In this study the influence of hydraulic retention time (HRT) and sludge age on organic substances removal (COD), nitrogen removal and formation of activated sludge biocenosis in sequencing batch reactors (SBRs) treating municipal landfill leachate were investigated. Two series were performed (series 1 and 2). Each of them was conducted at HRT 12, 6, 3 and 2 d. Series 1 and 2 were differed in sludge age. In series 1, sludge age was over 2-fold longer than in series 2, which was obtained by volume control of suspended solids disposed in SBR operating cycle. The efficiency of organics removal from leachate (as COD) in series 1 decreased from 82.9% (HRT 12 d) to 70.5% (HRT 2 d) and in series 2 from 83.5% to 71.9%, respectively. Complete nitrification and the ammonia nitrogen concentration in the effluent below 1 mg dm⁻³ was noticed in series 1 at HRT of 3 d and longer. In activated sludge, there were examined 20 taxons. Their number depended mainly on sludge age. At sludge age shorter than 16 d only 5-4 taxons were present. The number of individuals depended both on HRT and sludge age. The richest communities were observed at 12 d HRT in both series. At HRT of 6, 3 and 2 d the total number of individuals was clearly higher in series 1 than in series 2. Type 0092 was dominated among filamentous bacterium.
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