Spatial Patterns of Urban Wastewater Discharge and Treatment Plants Efficiency in China

Phytosphere purification of urban domestic wastewater

Industrialization and overpopulation have polluted aquatic environments with significant impacts on human health and wildlife. The main pollutants in urban sewage are nitrogen, phosphorus, heavy metals and organic pollutants, which need to be treated with sewage, and the use of aquatic plants to purify wastewater has high efficiency and low cost. However, the effectiveness and efficiency of phytoremediation are also affected by temperature, pH, microorganisms and other factors. The use of biochar can reduce the cost of wastewater purification, and the combination of biochar and nanotechnology can improve the efficiency of wastewater purification. Some aquatic plants can enrich pollutants in wastewater, so it can be considered to plant these aquatic plants in constructed wetlands to achieve the effect of purifying wastewater. Biochar treatment technology can purify wastewater with high efficiency and low cost, and can be further applied to constructed wetlands. In this paper, the latest research progress of various pollutants in wastewater purification by aquatic plants is reviewed, and the efficient treatment technology of wastewater by biochar is discussed. It provides theoretical basis for phytoremediation of urban sewage pollution in the future.

A comprehensive assessment of energy efficiency of wastewater treatment plants: An efficiency analysis tree approach

Wastewater treatment plants (WWTPs) are energy intensive facilities. Controlling energy use in WWTPs could bring substantial benefits to people and environment. Understanding how energy efficient the wastewater treatment process is and what drives efficiency would allow treating wastewater in a more sustainable way. In this study, we employed the efficiency analysis trees approach, that combines machine learning and linear programming techniques, to estimate energy efficiency of wastewater treatment process. The findings indicated that considerable energy inefficiency among WWTPs in Chile existed. The mean energy efficiency was 0.287 suggesting that energy use should cut reduce by 71.3 % to treat the same volume of wastewater. This was equivalent to a reduction in energy use by 0.40 kWh/m³ on average. Moreover, only 4 out of 203 assessed WWTPs (1.97 %) were identified as energy efficient. It was also found that the age of treatment plant and type of secondary technology played an important role in explaining energy efficiency variations among WWTPs.

The evaluation of greenhouse gas emissions from sewage treatment with urbanization: Understanding the opportunities and challenges for climate change mitigation in China's low-carbon pilot city, Shenzhen

Sewage treatment provides a pathway for anthropogenic water purification that can address the growth in domestic sewage volumes due to urbanization and protect the aquatic environment. However, the process can also generate greenhouse gases (GHGs), which are sometimes termed "unrestricted" GHG emissions and are neglected by low carbon policies. A combination of a life cycle analysis (LCA), data envelopment analysis (DEA), and questionnaire survey was used to evaluate sewage treatment related GHG emissions and assess the GHG emission reduction efficiencies during 2005-2020, as well as determine the opinions of environmental managers regarding the threats to climate change mitigation posed by sewage treatment in the low carbon pilot city of Shenzhen, China. There were four main results. (1) GHG emissions from sewage treatment plants (STPs) in Shenzhen increased gradually from 0.22 Mt. CO₂-eq in 2005 to 1.16 Mt. CO₂-eq in 2020 with an emission intensity ranging from 0.41 to 0.58 kg CO₂-eq/m³, mainly due to the indirect emissions from sludge disposal (35-57 %). Longgang administrative district was the hotspot of these GHG emissions during the study period. (2) Reductions in GHG emissions were achieved in most years since 2012 with the greatest efficiency observed in 2020. (3) Beyond the environmental managers' perceptions of the challenges in GHG mitigation, future sewage treatment may create the potential for more substantial GHG emission growth compared to the emissions from energy combustion, due to policy deficiencies, growth in sewage volumes, and the enforcement of stricter effluent quality control. (4) Several opportunities to overcome these barriers were considered including innovational environmental management, planting of constructed wetlands, and the promotion of water-saving behavior. This case study of Shenzhen has valuable implications for the synergistic governance of water pollution and climate change mitigation in megacities in China and elsewhere, enabling a move towards a future carbon-neutral society.

A modified dynamic DEA model to assess the wastewater treatment efficiency: perspective from Yangtze River and Non-Yangtze River Basin

The wastewater treatment efficiency is crucial to constructing a livable ecological environment and promoting the sustainable development of economy and society. The differences in natural conditions, economic development and local policies between the Yangtze River Basin (YRB) and the Non-Yangtze River Basin (NYRB) increase the difficulty of wastewater treatment in governance. This study uses a modified Dynamic Data Envelopment Analysis (DEA) model to assess the wastewater treatment from 2013 to 2020, and divides the study period into two stages: the first stage (2013–2017) assesses the wastewater treatment efficiency of 18 provinces and cities in YRB and 12 provinces and cities in NYRB; the second stage (2018–2020) conducts statistical analysis of wastewater discharge pollutants in YRB and NYRB. The results conclude that the total wastewater treatment efficiency is generally low, but polarization is quite prominent. Among total wastewater treatment efficiency, NYRB scored 0.504, or slightly higher than YRB (0.398). In terms of expense efficiency, both NYRB and YRB scored below 0.4. In terms of chemical oxygen demand (COD) output efficiency, YRB (0.488) is better than NYRB (0.420). The second stage of statistical analysis presents that pollutant emissions are still high; the regions need to increase wastewater treatment investment and improve wastewater treatment efficiency.

Cost of raising discharge standards: A plant-by-plant assessment from wastewater sector in China

China has implemented increasingly stringent effluent standards for wastewater treatment plants (WWTPs) to protect the aquatic environment, but at the cost of more resource consumption and greenhouse gas emissions. To elaborate tradeoffs between the elevated standard and the additional burden, we compile a 10-year inventory of 6032 WWTPs across China to estimate the impacts of changes in effluent pollutant concentration on operating costs and electricity consumption. Coupled with the increasing demand for wastewater treatment, upgrading standards to the Special Discharge Limit (SDL) by 2030 would increase electricity consumption and operating costs of the wastewater treatment sector by 86.59% and 70.44% compared to the status quo in 2015. The electricity consumption-induced GHG emissions would also increase by 72.21%, which accounts for 29.16% of total emissions in the domestic wastewater treatment sector. Substantial regional differences exist in terms of upgrade-induced resource burden. Less developed regions generally suffer more stress when encountering similar standards elevation. With large-scale microdata, our findings deepen the understanding of the potential cost of raising standards and provide insights into region-customized pollutant effluent standards implementation.

Efficiency evaluation of urban wastewater treatment: Evidence from 113 cities in the Yangtze River Economic Belt of China

To resolve the increasing water pollution crisis, scientifically evaluating the urban wastewater treatment efficiency (UWTE) is an essential prerequisite to ensure the success of any policies aiming to decrease water pollution. Using 113 city-level panel data in the Yangtze River Economic Belt (YREB)-the biggest economic belt in China, during 2008-2017, this study aims to evaluate the UWTE based on a Bootstrap-DEA and to measure the efficiency changes through a Malmquist index model. The results show that the overall UWTE is at a low level, evidenced by the fact the average efficiency score is 0.51 during 2008-2017, and no cities have an efficiency score equal to 1. The UWTE is in the trend of decreasing, which is mainly caused by the decline of technical progress change. 69.02% of cities are in the state of decreasing returns to scale. The UWTE shows considerable disparities both between regions and city sizes, with the highest efficiency score observed in the midstream area and large-sized cities, the lowest efficiency score observed in the downstream area and small-sized cities. The findings of this study are expected to have great practical significance for governing wastewater pollution.

The Driving Forces of Point Source Wastewater Emission: Case Study of COD and NH4-N Discharges in Mainland China

Excess consumption of water resources and environmental pollution have become major challenges restricting sustainable development in China. In order to prevent the pollution of water resources, policymakers should have reliable emission reduction strategies. This paper aims to contribute new knowledge by analyzing the spatial-temporal characteristics and driving forces of point source emission. The chemical oxygen demand (COD) and ammonia nitrogen (NH4-N) emission variations in 31 provinces and municipalities of mainland China during the years 2004-2017 are analyzed. The results obtained using the logarithmic mean Divisia index (LMDI) method indicate that: (1) the COD and NH4-N emission effects have similar temporal characteristics. Technology improvement and pollutant emission intensity are the main factors inhibiting the incremental COD and NH4-N emission effects, while economic development is the main driving factor of COD and NH4-N emission effects. Population increases play a relatively less important role in COD and NH4-N emission effects. (2) The spatial features of COD and NH4-N emission effects show differences among provinces and municipalities. The reduction of COD emission effects in each province and municipality is obviously better than that of NH4-N emissions. (3) In the eastern, central, and the western regions of China, the total COD emission effect shows a downward trend, while apart from the central region, the NH4-N emission effect appears to be rising in the east and west of China. Therefore, increasing investment into pollution treatment, promoting awareness of water conservation, strengthening technological and financial support from the more developed eastern to the less developed central and western regions, can help to reduce the COD and NH4-N emissions in China.

Expected Rural Wastewater Treatment Promoted by Provincial Local Discharge Limit Legislation in China

Wastewater treatment in a rural region in China was undeveloped both in treatment capacity and legislation. The successful fast development of urban wastewater treatment plants (WWTPs) demonstrated the importance of legislation, including discharge limits. However, most provinces, with as high as 79.8% of the rural population in China, released no specific local discharge limits. Newly issued top-designed nationwide policy in September of 2018 by central China government required all provinces to issue their local rural wastewater discharge limits before June 2019. For the first time, this research analyzed the requirements of the newly issued policy and their inconsistence with several existing provincial limits. It proposed flexible principles for determination of discharge limits under various conditions to improve the rural residential environment as a whole. This study also proposed the use of the ratio between wastewater treatment cost and life expense to describe economic burden. Economic burden calculation for wastewater treatment in rural and urban regions was established respectively. Based on three conditions described in the new policy, the average burden for all urban residents was estimated as 0.122 ± 0.038% of the total life expense. In comparison, average nationwide rural burden was 0.087 ± 0.035% and 0.564 ± 0.196% for condition I (Total nitrogen(TN)/total phosphorus(TP) for resource recovery) and condition III (TN/TP for pollutant removal), respectively. It was also revealed that a stringent rural discharge limit lead to a Gini value as high as 0.38, indicting policy-related subsidies for rural residents should be carefully considered to ensure a balanced burden. Local discharge limit legislation and suitable financial policy is expected to promote rural wastewater treatment in China in the near future.