Wastewater sludge treatment: an innovative proposal for water utilities. Sewage sludge processing and disposal

The main task of sludge treatment Wastewater is to obtain the final product, the properties of which make it possible to utilize it in the interests of National economy or minimize damage to the environment. The technological schemes used to implement this task are very diverse.

Technological processes for the treatment of sewage sludge at all treatment plants for mechanical, physical, chemical and biological treatment can be divided into the following main stages: compaction (thickening), stabilization of the organic part, conditioning, dehydration, heat treatment, disposal of valuable products or elimination of sludge (Scheme 2 ) .

Figure 5 - Stages and methods of sewage sludge treatment

Precipitation compaction

Sediment compaction is associated with the removal of free moisture and is a necessary stage of all technological schemes precipitation treatment. During compaction, on average, 60% of moisture is removed and the sediment mass is reduced by 2.5 times.

For compaction, gravity, filtration, centrifugal and vibration methods are used. Gravity compaction is the most common. It is based on the settling of particles of the dispersed phase. Vertical or radial settling tanks are used as sludge thickeners.

The compaction of activated sludge, in contrast to the compaction of raw sludge, is accompanied by a change in the properties of the sludge. Activated sludge as a colloidal system has a high structure-forming ability, as a result of which its compaction leads to the transition of part of the free water into a bound state, and an increase in the content of bound water in the sludge leads to a deterioration in water loss.

By applying special treatment methods, for example, treatment with chemical reagents, it is possible to achieve the transfer of part of the bound water to a free state. However, a significant part of the bound water can only be removed by evaporation.

Sludge stabilization

Anaerobic stabilization

Anaerobic digestion is the main method of disposal of urban sewage sludge. Fermentation is called methane, because as a result of the decay organic matter precipitation as one of the main products formed methane.

The biochemical process of methane fermentation is based on the ability of microorganism communities to oxidize organic substances of sewage sludge during their life activity.

Industrial methane fermentation is carried out a wide range bacterial cultures. Theoretically, the fermentation of sediments is considered, consisting of two phases: acidic and alkaline.

In the first phase of acid or hydrogen fermentation, complex organic substances of sediment and sludge are first hydrolyzed to simpler ones under the action of extracellular bacterial enzymes: proteins to peptides and amino acids, fats to glycerol and fatty acids, carbohydrates to simple sugars. Further transformations of these substances in bacterial cells lead to the formation final products the first phase, mainly organic acids. More than 90% of the formed acids are butyric, propionic and acetic acids. Other relatively simple organic substances (aldehydes, alcohols) and inorganic substances (ammonia, hydrogen sulfide, carbon dioxide, hydrogen) are also formed.

The acid phase of fermentation is carried out by ordinary saprophytes: facultative anaerobes such as lactic acid, propionic acid bacteria and strict (obligate) anaerobes such as butyric, acetonobutyl, cellulose bacteria. Most of the bacterial species responsible for the first phase of fermentation are spore-forming. In the second phase of alkaline or methane fermentation, methane and carbonic acid are formed from the end products of the first phase as a result of the vital activity of methane-forming bacteria - non-spore-bearing obligate anaerobes, very sensitive to environmental conditions.

Methane is formed as a result of the reduction of CO 2 or the methyl group of acetic acid:

where AH 2 is an organic substance that serves as a hydrogen donor for methane-forming bacteria; usually these are fatty acids (except acetic) and alcohols (except methyl).

Many types of methane-forming bacteria oxidize the molecular hydrogen formed in the acid phase. Then the reaction of methane formation has the form:

Microorganisms using acetic acid and methyl alcohol carry out the following reactions:

All of these reactions are sources of energy for methane-forming bacteria, and each of them is a series of successive enzymatic transformations of the starting material. It has now been established that vitamin B 12 takes part in the process of methane formation, which is credited with the main role in the transfer of hydrogen in energy redox reactions in methane-forming bacteria.

It is believed that the rates of transformation of substances in the acidic and methane phases are the same, therefore, with a stable fermentation process, there is no accumulation of acids - products of the first phase.

The fermentation process is characterized by the composition and volume of the released gas, the quality of the interstitial water, and the chemical composition of the digested sludge.

The resulting gas consists mainly of methane and carbon dioxide. During normal (alkaline) fermentation, hydrogen as a product of the first phase can remain in the gas in a volume of no more than 1–2%, since it is used by methane-producing bacteria in redox reactions of energy metabolism.

The hydrogen sulfide H 2 S released during the breakdown of the protein practically does not enter the gas, since in the presence of ammonia it easily binds with the available iron ions into colloidal iron sulfide.

The end product of ammonification of protein substances, ammonia, binds with carbonic acid to form carbonates and bicarbonates, which cause high alkalinity of interstitial water.

Depending on the chemical composition precipitation during fermentation is released from 5 to 15 m 3 of gas per 1 m 3 of sediment.

The speed of the fermentation process depends on the temperature. So, at a sediment temperature of 25 - 27 ° C, the process lasts 25 - 30 days; at 10°C, its duration increases to 4 months or more. To accelerate fermentation and reduce the volume of facilities required for this, artificial heating of the sludge to a temperature of 30 -35 ° C or 50 - 55 ° C is used.

A normally occurring process of methane fermentation is characterized by a slightly alkaline reaction of the medium (pH? 7.b), high alkalinity of interstitial water (65–90 mg-eq/l) and a low content of fatty acids (up to 5–12 mg-eq/l). The concentration of ammonium nitrogen in the interstitial water reaches 500 - 800 mg/l.

Process disruption can result from overloading a facility, changing temperature regime, inflows with sediment toxic substances etc. Violation manifests itself in the accumulation of fatty acids, a decrease in the alkalinity of interstitial water, and a drop in pH. The volume of the resulting gas sharply decreases, the content of carbonic acid and hydrogen, the products of the acid phase of fermentation, increases in the gas.

Acid-forming bacteria responsible for the first phase of fermentation are more resistant to any kind of adverse conditions, including overload. Sediments entering for fermentation are largely seeded with them. Rapidly multiplying, acid-forming bacteria increase the assimilation capacity of the bacterial mass and thus adapt to increased loads. The speed of the first phase increases in this case, a a large number of fatty acids.

Methane bacteria multiply very slowly. The generation time for some species is several days, so they are not able to quickly increase the number of cultures, and their content in the raw sediment is insignificant. As soon as the neutralizing ability of the fermenting mass (alkalinity reserve) is exhausted, the pH drops sharply, which leads to the death of methane-forming bacteria.

Of great importance for normal sludge fermentation is the composition of wastewater, in particular the presence in them of such substances that inhibit or paralyze the vital activity of microorganisms that carry out the sludge fermentation process. Therefore, the question of the possibility of joint treatment of industrial and domestic wastewater should be resolved in each individual case, depending on their nature and physico-chemical composition.

When mixing domestic wastewater with industrial wastewater, it is necessary that the wastewater mixture has a pH = 7 - 8 and a temperature not lower than 6 ° C and not higher

30°C. The content of toxic or harmful substances should not exceed the maximum permissible concentration for microorganisms developing under anaerobic conditions. For example, when the content of copper in the sediment is more than 0.5% of the dry matter of the sludge, the biochemical reactions of the second phase of the fermentation process slow down and the reactions of the acidic phase accelerate. At a dose of sodium hydroarsenite of 0.037% by weight of the ash-free substance of fresh sediment, the process of decomposition of organic matter slows down.

Three types of structures are used for processing and fermentation of raw sludge: 1) septic tanks (septic tanks); 2) two-tier settling tanks; 3) digesters.

In septic tanks, water is clarified and the sediment that has fallen out of it is rotting at the same time. Septic tanks are currently used at stations with a small throughput.

In two-tier settling tanks, the settling part is separated from the putrefactive (septic) chamber located in the lower part. The development of the design of a two-tier settling tank is a clarifier-decomposer.

For sludge treatment, digesters are currently most widely used, serving only for sludge fermentation with artificial heating and stirring.

The digested sludge has high humidity(95 - 98%), which makes it difficult to use it in agriculture for fertilizer (due to the difficulty of moving with conventional vehicles without pressure distribution networks). Humidity is the main factor that determines the amount of sediment. Therefore, the main task of sludge treatment is to reduce its volume by separating water and obtaining a transportable product.

When carrying out wastewater treatment processes, the formation of various sediments occurs. They contain a large number of elements and substances that, if used correctly, can be of significant benefit. But before productive use, precipitation must be brought into a suitable quality condition. To solve this problem, various preparation and processing measures are used.

Disposal of sewage sludge is necessary for environmental safety

After the qualitative processing of the sludge, its properties are brought to the desired parameters, it can be disposed of. The treatment and disposal of sewage sludge is of significant environmental importance and requires detailed consideration.

Sewerage sludge: what is it?

AT sewer systems accumulate liquid effluents containing solid pollutants and neutral substances. Their concentration can reach 10% of the total volume. All wastewater, regardless of the source of origin (water supply system, production, etc.) and the degree of pollution, must undergo a pollution removal procedure. They must be 95-98% clean before being released into the environment. During the various processes removal of contaminants precipitates are formed.

Sewage sludge (SWS) – solids and elements that occur during settling and cleaning in storage tanks, settling tanks, aerotanks, metatanks, and other reservoirs for removing waste liquid contaminants.

In the process of removing contaminants, precipitation is formed with a total volume of 0.5 to 10% of the initial amount of incoming liquid substances.

Depending on the concentration structural compounds three main types of sedimentary formations are distinguished: with predominantly mineral composition, with a predominance of organic components, complex.

WWS are divided into six groups, depending on the source of income:

• large solid sediments selected by gratings;
• elements deposited in sand traps;
• heavy waste from primary settling tanks;
• bottom sediments from tanks with flocculants (substances that form loose flaky aggregates in liquids) and coagulants (substances that cause thickening in liquids, sticking together into large structures of solids);
• waste sludge from aeration tanks;
• waste film from biofilters.

WWS from gratings, sand traps and primary settling tanks have an unchanged structure. Sediments from the rest of the treatment units have a changed structure and composition as a result of exposure and treatment with biological and chemical reagents.

There are two main uses for sewage sludge:

1. Application as organomineral fertilizers.
2. As a raw material for generating heat by combustion.

Minor destinations modern application sedimentary elements are:

• slag and ash from the burning of wastewater serve constituent element in the production of numerous items of products intended for construction;
• sediments are used as sorbents (substances used to selectively absorb gases) in sewers;
• restoration of areas of depleted land.

Sewage sludge contains heavy metals

The main disadvantage of precipitation is that it contains heavy metals. Such precipitates are effectively used in the production of several types of bricks and cement mortars. The addition of sedimentary rocks with metals increases strength and adhesion.

WWS with heavy metals can be effectively used as fertilizers in areas of afforestation and urban greening. Such plants are not used for food, so the content of metals does not pose a threat. But rare elements affect the improvement of soil properties.

When entering the treatment systems, most of the sediment particles are formed in the primary settling tanks. Sedimentary elements are also formed in the blocks of biological (air and airless) removal of pollutants.

The settled layer from aerotanks (oxygen treatment) can be used to increase useful characteristics plots outside the city and in the country. But in metatanks (without air), the settling substances are too toxic. They are unsuitable as a means of improving soil properties and require processing.

Technological cycle of liquid sludge processing:

1. Receipt of initial sludge.
2. Seal.
3. Stabilization: Digestion or anaerobic treatment.
4. Dehydration: natural and thermal drying, filtration in vacuum and vibration chambers, centrifuges.
5. Conditioning: reagent, thermal, freezing, polyelectrolyte, polycoagulation treatment.
6. Elimination: incineration, oxidation, concentration in reservoirs, removal to soil voids, transportation to landfills.
7. Utilization: agricultural fertilizers, production of products for construction purposes, sorbents, isolation and recovery of metals.

Processing and stabilization

The treatment of sewage sludge includes the following procedures:

• thickening: up to 60% of all moisture is removed, the volume of sludge is reduced by 50%;
• seal;
• stabilization;
• conditioning.

The main purpose of the treatment is to remove liquid and obtain sludge (sediment of the smallest particles, recycled pollutants). The most common and simple method of dehydration is drying on platforms. But in order to process WWS by this method, large areas earth. This method is ineffective.

At the stages of thickening and compaction, all free moisture is removed. These methods are primary in the processing of sedimentary elements.

Compaction techniques:

• vibration;
• centrifugal;
• gravity;
• flotation;
• filtration;
• mixed.

The most simple and cheap way seals used in treatment systems everywhere - gravity. For its implementation, vertical and radial sedimentation tanks are used. The compaction process takes from several hours to several days. To give higher rates, coagulants containing ferric chloride are used, heated to a temperature of 90 degrees.

The flotation method uses air bubbles. Various elements stick to them and rise to the surface. A film is formed, which is quite easy to remove. The method is efficient and manageable, due to the simple regulation of the amount of oxygen supplied to the liquid. With the centrifugal method, cyclones and centrifuges are used.

Wastewater sludge is stabilized in the aerotank

In septic tanks, aerotanks, metatanks and certain types sedimentation tanks stabilize sewage sludge. This name refers to the process of splitting complex organic substances into simple components: H2O, CH4, CO2.

After completion of processes in oxygen and without oxygen conditions, the sediment loses its ability to rot, that is, it stabilizes. The stabilization process is necessary for the destruction of the organic components of wastewater.

Stabilization methods include: drying of waste sludge in specially equipped areas, the use of precipitation as nutrients for the soil. On the industrial enterprises for stabilization, anaerobic decomposition of organic matter in aerotanks is used. The decay and oxidation of biological elements occurs within 7-10 days.

As conditioning methods, the following methods are used: heat treatment, sequential freezing and thawing, electrocoagulation, and irradiation with radioactive radiation. Conditioning is necessary to destroy the natural structure of organic matter and increase the rate of fluid return during other activities.

This process, used in industry with the use of reagents, is an expensive procedure. The implementation of this procedure takes up to 40% of all costs for the treatment of precipitation. For this reason, other methods of treatment are being actively introduced: thermal, freezing, electrocoagulation.

Liquidation

Liquidation is carried out in cases where it is impossible or unprofitable, from a financial point of view, to carry out disposal or other procedures for their effective application. Used for elimination various methods. The choice of method depends on the composition of the sludge, the characteristics of the processing plant or treatment plant.

Destruction methods:

1. Incineration is the most effective way to quickly and cost-effectively reduce the amount of WWS with effective disinfection.
2. Regeneration (isolation and recovery of certain elements) of WWS at mechanical engineering enterprises.
3. Discharge of liquids into reservoirs and injection into special soil voids. These methods belong to the secondary methods of elimination.
4. Natural long-term processing and storage of sediments at special landfills.

Disposal

Before disposal, sewage sludge goes through a composting process. This concept refers to the processes of the natural biological-temperature process of the splitting of organic sediments under the action of aerobic microorganisms.

The purpose of composting is to stabilize, disinfect and prepare substances for use as fertilizers on agricultural land. This process is accompanied by the release of heat, which, with a competent approach, can also be effectively used.

Composting does an excellent job of minimizing fuel consumption for decontamination and improving sanitation and hygiene.

Composting involves the passage of two phases:

1. Phase, active development of microorganisms. Last from 5 to 20 days. The process is accompanied by an increase in temperature in the environment up to 75-80 degrees. The sediments themselves are disinfected, their volume is significantly reduced.
2. Compost formation. Accompanied by the active development of microorganisms in a temperature environment up to 50 degrees.
3. The duration of the period can take from 15 days to six months.

The most important factor in the composting procedure is a constant supply of fresh oxygen to maintain a favorable environment for developing microorganisms. There are three main composting methods:

1. Mechanical.
2. Static heaps.
3. Ridges.

A mandatory disposal procedure is the disinfection of wastewater. Elimination of dangerous microorganisms, viruses and bacteria from WWS is carried out in various ways:

1. Thermal (burning, drying and heating).
2. Chemical (reagents).
3. Biotemperature (obtaining compost).
4. Biological (cleaning with the help of plants, fungi and microorganisms).
5. Physical (high frequency waves, electricity, radiation, ultrasound, ultraviolet).

After composting and disinfection, WWS are used in agriculture as nitrogen-phosphorus nutrients. When placed in the ground, they mineralize and the organic matter is converted into compounds suitable for absorption by plants.

The usefulness of certain fertilizers from wastewater is determined by the parameters of the concentration of biogenic elements. The most valuable is activated sludge. The introduction of prepared compost allows you to reduce the acidity of the soil, increase the content of nutrients.

Precipitation is excellent for restoring the surface layers of the earth depleted as a result of agricultural activities. OSV fertilizers are actively used in the preparation of soils that have been transferred from industrial use into agriculture. The composition of the compost actively restores the top fertile soil layer.

Fertilizers from WWS contain a large amount of mineral trace elements. Each of the elements is essential for the growth and development of crops in various conditions cultivation. The required concentration of these substances helps to accelerate the development of plants, increase the resistance of many species.

Deficiency leads to disruption of metabolic processes. Thus, copper increases the degree of maturation of grain plants grown in swampy and sandy areas. Manganese is needed for active growth beets, corn. Fruit plants and grapes react painfully to a lack of iron and zinc. Clover, beans, peas, vegetables need access to boron-containing fertilizers.

A number of types of processed sludge are used as feed additives at livestock facilities. Sludge processing products are actively used as a raw material for production processes and for the production of thermal energy on an industrial scale.

modern and effective way recycling is pyrolysis. This concept refers to the process of processing substances that include hydrocarbons under special conditions (high-temperature exposure, vacuum space without oxygen access).

Under certain conditions, pyrolysis can be carried out without the use of third-party fuel, since the dry residue is able to create the necessary thermal conditions on its own.

As a result of pyrolysis, semi-coke is obtained - a crumbling black powder. Such semi-coke is actively used in various industrial sectors. It is efficient as a fuel.

The advantages of pyrolysis as a means of disposal include the following factors:

1. The absence of a large amount of ash and low level atmospheric pollution.
2. Minimum need for fuel consumption.
3. Process control.
4. Efficiency, regardless of incoming volumes.
5. Possibility of equipping pyrolysis plants at wastewater treatment plants.
6. The main disadvantage of equipment for pyrolysis reactors is an increased explosion hazard. Such threats require significant control measures in the equipment and careful fire control in operation.

Urban sewage sludge is large in volume, high in moisture, heterogeneous in composition and properties, and contains organic matter that can quickly decompose and rot. Sediments are contaminated with bacterial and pathogenic microflora and helminth eggs.

Sediment from primary settling tanks and excess activated sludge consist of 65–75% organic substances, which are 80–85% represented by proteins, fats and carbohydrates.

Sewage sludge is a hard-to-filter silt suspension. The water-releasing properties of sediments are characterized by specific filtration resistance and centrifugation index.

Technological process sludge treatment can be divided into the following main stages: compaction (thickening); stabilization of the organic part; conditioning; dehydration; heat treatment; disposal of valuable products or disposal of sludge.

Compaction of sludge and sewage sludge. Depending on the adopted scheme of the treatment plant, sediments from primary clarifiers, excess activated sludge, a mixture of primary clarifier sludge and excess activated sludge, flotation sludge, sediments and sludge after stabilization can be compacted.

To compact excess activated sludge at treatment plants, vertical and radial gravity-type sludge thickeners or flotation sludge thickeners operating on the principle of compression flotation are used.

Gravity compaction is the most common technique for reducing the volume of excess activated sludge. It significantly reduces the volume of facilities and the cost of electricity required for its subsequent processing. The designs of vertical and radial seals are similar to those of primary settling tanks.

Sediment collection and removal in radial sludge thickeners is carried out by sludge scrapers or sludge pumps. Comparison of the operation of vertical sludge thickeners with radial sludge thickeners equipped with sludge scrapers and sludge pumps showed that radial sludge thickeners with sludge scrapers are the most efficient. This is due to the slow mixing of activated sludge during compaction, as well as the lower height of radial sludge compactors compared to vertical ones. When mixing, the viscosity of activated sludge and its electrokinetic potential decrease, which contributes to better flocculation and sedimentation. Therefore, in modern designs sludge thickeners, low-gradient mixers are provided.

Flotation compaction of activated sludge prevents its decay, reduces the duration of compaction and the volume of structures. Flotation machines for compacting excess activated sludge are usually round tanks with a diameter of 6, 9, 12, 15, 18, 20, 24 m and a depth of 2–3 m, differing in internal equipment.

Stabilization of sewage sludge and activated sludge under anaerobic and aerobic conditions. Stabilization of primary and secondary sediments is achieved by decomposition of the organic part to simple connections or products that have a long assimilation period environment. Stabilization of sediments can be carried out by various methods - biological, chemical, physical, as well as their combination.

The most widespread methods of biological anaerobic and aerobic stabilization. With a small amount of precipitation, septic tanks, two-tier settling tanks and clarifiers are used - decomposers. For processing large volumes of sediments, digesters and aerobic mineralizers are used.

In digesters, the biochemical stabilization process is carried out under anaerobic conditions and is the decomposition of the organic matter of sediments as a result of the vital activity of a complex complex of microorganisms to final products, mainly methane and carbon dioxide.

According to modern concepts, anaerobic methane digestion includes four interrelated stages carried out by different groups of bacteria:

1. The stage of enzymatic hydrolysis is carried out by fast-growing facultative anaerobes that secrete exoenzymes, with
which participate in the hydrolysis of undissolved complex organic compounds with the formation of simpler solutes. Optimal value The pH for the development of this group of bacteria is in the range of 6.5–7.5.

2. The stage of acid formation (acidogenic) is accompanied by the release of volatile fatty acids, amino acids, alcohols, as well as hydrogen and carbon dioxide. The stage is carried out by fast-growing, highly resistant to adverse environmental conditions, heterogeneous bacteria.

3. The acetogenic stage of the conversion of VFAs, amino acids and alcohols into acetic acid is carried out by two groups of acetogenic bacteria. The first group, which form acetates with the release of hydrogen from the products of the previous stages, are called hydrogen-forming acetateogens:

CH CH COOH + 2H 2 0 CH3COOH + CO + 3H 2.

The second group, which also form acetates and use hydrogen to reduce carbon dioxide, are called hydrogen-using acetateogens:

4H 2 + 2C0 2 CH COOH + 2H 2 0.

4. Methanogenic stage, carried out by slow-growing bacteria, which are strict anaerobes, very sensitive to changes in environmental conditions, especially to a decrease in pH less than 7.0 - 7.5 and temperature. Miscellaneous groups methanogens form methane in two ways:

Acetate breakdown:

CH 3 COOH CH 4 + CO 2,

Recovery of carbon dioxide:

C0 2 + H 2 CH 4 + H 2 0.

The first path produces 72% of methane, the second - 28%.

The fermentation process is slow. To accelerate it and reduce the volume of structures, artificial heating of sludge is used. At the same time, the release of gas, methane, is much more efficient, which is captured and can be used as a fuel. Depending on the temperature, two types of process are distinguished: mesophilic (t = 30 - 35) and thermophilic (t = 50 - 55).

Methane tanks are sealed vertical tanks with a conical or flat bottom, made of reinforced concrete or steel.

Diagram of the digester is shown in fig. 3.2.17. The sediment level is maintained in the narrow mouth of the digester, which makes it possible to increase the intensity of gas evolution per unit surface of the fermenting mass and prevent the formation of a dense crust.

Rice. 3.2.17. Metatank :

1 - sediment supply; 2 − steam injector; 3 - release of digested sludge;

4 - emptying the digester; 5 - thermal insulation;

6 - gas collection and removal system; 7 - circulation pipe; 8 - sediment level

Aerobic stabilization of sewage sludge is the process of oxidation of organic substances under aerobic conditions. Unlike anaerobic digestion, aerobic stabilization proceeds in one stage:

C 5 H 7 N0 2 +50 2 -> 5C0 2 + 2H 2 0 + NH 3,

followed by the oxidation of NH 3 to N0 3 .

Aerobic stabilization can be applied to non-compacted and compacted excess activated sludge and its mixture with the sludge from the primary settling tanks.

Aerobic stabilization of sediments is usually carried out in structures such as aerotanks with a depth of 3–5 m. Settling and compaction of aerobically stabilized sediment should be carried out within 1.5–5 hours in separate sludge thickeners or in a specially allocated area inside the stabilizer. Humidity of the compacted sediment is 96.5 - 98.5%. The sludge water should be sent to the aeration tanks. The diagram of the aerobic stabilizer is shown in fig. 3.2.18.

Rice. 3.2.18. Scheme of the mineralizer: I - aeration zone; II - settling zone; III - sediment thickener; 1 - stabilized precipitate; 2 - release of settling water; 3 − air duct; 4 - emptying; 5 - silt mixture; 6 - centrifuge from the mechanical dehydration shop

Aerobic stabilization of precipitation ensures the production of biologically stable products, good performance moisture yield, ease of operation and low construction costs of structures. However, significant energy costs for aeration limit the expediency of using this process at treatment facilities with a capacity of more than 50 - 100 thousand m 3 /day.

Disinfection of sewage sludge. Urban sewage sludge contains a large number of pathogenic microorganisms and helminth eggs, so the sludge must be decontaminated before disposal and storage. Disinfection of sewage sludge is achieved by various methods:

Thermal - heating, drying, burning;

Chemical - treatment with chemical reagents;

Biothermal - composting;

Biological - destruction of microorganisms by protozoa, fungi and soil plants;

Physical influences - radiation, high frequency currents, ultrasonic vibrations, ultraviolet radiation etc.

general characteristics processes of disinfection of sewage sludge is given in table. 3.2.2. At large aeration stations, it is advisable to use thermal drying of mechanically dehydrated sludge, which makes it possible to reduce fare and get fertilizer from precipitation in the form bulk materials. To reduce fuel and energy costs at aeration stations with a capacity of up to 20 thousand m 3 /day, it is advisable to use deworming chambers, up to 50 thousand m 3 /day - chemical disinfection methods. In cases where the sludge cannot be disposed of as fertilizer, incineration using the resulting heat can be used.

Wastewater treatment produces sludge that poses a health hazard. They must be handled and/or disposed of. Treatment of sewage sludge is carried out by the following methods:

1. Stabilization;
2. Seal;
3. Conditioning;
4. Dehydration;
5. Destruction;
6. Disposal.

The choice of processing method depends on the type and properties of precipitation. By origin, precipitation can be classified into:
1. Primary:
1.1. coarse (formed on gratings and sieves; humidity - 80%);
1.2. heavy (on sand traps; humidity - 60%);
1.3. floating (on settling tanks; humidity - 60%);
1.4. raw (on primary clarifiers and clarifiers; humidity is about 93-95%);
2. Secondary:
2.1. raw (on secondary clarifiers; humidity is about 99.2-99.7%);
2.2. fermented (at sludge stabilization facilities; humidity - 97%);
2.3. compacted (on sludge thickeners and sediment thickeners; humidity - 90-96%%);
2.4. dehydrated (at dehydration facilities; humidity - 68-75%);
2.5. dry (after dryers).

Precipitation is also divided according to the degree of danger, toxicity:

1. low-hazard;
2. Moderately dangerous;
3. Highly dangerous;
4. Extremely dangerous.

Compaction and stabilization of sewage sludge

Compaction of sewage sludge is a decrease in their volume. Usually applied before dehydration. It can be carried out on sludge thickeners (compacting activated sludge) and sediment thickeners (compacting a mixture of activated sludge and raw sludge that forms in primary clarifiers). Structurally, there are two types of seals: radial and vertical. Also, flotators are used to compact sediments (before digesters or heat treatment).

Precipitation stabilization There are two types of wastewater:
1. Anaerobic:
1.1. Methane tanks (manufacturers: R.Lach GmbH, KRESTA);
1.2. Septic tanks (for individual buildings, without access to the alloy sewer), after which the water is disinfected and taken to the landfill; (manufacturers: GRAF, AUGUST-ECO, Akyop; UE "Polymerconstruction").
1.3. Two-tier settling tanks (with a capacity of up to 10,000 m3 / day; ;
1.4. Clarifiers-decomposers.
2. Aerobic:
2.1. Corridor aerotank;
2.2. Aerotank of prolonged aeration;
2.3. Aerotank-displacer.

Sewage sludge conditioning– treatment that improves their water-releasing properties, as a result of which the effect of subsequent mechanical dehydration increases.
Conditioning methods:
1. Reagent:
1.1 Coagulation (salts of iron, aluminum, lime);
1.2 Neutralization;
2. Heat treatment;
3. Defrosting;

Dehydration of sewage sludge- the separation of the wet part from them, for maximum drying, is carried out on:

1. Vacuum filters (manufacturers: Koch Industries, Eagle Group);
2. Vibration filters;
3. Filter presses (Ecoton);
4. Centrifuges (manufacturers Humbolt, Flottweq, Alfa, Laval);
5. drum dryers;
6. separators;
7. Counter jet dryers;
8. Vacuum drying installations;
9. Silt sites.

The destruction of sewage sludge is carried out in the following ways:

• "Wet" oxidation;
• Burning;
• Pyrolysis;
• Gasification.

After processing, most of the sludge is taken to the landfill. However, precipitation that is not hazardous in sanitary terms can be used, for example, for planting forests or sprinkling roads in winter (sediment from sand traps). In the past, the sludge from secondary clarifiers after treatment was used as fertilizer for plants.

Articles Figures Tables

Control of sludge treatment processes. Methane fermentation processes and control of digester operation

from "Water quality control"

Suspensions released from waste and waste water in the process of their mechanical, biological and physico-chemical (reagent) treatment are sediments.
It is advisable to divide the properties of sediments into those that characterize their nature and structure, as well as those that determine their behavior in the process of dehydration.
The ashless content is expressed as a percentage by weight of the dry matter content. It is determined by burning at a temperature of 550-600°C.
In hydrophilic organic sediments, this indicator is often close to the content of organic substances and characterizes the content of nitrogenous substances.
The elemental composition is especially important for organic sediments, primarily in terms of such indicators as the content of carbon and hydrogen to determine the degree of stabilization or to establish the total acidity of nitrogen and phosphorus to assess the fertilizer value of the sediment. heavy metals and etc.
For inorganic sediments, it is often useful to determine the content of Fe, M, Al, Cr, Ca salts (carbonates and sulfates) and S1.
According to the currently accepted maximum permissible concentrations, which take into account, along with toxicity, the cumulative properties of substances, cadmium, chromium, and nickel are the greatest danger to public health; copper and zinc are less dangerous.
Precipitation treatment facilities galvanic industries containing heavy metal oxides belong to the fourth hazard class, i.e. to less hazardous substances.
The formation of sludge with desired properties begins with the choice of those cleaning methods that provide the possibility of recycling or safe storage of sludge, reducing the cost of their dehydration and drying.
The apparent viscosity and the associated fluidity of sediments can be considered as a measure of the intensity of the forces of the relationship between particles. It also makes it possible to evaluate the thixotropic character of the precipitate (the ability of the precipitate to form a gel at rest and return to fluidity even with slight agitation). This property is very important for assessing the ability of the sludge to collect, transport and pump.
The sludge slurry is not a Newtonian fluid because the viscosity found is very relative and depends on the applied shear stress.
The nature of the water contained in the sediment. This water is the sum of free water, which can be easily removed, and bound water, including colloidal water of hydration, capillary, cellular and chemically bound water. The release of bound water requires considerable effort. For example, cellular water is separated only by heat treatment (drying or burning).
The approximate value of this ratio can be obtained thermogravimetrically, i.e. plotting a mass loss curve for a compacted sludge sample at constant temperature and processing under appropriate conditions. The point at which the thermogram has a break can be determined by plotting the K=G(.U) dependence, where V is the drying rate, g/min. Y - dry matter content in the sample,% (Fig. 2.6).
The ratio between free and bound water is a decisive factor in assessing the dewaterability of a sludge.
From fig. 2.6 it can be seen that the first critical current 5 determines the amount of water that can be removed from the sludge at a constant drying rate (phase 1), and represents the dry matter content in the sludge after the loss of free water. Next, bound water is removed, first to point S2 with a linear relationship between a decrease in the drying rate and an increase in the dry matter content (phase 2), and then with a sharper decrease in the rate of decrease in the drying rate (phase 3).
These factors include densification capacity resistivity numerical characteristics of sludge compressibility under increasing pressure (sludge compressibility) determination of the maximum dry matter percentage in sludge at a given pressure.
The ability to compact is determined from the analysis of the sedimentation curve for sediment. This curve is drawn on the basis of laboratory tests in a vessel equipped with a slowly working stirrer. The curve characterizes the degree of separation of the sediment mass in the vessel depending on the residence time in it.
Here t is the duration of filtration V is the volume of the deposited sediment.
Humidity. This parameter takes into account changes in the composition and properties of sludge during their processing and storage.