Top 10 FAQs on Common Sewage Treatment

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1. How to cause colloidal particles to precipitate? To make colloidal particles precipitate, it is necessary to bring them into contact with each other so that they form larger particles; in other words, they aggregate, resulting in a density greater than 1 and thus precipitation. There are many methods available, and the techniques commonly used in engineering include coagulation, flocculation, and coacervation. 2. What is cohesion? Coagulants with positive ions are added to wastewater; the presence of a large number of positive ions between colloidal particles eliminates the electrostatic repulsion between them, thereby causing the particles to aggregate. This process of causing colloidal particles to aggregate through the addition of positive ion electrolytes is known as coagulation. Common coagulants include aluminum sulfate, ferrous sulfate, alum, ferric chloride, etc. 3. What is flocculation? Flocculation involves adding polymer coagulants to wastewater; once these polymers are dissolved, they form polymer complexes. The structure of this polymer is linear; one end of the chain is attached to a tiny particle, and the other end is attached to another tiny particle. It acts as a bonding bridge between these two particles that are far apart from each other, causing the particles to grow larger over time and eventually forming large particle flocs (commonly known as alum flowers), which accelerates the settling of the particles. Common flocculants include polyacrylamide (PAM), polyferric compounds (PE), etc. 4. Why is polyferric sulfate used for flocculation and adsorption pre-treatment of wastewater? Ferric polymers form iron hydroxide flocs during the coagulation process, and these flocs possess a strong ability to adsorb organic substances in wastewater. Experimental data show that after treating wastewater with ferric polymer flocs for adsorption, approximately 10%-20% of the COD in the wastewater can be removed. This helps to reduce the operational burden on biological treatment tanks, thereby facilitating the proper treatment of wastewater before it is discharged. Furthermore, coagulation pretreatment using polyferric salts can remove trace substances in wastewater that are toxic and inhibitory to microorganisms, thereby ensuring the proper functioning of the microorganisms in the biological treatment tank. Among various coagulation agents, polyferric salts are relatively inexpensive (25–300 yuan per ton), resulting in low treatment costs; hence they are suitable for the pretreatment of process wastewater. Polyferric acid is an acidic substance with strong corrosivity; therefore, the treatment equipment must be properly protected against corrosion. 5. What is coagulation? The process of combining coagulation and flocculation is known as coagulation-flocculation. Coagulation is frequently used in experiments and engineering applications: first, chemicals such as ferrous sulfate are added to water to eliminate the electrostatic repulsion between colloidal particles; then polyacrylamide (PAM) is added, causing the particles to grow larger and form flocs visible to the naked eye, which eventually settle. 6. What is adsorption? Adsorption treatment involves using porous solids (such as activated carbon) or floc materials (such as polyferric compounds) to adsorb toxic and harmful substances in wastewater onto the surface or within the micropores of these solids or flocs, thereby purifying the water quality. The substances to be adsorbed can be either insoluble solid substances or soluble substances. Adsorption treatment has high efficiency and produces water of good quality, which is why it is often used for the advanced treatment of wastewater. Adsorption treatment can also be introduced into the biochemical treatment unit to improve its efficiency (the PACT method is one such example). 7. What is the iron-carbon treatment method? The iron-carbon treatment method, also known as iron-carbon microelectrolysis or iron-carbon internal electrolysis, is a form of technology for treating wastewater using metallic iron. It exhibits unique effectiveness when employed as a pretreatment technique for dealing with toxic and harmful wastewater with high COD levels. The treatment mechanism of the iron-carbon method is not yet fully understood. One widely accepted explanation is that under acidic conditions, numerous micro-current reaction cells are formed between iron and carbon, and organic substances are reduced or oxidized under the action of these micro-currents. The water exiting the iron-carbon process is then neutralized with lime or lime slurry; the resulting Fe(OH)2 colloidal flocs possess a strong flocculating and adsorbing capacity for organic substances. Therefore, the iron-carbon method makes comprehensive use of the reducing properties of iron, the electrochemical properties of iron-carbon, and the flocculation and adsorption effects of iron ions; it is precisely the combined action of these three properties that enables the iron-carbon method to achieve excellent treatment results. The disadvantages of the iron-carbon method are: (1) After being immersed in an acidic medium for a long time, iron shavings tend to agglomerate into lumps, causing blockages and channeling flow, which makes operation difficult and reduces the efficiency of treatment ； (2) More iron is dissolved under acidic conditions, resulting in a larger amount of sludge upon neutralization with alkali. 8. Why is lime powder used for neutralization treatment of the effluent from iron-carbon processes? After the wastewater, whose pH has been adjusted to 2 using sulfuric acid, is treated with iron and carbon, sulfuric acid is converted into ferrous sulfate, and the pH of the wastewater rises from 2 to 5–6. Then why is lime powder still used for neutralizing the effluent resulting from this iron-carbon treatment? Or could we use less lime powder during the neutralization process? The effluent from the iron-carbon process contains large amounts of ferrous sulfate; if this is not removed, it will affect the growth and reproduction of microorganisms in the subsequent biological treatment tanks. Therefore, we must use lime to raise the pH value of the wastewater from 5-6 to above 9, thereby converting the water-soluble ferrous sulfate into insoluble ferrous hydroxide and calcium sulfate. These substances are then precipitated through coagulation and sedimentation, ensuring that the wastewater entering the biological treatment tanks does not contain ferrous sulfate. Is it possible to add less lime powder during neutralization? We can conduct a comparative experiment in the laboratory. The same amount of iron-carbon influent (with a pH of around 2) and iron-carbon effluent (with a pH of 5–6) were placed into two beakers respectively. Then, lime powder was added in measured amounts to each beaker for neutralization and coagulation. When the pH values of the wastewater in both beakers were adjusted to 9, it was found that the amounts of lime powder added to the two beakers were identical. This is because iron is not a neutralizing agent; ferrous sulfate, which is formed from sulfuric acid, remains an acidic substance. Lime powder is essential for the conversion of ferrous sulfate into ferrous hydroxide and calcium sulfate during the neutralization process. Therefore, it is imperative to add lime powder when neutralizing iron-carbon effluent. 9. How to estimate the generation amount of chemical sludge? Sludge generated through chemical reactions (such as neutralization) and physicochemical treatments (such as coagulation with chemicals) is commonly referred to as chemical sludge. The sludge formed after neutralization and coagulation treatment of the iron-carbon effluent is mainly composed of ferrous hydroxide and calcium sulfate. The amount of sludge generated can be calculated based on the quantities of sulfuric acid and lime powder added. In engineering, experience can also be used for estimation. Generally speaking, if the pH of the iron-carbon treated influent is around 2, the amount of chemical sludge produced per ton of wastewater after neutralization and coagulation is approximately 50 kg (with a moisture content of 80%). 10. What is the biochemical treatment of wastewater? The biochemical treatment of wastewater is one of the most important processes in wastewater treatment systems; it is commonly referred to as biochemical treatment. Biochemical treatment utilizes the life processes of microorganisms to effectively remove soluble organic substances as well as some insoluble organic substances from wastewater, thereby purifying the water. In fact, we are not unfamiliar with biochemical treatment. In natural water bodies, there exists a food chain: large fish eat smaller fish, smaller fish eat shrimp, shrimp eat tiny insects, those insects eat microorganisms, and the microorganisms consume sewage. Without this food chain, nature would fall into chaos. In natural rivers, there are numerous microorganisms that rely on organic matter for survival. These microorganisms oxidize or reduce the organic substances that people discharge into the rivers day and night – such as industrial wastewater, pesticides, fertilizers, feces, and other organic materials – turning them into inorganic substances. Without these microorganisms, the rivers around us would become foul-smelling within a few months at the earliest, or even one to two years at the latest. It’s just that these microorganisms are so tiny and scattered that they cannot be seen with the naked eye. The biochemical treatment of wastewater is an enhancement of this process under artificial conditions. People concentrate countless microorganisms in one tank, creating an environment highly suitable for their reproduction and growth (such as temperature, pH level, oxygen, and nutrients like nitrogen and phosphorus), thereby allowing the microorganisms to multiply in large numbers and increasing the speed and efficiency with which they can break down organic matter. Then, wastewater is pumped into the tank, where the organic substances in it are oxidized and degraded as a result of the metabolic activities of microorganisms, thereby purifying and treating the wastewater. Compared with other treatment methods, the biochemical method features low energy consumption, no need for chemicals, good treatment efficiency, and low treatment costs.