Biological reject water treatment by using moving bed biofilm reactors (MBBR)
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- Master i teknologi 
In this study two moving bed biofilm reactors (MBBRs) (i.e. the reactors called R1 and R2 respectively) were subjected to treat reject water from sludge dewatering. Such biological treatment was investigated for possible improvements of discharge water quality to reduce disturbance on, and enhance the main coagulation process. Four mechanisms assumed to be involved in biological reject water treatment evaluated are I) Oxidation of dissolved and colloidal organics in the proposed bioprocess. II) Conversion of these organics in to live biomass through a cell synthesis process and this biomass will leave the reactors and follow reject water into the main inlet. III) The active biomass will capture more dissolved organics and colloidal solids from raw wastewater when introduced to the main inlet. IV) Biological treated reject water causes less disturbance than untreated on the main coagulation process. This study attempted to generate relevant experimental data for testing the hypothesized mechanisms. The reactors were fed from an equalization tank continuously. During the study period two main condition were observed, unstable and stable conditions. In the stable condition a mesh installed inside the equalization tank in order to reduce the fluctuation of organic loading rate (OLR) to the reactors. The HRT in stable condition was maintained at 24 h and then 12 h. Besides the experimental part, the process further analyzed by modeling and simulation using an activated sludge model (ASM1). The results show organics (measured as COD) confirming mechanism I. The highest average soluble COD removal in R1 and R2 were 50 % and 58 % respectively when the HRT was 12 h. The total COD removal at 12 h HRT were 43 % and 33 % for R1 and R2. The ammonium removal in R1 and R2 were 28 % and 25 % when HRT was 24 h and it was reduced to 3.5 % and 9.1 % when HRT decreased to 12 h. The simulations show that low alkalinity level in the reject water and low dissolved oxygen (DO) inside the reactors may have limited ammonium removal. In addition, simulations suggest that such bioreactors can obtain efficient ammonium removal and COD removal at much lower HRT than tested experimentally, when optimum condition achieved (i.e. when alkalinity level was 70 mmol HCO3/L and DO level was 7.5 mg/L). Lower HRTs reduce construction cost and capital investments for the implementation of biological reject water treatment. Optimum conditions can give high biomass production, which may increase coagulation efficiency according to proposed mechanisms II and III. Coagulation experiments in jar tests (carried out by another student) using a relevant mixture of raw wastewater and reject water from the experiments reported here, supports that mechanism IV can be important. The coagulation COD removal efficiency improved by around 10 % when using treated reject water compared to untreated. More COD removal as sludge may also increase biogas production potential in the anaerobic digestion process.