Javno podjetje Tel: +386 (0)1 5808 100 V O D O V O D Vodovod-Kanalizacija d.o.o. Fax: +386 (0)1 5808 403 K A N A L I Z A C I J A Vodovodna cesta 90, p.p.3233 E- pošta: voka@vo-ka.si SI-1001 Ljubljana Internet: www.vo-ka.si Mission of the Central WWTP Ljubljana - the release of treated wastewater back into nature and on-site preparation of the excess sewage sludge for its final utilization Vesna Mislej JP VODOVOD-KANALIZACIJA d.o.o. ZAG, 4.10.2016
The public company JP VODOVOD – KANALIZACIJA d.o.o. is legally held responsible for the collection and the treatment of domestic as well as rainwater in the Municipality of Ljubljana and in the six municipalities surrounding it. PUMPING STATION According to the data of the Operative program of collection and treatment of the municipal waste waters in the City Municipality of Ljubljana, the achieved connectivity to the sewage system within the concerned agglomeration in the year 2014 was 93,4%.
• The central sewage system of the capital city Ljubljana is mostly constructed in a mixed sewer system. Public sewer system JP VODOVOD-KANALIZACIJA d.o.o. (31. 12. 2014) Parameter Channel length Public sewer system 1.172 km - mixed sewer system 477 km - sewer system for municipal wastewater 363 km - sewer system for rain wastewater 329 km Capacity of WWTP Ljubljana 360,000 PE Number of Pumping stations 45 19,066,286 m 3 The annual quantity of municipal wastewater (2014)
• The location of the CWWTP is between the Sava River and the Ljubljanica River. • Ljubljanica is the right affluent of the Sava River, and the confluence is located approximately 1 km downstream from the CWWTP Ljubljana. • Sava is the longest river in Slovenia, more than half of the Slovenian territory belongs to its water catchment/river basin .
• The majority of generated wastewater, approximately 60%, is of urban origin, the part of industrial is about 10 %, the rest being water collected from precipitations or waste storm water. All wastewater is collected in the central sewage system flow to the Central treatment plant of Ljubljana (CWWTPL).
The existing inflow of the waste water to the CWWTPL and the quantity of precipitations 2010 2011 2012 2013 2014 Average annual inflow to the CWWTPL 90,912 67,883 78,557 82,347 82,998 (m³/d) Annual quantity of 1799 1000 1339 1531 1851 precipitations (mm)
The existing CWWTPL was designed for the capacity of 360,000 PE. The CWWTP Ljubljana is a single-stage mechanic-biological treatment plant, intended for the selection of the suspended solids, carbon compounds, and for nitrification.
1200 to 1400 t screenings per year MECHANICAL TREATMENT
MECHANICAL TREATMENT – GRIT TRAP
MECHANICAL TREATMENT – PUMPING STATION and BAR SCREEN
MECHANICAL TREATMENT – FINE SCREEN
MECHANICAL TREATMENT – AERATED TANK FOR SAND, FAT AND OIL REMOVAL
BIOLOGICAL TREATMENT – Aeration basin 39,034 m 3 2,750 m 3 The mechanically treated wastewater flows to the biological stage - into the aeration tanks, where the biological treatment is performed with the micro- organisms . For their activity the micro-organisms need oxygen, that is, air. The compressed air, produced by the blowers, is supplied for this purpose into the aeration basins with the help of membrane diffusor.
Sewage sludge, which provide the biological treatment of wastewater, is activate sludge. It is a mixture of bacteria and of Protozoa. The main part of sewage sludge consist of bacteria, which also provide the biological treatment of wastewater. Protozoa are defined as single-celled organisms such as ciliates, amoebae and flagellates and they are mainly microscopic organisms, ranging in size from 10 to 52 micrometers.
BIOLOGICAL TREATMENT – Settlement tank 25,200 m 3 2,750 m 3 The mixture of the treated water and the active sludge from the aeration basins flows out into the settlement tank, where the water is separated from the sludge. The treated water is then collected by the outflow channel into the water course - the Ljubljanica River. The sedimented sludge is partly returned into the aeration basin for the maintenance of the required concentration of the active sludge in the treatment process - the excess, surplus sludge is collected into the process of sludge treatment - sludge line.
BIOLOGICAL TREATMENT – LOAD IN VIEW OF BOD 5
BIOLOGICAL TREATMENT – TREATMENT EFFICIENCY The organic load of input wastewater is easily biologically degraded and satisfactory treatment efficiency is achieved. Review of the existing flow rates and loads at the CWWTPL does not state significant differences in the operation of the treatment plant. Operation indicators of the CWWTPL Parameter Unit 2012 2013 2014 Annual quantity of treated 1000 28,018 29,568 29,991 m 3 /year waste water m 3 /d Medium daily flow 76,500 81,596 78,933 Effect of COD % % 95.3 94.8 94.16 Effect of BOD5 % 98.2 97.7 97.73 Ptot effect % 58.7 50.0 47.78 54.42 Ntot effect % 63.1 59.1 Average conc. Inflow NH4-N mg/l 28.8 27.5 26.93 Average conc. Inflow COD mg/l 670 596 499 Average conc. Inflow BOD5 mg/l 362 315 271 Aver. conc. – inflow Ptot mg/l 10.1 8.4 7.5
BIOLOGICAL TREATMENT – TREATMENT EFFICIENCY The organic load of input wastewater is easily biologically degraded and satisfactory treatment efficiency is achieved. Review of the existing flow rates and loads at the CWWTPL does not state significant differences in the operation of the treatment plant. Operation indicators of the CWWTPL Load of Removed Released wastewater amount at WWTP amount into Parameter Unit (2015) Ljubljana (2015) Ljubljanica (2015) t/year t/year t/year COD t O 2 /L 16.048,9 15.119,102 929,81 BOD t O 2 /L 8.957,6 8.744,8 212,8 N tot t N/L 1.138,72 602,9 535,8 P tot t P/L 221,9 111,2 110,7
The surplus sludge discharge is a very dynamic process. It is discharged from the biological system daily, depending on the desired SRT (7 to 10 days) and the desired concentration of activate biomass (2.5 gL -1 in Sommer – 3.7 gL -1 in Winter) according to the seasonal temperature fluctuations of inlet raw wastewater (22 o C in Sommer - 12 o C in Winter).
Sewage sludge treatment • The procedure of preparing the biomass, before submitting to the final treatment of waste, includes: • gravitational and mechanical pre-thickening of the aerobic biomass with the addition of cationic polymer up to 6% dryness, • transformation of aerobic biomass to anaerobic form at the digestor by stabilisation at mesophilic conditions, • dehydration of digestate with the centrifuge and addition of cationic polyelectrolyte with consumption rate of 9.0 kgton -1 digestate (DS) to achieve the dry matter of 21 - 23 %.
• Afterwards, it is transported to the mixer, where it is mixed with the final granulates. This mixture reaches a moisture content of 57 – 64 % dry matter. In this rheological form it is suitable to be taken for thermal treatment in a convectional rotating drying drum. • At the inlet that leads into the drying chamber the off gas from the burner reaches temperature of 480 - 510 o C but it immediately drops down between 90 - 93 o C as soon as it comes in contact with wet sludge mixture. Both these temperatures are important “sets” technological parameters to perform a stable drying procedure and to keep a pellets diameter constant.
Monitoring of the drying process The control of important key parameters of the drying process Parameter Unit 2009 2010 2011 Mass flow, drying drum (drum working hours, h d , project capacity kg DS/h d 860 686 680 1050 kg/h) Drying hours, h b , for tons DS of pellets h b /ton DS 1.25 1.45 1.39 Drying drum, inlet - temperature, o C 486.1 ± 14.4 501.1 ± 10.9 499.4 ± 6.1 direct contact Evaporation rate kg H 2 O/h 2,485 ± 145 2,369 ± 160 2,770 ± 273 (project capacity 3000 kg H 2 O/h) Drying drum, outlet exhaust gases - o C 92.9 ± 0.6 94.4 ± 1.1 93.8 ± 0.5 temperature 91.9 ± 0.8 91.6 ± 0.5 91.4 ± 0.5 Pellets - dry solid content % In order to obtain predictable results, the process must be precisely performed.
The final product from the treatment of the sewage sludge is a • stabilised, as well as hygienised, non-hazardous waste, coming in the shape of pellets. They are 2 mm - 4 mm in size and have a bulk density of 650 kgm -3 to 700 kgm -3 . This makes it possible for the final product derived from the process to be easily and safely handled as well as transported. Pellets have a shape that is suitable to be marketed. From 2008 onwards, the pellets have been used as an additional alternative solid recovered fuel (SRF) in the Slovenian cement kiln.
At WWTPL, extensive analys es of the pelletised biomass have been carried out in order to obtain the necessary information about its composition , characteristics and a statistically supported specification such as SRF. A classification category of waste as a solid alternative fuel has been established according to the standard CEN/TS 15359: NCV 4; Cl 1; Hg 4
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