This Estonian standard is applicable, regardless of the form of wastewater treatment plants ownership, in the planning of the cleaning methods necessary for wastewater treatment and in the dimensioning of equipment, both in the construction of a new wastewater treatment plant and in the expansion or reconstruction of an existing one.
Wastewater treatment plants in Estonia are divided into three size classes:
- small wastewater treatment systems: up to 49 PE
- small wastewater treatment plants: 50 PE to 1999 PE
- large sewage treatment plants: 2000 PE to 100,000 PE and more.
In addition, a distinction is made between industrial wastewater treatment plants and pre-treatment plants, i.e. on-site treatment plants. Industrial wastewater treatment plants clean wastewater generated in industry or other production processes, the wastewater is discharged directly into the recipient.
Pre-treatment plants (separator systems) are mud, sand, grease and oil traps and their combinations and other technological devices for the partial purification of wastewater, after passing through which the wastewater is discharged into the common sewer. Pre-treatment can also be biological treatment, during which the load of organic and nutrients in wastewater is reduced before it is sent to the common sewer.
Wastewater treatment methods are divided into mechanical, biological and, if necessary, post-treatment. During mechanical cleaning, larger foreign objects are removed with the help of grids, sand is removed in the separator, which wears out the pumps due to its abrasive properties and settles in the tanks in the further stages of the process. In the case of chemical phosphorus removal, dosing of the precipitation chemical is applied in combination with biological treatment. Figure 1 shows a simplified principal diagram of wastewater treatment.
This standard describes the most common cleaning technology in small treatment plants in Estonian conditions - aerobic biological treatment, focusing only on the flow-through activated sludge process. When setting up a dose treatment plant, the specifics of the dose treatment plant and the relevant standard documents must be considered.
To select a treatment technology, information on the content of pollutants in wastewater must be available. If it differs from the norm e.g., the nitrogen content of the wastewater is significantly higher than the limit values, a technology involving nitrogen removal must be chosen (see, in addition to the standard, for example, DWA-A 131 instructions), or if the wastewater contains heavy metals, appropriate methods for their removal must be applied.
The standard specifies the requirements for the wastewater treatment plant in relation to planning, design, construction, operation and maintenance, as well as the activities to fulfil the requirements.
When choosing small wastewater treatment systems, be guided by the relevant legislation and the standard EVS-EN 12566 (all parts) and technical report CEN/TR 12566 (all parts). When choosing pre-treatment plants, be guided by the relevant legislation and standards EVS-EN 858 (all parts) and EVS-EN 1825 (all parts).
The standard does not deal with industrial wastewater treatment plants.
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