Wastewater treatment
Implementation of efficient wastewater treatment systems is one of the most crucial steps in preserving the environment.
Wastewater treatment is the process of removing pollutants and contaminating substances from wastewater generated in urban and industrial areas as a result of human activities. Wastewater contains mineral, organic, plant, animal, artificial, and biological substances.
The wastewater treatment process:
- Preliminary Treatment: In this stage, wastewater passes through screens that remove large solid particles such as branches and plastics.
- Sedimentation: Wastewater enters large reservoirs where solid particles settle down. The formed sludge is separated from the liquid, which proceeds for further treatment.
- Biological Treatment: This stage involves the use of microorganisms (bacteria and fungi) that decompose organic matter in wastewater at the biological level.
- Chemical Treatment: Chemical reagents are used to remove various pollutants such as phosphates and heavy metals.
- Disinfection: In the final stage, wastewater undergoes disinfection, which eliminates bacteria and viruses that might remain after previous treatment processes.
When characterizing wastewater, the following terms and concepts are used:
- Suspended Solids: These are the contents of solids that can be settled and measured. A conical container called the Imhoff cone with a height of 40 cm is used to determine their quantity. 1 dm3 of wastewater is poured into the cone, and the height of the settled solids is measured after 2 hours of settling.
- BOD (Biochemical Oxygen Demand): BOD5 is the biochemical oxygen consumption by bacteria present in wastewater over a 5-day incubation period at 20°C. Before determining BOD5, the initial dissolved oxygen content (8-9 mg O2/dm3) of the sample is measured. Then, after a 5-day incubation at 20°C, the dissolved oxygen content of the sample is measured again, and the difference between the two measurements gives the BOD5 value.
- COD (Chemical Oxygen Demand): COD is the chemical requirement for oxygen, which is determined by the oxidation reaction of a strong oxidizing agent with organic and inorganic substances in wastewater. Potassium permanganate (KMnO4) or potassium dichromate (K2Cr2O7) is used as a strong oxidant.
Principle scheme of material flows during biological wastewater treatment:
The scheme involves mechanical and single-stage biological treatment of wastewater received from the inlet well 1. Coarse mechanical impurities are separated on the screen 2, which is made of steel strips 8×50 mm with a distance of 12…16 mm between them. The screen is installed at a 60° angle to the flow direction of the wastewater. The impurities are collected in the collector 14, while the wastewater goes into the grit chamber 3 and the primary settling tank 4 for the removal of sand and coarse organic impurities like malt sprouts and grain particles, etc.
The grit chamber is a cylinder with tangential entry of wastewater. The sediment from the grit chamber is periodically removed to the drying bed 15.
In the first sedimentation tank, consisting of four parallel working settling channels, the wastewater is partially cleared of suspended impurities. To prevent the growth of filamentous bacteria and the consequent bulking of activated sludge, the flow is directed to the pre-aerator 5, where excess activated sludge from the secondary settling tank 9 is returned, and the wastewater is aerated for 20 minutes with air from the blower station 13. During this process, flocculation and adsorption of fine dispersed impurities occur, which are settled in the settling tank 6.
The pre-aerator is a rectangular-shaped tank equipped with tubular diffusers. At this stage of wastewater treatment, the air consumption is 0.5…1 m3/m3, the activated sludge concentration is 20 g/L, and the duration of pre-aeration is 30 minutes. After processing in the pre-aerator, the content of suspended substances in wastewater is reduced by 30…40%, and BOD5 by 20…25%.
The sludge from settling tanks 4 and 6 is periodically removed to the sludge drying bed 16. The clarified liquid goes to the buffer tank 7, which stabilizes the flow before sending it to the aeration tank 8.
In the aeration tank, wastewater undergoes biological treatment with activated sludge. These tanks have a rectangular shape and consist of two parallel working sections, with a total capacity of 280 m3. Aeration is provided through tubular diffusers with a diameter of 100 mm, using air from the blower station 13. The air consumption per 1 m3 of wastewater is 22…26 m3/h, and the concentration of activated sludge is maintained at 3…3.5 g/L. The biological treatment efficiency is 96% for suspended substances and 95% for BOD5.
The treated effluent goes to the secondary settling tank 9, designed to hold the liquid for 2.5 hours. The activated sludge from the sludge chambers of the tank is removed using a scum lift and returned to the pre-aerator and aeration tank.
Next, the water is mixed with chlorine water in the mixer 10, which is prepared in chlorinators 12, and directed to the contact tank 11. Here, the water is disinfected with chlorine. After a 30-minute retention time in this tank, the treated water is discharged into the river.
Wastewater treatment is a critical process for any production facility to prevent environmental pollution and ensure human health.
