Public concession contract for water supply and sewage services with an arbitration clause entered into after the creation of the Regulatory Agency for Water Supply and Sanitary Sewage Services (ARSAE). Regulatory agency that did not participate in the agreement nor in the arbitration proceedings, although notified thereof. Validity of the arbitration award that revised the tariffs solely in relation to the municipal concession-granting authority and the concessionaire. Inapplicability of the tariff revision as against the regulatory agency and, consequently, the users of the service provided: