Wastewater temperature changes in sewer outlets under severe climatic conditions

. Sewer outlets from buildings in a harsh climate are considered one of the main elements of the sewer network, determining its reliability and efficiency. The article considers the change in the temperature of wastewater at sewer outlets laid at a reduced depth in the frozen ground. Methods: The purpose of this study was to determine the temperature of wastewater at the sewer outlets in the city of Kyzyl during the day in winter. Result: New experimental data on wastewater temperature are presented and summarized. The range of change in the temperature of wastewater at the sewer outlets from the building in the city of Kyzyl has been determined. The results of measurements of a full-scale experiment during the day in winter are presented


Introduction
More than 60% of the territory of the Russian Federation is characterized by permafrost soils with harsh natural and climatic conditions as well as cold winters lasting up to 7-8 months [1][2][3][4][5]. The Republic of Tuva, located in the south of Eastern Siberia, is one such territory. Currently, the population of the Republic exceeds 310,000 people, the capital city being Kyzyl. The city operates a centralized sewage system. The length of sewage networks in the city is 88.3 km. The main problem of operating these networks is the elimination of failures, given that the depth of the pipeline is from 3 to 6 m. The reason for such embedding is the deep freezing of soils (from 3.2 m) [3].
Improving the reliability of sewerage operation, reducing operational costs, cutting the time of emergency works depend not only on a better quality of design, construction and operation of the network, but also on raising the culture of using indoor sewerage facilities by the population.
Reducing the depth of sewage pipelines while maintaining the reliability of operation under a normal operating mode, and ensuring frost-proof conditions in a potential emergency is a relevant challenge. Shallow laying of pipelines has a number of technical and economic advantages: reducing the time and cost of construction by reducing the amount of earthworks; reduction of labor intensity in repair and recovery efforts; rapid detection and elimination of breakdowns.
Of the entire sewage system, outlets from houses are particularly prone to frost penetration. Under minimum drainage in some parts of the sewer network, sediments can be deposited with subsequent icing.
In addition, in the lack of flow, the pipes get covered with hoar frost on the inside when cooled, and ice can form in them when new sewage comes in.
Sewer outlets are one of the main elements of the sewer network, determining its reliability and efficiency [6][7][8][9].
Sewer outlets from houses, when laid in the frozen ground, are one of the most dangerous and contrarious parts of the sewerage system [10][11].
In this case, it is especially important to determine the depth of the sewer outlet embedding, taking into account the temperature of the wastewater transported through the outlet.
In operational practice it has been found that intra-neighborhood sewer networks, as well as sewage outlets from houses, are most often exposed to the risk of freezing. This is due to the low temperature of wastewater in the pipeline, the presence of dead-ends and initial sections of the network, characterized by low water flow, and secondly, due to periodicity of operation during the day and the usual absence of wastewater flow at night. As a consequence, there is a high probability of an emergency situation, with greater efforts on the part of service personnel required to eliminate it. And thirdly, as for the dangerous impact upon the stability of ground at the foundations of buildings, the threat lies in the spreading of melted ground to the depth of a building's foundation: its stability can be disturbed by periodic subsidence as well as by ground swelling at the basement level.
Given the said circumstances, the construction of sewer outlets deserves most serious consideration, both in terms of choosing the most rational design options and in terms of meeting all the requirements for quality construction of sewer outlets.
Water seeping into the foundation soil poses a great danger. Therefore, any sewer leakage, no matter how small, is a major cause of pipeline failure and foundation subsidence; therefore, it must be eliminated immediately after detection [12].
In general, a total reduction in the embedment depth of street sewer pipes depends primarily on a decrease in the embedment depth of house sewer outlets.
In order to justify the reduction in the depth of sewer outlets, it is necessary to know the actual values of wastewater temperatures at the building's outlet and in downstream sections [13,14].
It was found from a review of specialized materials on this subject that there has been no field research done on the thermal operating mode of buildings' sewage outlets.

Materials and methods
The object of the study was Dormitory № 3 of Tuva State University, located at 1/1, Druzhba Street. The five-storey building was raised in 1984. Figure 1 shows the courtyard drainage of Dormitory № 3. Sewage outlet KK1 laid at the lowest depth of 1.9 m, having the diameter of 100 mm and the length of 5 m, was picked for investigation. There are 86 rooms in the dormitory overall.
As per the typical floor plan for a typical floor, two flush toilets and two sinks are connected to a soil stack on each floor. The total number of plumbing fixtures per outlet is 10 flush toilets and 10 sinks.
A Testo 872 thermal IR imager was used to measure the wastewater temperature at the house outlets in winter during the day. The device measures air and wastewater temperature with the accuracy of 2 degrees C or ±2% of the readings. The camera allows to measure infrared radiation, emitted by the surface of an investigated object, later transforming it into a two-dimensional image, to characterize the temperature distribution on this surface [13][14][15][16][17][18][19].
Field studies were conducted from February 10 to March 10, 2021. The experiment methodology was consistent with the purpose of the field studies, which was assessing a change in the temperature of the wastewater flowing over the sewage outlet during the day.
Values were taken in the basement at the outlet and in manhole KK1. Measurements of wastewater temperature were carried out every hour during the day.
Thus, the measurements show a decrease in the temperature of wastewater as it was flowing from the soil stack outlet inside the building to the manhole (inspection chamber).
To assess the dynamics of changes in the temperature of wastewater, it was measured in the basement (soil stack temperature) and at the outlet in manhole KK1. Fig. 2 shows the photo taken from a thermal IR imager with the morning value of wastewater temperatures in the manhole, taken at 7:00 a.m. The wastewater temperature at the outlet in manhole KK1 was found to be 20.1 degrees C. Such a sequence of measurements was chosen in order to trace the change in wastewater temperatures at the outlets during the day. The outlet depth is 1.9 m, the air temperature is minus 23 degrees C [4].
Thus, taking the temperature values in the manhole flume with a thermal IR imager, one can assume it being equal to the temperature of effluents at the end of the outlet [4].

Results
See Table 1 for the results of wastewater temperature measurements at the basement soil stack in manhole KK1. Based on the data in Table 1, a chart of hourly changes in wastewater temperatures in manhole KK1 was drawn (Figure 3). As a result of experimental investigation of the thermal operational regime at sewage outlets in winter, it was found that the change in wastewater temperatures in manhole KK1 during the day reveals a wave-like pattern and has three pronounced peaks: the first one occurs in the morning (7:00-8:00) -22.3 degrees C, the second one is at lunch time (13:00-14:00) -19.1 degrees C and the third one in the evening (20:00-22:00) -21.5 degrees C. The shape of the daily temperature chart changes cyclically, that is, the temperature in one day of the week repeats the temperature on another day of the week. For these days of the week, the minimum and maximum temperature peaks occur at the same time.

Conclusion
In order to reduce construction costs in areas with deep seasonal freezing of soils, sewerage pipelines can be laid in a layer of frozen soil above the depth of freezing. Studies of the temperature regime of a sewage outlet laid in a layer of frozen soil at the depth of 1.9 m showed that in the point of the area investigated the wastewater temperature during the day has a positive value in the range of +8.1 ÷+22.3 ° C, which indicates a sufficient thermal reserve against excessive freezing of the pipeline.