Biological Aerated Filter Design and Backwash Control: The Two Parameter Tables of HJ 2014-2012

1. Scope and influent conditions

HJ 2014-2012, Technical specifications for biofilter in wastewater bio-film treatment, was approved by the former Ministry of Environmental Protection on 2012-03-19 and became effective on 2012-06-01. Clause 1 is explicit: the standard covers only biofilter processes dominated by aerobic reactions and is not applicable to anaerobic biofilters; it addresses municipal wastewater and industrial wastewater of similar character. Chapter 9 is dedicated to the biological aerated filter, BAF.

Chapter 4 sets the influent conditions first:

• Clause 4.2: influent BOD5/CODCr should be above 0.3, pH preferably 6.5 to 9.5, water temperature preferably 12 to 35 degrees C, and the nutrient ratio BOD5:nitrogen:phosphorus preferably 100:5:1;
• Clause 4.5: where ammonia nitrogen removal is required, the ratio of total alkalinity (as CaCO3) to ammonia nitrogen should not be below 7.14 and the residual alkalinity of the aerobic tank or zone should exceed 70 mg/L, with alkalinity dosing where these are not met;
• Clause 4.6: where total nitrogen removal is required, the readily biodegradable carbon ratio BOD5 to total Kjeldahl nitrogen should exceed 4.0 and the ratio of total alkalinity (as CaCO3) to ammonia nitrogen should not be below 3.6;
• Clause 4.7: where phosphorus removal is required, the influent BOD5 to total phosphorus ratio should not be below 17.0, and chemical phosphorus removal should be added when the effluent total phosphorus fails to meet the design value.

Clause 4.8 and Table 1 give removal rates for use without test data: for municipal sewage after pretreatment followed by a biofilter, SS 75% to 98%, BOD5 80% to 95%, CODCr 80% to 90% and ammonia nitrogen 80% to 95%.

2. The loading table for five filter types (Table 2)

Clause 9.3.2.1 requires the volumetric and hydraulic loading to be established from test data, with Table 2 used only where such data are absent. The division of labour in Table 2 is very clear:

• Carbon oxidation filter: 3.0 to 6.0 kgBOD5/(m3.d), hydraulic loading 2.0 to 10.0 m3/(m2.h), empty bed contact time 40 to 60 min;
• Nitrification filter: 0.6 to 1.0 kgNH3-N/(m3.d), hydraulic loading 3.0 to 12.0 m3/(m2.h), empty bed contact time 30 to 45 min;
• Combined carbon oxidation and nitrification filter: 1.0 to 3.0 kgBOD5/(m3.d) together with 0.4 to 0.6 kgNH3-N/(m3.d), hydraulic loading 1.5 to 3.5 m3/(m2.h), empty bed contact time 80 to 100 min;
• Pre-denitrification filter: 0.8 to 1.2 kgNO3-N/(m3.d), hydraulic loading 8.0 to 10.0 m3/(m2.h) including recirculation, empty bed contact time 20 to 30 min;
• Post-denitrification filter: 1.5 to 3.0 kgNO3-N/(m3.d), hydraulic loading 8.0 to 12.0 m3/(m2.h), empty bed contact time 20 to 30 min.

Two notes to Table 2 matter. First, where the design water temperature is low, the influent concentration is low or the effluent requirement is stringent, the organic, nitrification and denitrification loadings should all take their lower bounds. Second, the hydraulic loading and empty bed contact time for denitrification filters are both determined on the flow including nitrified liquor recirculation, and the recirculation ratio is set by the required total nitrogen removal - the flow term most often omitted when sizing denitrification filters.

Clause 9.3.2.2 adds a frequently overlooked item: dissolved oxygen in the effluent of carbon oxidation and nitrification filters should be controlled at 3.0 to 4.0 mg/L. Clause 9.1.1 requires influent suspended solids not to exceed 60 mg/L; clause 9.1.6 requires the inlet to outlet water level difference, set by the distribution arrangement, filtration velocity and media head loss, to be not less than 1.8 m; clause 9.3.1.3 requires at least two cells with a single cell area of preferably 50 to 100 m2.

3. Tank dimensions and media

Clause 9.3.3.1 sets the method: calculate by volumetric loading and check by hydraulic loading. Clause 9.3.3.6 gives the six components of total tank height with values: media layer 2.5 to 4.5 m, support layer 0.3 to 0.4 m, filter floor thickness, distribution zone 1.2 to 1.5 m, clear water zone 0.8 to 1.0 m and freeboard 0.5 m.

Section 9.4 quantifies the media: particle size preferably 2 to 10 mm, with 4 to 10 mm preferred for first stage or denitrification filters and 2 to 6 mm acceptable for second and subsequent stages; bulk density preferably 750 to 900 kg/m3; specific surface area preferably above 1 m2/g; and the amount of media finer than the design minimum or coarser than the design maximum should not exceed 5% by mass. Clause 9.5.1.2 requires the special filter nozzle density generally to be not below 36 per m2, and clause 10.7 sets the ratio of total slot area of nozzles to filtration area at 1.2% to 2.4%.

4. Oxygen demand: formulas and coefficients

Clause 9.5.2.1 gives the unit oxygen demand with coefficients a = 0.82 and b = 0.28. Clause 9.5.2.2 converts nitrification and denitrification into oxygen terms using two stoichiometric constants: 4.57 g of oxygen consumed per gram of ammonia nitrogen nitrified, and 2.86 g of oxygen saved per gram of nitrate nitrogen reduced. For a post carbon oxidation filter in a pre-denitrification layout, the actual oxygen demand is computed as carbon oxidation plus nitrification minus the denitrification credit.

Clause 9.5.2.3 handles the correction from clean water to real sewage: for domestic sewage the oxygen transfer coefficient alpha is 0.8, the saturation correction factor beta is 0.9 to 0.95, the correction factor rho is 1, and the residual dissolved oxygen in the filter effluent is preferably 3 to 4 mg/L. Clause 9.5.2.4 computes the air supply using an oxygen utilisation efficiency of 5% to 15% and 0.28 kg O2 per cubic metre of air at standard conditions of 0.1 MPa and 20 degrees C.

5. Backwashing: the three stage sequence in Table 3

Clause 9.6.1 requires combined air and water backwash in the sequence air only, then air plus water, then water only, distributed through the special nozzles. Clause 9.6.2 requires backwash water to be treated effluent, with the backwash interval related to filter loading, filtration time and head loss, typically 24 to 72 h.

Table 3 gives the intensities and durations:

• Air only: 12 to 25 L/(m2.s) for 3 to 10 min;
• Air plus water: air 10 to 15 L/(m2.s) and water 4 to 6 L/(m2.s) for 3 to 5 min;
• Water only: 8 to 16 L/(m2.s) for 3 to 10 min.

The drainage side is equally quantified. Clause 9.6.4 requires the backwash drainage buffer tank to have an effective volume not less than 1.5 times the total backwash volume of a single cell; clause 9.3.1.5 requires backwash drainage and the effluent trough to be laid out separately, with effluent weir plates to prevent media loss. Clause 9.1.3 lists prevention of nozzle clogging and prevention of media loss as mandatory measures.

6. Sludge production and effluent

Clause 9.7.1: sludge production is the sum of the biomass increase from organic removal and the captured suspended solids, and varies with loading, with 0.18 to 0.75 kg of sludge per kg of BOD5 removed given as a reference. Clause 9.7.2 requires the backwash sludge water to discharge to the buffer tank and, after settling, to return to the sedimentation tank ahead of the filter, with all process sludge treated together. Clause 9.1.7 allows a simplification: where the effluent suspended solids already meet the downstream or discharge requirement, separate sedimentation or filtration may be omitted.

7. Items the standard leaves to testing or calculation

The gaps that must be stated honestly are: the Table 2 loadings give priority to test data and provide only empirical ranges; the nitrified liquor recirculation ratio is only described as set by the total nitrogen removal rate, with no formula or value; the external carbon dose is covered in clause 9.2.4 only by the words determined by calculation; the effective size d10, non-uniformity coefficient K80 or uniformity coefficient K60 are, per clause 9.4.1.5, determined by project conditions and user requirements; and the actual backwash intensity and duration must, per clause 14.3.4, be established from operating experience and then strictly controlled.

Treat Table 2 and Table 3 as a matched tool set: select the filter type by target pollutant, size the media volume by volumetric loading and check the area with the hydraulic loading including recirculation, assemble the tank from the six height components, and finally configure the backwash from Table 3. That is the complete design chain offered by Chapter 9 of HJ 2014-2012.