Wastewater treatment and sewage sludge process
in the Wastewater Treatment Plant of Łódź ,Ltd. (GOŚ ŁAM)
The Wastewater Treatment Plant of the Łódź Agglomeration is located on the south-western border of Łódź. Location on the right bank of the Ner river, at the lowest point of the city allows for the gravity flow almost all wastewater to the treatment plant
Along with the area of limited use, the GOŚ ŁAM covers an area of 366 ha (including 41.3 ha as a basic area) in the rural area of Pabianice, the city Łódź and Konstantynów Łódzki community.
The sewerage network in Łódź, as well as the network in Pabianice and Konstantynów Łódzki, is mainly combined sewers system, which in the case of heavy rainfall and thaw, causes a heavy peak flows into the treatment plant.
At present, the GOŚ ŁAM receives wastewater from Łódź, Pabianice, Konstantynów Łódzki and the community of Nowosolna and Ksawerów and servers about 820 000 population.
The treatment plant capacity is designed for a maximum flow of dry weather 215.300 m3 / d and a load of 1 026 260 PE.
Over the last five years, an average inflow was 195 000 m3/ day, including 170 000 m3/d of dry weather.
The treatment plant is processing about 51% of all the wastewater from the Łódź Voivodship
The wastewater treatment plant is a typical mechanical and biological plant with increased biogenic compounds removal. The biological process is periodically supported by a coagulant and an external carbon source
Wastewater flow part
The wastewater flows by gravity to the treatment plant, through one of the two parallel combined sewers measuring 3,35x2,5 m each.
In front of the inlet chamber a coarse screen is placed with 100 mm slot, protecting the fine screens against large particles.
In the main chamber, the wastewater flow is divided into 1÷4 lines, depending on the volume of the inflow. During dry weather, one line can be run only.
The inlet chamber is covered and odors from the chamber and screens are neutralized on a biological filter.
On each of the four lines, there is a set of two parallel screens. On two lines, there are four hook-bars screens with 6 mm slot and on the remaining two lines, four dynamic disk screens with 5 mm slot, equipped with grinders. After flushing and pressing, the removed screenings are burned or disposed in a landfill.
On each of the four lines, there is an unaerated rectangular grit chamber, equipped with a bottom scraper. The grit separating to the bottom is scraped into two of the scraper separators. Partly washed sand passes through two flushing scrubbers. The flushed sand containing less than 5% of organic matter is disposed in the landfill.
As a final stage of mechanical treatment, after the grit chambers, sewage flows into six rectangular settling tanks (4 000 m3 each) equipped with bottom scrapers. Sludge from the tanks' bottom is scraped to the tanks' hoppers and flows by gravity into the thickeners-fermenters or alternatively into the Main Pumping Station. It is then pumped into the four digesters.
Process is running in rectangular chambers of activated sludge (7 x capacity of approx. 19,900 m3 each). Organic and biogenic compounds (nitrogen, phosphorus) contained in wastewater are decomposed by microorganisms found in activated sludge. The decomposition cycle is a complex biological process dependent on many factors, including the oxygen content, temperature, type of bacteria, quality of incoming wastewater and the applied method of process.
There is a 3-phase process of activated sludge so-called MUCT method, carried out in three successive flow zones:
- anaerobic (KB dephosphatation) - consisting of two chambers, working with full mixing,
- anoxic (KDN denitrification) - consisting of two chambers divided in a ratio of 1: 2.8; the KDN I chamber works with full mixing, and plug flow
- oxygen (KN nitrification) - consisting of two chambers with plug flow, equipped with a fine-bubble aeration system.
The process is monitoring on-line by measuring O2, N-NH4, N-NO3, P-PO4, pH, potential redox and sludge concentration.
Basic design hydraulic parameters of biological lines:
- average daily flow Qdav = 215.300 m3 / d
- average hourly flow Qhav. = 8.971 m3 / h,
- maximum hourly flow Qhmax. = 11.500 m3 / h.
Each biological line has three recirculation circuits, which include:
- external recirculation RZ - activated sludge from secondary settling tanks is recirculated by activated sludge pumping stations to KDN I anoxic chambers or directly to the anaerobic chambers KB I,
- RW I internal recirculation – the denitrified activated sludge along with the wastewater from the KDN I predenitrification chambers is recycled to the KB I anaerobic chambers,
- RW II internal recirculation – the wastewater together with the activated sludge are recirculated from the KN II oxygen chambers to the KDN II anoxic chambers.
The final stage of the flow part are rectangular secondary settling tanks (7 x 2.160 m2), where the treated wastewater is separated from the sludge.
The four tanks are equipped with scrapers, central pipes removing sludge by the siphons. The other three settling tanks are equipped with chain scrapers.
The sludge is discharged into four pumping stations, where part of it is pumping back to the sludge chambers, and some is removed as a surplus sludge.
From the settling tanks the treated wastewater flows into an open chanel and then, after connection with the bypas channel to the Ner river.
The sludge treatment part
The sludge separated during the wastewater treatment process (primary and surplus) is firstly thickened. The primary sludge is thickened in the tanks' hopper and optionally in the gravity thickeners.
The surplus sludge is thickened on belt thickeners (5 units with a capacity of 91.5 m3 / h each), with a help of a polyelectrolyte.
The thickened sludge is stabilized in the process of methane fermentation, under mesophilic conditions (35 ÷ 38 ºC) in closed fermentation tanks (4 x 10 000 m3 each). The sludge retention time is 20 to 26 days.
The biogas produced in the fermentation process is desulfurized in two columns with a capacity of 800 m3 / h each, filled with granules containing a ferric trioxide.
Additionally the biogas is treated on a filter with activated carbon.
Then, the biogas can be:
- burned in cogeneration units (3 x an electric power of 0.933 MW each, and with a heat power of 1,165 MW each) .This is the basic operation,
- burned in oil and gas boilers (3 x with a heat power 1, 4 MW each
- burned as an additional fuel in ITPO,
- stored in a membrane tank with a capacity of 3.000 m3,
- burnt in a flare in an emergency situation.
The stabilized sludge, after degassing in expansion tanks is dewatered on seven belts presses ( 17,5 m3/h each), to a dry mass content of 18 to 22 % with a help of a polyelectrolyte, and its volume is reduced, nearly seven times
The dewatered sludge is directed to the Incineration Plant (ITPO), consisting of two technological lines. The designed capacity of the installation is 84,000 Mg / year (including 2,000 Mg screenings), which assuming the operation of both lines for 8,000 h in a year. It corresponds to a daily capacity of 246 Mg of sludge dewatered to 20% t.d.s. and 6 Mg of screenings. The dewatered sludge, through belt conveyors is fed to silos, followed by screw pumps feeding disc dryers, where total dry solids grows to 30 ÷ 32%. Such sludge is transported by the screw conveyors to the furnaces. After grinding, the screenings are transported by the screw conveyors directly to the furnances, where are neutralized by a combustion in the fluidized bed.
Oil and biogas can be used as an additional fuel .
Flue gas is passing through the heat recovery and cleaning installation.
Heat recovery is going in two stages:
- air combustion heating in a recuperator exchanger
- steam production in the boiler
Flue gas treatment consist of:
- dedusting in the cyclone and subsequent cooling
- chemical removal of acidic components and mercury with a dry method by injection sodium bicarbonate and activated carbon
- final dedusting on bag filters.
Flue gas quality is monitoring on-line by measuring of dust, SO2, Nox, CO, COT, HCL, HF.
Ashes and dust removed from flue gas are disposed by specialized container trucks in the separated landfill.
Because of high content of salt, dust is washed, settled with the help of coagulants and polyelectrolytes on the lamella clarifier, and than dewatered on a frame and plate press.