Public Sewage System
NOTES ON METHODOLOGY
Legal and methodological framework
The data on the public sewage system are collected by using the annual statistical survey entitled the Annual Survey on the Public Sewage System (VOD-2K form) on the basis of the Official Statistics Act (NN, Nos 25/20, 155/23, 124/25 – corr. and 69/26).
Coverage
Reporting units are public suppliers of public sewage system services. According to the Water Act (NN, No. 47/23), the public supplier of sewage system services is a company in which all shares or equity shares are held by local self-government units or companies in which all shares or equity shares are held directly by local self-government units or institutions founded by a local self-government unit.
The report is filled in by legal entities and parts thereof that are registered, according to NKD 2025 (NN, No. 47/24), in section E Water supply; sewerage, waste management and remediation activities, class 37.00 Sewerage.
The source for the address list is the Statistical Business Register.
The survey provides basic data on waste waters, their treatment and discharge as well as on the public sewerage network and waste water treatment plants.
Definitions
Public sewerage means the activity of collecting waste waters, their transport to a waste water treatment plant, treatment and direct or indirect discharge into surface waters, underground waters and/or the sea, treatment of sludge generated in the process of waste water treatment, if the above is conducted through public sewerage facilities, and management of these facilities; public sewerage also includes pumping and transport of waste water from sump pits.
Waste waters are all potentially polluted industrial, sanitary, rainwater and other waters.
Treated waste waters comprise all amounts of waste waters treated by using either primary (mechanical), secondary (biological) or tertiary (combined) treatment method.
Primary treatment includes the application of physical and/or chemical processes by which at least 50% of suspended solids are removed from the waste water, while the BOD5 value is reduced by at least 20%, as compared to the BOD5 value of influent waters.
Secondary treatment includes the application of biological and/or other treatment processes by which the concentration of suspended solids and BOD5 is reduced by 70% to 90% and the concentration of COD by at least 75%.
Tertiary treatment includes the application of physical and chemical, biological and other treatment processes that reduce the concentration of influent nutrients in waste waters by at least 80% and remove other specific indicators of waste substances within the limits of values that cannot be achieved by applying secondary treatment.
Discharged waste waters consist of discharges of treated and untreated waters. The waters can be discharged into ground waters, waterflows, reservoirs, lakes and the sea.
Public sewage network is a network of enclosed public drains and sewers used either for draining both waste and atmospheric waters together (general water sewage system), or for separate draining of waste and atmospheric waters (separation water sewage system).
Total length of the public sewage network is the length of enclosed public drains and pipes for draining wastewater and atmospheric waters from settlements, without connections and networks within buildings.
Waste water treatment plants are devices for the treatment of waste water. They are divided into devices for the primary, secondary and tertiary treatment.
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Abbreviations
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BOD5 |
biochemical oxygen demand |
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COD |
chemical oxygen demand |
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km |
kilometre |
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m3 |
cubic metre |
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NKD 2025 |
National Classification of Activities, 2025 version |
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NN |
Narodne novine, official gazette of the Republic of Croatia |
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'000 |
thousand |