Changes of the Content of Heavy Metals and PAH’s in Sewage Sludge Treated with Reed Bed Lagoons

In Poland, low-input methods such as composting, vermicomposting, reed beds, willow energy and solar driers are increasingly often being used in the processing of sewage sludge. The northeastern part of Poland has operated successfully for years using these methods. This paper presents the results of using low-cost methods of sludge treatment in the wastewater treatment plant located in Zambrow, Podlaskie Province. The results of sewage sludge studies on the PAHs and heavy metals content after treatment in a reed bed system are presented. Among the 16 examined PAHs, the lowest concentration was obtained for the dibenz (a,h)anthracene. Not a single sample exceeded a concentration of 100 μg/kg d.m. The highest concentration was exhibited by fluoranthene, benzo(g,h,i)perylene and indeno(1,2,3-c,d)pyrene. The concentration of these compounds exceeded 1000 μg/kg d.m. The obtained results for the PAHs in sewage sludge from the reed lagoon at the treatment plant in Zambrow showed that the average content of PAHs studied was approximately 8300 μg/kg d.m. The lowest concentration, below 1.3 mg/kg d.m. of the seven heavy metals examined was obtained for mercury (Hg). On the other hand, the highest concentration, exceeding 1300 mg/kg d.m. was found in the case of zinc (Zn). The obtained results for the heavy metals in sewage sludge from the reed bed lagoons in Zambrow show that the average content of heavy metals studied is approximately 1620 mg/kg d.m. The results of the study demonstrate a high efficiency of the low-cost methods used in the Zambrów WWTP in terms of the quality of processed sludge. The sewage sludge from the lowest layer of the reed lagoon (13–14 years of dewatering and transformation) are characterized by the lowest PAHs and heavy metals content. The higher a sediment layer lies, i.e. the shorter the time of processing, the higher are the PAHs and content of heavy metals content. This indicates a great role of reeds in the accumulation of these compounds.