Drying system of sewage sludge - wastewater treatment plant in Gdansk

Abstract

The 'Wschod' wastewater treatment plant, with average flow of sewage equal to 110 000 m³ /day, is the biggest plant in the voyevodship of Gdansk. The volume of treated sewage amounts to 60% of the total quantity of sewage in the community of Gdansk. Until 1993 treatment of sewage was carried out in the facilities that ensured only mechanical removal of contaminants. Poor sanitary conditions of the coastal water of the Gulf of Gdansk was the main reason for introducing primary chemical precipitation (dephosphatation), using coagulant called PIX. Chemical process of precipitation resulted first of all in effective elimination of phosphates, between 77.6 - 81.8% during the period 1993-95. The removal of organic matter also improved. Due to this the load of total suspended solids in 1995 decreased by 49.1% in comparison to 1992. The removal of organic matter expressed as BOD₅ and COD_cr equaled respactively 34.7% and 32,8%. However, after the introduction of chemical precipitation, the increase of the sludge
from 600 m³ /d (before introducing of chemical precipitation) to 1100 m³ /d (after introducing of chemical precipitation) was observed. For this reason it was decided to provide support for the coagulation process with anionic polimer. The anionic polimer was introduced in 1995, which resulted in the decrease of the total production of sludge to the amount of 600 - 650 m³ /d.

Modernization of the WWTP also caused the necessity of replacing the formerly used method of dewatering on drying beds (total area of 7.4 ha) by mechanical dewatering in centrifuges of capacity of 8-25 m³ /h and maximum load of sludge of 800 kg DS/h. As a result the operation of the treatment plant became easier, though the problems with utilization of the sludge are not completely solved yet.