3.1.1. CH4 Utilization

Reaching an "energy-positive WWTP" is the proclaimed goal of many operators of anaerobic sludge digestion plants, which can be primary realized by reducing energy consumption, mostly due to efficiency measures, and increasing biogas production. Therefore, the produced digestion gas is mostly used in CHP units in WWTPs to produce electricity. The present literature shows that increasing self-sufficiency is still the dominant topic rather than an interest in holistic and overarching utilization of the different energy forms (e.g., [33–35]). This is mostly caused by missing incentives and the regulatory framework and taxes, which makes selling gas/electricity not economically viable in contrast to a use on-site (e.g., in Germany; [9]). The economic feasibility of anaerobic digestion in WWTPs is highly dependent on the legal framework conditions. A large-scale study of the feasibility of a conversation from aerobic to anaerobic stabilization was conducted in [36]. It could be shown that it is feasible even for WWTPs at sizes of 20,000 PE120 under fitting circumstances (for Germany). Furthermore, the boundary conditions favoring conversation were evaluated, but it was also shown that, especially, taxation systems have a huge impact, and feasibility has to be examined individually for each plant and each country's framework [36].

In a future RE-dominated system, biogas should be used to generate electricity with the CHP units if the RE production in the grid is lower than its demand and vice versa. If the demand is below the actual production, the CHP unit would be shut down, and the electrolysis is used to convert the energy surplus into CH4 and stored in the NGI afterwards. Another possibility of a holistic gas usage could be a more centralized than decentralized electricity production for the CH4. The (electric) efficiency of modern CHP units in WWTPs is around 43%, with an electrical power scale in the range of 100 up to 1000 kWel [37]. At the same time, big natural gas power plants reach up to 60% [38]. Under a holistic point of view, the overall benefit from the utilization of the gas would be much higher to feed the (green) gas into the NGI and utilize it in large power plants. This would increase the electric energy output from the same gas by ~ 17% and replace fossil natural gas in the NGI at the same time.

Isolated feed-in endeavors of WWTPs are made, e.g., at Hamburg WWTP [39] or Pfaffenhofen WWTP [40]. First efforts of rethinking for plants larger than 30,000 PE120 of feeding in the biogas as a suitable alternative to CHP usage on a countrywide scale are made in Switzerland [19,41]. In 2019, a guideline [42] for the plant operator and planner was published, giving specific advice for decision-making and implementations in WWTPs. Both systems go<sup>t</sup> their advantages and drawbacks shown in Table 2. Finally, decision-making will be determined economically and differ widely due to local boundary conditions. Compared to both options, revenues of selling energy (gas, electrical

energy and heat) and energy trading on energy exchanges (e.g., control energy and load-management) have to exceed the incurring costs for construction, machinery, energy, maintenance and legal authorizations [42].

**Table 2.** Advantages and disadvantages of using digestion gas on-site and feeding into the natural gas infrastructure (NGI) (adapted and extended from [42]). WWTP: wastewater treatment plant.


(+) Advantage; (−) Drawback.

The waste heat from the CHP units usually covers the heating demand of the digester tanks and the operating buildings. Abandoning the standard of using the CHP waste heat could lead to a higher energy demand for heating purposes due to a feed-in instead of an on-site utilization of the gas. However, this fact may be the missing incentive for plant operators to integrate new renewable heat solutions, like heat recovery from wastewater. Furthermore, the most digestive tanks lack a proper insulation. Improving the reduction of heat losses could lower the thermal energy demand significantly, down to a manageable amount even without CHP units. For conventional WWTPs with fitting feed-in conditions, it is presumed that implementing PtG concepts are a considerable addition to trigger additional synergies and improve the overall efficiency. The authors of [32] showed that it is possible that the missing waste heat can be substituted by the heat production of an electrolyzer and the methanization reactor as well. Additionally, the produced metabolic by-water during the methanization process can be used in the digestion tanks to enhance biogas production [32].

#### 3.1.2. O2-Utilization: Usage of O2 From Electrolysis in Wastewater Treatment

Another synergy for PtG in WWTPs is the use of O2 from the electrolyzer. PtG systems are usually focused on H2 production, thus O2 is—as an unintended by-product—usually just vented-off due to a missing application [26]. During the different wastewater treatment steps, O2 is needed in the aeration tanks for the biological degradation of carbon and nitrogen (nitrification) and can also be utilized to produce ozone (O3) for micropollutant removal or sludge disintegration.

The intake of O2 for biological treatment is usually realized by inserting compressed air (blowers/compressors) into the system and supplying the bacteria with the needed oxygen. This part of wastewater treatment is the biggest energy consumer, with 50%–70% of the overall energy consumption [43,44]. Based on that fact, it is often claimed that big energy savings could be achieved due to a reduced gas volume needed, caused by higher O2 rates (pure O2 go<sup>t</sup> a five-times higher O2 content than air). However, there are contradictory scientific evidences for the actual feasibility and impacts of an implementation beyond theoretical considerations. Especially, the benefits of a pure O2 system in comparison to a conventional municipal system on a large scale are insufficient [45]. Based on a market analysis conducted in [46], using pure O2 is usually more interesting in treating industry wastewater than for municipal WWTPs.

The major benefits and drawbacks of using pure O2 instead of compressed air to enhance wastewater treatment are shown in Table 3.

**Table 3.** Benefits and drawbacks for using pure oxygen for aeration purposes in wastewater treatments.


In the following, promising conductions related to enhanced purification quality and/or energy savings are summarized.

One of the first worldwide electrolyzer implementations in WWTPs for O2 utilization was conducted in 2002 in Barth (Germany) to support the biological treatment during high-load phases and avoid capacity extensions. The annual average load amounts to 10,000 PE120, but during tourism season, the loads peak up to 24,000 PE120, and the plant was not able to handle those peaks with the existing plant configuration. With the electrolyzer-driven supportive O2 system, the seasonal influence was manageable, and no capacity extension was needed [50,53]. The PEM electrolyzer was used from 2002–2007, and since 2009, an alkaline electrolyzer generates O2 for treating the wastewater. However, the H2 utilization path (fuel-cell bus) was abandoned due to two major break-downs and the lack of further funding [53].

Regarding purification quality, approaches for a pure O2 system in Spain showed no significant increase in COD (chemical oxygen demand), BOD (biochemical oxygen demand) or TN (total nitrogen) removal, whilst energy savings were assumed [54]. In contrast, a study in Germany showed that, in comparison with a conventional system, an additional reduction of up to −20% for COD and TN is possible, but no energy savings could be achieved [47]. At the Nürnberg WWTP (Germany), pure oxygen is successfully used for years as the first biological treatment step for COD removal [55], and at the Lynette WWTP (Copenhagen, Denmark), pure O2 was used to support the biological treatment step for 15 years. In Neustadt (Germany), a pure oxygen system was implemented and used for years to handle and ensure the legal discharge values due to high viticulture-related load peaks of nearly 100% compared to the usual loads [51].

At the PtG implementation in Avedøre (Denmark), it is shown via laboratory experiments that the use of pure O2 is possible, and purification processes are not inhibited by the use of pure O2. Furthermore, the already pressurized O2 from the electrolyzer is able to displace the corresponding air blower operation to supply the aeration basins [32]. The implementation of an O2 utilization would increase the overall system economy but was not implemented in full scale due to high investment costs caused by misfitting local plant circumstances. Nevertheless, in combination with an above-described PtG system, the feasibility in WWTPs could be reached towards a single investment (cf. [32]).

For pure oxygen systems, an additional mixing system is required, and deeper basins are favorable due to a longer contact of the O2 bubbles with the media. The authors of [46,47] showed that shallow basins or volatile fill levels might cause losses in the purification quality. Furthermore, the reduced gas volume of the pure O2 system is insufficient for mixing the media and preventing sludge settling in common aeration tanks. This also causes a lower CO2 stripping, which could result in a reduced pH level for wastewater with low buffer/acid capacity and inhibited nitrification processes at pH levels

under 6.6. To solve this problem, it could be required to add chemicals (e.g., milk of lime; cf. [47]) or add additional aeration times for the system (cf. [46]) to sustain the biological treatment. This issue could also occur with aeration tank depths of 6 meters or deeper [56]. Other cost savings could thereby be offset, leading to similar or higher total operating costs for the use of pure O2 systems. Claimed reduced overall basin volume or smaller basins must be viewed critically as well, because sludge concentration is not limited by the O2 input but of the thickening in the secondary clarifier. Nevertheless, due to the needed deeper basin construction, the space requirements are reduced and favorable for, e.g., very limited field conditions. Whilst [49] claims better sludge characteristics like sedimentation properties or sludge volume indices, in contrast, [48,51] could not observe a similar effect. Additionally, for an implementation of an O2 system, an expensive O2 storage and piping infrastructure is needed, with special arrangements regarding corrosion, higher fire risk and a special safety concept due to dealing with pure O2 [48]. Further instructions regarding handling O2 in WWTPs are given in [52].

In summary, the results of the evaluated scientific papers are partially contradictory, e.g., enhanced purification quality or space requirements. However, the long-term experiences of several WWTPs with supportive O2 utilization show that it is even on a large scale possible and reasonable if the local boundary conditions are fitting or require special arrangements. However, no economic statements were given in those publications. The popular claim that using pure O2 will result in energy (and cost) savings for the WWTP could not be proven in any practical implementation, but higher operating costs were mentioned in [51]. Nevertheless, it can be stated that at least similar or better purification results could be achieved with the use of pure oxygen regarding COD, BOD and NH4-N (e.g., [47,54]) and energy savings are theoretically possible (e.g., [32]).

#### 3.1.3. O2 Utilization: Micropollutant Removal via Ozone Produced from Renewable O2

At present, removing micropollutants is getting more and more important for the water sector. In Switzerland, it is even ordered by law for municipal WWTPs above 80,000 PE120 [57]. This is discussed in several other countries as well (e.g., Germany, [58]), or additional steps for micropollutant removal are set up on plants unsolicited (e.g., WWTP Mannheim, Sindelfingen, etc.; Germany). Besides adsorption on activated carbon, oxidation of the micropollutants is a suitable removal process. This leads to another future benefit in operating an electrolyzer in WWTPs by upgrading O2 to O3, which could be used in a further treatment step. This additional process stage will result in a substantially higher energy demand for WWTPs, e.g., for a 100,000 PE120, WWTP 22.5 kWh/(PE\*a) to 58.3 kWh/(PE\*a) (cf. [59]). In a future holistic WWTP concept, O2 is produced by a RE-driven electrolyzer and closes another loop of resource and overall efficiency. In the next steps, O3 is produced and the water treated, followed by a biological treatment step to remove the by-products due to the ozonization process to ensure the effluent quality [60]. Using the ozone to remove micropollutants is a commonly used technology in WWTPs [61,62]. Guidelines for the preinvestigation, design and operation are described in, e.g., [63]. This makes ozonization a preferable option for removing micropollutants in combination with PtG systems, leading to more feasibility for both applications. Like dealing with O2, O3 requires special precautions in handling as well, which should not be underestimated in daily business. Those are, for instance: use of specific materials, needed O3 detectors, ventilation systems and safety concepts [64]. First approaches in the form of feasibility studies for a full-scale application for such a system are conducted at WWTP Mainz (Germany) with a planned start in 2021 [65,66].
