Different methods for determining the toxicity and biodegradability of hazardous compounds evaluating their susceptibility to biological treatment were studied. Several compounds including chlorophenols and herbicides have been evaluated. Toxicity was analyzed in terms of EC50 and by a simple respirometric procedure based on the OECD Method 209 and by the Microtox® bioassay. The values of EC50 obtained from respirometry were in all the cases higher than those from the Microtox® test. The respirometric inhibition values of chlorophenols were related well with the number of chlorine atoms and their position in the aromatic ring. In general, herbicides showed lower inhibition, being alachlor the less toxic from this criterion. For determination of biodegradability an easier and faster alternative to the OECD Method 301, with a higher biomass to substrate ratio is proposed. When this test was negative, the Zahn-Wellens one was performed in order to evaluate the inherent biodegradability. In the fast test of biodegradability, 4-chlorocatechol and 4-chlorophenol showed a complete biodegradation by an unacclimated sludge upon 48 h. These results together with their low respirometric inhibition, allow concluding that these compounds could be conveniently removed in a WWTP. Alachlor, 2,4-dichlorophenol, 2,4,6-trichlorophenol and MCPA showed a partial biodegradation upon 28 days by the Zahn-Wellens inherent biodegradability test.

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The biodegradability of nitrochlorinated (diuron and atrazine) and chlorophenoxy herbicides (2,4-D and MCPA) has been studied through several bioassays using different testing times and biomass/substrate ratios. A fast biodegradability test using unacclimated activated sludge yielded no biodegradation of the herbicides in 24 h. The inherent biodegradability test gave degradation percentages of around 20-30% for the nitrochlorinated herbicides and almost complete removal of the chlorophenoxy compounds. Long-term biodegradability assays were performed using sequencing batch reactor (SBR) and sequencing batch membrane bioreactor (SB-MBR). Fixed concentrations of each herbicide below the corresponding EC50 value for activated sludge were used (30 mg L(-1) for diuron and atrazine and 50 mg L(-1) for 2,4-D and MCPA). No signs of herbicide degradation appeared before 35 days in the case of diuron and atrazine and 21 days for 2,4-D, whereas MCPA was partially degraded since the early stages. Around 25-36% degradation of the nitrochlorinated herbicides and 53-77% of the chlorophenoxy ones was achieved after 180 and 135 days, respectively, in SBR, whereas complete disappearance of 2,4-D was reached after 80 days in SB-MBR.

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