Characteristics of rural agritainment sewage in Sichuan, China

The rapid development of the rural economy in China brings tremendous improvements in farmer living standards. However, environmental pollution caused by rural domestic sewage is becoming more serious. In China, the annual volume of discharged wastewater was 73 trillion litres in 2015 (NBS 2016), of which urban domestic wastewater represented 41%, and rural sewage about 27% of the total (NBS 2016). The sewage quality in rural areas is characterized by high nitrogen and phosphorus contents, strong biodegradability and relatively low content of toxic metals and other noxious substances (Zhao 2010). Rural areas now emit approximately 50% of the entire Chinese discharge of major water pollutants, including 43% of the chemical oxygen demand (COD), 57% of the total nitrogen (TN), and 67% of the total phosphorus (TP) (MOEP 2010). At present, only 11.4% of sewage in the villages of China is being managed and treated; most of this rural sewage is naturally evaporated on the surface in comparison to an over 91.9% treatment rate in urban areas (MHURD 2017). Fortunately, treating sewage in rural areas of China has attracted increasing attention of scholars in recent years (Guo et al. 2014; Zhao et al. 2017).

At present, decentralized sewage in rural areas is made up mainly of domestic wastewater from residents, but sewage in rural areas from agritainment sites has not been investigated. Agritainment is a new form of tourism and recreation that originated from Chengdu in the Sichuan Province of China (Jiang et al. 2015). Since the 1980s, agritainment has become an important carrier for the economic prosperity of Sichuan Province, and has subsequently become popular all over China. Agritainment combines catering, entertainment and accommodation, and is similar to rural tourism. The reception capacity of each agritainment site, which includes rural eateries such as hot pot, barbecue, and other Chinese food sites, varies from a few to several hundred people per day. The greater the capacity of an agritainment site, the greater volume of sewage being discharged. Guo et al. (2014) found that agritainment sewage accounts for about 3% of domestic sewage discharge volume in rural areas. Geng (2013) found that total suspended solids (TSS), animal and vegetable oil (AVO) and COD are the main pollutants in agritainment sewage, with the concentration of pollutants being high and the concentration range large. Jiang et al. (2015) found that sewage from agritainment is typically discharged directly. A comparison of the different characteristics can be made between agritainment sewage and regular domestic sewage. In comparison with ordinary domestic sewage, agritainment sewage is of complex composition, of intermittent discharge and with high organic content, with especially high concentrations of pollutants such as TSS, COD and AVO (Zulaikha et al. 2014; Jiang et al. 2015). The pollutants from agritainment sewage cause partial sewer blockage and seriously affect the ability of the drainage pipeline to pass water in winter (Zulaikha et al. 2014; Jiang et al. 2015). The harmfulness of agritainment sewage to the environment, especially oily sewage, was described in detail by Sun & Jia (2012).

Agritainment sewage consists of black water and grey water. Black water is wastewater from toilets, which contains high amounts of COD, rich organic matter, and nitrogen, phosphorus and potassium. Grey water is defined as wastewater without any input from toilets, which means that it includes wastewater from cooking, dishwashing, baths, etc. Since agritainment sewage is mainly discharged directly into the environment and is more difficult to degrade naturally, the consequences are more serious than those with ordinary domestic sewage. This is because the composition of agritainment sewage is more complex and the concentration of pollutants in sewage from agritainment is several times higher than domestic sewage. Generally, the discharge of agritainment sewage is mismanaged and neglected by the government and the public. The key reason for this neglect is the lack of basic data on the characteristics of agritainment sewage, including discharge quantity and quality. Hence, it is necessary to do further research to obtain more survey data to provide guidance and raise public awareness. This article intends to obtain and report the current status and characteristics of discharge from agritainment sites in Sichuan Province as a representative area. The discharge survey as completed uses water samples from 280 agritainment sites in 15 cities and 25 surveyed sites in Sichuan Province to characterize agritainment sewage discharge quantity and quality.

Sichuan, located in the hinterland of southwest China and the upper reaches of the Yangtze River, consists of 18 cities and three states. According to different cities and different types of agritainment, we selected hundreds of agritainment sites for a random field survey to determine discharge characteristics. As shown in Figure 1, 15 cities were selected as representative research areas. The survey points have been randomly selected at a ratio of 5/1,000 agritainment sites. A questionnaire survey has been distributed by 50 volunteers. As shown, most of the agritainment sites are distributed in western and northern cities of Sichuan, like Chengdu and Mianyang.

The aim of this questionnaire survey is to understand the discharge characteristics of agritainment sewage in Sichuan Province. To ensure the authenticity and validity of the questionnaire, 280 questionnaires were screened, and data with abnormal or incomplete content were excluded. The final number of valid questionnaires received was 264 out of 280 mailed, which includes 48 classified as hot pot sites, 36 as barbecue sites and 180 as Chinese food sites (Figure S3 in the Supplementary Material, available with the online version of this paper).

The questionnaire template is shown in Figure S1 (Supplementary Material, available online) and includes basic information about agritainment sites, management status, water supply, drainage assessment and client understanding of the problem. The scale of agritainment is divided into three categories based on maximum capacity served and includes large-scale (>300 people), middle-scale (100–300 people) and small-scale (<100 people). Hotpot, barbecue and Chinese food are three selected types of agritainment which are most famous in the local countryside. The data analysis method is shown in Figure S2 (Supplementary Material, available online). According to water supply and drainage assessment in the questionnaire, the characteristics of the discharge of black water and grey water have been quantified and compared in Figures 2 and 3.

The characteristics of discharge sewage quantity from agritainment sites are discussed and the relationship between sewage discharge volume and operating income are analysed in Figure 4 through the water consumption of each site, the operating time and the discharge method. As agritainment develops, sewage discharge volume increases, but it is difficult to quantify the volume of sewage discharge from agritainment each year. However, it is feasible to predict the annual sewage discharge volume based on the relationship between the operating income and the volume of agritainment sewage.

Grey water samples are collected and monitored considering that black water is mainly sent to a septic tank or wastewater treatment plant (WWTP) for treatment and the directly discharged sewage of agritainment is mostly grey water. The characteristics of grey water quality from agritainment sites are shown in Table 2. The average direct discharge concentration of pollution indices in grey water have been calculated according to the ratio of direct discharge grey water to three discharge methods and the average concentration of pollution index. Twenty random samples of grey water, which included five hotpot sites, three barbecue sites and 12 Chinese food sites were collected to assess representative characteristics of water quality from agritainment sites. Every sewage sample was collected at the drainage outlet of the agritainment kitchen, and the collection time was from 11 a.m. to 12 noon, which is the normal peak time of cooking. Each sewage sample was collected in a 1.5 L polyethylene bottle and sealed to avoid contamination and damage. All indicators were measured in the laboratory within 48 hours.

Ammonium nitrogen (NH₃-N), TN, TP, COD, anionic surfactant (AS), AVO and TSS were analysed using standard methods for testing in China (Table 1). pH was measured using a pH meter.

Generally, there are three discharge methods for agritainment sewage: (1) directly into the natural environment; (2) discharged into the environment after septic tank treatment; (3) discharged into the environment after being treated by a WWTP (Geng 2013).

The discharge methods for black water and grey water in Sichuan agritainment sites are shown in Figures 2 and 3. Approximately 36% of agritainment sites discharged black water into septic tanks and 58% of agritainment sites discharged into WWTPs. Hence, more than 94% of agritainment sites sent black water to septic tanks or WWTPs before being discharged into the environment; only 5.3% of agritainment sites directly discharged black water into the environment without treatment. Figure 3 shows that 25.3% of agritainment sites discharged grey water directly, 31.8% of agritainment sites discharged grey water into septic tanks and 42.8% of agritainment sites discharged into WWTPs. The proportion of hot pot sites, barbecue sites and Chinese food sites directly discharging grey water was 18.0, 7.4 and 74.6%, respectively.

The discharge methods for black water and grey water from hot pot sites, barbecue sites and Chinese food sites are indicated in Figure 4. In three types, only 4.1% of hot pot sites, 2.7% of barbecue sites and 6.7% of Chinese food sites discharged black water directly. However, 25.0% of hot pot sites, 13.9% of barbecue sites and 27.8% of Chinese food sites discharged grey water directly.

With different types and reception capacity of agritainment sites, some can only accommodate dozens of people per day, while some larger scale agritainment sites can receive thousands of people per day. The annual sewage discharge volume per site including black water and grey water ranged from 12 thousand to 38 million litres in surveyed agritainment sites. The annual average discharge volume of sewage per site was 2.48 million litres. It is worth mentioning that the total discharge volume in 24% of the agritainment sites was 504 million L/y, which accounts for 77% of the annual discharge volume of the surveyed agritainment sewage. Accordingly, it is necessary to attach importance to agritainment sites with large discharge volumes so that they can be given priority treatment. The annual average ratio of sewage discharge to income is 9.24 L/yuan in this survey, with a lower average ratio for barbecue (6.27 L/yuan). Overall, the ratio was generally lower than 50 L/yuan, with only 3% of the Chinese food agritainment sites exceeding this value.

The linear correlation between operating income and sewage discharge volume in hot pot, barbecue and Chinese food sites is given in Figure 5. Each category shows a good correlation and the R² values were 0.97, 0.96 and 0.96 in barbecue sites, Chinese food sites and hot pot sites, respectively. Barbecue sites have the fastest growth trend but the maximum operating income is the lowest compared to the other categories. Chinese food sites have the largest range of sewage discharge volume and operating income, which illustrates the variety of scale and nature of Chinese food. The regression trend line and the R² values further confirmed that it is feasible to predict sewage discharge volume based on operating income.

In 2017, the total income of rural tourism in Sichuan reached 228.3 billion yuan (SPBS 2018). The annual total income of agritainment sites was approximately 13.4 billion yuan in Sichuan, based on the income of agritainment sites accounting for 6% of rural tourism income (SPBS 2018). According to average ratio of sewage discharge volume to operating income (9.24 L/yuan), the estimated annual discharge volume was 124 billion litres in 2017, 55% higher than in 2016 (80 billion) (MHURD 2017). Considering the increasing discharge of agritainment sewage, we appeal to the public and government to be aware of the pollution resulting from agritainment sewage. Consequently, it is suggested that the investigation of agritainment water pollution should be included as part of the third national pollution source survey in China (2027).

The concentration of TSS in agritainment grey water varied widely and ranged between 10 and 2,470 mg/L. The average concentration of TSS was 625 mg/L, which is similar to the TSS concentration of grey water in Jordan (644 mg/L) (Halalsheh et al. 2008) and Ghana (538 mg/L) (Oteng-Peprah et al. 2018), but much higher than the other studies in Table 3. The concentration of TSS in 45% of agritainment sites exceeds the Chinese standard by 55–1,135%. The average concentration of TSS is related to the different types of agritainment, in the order of hot pot (924 mg/L) > Chinese food (573 mg/L) > barbecue (327 mg/L). The average direct discharge concentration of TSS in grey water calculated according to Equation (1) was 436 mg/L, which exceeds the standard concentration of China by 118% (200 mg/L, GB8918-1996 Class II) (Tables S1 and S2, available with the online version of this paper).

The concentration of TP in agritainment grey water was in the range 0.07–17.1 mg/L, which is similar to the report from Boyjoo et al. (2013) (4–14 mg/L), but higher than in India (0.012 mg/L), Kuwait (<0.05–2.6 mg/L) and Germany (1.1–2.1 mg/L) (Merz et al. 2007; Parjane & Sane 2011; Alsulaili & Hamoda 2015). The concentration of TP in 75% of agritainment sites exceeded the standard by 7–1,610%. The difference in TP concentration is not great among different types of agritainment, in the order of Chinese food (5.5 mg/L) > hot pot (4.0 mg/L) > barbecue (2.3 mg/L) and the average direct discharge concentration of TP calculated by Equation (1) was 2.9 mg/L, which exceeds the standard concentration of China by 190% (1 mg/L, GB8918-1996 Class II).

The concentration of NH₃-N in agritainment grey water was in the range 0.3–60.4 mg/L and the average concentration was 12.7 mg/L, which is slightly lower than that reported in Jordan (32 mg/L). The NH₃-N concentration of grey water in other studies was 7.6–16.0 mg/L in Germany and 14.8 mg/L in Ghana (Merz et al. 2007; Oteng-Peprah et al. 2018), which are similar to this study. According to the standard in China, the NH₃-N concentration of grey water in 85% of agritainment sites can reach the standard and the other 15% of agritainment sites exceeded the standard by 6–142%. In the different types of agritainment sites, the average concentration of NH₃-N in Chinese food sites (16.5 mg/L) was highest and that in hot pot and barbecue sites was 10.4 and 1.6 mg/L, respectively. The average direct discharge concentration of NH₃-N was 7.46 mg/L through Equation (1), which is within the standard of China (25 mg/L, GB8918-1996 Class II).

The concentration range of COD in agritainment grey water was 144–2,580 mg/L, which is similar to the report from Jordan (2,244 mg/L) (Halalsheh et al. 2008), but much higher than that in other reported research in Table 3. More than 95% of agritainment sites exceeded the standard concentration by 16–1,620%. The COD concentrations of grey water in hot pot and Chinese food sites were similar, and the average concentrations were 1,289 and 1,223 mg/L, respectively. However, the average concentration of COD in barbecue (312 mg/L) sites was about four times lower than that in hot pot and Chinese food sites. It is likely due to a simpler way of cooking and a lower use of oil and fat in barbecue sites in comparison to the two other cooking modes. The average direct discharge concentration of COD calculated by Equation (1) was 705 mg/L, which exceeds the standard concentration of China (150 mg/L, GB8918-1996 Class II) by 370%.

The concentration of AS in agritainment grey water was in the range 3.5–411 mg/L, which is similar to the AS concentration of grey water in Germany (66–532 mg/L) (Merz et al. 2007), but much higher than that in Israel (3.3–61 mg/L) and Jordan (55 mg/L) (Halalsheh et al. 2008; Friedler & Alfiya 2010). About 80% of agritainment sites exceeded the standard and the highest over-standard rate was 4,008%. The average concentration in different agritainment types is in order of hot pot (207 mg/L) > barbecue (139 mg/L) > Chinese food (111 mg/L). The average direct discharge concentration of AS was 102 mg/L, which exceeds the standard concentration of China by 920% (10 mg/L, GB8918-1996 Class II).

The concentration of AVO in agritainment grey water ranged between 1.6 and 29.3 mg/L, which is much lower than the reported value from Israel (77–328 mg/L) and Ghana (67.2 mg/L) (Friedler & Alfiya 2010; Oteng-Peprah et al. 2018). The average concentration of AVO in agritainment grey water was 9.5 mg/L, which is close to the value reported in India (7 mg/L) (Parjane & Sane 2011). Only 10% of agritainment sites exceeded the standard and the maximum concentration was only 47% higher than the standard (20 mg/L). The average concentration of AVO is related to different types of agritainment and the average concentration for hot pot and Chinese food was very similar, in the order of hot pot (10.6 mg/L) > Chinese food (10.1 mg/L) > barbecue (5.8 mg/L). The average direct discharge concentration of AVO was 6.3 mg/L, which is within the standard of China (20 mg/L, GB8918-1996 Class II).

In summary, the main pollutants of concern in agritainment sewage were TSS, TP, COD and AS with high concentrations exceeding current Chinese water standards. Therefore, we should consider the environmental risk of agritainment sewage, especially that which is directly discharged, and we should investigate cost-effective treatment technologies.

Agritainment sewage with high pollutant concentrations and increasing volumes should be targeted for treatment. As rural domestic sewage negatively affects the rural environment, the treatment technology for decentralized rural domestic sewage is developing rapidly in China (Zhao 2010; Ding et al. 2017; Zhang et al. 2017; Gu et al. 2018). However, there is little research on sewage treatment in agritainment. According to the characteristics of quantity and quality in agritainment, the treatment options should be designed specifically for general rural domestic sewage treatment. Two kinds of treatment processes are suggested in Figure 6.

Mode 1, named integrated waste treatment, is a combination of an oil separation tank and integrated wastewater treatment equipment, which is suggested to treat agritainment sewage from barbecue or small-scale agritainment (<100 people) sites. The average concentration of pollutants in barbecue sites is much lower than hot pot and Chinese food sites. In order to separate sewage and oil, remove oil slicks and prevent oil from blocking the sewage pipeline, it is necessary to add oil separation tanks as a preliminary treatment. Oil is separated in an oil separation tank through the specific gravity of suspended solids and water in the sewage without consuming energy. Meanwhile, part of the TSS and AS can also be removed in the oil separation tank. The separated oil should be cleaned and transferred regularly to ensure long-term operation of the equipment. Integrated wastewater treatment technology has become a popular way to treat decentralized domestic sewage in recent years, because of its strong resistance to shock loading, small footprint, low investment, low energy consumption and stable effluent quality. The so-called integrated wastewater treatment technology refers to multiple processing units integrated in the same reactor (Ding et al. 2017), generally including pretreatment, biological treatment (anaerobic/anoxic or aerobic), sedimentation and disinfection (Li et al. 2015). TSS, TP, COD and AS can be effectively removed by integrated wastewater treatment reactors when the sewage concentration is low or the agritainment scale is small. The concentration of effluent can meet the current standard after treatment by mode 1, and can even be directly discharged into surface water or reused for irrigation. The technology is designed as an automatic flow to save energy consumption, and the equipment can be placed over ground or buried underground. Burying the equipment underground is suggested to avoid the temperature impact on treatment efficiency and the ground may still be used for agricultural production.

Mode 2, named integrated waste treatment and constructed wetland, consists of an oil separation tank, integrated wastewater treatment equipment and constructed wetland, which is suggested to treat agritainment sewage with high pollutant concentrations and large discharge volumes. Due to the high concentration of sewage, it is necessary to increase the number or scale of integrated sewage treatment equipment based on the quantity and quality of agritainment sewage. After processing by integrated sewage treatment, 70–90% of pollutants are degraded and removed, but the effluent concentration may not yet reach current standards. Consequently, constructed wetlands are used in mode 2 for further treatment. The vertical flow–horizontal flow combination system is applied typically. Vertical flow is more prominent for oxygen delivery capacity, which can efficiently remove organic matter and ammonia nitrogen. Horizontal flow is less affected by climate change or the surrounding environment. Furthermore, it is necessary to adopt intermittent influent to control hydraulic load, and to strengthen anti-clogging and anti-seepage measures to ensure long-term stable operation of wetlands (Zhang et al. 2014; Gu et al. 2018).

Considering that very little research has been done on sewage treatment in agritainment and the variety of agritainment sites in terms of scale and nature of the operations, the estimation of cost would be speculative. There is no doubt that the action of adding treatment would certainly have an impact on operation income and eventually on the costs for tourists visiting such agritainment sites.

The annual sewage discharge from Sichuan agritainment sites was 124 billion litres in 2017, 55% higher than that in 2016. The survey found that 25.3% of surveyed agritainment sites discharge grey water directly but only 5.3% of surveyed agritainment sites discharge black water directly. The annual sewage discharge volume per site ranged from 12 thousand to 38 million litres and the average value was 2.48 million litres. The annual average ratio of sewage discharge volume to operating income is 9.24 L/yuan, which can be used to help predict the sewage discharge volume in agritainment sites. The main pollutants in agritainment sewage are TSS (10–1,069 mg/L), TP (0.07–17.1 mg/L), COD (144–2,580 mg/L) and AS (3.5–411 mg/L). In view of the increasing volume and concentration of sewage in agritainment, we call for more attention of the public and government to the environmental risks of agritainment sewage discharge. Additional surveys on the characteristics of rural agritainment sewage are needed, and are suggested to be included in the third national pollution source survey in China (2027).

This work was supported by the Young Scientists Fund of the National Natural Science Foundation of China (No. 21307100), the International Cooperation Project of Science and Technology Department (No. 2017HH0042) and Southwest University of Science and Technology Longshan talent program (No. 18lzx620).