2340 IWA Publishing 2011 Water Science & Technology 63.10 2011 Aerobic biological treatment of thermophilically digested sludge M. V. Kevbrina, Y. A. Nikolaev, D. A. Danilovich and A. Ya. Vanyushina ABSTRACT Aerobic biological treatment of digested sludge was studied in a continuously operated laboratory set-up. An aerated reactor was filled with thermophilically digested sludge from the Moscow wastewater treatment plant and inoculated with special activated sludge. It was then operated at the chemostat mode at different flow rates. Processes of nitrification and denitrification, as well as dephosphatation, occurred simultaneously during biological aerobic treatment of thermophilically digested sludge. Under optimal conditions, organic matter degradation was 9.6, the concentrations of ammonium nitrogen and phosphate decreased by 89 and 83, respectively, while COD decreased by 12. Dewaterability of digested sludge improved significantly. The processes were found to depend on hydraulic retention time, oxygen regime, and temperature. The optimal conditions were as follows: hydraulic retention time 3 4 days, temperature 30 35 W C, dissolved oxygen levels 0.2 0.5 at continuous aeration or 0.7 1 at intermittent aeration. Based on these findings, we propose a new combined technology of wastewater sludge treatment. The technology combines two stages: anaerobic digestion followed by aerobic biological treatment of digested sludge. The proposed technology makes it possible to degrade the sludge with conversion of 45 volatile suspended solids to biogas, to improve nitrogen and phosphorus removal in reject water from sludge treatment units, and to achieve removal of malodorous substances after 8 9 days of anaerobic aerobic sludge treatment. Key words aerobic biological treatment of the sludge, malodorous emission, nitrogen, phosphorus, reject water from sludge treatment, thermophilic anaerobic digestion M. V. Kevbrina (corresponding author) Y. A. Nikolaev A. Ya. Vanyushina MPUE Mosvodokanal, 2, Pleteshkovsky lane, Moscow 107005, Russia E-mail: markev@mail.ru D. A. Danilovich JSC Institute Mosvodokanal NII proect, 22, Pleteshkovsky lane, Moscow 107005, Russia INTRODUCTION Anaerobic digestion is one of the commonly used methods of sewage sludge stabilization. Traditional mesophilic sludge digestion requires significant hydraulic retention time (HRT) 20 25 days. The rate of the process may be increased and the treatment time reduced to 5 6 days by the use of thermophilic digestion. Both modes have advantages and disadvantages. The mesophilic process, while low-cost, less sensitive to toxicants, and allowing easier temperature control, has lower loading rates and poor inactivation of pathogens. The thermophilic process has faster reaction rates, permitting better decomposition of volatile solids (VS), and better inactivation of pathogens. Thermophilic digestion is 4 times more intense, has higher VSS removal efficiency, and yields more biogas. However, the negative points of this process include higher energy requirements for heating; lower-quality supernatant, containing large quantities of dissolved materials; higher odor potential; poorer process stability, due to higher sensitivity of thermophilic bacteria to temperature fluctuations; and poor dewaterability (de la Rubia et al. 2002; Gavala et al. 2003; Gerardi 2003; Turovskiy & Mathai 2006; Forster- Carneiro et al. 2008; Vindis et al. 2009). The thermophilic setup is used on the Moscow wastewater treatment plant (WWTP) for intensification of the digestion process. There are some negative aspects of this intensification, such as worse dewatering characteristics of the digested sludge and high concentration of nitrogen and phosphorus in reject water. While dewatering characteristics of digested sludge may be somewhat improved by washing with the subsequent gravitational settling, this doi: 10.2166/wst.2011.492
2341 M. V. Kevbrina et al. Aerobic biological treatment of thermophilically digested sludge Water Science & Technology 63.10 2011 procedure does not remove nitrogen and phosphorus. Significant degradation of VS under thermophilic conditions results in high levels of ammonium (up to 1 g N-NH 4 /L) and phosphate (up to 50 mg P-PO 4 /L) in reject water. This water forms recycle water flow and increases by up to 20 the load of ammonium, phosphate, and total solids on the biological stage of wastewater treatment. Decreasing the concentrations of ammonium and phosphate in the liquid phase of the digested sludge is an important problem. One of the methods for minimizing the disadvantages of thermophilic methane digestion is an additional aerobic biological treatment of the digested sludge. Anaerobic aerobic sludge treatment was previously studied in order to stabilize the sludge and to increase its dewaterability (Gunter & Goldfarb 1992; Danilovich & Epov 1997). Recently, interest to this process arose in the context of minimizing nitrogen recycle in treatment plants (Novak et al. 2006; Parravicini et al. 2006, 2008a, b). The proposed new technological scheme of sewage sludge treatment is intended for elimination of the negative points of the existing intensive technology of the sludge digestion: high ammonium and phosphates concentrations in the reject water, as well as low dewatering properties of the digested sludge. The technology consists of intensive thermophilic digestion of sewage sludge followed by aerobic anoxic biological treatment. MATERIALS AND METHODS Digested sludge of the Moscow WWTP was used in lab-scale experiments. At this plant, the thickened primary and waste activated sludge are digested anaerobically under thermophilic conditions (53 W C) with hydraulic retention time (HRT) of 6 days. The laboratory setup consisted of two chemostat reactors with aeration (7.5 and 90 L). The smaller one was equipped with a gravity settler (3.8 L) (Figure 1). Effluent gas from the larger reactor was directed to the gas-purifying biofilter. The chemostat reactors were inoculated with specific activated sludge, which was obtained by 1 month selection of the activated sludge from the Moscow WWTP under aeration and feed-batch feeding by digested sludge. The chemostat reactors were operated at 35 37 W С with HRT of 2 4 days and dissolved oxygen (DO) concentration of 0.15 3.5. In the small reactor, the aeration was maintained at the continuous mode. In the larger one, the aeration was 70 at the intermittent mode. The treated sludge was directed to the gravity settler operated at room temperature and HRT of 1.5 days. During additional experiments under the same conditions (35 W C, HRT 3 days, DO 0.7 1 with 70 aeration), the larger reactor was fed consecutively with two types of digested sludge. The first was taken from the thermophilic digester with HRT of 12 days, the second, from the thermophilic digester with HRT of 5 days. Figure 1 Laboratory set-up for aerobic biological treatment of digested sludge.
2342 M. V. Kevbrina et al. Aerobic biological treatment of thermophilically digested sludge Water Science & Technology 63.10 2011 Heating of the water jacket-equipped reactors was performed by hot water from a temperature-controlled heater. The sludge was fed from the receiving reservoir using peristaltic pumps. Air flow was controlled by a rotameter. The concentrations of hydrogen sulfide and volatile organic sulfur compounds were measured above the reactor surface with a Kolion-1B-03 gas analyzer (Russia). DO level was measured by an FDO700IQ WTW luminescent oxygen sensor (Germany), connected to a DIQ/S 182 WTW controller (Germany). The biological processes were controlled by monitoring the concentrations of ammonium nitrogen, nitrite, nitrate, and phosphate, as well as the chemical oxygen demand (COD). Changes in dewaterability of the sludge were monitored by the optimal flocculant dosage for its mechanical dewatering and its specific resistance to filtration. RESULTS AND DISCUSSION Decreased concentrations of phosphates and ammonium nitrogen, as well as increased levels of nitrite and nitrate nitrogen (Table 1) during aerobic biological treatment resulted from the simultaneous processes of nitrification, denitrification, and dephosphatation. The BOD/N mineral ratio in the digested sludge was 5.6, indicating sufficient quantities of organic matter to support complete denitrification. The level of degradation for volatile matter during aerobic biological treatment was 8.8 11.2, COD decreased by 11 15, and phosphates by 62 83 (from 32.5 to 5.4 12.3 ). Lower content of metals (from 12 to 4 for aluminum, from 1.6 to 0.7 for iron, from 80 to 40 for calcium, and from 30 to 18 for magnesium) suggested phosphate removal by both biological consumption and physicochemical binding. Ammonium nitrogen level decreased by 65 89; the sum of ionic forms of nitrogen decreased by 30 79. This effect was caused by the biological process of nitrification and denitrification. In the bulk of the aerated reactor, ph was 7.5 7.9 and the temperature was 35 W C. Although the percentage of free ammonium nitrogen could have been high under these conditions and ammonium nitrogen might have been partially removed with the air flow through the mass of sludge, this was not the case. In the control experiment, aeration of digested sludge without addition of specific activated sludge resulted in removal of only 3 of ammonium nitrogen. Thus, biological nitrification and denitrification play the major role in the process of nitrogen removal. Parravicini et al. reported the same processes of nitrification Table 1 Effect of DO concentration on the main characteristics of the aerobic biologically treated digested sludge Total N P-PO4 3 COD N-NH4 þ Sum N (Nþ NH 4 ;N- NO3 ;N- NO 2 ) VSS degradation, VSS, TSS, N-NO2 þ, N-NO3 þ, N-NH4 þ, COD, COD, P-PO4 3, Sample DO, Digested sludge 32.5 18.8 1.0 680 0.6 1.1 22.1 12.5 681.8 1.7 3.2 12.3 16.7 1.02 239 146 286 21.8 11.3 9.6 871 65 11 62 0 1 2.5 7.4 16.0 0.61 85 221 173 19.5 11.1 11.2 478 88 15 77 30 0.4 1.2 6.4 16.6 0.60 107 186 125 20.6 11.2 10.4 419 84 12 80 39 0.2 0.4 5.4 16.5 0.59 76 69 68 20.5 11.3 9.6 212 89 12 83 69 0.15 0.2 9.2 16.3 0.85 367 6 8 18.0 11.4 8.8 380 46 13 72 44 Treated sludge at continuous aeration 0.05 a 15.6 16.8 0.62 190 1 10 20.5 11.2 10.4 201 71 11 52 71 0.5 b 0.05 a 9.8 16.8 0.83 75 25 45 19.8 11.3 9.6 145 89 11 70 79 0.7 1 b 0.05 a 9.1 16.5 0.58 79 76 145 19.6 11.1 11.2 300 88 12 72 56 1.5 b Treated sludge at intermittent aeration a Aeration was out. b Aeration was on.
2343 M. V. Kevbrina et al. Aerobic biological treatment of thermophilically digested sludge Water Science & Technology 63.10 2011 and denitrification in mesophilic digested sludge during aerobic post-stabilization, when the efficiency of ammonium nitrogen removal was 98 (Parravicini et al. 2008a). By varying the HRT and DO levels in the aerated reactor, it was possible to control the degrees of nitrification denitrification and dephosphatation and therefore, to regulate the levels of phosphates, ammonium nitrogen, nitrite, and nitrate in the reject water of sludge treatment (Table 1). The optimal HRT was 3 4 days and the optimal DO concentration was 0.2 0.5 for continuous aeration or 0.7 1 for intermittent aeration. In the case of the intermittent aeration mode, stirring of the sludge was necessary when the aeration was out. It was determined that 30 35 W C was the optimal temperature for the process. These values conform with the optimal temperature for growth of the nitrifying bacteria. The effect of the properties of digested sludge on the aerobic biological treatment was studied. Two types of digested sludge were used: HRT in the digester was extended to 12 days or decreased to 5 days. During the phase of aerobic biological treatment of the digested sludge with HRT 12 days, efficiency of ammonium nitrogen removal decreased to 77, and total ionic nitrogen decreased to 44 (Table 2), due to higher ammonium nitrogen concentration in the initial sludge (resulting from its deeper degradation). The reverse situation was observed during aerobic biological treatment of the sludge digested in the high-load regime (HRT 5 days): efficiency of ammonium nitrogen removal increased to 91 and total ionic nitrogen increased to 60, due to an increased content of biodegradable substrates for denitrification and lower ammonium nitrogen level in the incoming sludge. In the aerobic biological treatment process, colloidal matter was oxidized, resulting in significantly improved dewaterability of the thermophilically digested sludge. The flocculant dosage for mechanical dewatering decreased from 8 to 5 kg per ton of dry matter. Specific resistance of the sludge to filtration decreased from 4,200 to 990 cm/g * 10 10. Aerobic biological treatment of Moscow WWTP digested sludge made it possible to omit the stages of sludge washing with the subsequent gravitational settling. During the aerobic treatment process, foaming on the surface of aerated reactors was insignificant. Foam volume did not exceed 5 of the treated sludge volume. During aerobic biological treatment, 50 of hydrogen sulfide and 78 of organic volatile sulfur compounds was eliminated from the liquid phase with the air flow through the sludge column. The treated sludge lost its specific anaerobic smell and became of good organoleptic quality (slight smell of activated sludge). In the process of Table 2 Effect of the properties of digested sludge on the major characteristics of the aerobically treated digested sludge Sum N Total N (N-NH4 þ ; N-NO3 ; N-NO 2 ) COD P-PO4 3 N-NH4 þ VSS degradation, VSS, TSS, N-NO2 þ, N-NO3 þ, N-NH4 þ, P-PO4 3, COD, COD, Sample 8.7 0.5 13.7 683 0.2 0.98 18 9.5 684 Digested 12 days) 8.0 0.5 3.6 159.9 25.22 196 17.2 8.9 6.0 76.6 73.5 8.0 381 44.3 Treated digested 12 days) 15.1 0.7 13.8 422 0.0 0.85 23.1 13.0 423 Digested 5days) 10.1 0.3 1.6 36.8 54.70 78.9 17.6 9.1 30.1 91.3 88.3 32.9 170 59.7 Treated digested 5days)
2344 M. V. Kevbrina et al. Aerobic biological treatment of thermophilically digested sludge Water Science & Technology 63.10 2011 Figure 2 Technological scheme of the two-stage anaerobic aerobic treatment of sewage sludge. thickening, the sludge did not emit malodorous substances. Purification of discharge air from the reactor in the gaspurifying biofilter resulted in its complete deodorization. Based on experimental data, a new anaerobic aerobic technological scheme is proposed (Figure 2). Thickened primary and secondary sludge is digested anaerobically under thermophilic conditions (53 W C, HRT 6 days). Thermophilically digested sludge runs then to an aerated reactor for biological treatment with HRT 3 4 days. The outgoing air is directed from the aerated reactor to the gas-purifying biofilter for deodorization. If the total solids concentration of the sludge is sufficiently high, aerobically treated digested sludge is then routed to dewatering. If thickening is needed, the aerobically treated digested sludge is run to gravity thickener (for 36 h) and then to dewatering. The developed technology makes it possible to convert the disadvantage of intensive thermophilic processes (high content of unconsumed organic matter) to an advantage, since residual organic matter is used in the processes of denitrification and dephosphatation of digested sludge prior to the dewatering stage. The content of VS in the mix of primary sludge and waste activated sludge is reduced by 44 with significant removal of N (89) and P (83) as a result of the application of the anaerobic aerobic technology during 8 11 days. In the case of the traditional mesophilic technology, VS degradation is 45 50 after 20 25 days (which is 2 3 times longer than for a new anaerobic aerobic technology). Addition of the post-aeration stage for mesophilically digested sludge resulted in an additional 16 removal of organic solids and 98 N-NH 4 removal after 35 days (Parravicini et al. 2008a, b), which was 3.5 times longer than in the proposed technology. Moreover, the authors did not report phosphate removal. Improved dewatering properties and odor of the sludge are additional advantages of the anaerobic aerobic technology. CONCLUSION A new technological scheme of two-stage treatment of sewage sludge was proposed. This technology includes intensive thermophilic digestion followed by aerobic biological treatment of digested sludge by specific activated sludge. The optimal parameters for the process were determined. The proposed technology results in the shortest time required to decrease nitrogen and phosphorus content in reject water, to oxidize excessive colloid and dissolved organic substances, to eliminate malodorous substances with localization and purification of discharged gas, and to improve dewatering properties and odor of the treated sludge. REFERENCES Danilovich, D. A. & Epov, A. N. 1997 Treatment method for sewage sludge (versions). RF Patent No. 95108573, claimed 25.05.1995, published 27.12.1997. de la Rubia, M. A., Perez, M., Romero, L. I. & Sales, D. 2002 Anaerobic mesophilic and thermophilic municipal sludge digestion. Chemical and Biochemical Engineering Quarterly 16 (3), 119 124.
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