designers' favorites.2.1.3.Performances of WWTPs Processes.Table2shows the different performances of the major processes in

Table 2 :
Performance of major WWTP processes in China in 2006 (unit, mg/L).

Table 3 :
National discharge criteria of municipal WWTPs in China.
a Criteria for chilly weather lower than 12 Celsus degree; b COD Mn in surface water instead of COD Cr in wastewater; c Standards only for lake water and reservoirs.

Table 4 :
Typical processes applied in municipal WWTPs and their performances in 2006.
[3]everse AAO; b AAO with preanoxic zone; c VIP process; d Cyclic activated sludge technology; e UNITANK process; f Demand aeration tank-intermittent tank; g Intermittent cycle extended aeration; h Triple channel oxidation ditch; i Dual channel oxidation ditch; j Anaerobic zone before Carrousel oxidation ditch; k Biological aeration filter; l BioLak is a process developed by a German Company; m The time of operation started; n Removal rates calculated approximately by values in 2006.WWTPs in 2006[3].AAO process group in China usually included conventional AAO, reverse AAO, AO, AAO with preanoxic zone (A-AAO), and UCT/VIP processes.SBR process group consisted of CAST, DAT-IAT, UNITANK, and ICEAS processes.OD process groups adapted in China were Orbal, Carrousel, T-OD, DE-OD, and other modifications like Carrousel with preanaerobic zones (A+OD)