The Potential of Heat Recovery from Wastewater Considering the Protection of Wastewater Treatment Plant Technology

Abstract

Energy efficiency is extremely significant for industrial processes and technologies. Rising energy prices, depleting fossil fuels, as well as tightening regulations that impose the need to reduce GHG emissions incentivize companies to look for energy-efficient solutions. This also applies to wastewater treatment plants, which, on the one hand, are consumers of very large amounts of energy, and on the other hand, have significant potential to retrieve waste energy in the form of heat accumulated in wastewater. The authors of this publication have recognized the benefits of managing this heat. However, they have also pointed out several problems and difficulties associated with this process. By means of measured data, this publication provides a comprehensive analysis of the heat that can be recovered from wastewater treatment plants. As a result of the analyses, the locations of sites for collecting heat from wastewater have been determined, and potential technologies for this purpose have been identified. Moreover, the impact of the proposed heat recovery technology on the process of biological wastewater treatment has also been analyzed. As a result of the research, the authors developed generalized guidelines for selecting an optimal heat recovery site and the technological system designed for this purpose.

1. Introduction

Energy efficiency is extremely important in building construction and industrial processes. With rising energy prices and the uncertainties related to their supply, industrial technologies and processes are subject to special supervision in terms of their energy consumption. This also applies to wastewater treatment plants, which consume very large amounts of energy during the implementation of wastewater treatment processes [1,2,3]. In addition, the amount of sewage increases each year due to the constant expansion of cities, population increases, and improved hygiene standards. This also results in an increased amount of energy used by treatment plants [4,5,6]. In this context, it is extremely important to look for solutions that, on the one hand, make it possible to reduce the energy consumption of the processes, and on the other hand, increase their share of energy from renewable energy sources. This is particularly important in light of the Directive of the European Parliament and of the Council of 11 December 2018 on the promotion of the use of energy from renewable sources, which recognizes energy from wastewater as renewable energy [7,8,9]. Therefore, there are many benefits of using wastewater energy, especially in the form of recovered heat, which can be used both in the technological processes of sewage treatment plants and in such areas as building construction, agriculture, and industry. Naturally, the acquisition of this energy can result in problems, which have become subjects of extensive research studies by many teams around the world.

Many studies and publications have been conducted on the technology of heat recovery from wastewater directly in municipal sewage systems. Kretschmer et al. [10] identified wastewater as a significant source of heat for heating buildings. However, they emphasize that wastewater treatment plant technologies are sensitive to changes in wastewater temperature. In this publication, the authors introduce a methodological framework for assessing the suitability of potential heat recovery locations in urban sewage systems. This framework includes decision support criteria and planning principles for engineers, designers, and local authorities. However, they do not solve the problem of synchronization of dispersed heat collection from wastewater, which significantly affects the inability to control the temperature of wastewater flowing to the treatment plant. This makes the methodology proposed by Kretschmer and his team debatable. Similar studies were presented by Nagpal et al. in [9], in which the authors presented a review of solutions, focusing mainly on heat recovery by means of locally built installations. The authors emphasized the advantages of solutions based on receiving heat directly in buildings, as well as in local municipal sewage networks. Similar solutions were proposed by Bertrand, Cecconet, and Cippola with their research teams in publications [11,12,13]. As in the case of [9], the authors did not analyze the impacts of the applied solutions on the parameters of sewage flowing to the sewage treatment plant.

Cecconet and his team published interesting data on the reduction of energy consumption in a building as a result of the use of wastewater heat recovery by as much as 59%. These are extremely encouraging values, but it should be emphasized that the publication does not present any economic analyses related to a scale larger than a single facility. It should be expected that the costs of distributed heat recovery from wastewater will be much higher than a single larger installation located in a wastewater treatment plant. An interesting analysis was presented by Golzar and Silveira in [14], in which, based on the constructed hybrid mathematical model, they showed that the use of local wastewater heat recovery technology may have destructive effects at the level of the wastewater treatment plant technology. The analyses presented in this paper showed that the use of local heat recovery from wastewater reduces the heat demand for buildings by 2–4%, and at the same time increases the heat demand for the needs of wastewater treatment plant technology by 2–3%, reducing the potential for heat recovery from wastewater in the treatment plant itself by 5–9%. This justifies the balance between local and systemic solutions, which is also indicated by other researchers, including in works [15,16,17].

The problem of sewage flowing into sewage treatment plants has been the subject of numerous studies. In [18], Durrenmatt and Wanner presented a mathematical model that calculates the flow in the canal as well as spatial profiles and temperature dynamics in wastewater. Wanner et al. have shown that lowering the wastewater temperature at the inlet to the treatment plant can have a negative effect on nitrification, as this process is highly temperature dependent. These authors analyzed the temperature regime in the wastewater treatment plant in Zurich and proved that nitrification is significantly affected by temperature lowered for longer than several hours, and such a situation should be taken into account in the case of heat collection from wastewater in buildings and the sewage network [19]. Similar conclusions from studies were published by the authors of other studies, including [20,21,22,23]. An interesting and extensive analysis of the potential of recovering resources, including energy in the form of heat, was presented by Kehrein et al. [24]. These authors showed that the collection of heat from wastewater in the sewage treatment plant with the use of heat exchangers constituting the lower source of heat for heat pumps is justified both technically and economically. In their publication, they described solutions consisting of the installation of such devices at the inlet to the technological system, which results in the above-mentioned lowering of the wastewater temperature, with all of its consequences. The publication does not propose any alternative to such a solution, and also points to the problem of a mismatch between the supply and demand in terms of time and location, which is a potential bottleneck hindering the recovery of thermal energy. Identical problems were indicated by other authors, among others, in references [25,26,27].

In light of the analysis presented above, the authors of this publication conducted research based on the findings of measurements of sewage parameters at an existing and functioning sewage treatment plant. As a result of the conducted research, they determined the locations of sites to receive heat from wastewater and indicated potential technologies for this purpose. They also analyzed the impact of the proposed heat recovery technology on the process of biological wastewater treatment. As a result, the authors proposed general guidelines for the selection of the optimal heat recovery location, as well as the technological system designed for this purpose.

2. Materials and Methods

The research was carried out in a sewage treatment plant for the city of Wrocław and measurement data were provided by the Wroclaw Municipal Water and Sewage Company. The facility is a mechanical–biological treatment plant with a capacity of 155,000 m${}^3$d and a $PE$ (population equivalent) of 1,050,000. It is located on the outskirts of the northwestern part of Wrocław. Wastewater treatment is carried out by means of the activated sludge method with the possibility of chemical precipitation of phosphorus.

The process includes several stages. The wastewater, collected through an extensive sewage network, is channeled into the main canal „Odra”, which transports it to the treatment plant. It is divided in the input chamber into three technological lines for preliminary mechanical treatment. These are open canals; each is equipped with a coarse screen with a clearance of 10 $\mathrm{m}$$\mathrm{m}$, a fine bar screen with a clearance of 5 $\mathrm{m}$$\mathrm{m}$, and an aerated grit chamber. Then, the pre-treated raw sewage is pumped through the main pumping station ($B9$) to a separation chamber located before the primary settling tanks. In the four primary settling tanks, quickly settling suspension is retained by means of sedimentation in the form of primary sludge. This is followed by biological treatment carried out by means of a multi-phase activated sludge method in the five biological blocks, each with a capacity of approximately 30,000 m${}^3$. The compressed air, necessary in biological oxidation processes, is pumped to the blocks by means of 10 blowers. After the biological treatment, the wastewater is directed to 7 secondary settling tanks, where the activated sludge suspension is separated from the wastewater. Finally, the treated sewage is discharged through a canal to the Oder River. Part of the treated sewage by the pumping station system is returned as processed water to the following facilities:

• The screen hall;
• The building for mechanical thickening of excess sludge;
• The building for digested sludge dewatering;
• The sludge dryer.

The primary and excessive sludge generated in the technological processes are subjected to gravitational and mechanical densification processes, respectively, and then, after mixing, they feed into separate fermentation chambers, in which the process of mesophilic methane fermentation (resulting in biogas) is carried out. After the treatment, the biogas is used to produce electricity and heat for the treatment plant’s own needs.

In order to enable the analyses of the potential for heat recovery from wastewater, measuring sensors were installed in the wastewater treatment plant to collect and archive operating parameters (Figure 1).

The collected and archived measurement data included the temperature of raw sewage at the inlet to two technological lines ($CT2$ and $CT3$) and the temperature of the treated sewage at the outlet of the treatment plant. Temperature sensors were located in the open canals of individual technological lines at the stage of the initial cleaning. In addition, wastewater flows were also measured by means of the built measurement infrastructure. Raw wastewater treatment processes, which were carried out before the temperature sensors, were installed, and did not involve the supply or removal of heat from raw wastewater, as they included only mechanical treatments on coarse screens and fine bar screens. The measurements of the operational parameters were made in different periods with differing frequencies:

• 1 April 2020–1 December 2020: recorded in 5-s intervals;
• 1 December 2020–1 December 2021: recorded in 20-s intervals;
• 1 December 2021–today: recorded in 300-s intervals.

The research and analyses covered measurement data for a period exceeding 1 year. However, only results for a period of approximately 12 months were submitted for publication, as the data for this period were almost complete (the missing data concerned single hours or days). As emphasized in the manuscript, there are also low variabilities of those parameters, which are important for the possibility of heat recovery. This feature can be observed in the long term, i.e., in the course of the following years. Significant changes, such as the amount of inflowing sewage, are related to the expansion of the sewage treatment plant and take place in a planned manner at long intervals (10–15 years). The data presented in the publication should be considered representative, allowing for the formulation of accurate and universal conclusions.

In CFCs, i.e., closed fermentation chambers, i.e., mesophilic fermentation of thickened sludge (preliminary and excessive) was carried out, the product of which was biogas. In this process, methanogenic bacteria require a temperature range of 30–38 degrees Celsius. The heat supplied to the CFCs is waste heat from cogeneration. For all fermentation chambers, the heating of the recirculated sludge takes place in tubular exchangers in the exchanger room, in which water is the heating medium. In low temperatures, a gas boiler room powered by biogas constitutes an additional source of heat. Electricity is necessary to power all devices throughout the entire process line, both in the sewage and sludge sections. It provides power to pumps, mixers, gate drives, grates, sieves, devices for mechanical thickening of sludge, dewatering presses, etc., and control and measure the fittings. Blowers (10 pcs), which turn off air to the nitrification part of the biological blocks, consume the largest amounts of electricity.

3. Results

The analysis of the recorded measurement data, the purpose of which was to determine the potential for heat recovery and the specific locations of places to receive heat from wastewater, was divided into two sections resulting from the adopted research assumptions. The parameters of raw sewage at the entrance to the treatment plant (flow in chamber $B9$) and the parameters of the treated sewage (at the exit from the treatment plant) were analyzed separately.

3.1. Raw Sewage Parameters

The average daily temperature values of raw sewage on the $CT2$ and $CT3$ lines were compared (Figure 2).

The collected data made it possible to confirm the expected seasonal variability of raw sewage temperatures inflowing into the treatment plant. The lowest temperatures were recorded in the winter months, and the highest in the summer, with the highest average monthly temperature values recorded in August each year for the period covered by the research. Comparing the values of the average daily temperature of raw sewage on both technological lines, it should be stated that there are some differences, but both the trend and the variability revealed a great degree of convergence. The average temperature difference for the entire period covered by the research was $0.76$ ${}^{\circ }\mathrm{C}$, and for 673 days it did not exceed 1 ${}^{\circ }\mathrm{C}$. In addition, it can be noticed that the wastewater at the entrance to the treatment plant revealed a slight variability of the average daily temperature (Figure 3); annually, the values of the temperatures were higher than the values of the ambient air temperature (

Table 1. Comparison of raw sewage temperature values in technological lines $CT2$ and $CT3$ with ambient outdoor temperatures.

Month	Year	Average Raw Sewage Temperature at the Inflow ${}^{\circ }\mathbf{C}$		Average Ambient Temperature ${}^{\circ }\mathbf{C}$
		Technological Line $\mathit{C}\mathit{T}2$	Technological Line $\mathit{C}\mathit{T}3$
4	2020	17.2	15.4	10.6
5	2020	18.1	17.0	12.4
6	2020	19.5	19.0	18.4
7	2020	20.9	19.9	19.3
8	2020	22.0	21.6	21.1
9	2020	21.8	20.4	15.9
10	2020	19.5	18.0	11.4
11	2020	18.6	18.2	6.3
12	2020	17.0	16.2	2.8
1	2021	14.7	14.5	0.1
2	2021	13.9	13.6	0.1
3	2021	14.7	14.6	4.5
4	2021	15.5	15.0	6.8
5	2021	16.8	16.4	12.7
6	2021	20.0	19.7	20.5
7	2021	21.6	21.2	21.1
8	2021	21.4	21.4	18.1
9	2021	20.9	21.4	15.8
10	2021	19.2	20.1	10.5
11	2021	14.8	16.3	5.6
12	2021	14.8	16.3	1.2
1	2022	14.2	15.7	2.4
2	2022	14.1	14.5	4.8
3	2022	15.1	15.4	15.8
4	2022	16.2	16.1	20.5
5	2022	18.4	18.4	20.4
6	2022	20.1	20.4	21.2
7	2022	21.2	22.0	13.6
8	2022	22.1	22.7	10.6
9	2022	20.7	21.6	12.4

).

From the point of view of heat supply, sewage temperature values in the winter period are particularly important, i.e., when the demand for heat is the highest. In addition to the values themselves, temperature stability and little dependence on weather conditions or other external factors are important features of this medium. Moreover, on a daily basis, fluctuations in sewage temperature values are not significant problems in terms of the operation of systems, such as heat pumps.

3.2. Parameters of Treated Sewage

In order to analyze the potential of heat recovery from treated wastewater and the potential for use as a lower source for a heat pump, a study was carried out to measure the parameters of these wastewater values, including, temperature and flow. The measuring apparatus described in Section 2 was used. Figure 4 presents the average hourly temperature values of treated wastewater (leaving the technological system of the treatment plant), and

Table 2. Temperature values of treated sewage.

Month	Year	Average Daily Volume of Treated Sewage m${}^3$	Average Monthly Adjusted Temperature of Treated Wastewater ${}^{\circ }\mathbf{C}$
5	2021	-	19.9
6	2021	-	24.0
7	2021	142,987.3	24.6
8	2021	146,677.8	23.0
9	2021	146,547.0	23.6
10	2021	138,349.8	22.4
11	2021	138,377.3	22.5
12	2021	146,279.7	14.9
1	2022	142,450.9	13.0
2	2022	153,232.2	12.5
3	2022	146,145.2	13.7
4	2022	150,977.3	14.6
5	2022	145,686.2	20.3
6	2022	143,489.8	23.0
7	2022	144,770.0	22.6
8	2022	155,683.4	23.6
9	2022	156,186.5	21.9

additionally presents them in numerical form, which made it possible to show that the average monthly temperature values of treated wastewater in most months are higher than those of raw sewage. As in the case of raw sewage, after treatment, they showed small variabilities in the average daily temperature values.

The temperature of the treated sewage was measured in a dedicated measuring tub at the exit of the sewage treatment plant. The measurement was carried out by means of an immersion temperature sensor integrated with a pH measuring probe. The measurement sample was taken from the sewage discharge canal to the receiver (river), and then pumped to the measurement site (measuring tank). The flow of the treated sewage was carried out by means of an ultrasonic flow meter, which consists of a transducer, speed sensor, and height sensor, and is located directly on the sewage discharge canal of the receiver. This flow meter is calibrated regularly.

4. Discussion

The analysis of parameters of raw sewage at the entrance to the treatment plant as well as the treated sewage at the exit, as presented in Section 3, revealed that both places of the technological system of the sewage treatment plant make it possible to collect heat for subsequent use. However, from the point of view of the operational correctness of the technological system of the treatment plant, the temperature of raw sewage (at the entrance to the treatment plant) is extremely important. It cannot exceed the limit values, and it is particularly important to prevent excessive cooling of wastewater, which may even stop technological processes, especially in the biological part of the treatment plant. This is of course completely unacceptable. Therefore, other places in the technological line of the treatment plant should be considered for waste heat collection.

In order to correctly calculate the amount of heat that can be obtained from treated wastewater, their flows were analyzed, as shown in Figure 5.

No clear seasonality was observed in the flows, which is a good property of the lower source for a heat pump. The highest monthly volume of treated sewage was recorded in September 2022 and was only 12.9% higher than the lowest recorded value in October 2021. The daily variability of the amount of treated sewage discharged to the receiver was also analyzed (Figure 6 and Figure 7). The obtained results indicate that the daily minimum occurred between 10 a.m. and 1 p.m. when an average of 3% of the daily amount of sewage was discharged per hour. In the winter period, the minimum shifted toward 12 o’clock, and in the summer and autumn months, toward 10 o’clock. Such daily profile patterns are beneficial from the point of view of heat reception by residential buildings, where increased demands for heat are observed in the afternoon and evening hours. Therefore, it can be concluded that it is justified to use solutions that make it possible to collect this heat, e.g., through the use of heat pumps.

As mentioned, the temperature of this medium constitutes an important parameter from the point of view of the potential for heat recovery from treated wastewater by using it as a lower source of heat for heat pumps (Figure 4). Due to failures in the value data recording system, the temperature values of treated wastewater are available from September 2021. The specific nature of this measurement size results in errors such as rapid increases or decreases in temperature. For further analysis, the values considered defective were removed and replaced with average values from neighboring values. Methods and effects of supplementation of the missing data are not the subjects of this publication, although they are important elements in the analyses regarding the estimation of available heat in the treated wastewater.

An important observation is that the treated sewage has a higher temperature than the raw sewage regardless of the season (Figure 8). In addition, wastewater at the exit of the wastewater treatment plant is characterized by smaller daily temperature fluctuations. These conditions constitute another potential premise to use a wastewater heat pump as a lower heat source installed on the cleaned side.

Holistically, it seems more reasonable to use heat pumps that use treated wastewater as lower sources of heat. Contrary to the restrictions regarding the temperature of wastewater at the entrance to the technological processes of the treatment plant, the theoretical lowest limit temperature of wastewater is 0 ${}^{\circ }\mathrm{C}$, because there are no legal restrictions on the minimum temperature of treated wastewater discharged to the receiver (river). There are only regulations that limit the maximum temperature of treated sewage. However, it seems that excessive cooling of treated wastewater can have a negative impact on the biology of the receiver, hence the analyses allowed for a maximum cooling of 2 $\mathrm{K}$ in relation to the values measured at the outlet of the sewage treatment plant.

Based on the data on the amount of sewage discharged to the receiver and the temperature of the treated sewage, the amount of heat possible to be collected by the heat pump was estimated. The analyses were performed in two scenarios. In the first one, it was assumed that the treated wastewater could be cooled down by 2 $\mathrm{K}$ (Figure 9).

The adopted assumption is conservative; even in the winter, the temperature of the wastewater after cooling down does not fall below 8 ${}^{\circ }\mathrm{C}$. Therefore, it does not pose a threat or technical difficulties with regard to the discharge to the receiver. The obtained results are promising because only during 5 days in the period covered by the research was the heat recovery potential below 1000 $\mathrm{G}$$\mathrm{J}$/d, while in the remaining days, it was higher, up to 1747 $\mathrm{G}$$\mathrm{J}$/d (11 September 2021).

In the second scenario, it was assumed that the treated wastewater was cooled by 7 $\mathrm{K}$, but with the restriction that its temperature will not drop by more than 3 ${}^{\circ }\mathrm{C}$ (Figure 10).

Moreover, in this case, the assumptions are conservative, because in the winter, the temperature of the water in the receiver (river) is lower, and in the summer, the discharge of treated sewage has a temperature lower than the temperature of the receiver, but in a limited amount compared to the flow in the river, and it does not significantly affect the biological conditions in the natural watercourse. In the scenario that assumes the cooling of the treated wastewater by 7 $\mathrm{K}$, a slight seasonality is noticeable. The biggest amounts of heat were recovered on 11 September 2021, as they amounted to as much as 6114 $\mathrm{G}$$\mathrm{J}$/d.

For comparison and to enable better inference, an analysis of the potential for heat recovery from raw sewage was carried out. In this case, a scenario assuming the cooling of raw sewage by only 2 $\mathrm{K}$ was adopted. Due to the requirements of the technological process of the sewage treatment in terms of raw sewage temperature, greater cooling of raw sewage can only take place based on extended, individual analyses for a given facility. The obtained results are similar to the analogous scenario for treated wastewater. The lower source of heat in the form of treated sewage seems to be an attractive medium for this type of application.

Sewage treated throughout the year is characterized by temperature values higher than those of raw wastewater, and at a level that meets the requirements for this type of application. Temperature variability of treated wastewater is limited, and what is important, in the winter, it is high compared to the other places that are considered valuable (and lower) heat sources (outside air and even the ground).

5. Conclusions

The authors have drawn several conclusions based on the results presented in this research, as presented below.

• Wastewater has many features that make it an attractive medium constituting a lower source of heat for heat pumps, leading to cheap and low-emission heat.
• In the case of heat recovery from wastewater, whether raw or treated, there are no periods when the amount of heat extracted drops to zero. This is clearly a different characteristic that distinguishes this type of renewable heat source from others. Both photovoltaics and wind turbines have periods of zero energy production.
• Because of the applied wastewater treatment technology using the activated sludge method, which is sensitive to low temperatures, it is safer to use treated wastewater than raw wastewater as a lower heat source. Excessive cooling of raw wastewater carries the risk of a decrease in the effectiveness of biological treatments, and in extreme cases, inhibits these processes. This may result in the treatment plant failing to meet the specified values of the quality parameters included in the legal specifications of the Water Law Act.
• In the case of large facilities located on the outskirts of cities, the amount of recovered heat may be too large in relation to their own needs. The biogas obtained in the technological process in some cases secures the facility’s own needs in terms of heat and, to some extent, electricity. Therefore, the heat potentially recovered from wastewater should go to other recipients; therefore, it should be possible to transfer the obtained heat to the district heating system. It may be unprofitable, due to the distance and low building density (and consequently low heat density). However, it should be remembered that there are facilities located at a short distance from some sewage treatment plants that have specific demands for heat. In such cases, wastewater heat recovery is an advantageous solution, providing access to cheap and emission-free heat.
• Wastewater, similar to other renewable sources, shows greater heat potential during the summer. However, what makes it different is that the differences in heat delivery potential between summer and winter are not as pronounced as for other zero-emission sources.
• On a daily and annual basis, the profiles of the recoverable amount of heat from wastewater (raw or treated) are best matched to the profiles of the thermal needs of consumers, including municipal consumers (space heating and hot water preparation).
• The sewage treatment plant in Wrocław, similar to other facilities of this type in large cities in Poland, is equipped with several specific solutions. However, the analysis method used for wastewater is universal, regardless of the size and location of a treatment plant, including one outside of Poland. The choice of where the heat pump is installed or the decision to cool raw sewage (in the case of heat recovery at this technological stage) are individual decisions, which take into account the specificity of a given sewage treatment plant. However, it should be emphasized that the assumptions adopted in the analyses are conservative, and the potential for recovering more heat in particular locations should be taken into account. This does not change the fact that the proposed methodology is strictly utilitarian.
• A wastewater treatment plant is a facility that consumes large amounts of energy in the form of electricity and heat. However, the main demand is for electricity. In terms of heat supply, the technological needs of such a treatment plant are limited to heating closed fermentation chambers. Waste heat from electricity production is used for this purpose. This form of energy is obtained in the process of burning biogas produced in the process of mesophilic fermentation.
• The supplied amount of energy is required to carry out the technological process and cannot be limited. This results in a slight increase in the temperature of the treated sewage, which is discharged to the receiver (river), and the heat is irretrievably lost.
• The amount of heat that can be recovered from wastewater, especially treated wastewater, usually exceeds the wastewater treatment plant’s own demand for this form of energy. If part of the heat recovered from wastewater is used in the treatment process, it translates into a reduction in the consumption of other primary energy mediums (usually gas or electricity). Therefore, regardless of the form in which the recovered heat is used, the proposed solution is justified, as there is no other method for reducing the energy input in the technological process. If heat is not recovered from raw or treated sewage, then a large amount of heat is dissipated in the receiver (river).