ro azio Keywords: Sludge stabilization Sequential anaerobic-aerobic digestion Volatile solid reduction con nve ment i alternative to be considered for the final destination of sewage sludge. Sewage sludge has been utilized in agricultural applications for several years as it represents an alternative source of nutrients for plant growth and is an efficient soil conditioner (Logan and Chaney, 1983). However, land application of sewage sludge is solid (VS) sludge fractions. This point is well addressed in Kumar et al. (2006a, 2006b) reporting a simple classification for the sludge constituent fractions in terms of VS degradability: - a fraction degradable only under anaerobic conditions - a fraction degradable only under aerobic conditions - a fraction degradable both under anaerobic and aerobic conditions - a non degradable fraction * Corresponding author. Tel.: þ39 06 90672800; fax: þ39 06 90672787. Contents lists availab Journal of Environm ls Journal of Environmental Management 92 (2011) 1867e1873 E-mail address:
[email protected] (M.C. Tomei). development of an integrated strategy for treating domestic wastewater. In fact, treatment and disposal of sewage sludge from wastewater treatment plants (WWTPs) accounts for up to 60%, of the total cost of wastewater treatment (Wei et al., 2003). Decreasing available land space, coupled with increasingly strin- gent regulations governing the design and operation of new land- fills (i.e. EU Landfill Directive 99/31), have caused the cost of siting, building, and operating new landfills to rise sharply. The increas- ingly restrictive targets for the continuous reduction of biode- gradable waste sent to landfills make land application a disposal salts, which may negatively affect the soil properties. Regardless of which sludge disposal alternative is employed, all can take advantage of more effective stabilization processes, and this justifies the increased attention paid to sludge stabilization processes aimed at increasing their efficiency and reducing costs. A promising strategy to improve the digestibility of sewage sludge is the application of a sequential process: anaerobic-aerobic digestion. The basicmotivation behind this approach arises from the different reaction environments (anaerobic and aerobic) provided to attain optimal biodegradability conditions for the different volatile Biogas production Proteins and polysaccharides 1. Introduction Improving sewage sludge manage 0301-4797/$ e see front matter � 2011 Elsevier Ltd. doi:10.1016/j.jenvman.2011.03.016 efficienciesof 32% in theanaerobicphaseand17% in theaerobic onewereobtained, and similarCODremoval efficiencies (29% anaerobic and 21% aerobic) were also observed. The aerobic stage was also efficient in nitrogen removal providing a decrease of the nitrogen content in the supernatant attributable to nitrifi- cation and simultaneous denitrification. Moreover, in the aerobic phase an additional marked removal of proteins andpolysaccharides produced in the anaerobic phasewas achieved. The sludge dewaterabilitywas evaluated by determining the Optimal Polymer Dose (OPD) and the Capillary Suction Time (CST) and a significant positive effect due to the aerobic stagewas observed. Biogas productionwas close to the upper limit of the range of values reported in the literature in spite of the lowanaerobic sludge retention time of 15 days. Fromapreliminaryanalysis itwas found that theenergydemandof theaerobic phasewas significantly lower than the recovered energy in the anaerobic phase and the associated additional costwas negligible in comparison to the saving derived from the reduced amount of sludge to be disposed. � 2011 Elsevier Ltd. All rights reserved. s a key objective for the restricted to prevent health risks to humans and livestock due to potentially toxic components, i.e. heavy metals, pathogens, and persistent organic pollutants and to the high amounts of soluble Accepted 10 March 2011 Available online 7 April 2011 teristics of the digested sludge in termsof volatile solids (VS), ChemicalOxygenDemand (COD) andnitrogen reduction, biogas production, dewaterability and the content of proteins and polysaccharides. VS removal Received in revised form 7 February 2011 plant. The objective of the work was to evaluate sequential digestion performance by testing the charac- Performance of sequential anaerobic/ae sewage sludge M. Concetta Tomei*, Sara Rita, Giuseppe Mininni Water Research Institute, C.N.R., Via Salaria km 29.300, C.P. 10, 00015 Monterotondo St a r t i c l e i n f o Article history: Received 7 July 2010 a b s t r a c t A promising alternative to digestion, was extensively i journal homepage: www.e All rights reserved. bic digestion applied to municipal ne (RM), Italy ventional single phase processing, the use of sequential anaerobic-aerobic stigated onmunicipal sewage sludge from a full scalewastewater treatment le at ScienceDirect ental Management evier .com/locate/ jenvman enta It follows that conventional one stage digestion (aerobic or anaerobic) does not always provide efficient VS biodegradation. Moreover, the combination of the two digestion phases can be advantageous in that they can alleviate some of the drawbacks characterizing the two individual digestion methods. Aerobic digestion is relatively simple to manage and produces biosolids useful for agriculture application but is characterized by high energy consumption. In contrast, anaerobic digestion allows for energy recovery in the form of biogas but is less stable and more complex to manage. Moreover, the efficiency of the digestion process strongly affects the next sludge treatment steps in that the demand for polymer conditioning agents results from colloidal proteins and polysaccharides released from sludge particles during digestion (Novak et al., 2003). Consequently a more effective VS degradation process should reduce the costs of sludge conditioning. Sequential digestion can potentially accomplish the above mentioned weak points of conventional processing, with potential advantages including a positive energy balance with partial energy recovery in the anaerobic stage, a reduction in conditioning costs, and the use of the specific metabolic pathways, aerobic and/or anaerobic, able to degrade the different VS frac- tions in the sludge. Recently multiestep digestion has received increased interest and previous studies have demonstrated the potential of this approach to improve the performance of conventional digestion processing (Kumar et al., 2006a, 2006b; Parravicini et al., 2008; Zupancic and Ros, 2008). Kumar et al. (2006a) observed that an aerobic digestion stage downstream from the anaerobic digestion unit improved the overall process performance both in terms of VS removal efficiency and dewatering properties of the biosolids. They operated the aerobic stage at SRT values in the range of 3e9 days and found that three days of additional aerobic digestion resulted in 20% additional VS removal beyond that achieved by the anaerobic stage while the dewaterability of the digested sludge increased with the aerobic SRT. Parravicini et al. (2008), in a full scale plant, ach- ieved additional VS removal in the range of 16% with post-aeration (SRTw 6d) of anaerobically digested sludge and found the relative increase of operational costs associated to the additional aerobic digestion phase to be negligible. Another positive effect of the sequential anaerobic-aerobic digestion is the ammonia nitrogen removal in the supernatant stream that can significantly reduce the nitrogen load recycled to the WWTP as observed by Zupancic and Ros (2008) with efficiency of ammonium nitrogen removal up to 85% in the aerobic stage. In this paper the results of an extended investigation into sequential digestion applied tomunicipal sewage sludge of a full scale urban wastewater treatment plant are reported. The objective of the paper is to verify the applicability of sequential digestion to a real sludgeby testing the characteristics of theproduced sludge in termsof VS, Chemical Oxygen Demand (COD) reduction, biogas production, dewaterability properties (by measuring Optimal Polymer Dose (OPD), Capillary Suction Time (CST), proteins and carbohydrates content), and coliform abatement. Moreover, to verify the contribu- tion of the supernatant from the digestion to the nitrogen load to the WWTP, TKN was followed in the sequential digestion process. Finally a preliminary energy and cost analysis was performed by comparing the sequential digestion to conventional anaerobic digestion. 2. Materials and methods 2.1. Sludge Primary and secondary sludges utilized in this study were provided by the Rome North wastewater treatment plant. The plant M.C. Tomei et al. / Journal of Environm1868 is a conventional activated sludge system including screening, 2.3. Analysis Regular sample collection and analysis were initiated one week after start up. Feed, anaerobic digested and aerobic digested sludge were analyzed for VS, COD, CST, OPD, total soluble nitrogen, proteins and polysaccharides, and total and fecal coliforms. Analytical methods and devices are reported in the following. Volatile solids - Volatile solid concentration was measured according to Standard Methods (APHA, 1998 2540E). Chemical Oxygen Demand - COD Cell Tests (MERCK- referring to EPA 410.4 method), based on potassium dichromate oxidation and spectrophotometric determination (Spectroquant Nova30), were employed. Total soluble nitrogen - Total soluble nitrogen was measured on filtered samples (0.45 mm). Nitrogen Cell Test (MERCK) was used. This test is based on the transformation of organic and inorganic nitrogen compounds into nitrate according to Koroleff’s method by treatment with an oxidizing agent in a thermoreactor. In a solution acidified with sulphuric and phosphoric acid, this nitrate reacts with 2,6-dimethylphenol to form 4-nitro-2,6-dimethylphenol that 2.2. Reactors Lab scale reactors utilized in this study were 7.4 L cylindrical glass vessels. The reactors were operated in series, with the sludge being fed to the anaerobic reactor once per day and an equivalent volume of digested sludge extracted from the anaerobic reactor and fed to the following aerobic reactor. Both of the reactors were equipped with mechanical stirrers fitted with helicoidal blades. The first reactor operated under anaerobic conditions and was equipped with a thermostatic jacket and a control device keeping the temperature at 37 � 0.5 �C. At the beginning of the start up phase it was filled with the inoculum and sewage sludge (ratio inoculum/sludge 1:1). The working volume was 7 L and the Sludge Retention Time (SRT) was controlled at 15 d. The second reactor, whichwas operated under aerobic conditions, has a working volume of 4.5 L, and air was supplied by a compressor able to maintain the concentration of dissolved oxygen (DO) at levels of w3 mg/L. The air flow rate, in order to reduce additional energy consumption, was kept at the minimum value necessary to maintain the dissolved oxygen in the reactor at the prefixed value. Such an operating strategy prevented ammonia stripping. The reactor was operated at room temperature with an SRT of 12 days. primary clarification and secondary treatment, and serves about 700,000 P.E. It is operated with a relatively high sludge age (20 d) and it is characterized by an unusual dilution of incoming sewage. The influent COD average value is 200 mg/L that is quite low in comparison to the range of COD values of 210e740mg/L reported in Henze et al. (1987) for settledwastewater in EU countries and to the COD values of 500 and 650 mg/L reported in Gray (1992) for raw wastewater in USA and UK respectively. Secondary sludge was obtained for each feed step from the recycle stream, then thickened for 18 h, while the anaerobic inoc- ulum was taken from the full scale digester of the plant fed with primary and secondary sludge. Primary sludge, due to practical reasons of accessibility, was obtained once a week, and was stored at 4 �C until used. The mixed sludge sample utilized in the exper- iments was prepared by mixing primary and secondary sludge in the ratio 3/2 on a VS basis. This ratio has been chosen on the basis of the data reported in Vesilind and Spinosa (2001) and is represen- tative of the primary and secondary sludge production observed in Italian WWTPs. l Management 92 (2011) 1867e1873 is determined photometrically. varied as a consequence of the variability of the fed sludge, but the improvement of the digestion performance, gained with the addi- tional aerobic stage, was maintained for the entire operation period. 3.1. VS removal Fig. 1 shows the VS concentration in the influent and in the effluent from anaerobic and aerobic digestion. A marked variation of the VS load is observed over the experimental period. For better legibility the VS data are reported in the form of a histogramwith a reference interval of 30 days. Average values and related standard deviations are reported in the graph. The removal efficiencies, as discussed below, refer to the entire operation period ental Management 92 (2011) 1867e1873 1869 Capillary Suction Time and Optimal Polymer Dose - CST was determined by a Triton Electronics apparatus, according to the StandardMethod procedure (APHA,1998 2710G). OPD is the dosage of polymer for sludge conditioning, which corresponds to the lowest value of CST. The sludgewas conditioned with Praestol 2540 (Stockhausen GmbH, Germany); stock solution of this polymer was prepared at a concentration of 1 g/L. Increasing doses of this solu- tion (from 0.5 to 2.5 mL) were added to 30 mL of the sample sludge and mixed for 5 min. The CST was then determined as described in the previous paragraph. Proteins and Polysaccharides - Samples were centrifuged for 10 min at 4000 rpm and the supernatant was passed through a 0.45 mm filter and analysed for biopolymer concentration. Protein concentration was determined according to the spectroscopic Bradford method (Bradford, 1976), using a wavelength of 595 nm. Polysaccharide concentration was evaluated by the Dubois method (Dubois et al., 1956), based on the reaction of the sample with phenol and sulphuric acid. Absorbance of the treated sample was measured at a wavelength of 490 nm. Total and fecal coliforms - The number of total coliforms and fecal coliforms in each sludge sample was determined according to the multiple tube fermentation procedure, using Hach-Lange kits. Methane - Methane in the biogas was determined by a gascro- matograph PERKIN ELMER AutoSystem equipped with a Carboxen 1000 (Supelco) column and a TCD detector. Sample volume was 50 mL, transport gas was Helium (3 bar) and operating Tempera- tures were 180 �C for the oven,150 �C for the injector and 250 �C for the detector. 2.4. Biogas detection device The flow rate of biogas produced by the anaerobic reactor was measured by a volumetric counter using a closedwater displacement systemwith electrical contacts and with an electromagnetic valve to discharge the produced biogas to the atmosphere (Mata-Alvarez et al., 1986). The measurement device was controlled by a Program- mable Logic Controller that also provides the recording of signals. 3. Results and discussion The SRT is a key parameter in determining the performance of the anaerobic digestion process and different criteria can be considered in its choice depending on the process objectives. The optimum SRT range of values suggested by Dohányos and Zábranská (2001) is 12e18 d while in previous studies on sequential digestion (Kumar et al., 2006a; Novak et al., 2011) the SRT of the anaerobic digester was in the range of 10e15 d for mesophilic and 15e20 d for thermophilic digestion. The interme- diate 15 d SRT value utilized in this study was chosen taking into account these data with the objectives of ensuring good perfor- mance of anaerobic digestion with a reduced reactor volume. Concerning the aerobic digester, previous studies on sequential anaerobic-aerobic digestion showed that SRTs values of 3e6 days in the aerobic stagewere enough to have a significant improvement in VS degradation (Kumar et al., 2006a) and biosolids dewatering (Subramanian et al., 2007) but these low SRT values are not suitable for nitrification so a more conservative value of 12 days was considered in this study. Reactors were operated in semi-continuous mode for approxi- mately 8 months in order to have an extended work period, which could represent real plant operation. The start up phase, until stable performance in the two reactors was observed, was quite rapid (�15 d), and this may be consistent with the fact that in both anaer- obic and aerobic reactors the biomass inoculum was already accli- M.C. Tomei et al. / Journal of Environm matised. After this start up period, the performance of the system of 240 days. Reasonably stable performance was achieved in the anaerobic reactor with an average VS removal efficiency, of 32 � 5%. The observed variation can be attributed to the variability of the feed VS concentration, which was 1.6 � 0.26 (expressed as %w/w). In the subsequent aerobic stage an additional VS removal of 17 � 5% was achieved, and therefore the global VS removal efficiencywas of 44%. The observed efficiency of the anaerobic stage is in the range of values (27e71%) reported in a survey of 30 anaerobic sludge digesters of municipalWWTPs (Speece, 1988) and in Bhattacharya et al. (1996) who found percent VS reductions in the range of 26e50% in conventional anaerobic digestion of mixed sludge of three WWTPs. In any case the anaerobic removal of VS in our experiments is not “high” as an absolute value but this digestion efficiency is good in relation to the operating conditions of the WWTP. The low influent COD combined with the high sludge age (w20d) give a secondary sludge partially digested from thewater line. In addition the primary sludge was sampled from the bottom of the thickener that was operated at a sludge volume ratio (volume of the sludge blanket/ volume of the thickened sludge removed daily) > 20 d. Also in this case the high retention time of the sludge in the thickener could cause a partial pre-digestion of the sludge. Additional VS removal in the aerobic stage is comparable to values reported in Parravicini et al. (2008) (16%) and Kumar et al. (2006a) (20%). Considering the poor digestibility characteristics of the fed sludge, the additional removal efficiency gained in the aerobic stage can be considered as a significant improvement of the entire digestion performance. 3.2. COD removal The same form of representation was chosen for total COD; concentration profiles are shown in Fig. 2 for a period of 210 days. The average removal efficiency in the anaerobic phase was 29 � 6% while an additional average removal in the aerobic phase of 21�3% 0.00 0.50 1.00 1.50 2.00 2.50 30 d 60 d 90 d 120 d 150 d 180 d 210 d 240 d V S % feed anaerobically digested aerobically digested Fig. 1. VS concentration during the entire period of operation. required to ensure favourable conditions for nitrification (Zupancic and Ros, 2008). The consequence may be the incomplete oxygen- ation of the biomass with a consequent presence of anoxic zones at microscopic level due to the partial penetration of the oxygen inside the bioflocs. In such anoxic conditions nitrate instead of oxygen can be utilized as an electron acceptor. For practical application of sequential digestion, this point is quite important in relation to the design criteria of the aerobic stage. In the literature (Kumar et al., 2006a, 2006b) low SRTs are suggested for the aerobic phase that can be considered to be a fin- ishing step to complete the removal process initiated in the anaerobic one. Instead, if it is desired to reduce the nitrate load recycled to the WWTP with the supernatant from the digester, the optimal SRT should be high enough to ensure efficient nitrification and simultaneous denitrification. This condition is critical also for the dissolved oxygen level considering that DO has to be high digestion, with a 3 day aerobic SRT, they observed that poly- 0 5 10 15 20 25 30 35 40 40 d 100 d 150 d 210 d g C O D /L feed anaerobically digested aerobically digested Fig. 2. COD concentration during the entire period of operation. M.C. Tomei et al. / Journal of Environmental Management 92 (2011) 1867e18731870 can also be observed. The parallel data analysis of VS and COD is useful to define the evolution and the mechanisms of biodegra- dation of the soluble and particulate organic matter in the subse- quent digestion phases. A comparison between the COD and VS patterns shows a lower COD removal efficiency in the anaerobic phase relative to the VS. The opposite is the case for the aerobic stage inwhich the efficiency of total COD removal is higher than for VS removal. This finding could be explained by considering the role of soluble COD. Total COD consists of a particulate fraction (the VS) and a soluble fraction; in the anaerobic phase there is net soluble COD production due to the hydrolysis of the particulate matter, that can account for the increase of soluble COD in the effluent and the lower COD removal efficiency. The soluble COD produced in the anaerobic phase is partially removed in the subsequent aerobic phase thus resulting in higher total COD removal when compared to VS removal. Soluble COD pattern values over a period of 20 days confirmed this hypothesis with an average increase of 35% in the anaerobic phase followed by a 42% average decrease in the aerobic one. 3.3. Nitrogen removal According to Zupancic and Ros (2008) the supernatant from anaerobic sludge digestion is characterized by a high nitrogen content and can represent a significant fraction (up to 50%) of the nitrogen load of a wastewater treatment plant. The presence of an aerobic phase in which nitrification and/or (depending on the oxygen concentration level) simultaneous nitrification-denitrifica- tion can take place, is potentially able to reduce this load. During the experimental campaign, the fate of total soluble nitrogen in the supernatant from the digesters was followed over a period of 2 weeks (6 samples) and the results are reported in Fig. 3. The marked decrease detected in the aerobic phase (51 � 8%) is presumably attributable to the effect of a simultaneous nitrifica- tion-denitrification process. In fact, the DO concentration of 3 mg/L maintained in the aerobic reactor is lower than the limit (�4 mg/L) 0 50 100 150 200 250 300 350 400 80th d 83rd d 86th d 89th d 92nd d 94th d m g N / L feed anaerobic digester aerobic digester Fig. 3. Total soluble nitrogen concentration in the feed and in the supernatant from the digesters over a period of two weeks (from the 80th to the 94th day). saccharides increase in the effluent while a significant removal (37%) was achieved with 9 days aerobic SRT. We worked with Table 1 Concentration of biopolymers in raw and digested sludge samples. Day Proteins (mg/L) Polysaccharides (mg/L) Feed Anaerobically digested Aerobically digested Feed Anaerobically digested Aerobically digested 12 30.0 55.7 31.1 59.2 148.3 71.7 20 30.0 57.2 20.0 59.2 135.8 51.1 23 24.3 34.3 14.3 62.5 153.3 77.0 35 28.3 37.8 12.1 73.3 85.9 75.0 68 23.8 54.4 21.0 73.3 85.8 71.2 84 23.8 31.2 13.4 85.8 134.2 91.0 100 23.0 32.7 26.8 58.3 137.5 83.3 112 19.1 36.7 20.8 74.2 101.7 85.8 131 21.7 38.7 11.9 52.5 100.8 78.4 154 28.28 44.6 15.0 52.6 124.2 93.3 159 35.72 50.0 20.5 74.2 140.0 39.2 Mean 26.2 43.0 18.8 65.9 122.5 74.3 enough to ensure the required nitrification efficiency and at the same time be compatible with simultaneous denitrification. 3.4. Biopolymers (proteins and polysaccharides) The protein and polysaccharide concentrations in the raw and digested sludge were periodically measured, and the data are shown in Table 1. In both cases, an increase in the anaerobic phase followed by a decrease in the subsequent aerobic one is observed. Proteins and carbohydrates are associated with the soluble and colloidal fraction of COD that increases in the anaerobic stage as a consequence of the hydrolytic process of organic matter and this COD is efficiently removed during aerobic digestion. The observed protein pattern is in agreement with the results of the experiments on anaerobic and aerobic digestion of Novak et al. (2003) and Kumar et al. (2006a): in anaerobic digestion the reduction of iron resulted in the release of large quantities of proteins into solution, that are available for subsequent aerobic degradation. With regard to the polysaccharides, as reported in Novak et al. (2003) in an aerobic environment the degradation of calcium and magnesium proteins results in a consequent polysaccharides release into solution that will be removed depending on the aerobic retention time. The effect of aerobic SRT on polysaccharide degra- dation was investigated by Kumar et al. (2006a): in sequential anaerobic mesophilic digestion (15 d SRT) followed by aerobic S.D. 4.7 9.7 6.2 10.8 24.6 16.3 anaerobic mesophilic digestion (15 d SRT) followed by an aerobic phase at a higher SRT of 12 days, that should be high enough to provide degradation of the released polysaccharides, which was observed. It is worth noting that the content of colloidal proteins and polysaccharides is a key parameter determining the demand for polymer conditioning agents so must be considered in the routine characterization of sludge in order to optimize the sequence of treatment operations. sludge from different plants so it is less significant as general parameter to quantify the process kinetics. The methane fraction of the produced biogas was measured over a one week period and was found to be equal to 66 � 3%, which is consistent with literature data (Bou�sková et al., 2005). 3.7. Total and fecal coliforms Preliminary tests were also performed to monitor the fate in the digestion stages of total and fecal coliforms, which are conventional parameters used as indicators of pathogens. Although this param- eter is of interest for sludge application in agriculture, the present EU regulations (Directive 86/278/EEC) on the use of sewage sludge in agriculture do not recommend a specific limit value of patho- genic microorganisms however, the “Working document on Table 3 Optimal polymer dose in three tests referring to three different periods of the experimental campaign (TS ¼ Total Solids). Test Feed (kgpol/tonTS) Anaerobically digested (kgpol/tonTS) Aerobically digested (kgpol/tonTS) 1 1.10 2.28 0.57 2 2.22 3.13 2.69 3 2.01 3.24 1.72 47.7 25 30 35 40 0 20 40 60 80 100 Praestol (mg/L) C S T ( s ) 32.3 100 120 140 160 0 20 40 60 80 100 Praestol (mg/L) C S T ( s ) a Fig. 4. CST profile vs Praestol dose (Test 3). (a) feed, (b) anaerobically digested sludge, M.C. Tomei et al. / Journal of Environmental Management 92 (2011) 1867e1873 1871 Table 2 CST values (sec) in raw and digested sludge samples. Day Feed Anaerobically digested Aerobically digested 30 72.3 176.8 81.5 54 48.6 163.7 84.0 68 48.6 124.5 75.2 93 43.8 103.3 89.8 103 54.1 119.2 69.5 112 48.7 121.6 66.5 121 29.5 105.7 87.2 Mean 49.4 130.7 79.1 3.6. Biogas Produced biogas was continuously monitored in all the experi- ments. The results in terms of daily and specific production (referred to the unit of destroyed VS) are reported in Fig. 5 and Fig. 6. After a short lag phase (about 10 days), a regular increase is observed. The average specific biogas production is 0.84 � 0.09 m3/ (kg/VS destroyed), a value that is within the range of 0.19e1.6 m3/ (kg/VS destroyed) reported in the literature (Speece, 1988; Bolzonella et al., 2005) for mesophilic digestion of sewage sludge. Thus, the 15 days SRT time adopted in our experiments is sufficient to have satisfactory biogas production that suggests acceptable performance of the anaerobic digestion process. Biogas data were also correlated with the added VS giving a specific production of 0.27 � 0.03 m3/(kg VS added). Also in this case the observed values are in the reported literature range for mixed sludge 0.14e0.91 m3/(kg VS added) (Speece, 1988). It has to be pointed out that the specific biogas production referred to the added VS, even if it is a significant parameter to express the process efficiency, is affected by the intrinsic variability characterizing the 3.5. Dewaterability assessment In order to provide a first estimation of dewatering character- istics, the CST was periodically measured in samples from the feed, anaerobically digested sludge and aerobically digested sludge, and the results are reported in Table 2. The sequential digestion causes an increase of the CST in the first anaerobic stage which is appreciably reduced in the aerobic stage. Sludge dewaterability was also assessed by determining optimal polymer dose, and the results are shown in Table 3, while in Fig. 4 a typical CST profile detected during the OPD test (3) is reported. It is evident from both CST and OPD data the positive effect of the aerobic phase on the sludge dewaterability overcomes the negative effect (increase of CST and OPD) observed in the anaerobic phase. The beneficial effect of the aerobic stage on sludge dewater- ability was also referred to by Kumar et al. (2006a) for mixed sludge (1:1 by weight) with a substantial reduction in the CST, and OPD after sequential mesophilic- aerobic digestion at aerobic SRTs as low as 3 days. S. D. 12.7 28.4 8.9 0 20 40 60 80 100 Praestol (mg/L) 180 200c 62.6 100 120 140 160 180 200 220 C S T ( s ) b (c) aerobically digested sludge. The optimal Praestol dose (referring to the sample volume) is reported in the graph. 0 500 1000 1500 2000 2500 3000 3500 0 50 100 150 m L /d M.C. Tomei et al. / Journal of Environmenta1872 sludge” (that is the basis of the new regulation) excludes land application for sewage sludge not hygienized by specific advanced treatments. Results of these preliminary tests are summarized in Table 4. A significant abatement is observed in the anaerobic stage of about one order of magnitude for both total and fecal coliforms. An additional abatement, 64% for the total and 78% for the fecal coli- forms, is gained in the subsequent aerobic stage. These results provide the first positive evaluation of sequential digestion in terms of coliform removal even if they need to be integrated with more specific microbial tests (beyond the scope of this paper) to have a more complete picture of the potential achievable degree of hygienization. 3.8. Energy balance and cost analysis To evaluate the proposed digestion scheme also in terms of energy consumption, a preliminary evaluation of the energy balance was performed comparing energy production in the anaerobic stage (conversion of methane into electric energy) to the energy demand in the aerobic phase. The following assumptions were considered: � energy consumption in the aerobic digestion 1 kWh/kg VS destroyed (Mininni et al., 1985); � biogas production in anaerobic digestion 0.84 Nm3/kgVS destroyed; � methane presence in biogas: 66% on volumetric basis; � lower calorific value of the methane: 8400 kcal/Nm3; � efficiency in electric energy production using a thermal cycle: 30%. This results in the amount of the recovered energy with the biogas to be 1.65 kWh/kg VS destroyed. According to generally adopted criteria, the energy balance is time (days) Fig. 5. Specific biogas production on daily base. referred to 1 kg TS fed to the digestion by assuming a ratio VS/TS ¼ 0.75. It was assumed that a VS removal efficiency of 32% 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 50 100 150 time (days) m 3 /k g V S Fig. 6. Specific biogas production referring to the destroyed VS. and 17% in the anaerobic and aerobic phase respectively were achieved according to the average data obtained in this study. Consequently the energy production in the anaerobic phase is of 0.39 kWh/kg TS and there is a demand of 0.087 kWh/kg TS in the aerobic phase. This results in an energy requirement in the aerobic phase that is significantly lower (22%) than the energy production in the anaerobic one. In addition to evaluate and quantify the advantages derived from the additional VS removal in the aerobic phase, a cost analysis was performed. The cost analysis also refers to 1 kgTS fed to the digestion. The cost data represent an average value for the Italian situation. An assumed cost of electric energy required in the aerobic phase of 0.12 €/kWh and a price of the sold energy of 0.20 €/kWh were used, typical of the situation in Italy. The price of the sold energy is derived by the added environmental value of renewable energy generated by biomass that is “green electricity generation” as recognized in many EU countries. The calculated value of the produced energy is 0.078 €/kgTS and the cost associated with the demand in the aerobic phase is 0.0104 €/kgTS. Therefore, in terms of costs the difference is even more relevant being the value of the produced “green” energy higher than the value of the energy supplied by the distribution line. For a more complete cost evaluation an important basis of comparison between single anaerobic digestion and the sequential one is the saving derived from the reduction of the amount of solids to be disposed of as a consequence of the additional TS removal in the aerobic phase. A simplified approach was considered by assuming a disposal cost of 0.1 €/kg sludge and final sludge concentration of 20% and 25% TS for the conventional anaerobic digestion and the sequential one justified by the better dewaterability properties of the digested sludge observed in our experiments. This results in a net cost saving (evaluated taking into account the energy cost for the aerobic digestion) of 0.10 €/kgTS that could be relevant considering that, for instance, in a plant serving 500.000 inhabitants the total saving is w920.000 €/year. 4. Conclusions An extensive investigation over a period of 8 months, was carried out to evaluate the performance of the sequential anaer- obic-aerobic digestion of sewage sludge. In addition to the classical efficiency parameters of VS, COD and nitrogen removal and biogas production, CST and OPD for the dewaterability assessment were also determined. The fate of biopolymers (proteins and poly- saccharides) was also tracked to provide additional information on the degradation mechanisms of the different sludge fractions. The main conclusions of the study can be summarized as Table 4 Total and fecal coliforms expressed as MPN/g suspended solids in the subsequent digestion stages. Parameter Feed (MPN/gSS) Anaerobically digested (MPN/gSS) Aerobically digested (MPN/gSS) Total coliforms 1.1$106 3.1$105 1.1 105 Fecal coliforms 4.2$105 9.4$104 2.1$104 l Management 92 (2011) 1867e1873 follows: � sequential anaerobic-aerobic digestion provides effective VS removal efficiency (32� 5% in the anaerobic phase and 17� 5% in the aerobic one) corresponding to an average global removal efficiency referred to the fed sludge of 44%; � COD removal efficiencies were 29 � 6% and 21 � 3% in the anaerobic and aerobic stage respectively (average global removal efficiency referred to the fed sludge of 44%); � total soluble nitrogen in the supernatant increases in the anaerobic phase and decreases after the aerobic stage (due to simultaneous nitrification-denitrification) thus reducing the nitrogen load recycled to the WWTP; � in the aerobic phase a marked removal of proteins and poly- saccharides was observed; � the final aerobic stage has a beneficial effect on sludge dew- aterability by recovering the negative effect of the anaerobic digestion as shown by the CST and OPD pattern; � biogas production (0.84 � 0.09 m3/(kg/VS destroyed)), is within the range of literature values reported for mesophilic digestion of sewage sludge; � the value of the produced energy (0.078 €/kgTS) is significantly higher than the cost associated with the demand in the aerobic phase (0.0104 €/kgTS) while the net cost saving, taking into account the cost for disposal is 0.10 €/kgTS. 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Performance of sequential anaerobic/aerobic digestion applied to municipal sewage sludge Introduction Materials and methods Sludge Reactors Analysis Biogas detection device Results and discussion VS removal COD removal Nitrogen removal Biopolymers (proteins and polysaccharides) Dewaterability assessment Biogas Total and fecal coliforms Energy balance and cost analysis Conclusions Acknowledgements References