EFFECTS OF BENTHAL SLUDGE ON OVERLYING WATER

Abstract

The large rivers of India, with highly variable flow, are the lifeline of people, satisfying all their needs of drinking, bathing and washing, irrigation and power generation. If the quantity of water in rivers and water bodies are inherently uncertain, its quality has been perennially suspect, because all have become very highly polluted, due to community wastes and drainage, made worse by tourism. Reasons for contamination of waters are obvious. Few settlements near lakes or rivers have systems to treat their wastes. The ability of the water body to clean itself has been affected because of the sheer quantity of wastes generated by the ever increasing populations. Settleable and colloidal organic solids discharged into rivers result in accumulation of sludge deposits. These accumulated deposits influence the quality of the overlying waters and have a very slow and long time reaction. In view of the stated importance of the sludge deposits, a comprehensive study on the mechanism of benthal sludge decomposition and related effects in a riverine ecosystem is necessary . Benthal sludges undergo continuous stabilisation in water bodies. During this stabilisation process, B.O.D. remaining in the benthal sludge continuously decreases with time. The B.O.D. in the deeper layers of the benthal sludge move upwards and finally leaches out into the overlying water. This results in the removal of B.O.D. from the benthal sludge ,layers and contribution to the overlying water. The present research project was conceived in 1989 with the objective of investigating into the reactions which take place in benthal sludges. The processes involved in benthal sludge decomposition, the conditions at which the decomposition gets affected, and their effect on the quality of the overlying water were analysed. To achieve the above, the following aspects were studied : 1) the factors affecting the B.O.D contribution by the benthal sludge to the overlying water. This included the effect of operating sludge related characteristics ( depth of benthal sludge, the initial B.O.D of the sludge), the operating overlying water related characteristics (different flow rates of the overlying waters, horizontal velocity, the ratio of the height of overlying water to the depth of benthal sludge), and the sludge related process variables (the rate constant of removal of organic matter in the benthal sludge, the age of sludge etc.). 2) contribution of nutrients by benthal sludge during the benthal sludge decomposition. 3) developing a comprehensive model involving the benthal sludge decomposition mechanism. 4) field investigations under different conditions. 5) comparison and analysis of the laboratory data with the field data. .In this thesis, the work has been presented in six chapters. The first chapter consists of the introduction to the thesis and the need for the study. In the second chapter, a review of the literature relevant to the work embodied in the thesis has been presented. The experimental methods and laboratory analytical techniques adopted in this study are presented in the third chapter. The results of the experimental observations and the field studies are presented in the fourth chapter. Chapter five consists of the discussions of the experimental and field study leading to the conclusions of the thesis. This chapter five, is further subdivided in the following order. 1. Stabilization of benthal sludge : In this section, the decomposition of the benthal sludge in the top layers and different layers below are discussed. The variations in the removal rates of the organic matter in the benthal sludge with the various stream conditions are also discussed. 2. Effects of operating sludge related characteristics on the oxygen consumption in the overlying water ( both at the top layers as well as the benthal sludge -overlying water interface), B.O.D contribution by benthal sludge, and the percent B.O.D contributions . in 3. Effects of operating overlying water related characteristics like the flow rates of overlying water, height of overlying waters on the B.O.D contribution by benthal sludge. 4. Effects of benthal sludge related process variables like the removal rate constant of organic matter in the top layers of benthal sludge, on the B.O.D contribution by benthal sludge. 5. Contribution of nutrients by the benthal sludge. 6. Dimensional analysis and comparison of the non-dimensional parameters. 7. Comparison and analysis of the results of field study. The present investigations were conducted in two phases. In the first phase laboratory scale studies were conducted on laboratory models. These models were 100 cm long, 10 cm wide and 60 cm deep, with ports at different heights for controlling the flow conditions, heights of overlying water, and for collecting samples for analysis. The laboratory models were charged with benthal sludge of variable characteristics. The depth of benthal sludge was varied from 0.04 m. to O .35 m. The B.O.D. of the benthal sludge also varied between 200 mg/L and 1000 mg/L. The operating stream characteristics were also varied, with the flow rate of overlying water between 0.108 m3 / day to 2.6 m3 /day, and height of overlying water between 0.04 mto 0.35 m. The experimental set ups were maintained at room temperature which varied between 19 to 22 ° C. IV The experimental investigation involved measurements of the dissolved oxygen (D.O.) and temperature of the overlying water; B.O.D., ammonia nitrogen, and phosphate concentration in the overlying water and different layers of benthal sludge. Samples were collected from the top layers of the benthal sludge (the top most 1-2 mm. layer), at 10,20 and 30 cm. depths within the benthal sludge. The samples from benthal sludge were collected with a special syringe so that the sludge layers would not get disturbed. The D.O. and temperature were measured daily, and the B.O.D., ammonia nitrogen and phosphate measurements were taken at a frequency of 5 days. Regression analysis of the observed values of the B.O.D. of the benthal sludge layers yield the rate constant of removal of organic matter from benthal sludge and the initial B.O.D. in the benthal sludge layers. The B.O.D contribution by the benthal sludge to the overlying water was determined through a model developed from mass balance for the experimental conditions. The observed values of D.O. and B.O.D. in the overlying water were used in this model. The percent B.O.D contribution (the ratio of B.O.D contribution to the B.O.D remaining in the benthal sludge at that instant of time) was also determined. The nutrients (ammonia nitrogen and phosphate) contribution by the benthal sludge was also determined in the same manner. In the second phase of the study, field studies were conducted at the river Ganga, near Kanpur, India, just downstream of the sewage outfall. Core samples of the benthal sludge were collected at three stations in the river, 500m away from the point where the outfall effluent mixes with the river water, 1500 m away, and 2500 m away respectively. The samples were brought to the laboratory at LIT Kanpur, and analysed for the D.O., B.O.D., ammonia nitrogen and phosphate concentrations in the overlying water and benthal sludge layers. These studies were conducted in two seasons: winter ( between January 1993 and February 1993) , and summer ( between June 1993 and July 1993). The summer temperatures increased from 38° Cat the start of study to 46° Cat the end and was almost av«nrBj« 6° C in winter. Analysis of the experimental data reveal that the rate of removal of organic matter was higher in the deeper layers of the benthal sludge, than the top layer in contact with the overlying water. The rate of removal in the top layer of benthal sludge decreased as the total depth of sludge increased. The benthal sludge layer at about 10 cm. from the top was found to be a very significant boundary layer in terms of benthal sludge stabilisation. The B.O.D. and nutrients in the benthal sludge was observed to be maximum at this layer. The B.O.D. contributions by benthal sludge to the overlying water decreased with increase in the depth of benthal sludge. The variation of B.O.D. contribution by benthal sludge with the velocity of overlying water depended on the h/d ratio. When the h/d ratio was more than 1.0, the B.O.D. contribution decreased with increase in velocity and flow rate, and increased with increase in velocity and flow rate if the h/d ratio was less than 1.0. This h/d ratio 1.0 was also a significant boundary, since the removal rate constants in the benthal sludge layers also changed at this h/d ratio. The field studies indicated a strong qualitative agreement with the laboratory results. Seasonal variations are exhibited in the field studies. The B.O.D in the overlying water of river Ganga was almost constant during the study period in winter. During summer, the B.O.D. was about five times higher in the overlying water. The B.O.D. in the top layers of benthal sludge was also higher in summer. However, in winter, the B.O.D. in the benthal sludge at depths of 10 and 20 cm. were about three times higher than that' in summer. The B.O.D. contributions by the benthal sludge to the overlying water was almost constant in winter. During summer, the B.O.D. contributions were higher than in winter and continued to increase as the atmospheric temperature increased during the study period. Hence, the B.O.D. contributions by benthal sludge are very sensitive to temperature variations. Further, results reveal that the B.O.D. contribution was higher (i) closer to the outfall in summers and (ii) farther away downstream during winter. vi Dimensional analysis of the variables considered for this study yielded four non dimensional terms: the percent BOD contributions ( i.e. the ratio of the BOD contribution on any day to the BOD remaining in the benthal sludge top layers on that day, multiplied by 100), the ratio of the height of overlying water to the depth of benthal sludge (h/d ratio), the t. Kb value ( where t is the number of days from start of experiment and Kb is the rate constant of removal of organic matter from the benthal sludge top layer), and the ut/d value ( where u is flow velocity of the overlying waters). These non-dimensional variables were used to analyse the effect of different parameters on the B.O.D. contributions and the percent B.O.D. contributions by benthal sludge. The BOD contribution and the percent BOD contribution by the benthal sludge on any day were also modeled with the different operating stream variables and the process sludge characteristics variables. Initially, empirical relationships were generated with individual variables like depth of benthal sludge, flow rate of overlying waters, etc. Finally, predictive models considering the integrated effects of all the studied variables were generated incorporating the non - dimensional variables. The data from the experiments were used to generate best fit equations at different "h/d" ratios. The "h/d" ratio for the river Ganga, where the field study was conducted was 4.0 in summer and 5.0 in winter. In order to use the results of the experimental study for the field study, the best fit equations for "h/d" ratios were extrapolated, which resulted in Eq. 1 which had "h/d" also as a variable. Percent BOD = exp {[-1.2 (h/d) + 6.2] + Contribution d (h/d) / [ut ( 3.7x10 "* (h/d) - 3.92x10 _4]}... (1) In the above equation, d = depth ofbenthal sludge, m. h = height of overlying water, m. t = number of the day from start of observations u = horizontal velocity of overlying water, m/day. vu This model gives comparative results with the experimental data when the h/d ratios are about 1.0. However, as the velocity increases, the model becomes limited in the sense that the percent B.O.D. contributions are affected more by the h/d ratios than any other value. In large rivers where the h/d ratios are very high, the percent B.O.D. contributions become very large. Also at very low h/d ratios, the model predicts very high percent B.O.D. contributions and needs to be further refined. The model gave good results with the field data observed. This model could be used to determine the BOD contribution by the benthal sludge on any day in the stream, if the operating stream and process sludge variables are known. Substitution of the field data studied in the above model gave almost correct values of B.O.D. contribution by benthal sludge to the overlying water during winter season. .The accumulated sludges in the river bottoms undergo continuous decomposition and exert an oxygen demand on the overlying water. Earlier studies have indicated that these sludges remain active for very long periods extending to more than a few years. Such decomposition of accumulated organic matter at the river bottoms act as hidden sources of B.O.D contributed to the overlying water continuously over along period. In situations where large scale industrial operations lead to continuous discharge of settleable organic matter to rivers, such accumulated sludges acting as hidden sources. In summer, particularly in the rivers with very little flow ( as in south Indian west flowing rivers) where the h/d ratio may approach 1.0 or less, this may lead to considerable B.O.D. release from the benthal sludge. Again, any disturbance to the top 10-20 cm. layers of benthal sludge exposes the layers where B.O.D. is concentrated and could be harmful to the health of the river. vin