**4. Heat-Sector**

PtH describes technologies that transform the electrical surplus energy of RES into heat. The scarce available publications on that topic substantiate that using electrical energy to utilize and store heat in a PtH concept is not an issue and not state-of-the-art in WWTPs so far. The dominant topics in the heat sector are mostly regarding internal optimization concepts using the waste heat for building heating, sludge treatment and in anaerobic sludge digestion (cf. [67]), as well as heat recovery from the

sewer system or influent/effluent of the WWTP (cf. [68,69]). Nevertheless, PtH and heat concepts hold a respectable potential for sector coupling, even if they are not in the actual focus for plant operators.

In addition to a high electricity demand, WWTPs with anaerobic sludge digestion usually also have a high demand for heat of 30 to 50 kWh/(PE\*d) with big seasonal fluctuations [70]. Compared to electrical energy, heat is an even more complex field to manage. This is caused by the di fferent parameters, which have to match in terms of the amount of needed heating energy, time of demand, di fferent temperature levels, various possible technologies and the nearly unique framework of each WWTP. Heat supply is usually handled as a by-product from the production of electrical energy from the CHP units and, therefore, not demand-driven and in competition with on-site electricity production [70]. Further information in this regard and specific proposals for the implementation of optimized heat and cooling concepts for WWTPs are given in [67].

Typical heat sources in WWTPs are exhaust gases from the CHP units and other combustion processes (e.g., pyrolysis and hydrothermal carbonization); waste heat from aerators/blowers and drying processes or heat recovery from wastewater [71]. Typical heat sinks are given in forms of sludge treatment (e.g., preheating, sludge drying and thermal disintegration); building heating and hot water preparation and heating of the digestion tank [67].

Especially, the digestion tanks are a reasonable heat sink, which is interesting for further investigations. They are usually operated on a mesophilic temperature level of 34 ◦C to 38 ◦C; however, a thermophilic temperature level of 48 ◦C to 55 ◦C is possible as well [72]. As a biological process, minor fluctuations are not harmful for the system, but e ffects in biogas production are possible, depending on the rate of increasing the temperature due to a needed adaptation time for the microorganisms. The authors of [73] showed that temperatures up to even 50 ◦C (change to thermophilic temperature level) are possible without losses in biogas quantity and quality with stable processes in the digestion tank. In this regard, existing studies show di fferent e ffects concerning methane yield, methane concentrations and needed adaptation time (cf. [73–75]). Thereby, a gradual temperature change is advisable and should not exceed 2 ◦C per week [76], whilst a direct heating within 24 h could result in a drop of methane yield and methane concentration [74]. Under the approach of using the digestion tanks for PtH purposes, a calculated raise or lowering of the usual operation temperature level is intended, and the digestion tank is used as a "hot water storage". The potential of this storage option is huge: with an average digestion tank volume of 50 l/PE [72] and a specific heat capacity of water of 4.19 kJ/(l\*K), the specific energy storage results in 58.2 kWh/(K\*PE) or nearly 17.5 GWh for a 100,000 PE120 WWTP for a temperature rise of 3 K.

A PtH concept can be implemented directly via heating rods/boilers or indirectly via di fferent types of heat pumps. However, electrode boilers are commonly used for PtH on an industrial level due to the need of a high-temperature process heat. It is possible to integrate such a system into existing heat cycles, but it depends highly on the local boundary conditions. Requirements are an all-the-year heat sink and a power grid connection with su fficient electrical power reserve [9].

Published practical implementations on a scientific level for "classic" PtH concepts in WWTPs are hardly available. Nevertheless, some holistic approaches in local energy concepts demonstrate promising interconnections between WWTPs and the heat sector. The authors of [77] showed for a commercial district located in Milan (Italy) that a PtH system complemented by sewage heat recovery is competitive to individual, distributed heat pumps. In addition, WWTPs are able to participate in local heat supply systems by feeding-in surplus heat [35,70,78,79]. This is realized, e.g., at the Hamburg WWTP, which is feeding heat into the district heating systems for the container terminals of the port of Hamburg. Furthermore, the Hamburg WWTP is testing an aquifer storage by using the groundwater to store heat surplus during the summer to compensate for seasonal fluctuations and save heating energy in the winter [79,80]. The aquifer storage is located beneath the WWTP and includes heat of the nearby industry as well—summing up to nearly 400 GWh/a. By using 100% green energy, this system is able to provide CO2-free district heating with economically acceptable costs of avoiding CO2 emissions below 100 € per ton and year [80].

Thermal energy potentials of the Austrian WWTPs are stated in [81], showing that WWTPs are able to contribute significantly in local district heating concepts by recovering energy from digester gas production and wastewater effluent. Based on that, the authors of [78] proposed a set of methods to integrate the potentials into local energy concepts, considering spatial, environmental and economic issues. It is shown that the heat generation from WWTPs offers an alternative to conventional heat generation at competitive costs.

Besides heat, cooling concepts are also an interesting approach to using energy surplus. Cooling is needed in WWTPs for the air conditioning of the operation buildings and cooling down the processed heat (e.g., blowers for aeration and digestion gas) [35]. This can be realized due to, e.g., sorption chillers, which work like heat pumps but just the other way around. Whereas, in heat pumps, a lot of heat is to be dissipated at a higher temperature, in chillers, the aim is to absorb as much heat as possible at a low temperature and, thus, provide cooling [9]. Further information regarding different cooling technologies are given in [82].

Regarding economic feasibility, it can be stated that single-handed PtH systems (even on an industrial level) were in the past, just in combination with high revenues from the control energy market profitable. On the basis of significantly decreasing prices, new business cases have to be found [18]. Nevertheless, compared to other general storage concepts (e.g., batteries, load-shifting, etc.), thermal storages are cheaper, even if prices for other technologies are dropping [9]. However, even if a single implementation of classic PtH systems is not ye<sup>t</sup> feasible, the chances to utilize surplus thermal energy from WWTPs are given but considerably underestimated or unknown. The actual benefit may not be using surplus energy from the subordinate energy grids but taking a more significant role in the local energy system. Integrating WWTPs in local district heating is possible without high investment costs compared to new developments in the vicinity of towns due to an intelligent use of the existing and surrounding infrastructure, provided that the spatial framework is suitable. In addition, thermal energy from WWTPs is a continuous and reliable source of energy, which can be substantially used in district heating, substituting fossil energy sources and reducing greenhouse gas emissions (cf. [78,81]).

#### **5. Mobility Sector and Power-to-Fuel**

The mobility sector is one of the most relevant factors and affected by the biggest challenges in reducing greenhouse gas emissions and energy consumption [83,84]. In perspective, it is predicted that, especially, road transport will rise even more and double in terms of CO2 emissions from 2010 to 2050, up to 14–18 Gt CO2 [85]. Therefore, a change in transport is mandatory to achieve climate protection goals. In fact, to reduce greenhouse gas emissions, decrease dependencies from crude oil imports and emissions of pollution and noise in cities, the mobility sector has to be nearly completely decarbonized [86]. The potential of RES is huge, e.g., using H2 in vehicles: 1.0 kWh H2 is able to replace 1.5 to 2.2 kWh fossil fuel in transport and prevents 465–680 CO2-eq [9].

To realize defossilization under the use of RES in the transport sector, different types of technologies, types of drive and fuels are available, each with different technology levels and market penetration rates. These include e-mobility, fuel-cell cars and combustion engines using fuel made from renewables. The comparison of the efficiency of drive concepts shows advantages for battery electric vehicles [87]. That is one reason why many car companies actually focus on this concept. Nevertheless, the comparison of total life-cycle greenhouse gas emissions shows that the difference is not as relevant as it seems when only comparing efficiency. All type of drives have advantages and disadvantages, and the assessment of life-cycle greenhouse gas emissions depends heavily on the assumed boundary conditions (e.g., battery size, range and RES) of the individual considered study [88]. Therefore, all types of drive will find its application area and will be part of a mix of technologies in the mobility sector. This variability will be needed to progress in achieving climate protection goals (cf. Table 4).


**Table 4.** Di fferent types of drives and used renewable fuels [87].

\* [88]: overall driving performance of 150,000 km, battery 60 kWh and further assumptions.

Though, advanced biofuels and biomethane are still represented just in small shares; especially biomethane is growing rapidly in some countries [6].

The production of fuels for the mobility sector in WWTPs is not state-of-the-art but demonstrated in several projects (e.g., [45,89]). Besides providing electrical energy for e-mobility, there are basically twotypesoffuelsusable:gaseousfuels(H2and CH4)andliquidfuels(biofuelandsyntheticfuel).

The production of gaseous fuels (CH4 and H2) by WWTPs using RES was discussed before in the PtG section of this paper. The utilization of these fuels in the mobility sector is successfully demonstrated in several practical implementations (e.g., [50]). The Henriksdal WWTP in Sweden produces and upgrades biogas for 280 buses used in public transportation [90]. Furthermore, the PtG plant at the Pfa ffenhofen WWTP (Germany) will serve as a CH4-fuel provider for 250 buses as of 2020 [91]. Further small-size mobility studies (1 to 6H2 buses) are conducted in Barth, Kaisersesch and Sonneberg, Germany [45,50,92]. Another way to produce H2 is possible via gas reformation from the biogas and was tested in Bottrop (Germany), as well as methanol synthesis (power-to-liquid), but not continued due to missing profitability whilst the technical feasibility could be shown (cf. [89,93]). Another way to produce H2 is the fermentative conversation of organic mass like sewage sludge or molasses, called "dark fermentation" (cf. [94]).

Besides WWTPs as power-to-mobility locations, there are a lot of ongoing and planned projects given in [21] related to the use of H2 and CH4 for transportation in Europe.

WWTPs are able to produce liquid biofuels as well (state-of-research), e.g., through microalgae using wastewater as a nutrient source. This o ffers a (theoretically) e fficient way to remove nutrients than conventional tertiary treatment together with a biomass production that does not use agricultural land or compete with food crops but still lacks feasibility on a large scale [95]. The drawbacks are a high demand of light and a subsequent poor performance during the wintertime for European climatic conditions. This would result in needed technical lighting and big basins, which can o ffset any feasibility.

Furthermore, the economic production of H2 and synthetic fuel from sewage sludge is examined in Rosenheim, Germany [96]. Advantages of synthetic liquid fuel and CH4 (as a natural gas substitute) is the usability in existing and established combustion technologies, which can be used as interim solutions to a future, e.g., H2-based mobility that di ffers substantially in the forms of the types of drive and needed infrastructure from the conventional system [97]. WWTPs are able to accompany that transformation by providing the currently required type of fuel and reducing the construction of interim solutions.

At first sight, fuel-based vehicles seem concurrent to electric vehicles. However, the existing energy system cannot provide enough capacity for a full-scale private e-mobility, especially not-controlled charging in local electric distribution grids. Furthermore, battery-driven vehicles are not suitable for transportation vehicles [98]. As a result, a mix of fuel-based mobility for long distances and transportation of goods, as well as electric vehicles for individual mobility in the cities, could be a suitable solution.

Apart from technical issues, the high demand of special raw materials for batteries and fuel cells raise additional ecological problems, which have to be taken into account. The authors of [99] state that tolerable environmental e ffects, working conditions in mining areas and violations of fundamental rights, as well as the needed amount of raw materials on a large scale, are not given for an abrupt transformation in transportation. Along with technical progress, the conditions of mining, the needed materials and their supply chains have to be developed gradually at the same time. This is transferable to other sectors, like catalysts in the electrolyzer or battery-storage systems.

From a realistic point of view, the capacities of WWTPs compared with the entire demands of fuel are just a contribution to a future solution. However, the authors of [45] showed that the provided H2 by WWTPs corresponds with the amount of needed fuel in the respective urban area with the size of the WWTP. Therefore, a WWTP of 50,000 PE120 that generates O2 for aeration purposes via a PtG system comprises the H2 potential of more than an average-sized filling station. The technical proportions between the PtF-concepts and WWTPs fit, but implementations are mostly in combination with other reasonable energy concepts. Nevertheless, this fact is no disadvantage, and WWTPs o ffer considerable opportunities to take a part in the local sector of mobility as a fuel provider in terms of production and location for filling stations. Combining fuel production and public transport, especially in combination with other synergies, seems reasonable.
