Waste water from the primary clarifier flows into four sequencing batch biological reactors. Here, aerobic biological treatment takes place. This process utilizes dissolved Oxygen delivered by the aeration system.
The SBR basins operate in three stages. The first stage comprises filling and aeration. The second stage involves separating the activated sludge from the treated water. In the third and final stage, decanting of the treated water and drainage through the outflow takes place. Waste water flows simultaneously into the individual SBR basins under a time delay. Normally, the SBR basins operate in four-hour cycles, however, during the rainy season, when the quantity of waste water increases due to the mixed sewerage system, reactors operate in three-hour cycles.
In the first stage, waste water enters a partially filled Sequencing Batch Reactor Basin, containing already acclimated biomass. Blowers blow air through the floor aerators, which assures there is a sufficient amount of oxygen. This biomass, known as activated sludge, needs this oxygen to decompose substances in the waste water, which in turn purifies the waste water. Microorganisms use these waste organic substances, expressed as biochemical oxygen demand together with inorganic substances like nitrogen and phosphorus to grow and reproduce. Microbial decomposition of organic and inorganic substances produces, activated sludge, carbon dioxide, elementary nitrogen, water and other substances, which are less harmful to the environment in comparison to the waste water.
HOW THE BIOLOGICAL TREATMENT TAKES PLACE
Within the advanced sequenced batch reactor, the biological decomposition of organic matter and the partial biological removal of phosphorus as well as the decomposition of nitrogen, through the process of nitrification and denitrification, take place simultaneously.
In the anaerobic selector at the intake of the SBR basin, waste water combines with the return flow from the activated sludge process. The selector ensures the partial biological removal of phosphorus and prevents the growth of anaerobic filamentous bacteria. In the selector, during a short retention time in the absence of oxygen, phosphate accumulating bacteria release phosphorus from their cells, while at the same time, accumulating large quantities of organic carbon. In the aerated part of the basin, they receive the large quantities of phosphorus released in the selector. If the value of phosphorus is above one milligram per litre, iron chloride will be added to the waste water to precipitate out the excess phosphorus.
In the aerated part of the basin, the removal of nitrogen takes place through the process of nitrification and denitrification. During nitrification, ammonium-nitrogen oxidizes into nitrate in the presence of oxygen. The process of denitrification follows, which takes place in the absence of oxygen. Microorganisms in the activated sludge then convert the nitrate-nitrogen to gaseous nitrogen.
Once the filling and aeration phases are complete, the water and sludge separation phase commences. When aeration stops, the activated sludge settles to the bottom where pumps pump the sludge into the thickeners. The third phase involves decanting the treated water prior to its discharge into the Kamniška Bistrica river. At the outflow, online monitors continually measure the flow measurements and quality of the treated effluent.
SURPLUS SLUDGE PATH
Surplus secondary sludge from the SBR basins enters the sludge thickeners, where the sludge undergoes mechanical thickening with the aid of a polyelectrolyte coagulant. Pumps pump the mechanically thickened sludge through ultrasonic disintegrators into the digesters. Ultrasonic energy breaks down the microbial cell structures, which in turn improves sludge digestion, reduces the amount of organic matter and increases the production of biogas.
LIQUID WASTE RECEPTION
Located at the entrance of the plant, are facilities for receiving septic tank sludge from small wastewater treatment plants and for receiving industrial waste water, which arrives by tanker truck. Here, sieves remove the coarse particles from the waste water. The septic tank sludge is then pumped into the digesters while the industrial waste water is pumped into the grease and sand trap.
Next to the primary clarifier, there is a facility for receiving the liquid biologically degradable waste. The liquid waste passes through a sieve and a crusher before entering the hygienisation tank were heaters heat the liquid waste at seventy degrees Celsius for one hour before entering into the digesters.
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