PERFORMANCE EVALUATION OF UASB PROCESS

Abstract

Upflow anaerobic sludge blanket (UASB) process offers great potentials for the treatment of a variety of industrial and domestic wastewater. In this process sludge immobilization is achieved through granulation and efficient settling. The various operational controls such as nature of the waste, organic loading rate (OLR), hydraulic loading, sludge occupancy (a ratio of sludge bed volume, v to r*eV:tor volume, V), sludge loading rate (SLR) alongwith design features like feed inlet system and gas solids separator (GSS) affect the performance of the reactor. Sludge occupancy in these reactors is expected to control the organic and sludge loading parameters. Viable sludge escape due to ex cessive gas formation at increased sludge occupancy may possibly limit the organic load to the reactor. However, at reduced sludge occupancy the reactor may develop stress due to the ratio of organic load to the amount of the sludge exceeding the critical value of sludge loading parameter. A survey of literature does not explicitly reveal design criteria for maintaining optimum sludge occupancy to fully utilize sludge activity. Compounding these factors into connldoration, investigations have been carried out (i) to study the performance of UASB reactors at different levels of sludge occupancy, (ii) to evaluate desludging schedule, and (iii) to assess the effect of desludging on the performance of UASB reactors. The efficiency of UASB process may also depend on the nature and concentrations of heavy metals in the feed. However, scanty information regarding this particular aspect is available in the literature. Therefore, in the next phase of the study synthetic (iii) wastewater spiked with chromium (III) and zinc (II) salts separately was fed to the reactor. The objective of this part of the study was (i) to investigate biochemical methane potential of chromium and zinc containing wastewater from batch bioassay test using serum bottle technique, and (ii) to study the effect of chromium and zinc on the efficiency of UASB process. The results obtained therefrom may be applicable in giving direction to the treatment of wastewaters from chrome tanning and viscose rayon industries. For convenience and clarity of presentation the subject matter of the thesis has been divided into following chapters: 1. GENERAL INTRODUCTION 2. LITERATURE SURVEY 3. MATERIALS AND METHODS 4. EVALUATION OF UASB PROCESS AT DIFFERENT LEVELS OF SLUDGE OCCUPANCY 5. EFFECT OF DESLUDGING ON THE PERFORMANCE OF UASB REACTORS 6. EFFECT OF CHROMIUM AND ZINC ON UASIJ PROCESS 7. CONCLUSIONS AND RECOMMENDATIONS In Chapter 1 the objectives of the work embodied in the thesis have been defined. It presents a brief background information on factors affecting the efficiency of upflow anaerobic sludge blanket reactors. Chapter 2 gives the details of literature on anaerobic process, high rate anaerobic systems, characteristic features of UASB process and its application for the treatment of industrial (iv) effluents and sewage. The effects of heavy metals on conventional and high rate anaerobic processes have also been highlighted. Chapter 3 describes the materials and methods employed in these investigations. Three USAB reactors charged with granular sludge housed in a constant temperature (30+1 C) chamber were used. Metal analysis were carried out using atomic absorption spectrophotometer (IL 751) and inductively coupled plasma atomic emission spectrophotometer (PLASMA LAB 8440). The resytts pertaining to the evaluation ofUASB process at different levels of sludge occupancy have been presented in Chapter 4. The performance of the UASB reactors was monitored at increasing sludge occupancy and different organic loading rates. It was observed that at relatively low sludge loading (kg COD per kg VSS per day) sludge washout limited the organic load to the reactors. On the basis of this the maximum or limiting sludge occupancy (v/V ratio) has been ascertained. The sludge accumulation monitored from sludge occupancy has boon empirically related to cumulative organic load- On this basis desludging schedule at maximum sludge occupancy has also been worked out. The findings given in Chapter 4 demonstrate that the sludge potentials at increased sludge occupancy are not fully utilized. Therefore, in order to ascertain optimum sludge occupancy the investigations were carried out to study the effect of desludging on UASB process. By desludging the reactors the organic load could be increased to utilize sludge activity to its maximum possible extent (critical value of sludge loading (v) parameter). The resluts obtained therefrom have been analysed and incorporated in Chapter 5. Chapter 6 deals with the effects of chromium and zinc on anaerobic digestibility of wastewater in batch bioassay tests and UASB reactors. Inhibition and toxic levels of chromium and zinc in both batch and UASB reactors were determined. The results demonstrate the practical suitability of the process and toxic limits cover the range of chromium and zinc concentrations generally present in wastes from chrome tanning and viscose rayon industries. The limitations and conclusions derived from the present study and scope for future investigations form the subject matter of Chapter 7.