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Paper N0: IV.08

Impact of the Zagreb Wastewater Treatment and Disposal on Sava River Dražen Vouk Davor Malus

Abstract: This paper gives the Sava River water quality analysis of the upper section on its course through Republic of Croatia. Analyzed section covers the area between hydrological stations Jesenice (state border with the Republic of Slovenia) and Galdovo (city of Sisak). Since Zagreb and its nearby settlements wastewaters are not treated, the studied section of the Sava is consequentially heavily loaded with waste (organic) matter. Water quality simulations were undertaken using QUAL2E mathematical model. Detailed analyses of available measurements, based on data from the National Watercourse Quality Monitoring Program, have preceded the simulation (forecasting). Concerning the input data availability, the given simulation has included the following water quality parameters: BOD-5 and dissolved oxygen concentrations, saturation of water with oxygen and water temperature. Besides the analysis with untreated wastewater effluents (discharges) characterizing current state, the analysis of planned state with treated effluent were undertaken as well. Current state analysis was also used for model calibration. Furthermore, simulations were undertaken for two hydrological regimes of Sava River: the current regime under which Sava maintains (more or less) natural flow condition and the planned regime that assumes the construction of water spillovers (forming river-run lakes). From computed results of autopurification potential of the Sava, the necessity for the planned wastewater treatment plants construction is estimated. The emphasis is on determination of necessity and justifiability of the Zagreb central wastewater treatment plant (CWWTPZ) construction, as well as the impact of the CWWTPZ on the Sava River water quality. Keywords: water quality, Sava River, CWWTPZ, relevant low flow (Q30,95%)

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1.

Introduction

Analyzing the Sava River as a significant tributary of the Danube watershed, on its course through Republic of Croatia, the single segments heavily loaded with waste matter are easily distinguished. Special interest is given to the upper Sava course with the urban territory of Zagreb and its nearby settlements as well as Medvednica and Žumberak mountainous streams. The current disposal of untreated wastewaters and runoff has systematically deteriorated Sava River water quality, on the observed location. Natural biological balance of Sava River has been constantly disturbed by significant amounts of organic waste discharge. Concerning the section downstream the discharge point of Zagreb main drainage channel (GOK), special circumstances such as small flows may result with total useless of Sava water for any purpose except for navigation. Consequent upon mentioned a necessity to prevent further deterioration and to protect and upgrade the Sava River water quality for its planned purpose is appearing to become urgent. In accordance with the relevant Water Protection Plans, several wastewater treatment plants have been planned to reduce the total waste loads of the Sava. The scope of this investigation covers the Sava River water quality analyses on the section from the border with the Republic of Slovenia (hydrological station Jesenice) to the City of Sisak (hydrological station Galdovo) in total length of nearly 123,0 km. The final goal is to forecast (simulate) certain water conditions of the Sava River (from Jesenice to Galdovo) in the near and far future, depending upon the impact of future plants operation, or upon the discharge of wastewaters and runoff. Analyze on the impact of the CWWTPZ on the Sava River water quality will be specially emphasized.

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Current state condition

The current state of Sava River water quantity and quality, concerning the studied section from Jesenice to Galdovo, has been assessed for low, medium and high flows. It should be pointed out that low flows are relevant for the assessment of watercourse quality and the impact of wastewaters discharge. Namely, wastewater discharge during higher flows has no considerable impact due to significant dilution. There are several methods (norms) to determine the relevant low flows. In Croatia, low flows during 30 consecutive days (any series within a year) with a return period of 20 years and 95% occurrence probability (Q30,95%) are determined as relevant. The relevant low flow will be determined in accordance with elaborates [1][2]. The relevant low flow value at Jesenice Station (beginning point of this analysis) is defined as: Q30,95% = 68,0 m 3 / s

Complete analysis comprises the simulations with medium and high flows as well. The values for these flows at Jesenice Station were chosen according to data from elaborate [3]:

Impact of the Zagreb Wastewater Treatment and Disposal on Sava River

Qmean = 300,0 m 3 / s

;

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Qmax = 800,0 m 3 / s

Wastewater loads of Sava River have been analysed for "point" and "dispersed" contamination sources. Point sources include all communities with public sewerage system that is used to collect and dispose of the fecal, industrial and runoff wastewater into the Sava River (or its tributaries), regardless whether wastewater treatment plants exist or not. This group of sources includes the mountainous streams that flow directly in Sava, as well. Dispersed sources include runoff waters collected on agricultural and other surfaces, including the number of cattle on the pertaining area and the number of inhabitants in small communities without the public sewerage system. Regarding the fact that the city of Zagreb has a system of embankments that protect the urban area against flooding, impact of the dispersed sources is insignificant. Runoff that washes out nearby agricultural fields flows into the Sava together with mountainous streams and other tributaries. Small amounts of this runoff still manage to influence the Sava directly as it drains the groundwater. Current quantity and quality of wastewaters and runoff flowing into the Sava have been estimated based on the following elaborates: [1][3][4][5][6][7][8][9][10][11][12][13]. Current state of water quality is given based on data from the National Water Quality Monitoring Program prepared by “Croatian Waters”. Studied section is covered with six hydrological stations on the Sava and three stations on the tributaries Sutla(2) and Krapina(1). Regarding time period of recordings, the 1995-2001 period is relevant. A longer period of time is not relevant due to the Homeland War and changes in water quality that took place as a consequence of war operations. Regarding the use of measurement results, the positioning of sampling stations, as well as sampling times, is not favourable for the preparation of this type of study. Namely, the sampling time is not synchronized with the hydraulic regime of the watercourse, but is determined in advance for regular time periods. Therefore, one particular mass of water passing the individual stations could not be monitored from which organic matter decay rate and watercourse reaeration could be determined. This was considered as a very sensitive part of the analysis and compensation was made with the application of averaged reference data [15][16][17][18][19]. Analysing positions of hydrological stations it is clear that there are no stations downstream from the mouth of the Main Drainage Channel of Zagreb, where the impact of discharged Zagreb wastewaters could be better monitored. The closest station downstream of GOK is situated some 22 km from the discharge point.

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Evaluation of planned state condition

The year 2015 has been taken as the planned year. In this period it’s been assumed that second-degree wastewater treatment plants would be constructed in all cities on the subject section. In comparison to the current state, the outlet of the Zaprešić plant will be directly in the Sava River. The discharge of the Samobor plant will be into the Gradna Channel, connecting Gradna stream and Rakovica stream, with the discharge point into the Sava

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common with the Rakovica stream mouth. In Zagreb, wastewaters from the right bank will be connected to the central wastewater plant (CWWTPZ) on the left bank and discharged by GOK into Sava River. The outlet of the Velika Gorica plant has already been constructed at its final position. During the rainy periods with heavy rains, high flows will be relieved from the Zagreb drainage network via Črnomerec and Savica spillovers, as well as number of spillovers connected to GOK. It is further assumed that upstream of Jesenice Station, wastewater treatment plants of at least second degree treatment would also be in operation, so that the Sava water quality at the Jesenice Station would meet the second degree requirements. Data on point and dispersed sources have been presented according to the same sources as it was done for the current state analyses. Besides the planned steady-state analysis with the current hydrological regime, the water quality state in the case of multi-purpose water spillovers construction is simulated as well. Under such circumstances, the hydrological regime of the Sava would change.

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Simulations of the Sava River water quality

Sava River water quality simulations were undertaken using QUAL2E mathematical model. The key water quality parameters (BOD-5 concentration, dissolved oxygen balance and the water temperature) have been modelled. Concerning the changed hydrological regime, total phosphorus has also been modelled as a good indicator of the future trophic state. Regarding this case, it can be stated that besides hydrological and hydraulic data on watercourses and some of the data on measured concentrations of key water quality parameters, there are no customized investigations and measurements applicable in environmental modelling. By establishing a mathematical model, which is based on series of input data that have been taken as averaged values from references, the probability of mistake is increasing. The model was calibrated by checking it for the measured state of the Sava River water quality, carried out according to the Sava water quality-monitoring program [7]. After establishing that the model appropriately describes current state of quality, as recorded at observing hydrological stations, the state for planned loadings of the Sava River was simulated. Simulation was undertaken for three different state of Sava water flows – low, medium and high flows. Regarding the meteorological conditions in Zagreb, each of the mentioned water flows was analysed during dry and rainy periods in Zagreb. It should be pointed out that during the rainy period this analysis assumes there will be rain in the Zagreb district, and on the other parts of the watershed dry weather would remain. However, there are no data on the probability of occurence of extended heavy rain in Zagreb and low flow of the Sava. Based on past experience, such an occurence has low probability. The hydraulic state characteristic for summer months (May-September) has been modelled. Clearly, such typically summer conditions concerning the environmental aspect would be most unfavourable. Simulation of the Sava River water quality was undertaken for:


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Figure 1 Sava River – studied section

• Current state of loading and current hydrological regime; • Planned state of loading and current hydrological regime; • Planned state of loading and changed hydrological regime.

4.1. Current state of loading and current hydrological regime Regarding the current state analysis, the year 1999 has been taken as the planned year. Simulation results has shown that with the current disposal of untreated wastewaters, the Sava water quality upstream from the GOK mouth can be classified as Category III (according to BOD-5), and occasionally bounding between Category II and III during dry periods with low Sava flows. The Sava water quality would significantly deteriorate after the discharge of untreated wastewaters of Zagreb, and would be classified as Category IV water (according to BOD-5) somewhere to Martinska Ves (cca 50,0 km from the GOK mouth). The status with dissolved oxygen is more favourable, and accordingly the Sava along the entire section can be classified as Category II. During medium and high flows the Sava meets the present provisions on water categorization as given in the National Water Protection Plan, due to significant dilution.

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Figure 2 Simulation results for BOD-5 – current state of loading with low Sava flows during dry period

In the case of rain in Zagreb and low flows of the Sava, the water quality status is even less favorable. The whole section downstream the GOK mouth all up to Galdovo would be classified (according to the BOD-5) as Category IV. Consequentially, Sava River would not meet water categorization requirements. Along the entire section, oxygen meets the requirements of Category II, except directly after the GOK discharge point, where it drops to Category III. During medium flows of the Sava and rain in Zagreb the subject section of the Sava from the GOK discharge point to Galdovo is classified as Category III according to BOD-5. On other parts the Sava is classified as Category II. According to the dissolved oxygen indicator, as well as oxygen saturation, the entire section of the Sava is classified as Category I. During high flows of the Sava and rain in Zagreb, the Sava belongs to Category II, along the whole section. At the same time, current state analyses were used for model calibration. From the computed simulations it is obvious that the results are residing in the range of measured values from the National Watercourse Quality Monitoring Program.

4.2. Planned state of loading and current hydrological regime The state of Sava River water quality for the planned waste loadings (year 2015) is forecasted under the assumption that all wastewaters on the monitored section prior to discharge into the Sava are previously secondary treated. Under such circumstances, Category II water characteristics could be maintained along the entire monitored section, with exception of the very short section downstream the GOK mouth, where the Sava during low flows would be classified as Category III, according to the BOD-5 parameter (Figuress 3 and 4).


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During medium and high flows, the Sava would meet Category II requirements on the entire section. In the case of rain in Zagreb and low Sava flows, the Sava would have Category III water quality up to the GOK mouth and Category IV water quality from GOK to Galdovo. On the results basis, the Sava River would not meet water categorization requirements from the National Water Protection Plan according to the BOD-5 indicator. During medium flows and rain in Zagreb, the water quality of Sava River would be Category III, from the GOK mouth to Galdovo. Only during high water flows and rain in Zagreb, Sava River would meet Category II requirements along the entire monitored section.

Figure 3 Simulation results for BOD-5 – planned state of loading with low Sava flows during dry period

Figure 4 Simulation results for dissolved oxygen – planned state of loading with low Sava flows during dry period

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4.3 Planned state of loading and changed hydrological regime The construction of water spillovers would change the hydrological regime in the watercourse on the monitored section of Sava River. Under such circumstances in the Sava, the water quality was forecast for the future wastewater loading with wastewaters previously treated to the second-degree treatment. During the dry period and low flows of Sava River, the critical indicators are total phosphorus and dissolved oxygen. Regarding total phosphorus, Sava River would be classified as Category IV downstream the GOK mouth. According to dissolved oxygen, Sava is at the same time classified as Category III. Under medium flows, according to total phosphorus, Sava River would be classified as Category III downstream the GOK mouth. For other indicators, Sava River would meet Category II, and Category I requirements. In the case of rain in Zagreb and low Sava flows, categorization requirements would not been met, downstream the GOK mouth. Under medium flows, Sava River would be classified as Category III downstream the GOK mouth all the way to Galdovo. Therefore, it would not meet the Sava categorization requirements even at medium flows. Additionally, the implementation of third-degree treatment on wastewater treatment plants prior to discharge into the Sava has been analysed. In that case, all indicators except dissolved oxygen meet the categorization requirements along the entire section. According to this indicator, Sava should be classified as Category III.

Summary Based on the undertaken conducted analyses it can be concluded that: • The current discharge of untreated wastewaters on the section from Jesenice to Galdovo does not meet the water categorization requirements according to the National Water Protection Plan – at the relevant flow (Q30,95%). • The proposed mathematical model for the simulation of planned water quality conditions in the Sava is appropriately chosen, which is proved by the “simulation” of the existing state, set by the Sava River Quality Monitoring Program. • The construction of the Zagreb plant (CWWTPZ) would significantly upgrade the water quality of Sava River on the section from Zagreb to Sisak at relevant flows. • The water quality upstream of Zagreb improves by treating Samobor and Zaprešić wastewater, and, at the same time, contributes to the upgrading of the water quality downstream of Zagreb. • In the case of rain in Zagreb and simultaneous low flows (relevant) of Sava River, under heavy rain over an extended period of time, the quality of water would significantly drop with regard to loading according to BOD-5. However, even under such circumstances, the value of dissolved oxygen on the whole section would not be lower than 6 mg O2/l, and there would be no reason to proclaim the “state of


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extraordinary contamination” and call for appropriate protection measures. It should be pointed out that such a state of increased loading in the watercourse could last up to one day or less, so unfavourable impact on the living communities in Sava River would not be expected. • The change in the hydrological regime of Sava River, after the construction of water spillovers on the monitored section, the secondary wastewater treatment plant in Zagreb will not fulfill the water categorization requirements on the subject section. • The construction of a tertiary treatment plant and water spillovers in the Zagreb area would ensure that most of the Sava River categorization requirements are met. However, it should be pointed out that in this case a reduced quantity of dissolved oxygen in the Sava water can be expected along the entire section from Podsused HEP to Galdovo. This is the consequence of the changed reaeration conditions and dissolved oxygen consumption, in the river. It is stressed that the simulation of water quality conditions after the construction of water spillovers could not be tested on the actual state, as such circumstances on the Sava are not yet existing. It will be necessary to undertake additional investigations after the construction of the first water spillover in the Zagreb area (probably Podsused HEP), in order to confirm the above statements.

References [1] Faculty of Civil Engineering - Zagreb (2000.): Hydrological Processing of Relevant Flow and Calculation of Recipient Capacity of Watercourses [2] Elektroprojekt (1982.): Water Management Plan of the City of Zagreb, Zagreb [3] Japan International Cooperation Agency - JICA (2001): The Study for Water Pollution Reduction on the Sava River Basin in the Republic of Croatia [4] Faculty of Civil Engineering - Zagreb (2000.): Environmental Impact Study of the Central Wastewater Treatment Plant of Zagreb [5] Faculty of Civil Engineering - Zagreb (2002.): Environmental Impact Study of the Public Drainage System of the City of Samobor [6] Elektroprojekt (1986.): Water Management Plan of the City of Zagreb - Samobor i Sesvete Municipalities, Zagreb [7] Hrvatske vode (2002.): Sava River Water Quality Monitoring Results from 1995 to 2001, Zagreb [8] Proning DHI (2000.): Analysis of the Wastewater Quality Impact on Future Runoff Spillovers in the Zagreb Sewerage System, Zagreb. [9] Proning DHI (2001.): GOK System Optimization – Preliminary Design of Connections of Main Sewers to the GOK System, Zagreb [10] Proning DHI (2003.): Preliminary Design of the Main Drainage Channel, Zagreb [11] Zagrebačke otpadne vode (2002.); Final Monitoring Report – CWWTP of Zagreb [12] Municipal Department for City Planning and Environmental Protection (2000.): Zagreb 2000 + New Urban strategy [13] Hrvatska vodoprivreda (1992.): Water Management Plan of the City of Zagreb, Modifications and Amendments – Valorization and Definition of Harmonized Solution, Zagreb [14] Proning DHI (1998.): Zagreb Sewerage System Optimization, Phase II, Zagreb [15] EPA (1980.): "Modeling Phosphorus Loading and Lake Response Under Uncertainty: A Manual and Compilation of Export Coefficients" - EPA/440/5-80-011 [16] EPA (1985.): "Rates, Constants and Kinetics Formulations in Surface Water Quality Modeling (Second Edition)" - EPA/600/3-85/040

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[17] EPA (1987.): "The Enhanced Stream Water Quality Models QUAL2E and QUAL2E-UNCAS: Documentation and User Manual" - EPA/600/3-87/007. [18] EPA (1993): "The Water Quality Analysis Simulation Program, WASP5" - EPA/Environmental Reaearch Laboratory, Athens, Georgia 30613 [19] EPA (1995): "QUAL2E Windows Interface User's Guide" - EPA/823/B/95/003. Authors Assistant Dražen Vouk: University of Zagreb, Faculty of Civil Engineering, Kačićeva 26, 10000 Zagreb, Croatia, [email protected] Professor Davor Malus: University of Zagreb, Faculty of Civil Engineering, Kačićeva 26, 10000 Zagreb, Croatia, [email protected]