The GHG mitigation opportunity of sludge management in China.

Greenhouse gas (GHG) mitigation in wastewater treatment sector is indispensable in China's carbon neutral target. As an important component of wastewater system, sludge generation is rapidly increased with the acceleration of urbanization in China. It is crucial to investigate the carbon footprint of various sludge management strategies and quantify the potential optimization of GHG reduction effect at national scale. Therefore, this study conducted a comprehensive analysis of sludge distribution and GHG profiles of various sludge systems. The overall dry sludge generation in China is 12.15 Mt, with spatial resolution at city level. Different sludge treatment options were categorized into four types: energy recovery, nutrient recovery (e.g. phosphorus and nitrogen), material valorisation (e.g. brick, biochar) and conventional disposal. With various sludge treatment options, the GHG profile of annual sludge management in China ranges from -35.86 Mt/year to 57.11 Mt/year. The best GHG mitigation can be achieved through energy recovery by co-incineration system and the greatest reduction opportunity is concentrated in highly urbanized regions, such as Yangtze River Delta, Pearl River Delta, and Beijing-Tianjin-Hebei urban agglomerations.

Performance of a green wall (Total Value Wall™) at high greywater loading rates and Life Cycle Impact Assessment.

Nature-based greywater (GW) treatment and reuse in urban areas has become an up-and-coming option. A 14.4 m2 green wall system called Total Value Wall (TVW) was installed at a terraced house in Gent (Belgium) for treating GW and reusing the effluent for toilet flushing. In a previous study, the TVW was loaded at 7 L.m-2.d-1 and efficiently removed TSS (67%), COD (43%), BOD5 (83%) and total coliforms (log 2), but a number of issues were reported related to nutrient leaching from the substrate, and the excessive retention time in the storage tanks. In this study results are reported from a follow-up study during which an adapted TVW was subjected to both higher hydraulic and pollutant loading rates in order to investigate the treatment capability of TVW. The design of the system, i.e. substrate contained in geotextile bags, did not sustain the higher hydraulic loading rates as excessive leakage occurred. Despite this, the higher pollutant loading rates still resulted in an acceptable effluent quality with 15 mg.L-1 TSS (90%), 85 mg.L-1 COD (82%), and 15 mg.L-1 BOD5 (95%). Ammonium, E. coli and total coliforms were removed with removal rates of 98%, 63% (0.4 log units), and 36% (0.2 log units), respectively. Finally, a life cycle assessment (LCA) was performed for the TVW with and without treating GW to analyze the environmental burden. The LCA impacts showed that replacing tap water and chemical fertilizer by GW, and the reuse of effluent, have a positive impact. However, the energy use for pumping has a major impact and should be minimized by using an efficient pump and distribution system to reduce the overall footprint.