INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 1, No 4, Copyright 2010 All rights reserved Integrated Publishing services

Transcription

1 Experimental studies on effect of Heavy Metals presence in Industrial Wastewater on Biological Treatment Majid Sa idi MS Student of Chemical Engineering, Faculty of Engineering, University of Tehran ABSTRACT Biological Wastewater Treatment is one of the most important parts of wastewater treatment and microorganism has importance role in biological treatment. Microorganisms effect in metabolism of various organic compounds and other elements. Such microbiological parameters as the number, weight and activity of microorganisms can be good indicators of wastewater contamination with heavy metals. Heavy metals are known as harmful pollutants in Wastewater having a negative effect on Biological Wastewater Treatment plant including microorganisms. Some heavy metals, e.g. lead, even at low levels are toxic for microorganisms. As a rule, heavy metal has a negative effect on the growth of water microorganisms as it can greatly depress their numbers. On one hand, the number of microorganisms depends on the total content and concentrations of particular forms of heavy metals. On the other hand, it is conditioned by several other factors, quantity and quality of organic matter, especially carbohydrate rich organic matter, ph, total exchange capacity, nutrient availability, moisture, temperature and oxygen availability. Heavy metals shift the structure of microbial populations, impoverish their diversity, and affect species composition, reproduction and activity of indigenous microorganisms. Contamination of wastewater with high rates of heavy metals caused a significant decrease in the numbers of bacteria in biological system. It is obvious that heavy metals are very danger contaminant in wastewater and disorder of biological wastewater treatment is as a result of this pollution. Keywords: BOD5, COD, PH, TDS. 1 Introduction There is a lot of Heavy Metal in our environment: cadmium, chromium, cobalt, copper, lead, mercury, etc. Interestingly, small amounts of these elements are common in our environment and are actually necessary for good health, but large amounts of any of them may cause acute or chronic toxicity. In small quantities, certain heavy metals are nutritionally essential for a healthy life but they become toxic when they are not metabolized by the body and accumulate in the soft tissues. Living organisms require varying amounts of heavy metals. Iron, cobalt, copper, manganese, and zinc are required by living organisms. Other heavy metals such as mercury, plutonium, and lead are toxic metals that have no known vital or beneficial effect on organisms, and their accumulation over time in the organism can cause serious illness. Certain elements that are normally toxic are, for certain organisms or under certain conditions. Heavy metal can be exist in wastewater and enter the wastewater treatment plant. Heavy metals have dangers effect on biological treatment, because they have toxic effect on microorganism, e.g. bacteria. 666

2 2. Toxic Effects of Heavy Metals on the Activated Sludge in Biological Wastewater Treatment Heavy metals are toxic to most microorganisms at specific concentrations and often cause serious upsets in biological waste treatment plants. The mechanism by which heavy metals affect the microorganisms is not clear. It has nevertheless been suggested that heavy metals block the enzyme systems or interfere with some essential cellular metabolite of bacteria and protozoa. The toxicity of heavy metals in activated sludge mixed liquor depends mainly upon two factors, namely, metal species and concentration. Other factors such as ph, sludge concentration, influent strength are also reported to affect the toxicity of metals, though to a lesser degree. It has been reported that only soluble metal ions are toxic to the activated sludge. Acclimatized sludge s maintain high removal efficiency of dissolved organic matter though exposed to constant input level of heavy metals, conversely shock loads manifest remarkable effects on sludge s whether acclimatized or not. The importance and role of the microbial community in the purification process of activatedsludge plants has been well documented. Ciliated protozoa are commonly found in densities of about 104 cells/ml of activated sludge and improve the quality of the effluent because of their involvement in the regulation of the bacterial biomass by removing, through predation, most of the bacteria dispersed in the mixed liquor. Moreover, the community structure of protozoan species is an effective biological indicator of functional conditions of waste treatment plants. Differences among species in sensitivity to a single chemical can vary an order of magnitude and possibly up to seven orders of magnitude. Moreover, the sensitivity of a species can vary from a toxicant to another one. Consequently, it is likely that a model of action for chemicals derived from one species may not directly apply to the other species. However, by testing many species simultaneously such as the ciliate community inhabiting the activated sludgethe concentration of chemicals damaging the whole community, in terms of depletion of both organisms and species, can be efficiently defined. It was the intent in this part to determine the shock load effect of cadmium, copper, chromium (VI), lead and zinc on the ciliate community in the activated sludge mixed liquor. Table 1 shows the nominal heavy metal concentrations in activated sludge samples and the corresponding concentrations of soluble metal observed after 24 h. The behavior of heavy metals during wastewater treatment is well documented. In the present study the proportion of heavy metals persisting as soluble chemical forms in the mixed liquor after 24 h of treatment did not exceed 30% (Table 1). The proportion of soluble Zn, Cu, and Cd increased with the increase of the initial concentration, while the proportion of soluble Pb did not vary. Soluble Pb and Cd showed the lowest percentages after 24 h. As only soluble metals are regarded as toxic, toxicological data are reported referring to soluble metal concentrations. The effect of the five metals on both cell density and number of species of the protozoan community is shown in Figure

3 Table 1: Heavy Metal Nominal Concentrations and Concentrations measured after 24 h in the Activated Sludge Mixed Liquor Solution Nominal Solution Metal Concentration (mg/l) mg/l % Cadmium Chromium (VI) Copper Lead Zinc

4 Figure 1: Effect of Heavy Metals upon the Activated Sludge Survival and species diversity are expressed as percentage of living individuals and number of species, respectively, present after 24 h of metal exposure. Metal concentration is referred to mg/l of soluble metal ions present in the mixed liquor 24 h after treatment. In the presence of 5.23 mg/1 Cd, the disappearance of 8 of the 16 species and the diminution of the cell density of the remaining species were observed. As a consequence, cell mortality of the whole protozoan community was 50%. A concentration of 6.12 mg/1 Cu caused 89% of cell mortality in the whole community and the disappearance of 7 out of 16 species. A concentration of 6.98 mg/1 of Pb killed 65% of the individuals, but caused the disappearance of only one species (C. uncinata). An 80% mortality and the reduction of species richness to 9 species out of 16 was observed in the presence of 81 mg/1 of soluble Zn, and two species were still surviving in the presence of 145 mg/1. 55% mortality in the whole protozoan community and the reduction of species richness to 11 species out of 16 was observed in the presence of 150 mg/1 of soluble Cr (VI). A concentration of 293 mg/1 Cr was necessary to cause 90% reduction in the number of organisms and to reduce richness to 8 species. 669

5 A peculiarity of the present work is that the experiments were carried out on an indigenous microbial community. becological communities, in fact, are more than just aggregations of species but rather composed of co evolved, interacting populations. The use of indigenous microbial communities increases the likelihood that important community and ecosystemlevel processes will be functioning in the test system. The results of toxicological tests conducted on communities depend, to a great extent, on the species that constitute the community. The higher the number of sensitive species (especially the dominant species), the more acute the toxic effect upon the community. The following sequences of heavy metal toxicity on bacterial communities in activated sludge have been reported: Cd > Cu > Zn > Cr > Pb Cd > Cr > Cu > Pb > Zn This discrepancy between the two studies depended on both the type and structure of the bacterial community. Heavy metals change the structure of the activated sludge microfauna by modifying both cell density and species richness. Chromium (VI) and zinc were considerably less toxic than lead, copper, and cadmium. The order of toxicity of the five metals to the microbial community was generally Cd>Cu > Pb > Zn > Cr. Ciliated protozoa contribute over 9% to the activated sludge biomass and together with bacteria, play an important role in the purification process by removing the dissolved organic matter (Floc forming bacteria), and by clarifying the treated effluent (filter feeding ciliates). The finding that these organisms are able to survive in the presence of heavy metal concentrations higher than those normally observed in the activated sludge mixed liquor, increase the likelihoods that the purification process will be functioning when additional amounts of heavy metals enter the plant. Furthermore, this study provided information about the metal concentrations that affect the protozoan community in the activated sludge. 3. Metal Ion Effect on BOD Exertion at Different Temperatures Since both the aerobic biological treatment process and BOD measuring technique are based on the same principle, the presence of metals like copper, zinc, lead and other heavy metals will have an influence on both processes. Because of the limited solubility of oxygen in the aqueous medium in BOD bottle, the effect of heavy metals will be sufficiently large as compared to that in the treatment plant because the effluent in the treatment plant gets a regular supply of oxygen from continuous aeration. Trace quantities of heavy metals such as nickel, manganese, lead, chromium, cadmium, zinc, copper, ferrous and mercury are common constituents of most wastewaters. Some of these metals are necessary for growth of biological life and the absence of sufficient quantities of them could limit growth of Bacteria. However, the presence of any of these metals in excessive quantities will interfere with many beneficial uses of water because of their toxicity. 670

6 Very little work is available in literature on the effect of temperature on heavy metal toxicity to BOD. Apparently, BOD is expected to increase with the increase in temperature of incubation because of the loss of dissolved oxygen from the medium of heavy metal ions. It will be quite interesting to see the effect on BOD particularly with an increase in temperature. It has been reported that BOD5 is suppressed significantly by even small concentrations (1 2 mg/l) of copper or chromium. Scientific measured the percentage of suppression caused by 1 mg/l of selected heavy metals on the BOD of domestic sewage. According to another study, the presence of metal ions like Al, Co, Ni, Cu, Zn, Pb and Hg in the effluent samples significantly affects BOD values. Metal ion addition results in the inhibition as well as increase in BOD, depending on its concentration. The ambient temperature range (15 30 with an incremental change of 5 ) was selected for the study, as a variety of microbes responsible for BOD exertion are optimally active in this temperature range. 4. Estimation of Dissolved Oxygen (DO) Initial and final DO values were determined by titration method. Percentage of inhibition in BOD values (due to toxicity of metal ions) were calculated for each metal ion at different temperatures, and are shown in Table 2. Table 2: Percentage Inhibition in BOD Exertion by Metal Ion Temperature Percentage inhibition in BOD exertion by metal ions ( ) Mean BOD value in presence of a given metal ion is subtracted from the mean BOD without the metal ion to calculate the inhibition. Histograms of the BOD without metal ion as well as those in presence of metal ion are plotted. Error bars for each histogram represent the standard deviation in the BOD values. Inhibitions in BOD due to the presence of metal ion 671

7 are shown in the figure as separate columns. No error bars can be given for BOD inhibition as the results are the differences in mean BOD values. BOD exertion is affected by factors like temperature, availability of organic matter as food, seed, and ph of the medium. Present study is undertaken to study the effect of some heavy metal ions like Cr3+, Co2+, Ni2+, Cu2+, Cd2+ and Pb2+ on BOD at different temperatures, i.e., 15 C, 20 C, 25 C and 30 C. All metal ions are found to inhibit the BOD (Table 2). Absolute values of BOD in presence of metal ions are compared with those in the absence of metal ions at different temperatures and are shown in Figures 3 to 6. These Figures also show inhibition in BOD in the presence of metal ions at a given temperature. Relative percentage inhibition increases with increase in temperature (Table 2) up to 25 C and further up to 30 C it tends to decrease, probably because of the relatively large increase in BOD exertion for the blank system from 25 C to 30 C. In all the metal ions, relative percentage inhibition is more at 25 C than at all other temperatures. Lead is found to be the most toxic element at all temperatures. Cd2+ and Pb2+ show relatively larger inhibition than that of transition elements. It shows that transition metals as well as Cd2+ and Pb2+ are highly toxic to microbes. Figure 3: Histogram of BOD in presence of Metal Ion at 15 C 672

8 Figure 4: Histogram of BOD in presence of Metal Ion at 20 C Figure 5: Histogram of BOD in presence of Metal Ion at 25 C 673

9 Figure 6: Histogram of BOD in presence of Metal Ion at 30 C There are two reasons for the decrease in BOD from 15 C to 30 C: I. Depletion of dissolved oxygen (mg/l) with increase in temperature. Solubility of oxygen in water at 760 mm of mercury pressure and 100% relative humidity are: at (15 C), 9.08 at (20 C), 8.27 at (25 C), and 7.59 at (30 C). II. Due to the toxic behavior of metal ions towards microbes (metal ions complexion with microbial cells) resulting in a lower demand of dissolved oxygen. We know that Biochemical oxygen demand or (BOD5) is the amount of oxygen consumed by a fixed quantity of organisms over 5 day period. By using above figures we can conclude that when the concentration of heavy metal increases in wastewater, there is toxic effect on microorganism, because heavy metal adheres to the microorganism and produce complex with bacteria. Metal ion complexion with heavy metal effects the growth of bacteria and it s the cause of dead of Bacteria. When the bacteria dead, the BOD will be increase, because the population of Bacteria decreases and the amount of oxygen for growth of bacteria will be decrease. Therefore in the present of heavy metal in wastewater, increasing in BOD is a bad sign for the system, because this is a threat for microorganism. A peer look into Table 2 shows that the relative percentage inhibition increases almost linearly with increase in temperature up to 25 C in the presence of each metal ion. It indicates that the metal microbe complex is stabilized with increase in temperature up to 674

10 25 C. For most of the metals, except copper, the relative percentage inhibition lies between 40% and 60%. Presence of lead in the BOD bottle leads to an exceptionally large inhibition with very little dependence on temperature. The recorded inhibition at 30 C is besides the effect of nitrifying bacteria. In their absence the inhibition would have been still larger. These bacteria are known to be most active in a temperature range 30 C 40 C. Measurements of BOD can be influenced by the temperature of the solution by acting on properties of substances present like microbes, metal ions in the sample. Complexion phenomenons are strongly dependent on temperature. It is therefore recommended that BOD be measured for different temperature values. This part provides general guidelines on how to remove dissolved metals from wastewaters for discharge to sanitary sewer systems. Each of the various stages or operations of wastewater treatment will be discussed with their role in the metals removal process. The treatment train described is general for metals removal. Some variations will exist among different systems. 5. Conclusion Biological treatment the use of bacteria and other microorganisms to remove contaminants by assimilating them has long been a mainstay of wastewater treatment in the chemical process industries. Because they are effective and widely used, many biological treatment options are available today. Heavy metals are toxic to most microorganisms at specific concentrations and often cause serious upsets in biological waste treatment plants. Contamination of wastewater with high concentration of heavy metals caused a significant decrease in the number of microorganism e.g. Bacteria. The toxicity of heavy metals on microorganism depends mainly upon two factors, namely, metal species and concentration. Other factors such as ph, bacteria sludge concentration, influent strength are also reported to affect the toxicity of metals, though to a lesser degree. Heavy metals change the structure of the activated sludge micro fauna by modifying both cell density and species richness. Due to the toxic behavior of metal ions towards microbes (metal ions complexion with microbial cells) resulting in a lower demand of dissolved oxygen. When the concentration of heavy metal increases in wastewater, there is toxic effect on microorganism, because heavy metal adheres to the microorganism and produce complex with bacteria. Metal ion complexion with heavy metal effects the growth of bacteria and it s the cause of dead of Bacteria. When the bacteria dead, the BOD will be increase, because the population of Bacteria decreases and the amount of oxygen for growth of bacteria will be decrease. Therefore in the present of heavy metal in wastewater, increasing in BOD is a bad sign for the system, because this is a threat for microorganism. 6: Nomenclature IJES: International Journal of Environmental Science PUT : Petroleum University of Technology AORC: Abadan Oil Refinery Company 675

11 7. References 1. Metcalf and Eddy. Wastewater Engineering, Treatment and Reuse. Mc Graw Hill, International Edition, 3 rd edition. 2. Edward E. Baruth. Water Treatment Plant Design. Mc Graw Hill, 4 th edition. 3. C. Crittenden. Water treatment, principle and design. WILEY, 2 nd Edition. 4. W.Weseley Eckenfelder. Industrial Water Pollution Control. Mc Graw Hill, 3 rd Edition. 5. Gary Benjamin Tatterson. Scale up and Design of Industrial Mixing Processes. Mc Graw Hill International Edition, 3 rd edition. 6. Stanley E. Manahan. Environmental Chemistry. Florida, CRC Press, 1994, 1 st edition. 7. Ron Crites, George Tchobanoglous. Small and Decentralized Wastewater Management Systems. Hall International, 1 st edition 8. Ron Crites, George Tchobanoglous. Small and Decentralized Wastewater Management Systems. Hall International, 1 st edition. 9. Craig Cogger. Recycling Municipal Wastewater Sludge in Washington. Washington State University, November Cheremisinoff, N. P. and P. N. Cheremisinoff. Water Treament and Waste Recovery. Advanced Technologies and Application: Prentice hall Publishers, New Jersey, P. Aarne Vesilind. Treatment and Disposal of Wastewater Sludge s. Michigan: Ann Arbor Science, H. Polat, D. Erdogan. Heavy metal removal from waste waters by ion flotation. Journal of Hazardous Materials 148 (2007), pp Omid Tavakoli, Hiroyuki Yoshida. Application of sub critical water technology for recovery of heavy metal ions from the wastes of Japanese scallop Patinopecten yessoensis. Science of The Total Environment, 368 (2008), pp John Davidson, Christopher Good, Carla Welsh, Brian Brazil, Steven Summerfelt. Heavy metal and waste metabolite accumulation and their potential effect on rainbow trout performance in a replicated water reuse system operated at low or high system flushing rates. Aqua cultural Engineering 41 (2009), pp