Relationship Between Centrifugation and Drying of Sludge and The Organic Halogenes

185 ACTA UNIVERSITATIS AGRICULTURAE ET SILVICULTURAE MENDELIANAE BRUNENSIS Volume LVIII 21 Number 5, 2010 RELATIONSHIP BETWEEN CENTRIFUGATION AND DRYING OF SLUDGE AND THE ORGANIC HALOGENS K. Hrich, B. Groda Received: May 12, 2010 Abstract HRICH, K., GRODA, B.: Relationship between centrifugation and drying of sludge and the organic halogens. Acta univ. agric. et silvic. Mendel. Brun., 2010, LVIII, No. 5, pp. 185–190 This work is focused on determination of adsorbable organic halogens (AOX) concentration in the digested sludge from the sewage treatment plant and the losses of this component during dewatering and drying of sludge. Drying of the sludge from wastewater treatment plant is not extended too much in Czech Republic. In this work, the AOX are monitored, because AOX is one of the limits restraining use of the sludge on an agricultural land. Another reason is technological demand for using the sludge in cement processing, because chlorine in AOX can cause decrease in a heat transfer eff ect in a cement kiln. It is clear from the results that both centrifuged and dried sludge from the sewage treatment plant Brno fulfi lled limits for using sludge on agriculture land. They can also be composted, in case they meet other requirements. If not, it is a possibility of co-incineration in cement kiln. In such case, limit for total chlorine including the AOX is required too. This limit was not exceeded. Another aim was to calculate a mass balance of AOX during the centrifugation and drying processes. It was found out, that a er centrifugation the main part of AOX remained in the centrifuged sludge (96.4 %). The rest was drawn-off with reject water. 60 % of AOX in the reject water were dissolved compounds. A similar situation occurred during the drying process. More than 99 % of AOX was bound in the dried sludge. The air and vaporised water contained such quantity of AOX, which corresponded with the amount of the dust in the air and the amount of particles of sludge in vaporised water. adsorbable organically bound halogens, centrifugation, drying, sludge, waste water Sludge is a mixture of water and solids separated from the waste water in various ways. Sludge is about 1–2 % of the treated waste water; however, it contains 50–80 % of the initial pollution in the waste water (Kutil, Dohányos, 2005; Malý, Malá, 2006). The ideal status would be to use all sludge as fertilizer for agricultural land to replace mineral fertilizers. However, sludge contains pollution originally present in the sewage. This limits its use on agricultural land, which is modifi ed by regulation in Czech Rep. (corresponds with European regulation), specifying the concentration of heavy metals, PCBs, and AOX. Composting is another option for treating the sludge. The requirements on the sludge are similar. If the sludge contains the above pol lutants, it is possible to dispose it in a hazardous waste landfi ll. In this case, the producer of sludge pays for it and therefore he looks for other choices. One of them is co-incineration of the sludge in the cement work. During the incineration process, chlorine from sludge reacts to chlorides and further with substances in the combustion gases. Arising pro ducts adhere to the furnace, cause deterioration of the heat transfer in the furnace, and reduce its eff ectiveness. Main part of this chlorine is of inorganic origin, but there are the adsorbable organic bound halogens – AOX, too. They are organic compounds containing halogen elements, usually chlorine atoms. Many articles were published about AOX, but the majority is aimed at analysing concentration of AOX in industrial waste water and sludge. Commonly higher amounts of AOX are in effl uents from textile or paper factories. These effl uents are usually 186 K. Hrich, B. Groda being treated directly in the own treatment plants. Content of AOX in sludge from urban waste water was mentioned in (Shomar, 2007), who studied the concentration of AOX in waste water and sludge in sewer system of Gaza Strip. It follows from this article that some AOX compounds can form directly during the treating process, especially in anaerobic tanks. Other authors focused rather at the me tho dolo gy AOX analysis in the sludge (Krysell et al., 2008) and the AOX concentration in rivers, such as (Kaczmarczyk, Niemiycz, 2005). As already noted, this work was aimed on monitoring the AOX in the centrifuging and drying processes. Sampling was made at the municipal waste water treatment plant (WWTP) in Brno-Modřice, which has sludge treatment including centrifugation and drying. Its capacity is 400 thousand population equivalents (p. e.). Sludge treatment at WWTP Modřice Total volume of the digestion tanks is 14 960 m3. They are equipped by gaseous seal laminated roof. Tank diameter is 20 meters with conical bottom with 8 m depth. Digestion tanks are fi tted with a vertical, slow, normal stirrer. Heating of the tanks on tem pera tu re 35 °C is carried out by three heat-exchangers sludge/sludge and two heat-exchangers sludge/water. Two parallel centrifuges Guinard D5LLC with capacity 36 m3 . h−1 per unit were used for dewatering of sludge. About 670 m3 . d−1 of sludge with 57 g . l−1 dried matter are processed on average. Drier NARA NPD14W type with indirect heating consists of hori zon tal case trough equipped with two hollow rotating spindles with hollow paddles. Hot oil (180– 210 °C) fl ows inside the case, spindles, and paddles. Retention time of the sludge in the drier is about 4 hours. Steam with captured dust from the sludge (3.43 m3 . h−1) is lead to a gas scrubber. Condensate with temperature of 85 °C is fi lled into sludge/water heat exchanger and serves as main source of heat for digestion tanks. Dried sludge of the amount 28 t . d−1 and 88–92 % dry matter is cooled to 40 °C and carried with the system of spindles out to prepared con tai ners. Produced sludge is sampled and carried away for its fi nal use. The whole production of dried sludge is used as an alternative fuel in a cement work. At the time of cement factory stoppages, centrifuged sludge is used as a material for industrial composts. According to the values obtained from the WWTP operator, the average AOX concentration in raw waste water was 0.138 mg . l−1 in 2007. The concentration decreased to 0.050 mg . l−1 in treated water. The material balance was calculated on the basis of knowledge of water fl ow through the WWTP and it can be said that 59.6 % of AOX are captured in the sludge and 40.4 % remain in water. The daily quantity of AOX in sludge was 7359 g. MATERIAL AND METHODS Selection of sampling sites Eight sites were selected (see fi g. 1): (1) stabilized sludge entering the centrifuges, (2) water from centrifuges, (3) sludge entering the dryer (centrifuged sludge), (4) the cooling water entering the gas scrubber, (5) industrial water entering the gas scrubber, (6) the mixture of water and condensate leaving the gas scrubber, (7) dried sludge, (8) air in the dryer building before it enters the fi lters for deodorizing (not marked in the fi gure). Concentrations of AOX were analysed im me diate ly a er sampling. In case this was not possible, the samples were frozen and analysed as soon as possible. Air was sampled by the HS-SPME method (head space – solid phase micro extraction), in which the sampled air was caught into a prepared container and allowed to react on a fi bre impregnated sorbent. The sample was then analysed by gas chromatography. 1: Sampling sites in the centrifugation and drying processes Relationship between centrifugation and drying of sludge and the organic halogens 187 Analysis of AOX Analysis of aqueous samples was carried out according to ČSN EN 1485: Water quality: The determination of adsorbable organically bound halogens. The ECS 1200 Euroglas analyzer was used. In case of sludge samples treatment, the sludge is pre-dried to 130 °C, mixed with water and further treated similarly to aqueous samples. RESULTS AND DISCUSSION Results published in this work were measured during the year 2007. Daily average of stabilized sludge dry matter in measured samples ranged between 3.12 up to 3.57 % (table Ia and Ib) with the avera ge of 3.38 %. The common value is not overreaches 10 % (Dohányos et al., 2007), usually average dry matter in the sludge is 5.1 % (Malý, Malá, 2006). Higher dry matter content causes transport pipeline problems. Organic part of the dry matter in the monitored sludge was about 52.1 % (usually about 46 % – Chudoba et al., 1991). As shown in fi g. 2, the AOX content in sludge is below the limit value for sludge disposal as fer ti lizer, and thus from the perspective of AOX content, the sludge can be used to fertilize agricultural land. As already indicated, co-incineration in a cement furnace is possible. The cement factory is required to have a maximum total chlorine level which does not exceed 0.08 % wt. This is equivalent to 0.8 g total chlorine in one kilogram of dry matter. The measured samples of sludge were suitable for the cement factory. It corresponds with the data obtained from the WWTP operator: in 2008, the total chlorine level was usually only 0.052 % wt. Data that represent the AOX way through centrifugation process are sorted in table Ia and Ib. Table II shows the values from sludge drying. During centrifugation, water was removed and the dry matter of sludge increased. The water content in dewatered sludge was between 72,6 % and 79,0 % (ave ra ge 77,1 %). More than 99 % of dry matter remained in the sludge and 29.97 m3 . h−1 centrifuged water fl ew off . The centrifuged sludge contained more than 96 % of AOX fed into the centrifuge. 99.6 % of AOX in the centrifuged sludge was bound in solid phase, while in case of stabilized sludge the percentage was only 96.8 %. Proportion of insoluble AOX in reject water was about 32 % of total AOX content. The avera ge concentration of dissolved AOX in sludge was 242 mg . m−3, the average concentration of AOX in sludge solids was about 260 mg . kg−1, which corresponds with the literature data: 300 ± 162 mg . kg−1 (min. = 68 mg . kg−1, max. = 199 mg . kg−1) (Malý, Malá, 2009). Adsorption capacity of solid sludge may cause the accumulation of certain substances contained in waste water, such as non-polar hydrocarbons, tensides, polycyclic aromatic hydrocarbons (PAHs), AOX, and heavy metals. Therefore, their proportion in solids of sludge is higher than in the aqueous phase. PAHs, AOX and PCB are not signifi cantly decomposed. The situation during the drying is a similar as in case of the centrifugation. The majority (about 99 %) of AOX contained in centrifuged sludge remains in the dried sludge. A negligible amount is retained in vaporised water. These AOX are probably dis2: Concentration of AOX in centrifuged and dried sludges: comparison with the Czech regulation No. 504/2004 Coll. 188 K. Hrich, B. Groda solved, just like in the reject water. Higher quantities than in vaporised water are contained in the air outfl ow. This is caused by high concentration of dust in the air outfl ow. The daily production of dust is on average 120 kg. About 17 g AOX escape into the air with the dust every hour. This dust is separated in a gas scrubber and thus the air in the dryer building meets the permissible exposure limit (PEL) for dust specifi ed in Government Decree No. 361/2007 Coll. (the AOX limit is not set). I: Centrifugation process Date 04/14 04/15 06/09 06/10 10/10 10/11 10/12 10/13 10/14 10/15 10/16 10/ 24–25 Interval [h] 20 20 20 20 24 24 24 24 24 24 24 20 Stabilized sludge, inlet to the centrifuge Quantity [m3]829 825 808 806 734 723 739 720 821 755 795 716 Dry matter [kg.m−3]30.4 32.1 39.3 35.7 33.8 35.4 33.3 34.6 31.2 33.0 32.0 34.9 AOX [mg.kg−1 d.m.*] 186 202 300 189 253 279 274 246 281 256 285 321 AOX [g] 4687 5349 9526 5438 6277 7141 6743 6128 7198 6378 7250 8021 Centrifuged sludge Dry matter [%] 21.0 22.3 27.4 25.4 22.9 22.3 23.2 23.4 22.6 23.0 23.9 24.4 AOX [mg.kg−1 d.m.*] 151 141 138 156 242 259 255 235 268 246 273 312 AOX [g] 4255 4628 9331 5213 6131 7006 6581 5990 7043 6239 7061 7826 Reject water Quantity [m3]710 708 693 694 626 609 633 614 709 648 690 614 Suspended solids [g.m−3]395 493 542 447 380 450 300 290 450 500 580 475 AOX [mg.m−3]609 1020 282 325 233 222 256 225 219 215 274 319 AOX [g] 432 722 195 225 146 135 162 138 155 139 189 196 * d.m. – dry matter II: Drying process Date 10/10 10/11 10/12 10/13 10/14 10/15 10/16 10/24–25 Interval [h] 24 24 24 24 24 24 24 20 Centrifuged sludge, inlet to the drier Quantity [t] 107 114 105 106 112 107 104 101 Dry matter [%] 22.9 22.3 23.2 23.4 22.6 23.0 23.9 24.4 AOX [mg.kg−1 d.m.*] 249 276 269 242 278 253 281 316 AOX [g] 6131 7006 6581 5990 7043 6239 7061 7826 Dried sludge Dry matter [%] 91.4 91.5 91.7 92.1 91.5 92.3 91.9 90.8 AOX [mg.kg−1 d.m.*] 239 256 253 231 264 241 269 300 AOX [g] 5833 6458 6148 5677 6660 5892 6622 7390 Vaporized water Quantity [m3]80.2 85.9 78.6 79.0 84.4 80.4 77.2 73.7 AOX [μg.m−3]0.110 0.136 0.178 0.114 0.146 0.167 0.165 0.269 AOX [g] 8.84 11.70 13.99 8.99 12.34 13.44 12.75 19.80 Air from drier Quantity [m3]31104 31104 31104 31104 31104 31104 31104 31104 Dust in air, dried matter [kg] 118 97 106 124 72 121 132 174 AOX [mg.kg−1 d.m.*] 239 256 253 231 264 241 269 300 AOX [g] 28.2 24.8 26.8 28.7 19.0 29.1 35.6 52.3 * d.m. – dry matter Relationship between centrifugation and drying of sludge and the organic halogens 189 CONCLUSIONS The average content in solids of digested sludge in waste water treatment plant Brno is about 260 mg . kg−1.The most of AOX (96.8 %) is bound in suspended parts of sludge during the centrifugation and. 3.2 % of AOX remains in reject water, 68 % of total amount in form of dissolved substances. During the drying, 99 % of AOX remained in dried sludge. The rest (1 %) was divided between vaporised water and dust in outfl ow. From AOX concentration point of view, both centrifuged and dried sludge fulfi ls the legal requirements set for their use on agricultural land, into compost, and co-incineration in cement works. SOUHRN Vztah mezi odstřeďováním a sušením kalu a organickými halogenidy Tato práce byla zaměřena na zjišťování koncentrace adsorbovatelných organických halogenidů (AOX) v kalu z čistírny odpadních vod a hodnocením koncentračních změn AOX ve vyhnilém kalu při jeho odvodňování a sušení kalu. V České republice není sušení čistírenských kalů příliš rozšířeným způsobem. Jedním z důvodů, proč byly sledovány AOX v kalu, byl ten, že koncentrace těchto látek je jedna z limitujících hodnot při využití kalu na zemědělské půdě. Obdobně je tomu u použití kalu do zakládek kompostu. Sušený kal může být docela dobře spolu-spalován v cementářském procesu. I zde však je z technologického hlediska omezeno množství celkového chloru, respektive AOX obsaženého v kalu. Na celkovém chloru v kalu se podílejí vedle AOX i anorganické chloridy. Chlor se při spalování přeměňuje na takzvané „nálepy“ a zhoršuje přestup tepla přes stěny pece. Pro vypočítání bilančních rovnic pro odstřeďování a sušení bylo zvoleno sedm odběrných míst, tak, aby byly pokryty všechny vstupy a výstupy z těchto procesů. Vzorky vody a kalu byly podrobeny analýze na přístroji ECS Euroglas 1200. Dále byla získána provozní data, jako množství vody a kalu, které procházelo procesy. Z výsledků analýz je jasné, že odstředěný a sušený kal splňuje limity AOX pro použití kalu na zemědělské půdě. Mohou být tedy i kompostovány, pokud splňují ostatní požadavky. Z hlediska obsahu chloru, respektive AOX, je možné využít kal při spolu-spalování v cementárně. Z bilančních výpočtů vyplynulo, že během odstřeďování zůstává většina podílu AOX v kalu (96,4 %). Zbytek je odstraněn v odstředěné vodě. Přes 99 % pevných částic zůstává v odstředěném kalu. V procesu sušení zůstává v sušeném kalu přes 99 % AOX. Zbytek je rozdělen mezi ostatní výstupy (odpařená voda, odváděný vzduch). V odstředěné vodě tvoří hlavní podíl rozpustné formy AOX. Podobně tomu bude i v odpařené vodě. AOX v ovzduší odváděném ze sušárny jsou vázány na prachové částice. adsorbovatelné organické halogeny, odstřeďování, sušení, kal, odpadní voda REFERENCES DOHÁNYOS, M., KOLLER, J., STRNADOVÁ, N., 2007: Waste water treatment. VŠCHT, Praha (in Czech). GOVERNMENT DECREE No. 361/2007 Coll., kterým se stanoví podmínky ochrany zdraví při práci (in Czech). CHUDOBA, J., DOHÁNYOS, M., WANNER, J., 1991: Biological waste water treatment. SNTL Praha (in Czech). KACZMARCZYK, A., NIEMIYCZ, E., 2005: Ad sorba ble Organic Halogens (AOX) in Polish Rivers – Levels and Changes. Acta hydrochim. hydrobiol. 33: 324–336. KRYSELL, M., BECKER, R., WEGENER, J. W., BUGE, H. 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Bořivoj Groda, DrSc., Ústav zemědělské potravinářské a environmentální techniky, Mendelova univerzita v Brně, Zemědělská 1, 613 00 Brno, Česká republika, e-mail: borivoj.gr[email protected] 190