Upgraded Carrousel Concept

13-jun-2006, STOWA





The Netherlands

Stage of development

Full scale

Process  -





BOD removal - Nutrient removal



Raw wastewater - Effluent from primary treatment



Activated sludge process

Keywords: decrease area demand; water line; BOD removal; nutrient removal




The Carrousel system is based on the principle of an oxidation ditch where primary settlement, the acti­vated sludge process, secondary settlement and sludge mineralization can take place simultaneously. The original Carrousel system was developed in the Netherlands in the 1970s for the treatment of municipal and industrial wastewater. The treatment concept has been further developed to meet more stringent discharge requirements for wastewater treatment plants. The first Carrousel 2000 system, a Carrousel with internal pre‑denitrification for enhanced nutrient removal, was put into operation in 1993. In a separate development in recent years, the introduction of draft tube mixers has facilitated the construction of deep Carrousel basins.


Description and working principle

Carrousel 2000

The principle of the Carrousel 2000 system is based upon the general oxidation ditch technology. A Carrousel basin is shaped like a race track and has a central, longitudinal partition wall. While the wastewater is circulating around the channel, micro-organisms break down the organic compounds, nitrogen and phosphorus containing wastewater. Depending on which mode of system is employed, the intensive internal circulation exposes the activated sludge to a sequence of aerobic, anoxic as well as anaerobic conditions.

In the majority of cases, the system will be used to treat wastewater that has only been screened and degritted. Primary clarifiers can be omitted. To stimulate phosphorus removal and to prevent the generation of bulking sludge the Carrousel-2000 system can be preceded with a sludge-loading anaerobic tank (see also separate sheet Bulking sludge prevention). Usually, the process design is based on long sludge retention times with extended aeration for the production of stabilised sludge. The waste activated sludge is separated from the effluent in secondary clarifiers.


WWTP Zwijndrecht

The Carrousel 2000 process is based on the combination of a standard Carrousel system with an internal pre-denitrification tank. In the pre-denitrification tank the influent and return activated sludge are mixed with recirculated nitrified wastewater taken from the Carrousel system.


External pre-dentrification (left) and internal pre-denitrification in a Carrousel 2000 (right)

The grey-coloured parts denote oxygenated zones.


The pre-denitrification tank plays an important role in nutrient removal. On the one hand, pre-denitrification makes optimum use of the easy biodegradable organic carbon for the denitrification process. On the other hand, pre-denitrification in combination with an upstream located anaerobic tank is beneficial for biological P removal. The continuous low nitrate concentration will promote the selection of phosphate-accumulating bacteria, accomplishing phosphate removal even at low temperatures. The setup with pre-denitrification will also promote biological phosphate uptake under denitrifying conditions. This type of phosphate uptake is preferable to phosphate uptake in the aeration zone, in case of insufficient amounts of COD available for both denitrification and phosphate uptake.


Usually, the mixed liquor in the Carrousel system is oxygenated by low-speed vertical shaft aerators. These ensure proper mixing while generating the horizontal velocity and turbulence necessary to prevent sludge settling in the circuit. To obtain the most efficient level of BOD and nutrient removal, the power input is adjusted in relation to the actual oxygen demand and load conditions, by varying the speed and the submergence of each aerator. In some cases it is necessary to install submerged, low-energy consuming flow-boosters propellers to maintain the required minimum flow velocity in the channels. The use of such propellers allows an increased operational flexibility.

Orbital flow systems, such as the Carrousel can also be operated successfully with other types of aeration process, like diffused air systems. In addition, combinations of mechanical and diffused air aeration have been implemented and are currently in operation. When diffused air systems are used, flow boosters need to be installed for effective propulsion of the sludge-water mixture.


Deep carrousels - Carrousel 3000

When the surface aerators are equipped with draft tubes, depths of the aeration zone of up to 8 meters can be realised, resulting in a considerably reduced footprint. A draft tube is a vertical cylinder underneath an aerator, which extends almost down to the bottom. It enables the aerator to draw (oxygen deficient) water from the bottom of the tank, thereby ensuring proper mixing over the entire tank depth. This principle has proven to work well in practice for rectangular aeration tanks. However, the necessary propulsion effect of an aerator in a Carrousel basin is greatly reduced by the draft tube. This is overcome by installing additional propulsion means, propellers, in the channels of the Carrousel basin.

See heading Graphics for photographs of a draft tube mixer and the WWTP Leidsche Rijn where it is used.



Design guidelines / Technical data

As a result of the hydraulic characteristics of the Carrousel ditch and the position of the aerators, it is feasible to construct a Carrousel having a channel somewhat larger than 10 m wide by 5 m deep. If draft tubes are used areation depths of up to 8 meters can be realised.

Depending on the capacity of the WWTP, Carrousel systems having 2, 4, 6 or 8 channels can be incorporated and where mechanical aeration is considered a variable number of surface aerators can be installed. The capacity which can be treated in one system can range from 5.000 to 100.000 population equivalents. Plants with  greater capacities are designed as parallel systems.

In the table below some design parameters are given. The WWTP Wervershoof is a Carrousel system; the WWTP Veenendaal and Fernandina Beach are Carrousel 2000 systems; at the WWTP Leidsche Rijn a deep Carrousel has been realised.



p.e. 1)



flow max.






min [

sludge load

[kg BOD5/kg.d]


Wervershoof (NL)









Veenendaal (NL)




17.400 2)





Fernandina B.(US)




4.580 2)





Leidsche Rijn (NL)

70.000 4)



18.900 2)





1)    One population equivalent (p.e.) is 180 g O2

2)    Total volume of the aeration basin including the pre-denitrification volume

3)    Pre-denitrification volume expressed as a percentage of the total volume

4)    p.e. here is 54 g BOD



Performance at specific installations

In the table below some removal efficiencies are given. The WWTP Wervershoof is a Carrousel system; the WWTP Veenendaal and Fernandina Beach are Carrousel 2000 systems; at the WWTP Leidsche Rijn a deep Carrousel has been realised.


Actual load

Influent conc. [mg/l]

Effluent conc. [mg/l]












SVI [ml/g]











Veenendaal 1)










Fernandina Beach








< 100


Leidsche Rijn


  The installation is not yet operated at design capacity.

1)    Influent concentrations and removal efficiency are based on pre-settled influent     (- = not available)


Operational stability and maintenance

The Carrousel system can be fully automated and thus operated without operator attendance (however, operators are usually required for total plant control and surveillance).

In comparison with other systems, the internal recirculation factor for the sludge-water mixture relative to the influent flow is very high, which has major and distinct advan­tages. For a system with channel dimensions of 8 m by 4 m, and an influent flow of 1.000 m3/h, this internal recirculation factor comes close to 35. Due to this high recirculation rate the system can be considered as being completely mixed. As a consequence, the performance is affected to a lesser extent by peak loadings (e.g. hydraulic, toxic compounds, pH) compared to plug flow systems.

Generally surface aerators are preferred to diffused air systems because of their oper­ational reliabil­ity (no clogging and ragging) and a relatively low level of investment. The current generation of surface aerators and gear boxes require minimum mainten­ance compared with dif­fused air systems.

The energy requirements of a Carrousel system may be slightly higher than a conventional activated sludge system because of the necessary propulsion, but this is compensated by the fact that no recirculation pumps are required for denitrification.

Capital and operating cost

Cost estimates for Carrousel2000 system (100.000 p.e., P <1 mg/l, N <10 mg/l)

A system with 100.000 p.e. with a dry weather flow (DWF) of 1.000 m3/h (SWF=4500 m3/h) requires a surface area of 2.200 m2 (volume 8.800 m2 and 4 m deep) with related costs for construction. The investment costs of a Carrousel of the above dimensions would entail ca. 5 million euro. The all-in treatment costs would be ca. 0,5 to 1 euro/m3 of effluent, dependent on other facilities, such as sludge digestion, treatment and disposal and final polishing.

Operating costs are mainly linked to the aeration and mixing requirements of the activated sludge process.


Reference installations

About 900 Carrousel systems have been realised world-wide. About 100 Carrousel2000 systems have been realised, with treatment capacities ranging from 1.000 up to 100.000 m3/day. Countries, where most Carrousel2000 systems have been installed, are: Greece, Netherlands, Poland, China, USA.

Deep Carrousel systems have been installed in the Netherlands (1 reference) and the US (3 references).


Suppliers / Patents

The Carrousel 2000 system was developed by DHV in co-operation with EIMCO Process Equipment of Salt Lake City. DHV holds the patent on the Carrousel and Carrousel 2000 design. The Carrousel2000 design concept is marketed in the USA under the tradename DenitRŽ by EIMCO.

DHV Water BV

Postbus 484  - 3800 AL Amersfoort - The Netherlands

Tel: +31 - 33  468 2200  -  Fax: +31 33 468 2301  -  E-mail: info-water@dhv.nl    -  Web: http://www.dhv.com/


Literature references

[1]   DHV Leaflet: Carrousel and Carrousel 2000

[2]   DHV International: Standardisation design Carrousel STP




Carrousel 2000 - external pre-denitrification tank

WWTP Leidsche Rijn - Deep Carrousel

WWTP Leidsche Rijn - Draft tube mixer