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Sludge Thickening: A Biotechnological Approach Vfr MATfcJU SIMONA 6l2lNSKA Institute of Microbiology (CSAV) In recent decades rapid population and industrial growth have required more rigorous water pollution control measures which in turn have caused a considerable increase in the use of biological wastewater treatment facilities. Even after preliminary treatment, many municipal and indus- trial wastewaters would be highly polluting if discharged into a watercourse because of their biological oxygen demand (BOD) con- tent. The activated sludge process is frequently chosen as the sec- ondary treatment process because of the high BOD reductions usu- ally achieved, and in many countries it is the most commonly used method for biological treatment of wastewater. In order to maintain optimum process conditions, waste impu- rities that are converted into new biomass (activated sludge) have to be considered against the rate at which solids are wasted from the activated sludge process. This so-called waste-activated sludge is removed from the system as a very dilute suspension containing 0.3 to 1.2 percent of suspended solids, depending on the type of activated sludge plant and the plant loading. Waste-activated sludge contains between 98.7 and 99.7 percent water. Water is found in the solid particle cells adhering to and absorbed by the particles, in the capillary spaces, and in the cavities formed by a number of solid particles (Miiller 1971). It is the water in these different locations that has to be removed to reduce the volume of the sludge. Many existing treatment plants employ gravity thickeners, air 207
208 TABLE 1 A comparison of sludge thickening methods Method Polyelectrolyte addition Space requirement Energy Thickened consumption sludge concentration Gravity thickener No Large Low < 3 % DS Air flotation thickener Yes Large High < 5 % DS Centrifugation No Small High < 8 % DS Bioflotation No Small Low 5-8 % DS flotation thickeners, or centrifugation for sludge treatment. But there is another method of sludge thickening which has been de- veloped at the CSAV Institute of Microbiology (Barta et al. 1984a). This process involves two biological stepsâflocculation and flotation. Both processes are based on enzymatic activity of microorganisms present in activated sludge. A comparison of the available techniques is summarized in Table 1. Bioflotation thickening of sludge has some advantages over other alternatives, especially very low energy consumption and small space requirements. The biological processes which are utilized during bioflotation are dissimilatory denitrification or nitrate respiration. In dissimilatory denitrification, nitrate serves as the hydrogen ac- ceptor in the oxidation reduction reactions of the carbon substrate to provide energy for cell growth. It is converted to gaseous end productsâprincipally nitrogenâby heterotrophic bacteria. Biologi- cal denitrification is achieved under conditions of low (< 1 mg/L) or zero oxygen concentrations. A wide variety of common facultative bacteria present in activated sludge, such as Pseudomonas, Micro- coccus, Achromobacter, Denitrobacillus, Spirillum, and Bacillus, have been reported to accomplish denitrification under anoxic conditions (McCarty and Haug 1971). Dissimilatory nitrate reduction (denitrification) proceeds via ni- trite to nitrogen oxide and nitrogen. The end product depends on the system pH. Nitrogen is the end product if the pH is above 7.3; below 7.3 nitrous oxide is produced (du Toit and Davies 1973).
209 In the bioflotation process it is necessary to enhance denitri- fication activity by controlled addition of nitrates into the waste activated sludge. In the first stage of the process the physical struc- ture of the sludge is changed, and flocks are formed. In the second stage gaseous products of the denitrification are evolved and flota- tion occurs. Organic substances present in the sludge liquor serve as a carbon source of dissimilatory denitrification. It is not, therefore, necessary to add an external source of organic carbon in most cases. After laboratory tests with good results the new method of bioflocculation and bioflotation was tested in a pilot plant installa- tion. The pilot plant was built at the mechanical biological waste- water treatment plant which treats approximately 15,500 m3/d of wastewater. The wastewater was a mixture of municipal wastewater from a town with a population of 27,000 and industrial wastewater mainly from slaughterhouses and the textile industry. The total load was a 76,000 population equivalent. The mean BOD6 was 164 mg O2/L. A diagrammatic layout of the pilot plant is shown in Figure 1. The process starts with nitrate addition to the waste-activated sludge. The nitrate is added into the mixing tank in the form of a 10 percent solution of calcium nitrate. The hydraulic retention time in this vessel is very short, approximately five minutes. In the reaction tank the activity of enzyme nitrate reductase is induced and flocculation occurs. At the same time coagulation of colloid particles takes place. The hydraulic retention time in this tank ranges from 30 to 180 minutes according to different sludge properties. Flocculated waste-activated sludge is then pumped into the flotation tank. The volume of this tank in the pilot plant is 7.84 m3. The hydraulic retention time ranges from 50 to 180 minutes. In this tank the denitrification proceeds. The bubbles of gaseous end products of denitrification lift the sludge particles to the surface where a layer of thickened sludge is formed. The flotated thickened sludge is wiped away using hooks from the top of the flotation tank. The sludge liquor is drained off from the lower part of the flotation tank. This pilot plant was operated for more than two years. The results of the operation are summarized in Table 2; the values given are the mean of a one-year operation. The operation parameters of the flotation unit were as follows: The mean flow of the waste-activated sludge into the bioflotation pilot plant was 5.19 ms/h. Concentration of total suspended solids was 2.96 g/L, and the hydraulic retention time 1.41 h. The temperature
210 Ca(N03)2 Rake Waste- activated sludge FIGURE 1 Diagrammatic layout of the bioflotation pilot plant unit. TABLE 2 Results of bioflotation pilot plant operation with reaction tank Influent Thickened Effluent (Waste-activated sludge (sludge sludge liquor) Reduction Suspended solids (g/1) 2.96 42.82 0.22 .. BOD5(mg 02/1) 291 91 68.6 COD (mg 02/1) 495 232 53.0 NO_ (mg/1) 20.2 29.6 ^»
211 TABLE 3 Results of the bioflotation pilot plant operation without reaction tank Influent (waste Thickened Effluent Reduction activated sludge) sludge (sludge liquor) Suspended solids 3.15 45.88 0.36 (g/1) BOD, 372 â 94 74.7 (mg 62/l) COD 624 -- 267 57.2 (mg 02/1) NO, 24.0 ~ 27.0 (mg3/1) Nitrate dose = 95 - 110 mg N - NOg/1 Hydraulic retention time = 21/h Sludge temperature = 15 - 18° C of waste-activated sludge ranged from 12 to 24 degrees Centigrade. The dose of calcium nitrate was 128 g/m3 of waste activated sludge. The mean energy consumption was 0.25 per 1 m3 of waste-activated sludge (Barta et al. 1984b). In the next series of tests the bioflotation unit was changed. It was operated without the mixing tank and the reaction tank. Instead of these vessels, a static mixer for homogenization of the mixture of waste-activated sludge with calcium nitrate solution was used. All bioflocculation and bioflotation reactions took place in the flotation reactor. The hydraulic retention time in the flotation tank was prolonged to two hours, a period sufficient for satisfactory sludge thickening. In fact, this flotation unit arrangement decreased the hydraulic retention time of the whole system to nearly one-half. The results of the operation are summarized in Table 3. It can be seen that the performance of the bioflotation unit in this arrangement was nearly the same as in the first case. The BOD5 and chemical oxygen demand (COD) reduction was slightly higher. The waste-activated sludge volume reduction was higher than 90 percent. It is very important that during the bioflotation process the
212 BOD5 and COD values of sludge liquor which is commonly sent back for biological treatment are substantially decreased. In other thickening processes this effect cannot be achieved because all other processes utilize physical methods which cannot influence these val- ues. Even more promising results were obtained when the bioflotation unit was operated batch-wise. In this case the thickened sludge had a total suspended solids concentration of more than 80 g/L. This is particularly important when the bioflotation process is utilized at small wastewater treatment plants which serve communities with a few residents. In this case, only small volumes of waste-activated sludge have to be thickened and the batch process is very suitable. Longer hydraulic retention time helps attain a higher total suspended solids concentration in thickened sludges. In this case investment costs are much lower. The bioflotation process is suitable for nearly all plants which treat wastewater biologically. According to our experience it cannot be used in plants which treat wastewater with very low concentrations of nitrogen, e.g., wastewater from the pulp and paper industry. Biological flotation and thickening has the following advantages over other alternatives: ⢠Polyelectrolyte is not required. ⢠Energy consumption is very low. ⢠Space requirements are small. ⢠During bioflotation, the BODS and COD are substantially re- duced. Because of very low energy consumption, technical simplicity, and low investment and operation costs, bioflotation is a very promis- ing process for waste-activated sludge thickening. REFERENCES Barta, J., S. Ciiinska, V. HavUn, V. Mateju, J. Maixner. 1984a. Proc. d. Jahrestreffen 1984, Verein Deutscher Ingenieure, Miinchen, p. 763-772. Barta, J., M. Verner, I. Pardus, D. Vesely, G. Aronson. 1984b. Czechoslovak Patent No. 228 403 du Toit, P.J., T.R. Davies. 1973. Water Res. 7:489-500. McCarty, P.L., R.T. Haug. 1971. Microbiol. Asp. of Poll., Academic Press, London. Muller, V. 1971. Gewasserschutz, Wasser, Abwasser. Aachen 6:325-357.
