Wastewater Treatment Knowledge: What a Biological Filter Means and Its Role as a Biological Treatment Structure in Sewage Purification
biological filter, trickling filter: the biological filter is an artificial biological wastewater treatment technology developed on the basis of the soil self-purification principle and the practice of sewage irrigation, evolving from the earlier intermittent sand filter and contact filter.
(1) large specific surface area; (2) high porosity; (3) high material strength; (4) stable; (5) inexpensive
The main structure of the works, providing support.
3. Tank bottom: ventilation system, sludge discharge system, supporting and percolating structure
4. Water distribution system: rotating water distributor
(4) The biological filter has a large buffering capacity, can automatically absorb concentration peaks so that the microorganisms keep working normally, and has strong shock-load resistance.
(5) Operation is fully automatic and very stable, with no manual operation required. There are few vulnerable parts and maintenance is very simple; the plant can basically run unattended, with workers only needing to inspect for equipment failures.
(6) The biological filter tank is of modular construction, easy to transport and install; treatment capacity can be increased simply by adding modules, which is easy to implement, and it also suits separate treatment under dispersed air supply conditions.
1. When the main goal is removing carbonaceous organic matter from the sewage, a single-stage carbon-oxidation biological aerated filter should be used;
2. When carbonaceous organic matter removal and nitrification of ammonia nitrogen are both required, a single-stage carbon-oxidation biological aerated filter may be used with an appropriately reduced load, or a two-stage series process of a carbon-oxidation filter and a nitrifying biological aerated filter may be adopted;
3. When the influent carbon source is sufficient and the effluent total nitrogen requirement is strict, the pre-denitrification filter + nitrification filter combined process is recommended;
4. When the influent total nitrogen concentration is high, the carbon source is insufficient and the effluent total nitrogen requirement is strict, a post-nitrification process with carbon source supplementation may be adopted, or a pre-denitrification filter with external carbon addition; the nitrified-liquor recirculation ratio of the pre-denitrification filter is determined according to the design NO3-N removal rate and the influent carbon-to-nitrogen ratio, and the external carbon dosage must be determined by calculation.
3.0~6.0kgBOD5/(m‧d)
2.0~10.0 m/ (m‧h)
3.0~6.0kgNH3-N /(m‧d)
3.0~12.0 m/ (m‧h)
3.0~6.0kgNO3-N /(m‧d)
6.0~12.0 m/ (m‧h)
1.0~3.0kgBOD5/(m‧d)0.4~0.6kgNH3-N /(m‧d)
1.5~3.5 m/ (m‧h)
0.8~1.2kgNO3-N /(m‧d)
8. When denitrification is required but the carbon source is insufficient, the denitrification tank can be placed before the nitrification tank, with part of the nitrification tank effluent recirculated to the denitrification tank to form pre-denitrification. The advantages are as follows: a) organic matter in the wastewater serves as the carbon source, reducing external carbon addition; b) organic matter is removed in the denitrification tank, ensuring the nitrification capacity of the carbon-oxidation/nitrification tanks; c) the aeration demand of the system is relatively low; d) less sludge is produced. For domestic sewage with sufficient BOD5 that requires denitrification, the pre-denitrification process has clear advantages in terms of operating cost.
1.5~3.0kgNO3-N /(m‧d)
8.0~12.0 m/ (m‧h)
1. Dissolved oxygen in the effluent of carbon-oxidation and nitrification filters should be controlled at 3.0~4.0 mg/L.
2. Increasing the filtration rate is unfavourable for carbon oxidation, as part of the insoluble organic matter is discharged before being degraded; 6 m/h is recommended.
3. Within a certain volumetric loading range, however, increasing the filtration rate not only does not reduce the removal rate of the biological aerated filter but also raises nitrification-denitrification efficiency. There are three main reasons: first, a high filtration rate enhances the mass-transfer efficiency inside the filter, giving air, wastewater and biomass more contact opportunities; second, at a high filtration rate the biofilm renews faster, enhancing biological activity; third, at a low rate the filter media clogs easily, shortening the backwash cycle, and frequent backwashing is very unfavourable for nitrifying bacteria, which reproduce slowly.
4. When the filter is mainly used for carbon oxidation: for effluent BOD5 = 10~20 mg/L, a volumetric load of 3.5~5.0 kgBOD5/(m·d) is recommended; for effluent BOD5 = 5~10 mg/L, a volumetric load of 2.5~3.2 kgBOD5/(m·d) is recommended.
5. When the filter is mainly used for carbon oxidation and nitrification, a volumetric load of BOD5 ≤ 3.0 kgBOD5/(m·d) is suggested. Research shows that when the BOD5 volumetric load exceeds this value, ammonia nitrogen removal is inhibited, and at BOD5 ≥ 4.0 kgBOD5/(m·d) ammonia removal is clearly suppressed.
6. For effluent CODcr at 60 mg/L, the influent load should be 4.0~5.0 kgCODcr/(m·d); when CODcr ≤ 50 mg/L, the influent load should be controlled below 3.0 kgCODcr/(m·d).
7. When the filter has nitrification and denitrification nitrogen-removal requirements, the volumetric loads of both nitrification and denitrification must be checked. Suggested volumetric loads are below 2.0 kgNH3-N/(m·d) and 5.0 kgNO3-N/(m·d) respectively; 0.3~0.8 kgNH3-N/(m·d) and 0.8~4.0 kgNO3-N/(m·d) are recommended.
8.0~10.0 m/ (m‧h)
9. Post-denitrification is more suitable for the following situations: a) wastewater with markedly low BOD5 content (high proportion of industrial wastewater); b) plant upgrades where nitrification targets were not considered previously, with low effluent BOD5 but relatively high ammonia nitrogen.
10. To avoid carbon removal affecting nitrification, post-denitrification should remove part of the BOD5 at the pretreatment stage. The C/N tank design filtration rate should preferably be 6~10 m/h, and the nitrification load should satisfy: for influent BOD5 ≥ 60 mg/L, about 0.3 kgNH3-N/(m·d); for BOD5 = 20~50 mg/L, about 0.6 kgNH3-N/(m·d); for BOD5 ≤ 20 mg/L, about 1.0 kgNH3-N/(m·d). If methanol is used as the external carbon source, the DN dosage is 3.3 kgCH4O/kgNO3-N.
