Solar-driven Membrane Distillation Research Articles

Plasmonic titanium nitride-based membranes for solar-driven domestic wastewater distillation treatment

Membrane separation processes have emerged as efficient wastewater treatment technologies due to their reduced footprint, sustainability, cost-effectiveness, ease of operation, high filtration selectivity, and compliance with end-user requirements. Especially solar-driven membrane distillation, known for its low energy requirements and high pollutant rejection rates, has gained prominence as a potential wastewater treatment process that does not need external thermal energy input for water treatment. Herein, we demonstrate plasmonic titanium nitride nanoparticle-incorporated polymeric membranes for the solar-driven membrane distillation of domestic laundry wastewater, specifically focusing on SDBS (sodium dodecylbenzene sulfonate) surfactant removal. Under solar illumination, the plasmonic element within the polymeric matrix enables a localized heating effect driven by photothermal conversion in the proximity of the surface of distillation membranes, thereby producing a vapor pressure enough for water distillation across the hydrophobic membranes. The fabricated photothermal mixed matrix membranes showed incremental distillation flux and derived energy efficiency with the loading of titanium nitride nanoparticles under light illumination, reaching up to 0.47 kg/m2h and 29.79 %, respectively. The rejection rate against SDBS achieved up to 99 %. We also verified that its distinctive surface photothermal heating effect is associated with enhanced plasmonic absorption, surface thermal conductivity, and improved interfacial heat transfer of the membranes in the presence of plasmonic elements. The photothermal membranes proved that the permeate produced by photothermal distillation of real laundry wastewater fulfilled all required parameters for safe use as irrigation water, demonstrating its practical feasibility and viability of solar-driven distillation for domestic wastewater treatment and reuse.

Enhanced permeability and anti-wetting performance in solar-driven membrane distillation: Double-play of light-heat management and steric hindrance effect

Solar-driven membrane distillation (SDMD) opens up promising opportunities to solve freshwater supply predicament in remote and off-grid locations. However, such technology suffers from low permeability and poor resistance to wetting, which impedes its extensive application. Herein, we provided a novel MXene-coating photothermal layer design protocol by inducing dual-spacing channels between their nanosheets through a hydrazine-induced way. For one thing, the hierarchical MXene layer structure possessed unique merits in light-heat management, including improved light adsorption, high light-heat conversion efficiency, and inert heat dispersion, which facilitated vapor generation. For another, the dual-spacing channels relieved the resistance for vapor transportation by the wide region, while the high surface hydrophobicity and narrow region of the channels performed as double barriers to prevent sault invasion to resist wetting. Based on the synergistic effect, a preeminent photothermal efficiency of 94% with a vapor permeance of 1.66 kg/m2h under 1 sun illumination for 3.5% salinity solution was achieved, which was 1.4 times higher than that of pure MXene membrane. Meanwhile, the MXene foam membrane could fulfill a stable freshwater intake of 10.6 L m−2⋅day−1 for a household under natural sunlight illumination using Huanghai seawater. This research provides a delicate one-stone-two-birds strategy in constructing high permeability and anti-wetting SDMD membrane for potable water production.

Liquid-lubricated nanofibrous membrane for scaling mitigation in solar-driven membrane distillation

Recently, localized heating membrane distillation (MD) driven by solar energy has attracted much attention for desalination, but salt accumulation on membrane surface is still challenging and needs to be tackled properly. In this work, liquid-lubricated nanofibrous membrane was fabricated by dip-coating of liquid polydimethylsiloxane (PDMS) whilst maintaining the interconnected pore structure for scaling mitigation in solar-driven membrane distillation. Due to the nanoscale dynamic characteristics of liquid PDMS molecules, the coated membrane demonstrated excellent properties in minimizing salt scaling with prolonged lifespan by a factor of 2 or more. AFM measurement confirmed that decreased attachment strength of the liquid-lubricated surfaces plays a vital role in obstructing the adsorption of salt crystals on the surface. Furthermore, the interconnected structure of the nanofibrous mat endowed the membrane with a distillate flux of as high as ∼ 8.3 L·m−2·day−1 under natural light. These results evidenced that the as-prepared membrane can be taken as a potential candidate for durable MD applications by fully taking advantage of renewable solar energy.

Harnessing the power of neural networks for the investigation of solar-driven membrane distillation systems under the dynamic operation mode

Accurate modeling of solar-driven direct contact membrane distillation systems (DCMD) can enhance the commercialization of these promising systems. However, the existing dynamic mathematical models for predicting the performance of these systems are complex and computationally expensive. This is due to the intermittent nature of solar energy and complex heat/mass transfer of different components of solar-driven DCMD systems (solar collectors, MD modules and storage tanks). This study applies a machine learning-based approach to model the dynamic nature of a solar-driven DCMD system for the first time. A small-scale rig was designed and fabricated to experimentally assess the performance of the system over 20 days. The predictive capabilities of two neural network models: multilayer perceptron (MLP) and long short-term memory (LSTM) were then comprehensively examined to predict the permeate flux, efficiency and gain-output-ratio (GOR). The results showed that both models can efficiently predict the dynamic performance of solar-driven DCMD systems, where MLP outperformed the LSTM model overall, especially in the prediction of efficiency. Additionally, it was indicated that the accuracy of the models for the prediction of GOR can be significantly improved by increasing the size of the dataset.

Photothermal dual-layer hydrophilic/hydrophobic composite nanofibrous membrane for efficient solar-driven membrane distillation

Solar-driven membrane distillation (SDMD) had shown greater potential for desalination due to low energy consumption and convenient energy source compared to traditional membrane distillation. In this study, an innovative photothermal dual-layer hydrophilic/hydrophobic composite nanofibrous membrane consisting of polyacrylonitrile (PAN) nanofibrous membrane embedded with silver nanoparticles (Ag NPs) as hydrophilic layer and polyvinylidene fluoride (PVDF) nanofibrous membrane modified by polydimethylsiloxane (PDMS) as hydrophobic layer was constructed to be applied for efficient SDMD in desalination field. The photothermal particles loaded in the hydrophilic layer not only avoided clogging of the pores of the hydrophobic layer but also could be wetted completely to fully heat the feed fluid for achieving great photothermal conversion efficiency while the porous hydrophobic layer could prevent leakage of salt solute to attain high retention and vapor flux. Hence, the optimized composite nanofibrous membrane exhibited a high flux of 1.2 kg/m2/h with energy conversion efficiency of 75.2% and stable permeate conductivity (2.5 μs/cm) during 10 h SDMD test period (3.5 wt% NaCl salt feed) under 1 kw/m2 solar irradiation. Therefore, the photothermal dual-layer hydrophilic/hydrophobic composite nanofibrous membrane had promising application for desalination and provide a valuable inspiration in the development of membranes for membrane distillation.

Plasmonic Phenomena in Membrane Distillation.

Water scarcity raises important concerns with respect to human sustainability and the preservation of important ecosystem functions. To satisfy water requirements, seawater desalination represents one of the most sustainable solutions. In recent decades, membrane distillation has emerged as a promising thermal desalination process that may help to overcome the drawbacks of traditional desalination processes. Nevertheless, in membrane distillation, the temperature at the feed membrane interface is significantly lower than that of the bulk feed water, due to the latent heat flux associated with water evaporation. This phenomenon, known as temperature polarization, in membrane distillation is a crucial issue that could be responsible for a decay of about 50% in the initial transmembrane water flux. The use of plasmonic nanostructures, acting as thermal hotspots in the conventional membranes, may improve the performance of membrane distillation units by reducing or eliminating the temperature polarization problem. Furthermore, an efficient conversion of light into heat offers new opportunities for the use of solar energy in membrane distillation. This work summarizes recent developments in the field of plasmonic-enhanced solar evaporation with a particular focus on solar-driven membrane distillation applications and its potential prospects.

Experimental validation of membrane distillation unit coupled with direct contact membrane using solar energy

Abstract This paper presents an experimental validation of membrane distillation unit using solar energy which is coupled with direct contact membrane, which is placed at Kairouan University, Tunisia (35 N, 10 E) and tested on several sunny days. This unit is located as part of a cooperation project research and development between German Institute for Solar Energy Systems and Tunisian Electromechanical Systems Laboratory named: Solar driven membrane distillation for resource efficient desalination in remote areas. A theoretical model investigation as well as experimental is carried out. A mathematical model based on heat and mass transfers of the membrane distillation unit has been presented in this paper. The obtained global model of the membrane distillation unit has been converted to a set of algebraic system of equations to render them ordinary. To compare the experimental and numerical data of the mathematical model of the membrane distillation unit an example of the validation process that has been presented to assess the credibility of the obtained numerical model of membrane distillation unit, a laptop simulation program based on the global model of the unit is simulated by C++ software to solve the model of solar irradiation and all temperature on the journal productivity of the membrane distillation unit. It was shown by this study that the global mathematical model of the unit is able to predict accurately the trends of the thermal characteristic of the membrane distillation unit.

Nickel Chalcogenide Nanoparticles-Assisted Photothermal Solar Driven Membrane Distillation (PSDMD)

Developing photothermal solar driven membrane distillation (PSDMD) is of great importance in providing fresh water for remote off-grid regions. The production of freshwater through the PSDMD is driven by the temperature difference between feed and distillate sides created via the addition of efficient photothermal nanostructures. Here we proposed nickel sulfides and nickel tellurium nanoparticles (NPs) to be loaded into the polymeric membrane to enhance its performance. Ag and CuSe NPs are also considered for comparison as they are previously used for membrane distillation (MD). Our theoretical approach showed that all of the considered NPs increased the temperature of the PVDF membrane by around a few degrees. NiS and NiTe2 NPs are the most efficient solar light-to-heat converters compared to NiTe and NiS2 NPs due to their efficient absorption over the visible range. PVDF membrane loaded with 25% of NiCs NPs and a porosity of 32% produced a transmembrane vapor flux between 22 and 27 L/m2h under a 10-times-amplified sun intensity. Under the same conditions, the PVDF membrane loaded with CuSe and Ag NPs produced 15 and 18 L/m2h of vapor flux, respectively. The implantation of NPs through the membrane not only increased its surface temperature but also possessed a high porosity which provided a higher distillation and energy efficiency that reached 58% with NiS NPs. Finally, great agreement between our theoretical model and experimental measurement is obtained.

New Materials and Phenomena in Membrane Distillation

In recent decades, membrane-based processes have been extensively applied to a wide range of industrial processes, including gas separation, food industry, drug purification, and wastewater treatment. Membrane distillation is a thermally driven separation process, in which only vapour molecules transfer through a microporous hydrophobic membrane. At the operational level, the performance of membrane distillation is negatively affected by wetting and temperature polarization phenomena. In order to overcome these issues, advanced membranes have been developed in recent years. This review, which focuses specifically on membrane distillation presents the basic concepts associated with the mass and heat transfer through hydrophobic membranes, membrane properties, and advances in membrane materials. Photothermal materials for solar-driven membrane distillation applications are also presented and discussed.

Construction of electrospinning Janus nanofiber membranes for efficient solar-driven membrane distillation

Low permeate flux and membrane fouling are major limiting factors for solar-driven membrane distillation (SDMD) industrial applications. Here, we report a Janus nanofiber membrane with electrospinning CB/PVA as hydrophilic photothermal layer and electro-blown spinning PTFE nanofiber membrane as hydrophobic layer, which exhibits excellent photothermal conversion performance, anti-fouling performance and durability. The hydrophilic photothermal layer simultaneously captures solar energy and performs photothermal conversion, repels chemical/oil-based pollutants and promotes steam flow. At the same time, the low surface of PTFE layer can guarantee the long-term durability of the composite nanofiber membrane. Under the light intensity of 1 kW·m−2, the Janus nanofiber membrane can obtain a permeate flux of 1.05 L·m−2·h−1 and a salt rejection of > 99.99 %. And the photothermal conversion efficiency reached 71.4 %. More importantly, the permeate flux of the Janus nanofiber membrane was still stable at 1.035 L·m−2·h−1 after continuous treatment of pollutant-containing feed solutions for 60 h, and the salt rejection was > 99.98 %. This indicates that the efficient photothermal conversion performance and anti-fouling properties make the CB-PVA/PTFE photothermal nanofiber membrane promising for generating fresh water from feed solutions containing various pollutants in SDMD systems.

NiSe and CoSe Topological Nodal-Line Semimetals: A Sustainable Platform for Efficient Thermoplasmonics and Solar-Driven Photothermal Membrane Distillation.

The control of heat at the nanoscale via the excitation of localized surface plasmons in nanoparticles (NPs) irradiated with light holds great potential in several fields (cancer therapy, catalysis, desalination). To date, most thermoplasmonic applications are based on Ag and Au NPs, whose cost of raw materials inevitably limits the scalability for industrial applications requiring large amounts of photothermal NPs, as in the case of desalination plants. On the other hand, alternative nanomaterials proposed so far exhibit severe restrictions associated with the insufficient photothermal efficacy in the visible, the poor chemical stability, and the challenging scalability. Here, it is demonstrated the outstanding potential of NiSe and CoSe topological nodal-line semimetals for thermoplasmonics. The anisotropic dielectric properties of NiSe and CoSe activate additional plasmonic resonances. Specifically, NiSe and CoSe NPs support multiple localized surface plasmons in the optical range, resulting in a broadband matching with sunlight radiation spectrum. Finally, it is validated the proposed NiSe and CoSe-based thermoplasmonic platform by implementing solar-driven membrane distillation by adopting NiSe and CoSe nanofillers embedded in a polymeric membrane for seawater desalination. Remarkably, replacing Ag with NiSe and CoSe for solar membrane distillation increases the transmembrane flux by 330% and 690%, respectively. Correspondingly, costs of raw materials are also reduced by 24 and 11 times, respectively. The results pave the way for the advent of NiSe and CoSe for efficient and sustainable thermoplasmonics and related applications exploiting sunlight within the paradigm of the circular blue economy.

PPy nanotubes-enabled in-situ heating nanofibrous composite membrane for solar-driven membrane distillation

• A superhydrophobic photothermal composite membrane was demonstrated. • PDMS binder enhanced the structural stability and hydrophobicity of dual-layer membrane. • The PPy NTs/PVDF nanofibrous composite membrane showed outstanding solar-driven DCMD performance. Solar-driven membrane distillation is an emerging desalination technology with low energy consumption and efficient vapor collection, but the low vapor flux and limited efficiency hinders its practical application. Here, we demonstrated an innovative photothermal composite membrane by depositing polypyrrole nanotubes (PPy NTs) as functional coating layer onto an electrospun poly (vinylidene fluoride) (PVDF) nanofibrous support layer via vacuum filtration stabilized by adhesive polydimethylsiloxane (PDMS). According to the proportional relationship between the fibrous materials diameter and the pore size, the large-scale surface gaps between adjacent nanofibrous could be replaced by smaller pores from the PPy NTs layer on the PVDF substrate. The PDMS with low surface energy was used as an adhesive to in-situ solder the loose PPy NTs together as well as to enhance the adhesion between the PPy layer and the PVDF substrate to form an integrated and robust dual-layer composite membrane with superhydrophobicity, high porosity and controllable pore size. By exploring the optimal match of PPy loading and PDMS concentration, the solar-to-thermal performance and structural stability could be significantly improved. With 1 sun irradiation, the selected composite membrane thus showed a high-water vapor flux of 1.3 kg/(m 2 ·h) and a photothermal conversion efficiency of 81.6%, specially, the membranes maintained stable permeate conductivity during the 10 h solar-driven DCMD seawater desalination test. These results indicated that the photothermal dual-layer superhydrophobic composite membranes has promising potential for desalination with the help of sustainable energy sources such as sunlight.

Janus Photothermal Membrane as an Energy Generator and a Mass-Transfer Accelerator for High-Efficiency Solar-Driven Membrane Distillation.

Membrane distillation (MD) is an emerging membrane-based evaporation technology with great promise for the desalination and separation industries. However, its widespread application still depends on substantial development to increase the distillation flux, reduce the energy consumption, and extend the lifespan of the membrane. Herein, we report for the first time the integration of multiple functions, that is, energy-saving, flux-enhancing, and anti-fouling properties, into a single membrane. Such a membrane was fabricated by coating the top surface of a poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) nanofibrous mat with photothermal and hydrophobic graphitic carbon spheres and subsequently coating the bottom surface with a hydrophilic polydopamine layer, yielding a novel Janus photothermal membrane (JPTM). Owing to the high photothermal efficiency and accelerated mass transport across the membrane, the JPTM demonstrated an excellent desalination performance when assembled into a solar-driven MD system, with a distillation flux of 1.29 kg m-2 h-1, which is 10 times higher than that of the conventional un-modified PVDF-HFP membrane, requiring only 1 kW m-2 solar illumination as the energy input.

Plasmonic Titanium Nitride Nano-enabled Membranes with High Structural Stability for Efficient Photothermal Desalination.

Herein, we demonstrate the desalination performance of a solar-driven membrane distillation (MD) process, where upon light illumination, a highly localized heating of plasmonic titanium nitride nanoparticles (TiN NPs) immobilized on a hydrophobic membrane provides the thermal driving force for the MD operation. The engineered TiN photothermal membrane induces vapor generation directly at the feed-membrane interface upon solar irradiation, thereby eliminating the need to heat the entire bulk feed water. The results indicate that the average vapor flux through the TiN photothermal membrane without any auxiliary feed heating was recorded as 1.01 L m-2 h-1, which corresponds to the solar-thermal efficiency of 66.7% under 1 sun solar irradiance. The superior performance of the photothermal MD process is attributed to the broadband optical absorption and excellent light-to-heat conversion properties of the plasmonic TiN NP layer, which enabled efficient interfacial water heating at the membrane surface and increased the net driving force for vapor transport. Results also reveal the high mechanical stability of the TiN photothermal coating layer during long-term photothermal MD operations. We believe that the TiN photothermal membranes fabricated using a relatively inexpensive and nontoxic material via the simple technique with high stability and photothermal conversion efficiency will provide a path forward for developing the solar-driven MD applications.

Economic Design of Solar-Driven Membrane Distillation Systems for Desalination

Solar-driven membrane distillation (SDMD) for desalination is a feasible method to solve water and energy resource issues. The design and operation of SDMD is different from continuous and steady state processes, such as common chemical plants, due to the intermittent and unpredictive characteristics of solar radiation. Employing the steady state and dynamic simulation models developed on the platform of Aspen Custom Modeler®, this paper presents a two-stage design approach for the SDMD systems using different types of membrane distillation configurations, including AGMD (air gap MD), DCMD (direct contract MD) and VMD (vacuum MD). The first design stage uses the steady state simulation model and determines equipment sizes for different constant-value solar radiation intensities with the objective of minimizing total annual cost. The second design stage is implemented on the SDMD systems with process control to automatically adjust the operating flow rates using the dynamic simulation model. Operated with the yearly solar radiation intensity of Taiwan, the unit production costs (UPCs) of the optimal SDMD systems using AGMD, DCMD, and VMD are $2.71, 5.38, and 10.41 per m3 of water produced, respectively. When the membrane unit cost is decreased from $90/m2 to $36/m2, the UPC of the optimal solar-driven AGMD system can be reduced from $2.71/m3 to $2.04/m3.

Experimental and economic analysis of a permeate gap solar-driven membrane distillation system

Experimental and economic analysis of a permeate gap solar-driven membrane distillation system

Fe3O4/PVDF-HFP photothermal membrane with in-situ heating for sustainable, stable and efficient pilot-scale solar-driven membrane distillation

Solar-driven membrane distillation (SDMD) is a newly developed desalination technique with promising potential to address the global shortage of freshwater resources. However, this technique still suffers from tedious and high cost synthesis process for the photothermal composite membrane, and low conversion efficiencies for photothermal energy. Herein, we prepared a Fe3O4/polyvinylidene fluoride-co-hexafluoropropylene (Fe3O4/PVDF-HFP) absorber by vacuum-assisted filtration, which exhibited strong interfacial adhesion due to coordination between Fe3O4 nanoparticles and PVDF-HFP nanofibers, thus showing excellent performance for SDMD. Benefiting from the excellent absorption of solar energy by Fe3O4 nanoparticles and the high porosity of Fe3O4/PVDF-HFP membrane, the transmembrane temperature was increased and the transmembrane resistance of vapor was reduced significantly. The photothermal membrane showed a permeate flux of 0.97 kg m−2 h−1 with a salt rejection rate of 99.99% under 1 kW m−2 solar irradiation, and the photothermal conversion efficiency is among the highest (53%) reported so far. Furthermore, such a composite membrane worked stably in a pilot-scale system and demonstrated a 21.99 kg m−2 h−1 which is 11% higher than the solar-free system, thus manifesting its great prospect for practical applications.

Fabrication and Performance Evaluation of Integrated Solar-Driven Membrane Distillation System with Serpentine-shape of Flat Plate Solar Collector for Seawater Desalination

Solar-powered membrane distillation (SPMD) system has gained its popularity in desalination application for past decade credit to the system efficiency in producing pure water and the utilization of renewable energy. However, most of the past SPMD works used commercial solar thermal collector (STC) as the thermal energy supply to the feed solution and the study only focused on the performance of the system in terms of flux and salt rejection. In this work, a self-made flat plate solar collector (FPSC) with the serpentine-shape pipe was designed and fabricated to study the effect of the STC towards the membrane performance. Before testing, a simulation work of the fluid flow inside the serpentine-shape pipe of the FPSC was analyzed using NX 10.0 computer-aided design simulation. After that, the efficiency of the self-made FPSC system was tested directly to sunlight in order to identify the maximum irradiance and the temperature of the feed solution. Due to the fluctuation of solar irradiance, the experimental setup of the SPMD system was tested using a solar simulator, and the performance was compared with the membrane distillation (MD) system without integration with FPSC system. Based on the simulation data, it can be concluded that the heat losses across the pipe are due to the slower fluid velocity and sudden pressure drop, which attributed to centripetal force and pressure differences. Meanwhile, the outdoor evaluation data showed that the temperatures of collector and water inside the feed tank could reach up to 84°C and 64°C, respectively when the maximum irradiance of 938 W/m2 was applied. For the performance evaluation between with and without the self-made FPSC system, it can be seen that only marginal difference can be observed for the permeate flux and salt rejection with an average difference of 6.06% and 1.29%, respectively.

Experimental parametric study of membrane distillation unit using solar energy

Solar distillation is a promising method for the supply of freshwater to rural communities because water is a vital factor for the survival of humans. Therefore potable water demand is increasing continuously because of the industrial, population and, fast agricultural applications. Nowadays, different techniques for water purification have been created to provide freshwater from salty and polluted water such as solar distillation method to distill brackish/saline water. This work presents an experimental study of the solar membrane distillation unit which is coupled with direct contact membrane (DCMD) which located at Kairouan University. The system is installed as part of a cooperation project research and development between Tunisian Electromechanical Systems Laboratory and German Institute for Solar Energy Systems entitled: Solar driven membrane distillation for resource efficient desalination in remote areas. For the heating of the hot feed water stream, a heat exchanger is included which can be operated with any external heat supply between 60 and 80 °C. For the cooling of the cold feed water stream, no heat exchanger is included. The cooling can be provided over the intake of cold feed water. The effect of solar irradiation and all temperature on the journal productivity of the unit has been presented.

An integrated, solar-driven membrane distillation system for water purification and energy generation

Utilising solar thermal energy for membrane distillation desalination represents a green and sustainable solution for building environments in regions with a high correlation between water shortage and high solar irradiance. Today’s solar thermal-driven membrane distillation systems are designed with physically separated solar thermal collectors (e.g. flat plate or evacuated solar thermal collectors) and membrane distillation modules. In these systems, a thermal storage tank, a heat exchanger, and complex plumbing arrangements are required to control the heat and mass transfer between the solar collectors and the membrane distillation unit(s). Due to their high complexity and high capital/operational costs, these systems are rarely installed in buildings. To overcome these weaknesses, the present work conducts an experimental and numerical feasibility study of an integrated solar membrane distillation prototype (with the membrane distillation modules built directly into the evacuated solar tubes) for both potable water and/or thermal energy production. To the best of the authors’ knowledge, this elegant combination of an evacuated tube solar collector and a membrane distillation unit represents an innovative approach which couples two well-developed technologies into an efficient, yet relatively low cost, hybrid energy-water production system.Our experimental results revealed that 4–10 L per square meter of membrane area per hour of permeate flux is achievable when the feed temperature ranges from 50 to 70 °C, achieving a salinity level of 10–200 ppm from a 35,000 ppm (e.g. the salinity of seawater) feed. It was found that a system with a solar absorbing area of 1.6 m2 integrated with ∼0.2 m2 of membranes can produce ∼4 L of drinkable water and ∼4.5 kWh of heat energy (at 45 °C) per day (with an average daily solar exposure of 4 kWh/m2). We envision that this design could be beneficially deployed on the rooftops of residential and commercial buildings—buildings which require a continual supply of both potable water and domestic hot water.