Q&A (Part 1)

Water self-purification: The process by which polluted rivers, through physical, chemical, and biological processes, reduce or transform pollutant concentrations, restoring the water body to its original state or reducing it from exceeding water quality standards to meeting them.

Basic methods of wastewater treatment: Employing various methods and technologies to separate and remove pollutants from wastewater, recycle them, or transform them into harmless substances, thus purifying the wastewater. This generally includes water supply treatment and wastewater treatment.

Modern wastewater treatment technologies can be divided into physical, chemical, and biological treatment methods according to their working principles.

Biochemical Oxygen Demand (BOD): The amount of oxygen required to degrade organic matter through microbial action under aerobic conditions. It is a comprehensive indicator of organic pollution in wastewater.

Theoretical Oxygen Demand (ThOD): The theoretical oxygen demand of a specific organic substance in water. This usually refers to the theoretical value of oxygen required to completely oxidize the carbon and hydrogen elements in the organic matter into carbon dioxide and water (i.e., the oxygen demand calculated according to the complete oxidation reaction formula).

Total Oxygen Demand (TOD): The amount of oxidizable substances in water, mainly the oxygen required for organic substances to become stable oxides during combustion, expressed as mg/L of O2.

Chemical Oxygen Demand (COD): Measured chemically, it represents the amount of reductive substances in a water sample that need to be oxidized. In wastewater, wastewater treatment plant effluent, and polluted water, it represents the oxygen equivalent of substances (generally organic matter) that can be oxidized by strong oxidants.

Total Organic Carbon (TOC): The total amount of carbon in dissolved and suspended organic matter in a water body.

5. Under what circumstances is biochemical treatment used?

It is generally considered that only wastewater with a BOD/COD value greater than 0.3 is suitable for biochemical treatment.

Physical indicators of potable water hygiene standards: color, turbidity, odor, and taste.

Eutrophication of water bodies is a natural phenomenon occurring in freshwater, caused by excessive nitrogen, phosphorus, and potassium levels leading to sudden overgrowth of algae.

The main cause of water body eutrophication is the discharge of nitrogen, phosphorus, and potassium into slow-flowing surface waters with long renewal cycles, causing excessive growth of algae and other aquatic organisms. This leads to organic matter production far exceeding consumption, resulting in organic matter accumulation and disruption of the aquatic ecological balance.

Oxygen dissolved in water is called dissolved oxygen. Aquatic organisms and aerobic microorganisms rely on dissolved oxygen for survival. Different microorganisms have different dissolved oxygen requirements.

Modern wastewater treatment technologies can be divided into physical, chemical, and biological treatment methods according to their working principles.

Colloid stability: The characteristic of colloidal particles remaining dispersed and suspended in water for an extended period.

Electrokinetic potential: The potential at the colloid slip plane, also known as the zeta potential.

For hydrophobic colloids, to allow colloids to collide with each other through Brownian motion and form larger particles, the repulsive energy peak must be reduced or eliminated. This can be achieved by reducing or eliminating the zeta potential of the colloid particles.

Adsorption bridging refers to the adsorption and bridging of macromolecules with colloid particles.

Function of a screen: To intercept larger suspended solids or floating impurities.

Main factors affecting coagulation efficiency: water temperature, water pH and alkalinity, suspended solids concentration, and hydraulic conditions.

There are four types of sedimentation:

Free settling: Particles remain discrete during sedimentation, their shape, size, and mass remain unchanged, and their settling speed is not affected, each completing the sedimentation process independently.

Flocculent settling: During sedimentation, the size, mass, and settling rate of particles increase with depth.

Hindered settling: The concentration of particles in the water is high, and during settling, they interfere with each other, forming a clear interface between clear water and turbid water, which gradually moves downward.

Compression settling: The concentration of particles in the water is very high, and during sedimentation, the particles come into contact with each other and are largely supported by the compressed material, with the gaps between the lower particles being squeezed out.

According to the direction of water flow in the tank, sedimentation tanks can be classified as horizontal flow, inclined flow, radial flow, and vertical flow.

Distribution Pattern of Impurities within the Filter Layer: In the initial stage of filtration, the filter material is relatively clean, with larger pores and smaller water flow shear force, resulting in stronger adhesion. At this time, particles in the water are primarily retained by the surface filter material. As filtration time increases, impurities accumulate in the filter layer, and porosity gradually decreases, especially in the surface fine filter material. The water flow shear force increases, enhancing the detachment effect, and the particles initially adhered are the first to detach and move downward, being retained by the lower filter material.

The result is: under a certain filtration head, the filtration rate will decrease sharply; or, under a certain filtration rate, the head loss reaches its limit; or, due to uneven stress on the filter layer surface, cracks appear in the mud film, causing a large amount of water to flow out through the cracks, resulting in the penetration of impurities into the filter layer and deterioration of the effluent water quality.

Ways to Improve Filtration Efficiency: To change this situation and improve the dirt-holding capacity of the filter layer, "reverse particle size" filtration has emerged, where the filter material particle size decreases in the direction of water flow. However, due to the complex structure of upward flow and two-way flow filter pools and the inconvenience of backwashing, etc., this method is not widely used.

Composition of Homogeneous Filter Material: Homogeneous filter material refers to a filter material where the composition and average particle size are uniform across any cross-section of the filter layer depth, not that the particle sizes are exactly the same.

Negative Head Phenomenon: During filtration, when the filter layer retains a large amount of impurities, causing the head loss at a certain depth below the sand surface to exceed the water depth at that location.

Methods to Avoid Negative Head: Increase the water depth above the sand surface, or ensure that the filter pool outlet is at or above the filter layer surface. Siphon filters and valveless filters do not experience negative head for this reason.

There are two methods for supplying backwash water to ordinary rapid filters: wash water pumps and water towers.

When the main organic matter in the water is ammonia and nitrogenous compounds, after the actual chlorine demand is met, if the amount of chlorine added is increased, the residual chlorine will also increase, but the latter increases slowly. After a period of time, if the amount of chlorine added is further increased, the residual chlorine will decrease instead. After that, if the amount of chlorine added is increased again, the residual chlorine will rise again. After this breakpoint, free residual chlorine appears, and further chlorination achieves the best disinfection effect, which is breakpoint chlorination.

The activated sludge process consists of an aeration tank, a sedimentation tank, sludge return, and excess sludge removal systems.

Sludge settling ratio (SV%): The volume ratio (%) of settled sludge to the mixed liquor in a 1000ml cylinder after settling for 30min.