Wasting water in toilets flushing is the largest source of water wasting through the use of old siphon boxes. It occupies the first place in domestic consumption. This study reports two trial approaches for optimizing the flushing system design. The first one employs a rotatable blade in the bottom of the bowl. This blade pushes materials in the bowl to cross the trapway thus; less toilet flushing water can be used. The second approach depends on using a rotatable trapway such that it can be tilted down to enable discharging materials in the bowl directly by its gravity. This facilitates the discharge and reduces the flushwater amount which is just used to overcome friction and to clean the passage. Both are mechanical systems actuated by an external pedal mechanism that triggers the water flushing valve system in the same time. Real experiments revealed that the first approach needs more developments to work efficiently. Experiments with restricted conditions revealed that with using the rotatable trapway system approach, less than one liter of flushwater is sufficient. The required flushwater flow rate can be obtained directly from the water supply system without the need to install siphon boxes. This study can help more to design better water saving systems. Keywords Water saving · Toilet flushing · Pedal operated flushing system · Fixed trapway · Rotatable trapway Introduction is the total volume of water generated from washing food, clothes and dishware, as well as from bathing, but not from According to surveyed studies, toilet flushing consumes toilets. Greywater accounts for approximately 65% of the between 25 and 35% of the total house consumption so; it wastewater produced in households with u fl sh toilets (Tilley occupies the first place in domestic consumption. A tradi - et al. 2014 pp. 10–11). The organic matter contributed per tional lavatory bowl has a fixed S-shape outlet passage by person per day in domestic wastewater is approximately which its exit level is elevated than the entrance level to 110 g of suspended solids and 90 g of biochemical oxygen guarantee sealing. The level difference is called seal depth. demand (Mark et al. 2014) Without sealing, generated and accumulated awful gases are Numerous proposal trials have been made to minimize leaked from the sanitary system inside toilets. water wasting in toilet flushing. Proposed solutions in sur - Excreta consists of urine and feces that is not mixed with veyed studies can be generally categorized into three groups any flushwater. Blackwater is the mixture of urine, feces and as follows: flushwater along with anal cleansing water (if water is used for cleansing) and/or dry cleansing materials. Depending on (1) Using alternative water source such as greywater or diet, each person produces approximately 50 l per year of seawater instead of fresh water fecal matter. Fresh feces contain about 80% water. Greywater (2) Optimizing the siphon and flushing system design (3) Using an alternative flushing way rather than using water such as vacuum. * Roubi A. Zaied email@example.com; firstname.lastname@example.org The use of reclaimed water, in one form or another, has long been a common practice all over the world in times of Department of Industrial Engineering, Faculty drought. When and where water resources are scarce, people of Engineering, Northern Border University, Arar, Saudi Arabia will have no choice but to use as little water as possible, and will naturally save the less dirty water for toilet flushing, Department of Mechanical Engineering, Faculty of Engineering, Benha University, Benha, Egypt Vol.:(0123456789) 1 3 53 Page 2 of 10 Applied Water Science (2018) 8:53 floor cleansing, etc.(Tang et al. 2007 p 28). Hranova (2010) is not sewage but water in the trap seal. Then, by simplifying has studied on-site greywater treatment and reuse with the mixture of sewage and water in the trap seal as the third respect to toilet flushing. Results show that the toilet flush- phase with high viscosity, a three-phase flow was simulated. ing alternative becomes cost effective at larger population Modeling the flushing process helps studying the process densities. Abu Rozaiza (2002), Al Mamun et al. (2014) and variables and performing sensitivity analysis through simu- Suratkon et al. (2014) studied using greywater resulting from lation. Flushing process involves turbulence and nonlineari- ablution facilities in toilet flushing. Recycling of greywater ties especially; more than one fluid and semisolid materials is a good treatment of potable water wasting problem but, it are involved. In this work, three phases are considered. In is better to make prevention (Zaied 2016). Using seawater as the initial phase, the bowl is filled with fresh water and the an alternative water source is limited to just coastal zones. mass of the water confined in the bowl (M ) is calculated as: bl Ng (2015) states that seawater, with minimal treatment, can M = V bl w bl (1) be used for toilet flushing reducing the demand for freshwa- where ρ is the water density, V is the bowl volume. w bl ter in coastal cities. But seawater flushing requires a separate The second phase is considered during the toilet use; i.e. network of mains and, therefore, a greater capital cost and filling with excreta and cleansing water. During this phase, wastewater recycling has a higher ongoing treatment cost. urine and cleansing water rapidly mix with water in the More engineering work has been achieved to optimize bowl. Soon, semi solid feces displace an equal water weight toilet flushing system design. An et al. (2012) have investi- from the bowl to the sewer but, they take long time to absorb gated toilet flushing performance with volume of fluid model water, change their density and consequently float or sink. to obtain the basic design data for the development of high- The third phase starts when fresh flushwater flows down efficiency toilets. Suh et al. (2009) proposed a flexible trap into the bowl, displaces the blackwater and replaces it. This system that discharges feces directly from bowl hence, saves phase is our concern to determine and optimize the flushwa- water to less than 4.5 kg. Their flexible-trap toilet uses a ter flow. As blackwater is a multi-phase media contains liq- straight trap and cover without the traditional trapway. uids, semisolid and/or solid materials, any water added to the Watari et al. (2013) studied the 4 l toilet with new flushing bowl displaces an equal water volume from the bowl to the technology in Japan and evaluated its drainage characteristic sewer. Because of mixing with blackwater, more flushwater and the drainage-transportability. An et al. (2014) conducted is required to dilute the blackwater till replacing it. Because a comparative analysis of the flushing and water-saving per - of the difference in densities of liquids and semisolid (and/or formances of a flexible-trapway toilet. The flushing perfor - solid) wastes, they do not move in phase during the flushing mance of the toilet was quantized through the development process. Relative velocities cause internal friction between of a measurement method to measure the accumulated flow the semisolids and the liquid which requires more energy for rate and mass flow rate of the trapway with respect to time. flushing. Flushwater must have sufficient mechanical energy to The flexible-trapway toilet yielded stable flushing and good drive out the wastes from the bowl. There are two key factors filth emission performance with an inflow of 4 kg. However, affect the required amount of flushwater to drive out semi- the fixed-trapway toilet failed to generate a steady siphon solids from the bowl. These factors are the density and total with an inflow of 5 kg. weight of the semisolid and/or other solid wastes. When a low This study deliberates two trial approaches for optimizing specific weight-material enters the bowl from its right side in the flushing system design. Both are mechanically actuated the front zone it floats (Fig. 1). Hence, to overcome the buoy- by an external pedal mechanism. The pedal mechanism trig- ancy force, more energy is required to push it down to pass gers, in synchronization, the water flushing valve system. the lower zone. Improperly, the flushwater usually enters the It is applicable for ground base lavatory and seat lavatory systems. Modeling of flushing process Flushing flow of the siphon jet toilet belongs to the fluid flow problem with free surface whose shape and location would vary intricately and continually (Wang et al. 2011). Hu et al. (2014) used the finite volume method (FVM) to discrete three governing equations in space and time. The realizable turbulence model was chosen as the viscous model to treat the fluid flow with large bending curvature wall. First, a two-phase flow was simulated on the assumption that there Fig. 1 A schematic diagram of wastes path in the lavatory bowl 1 3 Applied Water Science (2018) 8:53 Page 3 of 10 53 bowl from its boundary wall and its stream does not enter as one stream having the same cross sectional area of the bowl. F = m g 1 − (6) z s Thus, light wastes resist flushing by escaping to the center. In this case, after passing sufficient amount of flushwater to gen- where F is the buoyancy force (upward), W is the weight B s erate a siphon effect, it causes the level of material in the bowl of semisolid material (downward), M is the mass of the to be lowered from Z to Z (Fig. 1a) and so, flushing occurs. 1 2 submerged semisolid material, ρ ρ is the densities of the f, s Then to secure sealing, additional flushwater is needed to fill fluid in the bowl and the semisolid material, respectively, g the bowl to Z level. 1 is the gravitational acceleration, F is the net vertical force High specific weight-material sinks down to the lower zone acting on the semisolid material. and more energy is required to overcome the gravity force to Based on physics principles published by Halliday: A push it up to pass the back zone (Fig. 1b). To estimate the totally submerged object that is less dense than the fluid in us fl hwater amount required to overcome inertia of semisolid which it is submerged experiences a net upward force and a wastes, the flushwater is considered free falling fluid body totally submerged object that is denser than the fluid sinks accelerates under the influence of gravity. Sufficient amount (Halliday 1999 p. 467). So, net F is upward for > 1 and of flushwater must be more than the blackwater in the bowl downward for < 1 ). Now, the net vertical force is to be (V ) to replace it. The extra water amount is used to provide bl kinetic energy to overcome inertia and friction forces besides integrated with respect to the vertical travel to estimate the an additional part to clean the bowl walls. The last part is used work required against the buoyancy force to move the semi- to overcome adhesion forces between the sticky feces and the solid material from z to z . The energy required to overcome 1 2 bowl walls. the semisolid inertia (E ) is estimated as follows: When the air resistance is negligible, the acceleration of the z z 2 2 body equals the gravitational acceleration, and acceleration in E = f dz = m g 1 − dz i z s (7) any horizontal direction is zero. The major energy loss is due z z s 1 1 to friction between the moving water and pipe wall; however, energy losses also occur from flow disturbance (Mark et al. 2014). If a flushwater of mass m drops from a level of z to fw 0 E = m g 1 − (z − z ) (8) i s 2 1 the upper level of the front zone (z in Fig. 1a) then, by neglect- ing friction, its kinetic energy (E ) can be estimated as: These equations are applicable for both cases (Fig. 1a, b) E = m g(z − z ) with noticing the positions of z to z for each case. (2) k fw 0 1 1 2 By equating the energies in Eqs. 2 and 8, the relative The energy required to move the semisolid material to mass of the flushwater (m ) needed to overcome inertia of the back zone is intended to be the integration of forces with fw semisolid wastes (m ) can be approximated as: respect to movements in their two directions as, it is two dimensional flow, i.e.: fw f = 1 − (z − z )∕(z − z ) (9) 2 1 0 1 s s x z 2 2 Flushing Energy = f dx+ f d x z z (3) Some typical data are used to realize the effect of relative x z 1 1 elevation of flushwater source (Z − Z ) and relative densi- 0 1 The vertical inertia force component is our concern in this ties of blackwater and semisolid (ρ /ρ ) on relative mass of f s work thus, it is the only force modeled. The buoyancy force flushwater required (m /m ) as shown in Fig. 2. Data pre- fw s acting on a body of uniform density immersed in a fluid is sented in this figure are based on setting (Z − Z ) = 10 cm 2 1 equal to the weight of the fluid displaced by the body and it for ρ /ρ > 1 and (Z − Z ) = 20 cm for ρ /ρ <1 as depicted f s 2 1 f S acts upward through the centroid of the displaced volume. For in Fig. 1. It can be concluded that the relative mass of flush- floating bodies, the weight of the entire body must equal to the water required highly depends on the potential energy of the buoyant force, which is the weight of the fluid whose volume falling flushwater and the relative densities of blackwater is equal to the volume of the submerged portion of the floating and semisolid. For instance, based on the supposed values body (Ҫengel and Cimbala 2014). Hence, the vertical forces of Z and Z , if the flushwater tank or source is elevated 1 2 acting on the immersed semisolid material during its crossing only 10 cm above the level of water in the bowl, the relative from left to right (Fig. 1) are: mass of flushwater is equivalent to ρ /ρ for light semisolids f s whereas but, it is multiplied 10 times for dense semisolids F ↑= g when ρ /ρ changes from 1.5 to 2. This means that dense B f (4) f s semisolids needs more flushing energy because their upward travel against gravity in the bowl is longer. W ↓= m g (5) s s 1 3 53 Page 4 of 10 Applied Water Science (2018) 8:53 Fig. 2 Effect of flushwater source elevation and relative densities on relative mass of flushwater required Fig. 3 A schematic diagram of a proposed bowl-rotatable blade sys- Fig. 4 Application of bowl-rotatable blade on a ceramic seat tem return spring. The pedal actuates the flushing system in the same time when it is depressed through a lever mechanism Methodology consisting of two steel levers and a plastic pipe. There is a time shift such that the pedal movement releases the flush- First trial water when the blade rotation angle exceeds 180°. Two sorts of blades are tried (both are made from aluminum); The first trial to decrease flushwater wasting depends on a solid one and another perforated one as shown if Figs. 5 using a mechanical system to push out waste materials and 6, respectively. The solid blade can sweep urine in from the bowl. A rotatable blade is installed in the bot- addition to feces where perforated blade is intended to tom of the bowl that pushes out materials at onset of the flush feces only to decrease the required actuation force flushing action thus; less toilet flushwater can be used. by decreasing the total exposed area. This proposed mechanism is designed and implemented Dry run experiments are used to check the mechanical on a common lavatory seat. Installation of the mechanism system functionality and other wet runs are done to investi- requires only some holes in the ceramic seat. The key idea gate the real functionality and flushwater saving. of the system is illustrated in Fig. 3 and the actual applica- tion is pictured in Fig. 4. Second trial The system is composed of an external pedal (plastic pad and steel lever) drives a mechanism that pulls the In traditional lavatory systems, flushwater drives blackwater string to turn the blade counterclockwise and the return till it passes the back zone and enters the exit pipe. In the spring retain the blade to its initial position when the pedal study of An et al. (2014), weight of some water from the is released. The string is a threaded stainless steel wire and flushing box is used in tilting down a U -shape trapway. It the blade and its bracket are made from aluminum to ease has a spring located in large corrugated flexible tube that their manufacturing. For real application, stainless steel contracts prior to flushing due to the elastic strain energy of is recommended for all parts including the screws and the the spring to maintain a certain slope. A certain amount of 1 3 Applied Water Science (2018) 8:53 Page 5 of 10 53 Fig. 5 Steps of moving down solid rotatable blade Fig. 6 Steps of moving down perforated rotatable blade water flows into the corrugated flexible tube once flushing begins. Once the mass of the inflowing water becomes larger level of the trapway pipe; any more water passes directly than the elastic energy of the spring, the corrugated flexible to the exit pipe. The flushing gate valve starts to open and tube relaxes to the bottom, making the slope of the trapway release water after certain rotation angle of the pedal. The horizontal and facilitating the discharge of waste. tension spring between the lavatory base and the trapway In our second approach, this idea is developed and simpli- pipe retains it to the ordinary position when the pedal is fied. The trapway is rotatable by which it can be tilted down released after depressing it. In extreme discharge position to enable discharging blackwater from the bowl directly by (Fig. 7a), the trapway pipe is tilted down and the flushing gravity. Hence, flushwater is just used to overcome friction vale is at its maximum opening position, thus blackwater and to clean the passage. The method is illustrated in Fig. 7 moves to left side and running flushwater helps to overcome and the experimental setup for the proposed system is pic- friction and cleans bowl and the whole trapway. The timing tured in Fig. 8. A mechanism is installed inside the seat diagram of the system is illustrated in Fig. 9 that depicts between the bowl and the exit pipe fitting. Angles α , β the synchronization between the pedal movement, trapway O O and γ are rotation angles, from vertical axes in the ordi- rotation, flushwater valve and flushwater flow. Table 1 sum- nary position, of the Pedal, trapway pipe and water valve marizes the specifications of the experimental apparatus. handle, respectively, where α , β and γ are there angles in To analyze the system performance, dices of internal f f f the extreme position. melon shell is found suitable to simulate feces regarding The trapway pipe is installed between two flexible tubes its density. Its specific weight is 0.9 (150 g has a volume of and rotates about fixed hinge. The pedal mechanism, when 167 cm ) and each dice is about 3 g. Figure 10 shows the depressed, pushes the trapway pipe down; the right side flex- melon dices used in the experiment inside the measuring cup ible tube expands and the left side flexible tube contracts. and inside the bowl while running the experiments. The gate valve handle is linked to the pedal; the vale is nor- Two different quantities of dices are used in the experi - mally closed under the weight of water column over it in ments; 99 and 150 g. In each experiment run, the siphon ordinary position (Fig. 7a). In this position, when blackwater box is filled with definite amount of flushwater e.g. 500 ml is filling the bowl and its level does not exceed the bottom or 1000 ml to be discharged as one time batch instead of 1 3 53 Page 6 of 10 Applied Water Science (2018) 8:53 Fig. 7 A schematic diagram of a proposed rotatable trapway system 1 3 Applied Water Science (2018) 8:53 Page 7 of 10 53 amounts of water filled in the siphon box. Each experiment is repeated 5 times and the average values are considered. Table 2 summarizes the obtained results of different 12 experiments. Experimental results and discussion When the proposed bowl-rotatable blade system was testes by dry run it functioned well but, when the bowl is filled with water and melon dices it functioned badly. The melon dices tend to escape from the blade side to wide gap and some dices are stuffed between the blade and the bowl wall. In sometimes, the blade was stuck in its vertical position and, therefore, the design needs major revising. No more trials are made to redesign bowl-rotatable blade system. The sec- ond approach of rotatable trapway seems more promising. Analyzing the data in Table 2 confirms that the rotatable trapway has superior performance in water saving. To be noticed that according to the mentioned procedure of the experiments, the amounts of flushwater used do not express Fig. 8 The experimental setup for the proposed rotatable trapway sys- truthfully the real amount of water required to replace the tem blackwater. The g fi ures in this table give only a comparative aspect of flushing in the two cases of fixed trapway and the rotatable one. For example, the last figure in the Table (98%) controlling the time of flow. In each run, the bowl is filled means that 2 l of flushwater when being discharged from with fresh water and then dices are added. For fixed trapway the siphon box at flowrate of 10.7 l/min could displace out experiments, flushing valve only is opened to discharge the 98% of the dices to the exit pipe. But in real application siphon box water to the bowl without depressing the pedal more water is required to replace all the blackwater in the and, for the rotatable trapway the pedal is depressed to rotate bowl. Also 0.5 l of flushwater at flowrate of 3.2 l/min could the trapway pipe and to open the flushing gate vale simulta- displace out 100% of the dices to the exit pipe of a tilted neously. Four experiment groups are implemented for dif- trapway. Really, additional amount of at least 365 ml is ferent trapway type and dices weight mutually at suitable Fig. 9 Timing diagram of the proposed pedal actuated-rotata- ble trap way system 1 3 53 Page 8 of 10 Applied Water Science (2018) 8:53 Table 1 Specifications of the experimental apparatus Parameter Value Parameter Value Capacity of the bowl 365 mL Capacity of the siphon box 10 L Minimum pressure Head (from the Lavatory base To the 50 cm Maximum downward vertical travel of the pedal 5 cm siphon box) Range of pedal rotation angle (α − α ) 120°–150° Nominal trapway diameter 8 cm O f Range of trapway pipe rotation angle (β − β ) 50°–120°Seal depth 2 cm O f Range of water valve-handle rotation angle (γ − γ ) 90°–120° Material of flexible tubes and pedal rollers Rubber O f The selection of this parameters is based on: “Although higher water level in the water tank shows better performance in flushing, lower water level is preferred to save water. Too low water level may cause dirt to clog the trapway due to relatively weak siphon” (An et al. 2012) The selection of these parameters is based on: “The optimal depth of the water seal head is approximately 2 cm to minimize the water required to flush the excreta. The trap should be approximately 7 cm in diameter” (Tilley 2014 p 50) Table 2 Results obtained of experiments on the pedal actuated-rotat- able trap way system Flushwater Flushwater Fixed trapway Rotatable amount (l) flowrate (l/min) Average percent- trapway ages of melon Average dices flushed of percentages of melon dices flushed of 99 g 150 g 99 g 150 g 0.250 1.2 − − 45.7% − 0.300 1.9 − − − 25.5% 0.350 2.1 − − 72.7% − 0.400 2.7 − − − 71.9% 0.500 3.2 19.7% − 100% 100% 1.000 5.5 78% 33.6% − − 1.500 7.5 82.7% 40.6% − − 2.000 10.7 − 98% − − Actually, blackwater sewer pipes are more prone to clog- ging than greywater sewer pipes because of higher viscosity of blackwater. In typical domestic sanitary systems, large pipes of 4–5 inches dimeter are used for blackwater sewer while thinner pipes of 1.5–3 inches dimeter are used for greywater sewer. In multistory buildings, main sewer pipes are installed vertically behind toilets and kitchens as separate columns for blackwater and separate columns for greywater and mixing them occurs at end points before sending them to the main municipal sewer network. This separation secure hindering of blackwater reflec- Fig. 10 Melon dices used in the experiment tion to greywater drains in case of clogging of their passages. Using small amount of flushwater may cause high viscosity of the black water in blackwater pipes causing clogging. To avoid required to refill the bowl after retaining the trapway pipe to this, pipes of blackwater must be joined with greywater pipes its ordinary position. The required flushwater flowrate can be at the closest point in the ground level. Mixing greywater with obtained directly from the fresh water supply system with- blackwater before sending them to the main sanitation duct can out the need to install siphon boxes. The final result of this help in prevention of system clogging. analysis is that less than one liter of flushwater is sufficient with using the rotatable trapway system approach. 1 3 Applied Water Science (2018) 8:53 Page 9 of 10 53 Table 3 Relative costs of traditional, rotatable blade mechanism and Conclusions rotatable trapway systems Through the literature survey and experimental work, it Cost category Relative Relative Relative Flushing system installation maintenance flushwater was found that: cost cost cost Plentiful studies, trials and proposals have been made Traditional system Low Low Very high to minimize water wasting in toilet flushing and the Rotatable blade system High High Low proposed solutions included using greywater or seawa- Rotatable trapway system High High Very low ter water as alternative source instead of fresh water, optimizing the siphon and flushing system design and Cost analysis and economic feasibility using an alternative rather than water such as vacuum. Flushwater must have sufficient mechanical energy to The costs of first proposed system (the rotatable blade drive out waste materials from the bowl. mechanism) include cost of materials and that of chang- There are two key factors affect the required amount of ing the typical manufacturing routine. In the experimen- flushwater required: the density and total weight of the tal work of this study, the cost of the mechanism parts feces or semisolids. is about 20% of the ceramic base price. Improving the • This study deliberates two mechanically actuated trial mechanism design may include additional costs, thus its system approaches for optimizing the flushing system feasibility depends on the final applicable design. The sec- design applicable for ground base lavatory and seat ond approach is considered seems more promising and fea- lavatory systems. sible. Manufacturing, installation and maintenance of the • The first trial depends on using a rotatable blade in the rotatable trapway system will rise its cost but it reduces the bottom of the bowl as a mechanical mean to push solid cost of flushwater considerably as well. As to the author or semisolid materials from the bowl but, real testing experience, the cost increment can be 40–80%. As the of one design revealed poor performance. mechanical pedal system is prone to malfunctions and its • The second approach depends on making the trapway appearance is not desired in hotels and some facilities, the rotatable by which it can be tilted down to enable dis- actuation system can be automated. A small electric motor, charging blackwater from the bowl directly by its grav- a cam mechanism, a solenoid valve and a minor control ity. Hence, water is just used to overcome friction and circuit can be used instead with additional cost of 20–50%. to clean the passage A system is considered feasible if the cost of water saved Experiments with restricted conditions revealed that by it surpasses its installation, running and maintenance less than one liter of flushwater is sufficient with using costs during its useful life. Relative cost comparison is the rotatable trapway system approach. The required presented in Table 3 for costs of traditional, rotatable blade water flowrate can be obtained directly from the water mechanism and rotatable trapway systems. supply system without the need to install siphon boxes. A system total annual cost can be modeled as follows: T = W + D + M c c c c (10) where T is the Annual total cost, W is the Annual flush- c c Recommendations and future work water cost, D is the Annual deterioration cost (Installation cost divided by its useful life in years), M is the Annual Researchers are encouraged to enhance the rotatable blade maintenance cost. system design as its application will be easier on the exist- Change in Annual cost when replacing a traditional toi- ing lavatories without major change in the manufactur- let and its flushing system: ing and installation systems. The rotatable trapway seems promising in super saving of water but, more research work ΔT = T − T (11) C CN CO is required to improve its reliability and manufacturability. where T and T are the total costs for the new and old cN cO. system, respectively. Acknowledgement Praise is due to Allah for his refinement. Then, For economic feasibility decision to be made, definite thanks to the Northern Border University for sponsoring the research; figures are required for real costs of manufacturing, instal- especially the Deanship of Scientific Research. Thanks to my family and my colleagues who supported me to complete it. I ask Allah that lation and maintenance. the results of this research promise and benefit everyone. Open Access This article is distributed under the terms of the Crea- tive Commons Attribution 4.0 International License (http://creat iveco 1 3 53 Page 10 of 10 Applied Water Science (2018) 8:53 mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- Mark J, Hammer Sr, Mark J, Hammer Jr (2014) Water and wastewater tion, and reproduction in any medium, provided you give appropriate technology, 7th edn. Pearson, London. ISBN 978-1-292-02014-1 credit to the original author(s) and the source, provide a link to the Ng TL (2015) Cost comparison of seawater for toilet flushing and Creative Commons license, and indicate if changes were made. wastewater recycling. Water Policy 17(1):83–97 Suh KW, Won YJ, Lee YH (2009) A study on the water saving device using a variable position straight trap in water closet system. The Society of Air-Conditioning and Refrigerating Engineers of References Korea, pp. 465–470 Suratkon A, Chee MC, Ab Rahman TST (2014) SmartWUDHU: recy- Abu Rozaiza OS (2002) Ablution water: prospects for reuse in flushing cling ablution water for sustainable living in Malaysia. J Sustain of toilets at mosques, schools, and offices in Saudi Arabia. J King Dev 7(6):150–157. https ://doi.org/10.5539/jsd.v7n6p 150 Abdul Aziz Univ-Eng Sci 14(2):3–28 Tang SL, Yue DPT, Ku DCC (2007) Engineering and costs of Al Mamun A, Muyibi SA, Abdul Razak NA (2014) Treatment of used dual water supply systems. 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Published: Apr 2, 2018
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