All wastewater of the population, institutions, crafts and industry, as well as the storm water of the city of Subotica, is collected in the combined sewerage system and reaches the Wastewater Treatment Plant through the main collector line.
The plant consists of two lines:
- Water line – mechanical and biological wastewater treatment
- Sludge line – anaerobic mesophilic stabilization of sludge (generated during biological wastewater treatment) in digesters, along with the production of biogas and dewatering of the generated digested sludge
The process of wastewater treatment – “at hand” in simple terms
The “mouth” of the treatment plant represents the canal at the very entrance to the plant. It is the lowest gravitational point in the city where all wastewater and rainwater meet. There is a rough grid with a density of 5 cm, which serves as the “ramp” for the large waste. This grid prevents the large waste to pass, which is then disposed of in containers.
The “teeth” of the treatment plant are located at the highest point to which the wastewater comes through screw pumps/water screws. The wastewater passes here through fine grids with a density of 6 millimetres that separates small waste, and then it goes to the sand trap that separates the sand and which goes down from the surfaces and enters the sewer due to heavy rains and extreme precipitation.
The “comb” of the treatment plant is the previous sedimentation tank – ring-cone shaped – which pushes the organic matter from the wastewater to the conical bottom by making a full circle through the bridge at a “speed” of one hour. Everything that falls to the bottom, which is called settled sludge, pushes the bridge into the middle, from where it is transported for further treatment, to the sludge line. What is left is water with dissolved substances, and it goes to biological tanks.
Biological tanks are the “heart” of the treatment plant. They set special conditions suitable for the reproduction of microorganisms, which have the task of “eating” nitrogen, phosphorus and other substances, and thus treat/clear the water. Furthermore, the tanks consist of three parts, and in each of them, the water should be kept for a strictly specific time so that the sludge and substances could be separated the right way.
The water from the biological tanks passes through the last sedimentation tank, and then it flows through the exit channel into the first sector of Lake Palić as a completely clear liquid.
The sludge, separated from the water during the treatment process, is divided into two parts, one of which is used to multiply microorganisms in biological tanks, and the other one is directed to the so-called “big digester”, i.e. digesters in which it decomposes and rots.
During this process, biogas is extracted from the sludge, which is then used to obtain the electrical and thermal energy (“green energy”), used for the operation of the plant, and heating the digester.
What happens to the excess sludge resulting from wastewater?
The excess sludge, which is not used for internal needs, is dewatered on a belt filter press and transported as a by-product to the regional landfill Bikovo, where it is used for composting and phytoremediation.
To efficiently and safely transport the sludge from the wastewater treatment process from the wastewater treatment plant – Subotica City Treatment Plant to the Bikovo regional landfill (the sludge used to be delivered to the local landfill in Subotica in the previous period), a new IVECO truck of the larger capacity with a suitable container was provided from the funds of the project “Biodiversity and water protection lake Palić and lake Ludaš ”.
What are the effects of this investment? The capacity of the current container is 14 cubic meters/tons. Three rounds of 10 or 30 tons of sludge are transported to the 30 km distanced landfill daily. If it were not for this investment, the sludge would have been transported in containers with a volume of 5 cubic meters/tons to the landfill at least six times a day with obsolete trucks, which, by the way, were also breaking down. It is estimated that about 10,000 tons of sludge from the Subotica wastewater treatment plant will be transported to the regional landfill by the end of 2021.
Sludge is not a waste – it is a resource! Compost
The regional landfill of Subotica uses special composting technology to produce compost. The compost material contains anaerobically stabilized sludge, which is transported from the Subotica Wastewater Treatment Plant according to a certain schedule, as well as the biomass (green waste) collected within the regional waste management system.
As stated in the records, about 6.8 thousand tons of stabilized sludge was delivered from the city wastewater treatment plant in 2020, while exactly 6,664.17 tons were recorded by the end of July 2021. It means that, according to the estimates, there will be 10,000 tons of this by-product residing from the wastewater treatment process by the end of the year.
A modern control system (SCADA) is used in the compost material obtaining process. The process of adding oxygen and certain parameters are constantly monitored through sensors. The composting cycle lasts 41-42 days at a temperature of about 70 C.
After industrial processing, the compost material has its own recycling course and reuse.
Sludge is not a waste – it is a resource! Phytoremediant
The obtained stabilized sludge is mostly composted, but the smaller part of it is used during phytoremediation for feeding energy plants: energy willow and “arunda” (Italian cane). These plants are also used as their own biomass reserves to ensure a continuous composting process. Naturally, along with the positive effect in terms of agroecology, energy plants will also provide a basis for the possibility of an alternative energy supply.
Energy willow and Arunda (Italian cane)