Vacuum Toilets Research Articles

Innovative Energy Design for Energy Efficient Buildings - An Example from the Bullitt Center

In recent years, the extreme climate caused by global warming has brought serious consequences to the world. In order to alleviate the problem of global warming, many scholars have started to analyse energy-saving green buildings. Taking the Bullitt Center as an example, this paper illustrates the innovative applications of energy-efficient buildings in solar power generation, rainwater collection and utilization, toilet water conservation and resource utilization and room temperature regulation. It focuses on the differences in the use of solar power generation in winter and summer, the process of purification of rainwater into water, the comparison between composting toilets and vacuum toilets and the steps of regulating the room temperature by windows and ground-source heat pumps. This paper also talks about the limitations of these innovative applications. It could probably give reference to the subsequent construction of buildings, thus promoting energy saving and emission reduction in buildings and mitigating global warming.

Life Cycle Assessment of Resource-Oriented Sanitation Based on Vacuum Toilet and Vacuum Kitchen Waste Shredder: A Case Study in Rural Southeastern China

The resource value of feces and kitchen waste has not been fully emphasized and utilized in rural sanitation management in China. In this paper, we propose a new ecological sanitation model with the core technology of “vacuum toilet and vacuum kitchen waste shredder—vacuum collection—resource treatment”, i.e., the modern home–farm cycle sanitation (MHFCS) system. We compared the environmental performance of the MHFCS system with that of a typical end-pipe treatment sanitation (EPTS) system (Johkasou—small onsite wastewater treatment system) in rural China using a life cycle assessment (LCA) approach. The results showed that the main source of environmental impacts of the MHFCS system was the collection and treatment process of domestic organic liquid wastes; the greenhouse gas emissions were 64.543 kg CO2eq·PE−1·year−1, and the MHFCS system indirectly gained a fertilizer substitution benefit of 65.960 kg CO2eq·PE−1·year−1 through nutrient element recycling. The MHFCS system has significant advantages in terms of net GHG emissions. Sensitivity analyses showed that resource consumption of vacuum facilities was a key factor for the MHFCS system. This system offers the potential to break down the barriers of the EPTS system in order to meet environmental sustainability and market demands for systemic diversity.

Characterisation of blackwater from human transportation systems equipped with vacuum toilets and controlled emissions tanks and its impact on solid/liquid separation technologies

The first ever detailed characterisation of blackwater from human transportation systems equipped with vacuum toilets and Controlled Emissions Tanks (CET) revealed a stream that is very concentrated in salts and nutrients when compared to other blackwater sources. Escherichia Coli (E. coli) levels in the blackwater characterised were comparatively lower than the range reported for septic tanks, for example. Suspended solids were significantly lower than the levels found in pit latrines, but closely comparable to those reported for gravity toilets. The average COD of 3566 ± 2049 mg/L was typically lower than in pit latrines and gravity flush toilets. These findings then show that the characteristics of the blackwater from human transportation systems are directly impacted by a combination of the low flush vacuum toilets, storage in the CET, and different behaviour for passengers with increased usage for urination over defecation when compared to common uses of toilets. Such factors collectively dictate the nature of the blackwater and require that a solid/liquid separation technology for further processing is fundamentally robust in operation. The greater solids pulverisation, largely resulting from vacuum flushing combined with the storage environment, reduces the suitability of centrifugal and settling technologies whilst making filtration the most suitable option.

Comparison of anaerobic co-digestion of vacuum toilet blackwater and kitchen waste under mesophilic and thermophilic conditions: Reactor performance, microbial response and metabolic pathway

Co-digestion of kitchen waste (KW) and black water (BW) can be considered as an attractive method to efficiently achieve the clean energy from waste. To find the optimal operation parameters for the co-digestion, the effects of different temperatures (35 and 55 °C) and BW:KW ratios on the reactor performances, microbial communities and metabolic pathways were studied. The results showed that the optimum BW:KW ratio was 1:3.6 and 1:4.5 for mesophilic and thermophilic optimal reactors, with methane production of 449.04 mL/g VS and 411.90 mL/g VS, respectively. Microbial communities showed significant differences between the reactors under different temperatures. For bacteria, increasing BW:KW ratio significantly promoted Defluviitoga enrichment (1.1%–9.5%) under thermophilic condition. For Archaea, the increase in BW:KW ratio promoted the enrichment of Methanosaeta (8.6%–56.4%) in the mesophilic reactor and Methanothermobacter (62.0%–89.2%) in the thermophilic reactor. The analysis of the key enzymes showed that, acetoclastic methanogenic pathway performed as the dominant under mesophilic condition, with high abundance of Acetate-CoA ligase (EC:6.2.1.1) and Pyruvate synthase (EC:1.2.7.1). Hydrogenotrophic methanogenic pathway was the main pathway in the thermophilic reactors, with high abundance of Formylmethanofuran dehydrogenase (EC:1.2.99.5).

Partial nitritation and nitrogen removal of vacuum toilet wastewater from high-speed trains in a sequential batch reactor

Owing to the high contents of organics and nitrogen in vacuum toilet wastewater (VTW) generated from high-speed trains, onsite pretreatment is usually required before VTW can be discharged into municipal sewers. In this study, a partial nitritation process was stably established in a sequential batch reactor to efficiently utilize the organics in synthetic and real VTWs for nitrogen removal and to produce an effluent suitable for anaerobic ammonia oxidation. In spite of the high fluctuations of COD and nitrogen in VTW, the organics used for nitrogen removal stabilized at 1.97 ± 0.18 mg COD mg N−1 removed, and the effluent NO2−-N/NH4+-N ratios were maintained at 1.26 ± 0.13. The removal efficiencies of nitrogen and COD were 31.8 ± 3.5% and 65.2 ± 5.3% under the volumetric loading rates of 1.14 ± 0.15 kg N m−3 d−1 and 1.03 ± 0.26 kg COD m−3 d−1 for real VTW, respectively. Microbial community analysis revealed that Nitrosomonas (0.95%–1.71%) was the dominant autotrophic ammonium-oxidizing bacterial genus, but nitrite-oxidizing bacteria, Nitrolancea, was severely inhibited, with a relative abundance less than 0.05%. The relative abundance of denitrifying bacteria increased by 7.34% when the influent was switched to real VTW. Functional profile predictions of the biomass showed that the decrease in the COD/N ratio and the switch of reactor influent from synthetic to real VTW increased the relative abundance of enzymes and modules involved in carbon and nitrogen metabolisms.

The Electrochemical Reaction Kinetics during Synthetic Wastewater Treatment Using a Reactor with Boron-Doped Diamond Anode and Gas Diffusion Cathode

A system of boron-doped diamond (BDD) anode combined with a gas diffusion electrode (GDE) as a cathode is an attractive kind of electrolysis system to treat wastewater to remove organic pollutants. Depending on the operating parameters and water matrix, the kinetics of the electrochemical reaction must be defined to calculate the reaction rate constant, which enables designing the treatment reactor in a continuous process. In this work, synthetic wastewater simulating the vacuum toilet sewage on trains was treated via a BDD-GDE reactor, where the kinetics was presented as the abatement of chemical oxygen demand (COD) over time. By investigating three different initial COD concentrations (C0,1 ≈ 2 × C0,2 ≈ 4 × C0,3), the kinetics was presented and the observed reaction rate constant kobs. was derived at different current densities (20, 50, 100 mA/cm2). Accordingly, a mathematical model has derived kobs. as a function of the cell potential Ecell. Ranging from 1 × 10−5 to 7.4 × 10−5 s−1, the kobs. is readily calculated when Ecell varies in a range of 2.5–21 V. Furthermore, it was experimentally stated that the highest economic removal of COD was achieved at 20 mA/cm2 demanding the lowest specific charge (~7 Ah/gCOD) and acquiring the highest current efficiency (up to ~48%).

A multifaceted screening of applied voltages for electro-assisted anaerobic digestion of blackwater: Significance of temperature, hydrolysis/acidogenesis, electrode corrosion, and energy efficiencies

A microbial electrolysis cell-assisted anaerobic digester (MEC-AD) was operated with vacuum toilet blackwater at different applied voltages (0–1.6 V) at room temperature (R20). A parallel MEC-AD was operated at 35 °C (R35) to provide a kinetics index at mesophilic temperature. Both reactors failed at 1.6 V due to the alkaline pH created by anodic corrosion. In R20, the best performance was observed at 1.2 V, with methane yield, COD removal, hydrolysis and acidogenesis efficiency increased by 59.9%, 27.0%, 52.0%, and 44.9%, respectively, compared to those of 0 V. Enrichment of hydrolytic and syntrophic bacteria (e.g., Clostridium, Bacteroidales, Sedimentibacter, Syntrophomonas) and increased abundance of genes encoding complex organics (e.g., proteins, carbohydrates, lipids) metabolism in R20 at 1.2 V corresponded to the enhanced hydrolysis/acidogenesis processes. R20 at 1.2 V generated 1.16 times more net energy than R35 at the optimum voltage for methane yield (0.8 V), indicating ambient temperature operation of MEC-AD systems would be a more sustainable strategy.

Modeling and optimization of an upflow anaerobic sludge blanket (UASB) system treating blackwaters

In the present work, a novel mathematical dynamic model describing the upflow anaerobic sludge blanket (UASB) has been developed. The model incorporates both the biological processes and the transport of sludge particles specifically related to the UASB treatment of blackwater. The processes occurring in the reactor are modeled as a sequence of two completely-mixed reactive tanks, one representing the sludge bed where most solids concentrate and the other representing a less solid-concentrated liquid volume. Solids from the sludge bed are assumed to be dragged to the upper zone of the reactor by the methane gas produced. The time variation of the sludge bed volume is included by assuming a constant sludge density. The model is calibrated against published data obtained from a stable operation of a lab-scale UASB system treating at different organic loading rates blackwater collected from real vacuum toilets. Good model prediction compatibility with the measurements of emitted biogas and methane gas flow rates, effluent COD, volatile fatty acids, pH and sludge bed volume was revealed. Furthermore, the model was successfully validated against another independent set of data from a UASB reactor treating a more diluted type of blackwater collected from conventional toilets. A scenario analysis performed using such a validated model reveals higher methane production yield but also higher effluent COD concentration at increasing organic loading rates and sludge bed volumes. It is found that longer HRTs can improve COD removal efficiency and methane production in blackwater treatment processes.

Numerical Investigation of Bioaerosol Transport in a Compact Lavatory

The lavatory is a fertile area for the transmission of infectious disease through bioaerosols between its users. In this study, we built a generic compact lavatory model with a vacuum toilet, and computational fluid dynamics (CFD) is used to evaluate the effects of ventilation and user behaviors on the airflow patterns, and the resulting fates of bioaerosols. Fecal aerosols are readily released into the lavatory during toilet flush. Their concentration rapidly decays in the first 20 s after flushing by deposition or dilution. It takes about 315 s to 348 s for fine bioaerosols (<10 µm in diameter) to decrease to 5% of the initial concentration, while it takes 50 and 100 µm bioaerosols approximately 11 and <1 s, respectively, to completely deposit. The most contaminated surfaces by aerosol deposition include the toilet seat, the bowl, and the nearby walls. The 10 µm aerosols tend to deposit on horizontal surfaces, while the 50 and 100 µm bioaerosols almost always deposit on the bowl. In the presence of a standing thermal manikin, the rising thermal plume alters the flow field and more bioaerosols are carried out from the toilet; a large fraction of aerosols deposit on the manikin’s legs. The respiratory droplets generated by a seated coughing manikin tend to deposit on the floor, legs, and feet of the manikin. In summary, this study reveals the bioaerosol dilution time and the easily contaminated surfaces in a compact lavatory, which will aid the development of control measures against infectious diseases.

Practical Performance and User Experience of Novel DUAL-Flush Vacuum Toilets

Vacuum toilets have gained increasing attention in circular urban development projects, because of their marked water saving qualities compared to conventional flush toilets and the increased resource recovery potential for energy in the form of biogas and phosphorous as, e.g., struvite from the resulting concentrated wastewater. A further reduction of the flushing volume of vacuum toilets would also bring nitrogen recovery options in reach. In the framework of the EU Horizon 2020 project Run4Life, a novel dual-flush vacuum toilet was developed and tested at two sites and combined with an analysis of the flushing patterns and a qualitative user survey. The results show that a 25–50% lower flushing water consumption and accordingly 1.5–2 times higher nutrient concentrations are achievable with this novel type of vacuum toilet. The usage frequency of the dual flush feature was higher in residential homes than in an office building, which also had urinals installed at the men toilets. A notable fraction of toilet visits in which the toilet was flushed twice as well as user feedback on dissatisfactory cleaning effects suggest that the applied reduction in water use is most likely the upper limit of what can be achieved in this type of toilet.

Constructing and selecting optimal sustainable sanitation system based on expanded structured decision-making for global sanitation and resources crisis

Facing with global sanitation and resources crisis, developing and identifying optimal sustainable sanitation system (SSS) is particularly challenging in most of global areas through conventional solution. A novel method by coupling macro-environmental content analysis, compatibility assessment, and multi-criteria decision analysis into structured decision-making is developed and applied to select the optimal SSS to improve the rural human settlements environment of Beijing, China. A predictive model of weighting coefficient has been proposed to deal with the interactions based on the data-driven method considering temporal (56-y time frame) and geographic (87 countries) factors. The technologies space with mature technologies of SSS came from 402 demonstration projects of the world. The results show that vacuum toilet + vacuum sewer + anaerobic digestion/composting (for blackwater) + struvite crystallization (for yellowwater), and biogas/composting toilet + gravity sewer + anaerobic digestion/composting is suitable for high and middle income Districts of Beijing, respectively, in the coming 5–10 years. Although the novel method is the first application in SSS planning, the proposed method can also be extended to apply to other complex infrastructure decision-making.

Thermophilic (55 °C) and hyper-thermophilic (70 °C) anaerobic digestion as novel treatment technologies for concentrated black water

Thermophilic and hyper-thermophilic anaerobic digestion (AD) are promising techniques for the treatment of concentrated black water (toilet fraction of domestic wastewater collected by low flush volume toilets; BW), recovery of nutrients and simultaneous pathogen removal for safe recovery and reuse of those nutrients. This study showed that thermophilic AD (55 °C) of concentrated BW reaches the same methanisation and COD removal as mesophilic anaerobic treatment of BW (conventional vacuum toilets) and kitchen waste while applying a higher loading rate (OLR) (2.5–4.0 kgCOD/m3/day). With a retention time of 8.7 days, and an OLR of >3 kgCOD/m3/day, COD removal of 70% and a methanisation of 62% (based on CODt) was achieved during thermophilic AD. Hyper-thermophilic (70 °C) reached lower levels of methanisation (38%). Start-up time of thermophilic AD was 12 days. And during thermophilic AD, a shift from acetoclastic methanogenesis towards syntrophic acetate oxidation was observed.

Pushing the organic loading rate in electrochemically assisted anaerobic digestion of blackwater at ambient temperature: Insights into microbial community dynamics

Decentralized blackwater treatment by anaerobic digestion is being considered as a sustainable sanitation concept. However, the low biodegradability and complex composition restrictedly limited the treatability of blackwater, resulting in requirements of low operational organic loading rates (OLRs). In this study, a microbial electrolysis cell assisted anaerobic digester (MEC-AD) treating vacuum toilet blackwater was successfully operated for 420 days at OLRs ranging from 0.77 to 3.03 g COD/L-d in 6 stages (including an open-circuit Stage 5) at ambient temperature. Based on the steady-state results from different stages, the highest methane yield (42.4% out of 45% biochemical methane potential value) was achieved in Stage 1 with an OLR of 0.77 g COD/L-d. At the same OLR of ~3.0 g COD/L-d, Stage 4 (32.4%) and Stage 6 (35.2%) showed significantly higher methane yield (p < 0.01) than open-circuit Stage 5 (24.1%). The lowest COD removal efficiency of 31.8% was observed in Stage 5 with short-chain volatile fatty acids (SCVFAs) accumulated to ~1000 mg/L, which was more than double the values of Stage 4 and 6. The microbial community analysis revealed that the applied potential did not significantly affect archaeal diversity but largely increased the archaeal abundance on the cathode, and led the bacterial community shift with the enrichment of specific electroactive bacteria. Microbial co-occurrence network analysis further confirmed the positive correlations between known electroactive bacteria and electrotrophic methanogens. Moreover, electric energy consumed by the MEC-AD system was fully recovered as biomethane.

Towards the Removal of Antibiotics Detected in High-Speed Railway Vacuum Toilet Wastewater by Pilot-Scale Advanced Technologies

Towards the Removal of Antibiotics Detected in High-Speed Railway Vacuum Toilet Wastewater by Pilot-Scale Advanced Technologies

Impact of Total Suspended Solids on Struvite Precipitation from Source-Diverted Blackwater

AbstractSource-diverted blackwater, especially concentrated blackwater collected from vacuum toilet systems, has been demonstrated to be an ideal source for struvite (NH4MgPO4·6H2O) production. How...

Combination of magnetically actuated flexible graphite–polymer composite cathode and boron-doped diamond anode for electrochemical water softening or wastewater treatment

An electrochemical reactor based on a moving graphite–polymer composite (GPC) cathode and boron-doped diamond anode is developed and used for the electrochemical water softening and treatment of artificial vacuum toilet wastewater. The magnetically actuated cathode is designed for the in situ removal of insoluble compounds, which usually precipitate on the cathode during electrochemical water treatment processes. To obtain a suitable GPC cathode, the chemical stability and conductivity of various composites and their flexion and magnetic actuator characteristics are investigated. The most suitable GPC cathode is a 0.5 mm thick polypropylene-based composite, which presents chemical stability, the lowest resistivity of all analyzed samples (5.06 ± 1.80 mΩ cm), and enables flexions up to 2.4 mm. The water softening performance of the reactor featuring this electrode was evaluated using two configurations. Water hardness was decreased up to 72% and more than 90% in mixed and separated electrolyte modes, respectively. Further investigations demonstrate that this reactor can also be used for wastewater treatment. Artificial toilet wastewater was successfully discolored for reuse as toilet flushing water. Lastly, a treatment test over 120 h demonstrates that the magnetically actuated flexible GPC cathode removes in situ deposits on its surface and requires low maintenance. The performance of the new electrode configuration is similar to that of the state-of-the-art polarity reversal system and does not shorten the electrode service life.

Phosphorus recovery from source-diverted blackwater through struvite precipitation

Phosphorus recovery from wastewater through struvite precipitation is becoming a promising strategy to both mitigate eutrophication risk due to excess phosphorus discharge into water bodies and alleviate the global phosphorus crisis by producing value-added fertilizer. However, the composition and quality of wastewater differ among regions and home to home. Source-diverted blackwater, especially concentrated blackwater collected from vacuum toilet systems, typically has a moderate phosphate-phosphorus (PO4-P) content, high ammonia-nitrogen (NH4-N) content, strong buffering capacity as a result of high alkalinity, and a high pH close to 9. Thus, concentrated blackwater is a good source for phosphorus recovery through struvite precipitation. In this study, we examined the feasibility of recovering phosphorus from concentrated blackwater through struvite precipitation by conducting batch experiments. The characterization of the struvite produced from concentrated blackwater was performed via X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) techniques. The metal contents of the struvite product were analyzed by inductively coupled plasma mass spectrometry (ICP-MS), and its purity was determined. A phosphorus removal efficiency exceeding 90% was achieved as a result of both an appropriate supersaturation ratio of struvite (>4.0) and a high initial pH. The high quality of the produced struvite was demonstrated by a purity of 94.9%, the Mg/P/N mass ratio of 10:12:4.7 based on the EDS analysis, and low heavy-metals contents. This feasibility study provides evidence for the efficacy of phosphorus recovery from concentrated blackwater through struvite precipitation in decentralized wastewater management systems.

A high-rate anaerobic biofilm reactor for biomethane recovery from source-separated blackwater at ambient temperature.

Anaerobic bioreactors for source-separated blackwater are mostly operated at low organic loading rates (OLRs) due to low biodegradability and the potential of ammonia inhibition. In this study, an anaerobic biofilm reactor having conductive carbon fibers as the media was investigated for the high-rate treatment of blackwater collected from vacuum toilets. The bioreactor was operated at different OLRs ranged from 0.77 to 3.01g COD/L-d in four stages for a total operating period of~250days. With the increase of OLRs, the specific methane production rate increased from 105.3 to 304.6ml/L-d with high methane content in biogas (75.5%-83%). The maximum methane yield was achieved at hydraulic retention time (HRT) of 15days. Highest organics and suspended solids removal (80%-83%) were achieved at 20-days HRT, while increased OLRs resulted in diminished removal efficiencies. The state variables, including pH, total ammonia nitrogen, short-chain volatile fatty acids, and soluble chemical oxygen demand, indicated the system had a great capability to withstand the high OLRs. Microbial community analysis revealed that the high performance might be attributed to direct interspecies electron transfer (DIET) facilitated by potentially electroactive bacteria (e.g., Syntrophomonas, Clostridium) and electrotrophic archaea (e.g., Methanosaeta and Methanosarcina species) enriched on the carbon fibers. PRACTITIONER POINTS: An anaerobic biofilm reactor was investigated for biomethane recovery from source-separated blackwater. Conductive carbon fibers were utilized as the media to stimulate enrichment of potentially electroactive methanogenic communities. The bioreactor was operated at ambient temperature for over 250days. High methane production rate and high-quality biogas were achieved at OLRs ranged from 0.77 to 3.01g COD/L-d. Microbial community analysis suggested direct interspecies electron transfer (DIET) between specific electroactive bacteria and electrotrophic archaea.

Enhancing biomethane recovery from source-diverted blackwater through hydrogenotrophic methanogenesis dominant pathway

Developments in energy recovery from decentralized wastewater streams provide a critical contribution to reducing greenhouse gases (GHGs) and promoting resource recovery. Source-diverted blackwater collected from vacuum toilets contains high solids organic content and is ideal for wastewater resource recovery. In the present study, an upflow anaerobic sludge blanket (UASB) reactor was operated under the mesophilic temperature condition (35 °C) to investigate methanogenic pathways and microbial development under high-loading conditions. We achieved the highest maximum organic loading rate (OLR) reported to date, at 4.1 kg chemical oxygen demand (COD)/m3/day (hydraulic retention time [HRT] 2.6 days), with a CODt removal efficiency of 84% (±5%), and methane production of 0.68 (±0.08) m3 CH4/ m3 reactor/day. This high efficiency resulted from domination of the H2/CO2 methanogenic pathway. Hydrogenotrophic methanogens with the genus Methanogenium were specifically enriched with the continuous treatment of blackwater.

Microbial community dynamics in anaerobic digesters treating conventional and vacuum toilet flushed blackwater

Decentralized wastewater treatment represents a promising sustainable option for future wastewater management. Blackwater collected from toilets contains high concentrations of organic matter, ideal for energy recovery using anaerobic digestion. Up-flow anaerobic sludge blanket (UASB) reactors treating conventional toilet (CT, 9 L water per flush) and vacuum toilet (VT, 1 L water per flush) blackwater with increments of loadings were successfully operated to steady state in three phases. The organic loading rates were maintained at comparable levels between the two reactors. The methanisation rates were 0.23–0.29 and 0.41–0.48 gCH4-COD/gfeedCOD in the CT and VT reactors, and the COD removal rates were 72% and 89%, respectively. The enriched microbial consortia and the community dynamics under different loading phases were compared. The rank abundance distributions and alpha-diversity showed that archaeal communities were predominated by mono-enrichments in both CT and VT reactors, while bacterial communities showed lower diversity in the VT reactor. Through principal coordinates analysis (beta-diversity), clear divergences of archaeal and bacterial communities between the CT and VT reactors were revealed, and the archaeal community developed at a slower rate than the bacterial community. The enriched archaea were hydrogenotrophic methanogens, Methanolinea in the CT reactor (56.6%), and Methanogenium in the VT reactor (62.3%). The enriched bacteria were Porphyromonadaceae in both CT (15.9%) and VT (13.4%) reactors, sulfate-reducing bacteria in the CT reactor, and Fibrobacteraceae in the VT reactor (13.8%). Links between enriched consortia and ammonia stress were discussed. Isotope fraction analysis of the biogas showed a slight shift from acetoclastic methanogenesis to hydrogenotrophic methanogenesis. A closer look into the predicted metagenomic functional profiles showed agreeing results, where hydrogenotrophic methanogenesis and fhs gene abundances were higher in the VT reactor. We demonstrated that different blackwater types enriched different microbial consortia, probably due to ammonia concentrations and sulfate loadings, which should be taken into consideration for practical applications.