Practical application of diatomite in water treatment processes

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How to change the urban water-saving model, which involves first carrying out secondary biological treatment and then intermediate water treatment, in urban sewage treatment? By developing a process technology that can treat sewage directly into intermediate quality water, it is possible to save valuable financial and land resources for society. In order to create ecological cities and protect our living environment, Mr. Wang Qingzhong from the Dali Bai ethnic group, under the guidance of Academician Wang Xiji, developed the \"diatomaceous earth water treatment process,\" which has been advanced to the sixth generation of this technology – enabling urban sewage to be treated to meet the Grade A standards specified in GB18918-2002 with just one treatment step. Taking the \"Denglong River Project\" for urban wastewater in Dali City, Yunnan Province as an example, the diatomite water treatment process and the \"Qingzhong biochemical process\" are introduced as follows: I. Mechanism of pollutant removal in the diatomite water treatment process (1) Removal of SS: The removal of suspended solids from wastewater relies mainly on the adsorption, flocculation, and sedimentation effects of diatomite. In wastewater treatment, the concentration of suspended solids is not only related to the SS value of the effluent, but also to parameters such as BOD5 and CODcr in the effluent. Since the suspended solids in the effluent are primarily composed of activated sludge flocs, controlling the SS level in the effluent from wastewater treatment plants is a fundamental and crucial aspect. To remove suspended solids from water as effectively as possible, appropriate measures must be adopted in engineering. Common traditional biochemical methods include using an appropriate sludge load rate to maintain the coagulation and sedimentation properties of the activated sludge, employing a lower secondary sedimentation surface load, using a lower effluent weir load, and making full use of the adsorption capacity of the activated sludge suspension layer. Through comprehensive comparison, the best approach is to use diatomite water treatment agents in combination with high-efficiency hydraulic circulation clarifiers, which can achieve a SS removal rate of 99% in wastewater. (II) Removal of BOD5: The removal of BOD5 from wastewater is primarily achieved through the adsorption and metabolic actions of diatomaceous earth water treatment agents, followed by the separation of the adsorbed metabolites from the sludge. At the initial stage of contact between diatomaceous earth water treatment agent and wastewater, a very high BOD5 removal rate is observed; this is because the organic particles and colloids in the wastewater are adsorbed onto the surface of the diatomaceous earth water treatment agent, thereby being removed. However, this adsorption effect only acts on suspended solids and colloids in wastewater, and has no effect on dissolved organic matter. The breakdown of soluble organic matter relies on the ion-exchange function of diatomaceous earth to carry out metabolic processes; under aerobic conditions, some of the organic matter in wastewater is used to synthesize new cells, while another portion is broken down metabolically to obtain the energy required for cell synthesis, with the end products being stable substances such as CO2 and H2O. Therefore, the residual BOD5 concentration in the treated wastewater can be kept very low. The effluent quality is below 10 mg/L. "xD}6(NL(r (III) Removal of CODcr @9_nwf~X4 The principle of removing CODcr from wastewater is basically the same as that for BOD5, but the removal rate of CODcr depends on the composition of the domestic wastewater from Luoshuicun. For composite wastewater primarily composed of domestic sewage and its components as well as food service wastewater, the BOD5/CODcr ratio is often close to 0.5 or even greater than 0.5, allowing the CODcr level in the effluent to be kept at a low level. Urban wastewater whose components are primarily industrial wastewater has a low BOD5/CODcr ratio, meaning that its biodegradability is poor. For such wastewater, the use of diatomaceous earth as a water treatment agent, along with high-efficiency hydraulic circulation clarifiers, allows the advantages of this treatment process over other methods to be fully utilized. As a result, the residual CODcr in the treated wastewater can be reduced to very low levels, with removal rates exceeding 90%, and in some cases even over 95%. The test results for the inlet and outlet water parameters of the Denglong River project in Dali are as follows: Parameter, PH, Colority, BOD5, COD, SS, TN, TP. For urban sewage entering the Denglong River, the inlet water quality values are 6.31, 40, 150.68, 360, 250, 20.31, and 4.14 respectively; whereas the outlet water quality values are 6.87, 4, 10, 1, 33.7, 10, and 8.78, with a TP value of 0.02. As can be seen from the table above, the treated outlet water meets the requirement of CODcr ≤ 50 mg/L, thus fully satisfying the **Grade 1 discharge standards (GB18918-2002)**. GtC7^ Z&E (IV) Removal of nitrogen. Nitrogen is an essential component of proteins, and it is present in large quantities in urban wastewater. Nitrogen in wastewater generally exists in four forms: organic nitrogen, ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen. The main forms of nitrogen in domestic wastewater are organic nitrogen and ammonia nitrogen. Among them, organic nitrogen accounts for 40-60% of the content in domestic wastewater, ammonia nitrogen accounts for 50-60%, while nitrite nitrogen and nitrate nitrogen account for only 0-5%. The basic principle of biological nitrogen removal in traditional secondary biological treatment for wastewater is as follows: based on the conversion of organic nitrogen into ammonia nitrogen, ammonia nitrogen is transformed into nitrite nitrogen and nitrate nitrogen through the action of nitrifying and denitrifying bacteria; subsequently, nitrate nitrogen is converted into nitrogen gas through denitrification, thereby achieving the goal of removing nitrogen from the wastewater. The biological denitrification process for wastewater essentially applies the fundamental principles of nitrogen cycling in nature to the biological treatment of wastewater. Through the coordinated action of various microorganisms and appropriate human-controlled operations, it converts the ammonia nitrogen generated during biological carbon removal as well as that present in the original wastewater into nitrogen gas, thereby removing it from the wastewater. In the biological denitrification process of wastewater, first, under aerobic conditions, ammonia nitrogen in the wastewater is oxidized to nitrite nitrogen or nitrate nitrogen through the action of aerobic nitrifying bacteria ； Then, under anaerobic conditions, denitrifying bacteria reduce nitrite and nitrate to nitrogen, which escapes from the water. During nitrification and denitrification, the factors that affect their nitrogen removal efficiency are temperature, dissolved oxygen, pH value, and denitrification carbon source. In biological nitrogen removal systems, the growth rate of nitrifying bacteria is relatively slow; therefore, an adequate sludge age is necessary. This means that the system must operate under low sludge load conditions, so as to ensure that the sludge age is greater than the minimum value required for maintaining nitrification. The growth of denitrifying bacteria occurs primarily under anaerobic conditions, and an adequate supply of carbon sources to provide energy is necessary to facilitate proper denitrification. The nitration and denitrification reactions in biological nitrogen removal systems require the following conditions: During the nitration stage, there must be sufficient dissolved oxygen, with a DO level of over 2 mg/L; an appropriate temperature, ideally 20 degrees, with a minimum of 10 degrees; a long enough sludge retention time; and suitable pH conditions. Denitrification stage: presence of nitrate, anaerobic conditions, DO level around 0.2 mg/L, sufficient carbon source (energy), and appropriate pH conditions. It is necessary to control an adequate sludge age as well as the carbon and nitrogen content of the influent water. There are also the CODcr concentration entering the biological treatment tank, as well as the TKN/CODcr ratio and the P/CODcr ratio. When the TKN/CODcr ratio is less than 0.08, there is an effect on removing nitrates; if it is between 0.08 and 0.11, nitrates cannot be completely removed. When it is between 0.11 and 0.14, the UCT process also cannot eliminate all nitrates in the anaerobic tank, and it is necessary to control the recirculation ratio in order to reduce the impact of nitrates on the anaerobic process. If TKN/CODcr is greater than 0.14, biological nitrogen and phosphorus removal methods cannot be used for municipal wastewater. Due to the large number of treatment structures, significant land use, high investment costs, and complex operation and management requirements associated with traditional A2/O and UCT processes, these are rarely used in municipal wastewater treatment plants of medium size or smaller; therefore, they are not recommended. Traditional oxidation ditches do not have phosphorus removal capabilities, nor do they feature dedicated anoxic zones; nitrogen removal takes place in the anoxic areas formed between the aerators, which results in limited nitrogen removal capacity. CG.,/]_ utilizes diatomaceous earth as a water treatment agent, along with high-efficiency hydraulic circulation clarifiers for wastewater treatment. During the selection process of diatoms, impurities that coexist with them are separated and removed, which results in an imbalance in the electric potential on the surface of the diatoms. During water treatment, a small amount of this diatomaceous earth agent is added to the wastewater; under high-speed stirring or due to the rotation of the pump blades that draw up the wastewater, it disperses instantly throughout the water body. The uneven electric potential on the surface of the diatoms enables them to neutralize the charge of suspended ions, thereby disrupting their repulsive forces and allowing them to attach to the diatoms. Through electrostatic neutralization and precipitation, larger flocs are formed, which then settle to the bottom due to gravity. Thanks to the large surface area, high pore volume, and strong adsorption capacity of diatoms, this treatment agent can adsorb, flocculate, and filter out organic and inorganic substances, as well as fine and ultra-fine particles, from the wastewater, forming chain-like structures. Diatoms composed of amorphous active silica have the ability to aggregate in water bodies and settle freely to form diatom cakes. Furthermore, the flocculation effect resulting from the modification of the fine soil accelerates the aggregation of diatoms and similar organisms at the bottom of the water, forming diatom cakes; this process leads to the neutralization of the electrical potential when the diatoms are adsorbed, allowing pollutants and bacteria to sink instantly and separate from the water. (5) Removal of P: To remove phosphorus from wastewater, traditional methods include chemical approaches as well as biological methods. 7k ‘gt/#up Chemical phosphorus removal involves adding trivalent salts (usually aluminum and iron salts; ferrous iron must be oxidized to ferric iron in the aeration tank) to the wastewater, causing them to form insoluble compounds with the phosphates present in the water, which are then removed from the water through precipitation. The advantage of using chemical phosphorus removal is its simple process; no additional equipment is required beyond that for adding chemicals, making it particularly suitable for equipping existing plants with phosphorus removal capabilities. The disadvantages are high chemical consumption, increased amount of excess sludge, and higher treatment costs. The addition of chemical agents also consumes the alkalinity in the water. F_v-}bbcFQ Biological phosphorus removal involves polyphosphate-accumulating bacteria in wastewater releasing the phosphate stored within them under anaerobic conditions to generate energy for the absorption of rapidly degradable organic matter, which is then converted into PHB (polyβ-hydroxybutyrate) for storage. When these polyphosphate-accumulating bacteria are placed in aerobic conditions, they degrade the PHB stored within them to generate energy, which is used for cell synthesis and phosphorus uptake. This results in the formation of high-concentration sludge, which is then removed from the system along with the excess sludge, thereby achieving phosphorus removal. The downside is that selecting a sludge treatment process is somewhat limited in order to avoid the re-release of phosphorus in the excess sludge. The absorption of phosphorus in the aerobic stage depends on the release of phosphorus in the anaerobic stage, and the release of phosphorus in turn depends on the anaerobic conditions in that stage (which require the absence of both molecular oxygen and nitrate nitrogen) as well as the amount of rapidly degradable organic matter (this value is generally 1/4 to 1/3 of the inlet CODcr); in other words, the lower the P/CODcr ratio, the better. The efficiency of phosphorus removal is closely related to the concentration of biodegradable CODcr in the biological treatment tank; when the concentration of biodegradable CODcr is less than 50 mg/L (with the inlet CODcr concentration being around 150–200 mg/L), almost no phosphorus removal occurs. A P/CODcr ratio of less than 0.025 is required to achieve effective phosphorus removal. Meeting all these conditions is very difficult; as a result, biological phosphorus removal processes are generally not effective. ~Based on the quality of water entering conventional urban sewage treatment plants and the goals that need to be achieved, we believe that the optimal treatment process is one that utilizes diatomaceous earth as a water treatment agent, along with high-efficiency hydraulic circulation clarifiers. This approach ensures a removal rate of total phosphorus of over 90%, and in some cases even 99%, a level that cannot be achieved by any other existing treatment process ； Moreover, it effectively removes heavy metal ions from wastewater while simultaneously removing phosphorus. While meeting the requirements for phosphorus and nitrogen removal, the removal of BOD5, CODcr, and SS can all be achieved simultaneously. II. Selection of process schemes for producing reclaimed water in wastewater treatment plants. Considering the various main methods for nitrogen and phosphorus removal, processes such as A2/O, UCT, and oxidation ditches require dedicated sedimentation tanks, which results in larger floor space as well as higher investment and operating costs ； Although a T-type oxidation ditch does not require a secondary sedimentation tank, the depth of water in such systems is limited by the aeration equipment used, which results in a large footprint; moreover, both the utilization rate and the volume efficiency are quite low ； Traditional SBRs (including the cyclic aeration process CAST), although having fewer structures and lower head losses, have a volume utilization rate of only 50% ； The diatomaceous earth water treatment process and the Qingzhong biochemical process (a biochemical technology used under conditions of a very low BC ratio) feature low investment costs, small land requirements, low operating expenses, high removal efficiency, simple structure, low energy consumption, the ability to recycle the sediment, no secondary pollution, strong adaptability, and the capability to function unaffected by changes in climate, temperature, or wastewater concentration.