District heating based on biogas from wastewater treatment plant

District heating based on biogas from wastewater treatment plant Alberto Picardo a, Victor M. Soltero a, M. Estela Peralta a, Ricardo Chacartegui b a Department of Design Engineering, University of Seville, 41011, Seville, Spain b Department of Energy Engineering, University of Seville, 41092, Seville, Spain Highlights Methodology for evaluation biogas district heating. Circular economy and Energy recovery. Wastewater treatment plant. Analysis of the viability of biogas district heating in 88 WWTP. Abstract Wastewater treatment plants have a relevant role within the circular economy chain of the cities. They purify contaminated waters and materials and biogas can be recovered from their processes. This article analyzes the potential integration of District Heating systems with wastewater treatment plants, using the biogas generated, in combination with natural gas, to supply heat and domestic hot water to the municipalities integrated in the wastewater network. A methodology is proposed to analyze the potential for implementation of these systems based on the evaluation of the requirements of transmission and distribution networks and cost benefit analysis. As case study this methodology was applied to 88 sewage treatment plants located in the Atlantic and Continental regions of Spain. Each wastewater treatment facility provides services to more than 50,000 equivalent users. The analysis is oriented to district heating system in cities with high/medium urban wastewater load. Analytical expressions to estimate capital expenditures as function of district heating network capacity have been obtained from data of the municipalities under analysis. The results for the analysed region show a potential for CO2 emissions savings of 1.8 Mt per year with an overall investment of 2,854 M euros. 1. Introduction Building sector is one of the largest energy consumers with more than a third of the global energy consumption and with an estimated demand growth of 50% by 2050 [1]. The percentage of global energy consumption in residential areas is 40% with a high fraction of this energy used for heating and cooling, 70%. The energy supply is based mainly on fossil fuels, 75%, meanwhile a relevant part is produced by renewable energy sources (11% of biomass and 7% distributed in wind, photovoltaic, solar and geothermal). In addition, they contribute to climate change with a volume of 36% of the total greenhouse gases (GHG) generated [2,3]. With the aim of ensuring energy supply for the coming decades and reducing the impact related to climate change, the Energy Efficiency Directive (EED e 2012/27/EU) [4] defines a common framework for the promotion of energy efficiency in the European Union. The Renewable Energy Directive (RED) 2009/28/EC [5], where the objective of increasing up to 20% the contribution from renewable energy sources for the year 2020 is established. European Union energy policies include: i) strategies to facilitate the development and implementation of energy infrastructures, with the goal for 2050 of a low carbon economy with a reduction of 80% of GHG emissions in relation to 1990 levels (40% in 2030 and 60% in 2040) [6]; ii) increasing the share of renewable energy between 55% and 75% of final gross consumption in the EU [7]; iii) bioeconomy strategies [8] oriented to an efficient use of renewable resources [9]; iv) circular economy initiatives that establish the interest of reusing or remanufacturing any by-product [9]. Related to these goals, in February 2016, the first action plan to reduce the energy consumption of heating and cooling of buildings in Europe was published [10]. In this context, the technological development related to renewable energies has experienced a significant growth in the last decade. Technologies such as Combined Heat & Power (CHP), biomass and waste gasification, solar thermalsolar water heating collectors or district heating (DH) are contributing to these objectives within the regulatory framework. Among them, district heating and cooling (DHC) systems are of interest to improve energy efficiency for Domestic Hot Water (DHW) and heating systems. A DHC system [11] distributes hot/chilled water or steam at low pressure from a central station to the residential complex through a network of underground pipes. There are currently more than 5,000 DH systems in Europe [2] covering around 10% of the EU's energy demand. These systems can use different types of energy sources (fossil fuels [12], biomass [13], geothermal [14], thermal solar [15], residues [16,17], waste heat from industrial processes [18] or cogeneration of power and heat (CHP) [19]). Currently, natural gas is the main energy source in DH (40%), followed by coal (29%), biomass (16%) and solar (3.2%). Although the current level of implementation faces technical and economic challenges, it is one of the key technologies for the sustainable energy transition and an opportunity to reduce the environmental impact of the residential sector, decreasing the contribution of fossil fuels and CO2 emissions. In addition, it promotes the reduction of energy costs by reducing investment costs and operating and maintenance costs [20,21]. There is a relevant number of research projects [22] whose objective is to improve the efficiency and optimization of heating networks such as OPTi [23], FLEXNETS [24], STORM [25], InDeal [26], E2District [27] and H-DisNet [28]; and to improve their performance by the integration of renewable or residual heat sources such as Upgrade DH and KeepWarm [29]. Recent studies estimate that in the EU, a large part of renewable energy will come from agriculture, forestry and livestock [30]; with 25% of all bioenergy obtained from biogas generated from organic matter. Biogas presents different economic and environmental advantages compared to other renewable and non-renewable fuels. It is generated by anaerobic digestion or natural degradation in the absence of oxygen by microorganisms of organic material from a wide range of industrial products, municipal or agricultural organic sources [31]. The main by-product is a renewable gas composed by 50–65% CH4 and 35–50% CO2 as main components. Along the period 2000-2014 biogas production in the world average growth was 11.2% (58.7 Nm3 in 2014); with an average energy density of 21.6 MJ/Nm3 [32]. At European level, the consumption of biogas is 16% of the total thermal energy consumption [[31], [32], [33]]. In 2015, 26.6 PJ of heat from biogas supplied urban heating networks. In addition, it is estimated that by 2020, the energy consumption from biogas will reach 11 Mtep (approximately 28 Mtep of primary production). Although at European level there is not a specific policy on biogas, its generation and use is included in three regulatory frameworks related to renewable energies and bioenergy: Renewable Energy Directive (2009/28/EC), Directive on Recycling and Waste Recovery (2008/98/EC) and Directive on the dumping of waste (1999/31/EC). Renewable energy policies include strategies to achieve a biogas methane contribution by 2020 above 1.5% of the EU's primary energy mix (approximately 5% of natural gas consumption in equivalent energy terms) [34]. Regarding the sources of biogas production (landfill gas, sewage sludge, anaerobic digestion or thermochemical processes), the anaerobic digestion process in the Waste Water Treatment Plant (WWTP) has the greatest growth [35]. WWTPs are relevant energy consumers in cities, so in addition to the basic objective of cleaning and treating wastewater (for reuse, reduction of odors and pathogens), they must be designed to be energy efficient. Industrial facilities have intrinsically a residual energy potential, which allows the recovery of part of the consumed energy [36]. With effective strategies WWTPs are an opportunity for energy generation and recycling of by-products. Specifically, with the production and efficient use of biogas, they will contribute to the closure of nutrient cycles, a strategy fostered by the new regulatory framework for circular economy and bioenergy [37,38]. The use of WWTP biogas depends on the country and the type of exploitation. In Europe, a large part of the biogas is directly vented and burned [39]. There are different options for biogas valorization: (i) use of biogas as fuel for heating the digester sludge; (ii) the combined generation of heat and power in a cogeneration system (CHP) [40,41], being currently the most feasible technology for medium and large size WWTP; (iii) biogas conditioning and debugging processes (upgrading biogas) for injection into local natural gas networks, although the quality specifications for their injection are still not regulated; (iv) use in fuel cells, a technology not fully developed at industrial level [42,43]; and (v) transmission (transmission and distribution networks, trucks or train), decentralizing the energy production of the generation of the resource, with logistical challenges and potential additional GHG emissions linked to the transport [44]. One interesting option is the use of biogas in district heating technology. There is a lack of methodologies for the design, analysis and development of DH with WWTP biogas. Nowak et al. [45] study different ways to optimize the energy balance of the WWTP. This work shows that WWTPs can cover their power demand with biogas and offer electricity savings up to 45% in DH systems. There are studies that evaluate the potential of wastewater heat pumps as largescale district heating systems using waste heat [35]. Kollman et al. [46] conclude that it would be possible to provide significant amounts of heat to consumers adjacent to WWTPs at competitive costs and with clear environmental benefits. In addition, they develop a method to evaluate the integration of WWTPs as local energy suppliers. Silva et al. [47] analyze the energy potential of the generation of electricity from biogas in WWTPs. They conclude that viability is reached only for cities with a population over 300,000 inhabitants. Different works show the viability and technical feasibility of using the biogas obtained from the anaerobic digestion of the organic matter coming from the agriculture and livestock sector in power generation plants and DH for domestic hot water supply [[48], [49], [50]]. In this work the potential of using WWTP biogas as energy source for integrated waste water treatment plants-district heatings, WWTP-DH, to supply heat and DHW to the municipalities integrated in WWTP network (Fig. 1) was evaluated. It complements the studies carried out for the evaluation of the potential of DH [2,17,34,[51], [52], [53], [54]]. The technical feasibility and viability were analyzed according to the number of equivalent inhabitants and the distance between WWTP and the municipalities. A design method was developed to ensure efficient heating and DHW supply at different seasons, with the support of the natural gas network to meet the demand not covered by biogas. The methodology allows to evaluate the economic and environmental potential of the WWTP DH systems to be implemented. The methodology is oriented to WWTPDH systems in cities with high/medium urban wastewater load. Within the methodology analytical expressions have been derived to estimate capital expenditures as function of the district heating network capacity The structure of this papers is as follows. First the WWTP-DH methodology was developed, describing the different steps and assumptions. Then this methodology was applied to a case study, where a specific region was analysed, selecting 88 WWTPDH for analysis. Finally, the main conclusions of the study were included in section 4. Fig. 1. Schematic network for WWTP biogas DH. 2. Methodology In this section is presented a methodology for evaluation of the potential for developing WWTP-DH systems in a region. Fig. 2 shows the structure of the proposed methodology. It allows the design and analysis of the DH transmission and distribution networks. It provides technical, economic and environmental results for analysis of the viability the integrated systems. The methodology is defined for the analysis of the design and operation of WWTP-DH systems in groups of municipalities with high and medium urban wastewater loads. The main steps of the methodology are described in the following sections. Fig. 2. Methodology for evaluating the potential of implementing DH in WWTP. 2.1. Step 1. delimitation of the study area and WWTPs selection The study area is selected for WWTP urban wastewater treatment plants whose design population exceeds 50,000 equivalent inhabitants. This lower limit for the design population restricts the application in municipalities with a relatively small population, or with a dispersed urban structure or with reduced wastewater loads and then with a reduced biogas generation capacity. Usually, wastewater treatment is carried out jointly for several municipalities and wastewater treatment plants are relatively far from urban nucleus. In these cases, WWTP location constrains the heating network size, and a longer transmission network is required. It results in low values of linear heating density and long payback periods. 2.2. Step 2. determination of biogas generation potential for the selected WWTPs The anaerobic digestion of sludge is one of the most common forms of biological treatment in wastewater treatment plants with more than 50,000 equivalent inhabitants. It allows a significant degradation of organic matter through a biogas-producing bacterial fermentation in the absence of air that produces a gas with methane as main component. This slow procedure requires a combination of conditions for an efficient gas production: temperature, dry matter concentration in the sludge to be digested, residence time, pH, alkalinity, etc. One of the parameters used to measure sludge stability is the content in volatile solids as indicator of the organic matter in the sludge. The amount of volatile solids reduced in the stabilization process is usually used as a measure of the effectiveness of organic matter stabilization and it depends on the type of digested sludge (primary, biological, mixed), temperature and residence time. The production of biogas is proportional to the reduction of volatile solids, with usual values in the range from 0.75 Nm3/kg to 1.1 Nm3/kg (0 °C and 1 atm.). According to Refs. [55,56] biogas production can be estimated based on the number of equivalent inhabitants. In primary treatment plants treating domestic wastewater of medium concentration, the production usually ranges between 15 Nm3 and 22 Nm3 per every 1,000 equivalent inhabitants and day. In secondary treatment plants biogas production increases up to values close to 28 Nm3 per 1,000 equivalent inhabitants and day. Therefore, the potentially available biogas flow can be expressed as function of the estimated average normal production of biogas per equivalent inhabitant (ηg) and design population (pd): 𝑄𝑄𝐷𝐷𝐷𝐷 =𝜂𝜂𝑔𝑔𝑝𝑝𝑑𝑑 From this biogas flow has already discounted the gas consumed in the internal processes of the WWTP (i.e: for the acceleration of the internal digestion process). 2.3. Step 3. determination of the heat demand covered for each WWTP and associated DH residential areas Overall heat demand covered by the WWTP-DH system is estimated from the analysis of the potential energy available in the WWTP biogas and the number of dwellings in the WWTP area. Heating demand is affected by several factors: i) mean average occupancy of dwellings; ii) dwellings mean heating area; iii) demand correction factors due to buildings age and users characteristics. DHW demand can be evaluated with flow rate and supply temperature values per inhabitant set in the regulation and with the fresh water supply temperature that depends on location and season of the year, Fig. 3. Fig. 3. DHW demand and potential energy covered with WWTP biogas. The size of the population to be supplied with domestic hot water along these periods can be evaluated from the potential energy available from biogas. The overall heat demand covered by the DH system is determined from the DHW and heating demands. The selected criteria for DH design and sizing has been the use of the available WWTP biogas to cover the DHW baseline demand in the warmest period of the year, Fig. 3. For each dwelling, heat load is given by the expression [51]: P=Kbuilding⋅(tc,indoor−tout)=(∑i=ni=1Ai⋅Ui+Vainf⋅ρ⋅Cp)⋅(tc,indoor−tout) (2) where Kbuilding is the overall heat transfer coefficient of the building, Ai is the external wall area of the building, Ui is the overall heat transfer coefficient of building envelope, Vainf is the air infiltration flux, ρ is the external air density, Cp is the specific heat of external air, tc,indoor is the indoor confort temperature (base temperature 20 °C) and tout is the ambient air temperature. The estimated global heat load PDH can be expressed as: PDH=nusersDH⋅Kbuilding⋅Smed (3) where, Smed is dwellings mean area in the residential area under analysis and nusersDH is the number of dwellings to be supplied by the DH network. Fig. 4 shows the evolution of heating and DHW demands in a district of Zaragoza (Spain) along the year; the heat demand not covered by biogas, corresponding to heating, must be covered by the complementary contribution of another sources, usually natural gas. In cases with space constrains or absence of natural gas supply network, integration of other fuel supply systems is required (i.e. liquefied natural gas satellite plants). Fig. 4. Heating and DHW demads in a district in Zaragoza (Continental region of Spain). 2.4. Step 4. DH network design The following criteria have been considered for DH network design. Transmission lines are considered to be built with pre-insulated steel pipes and for heat losses calculations, a thermal gradient of 20 K has been considered. The length of transmission network is a determining factor for the economic viability of the DH system [57]. It is determined by the distance between the WWTP and the outer perimeter of the urban nucleus to be supplied. Within the urban core, the distribution network is considered as a whole, including service pipes, Fig. 5. Fig. 5. Example of transmission and distribution DH network for WWTP (Huesca, North of Spain). This price should be lower than the price of the baseline to make it attractive to the consumer. In many systems a fixed cost is set for the end user, which would imply fixed incomes. In this analysis, this option has not been considered. 2.6.4. Operating expenditures (OPEX) In the cost-benefit analysis, the operating costs (OPEX) are given by the cost of the natural gas and pumping cost, which depend linearly on the heating demand. Besides, the fixed operating costs must be added. OPEX=Ccentralfuel+CPumpingcostsE+CWWTPfixed (14) where, Ccentralfuel is the product of the natural gas price of each DH for the energy consumed by the system subtracted the demand of DHW. The pumping cost CPumpingcostsE depends on the heat demand and, to a minor dependence, on the topology of the network. A quick estimation of fixed costs CWWTPfixed in this application can be set at 5.5% of the investment. It includes the indirect costs as management, maintenance and insurances [65]. Pumping costs can be effectively calculated as presented in Ref. [66]. In this work, as climate data and networks are known for each location, annual degree hours, heat load, supply temperature drops and flowrates can be calculated for each system. From them, pumping energy can be obtained and, with the hourly variation of electricity prices, the associated pumping costs. As they are quite different systems, pumping costs associated with the transmission and distribution networks are analysed separately. The natural gas covers mainly heating demand. In this way, the biogas that is generated with a continuous constant production in the WWTP is responsible for covering almost all of the DHW demand. Therefore, fuel consumption will drastically vary along the year: in the hottest seasons of the year, hardly any natural gas will be required for the operation of the system; while in winter, the natural gas contribution will be high. Taking into account that the fuel is used mainly to meet the heating demand, the annual operating costs associated to natural gas can be determined. 2.6.5. Analysis of the viability and the return of the investment Facility lifetime will depend on DH thermal energy generation technologies and materials. As an infrastructure, lifetime designs periods above 30 years must be considered. For evaluating and comparing the investments the internal ratio of return (IRR) was selected as parameter: (15) 2.7. Step 7. environmental assessment of the DH system potential For the evaluation of CO2 emissions savings by the implementation of the WWTP DH systems the most extended thermal generation technologies for heating and DHW in the region under analysis must be identified and associated current CO2 emissions predicted. With DH systems, the centralized generation of thermal energy will be carried out more efficiently and with a decrease in fuel consumption. It implies to a significant reduction in CO2 emissions. Besides the WWTPDH system uses biogas for DHW, which provides neutral CO2 emissions. CO2 emissions savings are obtained comparing current emissions and the estimated demand from a WWTP DH system: (16) where, D is the demand covered by DH, CurrentEmissions are current CO2 emissions with boilers, ηBoiler is the individual boilers efficiency, DHEmissions are CO2 emission of the DH thermal station and ηDH is efficiency of the thermal station. Carbon dioxide emissions savings are given on one hand by the reduction in natural gas consumption due to the higher efficiency of DH compared with individual heating facilities and on the other hand by the replacement of part of the fuel by the WWTP biogas, with a neutral carbon balance. 3. Case study In this section the proposed methodology is applied for its validation. In the case study, a number of 88 Waste Water Treatment Plants were considered in the Continental and Atlantic regions of Spain. Each WWTP provides service to more than 50,000 equivalent inhabitants. Only in WWTPs with a minimum size the investment in the transmission network is justified. 3.1. Step 1. Delimitation of the study area and WWTPs selection The study area is distributed in the continental and Atlantic zone of Spain. For this study a total of 88 WWTPs were selected. Thy were selected as function of the WWTP design population. All the selected WWTPs were designed for a minimum of 50,000 equivalent inhabitants. In terms of biogas capacity, a design population equal to the population connected to the WWTP has been considered in this study. This value is lower than the WWTP design size, since the plants do not normally operate within the limits of their maximum capacity, unless the population growth reaches design values. This circumstance is on the side of safety in relation to the estimated biogas generation capacity. The usual long distance between WWTP and demand nodes implies an extra investment in this WWTP-DH networks, with respect to those that have the generator within the urban nucleus. On the contrary, the location of the DH generation plant within the plot of each WWTP has the advantage of being located on consolidated land according to the corresponding urban planning regulations for industrial or equipment use, avoiding the potential public rejection of an infrastructure of these characteristics [67]. Besides, an adequate planning when WWTP are designed, that takes into account the space required for DH piping in the WWTP connections, can drastically reduce the overall civil works and costs of the joint facilities. The WWTPs selected for the study were among 1,186,086 and 50,000 equivalent inhabitants. The WWTP with the highest treatment capacity corresponds to the WWTP SUR (Madrid), while the one with the lowest capacity is the WWTP of La Muela-Plaza (Zaragoza). Fig. 8 shows the locations and data of the set of WWTP selected for the study case. Main data of these WWTP are given in Annex 1. It includes population, location extracted from Refs. [68,69] as well as buildings occupation and socioeconomic data, extracted from Spanish Statistical Office [70]. Data about houses area and buildings age were extracted from the official register of Spanish buildings, that belongs to the Ministry of Treasury (“registro del catastro” in Spanish) [71]. Fig. 8. WWTPs selected for the case study. 3.2. Step 2. Determination of biogas generation potential for the selected WWTPs The biogas available for the DH network can be obtained by evaluating the biogas generated in each WWTP according to its capacity and taking into account the gas consumed to support the internal digestion process. For this case study, the annual energy available from biogas in the WWTPs varies in the range between 77,024 MWh/year and 3,247 MWh/year. The overall potential for WWTP biogas is 1,061 GWh/year. Taking into account the biogas potential for each WWTP and that it will be used to cover DHW demand, the number of homes to be supplied by each DH network can be calculated. For this, temperature of water in the network, that depends on location and average occupancy of houses in each municipality, are taken into account. Under the previous hypothesis and based on the data obtained for this case study, an analitycal expression was obtained in this work, eq. (17). It gives a linear correlation between the design population of the WWTP (pd) and the number of dwellings supplied (nusersDH), Fig. 9. nusersDH=0.0556⋅pd (17) Fig. 9. Correlation between design population and number of dwellings supplied by the WWTP-DH. 3.3. Step 3. Determination of the heat demand covered for each WWTP and associated DH residential areas From the number of inhabitants, the average inhabitants per dwelling, the average area per dwelling and the climatic data for each system, the maximum heating load to be cover by the DH can be calculated. For calculation, the minimum ambient temperature, the 99th percentile was used. The results are in the range between 17.8 MW and 465 MW with an average heat load of 74.7 MW. Fig. 10 shows the relationship between the design population of the WWTP and the heat load of the WWTP DH system. The expression that correlates these two magnitudes is the following: P(MW)=4⋅10−4⋅pd (18) Fig. 10. Relationship between the design population of the WWTP and DH system heat load. 3.4. Step 4. DH network design In this study, the transmission and distribution networks were clearly differentiated. The length of the transmission network depends on the distance between the corresponding WWTP and the nearest population center. This distance is variable for each case and it does not depend on the size or capacity of the WWTP. However, in this study a slight tendency has been obtained with WWTPs of lower capacity closer to urban nucleus, and reduced transmission network lengths. For this case study transmission network lengths are in the range between 0.14 km and 9.02 km with an average value 2.44 km. These values were determined from real data in each study area. Fig. 5 shows an example of the transmission network in one of the analyzed WWTPs. The distribution network was obtained evaluating the parameters (d1, d2 y h) in each location. Based on these parameters the area to be supplied by the DH system was evaluated. Under the assumption of a meshed grid distribution network, the expression given in eq. (5) was used to estimate the length of the distribution network. For this case study, distribution network lenghts were in the range between 71.37 km and 2.43 km with an average value of 14.37 km. 3.5. Step 5. Determination of the operating parameters of the DH system for each WWTP and urban residential area As indicated in methodology, overall energy demand will be affected by the percentage of dwellings without occupation, the energy poverty index and the age of the dwellings in each study area. The percentage of dwellings without occupation increases the number of dwellings to be covered by the DH network. For this case study dwellings without occupancy percentage varies from 1.93% to 28.79%. The energy poverty index and the age of the dwellings were used to correct heat demand. The annual demand in the WWTP-DHs is between 37,520 MWh and 1,062,099 MWh, with an average value of 170,415 MWh. These demand values include both heating requirements and those needed for the production of DHW. 3.6. Step 6. Cost benefit analysis of DH system In this application, where WWTP-DHs are integrated in large urban areas with natural gas supply, the reference of energy sale price is determined by the price of natural gas. The mean energy price for fuel in current natural gas boilers was estimated at 98.3 €/MWh. It includes the average seasonal performance of the boilers, natural gas price, fixed rates and special taxes. Taking into account this reference baseline, the energy sale price of the DH in this study was set at 85 €/MWh. It implies savings of 12.38% from current end user heating costs. This value was considered competitive enough and it is within the range of the studied in Ref. [72]. Fig. 11 shows a sensitivity analysis of IRR (Internal Rate of Return) of the system as function of the energy sale discount rate and the energy sale price. Fig. 11. IRR values for different heat sale prices and discount rates at the WWTP EDAR SUR (MADRID). Table 1 shows the most relevant economic results of the economic analyses, with WTTPs grouped by size. It includes the estimated investment, pumping costs, fuel costs and IRR. Table 1. Study case main results. Classification Large WWTP Medium WWTP Small WWTP Total Number of WWTP 7 32 49 88 Equivalent Inhabitants of the WWTP 1,186,086- 509,089 496,578- 127,500 119,170- 50,000 16,336,963 Biogas Available for DHW [MWh/year] 77,024-33,060 32,248-8,280 7,739-3,247 1,060,922 Percentage of Empty Homes 15.32%-10.00% 23.76%-2.86% 28.79%-1.93% 28.79%-1.93% Energy Poverty 25.99%-18.49% 32.85%-15.39% 49.18%- 13.17% 49.18%- 13.17% Inhabitants Supplied by DH 162,117-69,583 67,873–16,021 16,288-6,357 2,155,184 Heat Load (MW) 464.9–199.6 194.7–49.2 51.9–17.8 6,576 Transmission Network Length (km) 7.05–0.59 9.02–0.22 5.98–0.14 214.7 Distribution Network Length (km) 71.37–20.42 47.88–3.76 25.04–2.43 1,264 Diameter of the Transmission Network (mm) 1,200-900 900–450 450–250 1,200- 250 Overall Heat Demand (MWh/year) 1,062,099- 455,872 444,668-98,580 116,490- 37,520 14,99-6,544 LHD, Distribution Network (GJ/m) 113.0–38.5 170.3–14.3 95.87–9.81 170.3–9.81 LHD, Transmission Network (GJ/m) 4,516-467 3,187-91.9 2,274-25.6 4,516-25.6 Investment Distribution Network (k€) 27,045-8,522 13,956-1,667 6,663-1,054 427,723 Investment Transmission Network (k€) 18,762-1,311 12,158-198 4,566-92.3 219,358 Total Inversion (k€) 200,138-77,112 74,594-18,854 28,215-9,741 2,854,398 Total Pump Cost (k€/year) 2,526-790 1,288-155 621–101 2,526-101 Cost of Natural Gas for Heating (k€/year) 34,182-14,672 14,311-3,133 3,774-1,186 483,566 IRR30 14.89%-12.23% 15.70%-6.84% 14.70%-5.42% 15.70%-5.42% IRR30 Without Transmission Network 15.69%-13.12% 16.73%-7.96% 15.44%-6.19% 16.73%-6.19% CO2 Emissions savings(t/year) 126,920-54,477 53,137-12,115 13,710-4,670 1,815,085 Percentage of Renewable Energy 7.96%-6.39% 8.91%-4.91% 9.56%-4.52% 9.56%-4.52% The investment required for the distribution network ranges between 1,054 k€ and 27,045 k€, with an average value of 4,860 k€. The investment for transmission network varies from 92.3 k€ to 4,516 k€, with an average value of 2,493 k€. Pumping costs are separated for the transmission and the distribution networks. Pumping costs of the distribution network oscillate between 97.2 k€/year and 2,495- k€/year, with an average value of 448 k€/year and for the transmission network between 30,698 and 203 €/year, with an average value of 3,920 €/year. IRR values vary in the range between 5.42% and 15.70% with an average value of 11.05%. To identify the influence of the transmission network on these systems, an economic analysis of the networks under study has been perfomed disregarding the investment in the transmission network and the associated pumping costs, that is, considering that DH system is located next to the urban nucleus. In this case IRR values are in the range from 6.19% to 16.73%, with an average value of 12.17%. It shows that transmission network length is an important parameter for the economic viability of the project, but for the considered values, it is not definitive. For the evaluation of the impact of the investment in the transmission network, in terms of transmission network length, the use of the parameter Design Heat Power Index is proposed. It is defined as:(19)where P is the heat load required by WWTP-DH in MW and is the transmission network length in km. This parameter can be correlated with the transmission network investment. Evaluating this parameter for the 88 WWTPDH under study, Fig. 12, it has been derived the following expression, eq. (20):(20)where, is the transmission network investment (€) and is the design heat power index (MW· km). Fig. 12. Relationship between Design Power Index by network length and investment. In terms of linear heat density, there is a great dispersion of IRR values, Fig. 13. This is due to the influence of the transmission network on the overall investment and the relevant differences in transmission length for the WWTP-DH systems analysed. Fig. 13. Linear heat density ratio and IRR 3.7. Step 7. Environmental assessment of the DH system potential To evaluate the CO2 emissions in the systems under analyses, current heating and DHW technologies must be characterized. In the case of urban areas of Spain, electricity, natural gas and other petroleum products such as butane gas are the most used energy sources used for heating and DHW. Current CO2 emissions with these technologies were estimated using data extracted from the Institute for Energy Diversification and Saving (IDAE) [73]. This database applies different CO2 emissions for natural gas combustión as function of the region: 284.28 kgCO2/MWh (North Atlantic Region) y 276.13 kgCO2/MWh (Continental Region). For this study case, the implementation of these WWTP-DH systems would generate annual CO2 emissions savings in the areas covered by the WWTP-DH between 10,130 tons of CO2 and 126,920 tons of CO2, with an average value of 20,626 tons of CO2. The overall CO2 emissions savings would be 1,815,085 t of CO2. Table 1 shows main results of the application of methodology to 88 WWTP. They have been grouped in terms of the size of the DH system. There are 7 large DH systems with capacities above 500,000 equivalent inhabitants, 32 medium size DH with capacities comprised between 120,000 and 500,000 equivalent inhabitants and 49 small DH with capacities in the range between 50,000 and 120,000 equivalence inhabitants. From the results shown in Table 1 is derived that with the implementation of these 88 WWTPDH in the area under analyses, there would be a heat supply capacity for 2.2 Millions of inhabitants, applied to WWTPs with a design population of 16,3 Millions of equivalent inhabitants. The biogas produces annually 1,060 GWh. Heating power of the DH systems varies from 18 MW to 465 MW, to supply a total amount of 14,996 GWh per year. Transmission networks connecting WWTP with distribution networks and end users have lengths in the range between 0.14 km and 9 km. Viability of the DH system depends on this value together with the size of DH and the linear heat density of the heating network, that varies from 9.8 GJ/m to 173 GJ/m. Internal Rate of Return at 30 year varies in the different WWTP-DH systems from 5.42% to 15.70%. Without including the transmission network IRR30 values are in the range between 6.19% and 6.73%. From an environmental point of view, the 1.8 Millions of CO2 tons could be saved with an estimated investment of 2,854 M€. 4. Conclusions In this paper a methodology for the evaluation of DH systems integrated with Waste Water Treatment Plants is proposed. WWTP plants are considered as local energy sources next to urban areas and with a high potential for the integration with low carbon DH using the generated biogas in the WWTP processes. The methodology was applied as case study to the North Atlantic and Continental regions of Spain. The analyses show that there is potential for the implementation of integrated waste water treatment plants-district heating networks designed for using the biogas generated to supply DHW along the whole year. The application of the methodology to WWTP 88 resulted in an investment of 2,854 M€ which would generate 1.8 Mt of CO2 savings, and IRR values in the WWTP-DH that would be between 5.42% and 15.70%. The length of the transmission networks ranged from 0.14 km to 9.22 km, with an extra cost for the transmission network from 92.3 k€ to 18.8 M€. The additional cost due to the location of DH generation systems in the WWTP implies a greater investment in the transmission network to the urban nucleus. By other side, it has the advantage of being integrated in a location already prepared for an industrial use, and not be subjected to social rejection that could affect to the viability of the projects. Based on the results obtained, the relationship between the capacity of wastewater plant and the potential of the heating network has been studied showing a linear relationship between the design population of the WWTP and the number of houses of the DH. Besides, analytical expressions for evaluating capital investments have been developed to ease the application of the methodology. On this purpose parameters based on DH heat power have been defined. In this work, natural gas was used as a complementary source of energy, but at the future it would be of great interest to identify strategies for incorporating renewable energy sources and waste heat sources in the WWTP, with use for low temperature networks under development. 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