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Discussion on Water Quality Standards for Reusing Reclaimed Water in Landscape Water Bodies (Wang Heli, Chen Lei, Cheng Li, Li Xiangfeng) Abstract: This paper introduces the current practices of reusing reclaimed water in landscape water bodies in China, conducts a comparative analysis of the existing national standards regarding water quality in such water bodies, and explores technical measures such as water body eutrophication control and self-purification processes as well as their relationship with secondary treatment. Finally, reasonable suggestions are put forward. Keywords: reclaimed water reuse, landscape water bodies, water quality standards, nitrogen and phosphorus removal. Chinese Library Classification Number: X703. Document Code: B. Article ID: 1000-4602(2001)12-0031-05. With the acceleration of urbanization and the implementation of sustainable development strategies in China, comprehensive urban management and the protection of urban water environments have gradually become priorities. Many cities in our country have carried out renovation work on those old rivers that flow through urban areas and have become dark and smelly due to severe pollution. However, after investing huge amounts of money in measures such as sewage interception, river dredging, hardening of the riverbeds, and stone embankments, these smelly rivers end up without any water in them. Due to the scarcity of available surface water resources in the areas surrounding some cities, in order to maintain the water surfaces that are part of the urban landscape, multiple rubber dams have been installed in rivers to hold back or direct water flow, and sometimes wells are dug to extract water or tap water is used to keep the rivers in good condition after treatment. However, over time the quality of the water held back by these dams deteriorates, becoming dark and foul-smelling, necessitating its discharge so that the dams can be refilled with water again – a practice that is unreasonable from social, economic, and environmental perspectives. Therefore, in recent years, after building large-scale urban sewage treatment plants, some cities have begun to consider further treating the wastewater from these plants and using it as water for landscaping in these dried-up rivers. Cities in China such as Shijiazhuang, Cixi, Zhengzhou, and Beijing have already started or plan to carry out such projects. These projects have played a positive role in promoting the large-scale reuse of reclaimed water in landscape water bodies in our country. However, many problems have arisen due to the fact that the relevant water quality standards and technical measures are still in the exploratory stage, and there is an urgent need to establish more scientific and reasonable regulations for water quality standards and design guidelines in such situations. 1 Quality standards and design specifications for landscape water bodies In China, a number of quality standards related to landscape water bodies have been issued, and these standards are listed in Table 1 for comparison. In 1988, to control water pollution and protect water resources, the **Environmental Protection Bureau approved the issuance of GB 3838—88, the \"Standards for the Environmental Quality of Surface Water.\" Surface water bodies such as rivers, lakes, and reservoirs that are used for various purposes were classified into five categories based on their intended uses and protection requirements. Categories III, IV, and V take into account the quality standards required for water bodies used for recreational and aesthetic purposes. In 1991, in order to further protect and improve the water quality of waters used for landscape and recreational purposes, to restore and maintain the natural ecological systems in these waters, and to promote the development of tourism, the **Environmental Protection Bureau approved the issuance of GB 12941—91, the Water Quality Standards for Landscape and Recreational Uses**. These standards classify waters into three categories: A, B, and C, corresponding roughly to water quality categories III, IV, and V under GB 3838—88, but with stricter requirements. Both of the above standards were simultaneously replaced and repealed in 2000 by GHZB 1—1999 \"Environmental Quality Standards for Surface Water\". These water quality standards impose very strict controls on parameters such as nitrogen and phosphorus; they also set strict requirements for indicators like COD, BOD, and dissolved oxygen. As standards, they serve as a basis for assessing and evaluating landscape water bodies in terms of compliance with regulations. The GB 8978—1996 \"Comprehensive Wastewater Discharge Standards\" approved and issued by the **Environmental Protection Bureau in 1996 set higher requirements for the wastewater discharged from secondary urban wastewater treatment plants; it stipulates that discharges into Class III water bodies as defined in GB 3838—88 must comply with Class I standards ； Discharge into Class IV and V water bodies is subject to Level 2 standards. To prevent eutrophication in water bodies and preserve their value as sources of drinking water or for landscaping, the new standards impose stricter controls on parameters such as nitrogen, phosphorus, COD, BOD, and suspended solids. At present, most newly built urban sewage treatment plants in our country comply with first-class standards or emission standards close to that level. With the ongoing practice of recycled water reuse in China, in 1994 the China Association for Engineering Standardization issued CECS 61:94, the \"Design Code for Reuse of Urban Sewage\" (which is currently being considered for upgrading to a formal standard). In 2000, the Ministry of Construction issued the industry standard CJ/T 95—2000, the \"Water Quality Standards for Reusing Recycled Water in Landscape Water Bodies.\" Both standards set requirements regarding the water quality of sewage that has been treated and reused in landscape water bodies. However, taking into account the feasibility at the current stage, the recommended values are set relatively low, roughly on par with the first-level standards specified in GB 8978—1996, the \"Integrated Wastewater Discharge Standards.\" As for parameters such as phosphorus, the required levels are even lower than those specified in the first-level standards of GB 8978—1996, and are far below those set in GHZB1—1999, the \"Environmental Quality Standards for Surface Water.\" As the technology and practice of wastewater reuse in our country improve, these standards will surely be further enhanced and refined. 2 Discussion on water quality standards As can be seen from the table, China’s existing industry standards for urban construction, namely CJ/T 95—2000 \"Water Quality Standards for Reclaimed Water Used in Landscape Water Bodies\" and CECS 61:94 \"Design Code for Reuse of Urban Sewage\", are similar to the first-level standards set out in GB 8978—1996 \"Comprehensive Wastewater Discharge Standards\". The requirements regarding indicators such as phosphorus are relatively low. Moreover, no specific distinctions or detailed provisions are made for the use of reclaimed water in landscape water bodies, which makes these standards somewhat rudimentary; therefore, they need further improvement. 2.1 Debates in engineering practice: The effluent treated by secondary urban sewage treatment plants still introduces certain pollutants into the receiving water bodies. If these levels do not exceed the environmental capacity of those water bodies, their quality can still be maintained at its original level thanks to the natural self-purification processes of the water, preventing rapid deterioration. It is based on this principle that GB 8978—1996 stipulates that discharges into Class III water bodies must comply with Grade 1 standards ； Discharge into Class IV and V water bodies is subject to Grade II standards. Another potential premise for such a regulation should be that the receiving water body is a flowing body of water with a large capacity relative to the amount of water discharged into it. Otherwise, it will inevitably lead to serious consequences of cumulative water pollution degradation. As for CJ/T 95—2000 and CECS61∶94, which have water quality standards close to those of Grade 1 as specified in GB 8978—1996, the prerequisite for reusing such water in landscape water bodies is that these water bodies be flowing, thereby allowing for a certain degree of dilution of the pollutant concentrations in the water discharged into them. Therefore, for some cities that plan to reuse reclaimed water as landscape water in slowly flowing artificial water bodies without any dilution conditions, it is extremely unsafe to simply apply the water quality standards specified in CJ/T 95—2000 and CECS61∶94. When a city in East China reused the treated effluent from its municipal wastewater treatment plant (at a rate of 2×104 m3/d) for use in river landscape applications, it adopted the stricter standards for Class III water bodies as defined in GHZB1—1999. Furthermore, the environmental protection and construction departments in the same city often have very different opinions on this issue. For example, in a water-scarce city in central China, it is planned to subject the secondary wastewater from the municipal wastewater treatment plant to tertiary treatment (10×104 m3/d) before returning it to an upstream dried-up and abandoned reservoir (used as a storage tank). After being stored there for several days, the water is released intermittently into two treated urban rivers – several rubber dams are installed in these rivers to retain the water as a surface area for urban recreation. There are significant differences in opinions among the relevant departments regarding the water quality standards for the effluent from this proposed tertiary treatment facility: the construction management department believes that since the **Ministry of Construction has already established the Water Quality Standards for Reclaimed Water to be Used in Landscape Water Bodies**, these standards should be followed in accordance with the urban construction industry standard CJ/T 95—2000 ； The environmental protection authorities insist on applying the standards for Class IV water bodies (or even specific parameters for reservoirs and lakes), on the grounds that a certain section downstream of one of the urban rivers has been designated as the monitoring section for that city (classified as a Class IV water body); moreover, **standard GHZB1—1999 should have greater legal validity. The differences between the two in terms of water quality parameters are so significant that they will have a notable impact on construction costs as well as the choice of treatment processes. The author believes that in such cases, the requirements set by the former indicator are too stringent; if no appropriate measures are taken thereafter, there will be a lack of safety guarantees (this artificial water body may gradually deteriorate and even become foul-smelling), thereby losing its value as a water body for landscaping and resulting in a complete waste of substantial investments ； The latter does not take into account the self-purification capacity of water bodies; its indicator requirements are too high, and it lacks feasibility under constraints such as funding and operating costs. Considering both safety and feasibility, appropriate relaxations can be made on the basis of the latter, but the lack of applicable standards raises concerns regarding liability. Therefore, such engineering practices require more comprehensive, specific, thorough, and feasible water quality standards and design specifications to guide them. 2.2 Relationship between water quality standards, eutrophication, and water self-purification: When it comes to reusing reclaimed water in landscape water bodies, professionals first think of the control of parameters such as nitrogen and phosphorus. It is generally believed that nitrogen levels in water exceeding 0.2–0.3 mg/L, phosphorus levels exceeding 0.01 mg/L, and BOD5 levels exceeding 10 mg/L can lead to eutrophication [1]. From this standard, many of the landscape water bodies we use currently suffer from eutrophication to varying degrees, including even some water bodies classified as categories IV and V. However, as long as the water body does not become black, smelly, and deteriorated, it still retains significant value as landscape water. In fact, the goal of reusing treated water in landscape water bodies is not to prevent eutrophication, but rather to prevent the water from becoming black, smelly, and deteriorated. In fact, the so-called eutrophication is a concept in the taxonomy of lake evolution, indicating the aging of lakes (which occurs easily in closed or slowly flowing water bodies); severe eutrophication can ultimately lead to the water becoming black and smelly. In flowing water bodies, due to the continuous reoxygenation caused by the moving water flow, the environmental capacity of the water body is maintained to a certain extent under the influence of the aquatic ecosystem; as a result, mild eutrophication does not quickly lead to the formation of a foul-smelling, dark-colored condition in such water bodies. Based on this, when recycled water is reused in flowing landscape water bodies, it is not necessary to apply the water quality standards designed to control eutrophication, and the nitrogen and phosphorus limits can be relaxed accordingly. Especially in water ecosystems with a well-developed structure, aquatic plants can absorb large amounts of nitrogen, phosphorus, and organic nutrients, while releasing oxygen into the water, thereby helping to maintain the stability of water quality to a certain extent. Therefore, when mechanisms for reoxygenation such as aquatic ecosystems or hydraulic flow are well-developed, it is advisable to make full use of the water body’s self-purification capacity. 2.3 The relationship between water quality standards, technical measures, and feasibility: In principle, the secondary effluent from urban sewage treatment plants can be further treated and reused to meet higher standards through various existing technical methods such as activated carbon adsorption, bioactivated carbon, ozone-activated carbon, membrane separation technologies, and ion exchange. However, the high construction costs and operating expenses make it difficult for large-scale reuse projects in China to be implemented, which in turn hinders their widespread adoption. “During the Seventh and Eighth Five-Year Plans periods [2], researchers carried out extensive and fruitful studies using artificial water bodies for simulation [3]. The recommended standard was a total phosphorus level of ≤0.5 mg/L. However, for various reasons, the \"Quality Standards for Reclaimed Water Used in Landscape Water Bodies\" standard CJ/T 95—2000 ultimately set the standards at 1.0 mg/L (for non-systemic human exposure) and 2.0 mg/L (for non-direct human exposure), as listed in Table 1: ① Due to funding constraints on the research team, the final evaluation of the results was conducted via correspondence; there was no direct communication between the evaluators and the team members. As a result, many experts, considering the feasibility at that time, recommended relatively lenient standards for use in landscapes, based on the higher water quality levels that could be achieved through secondary treatment ； ②When drafting the aforementioned standards, many of the experts involved took into account China’s national conditions at that time and believed that the current situation in China was still far from enabling the large-scale reuse of treated wastewater for landscaping purposes; as a result, strict requirements were not set for certain water quality indicators. For this reason, during the 2001 annual meeting of the \"National Research Society on Sewage Reuse,\" organized by the Northeast China Municipal Engineering Design Institute, opinions from the experts present were sought. As a result, stricter regulations were established for the water quality criteria specified in the original \"Code for Design of Urban Sewage Reuse\" CECS61:94, particularly those related to the water quality standards for water to be reused in landscape water bodies. This code will be elevated from an industry standard to an official **standard that will be put into effect. For the reuse of secondary effluent from urban sewage treatment plants, the traditional three-stage process of coagulation, sedimentation, and filtration is widely used in China. This approach is effective at removing suspended solids, but it is not very effective at removing COD, BOD, ammonia nitrogen, and color; as a result, it is only suitable for reusing secondary effluent in industrial and domestic applications where lower water quality standards are sufficient. Considering the water quality discharged by urban sewage treatment plants in our country at present, to reuse such water for landscape purposes, it is necessary to effectively remove nitrogen, phosphorus, BOD, COD, suspended solids, etc., as well as to disinfect it. From a technical perspective of advanced treatment, it generally requires the combined use of biochemical methods and physicochemical methods for synergistic treatment. For the removal of ammonia nitrogen, there are generally two approaches: physical-chemical methods ( breakpoint chlorination) and biological methods ( nitrification). However, the chlorine dosage required for the former is more than ten times that of the ammonia nitrogen content, making it very uneconomical to operate it when ammonia nitrogen levels are high ； In the latter case, it is difficult to cultivate activated sludge due to the low organic matter concentration in the secondary effluent; therefore, biofilm treatment methods are generally used. These methods can remove around 90% of the ammonia nitrogen. For example, the use of aerated biological filters not only enables effective removal of ammonia nitrogen as well as part of the BOD and COD, but also eliminates most of the suspended solids, thus eliminating the need for a sedimentation tank. If the secondary effluent meets the first-class discharge standards specified in GB 8978—1996, then tertiary biological treatment can reduce ammonia nitrogen to below 1 mg/L or close to 1 mg/L during mild-temperature seasons. Given the low temperatures in winter in the north, the removal of ammonia nitrogen is greatly affected by temperature; therefore, breakpoint chlorination can be preset as a safeguard. At the same time, since landscape water bodies are less prone to deterioration in winter, the ammonia nitrogen limit can be appropriately increased to 1–3 mg/L. The author recommends using ammonia nitrogen rather than total nitrogen as the water quality indicator for reusing secondary effluent in landscapes, based on the following considerations: ① In water eutrophication, ammonia nitrogen plays a major and decisive role [4] ； ②Denitrification via denitrification in the secondary effluent treatment requires the addition of external carbon sources due to insufficient carbon supply, which increases operating costs ； ③The reflux required for denitrification will lead to a significant increase in electricity consumption ； ④The structures for the anoxic stage of denitrification significantly increase construction costs. For phosphorus removal, there are generally two approaches: biochemical methods and physicochemical methods. The former relies on phosphorus-accumulating bacteria to remove phosphorus by excess uptake, achieving phosphorus removal through sludge discharge; however, since biological phosphorus removal is not considered in the secondary treatment stage, it is extremely difficult to employ biological methods for phosphorus removal in the tertiary treatment stage. The latter method involves the use of metal salt coagulants, with the impurities being removed through processes such as precipitation and filtration. If the concentration of suspended solids at the inlet is not high (such as in the effluent from aerated biological filters), micro-flocculation followed by direct filtration can be considered. The advantage of this approach is that it eliminates the need for a reaction and sedimentation tank, thereby reducing construction costs; however, the downside is an increased load on the filter, a shorter backwash cycle, greater water consumption, and higher operating costs ； When the volume of water to be treated is relatively small, a pressure filtration approach can also be considered. Its advantages include a higher filtration rate, reduced space and land requirements for the filter tanks, lower construction costs, as well as enhanced dirt-trapping capacity. Backwashing is required only when the head loss reaches 10 m, whereas in the case of gravity filters backwashing is needed at a head loss of 2 m; as a result, the backwashing interval is longer and water consumption is reduced. The downside is that operating costs increase due to higher electricity consumption for the pumps. Generally, physicochemical methods can achieve a removal rate of 80% for total phosphorus. If the effluent from the secondary treatment stage of urban sewage treatment plants meets the first or second grade standards specified in GB 8978—1996, it is feasible to control the total phosphorus level at 0.2–0.5 mg/L through tertiary treatment. Even when the total phosphorus level in the secondary effluent exceeds 5 mg/L, it is possible to reduce the total phosphorus level in the tertiary effluent to 0.5 mg/L or lower through coagulation and filtration methods; however, this comes at the cost of a significant increase in the amount of chemicals required. Additionally, other relevant parameters such as BOD, COD, and suspended solids can be removed to a certain extent during the denitrification and phosphorus removal processes, thereby meeting the water quality requirements for landscape use. As for parameters like dissolved oxygen, it is necessary to improve the aquatic ecosystem and enhance the oxygenation mechanisms in the water in order to ensure satisfactory levels of these parameters. The two-stage process of biochemical and physicochemical treatment will become the main technical approach for reusing treated wastewater in landscape water bodies in China at the current stage; however, without proper optimization, it will also lead to unnecessary waste in terms of construction costs and operating expenses. In a city in East China, the secondary wastewater is subjected to advanced treatment (2×104 m3/d) before being reused for landscape watering in rivers. The water quality standards applied are the relatively strict GHZB1—1999 Class III water standards. The main design process involves: secondary wastewater → first stage of submerged aerated biological filter (anoxic operation) → second stage of submerged aerated biological filter (aerobic operation) → coagulation → sedimentation (clarification) → pressure fiber ball filtration → disinfection → wastewater after advanced treatment. If the aforementioned process flow is analyzed from the perspective of minimizing construction costs and operating expenses while ensuring water quality, several issues that require further optimization can be identified: ① The first stage of the submerged aerated biological filter, which operates under anaerobic conditions, can be omitted; as explained earlier, this approach helps to save investment and operating costs for this section ； ②A sedimentation (clarification) tank may not be necessary, as the suspended solids in the effluent from a upward-flow aerobic biofilter (BAF) are less than 10 mg/L. Such a low turbidity level is sufficient to meet the requirements for pressure direct filtration; therefore, micro-flocculation-filtration can be used in place of coagulation-sedimentation-filtration, eliminating the need for a sedimentation tank and thereby reducing capital investment significantly. Another advantage of this approach is that since the flocs only need to be suitable for filtration rather than requiring separation on a sedimentation scale, it is possible to save some chemicals ； ③In pressure fiber ball filtration, it is difficult to thoroughly backwash the binding points between the fiber ball fillers; as operation time increases, it becomes hard to ensure the quality of the effluent. Therefore, fiber bundle fillers can be used as a substitute to achieve better filtration results. By optimizing the aforementioned aspects, the following process flow, which is more cost-effective in terms of investment and operating expenses, can be recommended: secondary effluent → upward-flow aerobic biofilter (BAF) → microflocculation → pressure fiber bundle filtration → advanced treatment effluent. As can be seen from the above analysis and discussion, this process flow is a simplified and optimized version of a two-stage process combining biochemical and physicochemical methods for tertiary (advanced) treatment. When operated with appropriate parameters, this system can achieve removal rates of over 90% for both ammonia nitrogen and total phosphorus in the secondary effluent. In other words, at economically reasonable operating costs, the water quality of the effluent can generally be maintained at levels where ammonia nitrogen is ≤1–3 mg/L and total phosphorus is ≤0.5 mg/L. Therefore, taking into account both safety and technical feasibility, regarding the higher quality standards required for reusing reclaimed water in landscape water bodies, it is recommended that the total phosphorus level should be kept below 0.5 mg/L (otherwise it will exceed the water quality criteria specified in GB 8978—1996 for Class 1 standards, which is contradictory and unreasonable) ； Ammonia nitrogen should be controlled at 1–3 mg/L ； CODCr should be controlled between 20 and 30 mg/L ； BOD5 can be controlled below 6 mg/L. 2.4 The relationship between water quality standards and secondary treatment processes: Some wastewater treatment plants in China were not planned or prepared for reuse at the time of their construction; as a result, when reuse was considered, tertiary treatment could only be applied after the secondary treatment stage. This led to redundant construction of certain treatment units, resulting in significant waste of investment and operational costs. Meanwhile, some issues that could have been resolved properly during secondary treatment were left for tertiary treatment, thereby increasing the complexity of that stage greatly. For example, regarding the removal of nitrogen and phosphorus, it is well known that biological denitrification and phosphorus removal are the most economical and efficient methods. However, if a process capable of effective denitrification and phosphorus removal is not selected in the secondary biological treatment stage, biological phosphorus removal becomes difficult to implement in the tertiary treatment stage due to the challenges associated with culturing activated sludge. For nitrogen removal, only biofilm processes can be considered, but these are also uneconomical due to the high load requirements. If a process capable of nitrogen and phosphorus removal is selected during the construction of urban sewage treatment plants, enabling the secondary effluent to meet the first-class standards of GB 8978—1996 or even better water quality criteria, it becomes easier to choose a process for tertiary treatment, and both construction costs and operating expenses will be significantly reduced. Therefore, it is recommended that when constructing new secondary wastewater treatment plants, processes capable of effective nitrogen and phosphorus removal should be given priority. For existing urban wastewater treatment plants that plan to reuse reclaimed water in landscape water bodies, it is also advisable to first consider enhancing nitrogen or phosphorus removal capabilities through partial process modifications in these secondary treatment plants, as this approach is more economical and efficient. For existing municipal sewage treatment plants, it is possible to consider adjusting their operation modes and enhancing the original processes so that they can perform anaerobic-aerobic phosphorus removal or anoxic-aerobic nitrogen removal. Suspended or fixed biochemical fillers can also be added to the aeration tanks to facilitate the growth and reproduction of nitrifying bacteria, thereby strengthening nitrification and ammonia nitrogen removal. At the same time, this increases the microbial density and the volume load, which in turn enhances the overall effectiveness of biological treatment. 3 Conclusions Based on the discussions above regarding the water quality indicators and technical measures for landscape water use, the author proposes the following suggestions: ① The existing national standards for urban construction in China, namely CJ/T 95—2000 \"Water Quality Standards for Reused Water in Landscape Water Bodies\" and CECS 61∶94 \"Design Code for Reuse of Urban Sewage\", set relatively low requirements for water quality. Moreover, they do not provide differentiated treatment or specific guidelines for the reuse of reclaimed water in landscape water bodies, and thus require further improvement. ②The water quality target for reusing reclaimed water in landscape water bodies is not to prevent eutrophication, but rather to avoid the occurrence of dark, foul-smelling water conditions. Mild eutrophication does not lead to such conditions in flowing water bodies quickly, so criteria such as nitrogen and phosphorus levels can be set more loosely. ③In aquatic ecosystems where water flow and oxygenation mechanisms are well-developed, aquatic plants can absorb large amounts of nitrogen, phosphorus, and organic matter, thereby re-oxygenating the water and maintaining environmental capacity through self-purification. Therefore, artificial water bodies should aim to establish a complete ecosystem (such as planting aquatic plants like reeds and enhancing the oxygenation mechanism) in order to make full use of the water body’s self-purification capabilities. ④Enhancing the nitrogen and phosphorus removal capabilities of secondary biological treatment will significantly reduce the load on tertiary treatment, as well as lowering construction costs and operating expenses. Therefore, it is recommended that new secondary wastewater treatment plants prioritize processes that can effectively remove nitrogen and phosphorus. For existing urban wastewater plants that plan to reuse reclaimed water in landscape water bodies, it would be more economical and effective to consider modifying these plants through certain processes to improve their nitrogen or phosphorus removal capabilities. ⑤Taking into account both safety and technical feasibility, for the higher quality standards required for reusing reclaimed water in landscape water bodies, it is recommended that total phosphorus be kept below 0.5 mg/L, ammonia nitrogen between 1 and 3 mg/L, CODCr between 20 and 30 mg/L, and BOD5 below 6 mg/L. ⑥Different specific scenarios of reusing reclaimed water in landscape water bodies should be treated separately; existing water quality standards (which are still being improved) can be used as a reference, but they should not be applied mechanically. It is recommended to determine the appropriate water quality standards through more thorough analysis and simulation tests, in order to avoid wasting large amounts of investment. References: [1] Ren Nanqi, Ma Fang, et al. Microbiology of Water Pollution Control [M]. Harbin: Heilongjiang Science and Technology Press, 1993. [2] Environmental Protection Bureau. Techniques for Water Pollution Control and the Reuse of Urban Sewage [M]. Beijing: Science Press, 1992. [3] Chen Li. Discussion on the Reuse of Urban Sewage in Artificial Water Bodies [J]. China Water & Wastewater, 1999, 15(9): 16–18. [4] Takashi Asano. Wastewater Reclamation and Reuse [J]. Technomic Publishing Company, 1998, 465–471. Author’s affiliation: 1. School of Environment and Resources, Zhejiang University, Hangzhou 310027, Zhejiang, China ； 2. Institute of Municipal Environment, Jilin University of Architecture and Engineering, Changchun 130021, Jilin ； 3. School of Environment and Resources, Jilin University.