Potential of municipal sewage treatment plants to remove bis(2-ethylhexyl) phthalate

Abstract

Sanna Marttinen tutki väitöskirjatyössään muovin pehmentimenä käytettävän DEHP-kemikaalin pitoisuuksia ja käyttäytymistä yhdyskuntajätevedenpuhdistamolla. Täyden mittakaavan puhdistamoilla tehtyjen tutkimusten sekä laboratorio- ja pilot-kokeiden avulla Marttinen selvitti puhdistamojen kykyä poistaa DEHP:a jätevesistä ja siten vähentää DEHP:n ympäristökuormitusta.

The removal of bis(2-ethylhexyl) phthalate (DEHP) at municipal sewage treatment plants (STPs) was studied at full-scale plants and in experiments carried out with sewage, reject water and sludge containing native DEHP. DEHP concentration in Finnish sewages was typically at a level of 100 µg/l. Most of the DEHP was sorbed to colloidal or settleable solids, while less than 6% was in dissolved state or sorbed to soluble organic matter. Typically, more than 90% of DEHP was removed from sewage at STPs, and secondary effluent concentrations were lower than 8 µg/l. The removal was a result of both biodegradation in activated sludge stage and sorption to primary and secondary sludges. DEHP concentrations in primary and secondary sludges and anaerobically digested sludges were often higher than 100 mg/kg dw, which is the proposed maximum value for DEHP in sludges to be used in agriculture. Reject waters together with return secondary sludge are usually returned to the headworks of the STP, and give rise to an internal circulation of pollutants, which in the case of DEHP may be as high as the load in sewage. DEHP is effectively removed from reject waters by improving solids separation or in nitrogen-removing sequential batch reactors (SBRs), partly by biodegradation and partly by sorption to sludge. The results indicate that anaerobic biodegradation occurred during the digestion of sludge. Composting and aeration efficiently removed DEHP from both raw and anaerobically digested sludge. During sludge aeration, a part of the DEHP originally weekly bound to the sludge matrix became more strongly bound, but it was able to biodegrade in a later phase of the aeration. The results of the study suggest that DEHP that is sorbed to the sludge matrix is biodegraded after desorption, which is aided by the decomposition of organic sorption sites. In conclusion, if properly operated, STPs have the potential to efficiently remove DEHP from sewage and reduce its load to the environment.