Improved protection of lakes in Skanderborg

Skanderborg is located in one of Denmark’s most sensitive in-land aquatic environments, “Søhøjlandet”. The area is under protection and there is a need to comply with regulations about phosphorus discharge from wastewater treatment plants to the lakes. Phosphorus concentration of the discharged wastewater must therefore be decreased to comply with the strict outlet requirement.

Additionally, Skanderborg municipality expects population and industrial growth in the future combined with a centralization of their wastewater treatment plants.

In this case the wastewater treatment plant in Skanderborg was upgraded with SUEZ’ technology – Densadeg XRC™ - extreme rate clarifier.

It targets wastewater treatment plant effluents as well as the stormwater overflows (known as CSO) leading to discharge of untreated wastewater combined with sewer flows when exceeding the hydraulic capacity of the sewer and/or wastewater treatment plant.

Wastewater treatment plant in Skanderborg is subjected to phosphorous discharge requirement of 1,38 kg/day to Skanderborg Lake, which is equivalent to 0,265 mg/L in the effluent with the current discharge of 1.900.000 m³/year.

Skanderborg municipality expects population and industrial growth in the future combined with a centralization of their wastewater plants. The phosphorus concentration of the discharged wastewater must therefore be decreased to an average level below 0,15 mg/L to comply with the outlet requirement.

In this case wastewater treatment in Skanderborg upgraded the plant with SUEZ technology – Densadeg XRC™ - extreme rate clarifier.

About the technology
Densadeg XRC™ is a high-rate settling clarifier process combining solids contact, ballast addition (dense granular additive) and solids recirculation to enhance settling of solids, which results in rapid floc maturation and faster clarification.

The process consists of:

1. rapid mix / coagulation stage
2. flocculation zone
3. maturation zone
4. settling & clarification zone
5. hydrocyclone and ballast recovery
6. effluent collection

This video shows how our technology works:

We carried out a monitoring program from March to September 2020 to assess the operating capacity of the Densadeg XRC™.

These first months of operation as tertiary treatment showed:

• average outlet levels:
  - 0,11 mg/L for total phosphorus
  - 3 mg/L for suspended solids
• with an iron chloride dosage ranging from 35 to 55 mg/L
• proven lamellar velocity up to 100 m/h

A significant decrease has been demonstrated between the outlet total phosphorus concentration before the construction of the Densadeg XRC (0,30 mg/L on average) and since it has been in operation (0,11 mg/L on average).

Our data enabled us to estimate that the Densadeg XRC™ operation as tertiary treatment could remove additional 584 kg of phosphorus per year!