Developing acceptable sewage screening practices

Severn Trent Water screens approximately 1.9 million cubic metres of sewage every day, producing 46000 m³ of screenings annually. Although the company's sludge incinerators are able to burn small quantities of screenings most of the material has to be landfilled.

Over the last few years the company has investigated the areas relating to the development of an environmentally sound disposal strategy at minimum cost. These areas have included the volume and “quality” of screenings produced, the costs of collection, treatment and disposal and the variation of screenings production rates under high flow conditions. Various items of screenings handling equipment have been assessed to identify acceptable options for permanent installations.

The required quality of screenings for economic disposal has been established and a “Quality Factor”, combining biological oxygen demand, density and dryness developed to provide a scientific basis to judge the aesthetic and commercial acceptability of a screenings product. This factor has been tested widely and found to coincide with visual assessment.

Data on the variation of screenings production with sewage flow have been collected to assist with the sizing of future installations.

The information obtained has been used to develop an application matrix for screenings handling equipment at large sewage treatment works.