Common problems in sewage treatment

1. What are the reasons for the phenomenon of fine sludge rolling and turbidity in the secondary sedimentation tank?

① The aerobic tank has a low sludge load, excessive aeration, and self oxidation of the sludge, resulting in poor sludge flocculation and dispersed sludge structure (water is turbid and suspended solids are abundant);

② The aerobic tank has an excessive sludge load, insufficient dissolved oxygen, poor sludge adsorption performance, and incomplete decomposition of organic matter;

③ The load of the secondary sedimentation tank is too high, or there is gravity flow phenomenon due to uneven water distribution in the secondary sedimentation tank, and the local flow rate is too fast, which will lift up the sludge;

④ The reflux ratio of the secondary sedimentation tank is too high, the mud layer in the secondary sedimentation tank is too low, and the water flow agitates the mud layer too much (this reason accounts for a small proportion);

⑤ The excessive discharge of sludge from the aerobic tank results in a short age of sludge, making it difficult for newly synthesized sludge flocs to settle (with clear water and high suspended solids);

⑥ The aerobic tank sludge is too old, resulting in sludge aging;

⑦ Insufficient or imbalanced nutrient ratio (high N and P ratio) in aerobic sludge;

⑧ The sludge in the aerobic tank experiences sludge bulking, poor settling ability, high sludge layer in the secondary sedimentation tank, and water flow carrying the sludge out (this situation can occur if the SVI value is too high or too low);

⑨ The ammonia nitrogen content in the wastewater of the aerobic tank is too high.

2. What are the reasons for the occurrence of floating slag and mud in the secondary sedimentation tank?

① The reflux ratio of the secondary sedimentation tank is small, and the sludge retention time is too long. After anaerobic denitrification, the sludge is carried up by gas;

② A large amount of physicochemical and anaerobic sludge enters the aerobic tank, and some of them cannot be converted into aerobic sludge and become floating sludge for discharge into the system;

③ Aerobic tank sludge spoilage and deterioration;

④ The aerobic tank has a lot of foam, which floats up in the secondary sedimentation tank after mixing with sludge/suspended solids;

⑤ Low sludge concentration in aerobic tanks (high sludge load) or high dissolved oxygen (possible);

⑥ Aging or short sludge age in aerobic tanks, poor flocculation, poor COD removal rate and treatment effect.

3. What are the reasons for insufficient dissolved oxygen in aerobic tanks?

① The rapid increase in sludge concentration in aerobic tanks or the increase in oxygen consumption due to sludge aging;

② There is a lot of suspended solids in the effluent of the anaerobic tank, and a large amount of dissolved oxygen is consumed after entering the aerobic tank;

③ The blower malfunctions and stops running or the fan pressure is insufficient (this situation is rare);

④ The COD of the effluent from the anaerobic tank suddenly increases significantly, or the inflow suddenly increases, resulting in a large impact load and an increase in the load of the aerobic tank;

⑤ The aerator is seriously damaged or blocked, and the aerobic tank has more foam.

4. What are the reasons for sludge expansion in aerobic tanks?

① Long term low or high dissolved oxygen in aerobic tanks (possible);

② Excessive sulfide content in raw water or anaerobic effluent leads to the proliferation of sulfur bacteria;

③ The aerobic tank load is consistently low or high;

④ The water temperature in the aerobic tank is too high;

⑤ Imbalance or lack of nutrients (low N and P);

⑥ The issue of influent pH value;

⑦ The sludge age in the aerobic tank is too long, leading to an increase in oxygen consumption and insufficient dissolved oxygen.

5. What are the reasons for the phenomenon of sludge disintegration and excessive crushing of supernatant in aerobic tanks?

① The aerobic tank has a low sludge load, excessive aeration, self oxidation of sludge, deterioration of sludge flocculation, and loose sludge structure (clear, with more fine sludge);

② The aerobic tank has an excessive sludge load, resulting in poor sludge adsorption performance, incomplete decomposition of organic matter, and loose sludge structure (turbid, opaque, high COD) during microscopic inspection;

③ The excessive discharge of sludge from the aerobic tank leads to a short age of sludge in the aerobic tank (SVI value is suitable between 70-120, within which the secondary sedimentation tank has less finely crushed sludge);

④ The influent of the aerobic tank contains toxic substances or aged sludge, with a long sludge age (turbid, fine sludge, high COD, and many rotifers detected by microscopy);

⑤ Insufficient or uneven proportion of nutrients (low N and P) in the aerobic pool.

6. Why is there a lot of foam in the aerobic tank?

① The raw water contains a large amount of surfactant ingredients (from the substances added in the production process, the foam is white, the bubbles are small, light and without viscosity);

② The small bubbles generated after the installation of the new aeration head (short-term impact);

③ A large number of lipid substances or microorganisms are produced during microbial reproduction (from the growth and reproduction of microorganisms themselves, the foam is muddy, with large bubbles and viscosity);

④ Sludge denitrification foam (the foam produced after denitrification of aerobic sludge in the secondary sedimentation tank for too long is viscous and muddy).

7. What are the reasons for the low COD removal rate in aerobic tanks?

① Aging of aerobic sludge and long sludge age;

② The aerobic tank has high sludge load, short sludge age, high reflux rate, and short retention time;

③ The aerobic tank has a low sludge load, and the long-term high dissolved oxygen leads to self oxidation of the sludge (low removal rate, high dissolved oxygen), with more finely crushed sludge and less active sludge;

④ Insufficient dissolved oxygen in the aerobic tank;

⑤ Insufficient or imbalanced proportion of nutrients (N, P ratio too high);

⑥ The COD removal rate in anaerobic tanks is low, the anaerobic hydrolysis effect is poor, and the COD concentration in the effluent is too high;

⑦ Raw water contains toxic substances and sludge poisoning;

⑧ The accumulated value of inorganic salts exceeds the specified range;

⑨ The aerobic tank has a high impact load or sludge expansion phenomenon.

8. What are the reasons for the low COD removal rate in anaerobic tanks?

① Insufficient sludge concentration in the anaerobic tank (returning biochemical sludge to the anaerobic tank);

② The anaerobic tank enters a large amount of physicochemical sludge (with inorganic substances accounting for the majority);

③ Insufficient or uneven proportion of nutrients in anaerobic tanks;

④ The water temperature exceeds the range adapted by anaerobic microorganisms (over 40 ℃);

⑤ The influent pH exceeds 10.5 or is below 6.5;

⑥ The retention time of the anaerobic tank is too short to reach the anaerobic hydrolysis state (design issue);

⑦ Entering toxic substances.

9. What are the reasons for the high amount of finely crushed sludge in the supernatant of the aerobic tank and the difficulty of settling due to the rolling of the finely crushed sludge?

① Insufficient or uneven proportion of nutrients in aerobic sludge;

② The sludge load in the aerobic tank is too high (the effluent of the secondary sedimentation tank is turbid, with high COD, and the sludge in the aerobic tank is finely crushed after settling in the supernatant, resulting in turbidity);

③ The sludge load in the aerobic tank is too low, the aeration is excessive, and the fine sludge produced by the self oxidation of the sludge (the COD removal rate in the aerobic tank is low, and the effluent COD is high);

④ The sludge load in the aerobic tank is too low, the sludge retention time is long, and excessive aeration leads to poor sludge flocculation (loose sludge structure but high or not low COD removal rate).

10.How to solve the problem of excessive suspended solids (sludge) in the effluent of anaerobic tanks?

① Control the entry of physicochemical sludge from the initial sedimentation tank into the anaerobic tank (mandatory);

② Add a siphon sludge discharge pipe at the top of the anaerobic tank (it is not recommended to discharge anaerobic bottom sludge);

③ Add polypropylene or aluminum to the anaerobic tank;

④ Reduce the inflow or discharge the sludge at the bottom of the anaerobic tank.

11.How to solve the problem of sludge expansion in aerobic tanks?

① Firstly, increase the discharge of sludge to solve the problem of poor sedimentation effect, and then improve the sludge concentration and reduce the sludge load;

② Increase the discharge of sludge from aerobic tanks and reduce the age of sludge (in severe cases, it should be persisted for about two months);

③ Control the water temperature within an appropriate range, stabilize the inflow, and maintain sufficient dissolved oxygen in the aerobic tank (necessary);

④ Increase the addition of nutrients to the aerobic tank;

⑤ If the mud layer in the secondary sedimentation tank is high, the reflux rate can be increased, the inlet water volume of each secondary sedimentation tank can be adjusted, or polyaluminum and polypropylene can be added (temporary control measures).

12. What issues should be noted when designing papermaking wastewater treatment projects?

① The sludge concentration tank must be large enough to produce a large amount of physicochemical sludge;

② The mud press should meet the demand for mud production in the system;

③ The regulating pool must be large enough, because the drainage of papermaking is extremely unstable and fluctuates greatly (the instantaneous drainage of the paper machine during shutdown is large);

④ It is best to treat white water (white/talc powder) separately or add a small amount of it to the raw water for treatment;

⑤ It is necessary to consider the problem of calcium ions entering the aerobic tank and causing scaling of the aeration head (selection of physical and chemical treatment methods or aeration methods);

⑥ Consider the issue of the destination of a large amount of sludge generated from papermaking wastewater (water content below 35%~40% can be sent to the boiler for incineration, while also dealing with the problem of flue gas after incineration);

⑦ When selecting pumps, consideration should be given to the problem of high levels of suspended solids and debris in papermaking wastewater, which can easily cause blockages.

13. What are the symptoms of aging sludge in aerobic tanks?

① In the initial stage of settling ratio, the supernatant begins to become turbid, with fine sludge suspended and difficult to settle. Gradually, there will be floating slag and mud in the secondary sedimentation tank;

② Aging of sludge can lead to an increase in oxygen consumption of aerobic tank sludge (pay attention to signs of sudden decrease in dissolved oxygen);

③ Microscopic examination shows that the sludge structure is dispersed, with fewer filamentous bacteria, more rotifers, and fewer protozoa. The color of the sludge becomes light and yellow;

④ The foam produced by the sludge from the returned secondary sedimentation tank is between the surfactant foam and the biological foam, which is somewhat sticky;

⑤ The treatment effect of aerobic tanks deteriorates, oxygen consumption increases, effluent COD and suspended solids increase, and turbidity increases.

14. What are the reasons for the aging of sludge in aerobic tanks?

① Insufficient or unbalanced nutrients, high sulfide concentration in aerobic tanks, and insufficient dissolved oxygen;

② Mud age is too long (microscopic examination shows that there are many rotifers in the sludge, the sludge structure is dispersed, the effluent is turbid, and the supernatant mixed with clean water is still turbid, with signs of sludge disintegration);

③ The sludge stays in the secondary sedimentation tank for a long time, and after anaerobic denitrification, the sludge becomes viscous, producing lipid substances (in severe cases, there may be a foul odor in the secondary sedimentation tank).

15. How to solve the aging of sludge in aerobic tanks?

① Increase the addition of nutrients;

② Discharge more aerobic sludge from the tank, increase sludge reflux, and reduce the residence time of sludge in the secondary sedimentation tank;

③ Reduce the inflow of the aerobic tank appropriately, and gradually increase the water volume when the sludge activity improves.

16. What are the shortcomings of microporous aeration methods?

① Micro porous aeration membranes are expensive and the installation process is complex and troublesome;

② High maintenance costs and cumbersome maintenance processes;

③ When applied to papermaking wastewater engineering, it is prone to blockage (oxygen reacts with calcium ions to produce calcium oxide);

④ The microporous aeration membrane is prone to aging, and the clamps are prone to detachment after corrosion.

17. What are the advantages and disadvantages of using stainless steel pipes (or PVC pipes with high pressure resistance and strength) for direct aeration?

① Low cost, simple and easy installation, with almost no maintenance costs (the size of the opening hole can be calculated according to needs);

② Not aging, not prone to scaling and blockage, corrosion-resistant;

③ The generated bubbles are large, the oxygen utilization rate is low, and a large amount of gas supply is required (when used in the contact oxidation method, the suspended packing has the effect of shearing bubbles, and the bubbles will become smaller).

18.How to restore the processing capacity after the renovation and installation of the aerobic tank is completed?

① Firstly, let the water flow past the aeration head, then turn on the fan to allow the aeration head to ventilate and check if there is any leakage, breakage, or non aeration at the joints of the aeration head;

② Then reflux the sludge while entering the water, with the inlet water volume around 1/2 or 1/3 of the design. Slowly increase the load after the outlet water meets the standards;

③ Nutrients can be added as usual.

19.Operation parameter control and effectiveness of A/O process for 192000 cubic meters/day papermaking wastewater

① Stable water inflow, achieving physical and chemical effects;

② Improve the anaerobic COD removal rate by regularly returning aerobic sludge to the anaerobic tank;

③ The water temperature of the aerobic tank is below 38 ℃, the sludge concentration is controlled at 3.0-3.5g/L, the dissolved oxygen is controlled within the normal range, and the sludge age is controlled at 5-7 days;

④ The reflux ratio of the secondary sedimentation tank should be controlled at 60%~75% (to ensure smooth suction of the scraper);

⑤ Nutrient dosage (anaerobic+aerobic): 450Kg/day for flour, 450Kg/day for urea, and 225Kg/day for SanNa;

⑥ The secondary sedimentation tank has no floating slag or mud, and its appearance is very good;

⑦ The secondary sedimentation tank does not (or rarely) roll fine sludge (aerobic sludge has good activity);

⑧ Aerobic sludge has a compact structure, with a sludge settling ratio of 30% to 40% and a sludge index between 100 and 120. Aerobic sludge is brown and full;

⑨ The effluent color of the secondary sedimentation tank is light brown, with a COD of around 80mg/L, clear and transparent, and low turbidity.

20.What measures should be taken when the aerobic tank stops receiving water for maintenance? How to restore the processing effect?

① Increase the return flow rate of the secondary sedimentation tank;

② Reduce the number of fans running;

③ Increase the addition of nutrients;

④ Discharge a small amount of biochemical sludge externally;

⑤ Gradually increase the inflow and increase the number of fans operating with the increase of water volume;

⑥ Restore normal sludge return flow rate and gradually resume normal nutrient feeding.

21. What will happen if the dissolved oxygen in the aerobic tank is too high for a long time?

① Aerobic sludge will oxidize on its own and turn white in color;

② Aerobic sludge gradually ages, with a loose structure, small bacterial clusters, an increase in filamentous bacteria, and a large number of rotifers reproducing;

③ The supernatant contains more finely crushed sludge, resulting in poorer treatment efficiency and turbid effluent;

④ The color of the effluent will darken (the broken bonds after anaerobic treatment will reconnect under high oxygen oxidation).

22. What will happen if the dissolved oxygen in the aerobic pool is insufficient for a long time?

① The color of sludge turns black, and the treatment effect deteriorates;

② As the sludge load increases, filamentous bacteria are prone to proliferation, leading to sludge bulking;

③ Microscopic examination of sludge revealed a large number of rotifers breeding, disappearance of clock worms, ciliates, etc., and opaque bacterial gel clusters;

④ The effluent of the secondary sedimentation tank is turbid, the return sludge denitrification foam increases, and the sludge and foam become viscous.

23. What are the manifestations of sludge expansion in aerobic tanks?

① The color of the effluent darkens (possibly due to filamentous bacteria);

② The settling ability of sludge decreases, and the sludge index increases (SV30 ≥ 80-100, SVI ≥ 150);

③ The sedimentation of sludge is overall sedimentation, and the supernatant is clear, but the effluent COD will gradually increase with the development of sludge expansion, and the aerobic removal rate will gradually decrease;

④ Microscopic examination shows that filamentous bacteria in sludge proliferate in large numbers and extend out of the bacterial gel (the bacterial gel gradually becomes thinner and smaller, and the sludge structure becomes loose);

⑤ After the sedimentation of sludge, there is a feeling of loose expansion on the appearance (shaking makes the sludge feel light and floating);

⑥ The foam in the aerobic tank increases (possibly due to filamentous bacteria);

⑦ The color of the sludge becomes lighter (brownish to yellowish).

24. What abnormal phenomena may occur in aerobic tanks?

① Aerobic sludge turns black or white (low or high dissolved oxygen);

② The clear liquid on the aerobic tank is turbid (the sludge adsorption performance deteriorates or the dissolved oxygen is too high, leading to sludge disintegration, and the dissolved oxygen is too low to oxidize organic matter);

③ The sludge foam returned from the secondary sedimentation tank becomes viscous (sludge stays in the secondary sedimentation tank for too long, and the activity of sludge after denitrification becomes poor);

④ The increase of foam in aerobic tank (judging by the color and viscosity of foam, whether it is caused by the change of sludge itself or by substances added in production);

⑤ The removal rate of the aerobic tank has decreased (specific analysis reasons: sludge activity, sludge load, dissolved oxygen, sludge concentration, water temperature, etc.);

⑥ Aerobic tank sludge expansion (controlled by increasing sludge discharge and adjusting nutrient feed to stabilize inflow, ensure sufficient dissolved oxygen and suitable water temperature);

⑦ When using aerobic sludge for sedimentation ratio, the supernatant is turbid and has more fine and fragmented sludge (due to high sludge load or sludge disintegration, loose sludge structure and small bacterial gel clusters observed under microscopy);

⑧ The number of aerobic microorganisms decreases, the structure becomes loose, and the microbial communities become thin (with low or high loads, insufficient dissolved oxygen, sludge swelling, and insufficient nutrients);

⑨ The dissolved oxygen in the aerobic tank is high for a long time, and the effluent is turbid and has high COD (the sludge load is low for a long time, and the sludge disintegrates and bacterial flocs are oxidized, without consuming oxygen);

⑩ Sludge aging (the reasons for sludge aging include long sludge age, low load, etc. The aging of sludge leads to poor effluent, more fine sludge and rotifers, and an increase in oxygen consumption).

25. What abnormal phenomena may occur in the secondary sedimentation tank?

① The appearance of scum and floating sludge (sludge aging or short sludge age, sludge staying in the secondary sedimentation tank for too long);

② The effluent is turbid, with high COD and foul odor (insufficient dissolved oxygen in the aerobic tank and short residence time in the aerobic tank);

③ The effluent is turbid, the COD is not very high, and there is a lot of fine sludge (sufficient dissolved oxygen in the aerobic tank, low sludge load, and sludge aging);

④ The effluent is turbid, with high COD and a large amount of fine sludge (insufficient dissolved oxygen in the aerobic tank, aging sludge, and high sludge load);

⑤ Clear effluent with high COD (sludge expansion phenomenon in aerobic tank);

⑥ Rolling of fine sludge (if there is a problem with the sludge in the aerobic tank, it is recommended to add nutrients and adjust the appropriate sludge age);

⑦ The sludge layer in the secondary sedimentation tank is too high (sludge expansion or low reflux ratio in the aerobic tank);

⑧ Bubbles appear on the surface of the secondary sedimentation tank (sludge stays in the tank for too long);

⑨ Returned sludge turns black, odorous, and sticky (sludge retention time is too long, reflux ratio is small);

⑩ The effluent color becomes darker (due to poor physicochemical effect, anaerobic tank effect, or sludge bulking phenomenon in aerobic tank).

26.Why does the phenomenon of clear supernatant but high COD occur when the sludge in the aerobic tank expands?

① Filamentous bacteria have a strong adsorption effect, and a large number of filamentous bacteria have a net trapping effect, so the supernatant is clear;

② A large number of filamentous bacteria extend out of the bacterial gel, blocking the bacterial gel from obtaining sufficient oxygen and failing to oxidize and convert organic matter into inorganic matter;

③ The microbial communities do not receive sufficient oxygen, resulting in a decrease in reproductive activity and a reduction in their thinness and activity.

27. What is the reason for the turbidity of anaerobic tank effluent?

① The sludge load in the anaerobic tank is too high;

② There are many suspended solids in the effluent of the initial sedimentation tank;

③ The sludge concentration in the anaerobic tank is too high;

④ Imbalance of nutrients in anaerobic tanks;

⑤ The inlet water temperature of the anaerobic tank is too high.