## 2 Methodology
### District Heating Network
#### 2.3.5 Use of Renewable Energy Carriers Produced in WWTP for WWTP Needs  
For the successful development of the green course, it is important that the green energy producers themselves set an example first. In a wastewater treatment plant, the greatest need for heavy transport, and thus the consumption of transport fuel, is for transporting treated wet or dried sludge to places of use or storage. The need for heavy transport increases even more if untreated sludge is transported by road for treatment to the centralized wastewater treatment plant from surrounding municipalities. Biomethane can easily replace fossil fuels in transport, as there are already enough biomethane-powered heavy vehicles on the market, and biomethane production technologies have reached a high industrial level of development. In addition, using biomethane-powered vehicles from a Life Cycle Assessment (LCA) perspective, considering vehicle, fuel, service and operating costs and environmental taxes, is already economically superior to cars powered by fossil fuels. According to data presented by Scania Lithuania in March 2024, the average cost per km of a biomethane-powered truck was EUR 0.856, compared to EUR 0.863 for a diesel-powered truck. The cost of the truck itself, fuel costs, total operating costs and driver wages were assessed over a 7-year period of operation of the truck, covering 840,000 km [108]. Hydrogen is a beneficial substitute for fossil fuels due to its superior heating value (142 MJ/kg) and the absence of hazardous emissions during burning [109]. From an energy efficiency perspective, compressed hydrogen gas (CHG) heavy goodsvehicles (HGV) have the lowest energy use of the three engine types (6.6 MJ/km), as compared to diesel (12.2 MJ/km) and compressed biomethane gas (CBG) (13.8 MJ/km) [13]. A particular challenge at present is the significantly higher cost of hydrogen vehicles, which can be up to three times greater than diesel heavy-duty vehicles [110]. There are a few articles that assess the fuel consumption of WWTPs in transport. Most often, the combined potential of transport fuels produced by WWTPs--biomethane and hydrogen--is assessed. Article [13] provides data that a 426,400 PE WWTP with integrated green hydrogen production could produce fuel that would replace 100,000 t of diesel and sufficient fuel would be generated annually for 94 compressed biomethane gas HSVs and 296 compressed hydrogen gas fuel cell HSVs at an annual milage of 60,000 km [13]. Use of Biomethane (Biogas) and Hydrogen for Electricity and Heat Production  
In order to achieve complete energy neutrality of the WWTP, all energy consumed in the WWTP must be produced in the WWTP itself or externally, but in this case, it must be used exclusively for the needs of the WWTP and cannot be purchased [9]. When solar and wind power plants and electricity storage devices do not ensure a sufficient electricity supply, it may be appropriate to produce the missing electricity from biomethane or green hydrogen, which are produced in the WWTP. In many European countries, WWTPs have operating biogas cogeneration plants. Energy (electricity and heat) production based on biogas obtained in WWTPs, for example, in Germany, UK and Sweden was 3657, 1483 and 715 GWh/year, respectively [111]. .3.6 Sufficiency of Incoming Waste Streams for the Efficient Functioning of WWTP as an Energy System  
The total energy consumed in a wastewater treatment plant can be conditionally divided into two types: fixed, which does not depend on the amount of wastewater being treated, and variable, which directly depends on the amount of wastewater being treated. Fixed energy costs include, for example, heating of administrative, laboratory and auxiliary premises. Variable energy costs include, for example, biogas upgrading and sludge drying. Some costs, such as aeration, partially depend on the amount of wastewater being treated. For this reason, as the volume of treated wastewater increases, the energy consumption required per cubic meter of wastewater treatment decreases relatively.