26. What is the sludge volume index?

Sludge Volume Index (SVI): The volume occupied by one gram of dry sludge after 30 minutes of sedimentation of the mixed liquor from the aeration tank outlet, measured in mL.

A low SVI value indicates that the sludge particles are small, dense, and rich in inorganic matter, lacking activity and adsorption capacity; a high SVI value indicates that the sludge is difficult to settle and separate, is about to swell or has already swelled, and the cause must be investigated and measures taken.

Sludge bulking: When sludge deteriorates, it becomes difficult to settle, the SVI value increases, the sludge structure becomes loose, the volume expands, the water content increases, the clarified liquid is reduced, and the color also changes.

Sludge disintegration: The phenomenon of sludge disintegration is characterized by turbid treated water, fine flocculation of sludge, and deterioration of treatment efficiency.

Sludge decay: In the secondary sedimentation tank, anaerobic fermentation may occur due to long-term sludge retention, generating gas and causing large chunks of sludge to float.

Sludge flotation: The phenomenon of sludge floating in chunks in the secondary sedimentation tank.

Foaming problems: Foaming in the aeration tank is mainly caused by a large amount of synthetic detergents or other foaming substances in the wastewater.

Activated sludge microorganisms are a mixed population of multiple species, and their growth patterns are complex, but they can also be represented by a growth curve. This curve expresses the proliferation and attenuation of the number of microorganisms over time after a sufficient amount of nutrients is added once, when the environmental conditions such as temperature and dissolved oxygen meet the growth requirements of the microorganisms, and a certain amount of initial microorganisms are inoculated.

The change in the growth rate of activated sludge is mainly caused by the ratio of nutrients or organic matter to microorganisms (usually expressed as F/M). The F/M value is also an important influencing factor for the organic substrate degradation rate, oxygen utilization rate, and the coagulation and adsorption performance of activated sludge.

The four stages of the activated sludge growth curve are: adaptation phase, logarithmic growth phase, deceleration growth phase (maximum biomass), and endogenous respiration phase (best treated water quality).

Activated sludge purifies wastewater through three stages:

In the first stage, wastewater is mainly purified through the adsorption of activated sludge. Adsorption is very rapid, generally completed within 30 minutes, and the BOD5 removal rate can reach up to 70%. It also has a partial oxidation effect, but adsorption is the main effect.
The second stage, also known as the oxidation stage, mainly involves the continued decomposition and oxidation of organic matter adsorbed and absorbed in the previous stage, while continuing to adsorb some residual dissolved substances.
The third stage is the solid-liquid separation stage. In this stage, activated sludge is settled and separated in the secondary sedimentation tank. Both the anabolism and catabolism of microorganisms can remove organic pollutants from wastewater, but the products are different.

Characteristics of secondary sedimentation tanks: In terms of function, in addition to solid-liquid separation, it also performs sludge concentration, and due to changes in water quality and quantity, it also temporarily stores sludge.

Slow sand filtration allows wastewater to slowly pass through the land, purifying the wastewater through natural seepage and filtration. It is suitable for areas with good soil permeability, low evaporation, and a humid climate.

Suitable for soils with very good permeability, such as sandy soil and gravelly sandy soil. After the wastewater flows to the surface of the rapid infiltration field, it quickly infiltrates into the ground and eventually enters the groundwater layer.

Anaerobic reactions are divided into three stages:

The first stage is the decomposition of organic matter into fatty acids and other products under the action of hydrolytic and fermentative bacteria.

The second stage is the further conversion of hydrogen and acetic acid producing bacteria into hydrogen, carbon dioxide, and acetic acid.

The third stage is the methane fermentation stage (alkaline fermentation stage), where two different groups of methanogens act: one group converts hydrogen and carbon dioxide into methane, and the other group produces methane from acetic acid.

Two-phase digestion separates the acid-producing stage and the methane-producing stage of anaerobic digestion of organic substrates.

The material components of sludge can be divided into organic sludge and inorganic sludge.

According to the source of sludge, it can be divided into primary sedimentation sludge, excess activated sludge, humus sludge, digested sludge, and chemical sludge.

The water contained in sludge is divided into four categories: water in the intergranular voids, capillary water, water adsorbed by sludge particles, and water inside the particles.

Removal methods: gravity, flotation, centrifugation.

Mechanical dewatering: vacuum filtration dewatering, pressure filtration dewatering, roll pressing dewatering, and sludge centrifugation dewatering.

The purpose of sludge stabilization is to eliminate odors emitted from the sludge and kill pathogenic microorganisms in the sludge.

Adsorption treatment is a method that uses porous solids (such as activated carbon) or flocculent substances (such as polyferric) to adsorb toxic and harmful substances in wastewater onto the surface or micropores of the solid or flocculent, achieving the purpose of water purification. The object of adsorption can be insoluble solid substances or soluble substances.

Characteristics of physical adsorption: The adsorption heat is small, it can be carried out at low temperatures, the adsorption is reversible, and the adsorption has basically no selectivity.

Characteristics of chemical adsorption: The adsorption heat is large, the adsorption is irreversible, and the adsorption is selective.

Resin density: Generally refers to two methods of representation: wet true density and wet apparent density. Wet true density is related to the backwashing strength, expansion rate, and resin layering of the mixed bed and double-layer bed of the resin layer. Wet apparent density is used to calculate the amount of wet resin required for filling the ion exchanger.

The function of the water-spraying packing is to disperse the water droplets falling from the water distribution system into many tiny droplets or water films, increasing the contact area between water and air, extending the contact time, and thus ensuring good heat and mass exchange between air and water.

Mixed liquor volatile suspended solids (MLVSS) refers to the weight of volatile substances in the dry sludge contained in a unit volume of biochemical pool mixed liquor. The unit is also mg/L. Since it does not include inorganic substances in the activated sludge, it can more accurately represent the number of microorganisms in the activated sludge.

In the biochemical treatment process, the microorganisms in the activated sludge continuously consume the organic matter in the wastewater.

Of the organic matter consumed, part of it is oxidized to provide energy for the life activities of microorganisms, and the other part is used by microorganisms to synthesize new cytoplasm, thus enabling microorganisms to multiply and reproduce. At the same time as the metabolism of microorganisms, some old microorganisms die, thus producing excess sludge.

Iron-carbon treatment, also known as iron-carbon micro-electrolysis or iron-carbon internal electrolysis, is an application of metal iron wastewater treatment technology. Using the iron-carbon method as a pretreatment technology to treat toxic and harmful wastewater with high COD has a unique effect.

The iron-carbon effluent contains a large amount of ferrous sulfate. If it is not removed, it will affect the growth and reproduction of microorganisms in the subsequent biochemical pool.

Therefore, we must use lime to increase the pH of the wastewater from 5-6 to above 9, converting the water-soluble ferrous sulfate into insoluble ferrous hydroxide and calcium sulfate, and then using coagulation sedimentation to precipitate them to ensure that the wastewater entering the biochemical pool does not contain ferrous sulfate.

Flotation is divided into: dissolved air flotation (divided into vacuum dissolved air flotation and pressurized dissolved air flotation), dispersed air flotation, and electrolytic flotation.

Flocculation is the addition of high-molecular flocculants to wastewater. After dissolution, high-molecular flocculants form high-molecular polymers. The structure of this polymer is linear. One end of the line pulls a tiny particle, and the other end pulls another tiny particle, playing a bonding bridging role between two particles that are relatively far apart, causing the particles to gradually become larger and eventually form large flocculates (commonly known as alum flowers), accelerating particle sedimentation.

During coagulation, polyferric forms ferric hydroxide flocs, which have a good ability to adsorb organic substances in wastewater. Experimental data show that after flocculation and adsorption with polyferric, 10%-20% of COD in wastewater can be removed, which can greatly reduce the operating burden of the biochemical pool and is conducive to the discharge of treated wastewater that meets standards.

In addition, coagulation pretreatment with polyferric can remove trace substances that are toxic and inhibitory to microorganisms in wastewater, ensuring that the microorganisms in the biochemical pool can operate normally. Among many coagulants, the price of polyferric is relatively cheap (25-300 yuan/ton), so the treatment cost is relatively low, which is more suitable for the pretreatment of process wastewater.

Many impurities with a specific gravity greater than 1, large particles, and easily settleable suspended solids in wastewater can be removed by natural sedimentation, centrifugation, etc.

However, suspended particles with a specific gravity less than 1, tiny, or even invisible to the naked eye are difficult to naturally settle, such as colloidal particles, which are microparticles of 10-4-10-6 mm in size, are very stable in water, and their sedimentation rate is extremely slow, taking 200 years to settle 1 m.