Detailed explanation of a bread baking plant wastewater treatment case

The baking industry is one of the seven major food manufacturing industries in China. The food uses flour, yeast, salt, sugar and water as basic raw materials, and adds appropriate amount of oil, milk, eggs, and additives. With the rapid development of the bakery food industry and the scale of production, the drainage capacity of the wastewater produced by the baking enterprises is also increasing day by day, and the water quality is more complicated. The baking wastewater itself is non-toxic, but contains a large amount of biodegradable organic matter, high COD, BOD5, SS, and also contains a higher concentration of animal and vegetable oil. At present, there are few reports on wastewater treatment in the pastry industry at home and abroad. How to improve and improve the wastewater treatment technology of the pastry baking industry, adopting a scientific combination of technology and technology, and effectively solving the problem of wastewater treatment in the pastry industry has attracted more and more attention.

The waste water of a bread baking plant is 500 m3/d. The wastewater contains a lot of flour and cream. The original sewage treatment station can't handle the current sewage load. The original process uses air flotation + anaerobic + aerobic process to treat the production wastewater. The treatment effect is poor, the anaerobic residence time is short, and the original sewage treatment station cannot meet the actual production needs of the existing plant area, and the sewage treatment station of the plant area needs to be reconstructed. In this paper, combined with the hydrolysis acidification / UASB / A2O process to treat the drying wastewater treatment process and process design parameters, etc., can provide experience for similar projects.

1 original process

1.1 Project Overview

The wastewater influent water volume is 500 m3/d, and the water quality after treatment by the wastewater treatment station can meet the first-class water quality discharge standard stipulated by the comprehensive wastewater discharge standard (GB 8978-1996). The design influent water quality and emission standards of the project are shown in Table 1.

1.2 Problems with the original process

(1) Before the sewage enters the regulation tank, the pretreatment design is unreasonable. The solids of the large particles are not removed and the effective pool capacity is occupied by the sedimentation tank. It does not play a role in regulating the water quality and quantity.

(2) There is no oil separation and precipitation pretreatment process before the air flotation equipment, which causes high concentration of suspended pollutants and a large amount of oil to directly enter the air flotation equipment, resulting in high load of the air flotation equipment. The effect of air flotation equipment is too poor, which may be caused by unreasonable equipment configuration or design, and cannot meet the needs of wastewater pretreatment of this project.

(3) The design of the anaerobic treatment unit is not reasonable, resulting in poor water distribution effect and anaerobic three-phase separation, and sludge sludge drainage is not smooth, thus affecting the overall treatment effect of anaerobic. The anaerobic treatment unit was not designed according to the actual water quality. The volume of the anaerobic tank was too small, the hydraulic retention time was too short, and the organic pollutants could not be fully degraded, so that the actual water quality treatment requirements could not be met.

(4) CASS pool micro-hole aeration head is blocked or damaged too much, and the oxygenation effect is poor, which affects the treatment effect. The drowning length of the decanter of the CASS pool does not meet the design specifications, and it cannot guarantee that the suspended solids in the clear water will not exceed the standard.

(5) Because the water contains a lot of flour, the whole system will produce a large amount of sludge during the operation. The sludge dewatering device used in the original system can not solve the dehydration problem of the sludge, and the dewatering method needs to be changed; the dewatered sludge enters The mud process is too long and is not conducive to sludge treatment. Moreover, the current sludge treatment system has a small processing capacity and cannot meet the needs of sludge treatment.

2 Transformation process

In order to make the effluent reach the requirements of the first-class standard specified in the comprehensive discharge standard GB 8978-1996 for sewage after sewage treatment, and the current situation of the sewage treatment station, determine the oil separation + air flotation + UASB + A2O + biological filter The main body of the pool processes the process wastewater treatment process, and the process flow after the transformation is shown in Figure 1.

The production sewage is collected by the pipeline and then flows into the grid well, and the large debris is removed through the grid, and then enters the collecting basin. Install a self-contained submersible sewage pump in the collecting basin, and prevent the submersible sewage pump from being blocked by stirring and cutting. The sewage in the collecting basin is lifted into the rotary solid-liquid separator by the submersible sewage pump. The rotating solid-liquid separator passes through a 0.5 mm filter screen to remove most of the suspended solids in the sewage under the action of rotating centrifugal force, and then flows into the grease trap by self-flow, removes the oil in the sewage and enters the regulating tank. The sewage after adjusting the water quality and water volume is lifted into the vertical flow sedimentation tank by the lifting pump of the regulating tank, and the pollutants not removed by the filter net are further removed by the sediment, and then enter the air floating tank for further air flotation treatment [1] to ensure the oil in the sewage. Classes, flour and other suspended pollutants are effectively removed. Then, it flows into the hydrolysis acidification tank, and the macromolecular organic matter in the sewage is degraded into small molecular organic substances by the hydrolyzed bacteria, thereby improving the biodegradability of the wastewater. The sewage treated by the hydrolysis acidification tank enters the intermediate pool, is lifted into the UASB anaerobic tank by the lift pump for anaerobic treatment [2], and the organic pollutants in the sewage are converted into methane, carbon dioxide and water by anaerobic bacteria. The anaerobic treated wastewater from UASB flows into the anoxic + anaerobic + aerobic A2/O system for further biological treatment, thereby completely removing pollutants from the sewage.

The sewage treated by the secondary settling tank flows into the transfer tank and flows into the aerated biological filter through the aerated biological filter lift pump for further processing. The aerated biological filter further degrades the organic pollutants in the sewage through the biofilm and denitrifies and removes phosphorus. At the same time, the aerated biofilter also has the function of filtration. After treatment by aerated biological filter, the sewage can meet the requirements of the first-class standard specified in the comprehensive discharge standard of wastewater (GB 8978-1996), and it can be discharged after being measured by the total discharge of the sewage.

After the aerated biological filter runs for a period of time, more and more biofilms are aging and shed, and more and more pollutants are trapped, causing clogging of the filter material and increasing suspended solids. At this time, the biological aerated filter is required. Recoil. The backwash water is returned to the conditioning tank for reprocessing.

3 Main structures and design parameters

Grid room: reinforced concrete structure, structure size 3.0 m × 0.4 m × 2.0 m (new), the animal and vegetable oil in the sewage mixed with flour, adhered to the grille strips are easy to block, using mechanical grille to salvage. 1 mechanical grill machine.

Water collection pool: collection of incoming water, reinforced concrete structure, structure size 3.0 m × 2.0 m × 2.5 m (new construction), joint with the grille, the pool is equipped with agitated submersible sewage pump, and the effluent is connected to the rotary solid-liquid separator. 1 set with a flow rate of 25 m3/h, a filter aperture of 0.5 mm and a motor power of 1.5 kW. The access separator can remove most of the suspended pollutants, prevent them from entering the grease trap and regulate the sedimentation in the pool, which is convenient for clearing mud and saving floor space.

Grease trap: reinforced concrete structure, building size 2.0 m × 3.0 m × 2.5 m, newly built on the original conditioning tank.

Modification of the adjustment tank: connect the original sludge pool partition wall to the adjustment pool, and increase the adjustment pool capacity. After reconstruction, the structure size is 8.9 m×6.5 m×3.5 m, the effective pool capacity is 175 m3, and the hydraulic retention time is 8.4 h. It is used to adjust the water quality and the alkaline water quality fluctuates.

Vertical flow sedimentation tank: using the original anaerobic tank, the sedimentation tank has a diameter of 5.4 m and a surface load of 0.988 m3/(m2·h). In addition to the increase of the pool body, new equipment, a set of central sedimentation water distribution system, sludge collection mud 1 set of bucket and mud system.

Air floating pool: new supporting facilities, 1 pipeline mixer, 1 dissolved gas tank, 1 set of reflux dissolved air pump: Q=10 m3/h, H=40 m, N=4 kW, scroll air compressor 2 sets, air floating pool body, carbon steel anti-corrosion, processing capacity 21 m3 / h.

Hydrolytic acidification tank: The original anaerobic tank was used to renovate, the water retention time was 5 h, and 1 set of hydrolyzed gas distribution system and 1 set of sludge discharge system were added.

Intermediate pool: Reconstructed by the original sludge tank, the structure size is 2.5 m × 2.0 m × 3.5 m, the effective tank capacity is 13 m3, the intermediate tank lift pump, 2 sets, 1 with 1 preparation, the parameter is Q=21 m3/h, Head: 15 m, motor power 2.2 kW.

UASB anaerobic tank: single-cell structure size 6.0 m × 6.0 m × 7.0 m, 2 seats, effective tank capacity 468 m3, volume load 5 kg / (m3 · d), 8 sets of three-phase separator, anaerobic water distributor 30 sets, 2 internal circulation reflux pumps, parameters Q=9 m3/h, H=15 m, N=0.75 kW.

Anaerobic tank anoxic pool construction: anaerobic tank, effective volume 73 m3, steel-concrete structure, hydraulic retention time 3.5 h, anaerobic stage, release of phosphorus in sewage. The anoxic tank has an effective volume of 127 m3 and a water retention time of 6 h. During the denitrification and denitrification process, the anoxic tank consumes a large amount of nitrogen-containing organic matter in the wastewater, which greatly reduces the COD and BOD5 of the wastewater.

Renovation of aerobic aeration tank: use the original aeration tank to transform, increase the partition wall, need to replace part of the aeration head and pipeline, fan. The effective pool capacity is 500m3 and the hydraulic retention time is 24h. Mixture reflux pump, 2 sets (1 with 1 preparation), parameters: Q=100 m3/h, H=10m. 2 sets of sludge return pump (1 with 1) Parameters: Q=25 m3/h, H=10 m.

Second settling tank: 1 seat, steel-concrete structure, structure size 4.0m×6.5m×5.0 m, surface load 0.8 m3/(m2·h), muddy water separation for sewage, and inclined pipe sedimentation.

Transfer tank: 1 seat, steel-concrete structure, process size 1.5 m × 1.5 m × 4.5 m, receiving water from the second settling tank, so that the aeration biofilter inlet pump can be lifted. Aerated biological filter inlet pump, 1 set, Q=25m3/h, H=15m, N=2.2 kW.

Aerated biological filter, 2 blocks, steel-concrete structure, single structure size 3.0 m × 1.5m × 5.0m, further treatment of the secondary settling tank effluent. Increase blower, 2 units, 1 with 1 preparation, Q=6.2 m3/min, N=7.5 kW, shared with aerobic tank.

Clear water pool: 1 seat, steel-concrete structure, size 1.5m × 1.5m × 4.5m.

Backwashing pump parameters Q=100 m3/h, H=15 m, N=7.5 kW. The aeration filter is effluent and the aerated organism is backwashed.

Sludge tank: 1 seat, steel-concrete structure, process size 3.9 m × 2.7 m × 4.5 m, sludge pump parameters: Q = 6 m3 / h, H = 60 m.

Other structures: The dosing equipment room is newly built on the adjustment tank, and the dosing equipment is placed and installed. The size is 4.5 m × 2.7 m × 3.5 m, 1 seat, and the equipment room is newly built next to the second settling tank with an area of ​​50 m2.

4 operation effect and analysis

4.1 Operation effect

Since the sewage treatment station was started in March 2012, the operation effect is stable, and the effluent treatment effect is good, and all meet the design requirements and discharge standards. The wastewater treatment effect is shown in Table 2.

It can be seen from Table 2 that the combined process of hydrolysis and acidification/UASB/A2O for the bread-baked food waste water has good effluent effect after treatment, and has high removal efficiency, COD, ammonia nitrogen, SS, oil and total. The concentration of phosphorus effluent is 83.48, 11.61, 31.86, 0.26, 0.46 mg/L, which can meet the first-level discharge standard of comprehensive wastewater discharge standard (GB8978-1996). It can be seen that the process is suitable for the treatment of baked food wastewater.

4.2 Process treatment analysis

The bread processing food waste water is high-concentration organic sewage, its BOD5/COD is about 0.37, and its biodegradability is good. It is better to treat the sewage by anaerobic + aerobic main treatment process. However, pretreatment is particularly important because the sewage contains a lot of flour and oil. The reason why the original sewage treatment station can not effectively treat the sewage is that the pretreatment effect is not good.

In the front section of the sewage treatment, the grease trap and the rotating high-efficiency solid-liquid separator are added to remove oil slick and large-scale suspended pollutants (flour). However, considering that some of the flour is very fine, the solid-liquid separator is not completely separated. Therefore, after the adjustment tank, the vertical flow sedimentation tank and the air floatation unit are added for further removal [3] to ensure long-term stable operation of the subsequent biological treatment unit.

Due to the particularly high concentration of the sewage pollutants, it is required to have sufficient effective pool capacity to ensure effective treatment of sewage. According to the actual situation of the current sewage treatment station, the floor space is very limited, so it is necessary to adopt an efficient treatment process. The wastewater is subjected to anaerobic treatment by hydrolytic acidification + UASB anaerobic process to remove most of the organic matter, while improving the biodegradability of the wastewater and enhancing the effect of subsequent aerobic treatment.

The wastewater treated by the above anaerobic process still contains a relatively high concentration of ammonia nitrogen and total phosphorus, so the subsequent process is treated by an A2/O process with nitrogen and phosphorus removal. It can effectively reduce the floor space, improve the treatment effect of wastewater, and create conditions for subsequent advanced treatment.

5 Economic and technical analysis

5.1 Project Investment

The total investment of the project is 3.1 million yuan, of which the investment in the civil construction is 2.17 million yuan and the investment in equipment is 930,000 yuan.

5.2 Operating expenses

Power fee: The total installed capacity of the wastewater treatment station is 51.0 kW, and the actual total operating capacity is 24.6 kW. Some equipments are used intermittently, and the total operating capacity is reduced to 0.7 times. According to the local electricity price of 0.5 yuan/kWh, the power cost is 0.32 yuan/m3.

Pharmacy fee: flocculant, alkali, nutrient solution total 0.51 yuan / m3.

Labor cost: There are 3 transfer workers in this sewage treatment station. The monthly labor cost is 1,500 yuan, equivalent to 0.36 yuan/m3.

Water fee: mainly for the sewage treatment station staff water and medicated water, the daily water consumption is about 2.0 m3, the water fee is 5.2 yuan / m3, the sewage water fee is 0.02 yuan / m3.

Direct operating expenses include power, pharmacy, and labor, totaling 1.21 yuan/m3.

6 Conclusion

Treatment of baked food waste water by hydrolysis acidification / UASB / A2O, it has been proved that the combined process is highly targeted to the high concentration of food wastewater, has a certain impact resistance and high load bearing capacity, and the effluent can reach the comprehensive discharge of sewage. The first-level emission standard of the standard (GB 8978-1996).

The treatment process of this project covers the province, the structure of the structure is simple, the operation is simple, the investment in wastewater treatment project is saved, the operation cost is low, and the treatment cost per ton of water is 1.21 yuan/m3.

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