The comprehensive environmental performance evaluation of wastewater reclamation systems

The ‘New Sewerage Vision 2100’(MILIT Japan 2014) issued by the Ministry of Land, Infrastructure, Transport and Tourism and the Japan Sewage Works Association proposed making sewerage systems integrated bases for the supply of energy, water and resources. This was proposed in order to fully implement the concept of ‘the sewerage system as a road for circulation.’ This includes the idea of sewerage systems utilizing reclaimed water and heat as part of the water cycle to contribute to the creation of low-carbon and recycling-oriented communities. Such an approach would include energy management integrated with utilization of reused water and heat, and reducing energy consumed for water transportation.

The ‘Sewerage Technology Vision’ set forth in 2015 summarizes the technology development fields and the measures needed to achieve the long-term vision and middle-term targets set in the New Sewerage Vision. It identifies targets, and technology development tasks to be undertaken to solve critical problems described in a road map entitled ‘Eleven Technology Development Fields.’ The issues to be addressed in future sewage treatment system development include flexible responses to a declining population, and flood control measures to cope with changes in rainfall patterns due to climate change. The Sewerage Technology Vision cites reclaimed water utilization as a technology development task, proposing ‘a flexible system technology capable of supplying reclaimed water of required quality,’ and ‘international standards for of water reuse.’

However, in Japan, the situation for wastewater reuse in 2011, illustrated in Figure 1, shows that only approximately 1.3% of the wastewater was reused. This reuse percentage remained about the same in later years. (Approximately 1.5% of the country's wastewater was reused in 2012.) In terms of application, reuse as environmental waters accounts for more than half, followed by 20% used as municipal water (water for melting snow). On the other hand, agricultural applications, which are most widely used in other countries, were less than 10% in Japan. In addition, environmental water was used in urban areas such as the Tokyo metropolitan area (Kitamura et al. 2013) and the Kansai area, and as municipal water (for melting snow) in Hokkaido. Water was used in agriculture in outlying areas. Wastewater reuse was very uneven by area and by season. The NEWater project in Singapore (Lim & Seah 2013) and the Olympic park in China (Yi et al. 2011; Lyu et al. 2016) were reported on as examples in other countries.

A ‘Manual for Water Quality Standards of Wastewater Reuse’ (MILIT Japan and NILIM Japan 2005) issued set water quality standards for reclaimed water is supplied to facilities for use by unspecified numbers of persons in Japan. Table 1 shows water quality items according to wastewater reuse in other parts of the world (Fujie 2012). In addition to urban water and environmental water, some countries set water quality standards for agriculture water and industrial water. Suspended solids (SS) and the biological oxygen demand (BOD) are targeted in the water quality items, and more nitrogen and phosphorus items are set in water quality standards for urban water and agricultural water in China.

Recently, the removal of viruses and pharmaceutical products (PPCPs) and environmental endocrine disrupters have been requested, depending on the purpose of wastewater reuse (Siegrist et al. 2014). As a wastewater reclaiming system, ozonation (Singh et al. 2014; Mehrjouei et al. 2015) and filtration (Wintgens et al. 2004; Xue et al. 2010), advanced oxygenation processes (AOP) (Huber et al. 2003) and a combination of each technique (Im et al. 2014) are considered.

In regard to international standardization, as shown in Figure 2, Japan is a secretariat country of ISO/TC282 on Water Reuse as well as SC3 for Risk and Performance Evaluation (Tachi and Ohkuma 2015). Standards are developed in WG1: Health Risk and Water Quality Symbol and WG2: Performance Evaluation. The USA specifies treatment systems according to intended use (Asano et al. 2007), but there are no examples of implementing the performance evaluations of combinations of risk and water quality with energy consumption. I am attempting a risk and performance evaluation of wastewater reclamation systems as part of TC282 activity.

In order to evaluate the risk and performance of sewage reclamation systems, four treatment levels were established, as shown in Figure 3, with reference made to domestic and overseas reclaimed water quality standards:

• Level 1 (secondary treatment): CAS process + chlorine disinfection

• Level 2 (advanced treatment): A²O process + chlorine disinfection

• Level 3 (MBR): membrane bioreactor + chlorine disinfection

• Level 4 (reclaimed treatment): A²O process + treatment of water for re-use

The performance of a medium-scale treatment plant with a capacity of 48,000 m³/day was evaluated in terms of reclaimed water quality and power consumption by using the Performance Evaluation System (PES) treatment plant simulator (Fukushima 2015; Fukushima 2017). Sewage treatment was performed using the conventional activated sludge process, and sludge was treated according to a separated thickening–dewatering–incineration process. For influent water quality, chemical oxygen demand (COD) 360 mg/L, SS 160 mg/L, Total Nitrogen (TN) 35 mg/L, Total Phosphorus (TP) 4.0 mg/L’ (COD value as converted from the BOD value) were selected. This was based on median levels reported in Japanese national Sewerage Statistics for 2006. Note that 4,800 m³/day will be reclaimed, which is equivalent to 10% of the influent.

To calculate greenhouse gas (GHG) emissions from wastewater reclamation systems, there is a method according to IPCC guidelines (Intergovernmental Panel on Climate Change 2006). However there are indications that the evaluation of the N₂O emission from a wastewater treatment process is small, so the examination of the emission coefficient is pushed forward based on actual wastewater treatment plant surveys (Kampschreura et al. 2009; Global Water Research Coalition 2011).

In this study, I used the coefficient arranged in Table 2 in reference to the ‘global warming measures manual in sewerage’ (MOE Japan 2016) that reflected measuring results in Japan to simplify the calculation. For the electricity emission factor, it is recommended that the IEA (International Energy Agency) reported value be used: 0.5 kg CO₂/kWh (Delgado et al. 2012), but I used the averaged value for Japan in 2015.

Figure 4 shows the results of calculations of reclaimed water quality at each level. In the Level 1 secondary treatment, organics (COD) were removed, but relatively high amounts of SS, TN, and TP remained. In the advanced treatment (A²O process), Level 2, TN 12.5 mg/L, TP 0.5 mg/L, nitrogen, and phosphorus were removed to a satisfactory degree. In Level 3, MBR, SS, COD, and TP were almost completely removed, but the TN removal was 12.0 mg/L, which was similar to the A²O process, because the circulation rate was 100%. In the case of Level 4 sewage reclamation, combined use of coagulant helped attain satisfactory removal of SS and TP, but COD increased slightly because of solubilization by ozonation.

Assuming that the dissolved oxygen consumption at sites where reclaimed water is used will have an impact on the environment (environmental load), COD, TN, and TP left in reclaimed water were converted uniformly to COD by using the conversion factor, (Ishida et al. 2005) and the residual volume of water environmental load was calculated (conversion factors were TN: 19.7 and TP: 142.5). The water environmental load existing in influent decreased from 78,000 kg to 30,000 kg through secondary treatment, and to 11,000 kg through MBR and sewage reclaiming.

Figure 5 shows the results of calculating GHG emissions with the improvement of the treatment level. The highest levels of GHG emissions occurred in the Level 1 secondary treatment. It was 23,050 kg CO₂/day, and N₂O emission from wastewater treatment was about 10% more than with other levels. When advanced treatment was introduced, CO₂ associated with electricity consumption increased, but N₂O emission from wastewater and sludge incineration decreased. Remarkably, in MBR, GHG emission was as low as 19,740 kg CO₂/day. In the case of Level 4 sewage reclamation, it increased to 21,920 kg CO₂/day due to increased electricity consumption.

In addition to reclaimed water quality, the performance levels in removing health risks such as E. coli, viruses and PPCPs and environmental toxicity are numerically rated in five stages as shown in Figure 6.

Secondary and advanced treatments were limited to removing E. coli through disinfection. MBR could remove viruses, but the removal ratio of PPCPs remained at a medium rate. In sewage reclaiming, ozonation offered a high rate of removal of PPCPs, and with the combined use of a ceramic membrane filter, a high rate of removal of environmental toxicity (toxicity to aquatic life) was achieved. (The performance for ozonation + ceramic membrane filtration of this sewage reclaiming system was set on the basis of a performance evaluation of sewage reclaiming technology described in literature) (Tanaka 2015).

Each a is the weighting factor determined according to the usage of reclaimed water (1 for all cases in this paper).

The calculation results are shown in Table 3. The wastewater reclamation efficiency of secondary treatment, which was 19, was improved to 31 due to an increase in nitrogen and phosphorus removal through advanced treatment. The total removal score was improved to 21 due to improved removal of viruses and other health risks through MBR. In this way, the GHG emission ratio could be decreased to 0.41 kg CO2/m³, and the wastewater reclamation efficiency was raised to 51.

Sewage reclamation achieved a much higher removal performance for health risks, with the total removal score being a maximum of 25. The GHG emission ratio slightly increased to 0.46 kg CO₂/m³, however the wastewater reclamation efficiency was a maximum of 54. In this research, Level 4 (reclaimed treatment) was calculated as the most effective. It appears that high-quality reclaimed water is produced by the reclaimed treatment system (ozone and ceramic membrane filtration).

A sewage reclamation system was evaluated, using an index of wastewater reclamation efficiency determined from a health risk reduction and sewage reclaiming GHG emission ratio, while assuming four water reclamation levels. The following conclusions were reached:

• (1)

  Regarding the reclaimed water quality, organics (COD) could be removed, but SS, TN, and TP remained relatively high after secondary treatment, Level 1. In Levels 2, 3, and 4 in which advanced treatment was introduced, TN and TP could be removed to a satisfactory degree.

• (2)

  The water environmental load existing in influent decreased from 78,000 kg to 30,000 kg through secondary treatment, and to 11,000 kg through MBR and sewage reclaiming. Secondary treatment generated the highest levels of GHG emissions, 23,050 kg CO₂/day, and N₂O emission from wastewater treatment was at about 10%. CO₂ from electricity consumption increased with the introduction of advanced treatment, but N₂O emission from wastewater and sludge incineration decreased. Remarkably, in MBR, GHG emission was as low as 19,740 kg CO₂/day. CO₂ emission in sewage reclamation increased to 21,920 kg CO₂/day due to increased electricity consumption.

• (3)

  Evaluation in terms of wastewater reclamation efficiency showed improvement in secondary treatment, from 19 to 31. This was due to increased removal of nitrogen and phosphorus through advanced treatment. It rose further to 51 due to increased virus and health risk removal through MBR. Sewage reclaiming enhances removal of health risks further, with the efficiency being a maximum of 54.

Because GHG emissions increase in systems used to obtain high-grade reclaimed water, the selection of an appropriate wastewater reclamation system which yields water of acceptable quality for the purposes of reuse will be important.

In the future, the study will be continued using the indexes and a database for comprehensive evaluation of measures not only for reclaimed water, but also for resource and energy utilization. ‘Water’ and ‘energy’ will be the keywords.