Biological Contact Oxidation and Moving Bed Biofilm Reactor Design: Media and Loading Parameters in HJ 2009-2011

1. Standard Positioning of the Two Biofilm System Types

HJ 2009-2011, the Chinese MEE technical specification for the biological contact oxidation process, was approved on 24 October 2011 and entered into force on 1 January 2012 as a first edition. It applies to domestic and industrial wastewater treatment works using contact oxidation and its combined processes. Clause 3.1 defines biological contact oxidation as an aerobic biofilm method in which the system consists of media immersed in the wastewater, the biofilm on the media surface, an aeration system and the tank, a definition that covers both fixed suspended media and free-moving carriers.

Chapter 3 provides three terms that form the common yardstick for comparing the two system types:

• Clause 3.9, media volumetric loading rate: the amount of pollutant treated per cubic metre of media per day, including BOD5 volumetric loading, nitrification volumetric loading and denitrification volumetric loading.
• Clause 3.10, air-to-water ratio: the volume ratio of gas introduced per unit time to wastewater entering per unit time, generally an empirical value.
• Clause 3.11, media filling ratio: the ratio of media volume to the effective tank volume, generally an empirical value.

Clause 6.1.4 confirms that different media types may be used, including hanging media, suspended (free-moving) media and fixed media. Clause 6.1.5 requires high-efficiency media to be preferred, with the required total biomass and the biomass attached to the media determined from the treatment objective, taking account of biofilm thickness and biofilm activity. Together these clauses establish that media selection is not a matter of comparing material prices: the required biomass is calculated first, and the media type and filling ratio are then derived from it.

2. Influent Conditions and Pre-treatment Triggers

Clause 5.2.3 sets the influent conditions: water temperature 12 to 37 degrees Celsius, pH 6.0 to 9.0, and a nutrient ratio (BOD5 to ammonia nitrogen to phosphorus) of 100:5:1, with nitrogen and phosphorus supplemented where the ratio falls short. Where ammonia removal is required, the ratio of total alkalinity (as CaCO3) to ammonia nitrogen should not be less than 7.14; where total nitrogen removal is required, the readily biodegradable BOD5 to total nitrogen ratio should not be less than 4.0. Compared with the activated sludge specifications, the contact oxidation standard raises the upper temperature limit to 37 degrees Celsius, reflecting the better heat tolerance of biofilm systems.

Clause 6.2.3 gives a set of pre-treatment thresholds that can be applied directly at scheme stage:

• Where the influent COD exceeds 2000 mg/L, an anaerobic pre-treatment process must be added.
• Where BOD5 to COD is below 0.3, hydrolytic acidification should be added to improve biodegradability.
• Where the oil content exceeds 50 mg/L, oil separation or dissolved air flotation must be added.
• Where the pH falls outside 6.0 to 9.0, acid or alkali neutralisation must be added.
• For industrial wastewater with suspended solids above 500 mg/L, primary sedimentation or coagulation, sedimentation or flotation should be provided.
• Where the calcium content is 100 mg/L or more, pre-treatment should be provided to prevent media scaling.

The calcium scaling clause is specific to biofilm processes: once scale forms on fixed media or moving carriers, both the specific surface area and the fluidisation behaviour deteriorate irreversibly.

3. Design Removal Rates

Table 2 of clause 5.3 gives removal rates in percent. For municipal wastewater: suspended solids 70 to 90, BOD5 80 to 95, CODCr 80 to 90, ammonia nitrogen 60 to 90 and total nitrogen 50 to 80. For industrial wastewater: suspended solids 70 to 90, BOD5 70 to 95, CODCr 60 to 90, ammonia nitrogen 50 to 80 and total nitrogen 40 to 80.

Note that the upper bound for ammonia removal in the contact oxidation standard (90 percent for municipal wastewater) is lower than the 95 percent common in the activated sludge specifications of the same period, and that the total nitrogen band is wider. Design documents should not adopt activated sludge removal bands for biofilm systems.

4. Tank Sizing Equations

Clause 6.4.1.1 gives the tank volume centred on the media volumetric loading: V = Q(So - Se) / (1000 x Mc x eta), where V is the design tank volume in cubic metres, Q is the design flow in cubic metres per day, So and Se are the influent and effluent BOD5 in mg/L, Mc is the BOD5 volumetric loading of the media in kg BOD5 per cubic metre of media per day, and eta is the media filling ratio in percent.

This equation is shared by both system types, but the values of Mc and eta are entirely different. Clause 6.4.1.2 gives the separate nitrogen terms: the nitrification aerobic tank volume V = Q(NKNi - NKNe) / (1000 x MN x eta), where MN is the nitrification volumetric loading in kg TKN per cubic metre of media per day; and the denitrification anoxic tank volume VDN = Q(NIn - NEn) / (1000 x MDNL x eta), where MDNL is the denitrification volumetric loading in kg NOx-N per cubic metre per day. Clause 6.4.1.3 requires that where carbonaceous matter and ammonia are removed together, both volumes be calculated and the higher value taken, or the sum of the two used as the design volume. Clause 6.4.1.4 requires the calculated volume to be checked against the hydraulic retention time using HRT = 24 x V / Q.

5. Hanging Media versus Moving Carriers

5.1 Carbonaceous removal (Table 3, design water temperature 20 degrees Celsius)

• BOD5 media volumetric loading Mc 0.5 to 3.0 kg BOD5 per cubic metre of media per day, shared by both types.
• Filling ratio eta: 50 to 80 percent for hanging media, 20 to 50 percent for suspended (moving) media. This is the most important difference between the two.
• Sludge yield Y 0.2 to 0.7 kg VSS per kg BOD5; hydraulic retention time 2 to 6 h, applicable to domestic and municipal wastewater only.

5.2 Carbon removal with nitrogen removal (Table 4, design water temperature 10 degrees Celsius)

• BOD5 media volumetric loading Mc 0.4 to 2.0 kg BOD5 per cubic metre of media per day; nitrification media volumetric loading MN 0.5 to 1.0 kg TKN per cubic metre of media per day.
• Aerobic zone filling ratio 50 to 80 percent for hanging media and 20 to 50 percent for moving media; the same ranges apply in the anoxic zone.
• Hydraulic retention time 4 to 16 h including an anoxic stage of 0.5 to 3.0 h; sludge yield 0.2 to 0.6 kg VSS per kg BOD5; effluent recycle ratio 100 to 300 percent.

The key comparison: at the same volumetric loading, moving carriers occupy only about forty to sixty percent of the filling ratio used for hanging media, so the actual volume of carrier installed in a given tank is much smaller and must be compensated by a larger tank or by higher activity per unit of carrier. Conversely, the value of moving carriers lies in rapid biofilm renewal under fluidised conditions, resistance to clogging and the ability to replace media without shutdown, which suits upgrading projects that cannot be taken offline.

Clause 6.4.2.3 further requires that in a multi-stage contact oxidation process the first stage should account for 55 to 60 percent of the total hydraulic retention time. This matters most for two-stage carbon and nitrogen removal: the first stage carries most of the organic load, and a poor split will leave the second nitrification stage suppressed by heterotrophic competition.

6. Tank Stratification and Structural Parameters

Clause 6.4.3 gives the vertical stratification specific to hanging media tanks:

• Length to width ratio 2:1 to 1:1, effective water depth 3 to 6 m, and freeboard not less than 0.5 m.
• With hanging media the tank is arranged from bottom to top as an aeration zone, a media layer, a stabilising layer and freeboard, with the aeration zone 1.0 to 1.5 m, the media layer 2.5 to 3.5 m and the stabilising layer 0.4 to 0.5 m.
• A distribution channel should be provided at the inlet with a width of not less than 0.8 m; the distance from the lower edge of the baffle wall to the underside of the media should be 0.3 to 0.5 m and to the tank floor not less than 0.4 m.
• Vertical-flow contact oxidation tanks should discharge over a weir at a weir loading of 2.0 to 3.0 L per second per metre.
• Sludge withdrawal and emptying facilities must be provided at the tank bottom.

Moving carrier systems do not need this stratification, but they must solve carrier retention and fluidisation. Clause 7.3.2 requires that where moving media are installed in an anoxic zone, an adjustable-angle mechanical mixer be used with a mixing power of 2 to 4 W per cubic metre, protected by an external guard; clause 7.3.1 provides that where hanging media are used in an anoxic zone, hydraulic mixing or low-oxygen air mixing should be adopted.

7. Aeration and Oxygen Demand Checks

Clause 6.5.1 gives the oxygen demand equation: O2 = 0.001aQ(So - Se) - c dXv + b[0.001Q(Nk - Nke) - 0.12 dXv] - 0.62b[0.001Q(Nt - Nke - Noe) - 0.12 dXv], with a taken as 1.47, b as 4.57 kg O2 per kg N and c as 1.42. Clause 6.5.2 sets the standard-state demand Os = Ko x O2 and gives the terms: clear-water saturation dissolved oxygen Cs 9.17 mg/L, alpha generally 0.8 to 0.85, beta generally 0.9 to 0.97, residual dissolved oxygen Co generally 2 mg/L, mixed liquor temperature T generally 5 to 30 degrees Celsius, Ot the oxygen fraction of the gas leaving the tank, Pb the absolute pressure at the aeration device in MPa, and EA the oxygen utilisation efficiency.

Clause 6.5.3 gives the standard-state air supply for blown aeration as Gs = Os / (0.28 x EA), where 0.28 is the oxygen content of one cubic metre of air at standard state in kg O2 per cubic metre.

Clause 6.5.4 adds a check specific to biofilm processes: the calculated air supply should be verified against the air-to-water ratio. For domestic wastewater, municipal wastewater or industrial wastewater of similar character, the minimum air-to-water ratio should not be below 2:1 to 3:1 and the maximum should not exceed 15:1 to 20:1; for industrial wastewater the ratio should be established by testing or by reference to similar projects. The lower bound guarantees carrier fluidisation and biofilm scouring, while the upper bound prevents excessive shear from stripping the biofilm, a sensitivity that is particularly acute in moving carrier systems.

8. Media and Aeration Equipment Selection

Clause 7.1.1 requires hanging media and suspended media to comply with HJ/T 245 and HJ/T 246 respectively; clause 7.1.2 requires the technical performance and parameters of the media to comply with Annex A; clause 7.1.3 identifies the three technical parameters of media as attached biomass, attached biofilm thickness and biofilm activity; clause 7.1.4 requires the total biomass to be determined from influent quality and the required treatment level, the media type to be selected on the basis of attached biomass, and the media category to be chosen according to tank shape, flow pattern and construction conditions; clause 7.1.5 requires the media to be non-toxic to micro-organisms and easy to colonise, with low weight, high strength, ageing resistance, a large specific surface area and resistance to scaling.

Clause 7.2.1 matches aerators to media type: hanging media should use blown perforated pipes or medium-bubble diffusers, while moving media should use perforated pipes, medium-bubble diffusers, jet aerators or spiral aerators. Clause 7.2.2 covers blower selection, with centrifugal blowers preferred for medium-sized plants and Roots blowers acceptable for small plants and industrial wastewater stations; clauses 7.2.6 to 7.2.9 require single-stage high-speed centrifugal blowers to comply with HJ/T 278, Roots blowers with HJ/T 251, medium-bubble diffusers with HJ/T 252 and jet aerators with HJ/T 263.

Clauses 7.2.3 and 7.2.4 give pipework detail: blown aeration should use a combined main and branch pipe system, with bottom mains arranged as ring, straight, cross or double-cross patterns, the ends of straight, cross and double-cross mains being sealed; where perforated pipes are used, the horizontal length of each pipe should not exceed 5 m, the level error should not exceed plus or minus 2 mm per pipe and plus or minus 3 mm across the tank, and facilities for adjusting air flow and facilitating maintenance must be provided. Installation levelness determines air distribution uniformity and is the most common source of rework during commissioning of biofilm systems.

9. Carbon Source, Chemical Phosphorus Removal and Sludge

Clause 6.6.2.1 requires carbon source dosing in the anoxic zone where the BOD5 to TKN ratio of the contact oxidation tank influent is below 4; clause 6.6.2.2 gives the dose as BOD5 = 2.86 x delta-N x Q, where delta-N is the nitrate nitrogen removed in mg/L and Q the design flow in cubic metres per day. Clause 6.6.3.2 provides for chemical phosphorus removal using aluminium salts, iron salts or lime, with the molar ratio of iron or aluminium to total phosphorus at 1.5 to 3 to 1. Clause 6.6.1 requires not fewer than two dosing systems with precision metering pumps, and chemical storage sized for 4 to 7 days of theoretical dose.

Clause 6.7 gives the sludge system parameters: the surface loading of the sedimentation tank should be taken at 70 to 80 percent of the value used for the secondary sedimentation tank of a conventional activated sludge plant; sludge produced from organic matter removal should be taken as 0.2 to 0.4 kg VSS per kg BOD5 removed; a separate sludge digestion system is not recommended for contact oxidation tanks; and dewatering should preferably use a plate and frame filter press, or a centrifuge or belt filter press.

Selection conclusion: where the influent is stable, space is available and periodic shutdown for maintenance is acceptable, fixed hanging media carry more biomass in a given tank volume. Where the influent fluctuates widely, where upgrading must be carried out without shutdown, or where scaling from high calcium is likely, moving carriers are preferred, configured at a filling ratio of 20 to 50 percent with anoxic mixing power of 2 to 4 W per cubic metre, while holding the air-to-water ratio between the lower bound of 2:1 to 3:1 and the upper bound of 15:1 to 20:1.