Gaps between short-term tests and long-term tests
High level iron and aluminum ions have been suspected toxic to nitrifiers in biological wastewater treatment although there are no definitive evidences. Following are the claims found in literature, which are based on lab experiments.
- The ferrous ion (Fe2+) concentration higher than 20 mg/L appeared toxic to Nitrosomonas and as a result nitrification rate slowed down by 20% (Seyfried, 1988).
- When residual ferric ion in the primary effluent was 1.68 mg/L, nitrification rate in aeration basin decreased 20-34% (Lees, 2001).
- With 100 mg/L of PACl (poly-aluminum chloride), nitrification rate decreased by 16% while denitrification rate decreased by 43% (Iversen, 2009)
- Ferric ion (Fe3+) was found more detrimental than ferrous ion to microbial community in activated sludge process. The pH drop upon formation of iron hydroxides, the impairment of the floc structure, and the formation of nitrogen oxides could partially explained the toxicity of the iron. However, the reduced free phosphorus concentration due to the insoluble ferric phosphate formation did not appear a significant factor affecting the microbial activity (Philips, 2003).
In spite of the above observations in the lab, the toxicity of ferric and aluminum salts are rarely observed in long-term pilot- or full-scale applications. Following are the claims based on long-term full-scale tests.
- When alum was used at a Metal/P ratio of 1-2.5 in a one year MBR pilot tests in Broad Run WWTP in Asheville, Virginia, USA, NH4-N in the effluent was controlled at 0.03 mg/L on average while total phosphorus at below 0.05 mg/L (Daigger, 2010).
- In a long-term application of FeCl3 in a full-scale plant in Traverse City, Michigan, USA, no inhibition in nitrification has been reported while 30-150 mg/L of FeCl3 was added based on influent flow (Crawford, 2006; Daigger, 2010).
It is apparent that the negative impact on microbial community and the performance of the activated sludge is mostly observed in short term lab tests, where microorganisms do not have enough time to adapt with the chemical. In long-term full scale systems, microbial species can adapt to the new environment as a result of a long-term exposure to metal salts. It is also possible that the species with high tolerances against metal salts replace the other species with low tolerances.
Toxicity of aluminum salt to M.pavicella
Although aluminum salts are rarely toxic to wastewater microorganisms in long-term full-scale applications, their short-term toxicity to one of the prevalent filamentous microorganisms, M. parvicella, is well known (Roels, 2002; Nielsen, 2005). PAC (poly-aluminum chloride), AlCl3, and alum can be added directly to aeration tank, RAS line, or between aeration tank and clarifier at a dosage of 1.5-4.5 mg Al/g MLSS/day to reduce the foaming and the sludge bulking caused by M. parvicella. However, there is no enough evidence that aluminum salts are toxic to other filamentous organisms such as Nostocoida limicola and Nocardia spp.
The mechanism of the toxic effect is yet to be discovered, but it has been postulated that the coagulation of filamentous microorganisms reduced the accessibility to foods while physiology, particularly the lipase production, was partly inhibited. As a result, the uptake of long chain fatty acids can be interfered (Nielsen, 2005). Nonetheless, aluminum salts are suspected to lose their toxicity eventually due to the microbial adaptation.
© Seong Hoon Yoon