Feasibility of on-site grey-water reuse for toilet flushing in China

Although the total reuse rate of municipal wastewater was 8.8% in China in 2012, water crisis is forcing China to increasingly develop water reuse. Urban reuse is comparatively poor and has significant potential to be promoted in China. It is a sensitive matter whether to include kitchenwastewater in greywater reuse in water-deficient areas when kitchen wastewater accounts for a large proportion of total domestic water consumption. Concentrations of chemical oxygen demand, BOD5 (biochemical oxygen demand), and total organic carbon in kitchen wastewater are comparatively lower in China than in other countries, but a high concentration of nitrogen fromwashing tableware and ricemakes it difficult tomeet nitrogen requirements in Chinese guidelines. Whether kitchen wastewater should be included in greywater reuse in China needs further study. Aerobic biological processes combined with physical filtration and/or disinfection is preferred in grey-water treatment, and how to balance the investment and treatment costswith reuse criteria still needs to be researched further. The promotion of reclaimedwater for toilet flushing faces resistance in China. The necessity and effectiveness of existing restrictions in water reuse guidelines for toilet flushing in China are in doubt and need further discussion. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wrd.2016.086 ://iwaponline.com/jwrd/article-pdf/8/1/1/240167/jwrd0080001.pdf Jie Zhu (corresponding author) Martin Wagner Peter Cornel Institute IWAR, Technische Universität Darmstadt, Franziska-Braun-Str. 7, Darmstadt 64287, Germany E-mail: zhujie198461@163.com Jie Zhu Hongbin Chen Xiaohu Dai College of Environment Science and Engineering, Tongji University, No. 588, Miyun Rd, Shanghai 200092, China


INTRODUCTION
Drought and water shortage are the key factors for water scarcity in arid and semi-arid areas in the world. Nowadays, overexploited underground water reservoirs and seriously polluted surface water have intensified the water crisis; moreover, population growth coupled with ever-increasing urbanization has resulted in continuous growth of urban water demand. As a result, there is a growing need to manage water resources in a sustainable manner, and reclaimed water has become an increasingly important source of water.
Recently, centralized wastewater treatment and reuse systems have been confronted with many problems, such as high susceptibility towards overloading during storm events, unnecessary or insufficient treatment for different reuse applications and long-distance conveying systems (Hu et al. ).
Meanwhile, flexible on-site water treatment and reuse systems are considered a promising solution for Jordan (Al-Jayyousi ), Israel (Friedler ), and the USA (Seattle Public Utilities ; Pacific Institute & NRDC ). Since 2000, China has made good progress in water reuse with governmental policy support, but is still confronting difficulties in water reuse (Yi et al. ). Whether on-site water reuse systems are appropriate for China needs further discussion.
Due to low-level contamination and continuous availability, grey-water reuse has become a special focus for onsite reuse. Grey-water is defined as wastewater without any input from toilets, and it includes wastewater from bathtubs, showers, washing basins, laundry and kitchen sinks (Eriksson et al. ). Sometimes, kitchen wastewater is not classified as grey-water because of its high content of oil and food residuals Wilderer ). However, the characterization of grey-water has not been studied intensively in China. Based on the limited literature data available, quantity and quality of domestic wastewater in China are assessed here. In addition, this paper discusses whether kitchen wastewater should be classified as grey-water.
As for water reuse, current technology manages to treat wastewater into any desired quality (US Environmental Protection Agency ). Compared to household composite domestic wastewater, grey-water is more lightly polluted, thus the treatment process should be optimized; the key problem is how to balance the investment and treatment costs with reuse criteria. Hence, this paper summarizes the current greywater treatment systems and proposes advice on application.
Recently, the most commonly described and promising application for grey-water reuse has been toilet/urinal flushing, which can reduce water demand within a dwelling by up to 10-30% (Friedler et al. ; Gisi et al. ). However, water reuse for toilet flushing has been the subject of many complaints because of the undesired odor and color (Narasimhan et al. ), corrosion of pipelines (Li et al. a, b; Wang et al. ), and potential health risks (Jjemba et al. ), most of which result from microbial regrowth.
As a result, the reliability of water reuse guideline has been questioned. This paper compares guidelines for toilet flushing in China and other countries, and tries to calculate the key parameters leading to microbial regrowth in reclaimed water.
The purpose of this paper is: (1) to analyze the current state and potential of wastewater reuse in China and to propose possible solutions for improving water reuse rates; (2) to study the current situation regarding quantity and quality of grey-water in China and to analyze the feasibility of greywater reuse in China; (3) to summarize grey-water treatment technologies and propose advice on proper application; and (4) to compare the guidelines of water reuse in China and other countries, and propose suggestions for maintaining biological stability of reclaimed water in toilet flushing.

FEASIBILITY AND NECESSITY OF ON-SITE WATER REUSE IN CHINA
Assessment of the national water reuse potential Water recycling projects for non-potable end use are a common practice with more than 3,300 projects registered worldwide (European Commission ). Figure 1 shows freshwater resource quantities and water reuse rates in different countries.
Arid and freshwater-starved regions in the world are candidates for water reclamation. Israel, Malta, and Singapore are the top three countries in water reuse and reuse 80%, 40%, and 33% of total wastewater, respectively (World Bank ). In Israel, nearly all households have a double plumbing supply system (drinking water and reclaimed water). In Singapore, wastewater is being treated to such a quality level that it has been sold, inter alia, for use In China, the majority of available water is concentrated in the south, leaving the north and west to experience perpetual droughts and the need to develop water reuse. Beijing reused 0.75 × 10 9 m 3 wastewater in 2012, and the reuse rate reached 59.3%. However, the reused rate varies throughout China and in 65% of the provinces or municipalities the reuse rate is lower than 8.8%, while in water-scarce Tianjin it is only 2.9%. Population growth coupled with Also, a recently established government policy advocates more water reuse. According to Action Plan for Water Pollution Prevention (GOSC ), the utilization rate of recycled water should be more than 20% in water-deficient cities and 30% in Beijing, Tianjin, and Hebei by 2020. The Beijing government planned to increase the water reuse rate to 75% in 2015 (BWA a). According to assessment standards for water-saving cities, the water reuse rate should exceed 20% or the annual growth rate should be more than 5% (MOHURD b). As a result, more water reuse schemes have to be developed to meet the ever-increasing water recycling requirements throughout China.

Necessity of on-site treatment and urban reuse in China
With an annual precipitation of 585 mm in Beijing (BWA b) and 563 mm in California (NOAA ), drought undoubtedly motivates water reuse in these semi-arid areas In Japan, due to not facing immediate needs in agriculture, nearly 50% of the reclaimed water is directed towards urban miscellaneous reuse, using 27% in landscape irrigation, 18% in snow melting, and 3.5% in toilet flushing.  Without long-distance conveyance, on-site water treatment and reuse has been considered especially appropriate for high-density residential projects, such as multi-building complexes and commercial buildings in urban areas (Friedler & Galil ; Friedler ; Seattle Public Utilities ). With an ever-increasing population and urbanization, on-site water treatment and reuse is a good choice for improving the urban water reuse rate in China.

CHARACTERIZATION OF GREY-WATER IN CHINA Quantity of grey-water in China
Compared with rainfall and mixed domestic wastewater, grey-water was preferred for on-site treatment due to being a stable resource and low polluter (Friedler ). The quantity of grey-water depends on lifestyle, population structure (age and gender), water installation and the degree of water abundance (Morel & Diener ). Figure 4 shows the domestic water consumption in China and other countries.
As can be seen in Figure 4, the percentages of wastewater consumption for toilet flushing, baths/showers, and laundry are between 27% and 30%, 24% and 33%, and 9% and 17%, respectively, in different countries. Generally, bathing wastewater alone is not sufficient for toilet flushing.
However, in Germany, many people shower every day and bathing wastewater is being successfully reused in multibuilding complexes. Normally, bathing and laundry wastewater are collected and reused together to ensure sufficient grey-water quantity. Due to different dietary habits in Asia and Europe, kitchen water consumption accounts for more than 20% in Asia and no more than 8% in Europe. This is because in Asia, in most households, warm food is served three times a day, while in Europe, most people only have one hot meal a day and often eat out. Regarding the produced wastewater quantity, the relevance of including kitchen wastewater in water reuse is limited in Europe, while in Asian countries, especially for the arid regions, kitchen wastewater could be a stable grey-water resource. If kitchen wastewater were included in grey-water in China, 63.7% of domestic water could be reused.

Quality of grey-water in China
In China, there is an urgent need for more information regarding the characteristics of different types of grey-water, especially kitchen wastewater, in order to evaluate the reuse potential. The characteristics of different types of grey-water in China and in other countries is shown in Table 1.
As can be seen from Table 1 Table 2.

GREY-WATER TREATMENT TECHNOLOGIES
Although biogas production is an advantage, poor removal efficiencies of both organic substances and surfactants make anaerobic processes unsuitable for grey water recycling, and the mainstream biological treatment remains aerobic biological processes (Li ). hours makes it an attractive process for the treatment of grey-water. SBR technology was also considered to be promising when it was used in a sports center for grey-water reuse Total bacteria/CFU/L 2.0 × 10 7 -2.5 × 10 9 1.1 × 10 9 -9.5 × 10 9 1.4 × 10 8 -7.9 × 10 9 Total coliform/CFU/L 4.3 × 10 4 -3.5 × 10 7 5-2.4 × 10 7 56-8.9 × 10 5 1.7 × 10 7 -2.4 × 10 8 4.9 × 10 7 -9.4 × 10 8 10 7.2 × 10 8.8   As a result, the choice of a treatment system should be based on a careful evaluation of the local conditions (considering the beneficiaries of the system), the legislation as well as the socio-economic environment. It is suggested that an integrated design of the building (considering water and energy nexus) could make grey-water reuse system more economically sustainable (Gisi et al. ), and more fullscale grey-water reuse projects should be put into practice and studied over long time periods.

COMPARISON OF WATER REUSE GUIDELINE FOR TOILET FLUSHING
As current technology manages to treat wastewater into any  (Table 3). Many recent research projects have focused on identifying the key parameters leading to microbial regrowth.
Assimilable organic carbon (AOC), which is used to assess the biological stability of drinking water, is starting to be applied in reclaimed water projects as well (Ryu et

CONCLUSIONS
The following conclusions were drawn: (1) Although the total reuse rate of municipal wastewater was 8.8% in China in 2012, water crisis is forcing China to increasingly develop water reuse. Urban reuse is comparatively poor and has significant potential to be promoted in China. On-site reuse system seems a good way to promote urban water reuse throughout China.
(2) The quantity of kitchen wastewater varied greatly due to dietary habits. It is sensible to include kitchen wastewater in the case where it accounts for a large proportion of domestic water consumption in water-deficient areas. Owing to a large amount of low polluted water from washing uncooked food, the concentrations of COD, BOD 5 , and TOC in kitchen wastewater are comparatively lower in China than in other countries. High concentrations of nitrogen from washing tableware and rice makes it difficult to meet the   Chinese guideline. Whether kitchen wastewater should be included in grey-water reuse in China needs further discussion.
(3) The combination of aerobic biological processes with physical filtration and/or disinfection is considered to be the most economical and feasible solution for greywater recycling. Medium to high strength grey-water is suggested to be treated by RBC, SBR, CW, and MBR technology, but how to balance the investment and treatment costs with reuse criteria still needs further study.
(4) The Chinese guidelines for toilet flushing were compared with those of Japan, Great Britain, and the USA. Chemical restrictions in Chinese guidelines for toilet flushing are manifold and arduous; total coliforms alone are not suitable for indicating fecal contamination, and other bacteria should be taken into account. Also, the key restriction factors leading to microbial regrowth (e.g., AOC or MAP) should be identified and restricted in guidelines in future.