Water-saving urinals, such as waterless and low-flush urinals, have a great potential for water conservation by using 0 ∼ 0.8 l/flush compared with ordinary urinals, which use 2 ∼ 4 l/flush. However in some cases, water-saving urinals are not desirable because of technical problems, such as urine scale formation which makes the urinal dirty and blocks pipes. Also, some cultures do not allow the use of waterless urinals because of their notion of cleanliness. In this paper, factors causing urine scale formation have been identified from laboratory tests on pure urine and several types of flushing water. Some meaningful solutions for managing and solving urine scale problems have been suggested. In particular, the results show that mixing urine with seawater or high salinity groundwater will increase the potential of urine scale formation by increasing total dissolved solids (TDS) and pH. However, using rainwater for urinal flushing can significantly reduce the TDS and pH. These findings could support the use of water-saving toilets in Islamic societies by ensuring that the cleanliness of urinals can still be achieved.

INTRODUCTION

The main challenge for modern sanitation is to reduce water consumption while keeping the sanitary ware clean and hygienic. Men's urinals are commonly used to overcome this challenge and provide a facility which takes up less space and can reduce the water consumption (0 ∼ 4 l/flush) compared with ordinary toilets (6 ∼ 13 l/flush). Urinals can be categorized as ordinary urinals (2 ∼ 4 l/flush), low-flush urinals (0.5 ∼ 0.8 l/flush) and waterless urinals (0 l/flush) according to the amount of water used each time they are flushed (Vickers 2001).

Nowadays, waterless urinals are being studied and recommended widely owing to their two important advantages over water-flushed urinals: water saving and their potential to be able to collect undiluted urine which can be used to produce fertilizer. However, many surveys show that low-flush and waterless urinals are not accepted because of different practices or cultural notions of cleanliness (Romich et al. 2012).

Flushing urine with high total dissolved solids (TDS) water can increase urine scale formation depending on the characteristics of the flushing water, which in turn increases the potential for blockage in traps and connecting pipes, causes unpleasant odors, and makes the urinal look unsanitary, as shown in Figure 1 (Muskolus & Ellmer 2007). Good examples of these concepts are sanitation practices in Hong Kong and Muslim societies. In Hong Kong, as the majority of people proudly use seawater for flushing as an alternative water resource, the demand for waterless urinals is not being felt (Chau 2011).

Figure 1

(a) Scale formation in men's urinals using different amounts of flushing water; (b) an example of urine scale formation in spud housing of a urinal (Muskolus & Ellmer 2007).

Figure 1

(a) Scale formation in men's urinals using different amounts of flushing water; (b) an example of urine scale formation in spud housing of a urinal (Muskolus & Ellmer 2007).

Muslim societies do not accept waterless urinals because their cultural ideas require them to use water for body and facility cleansing practices. Wasting water is strongly interdicted by Islam (Hillenbrand 2009; Neirizi et al. 2014).

Although urine and its chemistry have been studied by many researchers in the area of medical science, the chemical behavior of urine and flushing water mixed in urinal systems (including their transport and storage) has not yet been studied at any significant technical depth; the same goes for the causes of scaling and possible solutions using the advantages of roof harvested rainwater such as no or too little TDS (Dao et al. 2013) or little energy demand for treatment (Han & Mun 2011). In this paper, factors that affect urine scale formation are studied by conducting laboratory tests using several types of water with different TDS including seawater, high salinity groundwater, groundwater, tap water and rainwater. Also, the ramifications of the results for the operational practices of urinals and water supply systems are discussed.

MATERIALS AND METHODS

Fresh urine was collected from the men's public waterless urinals installed in Seoul National University (SNU) for research purposes. Seawater was collected from the West Sea of Korea and groundwater was collected from two different places. One was from a residential well in Incheon City, which is a coastal city and slightly affected by salt intrusion, and the other from Bongeunsa temple, located in downtown Seoul. Tap water was collected from the Seoul Metropolitan water supply system. Rainwater was collected from the rainwater harvesting tank of SNU.

The fresh urine was used immediately after collection for the experiment. The different kinds of water were mixed with urine in different dilution ratios (1 unit urine to 1, 3, 8 and 10 units of diluting water) inside plastic tubes to make up 150 ml of volume in total, and were shaken intensively for 10 s. The samples were then left for 30 min to react before they were used for sample characterization. Using this sampling process, three different sets of samples were prepared for measurement.

Table 1 presents the physicochemical characteristics of all materials used in the experiment. The chemical composition of urine was examined based on the concentration of calcium, magnesium and phosphate ions, which were investigated because of their important role in forming urine scales. A dilution ratio 1:1 ∼ 10 was chosen because the amount of flushing water was 0.5 ∼ 4 l/flush while a healthy man usually urinates a maximum of about 500 ml (Larsen et al. 2001; de Jong et al. 2014).

Table 1

Characteristics of fresh urine and flushing water

Concentration of ions (mg/l)
MaterialspHTDS (mg/l)Ca2+Mg2+PO43−
Fresh urine 6.4 7,683 165.3 98.2 473.3 
Seawater 8.9 43,216 553.1 4,473.7 28.6 
Salty groundwater 8.4 13,698 289.2 155.4 39.2 
Groundwater 7.9 805 86.9 88.3 12.8 
Tap water 7.2 109 16.3 4.6 6.3 
Rainwater 6.2 12 1.2 0.9 0.1 
Concentration of ions (mg/l)
MaterialspHTDS (mg/l)Ca2+Mg2+PO43−
Fresh urine 6.4 7,683 165.3 98.2 473.3 
Seawater 8.9 43,216 553.1 4,473.7 28.6 
Salty groundwater 8.4 13,698 289.2 155.4 39.2 
Groundwater 7.9 805 86.9 88.3 12.8 
Tap water 7.2 109 16.3 4.6 6.3 
Rainwater 6.2 12 1.2 0.9 0.1 

The chemical concentration of the samples was measured following US Environmental Protection Agency (US EPA) standards using a UV/Visible Spectrometer, model HS-3300 (US-EPA 1983). For pH, temperature and TDS measurements, an Aquaread Aquaprobe model AP-2000 was used. The concentrations of phosphate, calcium and magnesium ions were measured for 6 h, starting from the time of sample preparation, at intervals of 1 h. After 6 h, the concentration of ions was shown not to have changed significantly.

Also, changes in pH with time were investigated because pH has a significant effect on urine scale formation (Udert et al. 2003). To study the effects of temperature, samples were left in a Cole-Parmer Standard Benchtop Chilling/Heating Block, model 100-230 VAC equipment to obtain temperatures of 5, 10, 20, 30 and 40 degrees Celsius for 30 min and then underwent sampling for chemical ion concentration measurement. Sampling for chemical concentrations at each temperature was done while the samples were still in the chilling/heating block. This was done to ensure the effects of temperature were accurately modeled.

Finally, TDS was measured for each of the dilution ratios (using the different water types) and each was compared with the pure urine results observed. For chemical measurements, experiments were triplicated and an arithmetic mean was taken as the final result. The standard deviation of the results was used as an indicator of the error bars.

RESULTS AND DISCUSSION

The urine scales form as a result of supersaturating minerals that are affected by many factors such as time, pH, and temperature, which can affect the solubility of solid ions. Moreover, the concentration of TDS can directly influence scale formation and the precipitation process.

Based on prior studies, such as by Udert et al. (2003), the urine scale formation process mainly occurs because supersaturated Ca2+, Mg2+ and PO43− ions precipitate as calcite (CaCO3), struvite (MgNH4PO4·6H2O) and hydroxyapatite (Ca5(PO4)3OH). The chemical reactions of these salts are presented in Table 2.

Table 2

Chemical reactions of urine precipitates (at 25°C)

SaltChemical reactionpKsp
Calcite (Stumm & Morgan 1996Ca2+ + CO32− ↔ CaCO3 8.48 
Struvite (Elliott 1994Mg2+ + NH4+ + HnPO43−n + 6H2O ↔ MgNH4PO4·6H2O + nH+ (n = 0, 1, 2) 13.15 
Hydroxyapatite (Taylor et al. 19635Ca2+ +3PO43− + OH ↔ Ca5(PO4)3OH 54.5 
SaltChemical reactionpKsp
Calcite (Stumm & Morgan 1996Ca2+ + CO32− ↔ CaCO3 8.48 
Struvite (Elliott 1994Mg2+ + NH4+ + HnPO43−n + 6H2O ↔ MgNH4PO4·6H2O + nH+ (n = 0, 1, 2) 13.15 
Hydroxyapatite (Taylor et al. 19635Ca2+ +3PO43− + OH ↔ Ca5(PO4)3OH 54.5 

Although chemical measurements and analysis were done for Ca2+, Mg2+ and PO43−, only the results for calcium ion are presented in this paper because similar results and trends are found for the magnesium ion and phosphate.

Precipitation process

Figure 2 illustrates how the precipitation of calcium ions decreased with time for different water types at the dilution ratio of 1:8. The Ca2+ concentration decreases with time for pure urine and all water types because of Ca2+ precipitation, and becomes stable after 6 h. The difference in Ca2+ concentration shows the amount of precipitation that is occurring.

Figure 2

Reduction of Ca2+ for different types of flushing water (dilution ratio = 1:8; temp = 25°C).

Figure 2

Reduction of Ca2+ for different types of flushing water (dilution ratio = 1:8; temp = 25°C).

After 6 h, calcium precipitation in urine diluted with seawater and rainwater was about 136 and 8.25 mg/l, respectively. This shows that not only was the amount of precipitation in the rainwater-diluted sample significantly smaller than the sample diluted with seawater, but also the precipitation process in urine diluted with seawater was about 16 times faster.

Change of pH with time and the effect on precipitation

Variations in pH of samples with time are shown in Figure 3 for the dilution ratio of 1:8. The pH of all samples increased because of a hydrolysis process of urea, described by Equation (1). However, only in the case of urine diluted with rainwater did the pH stay below 9.
formula
1
Figure 3

Change of pH of samples with time (dilution ratio = 1:8; temp = 25°C).

Figure 3

Change of pH of samples with time (dilution ratio = 1:8; temp = 25°C).

Studies show that in solutions with pH higher than 9, this typically leads to supersaturation of chemical ions and their eventual precipitation (Udert et al. 2003). This study shows that rainwater modifies the pH of the sample and so where urine is mixed with rainwater the potential for scale formation is reduced.

Effect of temperature on scale formation

Figure 4 illustrates the effect of temperature on calcium precipitation in the dilution ratio 1:8. The concentrations of all measured ions increased with increasing temperature; this indicates that the scale formation process was much slower at high temperatures, as the solubility of dissolved solids increases with temperature.

Figure 4

Effect of temperature on precipitation of Ca2+ (dilution ratio is 1:8).

Figure 4

Effect of temperature on precipitation of Ca2+ (dilution ratio is 1:8).

These results suggest that the observed practice of using ice cubes in some public urinals is not recommended. Instead, keeping the urinals warm can help reduce the amount of scale formation. The results also indicate that seawater and salty groundwater are affected most by temperature.

Effect of dilution ratio (TDS)

Water with higher TDS can produce more scale. Figure 5 presents the results of TDS measurements in urine diluted with different types of water with dilution ratios of 1:1, 3, 5, 8 and 10.

Figure 5

TDS measurement results for urine with different dilutions.

Figure 5

TDS measurement results for urine with different dilutions.

In the specific cases of urine diluted with seawater and salty groundwater, TDS increases with the dilution ratio, which could definitely increase scale formation potential. Conversely, dilution of urine with groundwater, tap water and rainwater resulted in a completely different trend; they decreased TDS and the potential for scale formation as well. Overall, mixing urine with water which has a lower TDS than itself can reduce the TDS of the mixed solution. Although diluting urine with higher dilution ratios using groundwater and tap water can reduce the TDS of the final solution, using rainwater is the most effective in reducing urine scale formation as it has the lowest TDS compared with other types of water. Using rainwater also has other advantages as it can be collected via rainwater harvesting systems free of charge, thus requiring little or zero energy for abstraction and treatment, unlike other flushing waters (Dao et al. 2013).

Discussion

The behavior of urine in scale formation due to the pH and the types of flushing water can be explained with a pC–pH diagram for Ca2+ solubility, as shown in Figure 6.

Figure 6

Precipitation process of calcium versus pH (dilution ratio = 1:8; temp = 25°C).

Figure 6

Precipitation process of calcium versus pH (dilution ratio = 1:8; temp = 25°C).

By considering the calcite formation process (Table 2) and the carbonate hydrolysis process (Equation (2)), as well as the molarity of calcium (40,078 mg/mole), Equation (3) can be developed. The dividing line for saturation is thus calculated. Precipitation occurs when the conditions are above the line (Stumm & Morgan 1996).
formula
2
formula
3
The symbols in the graph show the pH and concentration of Ca2+ of pure urine and urine mixed with different kinds of flushing water, respectively, and all stabilized after 6 h. The results suggest that urine mixed with rainwater is under-saturated, while others are in the supersaturation zone.

The diagram shown in Figure 6 can serve as a basis for criticism of several practices, as well as a suggested solution. Flushing with high TDS water such as seawater, which is practiced by the Hong Kong authorities (arrow 1), means moving into the supersaturation zone where urine scale formation will increase; therefore, in terms of scale formation, this is not a desirable practice. Although flushing with tap water (arrow 2) reduces precipitate composition (Udert et al. 2003) it increases the pH and requires high volumes of water to make it unsaturated. In contrast, flushing by rainwater (arrow 3) results in a lower Ca2+ concentration and lower pH; the Ca2+ is under-saturated stopping scale formation in this region. Another way to reduce urine scale formation is to add a weak acid (arrow 4) to the under-saturated zone. The amount of added acid can easily be calculated with simple chemistry.

CONCLUSION

Urine scale formation is affected by Ca2+ concentration and pH, which can be explained by a simple pC–pH diagram. To avoid scale formation, urine should be flushed with low TDS water such as rainwater or water with a lower pH by adding a weak acid. In terms of scale formation, flushing with seawater as practiced in Hong Kong is not a good practice, although it may seem practical to use an alternative water resource. Higher temperature can reduce the urine scale formation potential by increasing solubility. Our results also suggest that the sanitation practice of putting ice cubes in some public urinals is not good either in terms of urine scale formation, though it may help reduce water consumption and smell.

In addition to the increasing importance of rainwater as a sustainable water supply in many parts of the world, it also has great potential to solve the urine scale problem because of its low TDS and ability to reduce the pH of the final solution. Furthermore, the findings of this study can also support the use of low-flush urinals in Islamic countries as problems associated with urine scale can be reduced by using rainwater for flushing.

ACKNOWLEDGEMENTS

This study was supported by ‘Community Resource Oriented Source Separation Technologies (CROSS)’, the project of the Ministry of Environment in the Republic of Korea and Seoul National University.

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