Conditions and technologies of biological wastewater treatment in Hungary

1676 © IWA Publishing 2012 Water Science & Technology | 65.9 | 2012 Conditions and technologies of biological wastewater treatment in Hungary G. M. Tardy, V. Bakos and A. Jobbágy ABSTRACT A survey has been carried out involving 55 Hungarian wastewater treatment plants in order to evaluate the wastewater quality, the applied technologies and the resultant problems. Characteristically the treatment temperature is very wide-ranging from less than 10 C to higher than W 26 C. Influent quality proved to be very variable regarding both the organic matter (typical COD W concentration range 600–1,200 mg l 1) and the nitrogen content (typical NH4-N concentration range 40–80 mg l 1). As a consequence, significant differences have been found in the carbon availability G. M. Tardy (corresponding author) V. Bakos A. Jobbágy Department of Applied Biotechnology and Food Science, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary E-mail: gtardy@mail.bme.hu for denitrification from site to site. Forty two percent of the influents proved to lack an appropriate carbon source. As a consequence of carbon deficiency as well as technologies designed and/or operated with non-efficient denitrification, rising sludge in the secondary clarifiers typically occurs especially in summer. In case studies, application of intermittent aeration, low DO reactors, biofilters and anammox processes have been evaluated, as different biological nitrogen removal technologies. With low carbon source availability, favoring denitrification over enhanced biological phosphorus removal has led to an improved nitrogen removal. Key words | biological nutrient removal, carbon deficiency, wastewater quality INTRODUCTION AND GOALS The term of ‘population equivalent’ (PE) has been commonly used for simplifying design parameters for wastewater treatment plants (WWTPs). This practice suggests, that, at least as an average, a certain number of people consume about the same amount of water and contaminate it to about the same extent. However, during different surveys, very significant differences have been found in Hungary in this respect. It can be assumed that these differences derive from different social conditions and may increase with increasing water prices. Hungary joined the EU in 2004, and, as a consequence, adopted the EU Water Framework Directive. Based on the requirements of the Directive, decree 28/2004 (XII. 25.) of the Hungarian Ministry for Environment and Water classified the receiving bodies and the appropriate effluent limits of WWTPs into four main categories (see Table 1). In order to meet these requirements, several new treatment plants have been built and existing treatment facilities have been upgraded during the last decade. Taking into consideration that typical influent total nitrogen (TN) concentrations of domestic WWTPs in doi: 10.2166/wst.2012.062 Hungary were found to be in the range of 50–80 mg l 1 in a former survey (Tardy & Jobbágy ), it can be stated that WWTPs have to generally perform at least partial nitrification, and in category I, full nitrification is required in order to meet the 2 mg l 1 effluent NH4-N limit. Meeting the effluent TN criteria of 20 and 25 mg l 1, respectively, in categories I and III, and the even stricter summer period TN limit (10 mg l 1) applied to specific plants, also requires very efficient denitrification. As the wastewater quality has proven to be highly variable from site to site, appropriate design and efficient operation patterns can only be based on the thorough investigation of the local wastewater and environmental conditions. Regarding also the variably strict effluent criteria, the most promising treatment technology may be different from site to site. In this study, influent characteristics and effluent quality of 55 domestic Hungarian WWTPs have been evaluated and typical consequent problems of design and operation have been highlighted. 1677 Table 1 G. M. Tardy et al. | | Water Science & Technology Biological wastewater treatment in Hungary | 65.9 | 2012 Classification of the receiving water bodies with the emission criteria Category ! Limit. parameter ↓ I. Lake Balaton and its catchment area II. Other sensitive receiving bodies COD (mg l 1) 50 BOD5 (mg l 1) 15 1 NH4-N (mg l ) III. Intermittent and ephemeral streams IV. General receiving body 100 75 150 30 25 50 2 10 5 20 Tot. Inorg. N (mg l 1) 15 30 20 50 TN (mg l 1) 20 35 25 55 5 5 10 50 50 200 TP (mg l 1) 1 TSS (mg l ) 0.7 35 METHODS OF DATA ACQUISITION Collection of the basic data has been carried out by a survey. A questionnaire was created asking for answers on the following topics: (A) Basic parameters – capacity, category of the receiving body, year of startup of operation. (B) Influent parameters (2007–2008) – actual and design values of the wastewater amount and characteristics. (C) Effluent parameters (2007–2008) – effluent criteria and actual quality. (D) Pretreatment technology – grit/grease trap, primary clarification. (E) Technology of the biological treatment – arrangement of activated sludge basins, recirculations, biofilters etc. (F) Chemical treatment – used chemicals, amount, dosing strategy. (G) Secondary clarification – type and surface of the clarifier, SVI values. Figure 1 | Distribution of the capacity of the investigated plants. RESULTS AND DISCUSSION (H) Operational nuisances. Overall characteristics of the influent wastewater Questionnaires have been distributed to the operators of 55 domestic WWTPs of different categories with a wide variety of capacity (see Figure 1). As one of the main goals of the study has been the investigation of the larger WWTPs (with higher than 5,000 m3 d 1 capacity), in the selection, these plants have been over-represented compared with the overall Hungarian distribution. Typical technologies and nuisances have been investigated and demonstrated in case studies. For the plants chosen for the case studies further data have been collected from the operators and on-site measurements have also been carried out. During the past 20 years the rising water/wastewater fees have resulted in a definite decrease of water consumption in Hungary. Average wastewater discharges of the catchment areas of 14 out of the 55 investigated plants are less than even 100 l capita 1 d 1 (see Figure 2). As a consequence of the decreased wastewater discharge, the hydraulic load of the investigated plants proved to be generally low: the ratio of the average influent flow and the design capacity has been as little as 0.6 on average. The typical influent COD concentration of the investigated plants is between 600 and 1,200 mg l 1 (see Figure 3(a)), and 30 of the 55 investigated facilities (55%) 1678 Figure 2 G. M. Tardy et al. | | Distribution of the water consumption in the catchment area of the investigated WWTPs. receive influent wastewater with higher than 800 mg l 1 COD concentration, which is considered to be ‘high strength wastewater’ by Metcalf & Eddy et al. (). Onethird of the investigated plants can be characterized by an influent with COD concentration even higher than 1,000 mg l 1, which is the official sewer influent limit for industrial wastewater discharges. Characteristically, a relatively large part of the COD is in the suspended solid form (typically 50–70% of the total COD). Influent NH4-N concentrations (see Figure 3(b)) are typically in the range of 45–65 mg l 1, 76% of the investigated plants receive wastewater with higher than 45 mg l 1 NH4-N concentration, considered as ‘high strength wastewater’ by Metcalf & Eddy et al. (). As Figure 4 shows, higher than 80 mg l 1 NH4-N concentrations typically go Figure 3 | Water Science & Technology Biological wastewater treatment in Hungary (a and b) Distribution of the influent COD (a) and NH4-N (b) concentrations. | 65.9 | 2012 into small plants. Five small capacity (<1,000 m3 d 1) WWTPs receive wastewater with NH4-N concentration even higher than 100 mg l 1; the water consumption rates of the related catchment areas of these plants are below 100 l capita 1 d 1. NH4-N/TKN ratio of the wastewater has been typically found in the range of 0.7–0.8, suggesting that 20–30% of the influent TN is in organic form. As a consequence, it can be stated that low water consumption has led to very high influent nitrogen concentrations in most of the Hungarian WWTPs. For the appropriate elimination of this high amount of influent nitrogen, efficient biological nitrogen removal processes have to be established. As Hungary has a continental climate, during the year the influent wastewater temperature varies within a wide range (see Figure 5), low (<10 C) as well as high (>26 C) extremes may frequently occur. Thus, for efficient nitrification, in winter (typically with wastewater temperatures between 8 and 12 C) relatively high sludge retention time (SRT) values (10–20 d) have to be maintained. On the other hand, in the case of efficient nitrification the high nitrate concentrations have to be eliminated through denitrification. As carbon source availability may be the bottleneck for denitrification, the influent wastewater’s C/N ratio is a key parameter to determine the maximum achievable denitrification efficiency (Kujawa & Klapwijk ). Because of the water consumption habits of the population as well as the long retention time of the wastewater in the sewer systems (e.g. in the southern coastal side of the Lake Balaton), where the major part of the readily biodegradable substrates may be eliminated, the available carbon source may not be enough for efficient denitrification. W W W 1679 Figure 4 G. M. Tardy et al. | | Water Science & Technology Biological wastewater treatment in Hungary Influent NH4-N concentration of the investigated WWTPs as a function of influent flow rate. Figure 6 | | 65.9 | 2012 Distribution of the influent BOD5/NH4-N ratio of the investigated WWTPs. denitrification cannot be expected in these cases. This situation can be a serious problem when the 10 mg l 1 TN criterion has to be fulfilled in areas of category I. Nutrient removal processes and shortages Out of the investigated 55 WWTPs, 10 WWTPs only have a nitrification step without denitrification, 18 facilities have nitrification and denitrification technology and 27 WWTPs have biological nitrogen removal technology combined with biological phosphorus removal. Technologies with nitrification and without efficient denitrification Figure 5 | Distribution of the typical minimum, average and maximum influent wastewater temperatures. In order to illustrate this problem, the distribution of influent BOD5/NH4-N ratios of the investigated plants has been depicted in Figure 6. Marginal carbon availability for efficient denitrification can be defined by BOD5/NH4-N ratios between 4 and 6 and severe carbon deficiency can be assumed below 4 g BOD5/g NH4-N influent ratio (Grady et al. ). As Figure 6 shows, a remarkable number (42%) of the investigated WWTPs receive wastewater lacking the appropriate amount of carbon source and 16% have a severe carbon deficiency, thus efficient In some cases fulfillment of the TN limit of category IV (see Table 1) does not require denitrification. High effluent NO3-N concentration, however, may lead to rising sludge and thereby to severe sludge loss and deterioration of the effluent quality. As Henze et al. () concluded, at temperatures higher than 20 C, even as low as 8 mg l 1 NO3-N concentration in the effluent may lead to settling problems in the secondary clarifier. As Figure 7 shows, the characteristic maximum effluent NO3-N concentrations (detected generally in summer) exceed 8 mg l 1 in 29 WWTPs of the 55 investigated. As a consequence, rising sludge is a typical and severe problem in most of the WWTPs in Hungary in summer. Therefore denitrification has to be carried out even in cases when it is not required for the effluent quality. W 1680 G. M. Tardy et al. | Water Science & Technology Biological wastewater treatment in Hungary | 65.9 | 2012 the ratio of aerated/non-aerated phase) is eliminated through aerobic processes in the aerobic phase, thus intermittent aeration technologies can only be safely used for biological nitrogen removal with influent wastewaters with high (presumably >8) BOD5/NH4-N ratios. Biological nitrogen removal in low-DO environment Figure 7 | Distribution of characteristic effluent maximum NO3-N concentration ranges of the WWTPs investigated. In addition to this, in the summer period, decrease of the sludge concentration (with the decrease of the SRT demand of nitrification), as well as the decrease of the sludge layer in the secondary clarifier can be recommended. Denitrification with intermittent aeration Application of activated sludge basins with intermittent aeration is widely used in Hungary for nitrification/denitrification. In these cases, the operation time is divided into the nitrification and denitrification phases, and the two processes take place in the same bioreactor. In our case study, the intermittently aerated reactor received an influent of 462 mg l 1 BOD5 and 76 mg l 1 NH4-N concentration, with 35 min aerated (DO ∼2–2.5 mg l 1) and 15 min anoxic phases. At the given ∼15 d SRT, ∼2.3 g l 1 mixed liquor volatile suspended solids (MLVSS) at higher (>15 C) wastewater temperatures, full nitrification occurred, but the effluent NO3-N concentration was ∼35– 40 mg l 1. Mathematical simulation of the system using BIOWIN® software suggested that, even though the influent BOD5/NH4-N ratio was 6.1 (appropriate carbon availability based on Figure 6) lack of carbon source caused the limited denitrification. Results of the simulation suggested that rearrangement to a pre-denitrifying system (Modified Ludzack-Ettinger), would enable the major part of influent BOD to be used for denitrification, and effluent NO3-N concentrations may fall to ∼12 mg l 1. It can be concluded that application of intermittent aeration may severely hinder denitrification, as part of the influent BOD (determined by Shortcut biological nitrogen removal (SBNR – denitrification via nitrite) is a biological nitrogen removal alternative. Through the utilization of SBNR, the aeration may be decreased by ∼25% and the carbon source demand of denitrification may also be decreased by ∼40% (Turk & Mavinic ; Guo et al. ) through avoiding the oxidation of nitrite to nitrate and reduction of nitrate back to nitrite. In order to reduce the aeration and carbon source required, and meet the TN criteria, in certain WWTPs low DO conditions have been established in the bioreactors in order to achieve the simultaneous nitrification and denitrification (SND) and/or SBNR. Two large WWTPs (capacity: 48,000 m3 d 1 – Technology 1 and 21,500 m3 d 1 – Technology 2) with low DO operation have been investigated. Both WWTPs consist of carousel/ labyrinth type reactors in which a low DO environment is established. Technology 1 consists of four parallel carousel reactors (3,000 m3 each, typical MLVSS ∼2.5 g l 1), whereas Technology 2 has a UCT-type arrangement with a total volume of 12,500 m3 (typical MLVSS ∼2.0 g l 1) using a labyrinth basin with aerated and non-aerated zones as the last reactor (see Figures 8 and 9). Concentration profile measurements in the Technology 1 reactors showed that the 65 mg l 1 average influent NH4-N concentration is eliminated with higher than 95% efficiency while very low W Figure 8 | Reactor arrangement of Technology 1 – carousel type reactor. 1681 Figure 9 G. M. Tardy et al. | | Water Science & Technology Biological wastewater treatment in Hungary Reactor arrangement of Technology 2 – labyrinth basin with preceding anaerobic and anoxic reactors. concentrations of oxidized nitrogen forms (NO2,3-N concentration was below 3 mg/l) left the reactor, although the BOD5/NH4-N ratio of the influent was 5.8 suggesting marginal carbon availability. Simulation studies with BIOWIN© software suggested that the SBNR process takes place in the bioreactor and that the detected high efficiency of nitrogen removal would not have been possible with denitrification through nitrate at the given carbon availability. Technology 2 aims for N and P removal with an average of 7.4 BOD5/ NH4-N ratio in the influent. The 45 mg l 1 average influent NH4-N concentration is eliminated to below 1 mg l 1 in the system. On-site measurements and registered operation data showed that when maintaining low DO values in the labyrinth basin (DO concentrations in Figure 9) the effluent NO3-N concentration falls below 6 mg l 1, while with operation in the aerated zone with DO ¼ 1.5 mg l 1 or higher, the NO3-N concentration increases to above 10 mg l 1, suggesting that in a low DO environment SND may take place. Low DO operation has, however, a well known disadvantage: low DO bulking may occur and secondary clarification may deteriorate. As a consequence of the low DO environment, in Technology 1 the typical SVI value was between 230 and 250 cm3 g 1 and occasionally severe bulking (V30 > 950 cm3) hindered secondary clarification. In Technology 2 the bulking could be repressed to some extent by selector-effect provided by the anaerobic and anoxic reactors preceding the labyrinth basin, resulting in SVI values between 150 and 200 cm3 g 1. 65.9 | 2012 building of combined biological N and P removal technologies has been favored. The carbon source deficiency, however, has not usually facilitated efficient N and P removal together. Seven out of the 27 investigated WWTPs with biological N and P removal receive wastewater with BOD5/NH4-N ratio below 6, which suggests low achievable efficiency in either N or P removal. As a case study, a WWTP in the Lake Balaton area (limitation category I) has been investigated. The average of the influent total COD was 750 mg/l, but, especially in the summer period, partly as a result of biodegradation in the sewer system, the soluble fraction of COD proved to be below 25%. Therefore, the average values of 60 mg l 1 influent NH4-N and 285 mg l 1 BOD5 gave a low, 4.75 BOD5/ NH4-N ratio. In the original design idea, the influent was introduced to an anaerobic reactor followed by an anoxic and aerobic basin (see Figure 10) operated with the typical MLVSS value of ∼2.9 g l 1. Nitrified mixed liquor recirculation (NO3-N rec., having a ratio of ∼300% to the influent) was introduced to the anoxic reactor. Preceding the anaerobic reactor an additional anoxic reactor was constructed, receiving the sludge recirculation and a recirculation stream from the anaerobic reactor supposed to contain the carbon source for the denitrification of the nitrate in the return activated sludge (RAS) and providing anaerobic conditions in the anaerobic reactor. Effluent concentration values and concentration profile measurements, however, showed that with this arrangement, the WWTP was not able to fulfill the 10 mg 1 1 effluent TN limitation due to the inefficient denitrification in the anoxic basins (Jobbágy et al. ). The main cause of the low efficiency denitrification was the lack of carbon source for covering the readily biodegradable COD demand of the process. As a technological modification nitrified mixed liquor recirculation was switched over to the head of the system (marked with dashed line in Figure 10), converting the WWTP to an efficient, staged pre-denitrification arrangement. With the elimination of enhanced biological phosphorus removal Combined biological N and P removal technologies During the last decades, in the catchment areas of sensitive receiving water bodies (category I and II, see Table 1) | Figure 10 | Scheme of the Balatonújlak WWTP. 1682 G. M. Tardy et al. | Water Science & Technology Biological wastewater treatment in Hungary (EBPR), a larger part of the influent COD could be utilized for denitrification, and, as a result, effluent NO3-N concentration decreased by 10–15 mg l 1 from the original 20– 30 mg l 1 effluent NO3-N concentration of the original design. The results suggest that in the case of low carbon source availability, abandoning EBPR may be necessary for enhancing denitrification. 65.9 | 2012 CONCLUSIONS Based on the survey involving 55 Hungarian WWTPs, investigating the quality of the influent and effluent wastewater, the related technologies and nuisances, the following major conclusions can be drawn: • Special BNR technologies In the case of low influent C/N ratio and strict TN limitations, application of special technologies or dosing of excess carbon source may be required. In order to meet the limitations, the activated sludge system of the Southpest WWTP (design capacity 80,000 m3 d 1) was complemented by post-nitrification and denitrification biofilters (BIOFOR®). Nitrate formed in the system can be effectively removed in the post-denitrification biofilter with excess carbon source (methanol), and the 10 mg/l effluent TN limit can be safely fulfilled. However, due to the cost-effective upgrading as a combined system, backwashed excess biomass of the nitrification filter may efficiently seed the activated sludge reactors and nitrification can be achieved even at the given low SRT (∼2–3 d) and MLVSS (∼1.8–2.2 g l 1) (Jobbágy et al. , ). Therefore, through using the carbon source of the wastewater in activated sludge predenitrification, savings can be achieved in both air and methanol consumption and a better effluent quality can be provided. One of the state of the art approaches for biological nitrogen removal is the application of anaerobic ammonia oxidizing (anammox) biomass (Schmidt et al. ). Through this process both aeration requirement and carbon source consumption can be decreased compared with the conventional nitrification and denitrification process. In recent years in Switzerland, Germany and Austria DEMON® technology has been successfuly applied in fullscale for treating sludge process waters with high ammonia content (Wett ; Innerebner et al. ). In Hungary, the first application of this technology has been carried out in a 17,000 m3 d 1 capacity WWTP. The installed technology deammonifies 160 m3 d 1 sludge process water with 1,000 mg l 1 average NH4-N concentration providing higher than 90% NH4-N removal efficiency. As recirculation of sludge process waters may increase the TN load of WWTPs by even 10–30% (depending on the amounts and quality of cofermented materials) application of special technologies for high-strength wastewater may greatly increase the plant efficiency, decrease or eliminate the possible carbon deficiency and external carbon source requirements. | • • • • • • • Wastewater quality values may differ greatly in the different areas, thus, design practice based on the rigid PE should be revised and design parameters have to be carefully determined by on-site measurements and/or case studies. Treatment temperatures may vary over a wide range (from <10 C to >26 C). Most of the Hungarian WWTPs receive high-strength wastewater both in COD and NH4-N concentrations. The influent wastewater of more than one-third of the investigated WWTPs proved to be carbon deficient for efficient denitrification, which hinders the fulfillment of the strict effluent TN criteria, and may lead to rising sludge in the secondary clarifiers, especially in summer. Without efficient denitrification floating sludge in the secondary clarifier may cause severe technological problems and lead to the deterioration of the effluent quality. Several plants try to perform both biological nitrogen and phosphorous removal with carbon deficient wastewater. In these cases abandoning EBPR may lead to improved denitrification. Low DO technologies may provide enhanced nitrogen removal in case of marginal carbon availability, however, filamentous bulking caused by low DO conditions can be expected in the lack of appropriate selector systems. Special biological nitrogen removal technologies (e.g. DEMON® technology, combined AS-BIOFOR® system) provide efficient BNR, but have not been widely used so far. W W ACKNOWLEDGEMENTS The survey has been carried out within the frames of the National Platform of Water Technology directed by Prof. László Somlyódy and Dr Ernő Fleit, and funded by the Hungarian National Innovation Office. Valuable contributions of the co-workers of the Transdanubian Waterworks Co., Budapest Sewage Works Ltd., Bácsvíz Co. and UTB Envirotec Ltd. in facilitating the case studies are highly acknowledged. This work is connected to the scientific program of the ‘Development of quality-oriented and harmonized R þ D þ I strategy and functional model at 1683 G. M. Tardy et al. | Biological wastewater treatment in Hungary BME’ project. This project is supported by the New Széchenyi Plan (TÁMOP-4.2.1/B-09/1/KMR-2010–0002). REFERENCES Grady, C. P. L., Daigger, G. T. & Lim, H. C.  Biological Wastewater Treatment: Theory and Application, 2nd edition. Marcell Dekker, Inc., New York. Guo, J., Peng, Y., Wang, S., Zheng, Y., Huang, H. & Wang, Z.  Long-term effect of dissolved oxygen on partial nitrification performance and microbial community structure. Bioresource Technology 100, 2796–2802. Henze, M., Dupont, R., Grau, P. & Sota, A.  Rising sludge in secondary settlers due to denitrification. Water Research 27 (2), 231–236. Innerebner, G., Insam, H., Franke-Whittle, I. H. & Wett, B.  Identification of anammox bacteria in a full-scale deammonification plant making use of anaerobic ammonia oxidation. Systematic and Applied Microbiology 30, 408–412. Jobbágy, A., Tardy, G. & Literáthy, B.  Enhanced nitrogen removal in the combined activated sludge-biofilter system of the Southpest Wastewater Treatment Plant. Water Science and Technology 50 (7), 1–8. Jobbágy, A., Literáthy, B. & Tardy, G.  Comparative pilot-scale studies for upgrading denitrification at an activated sludge plant of the Lake Balaton area. In: Proceedings of the IWA Specialized Conference on Nutrient Management in Wastewater Treatment Processes Water Science & Technology | 65.9 | 2012 and Recycle Streams, Krakow, Poland, 19–21 September, pp. 1235–1240. Jobbágy, A., Tardy, G. M., Palkó, Gy., Benáková, A., Krhutková, O. & Wanner, J.  Savings with upgraded performance through improved activated sludge denitrification in the combined activated sludge – biofilter system of the Southpest Wastewater Treatment Plant. Water Science and Technology 57 (8), 1287–1293. Kujawa, K. & Klapwijk, B.  A method to estimate denitrification potential for predenitrification systems using NUR batch test. Water Research 33 (10), 2291–2300. Metcalf & Eddy, Tchobanoglous, G., Burton, F. L., Stensel, H. D.  Wastewater Engineering: Treatment and Reuse, (4th edition). McGraw-Hill, Boston, Montreal. Schmidt, I., Sliekers, O., Schmid, M., Bock, E., Fuerst, J., Kuenen, J. G., Jetten, M. S. M. & Strous, M.  New concepts of microbial treatment processes for the nitrogen removal in wastewater. FEMS Microbiology Reviews 27, 481–492. Tardy, G. & Jobbágy, A.  Characteristics of domestic wastewaters and treatment technologies in Hungary. Conference of the Hungarian Society of Hydrology, Pécs, Hungary, 5–6 July (presentation in Hungarian). Turk, O. & Mavinic, D. S.  Benefits of using selective-inhibition to remove nitrogen from highly nitrogenous wastes. Environmental and Technology Letters 8, 419–426. Wett, B.  Solved upscaling problems for implementing deammonification of rejection water. Water Science and Technology 53 (12), 121–128. First received 16 August 2011; accepted in revised form 4 January 2012 Copyright of Water Science & Technology is the property of IWA Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.