Renewable energy of wastewater heat recovery

Heat recovery (HR) from wastewater is a process that involves extracting heat from wastewater before it is discharged into the environment. The recovered heat can then be used for a variety of purposes, such as space heating, domestic hot water, or industrial processes. This process is often referred to as HR. There are several technologies used for HR, including heat exchangers (HEs), heat pumps (HPs), and heat wheels. HEs are the most common technology used for HR. They work by transferring heat from the wastewater to a heat transfer fluid, such as water or air. The heat transfer fluid is then circulated to a heat sink, where the heat is released (Sun et al. 2022; Wehbi et al. 2022). Moreover, HPs are another technology used for HR. They work by using electricity to move heat from the wastewater to a higher temperature level, which can then be used for heating. HPs are more efficient than HEs because they can produce more heat than the amount of electricity used to run them (Chae & Ren 2016; Serrat et al. 2023).

In addition, heat wheels are a less common technology used for HR. They work by rotating a wheel containing a heat-absorbing material through the wastewater, which then transfers heat to a heat transfer fluid. The heat transfer fluid is then circulated to a heat sink, where the heat is released. The benefits of HR from wastewater include reducing energy consumption and greenhouse gas emissions, as well as lowering operating costs for businesses and households (Jouhara et al. 2018; Ononogbo et al. 2023). Additionally, HR can help to reduce the demand for freshwater by using wastewater as a source of heat. However, it is important to note that the effectiveness of HR depends on several factors, such as the temperature of the wastewater, the amount of heat that needs to be recovered, and the available technology (Saagi et al. 2022).

Numerous countries aim to increase secondary energy sources, including HR. A reason behind that is strengthening energetic self-sufficiency and rescuing the environment affected by the exploitation of fossil sources. One effective solution is using warm wastewater's thermal energy as a secondary energy source (Stransky et al. 2016). Sewage HR applications are becoming widespread in energy-saving applications. A sustainable and low-emission operations in air conditioning and heating processes is achieved by harvesting the otherwise wasted energy in wastewater through specially developed HEs lying at the core of HPs (Culha et al. 2015). For instance, in central cities, 10% of the total energy spent in the whole urban water cycle is lost in the sewer systems and the primary energy consumption for collecting and transporting wastewater (Elías et al. 2014; Stransky et al. 2016). Therefore, HR from sewage is considered an efficient and energy-generating source. It is classified as a renewable energy source and can be optimally used in low-energy buildings for low-temperature heating, high-temperature cooling, and to preheat household hot water (Perackova & Podobekova 2013).

Cipolla & Maglionico (2014) reported that the amount of energy that can be obtained from wastewater and the optimal design of HR systems depend on the knowledge of the flow rate and the temperature in the sewer system in Bologna (Italy). The data analysis they used allowed them to identify the daily trend for the wastewater flow, whose coefficients in relation to the average flow (where average flow = 1) vary between 0.25 and 1.50, and for the wastewater temperature in which the coefficients is ranging from 0.90 to 1.05. Moreover, raw wastewater contains considerable amounts of energy that can be recovered by means of a heat pump and an HE installed in the sewer (Dürrenmatt & Wanner 2014). Hot wastewater is considered a valuable source of energy that could be recovered (Wehbi et al. 2022). Wastewater from domestic, industrial, and commercial developments maintains considerable amounts of thermal energy after discharging into the sewer system. It is possible to recover this heat by using technologies like HEs and HPs, and to reuse it to satisfy heating demands (Nagpal et al. 2021). In December 2018, wastewater was officially recognized by the European Union as a renewable source of energy, thus wastewater HR can be included in efforts to reduce greenhouse gas emissions. As the world's population continues to increase and the economy continues to develop, all scientists and research centers are looking for green energy sources and managing the current sources. The European Union Directive 2018/2001 recognized wastewater as a renewable heat source in compliance with the European environmental goals (Nagpal et al. 2021). Besides, global warming, greenhouse gas, and carbon footprint are deemed the most important, particularly in the water and wastewater sectors. Moreover, rising energy prices and concerns about global warming highlight the need to improve energy independence in wastewater treatment plants (WWTPs) (Chae & Kang 2013; Perackova & Podobekova 2013). Nevertheless, HR from wastewater can help decrease natural gas energy consumption, the associated energy costs, and greenhouse gas emissions, but the investment rate for the selected community size will be low (Garmsiri et al. 2014).

Many studies reported the mechanism of HR from wastewater using three components: household, sewer networks, and municipal WWTPs. Culha et al. (2015) reported that there are three different locations to recover heat from sewage. The first, HE, could be installed inside the facility to recover waste heat from household hot water, called domestic utilization. The second HE is located inside or outside the sewage networks, delivering considerable extra heat from wastewater to supply heating/cooling for multiple buildings. The principal requirement is that the sewer dimension should be more than 800 mm. Sewage flows are usually insufficient to get high heat efficiency in smaller sewer diameters, and the plant installation becomes complicated because of the diameter size of the sewer pipes (UHRIG 2022). The third HE can be installed downstream of WWTP to efficiently utilize the energy in the treated wastewater on a larger scale (UHRIG 2022). For instance, under some circumstances, 50% of energy could be stored or recovered using the HR system. In addition, all systems are considered economical as well as environmentally effective methods (Wehbi et al. 2022).

The objective of the study is to create a physical model to predict HR from wastewater, then what are the input parameters that should be provided for this model, and study the challenges. The model in this study has covered all aspects of HR from wastewater. The novelty of this study is to create a model to predict HR from wastewater. The model could be applied at any location in the world having similar issues.

The descriptions of the methods used in the study are given in the following:

The storm water management model (SWMM) keeps track of the flow rate, flow depth, runoff volume generated within each sub-catchment and the quality of water in each pipe and/or channel at different times of the simulation. SWMM Version 5.2.1, running under Windows was used in the present study, as it provides simulations, an environment for editing study area input data, running hydrologic, hydraulic and water quality simulations, and viewing the results in a variety of formats. These include the color-coded drainage area and conveyance system maps, time series graphs and tables, profile plots, and statistical frequency analyses. ArcGIS Pro 3.0 version was used as a tool for preparation of input data for SWMM5.

There are seven types of reinforced concrete manholes in the sanitary sewer of the Karbala city center, namely, AS, BS, CS, BD, CD, CD1, CD2. Table 1 lists the specifications and dimensions of these manholes (Directorate of Karbala sewerage 2022).

The topography of Karbala city has made it necessary to include pump stations in certain locations of the sewer network system, in order to facilitate wastewater flow through the sewers reaching the discharge point. Two types of pump stations were used in the design of the sewage project for Karbala city. These are submersible pump stations and screw pump stations (Directorate of Karbala sewerage 2022).

There are two WWTPs in Karbala city center, the old WWTP located east of the city with a maximum capacity of 70,000 m³/day. There is another new WWTP with a capacity of 700,000 m³/day. The plant has been designed to receive wastewater from Karbala neighborhoods. The location of the plant is about 13 km southwest of Karbala city center along the road of Karbala – Najaf as shown in Figure 3(b) (Directorate of Karbala sewerage, 2022).

This study presents the results obtained using physically based and statistical models. These results are selected and analyzed to fulfil objectives of this study. They are divided into two sections: results of sewer quantity modeling and results of HR modelling. Finally, the obtained results are analyzed and discussed to fulfil the objectives of the study.

The sewer network in the city center of Karbala is already fully implemented. Some outlying areas of the center, as well as nearby agricultural areas are still out of the sewer network. Both networks are more than half a century old. The sewer discharge is first modeled using the SWMM using input data to simulate WF for the period 1 November 2019 to 10 January 2022. The daily distribution of the total population in the city, including permanent residents, is shown in Figure 7. The figure shows that the population in the area varies between less than 305 and close to 40,000 persons during normal days.

The peak flow was calculated by multiplying the average flow with peak factor. The peak factors in the sewerage network generally decreased from upstream to downstream. Therefore, the value of the peak factor is decided based on sewer type or position in the sewer network. This research established that the peak sewerage flow factor (1.256 < P_f = 1.68 < 1.77) and significant difference (p < 0.001) during the period of this study. Gaines (1989) conducted a study for monitoring sewer wastewater in the city of Denver in Colorado. He plotted the rates of flow that are higher than the average rate of peak flow, approximated the normal probability distribution, and showed accurate prediction of flow rates. Following similar procedure in the present study, the peak factor value was estimated as 1.68 < 2.4 in critical condition. The value was similar to that reported by EPA (2004), i.e., maximum peak factor closes to 2.4.

Results obtained using statistical models namely, MLR analysis and structural equation modeling (SEM) are described in this section. For the modeling of HR from wastewater, the available data were divided into two parts: 70% of available data were used for model calibration and the rest 30% data were used for model validation, in addition the verification model with others previous studies. It was mentioned earlier that wastewater data in the study area is available only for a few days (from 1 November 2019 to 10 January 2020). Therefore, model results for both calibration and validation periods are presented together in this study. Variance inflation factors (VIF) for WF, TW, and TA were achieved with the requirement for a reflective measurement model. Since the model was reflective and flexible between the first and second order, the direction of causality from construct to measure was used. As such, it is required to check multi-collinearity, as shown in Table 3.

Correlations of TW, TA, WF, and HR are listed in Table 2. The table shows a strong correlation between WF and HR in the study area. This indicates that HR in the area is highly sensitive to WF. On the other hand, partial correlation between WT and HR was found to be low, which indicates low influence of WT in HR in Karbala. Since the sewer networks are basically installed underground, a low influence of TA on HR is expected (p-value = 0.53). Hence, in this study, it has been assumed the internal temperature of sewer networks equals to weather temperature which may it considered as a cap of this study.

Since the model was reflective and flexible between the first and second orders, the direction of causality from construct to measure was used, and as such it is required to check multi-collinearity (Table 3). Variance inflation factors (VIFs) for WF, TW, and TA was achieved with the requirement for reflective measurement model.

Table 4 shows the item's loading and t-statistic on their corresponding constructs for HR. The results showed that all the items considered in the present study have satisfactory indicator reliability, and t-value of HR → TW, HR → TA, HR → WF, TA → TW, TA → WF, and WF → TW are more than 1.28 (Hair et al. 2010). Also, the path coefficients of HR → TA, TA → TW, TA → WF, and WF → TW are less than 0.5. However, HR has most influenced by wastewater temperature and flow.

The physically based and statistical models were used to simulate HR form wastewater by some factors such as TA, TW, and WF. The obtained results revealed that the models were capable of estimating the influence of TA, TW, and WF on HR quantity. The study clearly showed that WF in Karbala, played a major role for HR. Conclusions can be made based on the results presented. A model has been developed in this research to estimate the performance of HR, which can be used for solving this major problem of heat consumptions in the cities across the world.

The results indicate that the models developed in the present study are capable of providing results which are in good agreement with the observed data. Therefore, the models can be used to simulate the HR from wastewater. A key requirement for integrated HR models is data to TW, TA, and WF, ideally including uncertainty estimates. In this model for the 70 days, the largest HR quantities form wastewater were from LINE4, it was 36,796 Mw, then LINE3 with amount 34,716 Mw, the third quantity of HR was 29,073 Mw from LINE5, then 23,756 and 10,066 Mw from LINE2 and LINE1 respectively, the lowest value of HR was 1,613 Mw from LINE6. However, the total amount of HR from wastewater in Karbala city center during the 70 days was 136,000 Mw.

HR from sewage is considered the most important source of renewable energy. Having proven to be a practicable and promising technology, it has been very successful and a very reliable and clean source of energy when proper management programs are followed. The associated harmful environmental, health, and social effects of traditional biomass, fossil fuel, and polluted other sources have enhanced the growing interest in the search for an alternate cleaner energy source globally. The major objective of this study is to develop a model to assess the impacts of WF, TW, and TA on the performance of HR. Sanitary sewer networks in the Karbala city of Iraq were chosen as a case study. SWMM, MLR, and SEM were used for this purpose. The model outputs were analyzed to assess the performance of HR in the context of changing WF, TW, and TA. The results showed that the total amount of HR from wastewater in Karbala city center during the 70 days was 136,000 Mw. The study clearly showed that WF in Karbala played a major role for HR. Basically, the heat from wastewater is CO₂-free and represents a significant opportunity for the energy transition in the heating market.

The authors thank Institut für Energietechnik – Technische Universität Berlin for their support in preparing this study. They would like to thank Directorate of Sewerage of Karbala for providing data and University of Karbala/Engineering College for supports.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.