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.

Total value wall: Full scale demonstration of a green wall for grey water treatment and recycling.

Greywater treatment and reuse for non-potable purposes in urban areas has become a widely researched topic to reduce the burden on fresh water resources. This study reports on the use of a green wall for treating grey water and reusing the effluent for toilet flushing, called Total Value Wall (TVW). Initially, the effectiveness of (mixtures of) different substrates, i.e. lava, lightweight expanded clay aggregates, organic soil and biochar was investigated by means of column tests. All substrates were first examined for hydraulic characteristics and later on the columns were fed with synthetic grey wastewater and followed up in terms of removal efficiency of COD and detergents. The mixture consisting of lava (50%), organic soil (25%) and biochar (25%) proved to be optimal both in terms of percolation rates and removal efficiencies, and was thus selected for the full-scale system. The full-scale TVW of 14.4 m2 was installed at a terraced house in Ghent (Belgium), and was loaded with grey water at 100 L per day. Influent and effluent quality were routinely monitored by grab sampling, water savings were monitored by means of flow meters, and electricity consumption was also accounted for. The TVW was further equipped with sensors that measure temperature, Particulate Matter (PM10) and CO2 in the air. The full-scale system obtained effluent concentrations of 13 mg.L-1 TSS, 91 mg.L-1 COD and 5 mg.L-1 BOD5. Ammonium and total coliforms were removed with removal rates of 97% and 99% (2 log units) respectively. However, an increase in effluent concentration of nitrate and phosphate was observed due to leaching from the selected substrate. Available data from the temperature sensors have clearly demonstrated the additional benefit of the TVW as an insulating layer, keeping the heat outside on warmer days, and keeping the heat inside on colder days. Overall, this study demonstrated that the TVW is a sustainable system for greywater treatment and reuse.

Water treatment and re-use at temporary events using a mobile constructed wetland and drinking water production system.

Temporary events such as music festivals are often organized in places that are not connected to a sewage network. As such, the waste (water) generated and discharged can place a heavy burden on the environment. A mobile vertical flow constructed wetland (MCW) with an area of 15 m2 was constructed, optimized and operated for three years treating grey water (GW) as well as grey and black water (GW + BW) at different festival locations to tackle this problem. During the initial development phase, the appropriate influent type (GW and/or GW + BW) was determined and challenge tests with pre-settled diluted domestic waste water (mimicking GW) were carried out to determine the maximal allowable loading rate. The MCW was able to treat both types of water. However, for further experiments GW was selected as the discharge limits could not be met when treating GW + BW. The challenge tests demonstrated that the MCW could be operated at a maximal allowable hydraulic loading rate of 1.1 m3·m-2·d-1, corresponding to mass loading rates of 68 gTSS·m-2·d-1, 160 gCOD·m-2·d-1 and 137 gBOD·m-2·d-1. During treatment of GW, the MCW achieved effluent concentrations for respectively chemical oxygen demand (COD), biological oxygen demand (BOD), total suspended solids (TSS) and total phosphorus (TP) of 43 mg·L-1, 16 mg·L-1, 2.7 mg·L-1 and 1.7 mgP·L-1. This corresponds to a removal of 90% (COD), 95% (BOD), 97% (TSS) and 76% (TP) respectively. Total nitrogen removal was 25% (from 45 mgN·L-1 to 34 mgN·L-1) as particularly denitrification was not complete. As a further development, the MCW was coupled to a drinking water treatment system using ultrafiltration and reverse osmosis (UF-RO) membranes to produce potable water on site. The drinking water system produced potable water that met the legislative criteria. As such, a sustainable and mobile water treatment system aiming at producing potable water at temporary events was demonstrated.