John Nurminen Foundation. Technical Audit of the Pomorzany Wastewater Treatment Plant

Transcription

1 November 26, 2010 John Nurminen Foundation Technical Audit of the Pomorzany Wastewater Treatment Plant Establishment of the Most Feasible Way for Accelerated Phosphorus Removal

2 i Table of Contents 1 INTRODUCTION BACKGROUND OBJECTIVES OF THE PROJECT POMORZANY WASTEWATER TREATMENT PLANT GENERAL...1 WASTEWATER FLOWS AND LOADS Wastewater flow Wastewater loads Temperature TREATMENT REQUIREMENTS AND STATE OF NUTRIENT REMOVAL Treatment requirements Effluent wastewater quality and loads PROCESS DESCRIPTION OPERATION Operation practice Aeration and DO control Solids retention time (SRT) and MLSS concentrations Return sludge circulation flow rates Secondary settling and sludge volume index Loading of the activated sludge process BOD 5 load Phosphorus balance Nitrogen balance ANALYSIS AND PROPOSED IMPROVEMENTS ANALYSIS OF PLANT CONDITIONS Technical condition and performance of wastewater treatment process Design issues Possibilities and effects of improving phosphorus removal Process stability and operational advantages and disadvantages RECOMMENDED ACTIONS General Operation of existing units Enhancement possibilities of phosphorus removal COST ESTIMATES SUMMARY... 31

3 ii DISCLAIMER This report has been prepared by Pöyry Finland Oy ( Pöyry ) for John Nurminen Foundation (the Recipient), in accordance with the Agreement between Pöyry and the Recipient. Any reliance any party other than the Recipient chooses to place on the information contained in the report is a matter of such party s judgment exclusively and solely at such party s own risk. Pöyry assumes no responsibility for any actions (or lack thereof) taken by any party as a result of relying on or in any way using any information contained in the report and in no event shall Pöyry be liable for any damages of whatsoever nature resulting from reliance on or use of such information. No representation or warranty, express or implied, is made by Pöyry as to the accuracy or completeness of any information contained in the report and nothing in the report is or shall be relied upon as a promise or representation as to the future. All rights to this report are reserved and exclusively defined in the Agreement between Pöyry and the Recipient.

4 1 1 INTRODUCTION 1.1 Background The John Nurminen Foundation, the Union of the Baltic Cities Commission on Environment and HELCOM (Baltic Marine Environment Protection Commission) have agreed to work together in order to improve the state of the Baltic Sea. In that purpose the Parties have applied and received financing from EU Baltic Sea Region Programme for three year project called PURE (Project on Urban Reduction of Eutrophication), started in December of The objective of the project PURE is to reduce phosphorus discharges to the Baltic Sea by enhancing phosphorus removal at municipal wastewater treatment plants and also improve the knowledge on the best available techniques on phosphorus removal in cities and water companies around the Baltic Sea. The PURE project was approved by the EU BSRP Monitoring Committee on 16 th of September The PURE project partners also include the following cities and/or water companies: Brest Vodokanal, city of Gdansk, Jurmala Water, Kohtla-Järve Water Company (Järve Biopuhastus OÜ) and Szczecin Water Company. The cities and/or water companies have agreed that a technical audit will be carried out at their wastewater treatment plants in order to assess the feasibility and cost efficiency of enhanced, chemical phosphorus removal and other low cost options to reduce phosphorus discharges to receiving waters. The Project Partners intend to achieve an average annual concentration of 0.5 mg phosphorus/litre in effluent wastewater on continuous basis. Also investments to achieve this value are included in the project PURE at the wastewater treatment plants of Brest and Jurmala, starting in The project will be carried out in harmony with national legislation, rules and environmental regulations of each participant and EU. 1.2 Objectives of the Project The overall objectives of the assignment are to review the current wastewater and sludge treatment processes especially in terms of phosphorus removal to develop the most cost effective plan to enhance phosphorus removal to the level of 0.5 mg/l and to estimate the additional O&M costs by the enhanced treatment. 2 POMORZANY WASTEWATER TREATMENT PLANT 2.1 General Szczecin (ca inhabitants) is the capital of the West Pomeranian voivodship. The city is located on the Oder River and Dabie Lake in the Szczecin Lowlands, about 65 km from the Baltic Sea.

5 For years, Szczecin has been dependent on inefficient mechanical-chemical wastewater treatment and as well as an outdated sewage network. The result has been heavy pollution of the Oder and the Baltic Sea. The low efficiency of wastewater treatment and sewage network was problematic not only for the city s inhabitants but it gave the popular perception of Szczecin as a major environmental polluter, which discouraged potential investors and stifled the city s economic growth. The Improvement of water quality in Szczecin program, which was approved for implementation in 2000, has been one of the largest pro-ecological projects in Central and Eastern Europe in recent years. Valued at 288 million EUR, including a 190 million EUR grant from the Cohesion Fund (formerly ISPA Instrument for Structural Policies for Pre-Accession) the program was aimed at upgrading the city s water supply and sewage treatment system. The key elements in the program was design, construction and commissioning of the mechanical-chemical-biological wastewater treatment plant Pomorzany ( PE) in Szczecin with capacity of m 3 /d with sludge and biogas management, supply of plant, starting up, tests on completion including trial operation along with a delivery of network of four pumping stations with pressure pipelines. The program also included design and modernization of the mechanical part of the existing wastewater treatment plant Zdroje ( PE) in Szczecin. The old Zdroje WWTP had only mechanical treatment, which has be extended by a biological stage with capacity m 3 /d, with sludge and biogas management, supply of plant, reinstatement works, starting-up and commissioning, tests of completion including trial operation. The new Pomorzany WWTP is located in the southern part of Szczecin. The time of realization the WWTP was from to and the value of this particular project was about 47 million EUR. The companies involved in this project were WTE Wassertechnik GMbH and its subsidiary in Poland, Pollmex Mostostal S.A. and OTV S.A. There are four main pumping stations (Bialowieska, Górny Brzeg, Grawbów and Dolny Brzeg), which transport wastewater to the Pomorzany WWTP. The pumping stations are equipped with 50 mm mechanical screens and small grit chambers. The pumping stations are situated in northern part of Szczecin whilst the Pomorzany WWTP is located in south. Grawbów and Dolny Brzeg pumping stations service a drainage area covering the central and northern parts of Szczecin. Sewage from both pumping station is transported to the Pomorzany WWTP by two parallel pressure pipelines, both 12 km long. During dry weather and periods of low precipitation only one of the pipelines will be in operation. During wet weather sewage is pumped through both pipelines. The capacity of the Grawbów pumping station is m 3 /h (0.95 m 3 /s) during dry weather and m 3 /h (1.8 m 3 /s) during wet weather. The capacity of the Dolny Brzeg pumping station is respectively m 3 /h (0.3 m 3 /s) and m 3 /h (1.38 m 3 /s). 2

6 The corresponding capacities of Biolowieska and Górny Brzeg are m 3 /h (0.29 m 3 /s) and m 3 /h (0.385 m 3 /s) during dry weather and m 3 /h (0.92 m 3 /s) and m 3 /h (2.0 m 3 /s) during wet weather. The pumped wastewaters are treated in the Pomorzany WWTP using mechanicalchemical-biological processes and the treated wastewaters from the Pomorzany WWTP are discharged to west branch of the Oder River (SNQ = 68.0 m 3 /s). Designed influent loads, anticipated effluent concentrations and reductions of the Pomorzany WWTP are shown in the Table 2.1 below: 3 Table 2.1. Design parameters of Pomorzany WWTP Designed Influent Loads Designed Effluent Parameters Designed Reduction (%) BOD 5 -load = ton O 2 /d BOD 5 = 15 mg O 2 /l 96.0 COD-load = ton O 2 /d COD = 125 mg O 2 /l 99.5 TSS-load = ton SS/d TSS = 35 mg/l 92.6 N tot -load = 4.39 ton N/d N tot = 10 mg/l 87.6 P tot -load = 0.66 ton P/d P tot = 1.0 mg/l 90.0 The basic values describing the average wastewater amount and quality from June to September 2010 compared to the design values are presented in Table 2.2. (1 P.E. = 60 gbod 5 /ca/d) Table 2.2. Wastewater data of Pomorzany WWTP Parameter Unit Design value Pop. Equivalent (BOD) ca Q average m 3 /d Q max m 3 /d q design/average m 3 /h BOD 5 COD Suspended Solids (TSS) Total Nitrogen (TN) Total Phosphorus (TP) Temperature, max Temperature, min kg/d g/ca/d mg/l kg/d g/ca/d mg/l kg/d g/ca/d mg/l kg/d g/ca/d mg/l kg/d g/ca/d mg/l o C o C Average Jun 10 Sep

7 4 During the 1 st technical audit ZWIK s (Water and Wastewater Works Ltd.) own organization had received the operation responsibility only a couple of weeks earlier from the constructor. 2.2 Wastewater flows and loads Wastewater flow Influent wastewater flow varied from m 3 /d to m 3 /d. The average flow was m3/d. Twenty two (22) times the daily flow values exceeded the dimensioning flow value of m 3 /d and twelve (12) times the flow was more than maximum day flow m 3 /d. The influent flows from the 1 st of June to the 27 th of September 2010 are shown in Figure 2.1. Figure 2.1. Influent wastewater flows from June to September Wastewater loads Organic load The influent organic load was measured by BOD 5 -measurements daily during June decreasing to five measurements per month during September The measurement interval with COD Cr -measurements was the same. BOD 5 -loads and concentrations varied as shown in Table 2.3. The influent BOD 5 - loads from June to September are shown in Figure 2.2.

8 5 Table 2.3 BOD 5 loads and concentrations from June to September 2010 BOD 5 Average Min Max Concentrations (mg/l) Loads (kg/d) Figure 2.2. Influent BOD 5 loads from June to September The influent COD Cr -loads and concentrations varied as shown in Table 2.4. The influent COD Cr -loads are shown in Figure 2.3. Table 2.4 COD Cr loads and concentrations from June to September COD Cr Average Min Max Concentrations (mg/l) Loads (kg/d)

9 6 Figure 2.3. Influent COD Cr loads from June to September The average COD/BOD ratio 1.9 of the influent wastewater correspond normal composition of the municipal wastewater. The wastewater is easily biologically treated and contains mainly easily biodegradable organic compounds. The COD/BOD ratios are shown Figure 2.4. Figure 2.4. COD/BOD ratios from June to September 2010.

10 7 Nutrient loads Influent phosphorus concentrations in Pomorzany WWTP are shown in Table 2.5. The influent P-loads from June to September are shown in Figure 2.5. Table 2.5. Influent phosphorus loads and concentrations from June to September 2010 Phosphorus Average Min Max Concentrations (mg/l) Loads (kg/d) Figure 2.5. Influent phosphorus loads from June to September Influent nitrogen concentrations in Pomarzany WWTP varied as shown in Table 2.6. The influent N-loads from June to September are shown in Figure 2.6. Table 2.6. Influent nitrogen loads and concentrations from June to September Nitrogen Average Min Max Concentrations (mg/l) Loads (kg/d)

11 8 Figure 2.6. Influent nitrogen loads from June to September Suspended solids Influent suspended solid concentrations were measured daily during June decreasing to five measurements per month during September The suspended solids loads and concentrations in Pomorzany WWTP varied as shown in Table 2.7. The influent SSloads from June to September are shown in Figure 2.7. Table 2.7. Influent suspended solid loads and concentrations from June to September Suspended solids Average Min Max Concentrations (mg/l) Loads (kg/d)

12 9 Figure 2.7. Influent suspended solids loads from June to September Temperature Influent wastewater temperature varied from 16.4 o C to 23.7 o C and was on average 20.3 o C. Minimum design temperature (nitrogen removal) is 12 o C. Figure 2.8 indicates that all measured temperatures were above the design value for nitrogen removal. Figure 2.8. Influent wastewater temperatures from June to September 2010.

13 2.3 Treatment requirements and state of nutrient removal Treatment requirements Technical Audit of the Pomorzany Wastewater Treatment Plant The achieved treatment results from June to September are compared to the EU s Urban Waste Water Treatment Directive of 15 May 1991 (91/271/EEC) concerning plants over P.E. (table 2-8). The achieved results are much under EU-directive requirements and are even fulfilling the HELCOM recommendations except the effluent total phosphorus concentration which was 0.61 mg P/l when the recommendation is 0.5 mg P/l but the reduction was 92.7% so the recommendation concerning the effluent total phosphorus is also fulfilled. However, the analysis result of total phosphorus is not entirely unambiguous, see Figure Table 2.8. Achieved effluent quality compared to EU-directive requirements from June to September Parameter Demand EU-Directive requirements Achieved average from June to September 2010 mg/l mg/l % mg/l % Suspended solids BOD COD Total nitrogen Total phosphorous *Note: annual averages, either concentration or percentage of reduction shall apply Effluent wastewater quality and loads EU s nitrogen removal requirements were reached without difficulties. The average effluent total nitrogen was clearly below the target value of 10 mg N/l and the reduction requirement 70-80% was easily fulfilled as shown in Figures 2-9 and 2-10.

14 11 Figure 2.9. Effluent total nitrogen concentrations from June to September Figure Effluent total nitrogen reductions and concentrations from June to September The on-line measurements in Figure 2.11 show that the average effluent NH 4 -N (ammonia) was 1.2 mg/l during June-September 2010.

15 12 Figure On-line measurements of NH 4 -N from June to September The on-line measurements in Figure 2.12 show that the average effluent NO 3 -N (nitrate) was 6. 5 mg/l during June-September From the the curves can be seen that effluent nitrate concentrations are clearly lower in September. Figure On-line measurements of NO 3 -N from June to September The effluent analysis of ammonia-nitrogen (NH 4 -N) and nitrate (NO 3 -N) during July- September show that the total nitrogen removal results were improving towards the end of September. Figure 2.13.

16 13 Figure Effluent ammonia and nitrate concentrations from July to September The effluent total nitrogen load to the Baltic Sea has been only 68.3% from the design load ton N/a. Figure Figure Effluent nitrogen loads from June to September In accordance with the EU-directive requirements the effluent total phosphorus must be 1.0 mg P/l or reduction 80% on yearly basis. According to HELCOM recommendations, the effluent total phosphorous concentration must be 0.5 mg P/l or reduction must be 90%.

17 The average effluent concentration of total phosphorous in the Pomorzany wastewater treatment plant from June to September was 0.61 mg P/l as shown in Figure Figure Effluent total phosphorus concentrations from June to September The average total phosphorus reduction during June-September was 92.7%. The total phosphorus reductions and concentrations from June to September are shown in Figure Figure Effluent total phosphorus concentrations and reductions from June to September. The online measurements of soluble orthophosphate (PO 4 -P) during June-September 2010 are shown in Figure Very low orthophosphate concentrations were measured in the end of September. The reason was that the dosing of iron sulfate was started to reduce total phosphorus concentration in the effluent under 0.5 mg P/l.

18 15 Figure On-line measurements of PO 4 -P from June to September There is a conflict between the analysis of effluent total phosphorus and online phosphate measurements. By comparing Figure 2.15 and Figure 2.17 we see, that effluent PO 4 -P has been, on average, 0,9 mg/l and at maximum as high as 2 3 mg/l. However, the total phosphorus analysed in the laboratory (which should, by definition, always be at least equal to PO 4 -P and in practice always higher) was on average 0,61 mg/l and at maximum 1,0 mg/l. The reason may be a systematic error in the analysis procedure, measurement errors or insufficient calibration of the online analyser. The effluent total phosphorus load to the Baltic Sea has been only 47.0% from the design load ton P/a (Figure 2.18). Figure Effluent total phosphorus loads from June to September 2010.

19 16 The effluent values (average concentrations and loads) of suspended solids and organic matter analyzed as BOD 5 and COD Cr were clearly below effluent requirements from June to September Process description The schematic diagram of treatment plant is shown in Figure Figure The layout drawing of the Pomorzany WWTP The plant has traditional mechanical pre-treatment including coarse and fine mechanical screens, aerated grit chambers and rectangular primary sedimentation basins. Secondary treatment consists of an activated sludge process of the A 2 O configuration, enabling biological nitrogen and phosphorus removal (BNR). There are three aeration lines, each consisting of a rectangular, completely mixed anaerobic zone, a carrouseltype anoxic zone and two oxidation ditches, in which there are several aerated and nonaerated zones for simultaneous BOD-removal, nitrification and denitrification. There are a total of six round secondary sedimentation basins. Raw sludge is thickened in two round gravity thickeners, and biological excess sludge in four sieve band filters. The sieve band filters need polymer to operate efficiently. After thickening, raw and biological sludge are mixed and pumped to two digesters, which are operated in series. From digestion, the sludge is transferred to a storage tank and dewatered with belt filter presses (4 units). The dewatered sludge is supposed to be treated with thermal drying and finally incinerated. All facilities are located at the WWTP site. However, because of technical problems with the sludge fired burners, incineration of sludge from the WWTPs of Zdroje and Pomorzany is not possible at the moment. Therefore, mechanically dewatered sludge is used for re-cultivation of an ash landfill.

20 Operation Operation practice Operation principles of nutrient removal process The biological wastewater treatment process at the Pomorzany WWTP is realized in three oxidation ditch units. In front of the oxidation stage are anoxic and anaerobic reactors. The biological nutrient removal process is so called A 2 /O process (the Johannesburg process). The pre-anoxic zone is protecting the anaerobic zone from nitrates, because bacteria consuming BOD are using oxygen from nitrate molecule. The oxidation reactor is consisting of oval-shaped channel equipped with diffused aeration systems and mixing devices. The tank configuration and mixing devices promote unidirectional channel flow. The mixing method used creates a velocity ( m/s), which is enough to keep the activated sludge in suspension. At these channel velocities mixed liquor completes a tank circulation in min so that when wastewater leaves the aeration zone there is enough time for the DO concentration to decrease and for denitrification to occur. The oxidation ditch process has a relatively long hydraulic retention time on average 58h varying from 14-77h (normally HRT is 18-36h in oxidation ditches), which makes possible to dilute influent wastewater flow by a channel flow considerably. The design value is 27.5 h. Oxidation ditches have sufficient volume available to accommodate both nitrification and denitrification at lower reaction rates under low DO conditions. In an oxidation ditch DO concentrations are maintained below 0.5 mg/l with automated DO control. The anoxic operating conditions, which make denitrification possible, are monitored using oxidation reduction potential (ORP) control and the intensity of aeration is regulated according to ORP level to guarantee efficient nitrogen removal. Effluent NO 3 -N and NH 4 -N concentrations of less than 3.0 and 1.0, respectively, have been achieved and nitrogen removals greater than 90% are possible with oxidation ditch processes. The oxidation ditch process of the Pomorzany WWTP has not yet reached these figures and the effluent total nitrogen is much higher than these processes are able to produce Aeration and DO control The aeration is realized using two aeration zones in each oxidation ditch. The dissolved oxygen concentration will be highest at the points of aeration zones and will subsequently decrease because of oxygen uptake by the biomass as the mixed liquor moves round the looped reactor. After sufficient travel time, anoxic zones will form upstream from aeration zone. The location and size of these anoxic zones will vary with time because oxygen uptake and transfer rates will vary with wastewater quality and flow. Therefore reliance on this mechanism for denitrification requires a comprehensive

21 control system to monitor dissolved oxygen throughout the tank and control the amount of oxygen transferred to the tank. Also the energy input for mixing and aeration must be carefully controlled to maintain the mixed liquor in suspension. The oxygen input has to match diurnal and seasonal changes in oxygen demand. Otherwise during periods of low loading, necessary anoxic zones will not develop Solids retention time (SRT) and MLSS concentrations The design value for solids retention time is days. The actual values are extremely high. The average SRT is about 100 days varying from 47 days to over 184 days as shown in Figure Normally oxidation ditch type rectors have SRTs from 10 days to 40 days. Activated sludge has been accumulated in the oxidation ditch system because it is not removed and processed fast enough by sludge handling facility. The bottleneck in sludge handling is the incineration process (see Item Error! Reference source not found.). The generated excess sludge is destroyed aerobically in the activated sludge process. The inventory of capacity of the oxidation ditch system will be easily exceeded and excess solids will exit in the secondary clarifier effluent, worsen the treatment results. Especially very high SRTs deteriorate the biological phosphorus removal results because of secondary release of polyphosphates from P-bacteria. 18 Figure Monthly average solids retention times (SRTs) from June to September Biomass solids concentrations or mixed liquor suspended solids (MLSS) concentrations in the oxidation ditches are quite normal. In the treatment line 1 the average concentration is 3.7 kg/m 3 and in the treatment line 2 the average concentration is 3.8 kg/m 3 and in the treatment line 3 the average concentration is 4.0 kg/m 3. The design concentration is 3.2 kg/m 3. Figure 2.21 shows that MLSS concentrations fluctuate mainly between 3 kg/m 3 and 4.5 kg/m 3.

22 With high MLSS concentrations and SRTs (extended aeration), the solids loading rates (SLRs) of the secondary settling tanks are able to increase over the value which uncontrollable affects to the effluent suspended solids concentration and the effluent quality deteriorates. Especially due to fluctuations in wastewater flow rate during peak flows. 19 Figure On-line measurements of MLSS from June to September Return sludge circulation flow rates The design return sludge rate (RAS) per one secondary settling tank (6 units) is 918 m 3 /h or m 3 /d. The total design recirculation rate is m 3 /d or 200% of the influent design flow. The return sludge is pumped to the head of the pre-anoxic zone of each treatment line (3 lines). The pre-anoxic tank plays important role in nutrient removal. On the one hand, pre-denitrification unit removes the rest of nitrates. One the other hand pre-denitrification in combination with anaerobic unit optimizes biological phosphorus removal. The continuous low nitrate concentration will promote the selection of phosphate accumulating bacteria, which accomplish efficient biological phosphorus removal. The average return sludge ratio during June-September 2010 in line 1 was 103% (46-153%), in line 2 was 92% (72-145%) and line 3 was 106% (73-152%). The average of lines 1-3 was 100%, which is low ratio for efficient nutrient removal. Figure The capacity of one return sludge pump (6) is 1000 m 3 /h or m 3 /d. The maximum RAS rate at the design flow is 218%. The internal recycle or nitrate recycle is not needed, because the simultaneous nitrification-denitrification (SNdN) happens when wastewater is cycling through anoxic and aerobic zones in oxidation ditch.

23 20 Figure On-line measurements of RAS ratios from June to September Secondary settling and sludge volume index The settling characteristics of the mixed-liquor solids must be considered when estimating solids separation efficiency of the secondary settling tanks. The settling characteristics of activated sludge are measured using the sludge volume index (SVI). A value of 100 ml/g or under is considered a good settling sludge and values above 150 ml/g are typically associated with filamentous growth. The design value for dimensioning the secondary settling tanks is 150 ml/g. With high MLSS concentrations the test gives significant errors because for example if MLSS concentration is 10 kg/m 3 and sludge did not settle at all the SVI value would be 100 ml/g. The only values of SVIs at the Pomorzany WWTP, which were available, were from the 28 th of July. In the line 1 the SVI was 101 ml/g, in the line 2 the SVI was 82 ml/g and in the line 3 the SVI was 148 ml/g. So the SVI values were varying from about from 100 ml/g to 150 ml/g but because MLSS concentrations were quite normal 4.15 kg/m 3, 3.77 kg/m 3 and 2.69 kg/m 3 the results are reliable. When the design concentration of mixed liquor suspended solids 3.2 kg/m 3 have been reached the SVI measurements will give the right information from the sludge characteristics, settling and thickening rates in the secondary settling tanks Loading of the activated sludge process BOD 5 load In the design calculations the following reductions of BOD 5 (29%), COD (29%), TSS (55.7%), N tot (9.1%) and P tot (11%) were assumed in the primary settling at the

24 Pomorzany WWTP. In practice the average BOD 5 -load reduction in the primaries between July-September 2010 was 42.4%, as is shown in Figure The suspended solids concentration dropped during the same time interval on average from 335 mg/l to 193 mg/l. The design BOD 5 -load after the primaries is kg/d and the actual load after the primaries was kg/d. The loading of the bioprocess is 51.4% of the design loading. 21 Figure BOD 5 -load reduction in primaries and loads to aeration from July to September 2010 before. The two important design and operating parameters for the activated sludge process are the food to microorganism ratio and the volumetric loading rate. The food to microorganism (F/M) ratio or sludge loading rate is defined as a rate of BOD 5 applied per unit of mixed liquor and organic volumetric loading rate is defined as the amount of BOD 5 applied to the aeration tank volume per day. The design value of sludge loading rate at the Pomorzany WWTP is kg BOD 5 /kg MLSS/d and the actual average value was kg BOD 5 /kg MLSS/d and the measured values varied between kg BOD 5 /kg MLSS/d. The design value and the measured values of the food to microorganism (F/M) ratios are shown in the Figure The normal design values of the food to microorganism ratio for the oxidation ditches are from 0.05 to 0.15 kg BOD 5 /kg MLSS/d. The design value of organic volumetric loading rate at the Pomorzany WWTP is kg BOD 5 /m 3 /d and the actual average value was kg BOD 5 /m 3 /d and the measured values varied between kg BOD 5 /m 3 /d. The design value and the measured values of the volumetric loading rates are shown in the Figure The normal design values of the volumetric loading rate for the oxidation ditches are from to kg BOD 5 / m 3 /d.

25 At moment both the food to microorganism (F/M) ratio and the volumetric loading rate are at a very low level and there is no danger of the process being overloaded. 22 Figure Sludge volumetric loading and organic loading rates from July to September Phosphorus balance A total phosphorus load of 660 kg P/d is used in the theoretical design calculations. The total quantity of phosphorus, which leaves with the effluent, is 66 kg P/d. Heterotrophic bacteria, with the BOD 5 -load kg/d is able produce, need 0.01 kg P/kg BOD 5 so the total phosphorus need is 219 kg P/d (3.32 mg P/l as concentration equivalents). The phosphorus uptake of P-bacteria is kg P/ kg BOD 5 so the total phosphorus uptake is kg P/d (4.98 mg P/l). From the phosphorus balance residual total phosphorus can be calculated and it is 46.5 kg P/d. The total quantity of phosphorus, which must be removed chemically, is 46.5 kg P/d or 0.7 mg P/l. The daily need of iron ( ) is kg Fe/d. The iron concentration of PIX-S (113) solution is 0,112 kg Fe/kg. The needed iron solution quantity is kg/d and when the density of iron solution is 1562 kg/m 3 and the needed daily dosage is m 3 /d and dosing points are the effluent channels from aeration before the secondary settling tanks or to the influent of the primary settling tanks. The estimated production of chemical sludge is 311 kg/d. The volume of the storage tank for one month consumption should be about 21.5 m 3. The volume of existing tanks (2 units) is 2x28 m 3, i.e. more than sufficient (Figure 2.25). The chemical assistance of biological phosphorus removal is not used at the Pomorzany WWTP, because at the moment the biological process is able to remove

26 total phosphorus concentration under the requirement (1 mg P/l). The total phosphorous load is 62.1 % of the design load at the moment. 23 Figure Iron sulfate (PIX-S) dosing tanks (V=2 x 28m 3 ) Nitrogen balance A total nitrogen load of 4076 kg N/d is used i the theoretical design calculations. Heterotrophic bacteria need 0.05 kg N/kg BOD 5 to reproduce new bacteria cells. When the BOD 5 -load kg/d is the total nitrogen need 1095 kg N/d (16.59 mg N/l as concentration equivalents). It is also assumed that 50% of this biomass nitrogen is coming back from sludge treatment processes or kg N/d. The effluent ammonia (NH 4 -N) is assumed zero and the effluent organic nitrogen concentration 2.0 mg/l or 132 kg N/d. The total nitrogen to be nitrified is received when from the total nitrogen load 4076 kg N/d coming to the aeration is removed the nitrogen used for biomass production 1095 kg N/d and the effluent total organic nitrogen 132 kg N/d. The recycled ammonia load kg N/d is added to the total nitrogen to be nitrified. Total nitrogen to be nitrified is thus kg N/d. Denitrified total nitrogen is received when from kg N/d is removed nitrates which leave with the effluent or 396 kg NO 3 -N/d or 6 mg NO 3 -N/l. The total nitrogen load to be denitrified is kg N/d. 3 ANALYSIS AND PROPOSED IMPROVEMENTS 3.1 Analysis of plant conditions Technical condition and performance of wastewater treatment process The plant is new and thus in excellent structural and mechanical condition. The machinery and instrumentation are modern.

27 The effluent targets (EU-requirements) have been reached with clear marginal. However, the results of total phosphorus cannot be considered certain due to conflicts between laboratory analysis and online measurements. Enhanced biological phosphorus removal (EBPR) has been enough to reach EUrequirements at least for most of the time. Effluent total phosphorus concentrations have exceeded HELCOM recommendation of 0.5 mg/l. With assistance of iron dosage the total effluent phosphorus can easily be lowered below 0.5 mg/l Design issues The distinguishing features of the oxidation ditch are the race track configuration of the aeration tank channel and the use of a long hydraulic residence time (HRT) and solids retention time (SRT) on the order of 24 hours and 30 days, respectively. For an oxidation ditch system like at Pomorzany WWTP with very long HRTs it is possible to achieve effluent TN concentration of less than 5 mg/l. The average monthly effluent ammonia-nitrogen (NH 4 -N) concentrations of less than 1 mg/l are also possible. No significant operating problems were exhibited by the oxidation process at Pomorzany WWTP. The oxidation ditches are modified at Pomorzany WWTP with an anaerobic basin before the ditch to accomplish EBPR in a combined system. Because of the very high internal recycle within the ditch, very low nitrate concentrations can be in the mixed liquor before settling, which minimizes nitrate in the RAS stream to the anaerobic selector zone, improving EBPR process efficiency. A pre-anoxic zone located before the anaerobic zone further improves process stability and control by protecting the anaerobic zone from nitrates. It also allows for optimizing ammonia and nitrate removal in the oxidation ditch. Some oxidation ditches consistently achieve very low annual average effluent TP concentrations ( mg/l) without chemical addition or effluent filtration. The good phosphorus removal at Pomorzany is the outcome of the low effluent suspended solids concentration in which case the phosphorus in particular form does not leave the process. Normally effluent suspended solids concentrations from oxidation ditches are high and phosphorus leaves the process with suspended solids. The usually high effluent solids concentrations from oxidation ditches are due to the bad settleability of sludge (bulking sludge). This, in turn, is usually caused by filamentous bacteria, which are favoured by gigh circulation rates and the resulting low BOD gradient characteristic traits of the oxidation ditch process. In the case of Pomorzany WWTP, the anaerobic reactor luckily acts as a selector, which suppresses many species of filamentous organisms. However, the notorious Microthrix organisms are not affected by anaerobic conditions, and problems with bulking sludge may thus occur at Pomorzany. However, the plant is equipped with facilities for adding chemicals to kill off the bulking organisms, so the operator can take corrective action before the bulking phenomenon affects treatment results. 24

28 3.1.3 Possibilities and effects of improving phosphorus removal Technical Audit of the Pomorzany Wastewater Treatment Plant The oxidation ditch process in Pomorzany WWTP has a good phosphorus removal capacity. The biological nutrient removal process is able to remove the total phosphorus under 1.0 mg P/l. Wastewater characteristics also favour efficient phosphorus removal, because the average BOD 5 /TP ratio of the influent was about 41 (varying from 22 to 62). About 87% of the values were over 25, which is requirement for the efficient biological phosphorus removal. The Pomorzany plant has no possibility to fermentate the raw sludge and produce the volatile fatty acids (VFAs) to increase the biological phosphorus removal efficiency. However, the plant has facilities for dosing ferric sulphate before secondary settling to improve phosphorus removal. This possibility was used at the end of September 2010 for the first time. The need for ferric sulphate can be estimated by performing the same calculations, which where presented in the phosphorus balance (2.5.3). In order to reduce the present total phosphorus concentration from 0.61 mg P/l under 0.5 mg P/l, about 500 l chemical solution per day or 180 m 3 /a is required. However, if the true total phosphorus concentration in the effluent is found to be higher than 0,61 mg/l (see note considering Figures 2.15 and 2.17 in Item 2.3.2), the needed amount of chemical increases in direct proportion. The information from the preceding data analysis of loadings has been used to create a plant capacity diagram as shown in figure Figure 3.1. This plant capacity diagram highlights possible process bottlenecks and reveals differences between nominal (design) loading parameters and actual loading parameters determined during process auditing. 25 Figure 3.1. Plant capacity diagram.

29 From the diagram some conclusions can immediately be drawn concerning nutrient removal: Effluent phosphorus concentrations of less than 0,5 mg/l are not possible to achieve without chemical precipitation, because the actual total phosphorus load is almost two thirds of the design load Nitrogen load is already quite high compared to the other loadings. Luckily the oxidation ditch process is more effective in the nitrogen removal so that the nitrogen removal is not going to be a problem in the near future. The diagram is useful also to stage and program the future process performance improvements. The annual loading of total phosphorus to the Baltic Sea 2010 has been estimated to be about 11,3 ton P/a. If the total phosphorus concentration in the effluent would be lowered to 0,5 mg P/l, the resulting net reduction would be about 1.3 tons of phosphorus per year. However, if the total phosphorus concentration in the effluent would be 1.0 mg P/l, the net reduction would be 8.6 tons of phosphorus per year. The situation of effluent phosphorus being 1,0 mg/l without precipitation can be considered either as a future scenario (due to increased influent load) or, as the case may be, the real situation of today, referring to the possible errors in analysis of effluent phosphorus Process stability and operational advantages and disadvantages The oxidation ditch systems are excellent for nitrogen removal when the solids retention time (SRT) is in excess of 15 to 20 days. At such SRTs, they provide more flexibility and longer periods of anoxic conditions between aeration points. At shorter SRTs (8 to 12 days), it is more difficult to balance the SNdN in the process to achieve complete ammonia removal as well as a high rate of nitrate reduction. Dissolved oxygen (DO) control maximizes oxidation ditch treatment efficiency. Dissolved oxygen (DO) control provides the anoxic zone within oxidation ditch. The size of the anoxic zone affects the degree of nitrate reduction and overall nitrogen removal efficiency. However, the DO concentration control depends on plant operator s skills and attention. A very skilled operator is able achieve complete nitrification and almost complete denitrification in an oxidation ditch. Using DO control and the knowledge how to vary DO levels in the aeration zone, the operator is able to maintain the sizes of aerobic zone for nitrification and anoxic zone for denitrification optimal to enhance nitrogen removal. Although the nitrification rate is significantly reduced at low DO levels, the long hydraulic residence time (HRT) and long SRT allows complete nitrification. At a DO concentration of 0.1 mg/l, nitrification is about 16% of its maximum rate. Phosphorus removal to well below 1 mg/l also requires that the operator keeps the SRT as short as possible. At longer SRTs (in excess of 15 days), there is more endogenous breakdown of the sludge; thus, less sludge production and the phosphorus content of the sludge must be higher for the same degree of removal. Also, some endogenous release may take place when all the carbon has been removed and the nitrates are less than 2 mg/l.

30 27 The main advantage of the oxidation ditch is the ability to achieve removal performance objectives with low operational requirements and operation and maintenance costs. Some specific advantages of oxidation ditches are: The large basin volumes compared with other secondary treatment processes add more stability, flexibility and reliability in biological wastewater treatment. The need of chemicals is minimized and the effluent is high quality especially in terms of nitrogen. Long hydraulic retention time (HRT) and complete mixing minimize the impact of shock loads (rapid fluctuations influent water conditions) or hydraulic surge. Less excess sludge is produced than in other biological treatment processes, owing to extended biological activity during the activated sludge process Simple automatic operation and optional real time BNR control is easy to realize. Energy efficient operations result in reduced energy costs compared other biological treatment processes. Some specific disadvantages of oxidation ditches are: The process requires more basin volume and a larger footprint than other activated sludge treatment options. This increases investment costs and limitis the feasibility of oxidation ditches especially in densely built areas Effluent suspended solids (TSS) concentrations are relatively high compared to other modifications of the activated sludge process. 3.2 Recommended actions General The contractor s commissioning program, which included startup and performance testing, has helped bring systems at the Pomorzany WWTP to their intended level of design. It is good to remember that initially newly completed plants or processes rarely perform as intended. After commissioning and starting steady state operation the contractor has handed the operation responsibility over to the ZWIK s O&M staff, which responsibility is to operate the plant to the best of their abilities. Operations and data management are the key activities, which guarantee the efficient operation and the good treatment results. A plant operation is clearly a knowledge-based industry. Documenting, communicating and managing operational data is vital for wastewater treatment plant operations efficiency and effectiveness. Operators need to understand how the oxidation ditch process is supposed to work. This is not because they will all be making process control decisions; rather, it is because they need to understand the significance of process fluctuations that they observe. Operators are the eyes and ears of the utility when it comes to process performance. They need to understand the difference between normal variability and circumstances requiring response.

31 The plant should have a set of written rules that govern how process changes are made and by whom. Certain changes can be made by operators directly in response to readings derived from instruments, observations or laboratory tests. Major changes in process variables should be under the control of one person, typically the plant operations manager. At a minimum, process control meetings should be conducted at least weekly to develop target ranges for operator-controllable process variables. These ranges should be developed with the participation of operators. Establish and implement a communications process among the lead shift operators to ensure that operators in each shift consistently manage process. Use trending to track the long term performance of processes. Document not only the process change but also indicate the conditions that prompted the change and the technological rational for the change. This information will be of value when you are considering future changes. Successful operation of the oxidation ditch process requires also good understanding of process performance, which makes adequate collection of water quality and process data necessary. Know what goes into the plant and trend performance parameters! Monitor influent as much as possible. The oxidation ditch influent is more important for the biological nutrient control than plant influent. The phosphorus and nitrate recycle loads can make up a significant fraction of the oxidation ditches influent load and it is therefore important to be monitored. The wastewater composition changes due to primary clarification, addition of recycle streams and possible chemical addition. Collect baseline data that are required to operate and control the plant. Catalog data in format that is easy to review (i.e. use software or spreadsheet that allows user to overlay historical data with variable timeframe between days and years). More data provides a better overall picture of plant operation. Advanced tools and consultation to support process operation and monitoring are strongly recommended. For example, a dynamic process model would be an excellent decision support tool. The conditions for constructing and calibrating a reliable model are good, because the plant is modern and has good instrumentation and laboratory facilities Operation of existing units In the future operation, a better understanding of the mechanisms and kinetic rates for simultaneous nitrification-denitrification (SNdN) is needed to develop improved designs and a more rational operating strategy. The oxidation ditches with pre-anoxic and anaerobic reactors are the most important unit processes for reaching efficient operation and good treatment results. Operational measures must be concentrated on these units. The most important control parameters are sludge retention time (SRT) and the concentrations of dissolved oxygen (DO) in the aeration basins. 28

32 The removal of excess sludge must immediately be developed so that SRT can be controlled at the correct level (10 30 d). If problems with the incinerators cannot be solved on short notice, and/or if there are other bottlenecks, alternative solutions for sludge disposal must be searched and found. Choosing the correct SRT is a balancing act between optimal nitrogen and phosphorus removal. The oxidation ditch systems are excellent for nitrogen removal when the solids retention time (SRT) is in excess of 15 to 20 days. At such SRTs, they provide more flexibility and longer periods of anoxic conditions between aeration points. At shorter SRTs (8 to 12 days), it is more difficult to balance the SNdN in the process to achieve complete ammonia removal as well as a high rate of nitrate reduction. However, phosphorus removal to well below 1 mg/l requires that the operator keeps the SRT as short as possible. At longer SRTs (in excess of 15 days), there is more endogenous breakdown of the sludge; thus, less sludge production and the phosphorus content of the sludge must be higher for the same degree of removal. Also, some endogenous release may take place when all the carbon has been removed and the nitrates are less than 2 mg/l. The DO concentration control depends on plant operator s skills and attention. A very skilled operator is able achieve complete nitrification and almost complete denitrification in an oxidation ditch. Using DO control and the knowledge how to vary DO levels in the aeration zone, the operator is able to maintain the sizes of aerobic zone for nitrification and anoxic zone for denitrification optimal to enhance nitrogen removal. Although the nitrification rate is significantly reduced at low DO levels, the long hydraulic residence time (HRT) and long SRT allows complete nitrification. At a DO concentration of 0.1 mg/l, nitrification is about 16% of its maximum rate. Although sludge bulking and high solids loss in the secondary effluent have not occurred so far at Pomorzany WWTP, they can occur in the future. Activated sludge TSS should be monitored continuously in the aeration basins. Moreover, the VSS/TSS ratio and sludge volume index (SVI) should be observed on a frequent basis, as this parameter may provide a clue to an impending or virtual upset condition. Should indications of bulking sludge be detected, the operator shall prepare to use chemical agents to suppress filamentous organisms, since this possibility exists at the plant. Loading rates for secondary clarifiers should be on the lower end of the recommend range for both hydraulic loading rates and solids loading rates. If SVI is controlled, higher loading rates are possible Enhancement possibilities of phosphorus removal EBPR can be optimised by the following measures: try to achieve as low effluent nitrate concentration as possible by optimising the aeration in the oxidation ditch avoid excessively high return sludge pumping and consequent transport of nitrate to the anaerobic zone keep sludge retention time as low as possible, however taking due care that SRT is still high enough for stable nitrification 29