Combined Sewer Overflow Screens for Overflow Structures

ROMAG specialises in the development and manufacture of screens for overflow structures. In times of excessive rainfall or storms, where do the contaminated solids present in the water go: on the ground or in the sewage treatment plant? Is it a question of expenditure, i.e. investment, or a question of choosing the right solution? ROMAG's high-performance combined sewer overflow (CSO) screen system presented here is a low-cost, efficient solution. It is a system where all requirements can be met by a diverse variety of types of construction and sizes with one modular basic structure.

How was this solution devised? When the Swiss Federal Office of the Environment, Forests and Landscape (FOEFL) investigated the question of CSO and SSO works, it quickly became clear that use of a CSO screen is far more efficient and environmentally friendly than constructing large catch basins. It simply required further development of screening technology.


ROMAG, one of the invited companies, was interested and it has committed itself fully to this technology ever since. By late 2001, more than 600 ROMAG CSO screens had already been installed in various countries. Today, ROMAG is one of the world's leading companies in CSO technology.

Experience is necessary when selecting the right solution. And this is where ROMAG's services shift into gear. Always on the lookout for new solutions, today ROMAG is a competent partner and advisor for the entire sector of water technology.


Basically, the effluent or sewage flows from the discharge culvert through the CSO screen into the relief sewer. The contamination retained in the CSO screen is cleared by a combing movement so that it is pushed with the sewage or effluent to the sewage treatment plant. The shape of the comb rungs is very important in this respect.

Depending on the type of works, the effluent or sewage flows horizontally, from top to bottom or from bottom to top, through the CSO screen. It is important that operational reliability is ensured with virtually low maintenance. A controlled weir wicket (Type RSW-K) is available for screens through which effluent or sewage flows horizontally. Type RSW is the most frequently used type of screen. This allows throughput to be rendered, dependent on the volume occurring and also allows conclusions to be drawn as regards the quantity relieved.


It is certainly interesting that the responsible Swiss Federal Agency has often halved the prescribed storage capacity per unit of drainage area in the catchment area when using a CSO screen. CSO screens are used almost everywhere today for new projects, dispensing with the need for complex and expensive structures for catch basins. Existing installations can be renovated very economically by installing a CSO screen. One important argument is certainly that no additional building area needs to be created for such projects. Thus, the required investments for new or existing installations are far lower when using CSO screens.


Owing to the large number of projects implemented, ROMAG has a wealth of experience. The rugged design of the CSO screens, installation of all control elements in a dry chamber and operation of the moving parts by hydraulic cylinders mean that this CSO works requires virtually low maintenance. It is recommended to conduct a monthly visual inspection and verifications after exceptional rainfalls. This has also proven the best policy in practice. Reference lists are available.

Contact Details

aquacare AG
Environmental Division
Haltaweg 3
CH-3186 Düdingen
+41 26 492 6500
+41 26 492 6565