Judgment;Summary — ECLI:EU:C:2023:939
Date: 2023-11-30
Language of the case: Slovenian
Court formation: Seventh Chamber
Subject matter: Environment
Procedure: Action for failure to fulfil obligations - successful;Action for failure to fulfil obligations - inadmissible
Origin: Slovenia

JUDGMENT OF THE COURT (Seventh Chamber) 30 November 2023 (*) (Failure of a Member State to fulfil obligations – Article 258 TFEU – Directive 91/271/EEC – Collection and treatment of urban waste water – Article 4(1) and (3), as well as Articles 5 and 15 – Sections B and D of Annex I – Secondary or equivalent treatment of urban waste water from agglomerations of a certain size – More stringent treatment of discharges into sensitive areas – Monitoring of discharges from waste water treatment plants) In Case C‑328/22, ACTION under Article 258 TFEU for failure to fulfil obligations, brought on 16 May 2022, European Commission, represented by B. Rous Demiri and E. Sanfrutos Cano, acting as Agents, applicant, v Republic of Slovenia, represented by A. Dežman Mušič, acting as Agent, defendant, THE COURT (Seventh Chamber), composed of F. Biltgen, President of the Chamber, J. Passer (Rapporteur) and M.L. Arastey Sahún, Judges, Advocate General: J. Kokott Registrar: M. Longar, Administrator, having regard to the written procedure and further to the hearing of 5 July 2023, having decided, after hearing the Advocate General, to proceed to judgment without an Opinion, gives the following Judgment 1 By its application, the European Commission requests the Court to declare that, by failing to take the measures necessary to ensure: – that urban waste water entering collecting systems is, before discharge, subject to secondary treatment or an equivalent treatment for the agglomerations of Kočevje, Trbovlje, Tržič and Ljubljana; – that urban waste water entering collecting systems is, before discharge into sensitive areas, subject to more stringent treatment for the agglomerations of Kočevje, Trbovlje and Tržič; and – appropriate monitoring of discharges from urban waste water treatment plants for the agglomerations of Ljubljana and Ptuj, the Republic of Slovenia has failed to fulfil its obligations under Articles 4, 5 and 15 of and sections B and D of Annex I to Council Directive 91/271/EEC of 21 May 1991 concerning urban waste water treatment (OJ 1991 L 135, p. 40), as amended by Regulation (EC) No 1137/2008 of the European Parliament and of the Council of 22 October 2008 (OJ 2008 L 311, p. 1) (‘Directive 91/271’). Legal framework Directive 91/271 2 Article 1 of Directive 91/271 provides: ‘This Directive concerns the collection, treatment and discharge of urban waste water and the treatment and discharge of waste water from certain industrial sectors. The objective of the Directive is to protect the environment from the adverse effects of the abovementioned waste water discharges.’ 3 Article 2 of that directive provides: ‘For the purpose of this Directive: 1. “urban waste water” means domestic waste water or the mixture of domestic waste water with industrial waste water and/or run-off rain water; 2. “domestic waste water” means waste water from residential settlements and services which originates predominantly from the human metabolism and from household activities; 3. “industrial waste water” means any waste water which is discharged from premises used for carrying on any trade or industry, other than domestic waste water and run-off rain water; 4. “agglomeration” means an area where the population and/or economic activities are sufficiently concentrated for urban waste water to be collected and conducted to an urban waste water treatment plant or to a final discharge point; 5. “collecting system” means a system of conduits which collects and conducts urban waste water; 6. “1 p.e. (population equivalent)” means the organic biodegradable load having a five-day biochemical oxygen demand (BOD5) of 60 g of oxygen per day; 7. “primary treatment” means treatment of urban waste water by a physical and/or chemical process involving settlement of suspended solids, or other processes in which the BOD5 of the incoming waste water is reduced by at least 20% before discharge and the total suspended solids of the incoming waste water are reduced by at least 50%; 8. “secondary treatment” means treatment of urban waste water by a process generally involving biological treatment with a secondary settlement or other process in which the requirements established in Table 1 of Annex I are respected; …’ 4 Article 3 of that directive provides: ‘1. Member States shall ensure that all agglomerations are provided with collecting systems for urban waste water, – at the latest by 31 December 2000 for those with a population equivalent (p.e.) of more than 15 000, and – at the latest by 31 December 2005 for those with a p.e. of between 2 000 and 15 000. For urban waste water discharging into receiving waters which are considered “sensitive areas” as defined under Article 5, Member States shall ensure that collection systems are provided at the latest by 31 December 1998 for agglomerations of more than 10 000 p.e. Where the establishment of a collecting system is not justified either because it would produce no environmental benefit or because it would involve excessive cost, individual systems or other appropriate systems which achieve the same level of environmental protection shall be used. 2. Collecting systems described in paragraph 1 shall satisfy the requirements of section A of Annex I. …’ 5 Article 4 of that directive is worded as follows: ‘1. Member States shall ensure that urban waste water entering collecting systems shall before discharge be subject to secondary treatment or an equivalent treatment as follows: – at the latest by 31 December 2000 for all discharges from agglomerations of more than 15 000 p.e., – at the latest by 31 December 2005 for all discharges from agglomerations of between 10 000 and 15 000 p.e., – at the latest by 31 December 2005 for discharges to fresh-water and estuaries from agglomerations of between 2 000 and 10 000 p.e. 2. Urban waste water discharges to waters situated in high mountain regions (over 1 500 m above sea level) where it is difficult to apply an effective biological treatment due to low temperatures may be subjected to treatment less stringent than that prescribed in paragraph 1, provided that detailed studies indicate that such discharges do not adversely affect the environment. 3. Discharges from urban waste water treatment plants described in paragraphs 1 and 2 shall satisfy the relevant requirements of section B of Annex I. … 4. The load expressed in p.e. shall be calculated on the basis of the maximum average weekly load entering the treatment plant during the year, excluding unusual situations such as those due to heavy rain.’ 6 Article 5 of Directive 91/271 provides: ‘1. For the purposes of paragraph 2, Member States shall by 31 December 1993 identify sensitive areas according to the criteria laid down in Annex II. 2. Member States shall ensure that urban waste water entering collecting systems shall before discharge into sensitive areas be subject to more stringent treatment than that described in Article 4, by 31 December 1998 at the latest for all discharges from agglomerations of more than 10 000 p.e. 3. Discharges from urban waste water treatment plants described in paragraph 2 shall satisfy the relevant requirements of section B of Annex I. … 4. Alternatively, requirements for individual plants set out in paragraphs 2 and 3 above need not apply in sensitive areas where it can be shown that the minimum percentage of reduction of the overall load entering all urban waste water treatment plants in that area is at least 75% for total phosphorus and at least 75% for total nitrogen. 5. Discharges from urban waste water treatment plants which are situated in the relevant catchment areas of sensitive areas and which contribute to the pollution of these areas shall be subject to paragraphs 2, 3 and 4. In cases where the above catchment areas are situated wholly or partly in another Member State Article 9 shall apply. 6. Member States shall ensure that the identification of sensitive areas is reviewed at intervals of no more than four years. 7. Member States shall ensure that areas identified as sensitive following review under paragraph 6 shall within seven years meet the above requirements. 8. A Member State does not have to identify sensitive areas for the purpose of this Directive if it implements the treatment established under paragraphs 2, 3 and 4 over all its territory.’ 7 Article 15 of that directive provides: ‘1. Competent authorities or appropriate bodies shall monitor: – discharges from urba[n] waste water treatment plants to verify compliance with the requirements of Annex I.B in accordance with the control procedures laid down in Annex I.D, – amounts and composition of sludges disposed of to surface waters. 2. Competent authorities or appropriate bodies shall monitor waters subject to discharges from urban waste water treatment plants and direct discharges as described in Article 13 in cases where it can be expected that the receiving environment will be significantly affected. …’ 8 Annex I to that directive, entitled ‘Requirements for urban waste water’, includes, inter alia, a section A, which sets out the requirements for collecting systems, a section B, which contains the requirements with which discharge from urban waste water treatment plants must comply, and a section D, which sets out the control procedures for waste water discharges. The pre-litigation procedure 9 Having found that, by failing to ensure that several of its agglomerations, including the agglomerations of Tržič, Ljubljana, Kočevje, Trbovlje and Ptuj, were provided with adequate collecting systems for urban waste water, the Republic of Slovenia had failed to fulfil its obligations under Articles 3 to 5, 10 and 15 of and Annex I to Directive 91/271, the Commission decided to issue it with a letter of formal notice on 16 February 2017, to which the Slovenian authorities responded by letter of 15 June 2017. 10 Several meetings between the Slovenian authorities and the Commission were held during 2017, as well as a ‘package’ meeting on 23 February 2018. The Slovenian authorities also sent the Commission interim progress reports on the agglomerations concerned on 29 September 2017, 27 February 2018 and 23 July 2018. 11 Having concluded, after examining the responses contained in those reports and taking into account all the information provided by the Slovenian authorities, that on certain points, despite the efforts made, the Republic of Slovenia was still failing to fulfil its obligations under Directive 91/271, the Commission, by letter of 8 March 2019, sent the Republic of Slovenia a reasoned opinion in accordance with Article 258 TFEU. 12 In that opinion, the Commission, first, found that, by failing to ensure that, inter alia, the agglomerations of Tržič, Ljubljana, Kočevje, Trbovlje and Ptuj were provided with adequate collecting systems for urban waste water, the Republic of Slovenia had failed to fulfil its obligations under Articles 3 to 5, 10 and 15 of Directive 91/271 and sections A, B and D of Annex I thereto, and, secondly, ordered that Member State to take the measures necessary to comply with that reasoned opinion within two months of receipt thereof, that is to say by 8 May 2019 at the latest. 13 The Republic of Slovenia responded to that reasoned opinion by letter of 25 April 2019, followed by further letters dated 2 September and 30 December 2019 and 6 March, 12 August and 15 December 2020. 14 Whilst it was able to conclude, in the light of the Slovenian authorities’ response to the reasoned opinion and the information provided after the ‘package’ meeting held in Ljubljana on 21 January 2020, that all the agglomerations referred to in the reasoned opinion now complied with the requirements of Article 3 of Directive 91/271, the Commission nevertheless found that, when the period set in the reasoned opinion expired, the Republic of Slovenia had still not fully complied with its obligations under that directive. 15 The Commission therefore decided to bring the present action. The action Admissibility Arguments of the parties 16 The Republic of Slovenia submits, as regards, first, the complaint alleging infringement of Article 4 of Directive 91/271 in respect of the agglomerations of Kočevje, Trbovlje and Tržič and, secondly, the complaint alleging infringement of Article 15 of that directive in respect of the agglomeration of Ptuj, that it was not given the opportunity to put forward its substantive arguments against those complaints in the pre-litigation procedure, which infringed its rights of defence. 17 The Commission refutes those claims. The differences in formulation between the content of the reasoned opinion and the forms of order sought in the application did not involve any amendment of the subject matter of the action. The Commission consistently referred to an infringement by the Republic of Slovenia of the same provision, in respect of the same agglomerations and in respect of the same period. The fact that the Commission initially relied on the hierarchy principle, according to which, where it is found that the requirements laid down in Article 3 of Directive 91/271 have not been complied with, nor, a fortiori, is the obligation laid down in Article 4 of that directive fulfilled, that cannot have the effect that, once the collection obligation laid down in Article 3 has been complied with by the Republic of Slovenia, the Commission can no longer criticise that Member State for having failed to fulfil its obligations under Article 4 of Directive 91/271. Nor were the rights of defence of the Republic of Slovenia infringed, since that Member State was informed from the outset that the Commission was alleging a failure to comply with Article 4 of that directive in respect of the agglomerations concerned. 18 Similarly, in the pre-litigation procedure, the Commission, inter alia, drew the attention of that Member State to an infringement relating to the agglomeration of Ptuj for failure to comply with the requirements of Articles 3, 4 and 15 of Directive 91/271 and of sections B and D of Annex I thereto. Findings of the Court 19 The Court has consistently held that the purpose of the pre-litigation procedure is to give the Member State concerned an opportunity, on the one hand, to comply with its obligations under EU law and, on the other, to avail itself of its right to defend itself against the objections formulated by the Commission. The proper conduct of that procedure constitutes an essential guarantee required by the FEU Treaty, not only in order to protect the rights of the Member State concerned, but also so as to ensure that any contentious procedure will have a clearly defined dispute as its subject matter (judgment of 2 April 2020, Commission v Poland, Hungary and Czech Republic (Temporary mechanism for the relocation of applicants for international protection), C‑715/17, C‑718/17 and C‑719/17, EU:C:2020:257, paragraph 91 and the case-law cited). 20 It follows, inter alia, that the subject matter of the proceedings under Article 258 TFEU is delimited by the pre-litigation procedure provided for in that provision. Accordingly, the application must be based on the same grounds and pleas as the reasoned opinion (judgment of 18 November 2010, Commission v Portugal, C‑458/08, EU:C:2010:692, paragraph 43 and the case-law cited). 21 Nevertheless, that requirement cannot be stretched so far as to mean that in every case the formal statement of objections set out in the reasoned opinion and the form of order sought in the application must be exactly the same, provided that the subject matter of the proceedings as defined in the reasoned opinion has not been extended or altered (judgment of 18 November 2010, Commission v Portugal, C‑458/08, EU:C:2010:692, paragraph 44 and the case-law cited). 22 In the present case, the Commission criticises the Republic of Slovenia for failing to comply with provisions of Directive 91/271 to which reference had earlier been made, as regards the agglomerations concerned, both in the letter of formal notice and in the reasoned opinion. 23 However, it must be pointed out that, in the course of the pre-litigation procedure, the Commission inferred the alleged infringement of Article 4 of Directive 91/271, in respect of the agglomerations of Kočevje, Trbovlje and Tržič, only from the alleged infringement of Article 3 of that directive and referred, in that context, only to the case-law of the Court according to which, where it is found that the agglomerations concerned do not have systems for collecting all the urban waste water discharged by those agglomerations, nor, a fortiori, is the obligation to subject all discharges to secondary or equivalent treatment fulfilled (see, to that effect, judgment of 25 October 2007, Commission v Greece, C‑440/06, EU:C:2007:642, paragraph 25 and the case-law cited). 24 It was only in the context of its application that the Commission put forward, for the first time, an allegation specific to the alleged infringement by the Republic of Slovenia of Article 4 of Directive 91/271, according to which it follows from a comparison of the data relating, first, to the total load of the agglomerations concerned and, secondly, to the maximum load treated, as communicated by the Slovenian authorities, that the total discharge of urban waste water from those agglomerations was not subject to secondary treatment or an equivalent treatment. 25 It is true that, in so far as, in the pre-litigation procedure, the Commission considered that, as regards the agglomerations concerned, the Republic of Slovenia had failed to fulfil its obligations arising from Article 3 of Directive 91/271, the Commission was entitled, at that stage of the procedure and in accordance with the case-law cited in paragraph 23 of this judgment, also to allege, as a consequence, an infringement of Article 4 of that directive. 26 It is also true that the fact that a Member State complies with the requirements of Article 3 of Directive 91/271 concerning the provision of agglomerations with collecting systems for urban waste water does not necessarily mean that the same applies to the requirements arising from Article 4 of that directive. Under the latter article, urban waste water entering collecting systems must, before discharge, be subject to secondary treatment or an equivalent treatment, those discharges having to satisfy the relevant requirements of section B of Annex I to that directive. Moreover, the Court has already held that, first, the requirements of Article 4(1) of Directive 91/271 are not complied with so far as concerns agglomerations suffering from under-capacity of secondary or equivalent treatment facilities for urban waste water and, secondly, since discharges of waters that have not been treated at all do not satisfy the requirements of section B of Annex I to that directive, the treatment of urban waste water by facilities suffering from under-capacity cannot be regarded as complying with Article 4(3) of that directive (see, to that effect, judgment of 28 March 2019, Commission v Ireland (System for collecting and treating waste water), C‑427/17, EU:C:2019:269, paragraphs 152 and 154). 27 However, the fact remains that, in the pre-litigation procedure, the Republic of Slovenia was not able usefully to avail itself of its right to defend itself against the Commission’s arguments referred to in paragraph 24 of this judgment, since those arguments were put forward for the first time by the Commission in its application initiating the proceedings and relied on a legal and factual basis completely different from that put forward by it in the pre-litigation procedure, referred to in paragraph 23 of this judgment. 28 Moreover, contrary to what the Commission maintains in its reply, it would have been sufficient, in order to respect the Republic of Slovenia’s rights of defence, to send that Member State a supplementary reasoned opinion, without it being necessary to reopen new infringement proceedings. 29 As regards the alleged infringement of Article 15 of Directive 91/271 so far as concerns the agglomeration of Ptuj, it must be noted that the reasoned opinion sent to the Republic of Slovenia did not contain any specific arguments in that regard. 30 In those circumstances, the Commission’s head of claim seeking a declaration that the Republic of Slovenia has failed to fulfil its obligations under Article 4 of Directive 91/271 in respect of the agglomerations of Kočevje, Trbovlje and Tržič and under Article 15 of that directive in respect of the agglomeration of Ptuj must be rejected as inadmissible. 31 Lastly, in so far as, in its application, the Commission merely maintains, as regards the agglomerations of Kočevje, Trbovlje and Tržič, that the existence of an infringement of Article 4 of Directive 91/271 entails an infringement of Article 5 thereof, it is also necessary to reject as inadmissible the Commission’s head of claim seeking a declaration of an infringement of the latter provision. 32 It may be added, for the sake of completeness, that, as regards the agglomeration of Ptuj, the Commission in any event acknowledged, at the hearing, that the information provided by the Republic of Slovenia in the annex to its rejoinder demonstrated, as regards the years in question, compliance by that agglomeration with the requirements of Article 15 of Directive 91/271. Substance Arguments of the parties 33 The Commission submits that, as regards the agglomeration of Ljubljana, the Republic of Slovenia has failed to comply with the requirements arising from Articles 4 and 15 of Directive 91/271. 34 It notes that the Republic of Slovenia stated, in its response of 25 April 2019 to the reasoned opinion, that changes would be made to waste water treatment systems in order to ensure that the situation in the agglomeration of Ljubljana complied with Article 4 of Directive 91/271 and specified that the urban waste water of that agglomeration was, at that date, treated by two waste water treatment plants and that only one, namely the smaller Brod plant, did not satisfy the requirements of Article 4 of that directive. That Member State did not therefore dispute, in that regard, infringement of the requirements of Article 4 owing to a lack of infrastructure. On the contrary, it asserts that the failure to comply with that provision was remedied by the closure of the Brod and Rakova Jelša treatment plants and the allocation of the remaining load to the Zalog waste water treatment plant as part of the project ‘Waste water discharge and treatment for the area of the Ljubljansko polje aquifer’, co-financed by the Cohesion Fund. In the that same response, the Republic of Slovenia stated, with regard to the Ljubljana agglomeration and the Brod waste water treatment plant, that the construction phase of that project was to be completed by December 2020 and that the operational phase was to begin the following year. However, in its letter of 6 March 2020, the Member State stated that the project would not be completed until the end of 2023. 35 Nor are the requirements of Article 15 of Directive 91/271 fulfilled, since it is apparent from the information provided by the Republic of Slovenia that, as regards the Brod waste water treatment plant, which treats urban waste water from the agglomeration of Ljubljana, four samples (out of a total of 12) have BOD5 and/or COD concentration values in excess of the limit values set out in Table 1 in section D of Annex I to Directive 91/271. In three samples, the BOD5 concentration values were exceeded by more than 100% and, in one sample, the COD concentration value was exceeded by more than 100%. The number of non-compliant samples also exceeded the limits set by that directive. 36 According to the Republic of Slovenia, the two complaints of the Commission are unfounded. 37 In its responses to the letter of formal notice and to the reasoned opinion, the Republic of Slovenia accepted the Commission’s assertion that there is an infringement of Article 4 of Directive 91/271 if the values produced by the waste water treatment plant exceed the thresholds laid down by that directive. That is, moreover, the reason why the Republic of Slovenia admitted infringement of Article 4 of Directive 91/271 during the pre-litigation procedure. However, it is clear from the Court’s case-law that the obligation under Article 4 of Directive 91/271 is fulfilled if the Member State concerned is able to submit a sample meeting the requirements of section B of Annex I to that directive. As regards the Brod urban waste water treatment plant, which has secondary water treatment technology, it is apparent from the results of the monitoring of the water treated by that plant, provided by the Republic of Slovenia in its response to the letter of formal notice and the reasoned opinion, that the samples taken in that context complied on several occasions with the requirements of section B of Annex I to Directive 91/271. Thus, the Republic of Slovenia submitted six compliant samples for 2016 and eight for 2018. 38 With regard to the Brod urban waste water treatment plant, with which paragraph 34 of this judgment is concerned, the Republic of Slovenia argues that the modernisation of that plant is neither appropriate nor economically justified because it will be closed as part of the implementation of the project ‘Waste water discharge and treatment for the area of the Ljubljansko polje aquifer’, financed by Cohesion Fund resources. The load from the Brod waste water treatment plant will then be diverted to the public collecting system, which is connected to the Zalog urban waste water treatment plant. 39 The Republic of Slovenia emphasises in that context that the part of the Ljubljana agglomeration that is connected to the Brod waste water treatment plant is not the source of uncontrolled discharges of untreated water into the environment. Measurements taken at the Brod waste water treatment plant show that most of the waste water is treated, with only four samples failing to comply in 2018 and only two in 2016. 40 For the same reasons, the obligation on the part of the Republic of Slovenia to ensure that discharges comply with the quality standards referred to in Article 15 of Directive 91/271 and section D of Annex I thereto is impracticable for the part of the agglomeration of Ljubljana connected to the Brod waste water treatment plant. However, until the final implementation of the project referred to in paragraph 38 of this judgment, the Republic of Slovenia states that it ensured that waste water from part of the Ljubljana agglomeration was treated in the Brod waste water treatment plant, which was regularly monitored by the inspection authorities, with 12 measurements being carried out each year for that purpose. It is in the context of those regular checks that the compliant results for 2016 and 2018, referred to in paragraph 39 of this judgment, were collected. Findings of the Court – Preliminary observations 41 It must be borne in mind that, although, in proceedings brought under Article 258 TFEU for failure to fulfil obligations, it is incumbent upon the Commission to prove the infringement alleged by providing the Court with all the evidence necessary to enable it to establish that the obligation has not been fulfilled, without being able to rely on any presumption for those purposes, account should be taken of the fact that, where it is a question of checking that the national provisions intended to ensure effective implementation of a directive are applied correctly in practice, the Commission, which does not have investigative powers of its own in this area, is largely reliant on the information provided by complainants or by the Member State concerned (judgment of 2 December 2010, Commission v Portugal, C‑526/09, EU:C:2010:734, paragraph 21 and the case-law cited). 42 It follows, inter alia, that, where the Commission has adduced sufficient evidence to establish that the national provisions transposing a directive are not applied correctly in practice in the territory of the defendant Member State, it is for that Member State to challenge in substance and in detail the information produced and the inferences drawn (judgment of 2 December 2010, Commission v Portugal, C‑526/09, EU:C:2010:734, paragraph 22, and the case-law cited). 43 It should also be noted that the question whether a Member State has failed to fulfil obligations must be determined by reference to the situation prevailing in the Member State at the end of the period laid down in the reasoned opinion and the Court cannot take account of any subsequent changes (judgment of 28 January 2016, Commission v Portugal, C‑398/14, EU:C:2016:61, paragraph 49 and the case-law cited). 44 In the present case, the reasoned opinion, dated 8 March 2019, set the Republic of Slovenia a period of two months from receipt of that opinion for complying with its obligations under, inter alia, Articles 4 and 15 of Directive 91/271. The period granted for that compliance thus expired on 8 May 2019. – The complaint alleging infringement of Article 4 of Directive 91/271 45 Article 4(1) of Directive 91/271 provides that all urban waste water entering collecting systems must, before discharge, be subject to secondary treatment or an equivalent treatment. 46 Under Article 4(3) of that directive, that secondary or equivalent treatment must be carried out by treatment plants the discharges of which satisfy the requirements of section B of Annex I to Directive 91/271. 47 In particular, it follows from paragraph 4 of section B of Annex I to that directive that discharge from urban waste water treatment plants must comply with the requirements shown in Table 1 thereof. 48 In accordance with Annex XIII to the Act concerning the conditions of accession of the Czech Republic, the Republic of Estonia, the Republic of Cyprus, the Republic of Latvia, the Republic of Lithuania, the Republic of Hungary, the Republic of Malta, the Republic of Poland, the Republic of Slovenia and the Slovak Republic and the adjustments to the Treaties on which the European Union is founded (OJ 2003 L 236, p. 33), those requirements were incumbent upon the Republic of Slovenia, as regards agglomerations with a p.e. of more than 15 000, as from 1 January 2011. 49 In that regard, it follows from the case-law of the Court that, where it is established that the BOD/COD standards prescribed in section B and Table 1 of Annex I to Directive 91/271 are not complied with, it must be held that the waste water of the agglomeration concerned is not subject to appropriate treatment before discharge, with the result that the complaint alleging non-compliance with Article 4(3) of Directive 91/271, read in conjunction with section B and Table 1 of Annex I thereto, must be regarded as well founded so far as concerns such an agglomeration (see, to that effect, judgment of 28 March 2019, Commission v Ireland (System for collecting and treating waste water), C‑427/17, EU:C:2019:269, paragraphs 155 and 157). 50 Moreover, the Court has already held that the requirements of Article 4(1) of Directive 91/271 are not complied with so far as concerns agglomerations which suffer from under-capacity of secondary or equivalent treatment facilities for urban waste water. Since discharges of waters that have not been treated at all do not satisfy the requirements of section B of Annex I to Directive 91/271, the treatment of urban waste water by facilities suffering from under-capacity cannot be regarded as complying with Article 4(3) of that directive (see, to that effect, judgment of 28 March 2019, Commission v Ireland (System for collecting and treating waste water), C‑427/17, EU:C:2019:269, paragraphs 152 and 154). 51 However, in its response to the reasoned opinion, the Republic of Slovenia did not contest the Commission’s assertion that the Brod waste water treatment plant did not comply with the requirements of Article 4 of Directive 91/271, since non-compliant BOD5 concentration values had been found on several occasions. In particular, it acknowledged that the results of the monitoring, referred to in paragraph 37 of the present judgment, showed that the Brod waste water treatment plant ‘d[id] not actually guarantee the elimination of nutrients (tertiary treatment)’. 52 Similarly, in its defence, in response to the Commission’s assertion in its application that the Republic of Slovenia was not contesting the infringement of the requirements of Article 4 of Directive 91/271 owing to a lack of infrastructure, that Member State acknowledges that there are ‘difficulties linked to the Ljubljana agglomeration’, while adding that this will be remedied in the context of the implementation of the project ‘Waste water discharge and treatment for the area of the Ljubljansko polje aquifer’. 53 In those circumstances, it must be held that the infringement of Article 4 of Directive 91/271 is established in respect of the Ljubljana agglomeration. 54 That finding cannot be called into question by the fact, put forward by the Republic of Slovenia in its defence, that the results of the monitoring of water treated by the Brod waste water treatment plant, relied upon by that Member State in its response to the letter of formal notice and to the reasoned opinion, ‘demonstrate that the samples were adequate on several occasions and therefore complied with the requirements of section B of Annex I to Directive [91/271]’. 55 The Court has indeed held that, where a Member State is able to present a sample which satisfies the requirements provided for in section B of Annex I to Directive 91/271, the obligations deriving from Article 4 of that directive must be regarded as having been complied with (judgment of 2 September 2021, Commission v Sweden (Waste water treatment plants), C‑22/20, EU:C:2021:669, paragraph 49 and the case-law cited). 56 However, that case-law concerns situations in which the existence, in the Member State concerned, of sufficient urban waste water treatment capacity was not disputed by the Commission, with the result that the Court rejected the Commission’s argument that, in order to demonstrate that its situation complied with the requirements of Article 4 of Directive 91/271, that Member State is also required to provide a sample obtained over a period of 12 months (see, in particular, to that effect, judgment of 10 March 2016, Commission v Spain, C‑38/15, EU:C:2016:156, paragraphs 24 and 35 to 39). 57 In the present case, as observed in paragraph 52 of this judgment, the Republic of Slovenia acknowledges that there is insufficient capacity for the treatment of urban waste water in the agglomeration of Ljubljana. 58 It should be added that the Court has also held that facilities cannot be regarded as complying with the concept of ‘best technical knowledge not entailing excessive costs’ within the meaning of section A of Annex I to Directive 91/271 where, first, the Member State concerned has embarked upon a large programme of upgrading works proving that there are technological solutions in order to overcome the problem of excessive spills of waste water, but that they have not been applied and, secondly, that Member State has decided to finance such works, so that the related costs cannot be regarded as excessive (judgment of 28 March 2019, Commission v Ireland (System for collecting and treating waste water), C‑427/17, EU:C:2019:269, paragraph 120 and the case-law cited). 59 It follows that nor can the concept of ‘best technical knowledge not entailing excessive costs’ be successfully relied upon by the Republic of Slovenia in the present case since, first, that Member State, in the context of the project ‘Waste water discharge and treatment for the area of the Ljubljansko polje aquifer’, has embarked upon the modernisation of the infrastructure for treating urban waste water, but this was not the case on the date on which the period referred to in the reasoned opinion expired and, secondly, that Member State has not sought to demonstrate the possibility that the costs of rendering compliant the facilities for treating urban waste water in that agglomeration may be disproportionate, in so far as it states that that financing will be provided ‘from its own resources and European resources’ (see, by analogy, judgment of 28 March 2019, Commission v Ireland (System for collecting and treating waste water), C‑427/17, EU:C:2019:269, paragraph 177). 60 Accordingly, the Court finds that, on the date on which the period set by the reasoned opinion expired, secondary treatment or equivalent treatment of all the urban waste water entering the collecting systems of the agglomeration of Ljubljana was not ensured and that the requirements laid down by Article 4 of Directive 91/271, read in conjunction with section B of Annex I to that directive, were therefore not complied with. – The complaint alleging infringement of Article 15 of Directive 91/271 61 In accordance with the first indent of Article 15(1) of Directive 91/271, the competent authorities or appropriate bodies of the Member State concerned are to monitor discharges from urban waste water treatment plants in order to verify compliance with the requirements of section B of Annex I to that directive in accordance with the control procedures laid down in section D of Annex I thereto. 62 In particular, it follows from paragraph 4 of section D of Annex I to Directive 91/271 that the treated waste water is deemed to conform to the relevant parameters if, for each relevant parameter considered individually, samples of the water show that it complies with the relevant parametric value, inter alia, in the following way: first, for the parameters specified in Article 2(7) of that directive and in Table 1 of that annex, the number of samples which are allowed to fail the requirements, expressed in concentrations and/or percentage reductions, set out therein, does not exceed the maximum specified in Table 3 of that annex, namely two out of a total number of 12 samples taken. Secondly, for the parameters of Table 1 of that annex expressed in concentrations, the failing samples taken under normal operating conditions must not deviate from the parametric values by more than 100%. 63 However, the Republic of Slovenia simply states that only two of the 12 samples taken from the Brod waste water treatment plant in 2016 did not comply with those requirements and that, for 2018, only four did not, without calling into question the Commission’s assertions that those four samples also had BOD5 and/or COD concentration values higher than the limit values set out in Table 1 in Annex I to Directive 91/271. The Republic of Slovenia also acknowledges that, pending the final implementation of the project ‘Waste water discharge and treatment for the area of the Ljubljansko polje aquifer’, its obligation to ensure that discharges comply with the quality standards referred to in Article 15 of that directive and section D of Annex I thereto is ‘impracticable’ for the part of the agglomeration of Ljubljana connected to the Brod waste water treatment plant. In those circumstances, the failure to comply with those provisions must also be regarded as having been established. 64 In the light of all the foregoing considerations, the Court finds that, by failing to take the measures necessary to ensure, so far as concerns the Ljubljana agglomeration: – that urban waste water entering collecting systems is, before discharge, subject to secondary treatment or an equivalent treatment, and – appropriate monitoring of discharges from urban waste water treatment plants, the Republic of Slovenia has failed to fulfil its obligations under Article 4(1) and (3) of Directive 91/271, read in conjunction with section B of Annex I to that directive, and the first indent of Article 15(1) of that directive, read in conjunction with section D of Annex I thereto. 65 The action must be dismissed as to the remainder. Costs 66 Under Article 138(3) of the Rules of Procedure of the Court, where each party succeeds on some and fails on other heads, the parties are to bear their own costs. 67 In the present case, since the Commission and the Republic of Slovenia have each been unsuccessful in certain claims, they shall each bear their own costs. On those grounds, the Court (Seventh Chamber) hereby: 1. Declares that, by failing to take the measures necessary to ensure, so far as concerns the Ljubljana agglomeration: – that urban waste water entering collecting systems is, before discharge, subject to secondary treatment or an equivalent treatment, and – appropriate monitoring of discharges from urban waste water treatment plants, the Republic of Slovenia has failed to fulfil its obligations under Article 4(1) and (3) of Council Directive 91/271/EEC of 21 May 1991 concerning urban waste water treatment, as amended by Regulation (EC) No 1137/2008 of the European Parliament and of the Council of 22 October 2008, read in conjunction with section B of Annex I to that directive, as amended, and the first indent of Article 15(1) of that directive, as amended, read in conjunction with section D of Annex I thereto; 2. Dismisses the action as to the remainder; 3. Orders the European Commission and the Republic of Slovenia to bear their own costs. [Signatures] * Language of the case: Slovenian.