ROLE OF METAL IONS ON AEROBIC AND ANAEROBIC BIODEGRADABILITY OF WASTEWATER

Abstract

The seriousness of the problem of pollution and its impact on the environment is presently the focus of third world countries as well as developed countries. The wastewaters which are discharged as effluents on lands and water bodies contain a wide variety of pollutants and toxicants such as heavy metals. Metals, in general, interfere in an undesirable manner in many of the environmental processes. Metals also interfere with biological waste treatment processes. To understand the impact of metal concentration on environment, it is necessary to study its bioavailability and potential for contamination of water. The aerobic degradability and anaerobic biodegradability of wastewaters are assessed by BOD (Biochemical Oxygen Demand) and BMP (Biochemical Methane Potential). Trace quantity of metal ions serve as nutrient. However, the presence of high concentration of metals may be detrimental to aerobic and/or anaerobic biodegradability. Quantitative impacts of these metals in different concentrations on biodegradability can be assessed by measuring BOD and BMP with metals in varying concentration. Metal containing wastewater from industries are, therefore, evaluated for their BMP and BOD before suggesting suitable treatment. With this in view a study was planned to investigate different concentrations of chromium, IV lead and nickel in the form of CrCl3,6H20 [Cr(III)]; K2Cr207[Cr(VI)]; Pb(N03)2[Pb(II)]; and NiCl2.6H20[Ni(II)]; NiS04.6H20 [Ni(II)] on BOD, acidification and methane generation. The distribution of the metal in different forms in the digested sludge was evaluated by selective extraction procedures. An attempt has been made to correlate BMP and BOD of synthetic wastewater prepared by diluting the molasses. For better presentation of the subjects the thesis has been divided into the following chapters General Introduction Chapter I Literature survey Chapter II Materials and Equipments Chapter III Effects of metals on BOD Chapter IV Effects of metals on BMP Chapter V Speciation of metals in digested sludge Chapter VI General Conclusions Chapter VII Chapter - I is a brief background of pollution in our sorrounding environment, toxicity of metal concentration in wastewater and their impact on environment. The need for detecting toxicity limit, their speciation, and methods for assessment are also presented. On the basis of above information the objectives of the work expressed tangibly in the thesis have been defined. V Chapter - II presents the details of literature on aerobic and anaerobic process, advantage of phase separation in anaerobic process for treatment f industrial effluents, the effects of heavy metal on conventional batch aerobic and anaerobic processes, the speciation of metals in digested sludge. Chapter - III present the materials and instruments generally used during the experimental procedure. For the ease of presentation, the methodology specific to a particular experiment has been included as a part of that chapter. Gas chromatography (GC 5700, NUCON Engineers) was used for detection of CH4 and C02 and Atomic Absorption Spectrophotometer (Perkin- Elmer Model 3100) undertaken for metal analyses. Chapter - IV describes the effects of chromium, lead and nickel in the form of their salts on BOD bioassay test. BOD exertion data at different metal concentrations was analysed for BOD kinetics. Thresholds and lethal concentrations of toxicants in carbonaceous stage of BOD exertion were determined. It is confirmed that the toxic limit (50% reduction in comparison to control) for Cr (VI) is between 17-20 mgf1 whereas for Cr(III) is <7 mgf . The toxic limit for Pb (II) is more than 35 mgf1 while for Ni(II) is less than 5 mgf . Chapter - V details the effect of added chromium (III, VI), lead(II) and nickel(II) in the form of their salts on methane generation as well as acid VI 0 production phase. The obtained results show the inhibition and toxic limit of these metals in the reactors. The inhibition level for chromium(lll), lead(II) and nikel(II) are >143 mgf1, 238 mgf1 and 48 mgf (20% reduction in gas production) respectively. Their toxic limits are > 190 mgf1, >278 mgf1 and 56 mgf (50% reduction in gas production) respectively. Chapter - VI contains the speciation of chromium, lead and nickel in digested sludges obtained from the termination of BMP^q experiment using stover sequential extraction procedure. The speciation profiles of digested sludge indicated that chromium is largely found as organically bound, carbonate form of lead and nickel were identified as major fraction effect of increasing concentration of metals in digested sludges. No marked difference was obtained in speciation profiles of nickel chloride and nickel sulphate. Chapter - VII attempts to integrate the observations recorded on BOD and BMP, and acidogenesis, BMP and metal speciation. The chapter gives an overall appraisal of the work carried out on BOD exertion, acid production, methane generation and the fate of the metal in the digested sludge.