The incoming raw wastewater from Kelenföld and Ferencváros first flows through fine grids of rods, 3 mm apart. The purpose of this equipment is to filter out small pieces of waste material, floating contaminants and solid particles. Removed grid trash is forwarded by conveyor screws; two units compact them and then they are placed into 10 m3 containers inside the building.
It is a combined hydraulic structure, which includes a fat and sand trap and a lamellar pre-settler, out of which 8 units are located in the plant. The advantage of this solution is its smaller space requirement, without reducing the effectiveness of removal.
Sand removal is also very important for the preservation of the engineering structures, since if sand gets into the further parts of the technology, it can cause a blockage or wear to the pumps and other mechanical equipment. This removal process occurs due to the greater specific weight of the sand in the first section of the SEDIPAC 3D. The sand removed by the pumps – approximately 7 m3 per day – is stored in watertight 10 m3 containers until it is transported.
Fat removal is carried out in the next section of the structure. The aim of this process is to protect the biological works in the equipment, and to prevent scum from floating up in the aeration reservoir. By blowing in air bubbles, the organic particles of low specific weight can be floated up to the water surface. An automated separator delivers the scum into the scum sump (one belongs to each structure), and before compression a pump lifts it into the fat collector container. After phase separation, the fat is collected in containers; this accounts for approximately 8.75 m3 per day.
Thanks to the built-in slides, the settling surface increases in the pre-settlers, and sludge particles that can be settled without chemicals can be held back. Raw sludge collected from the bottom of the reservoirs is delivered by pumps to the gravity thickener.
Bypass of the SEDIPAC 3D structures occurs when wastewater with a flow rate of more than 26,250 m3/h arrives, because this is the maximum hydraulic capacity of the pre-settler structures. A water amount between 26,250 m3/h and 37,500 m3/h is treated by three sand traps before it reaches the Danube.
The aim of biological treatment is to remove organic matter, nitrogen, phosphorous forms and remaining suspended solids from the water. These processes are performed on 18 (15 units + 3 backups) parallel connected structure lines.
Each structure line includes the following:
Due to the carousel type aeration basins being compact, the anaerobic basins could be placed under the post-settlers. The compact design made it possible to reduce the dimensions of the structure, and thanks to this, we can save space and operating costs, as we need to treat less contaminated air. In order to avoid odours, all biological structures are covered.
Removal of the organic material occurs through aerobic biological adsorption in the aeration basins. The fine bubble air intake is carried out by diffusers placed on the bottom of the basins providing an optimal level of dissolved oxygen to the microorganisms.
The first step of the nitrogen removal (after ammonification, which usually takes place in the sewage system) is aerobic nitrification and the subsequent denitrification under non-aerated conditions. These processes take place in the carousel basins – where aerated and non-aerated periods alternate – and, using internal recirculation, in the pre-anoxic zone.
The biological removal of phosphorous takes place in the anaerobic and aerobic basins. This is complemented by simultaneous chemical precipitation. This iron (III) chloride addition can be carried out in two points of the plant: either in the degassing pits of the aeration basins or for the precipitation of the released phosphorous during the digestion in the digester.
In the pre-denitrification and the anaerobic zones, mixing takes place with high-speed submersible stirrers, and low-speed banana-type flowmakers ensure the movement and mixing of the water.
Aeration basins are followed by degassing basins. Their purpose is the removal of the fine air bubbles, which are adhering to the sludge flakes after aeration, through natural flotation. That way, we can prevent sludge flocs from floating up to the surface in the post-settlers.
Removing sludge from the post-settlers located above the anaerobe basins is ensured using excavator bridges equipped with suction excavators. These suck up the sludge settled on the bottom of the basins, a portion of which returns to the biological reservoirs to maintain constant biomass concentration and another portion will be taken away. The so-called surplus sludge which was taken away will go to the thickening tables as a next step.
After this settling step, it is possible to start chlorination if the competent authorities lay down specific hygiene requirements. On each biological line a 1,000 m3 basin is available under the post-settlers; this provides sufficient reaction time, even in the event of hydraulic overload of the basins, which is in case of an incoming yield with a flow rate of over 26,500 m3/h.
Automatic sampling equipment ensures continuous control of the cleaned water quality before it is released into the Danube.