Long term comparison of trace organics removal performances between conventional and membrane activated sludge processes
Abstract:This project consisted in investigating the removal of trace organic compounds through membrane bioreactors (MBR) in comparison to conventional wastewater treatment plants (WWTP) in a long term study. Due to specific design characteristics such as high sludge age, high biomass concentration and complete particle retention, membrane bioreactors are expected to achieve enhanced performances to remove trace organic compounds.
Two MBR pilot plants of around 2m3 each have been operated in parallel to a full-scale WWTP over 20 months, fed with the same municipal raw waste water. The three systems were designed for extended biological phosphorous and nitrogen removal. The two MBR pilot plants featured respectively pre- and post-denitrification without carbon dosing.
Composite samples (12h) were analyzed up to 8 times per months on the raw waste water and the 3 plant effluents. Bulk organic parameters such as COD and TOC, high polar compounds (phenazone-type pharmaceuticals, their metabolites and carbamacepine), less polar estrogenic steroids (estradiol, estrone and ethinylestradiol) and Bisphenol A were quantified.
Carbamacepine, a broadly prescribed antiepileptic drug, was present in the range of 1.5-2.2 μg/L in the raw waste water and the three effluents. Thus, carbamacepine was not removed during conventional and membrane activated sludge treatment. Propyphenazone was not removed and the removal rate of phenazone was always below 15% by the WWTP. Significant removal rates started to be clearly monitored with the pilot plants after about 5 months only. This coincided with the summer time, and further analyses showed that both temperature and biological adaptation (related to sludge age), may affect the elimination of these compounds. The removal rate of the two compounds was monitored respectively up to 60-70% with pilot plant 1, and 20-30% with pilot plant 2. Biotransformation of the widely used analgesic drug metamizol in human bodies leads among other smaller molecules to acetylated (acetylaminoantipyrin) and formylated metabolites (formylaminoantipyrin). These metabolites can be found in raw waste water in the μg/L level and removal during conventional treatment was below 20%. In comparison, the removal of these compounds in both pilot plant effluents reaches 70%.
Estradiol was present in filtered raw waste water with an average value of 13ng/L. Estrone, natural metabolite of estradiol, was therefore much present in the raw waste water with about 182ng/L. The artificial steroid ethinylestradiol (contraceptive pill) is known to be more potent, but also more persistent in the environment. During the ten months of investigation, it was present in the raw waste water in average with 8ng/L. The conventional WWTP removed in average more than 90% of the natural steroids and about 80% of ethinylestradiol. The elimination of estradiol by the MBR processes was almost complete, with only about 10% of the samples above the detection limit of 0.4ng/L. The average removal rate of estrone in both pilot plants was about 99%. Ethinylestradiol, the potentially most hazardous compound, was removed by about 95%. The two MBR pilot plants achieved therefore significantly higher removal performances of steroids than the conventional treatment process.
Document Type: Research Article
Publication date: 2003-01-01
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