Oxidation Ditch Design and Denitrification Checking: Key Parameters and Calculation Routes in HJ 578-2010

1. Scope and Zone Definitions

HJ 578-2010, the Chinese MEE technical specification for the oxidation ditch activated sludge process, was approved on 12 October 2010 and entered into force on 1 January 2011 as a first edition, with Annex A normative and Annex B informative. It applies to municipal and industrial wastewater treatment works using the oxidation ditch activated sludge process.

Clause 3.1 defines an oxidation ditch as an activated sludge process in which the reactor is arranged as a closed endless loop channel equipped with aeration and flow-propulsion equipment. The principal configurations are single-channel, double-channel, triple-channel, vertical-shaft surface aerator and concentric centripetal-flow ditches, with the integrated oxidation ditch and the fine-bubble aeration ditch as variants. Chapter 3 also delimits three functional zones by dissolved oxygen, which is the basis of any denitrification check:

• Oxic zone: dissolved oxygen generally not less than 2 mg/L, serving organic matter degradation, ammonia nitrification and excess phosphorus uptake.
• Anoxic zone: dissolved oxygen generally 0.2 to 0.5 mg/L, serving denitrification.
• Anaerobic zone: dissolved oxygen generally below 0.2 mg/L, serving phosphorus release.

2. In-Channel Conditions and Removal Rates

Clause 6.1.2 requires the channel flow pattern to show overall mixing with local plug flow, with the influent flow far smaller than the circulating mixed liquor flow so that a dissolved oxygen gradient is formed. Clause 6.1.4 sets the in-channel sludge concentration at 2000 to 4500 mg/L, and clause 6.1.5 sets the minimum velocity at the channel bottom at not less than 0.3 m per second. These two clauses are what distinguish an oxidation ditch from a conventional plug-flow aeration tank: the strong dilution provided by the circulating flow gives the ditch its shock-load tolerance, while the bottom velocity is the hard constraint that prevents sludge deposition.

Table 2 of clause 5.3 gives removal rates in percent. For municipal wastewater with pre-treatment plus oxidation ditch plus secondary sedimentation: suspended solids 70 to 90, BOD5 80 to 95, CODCr 80 to 90, total nitrogen 55 to 85, ammonia nitrogen 85 to 95 and total phosphorus 50 to 75. For industrial wastewater: suspended solids 70 to 90, BOD5 70 to 90, CODCr 70 to 90, total nitrogen 45 to 85, ammonia nitrogen 70 to 95 and total phosphorus 40 to 75.

3. The Four Design Parameter Tables

3.1 Carbonaceous removal (Table 3)

• BOD5 sludge loading 0.14 to 0.36 kg per kg MLVSS per day, equivalent to 0.10 to 0.25 kg per kg MLSS per day.
• MLSS 2.0 to 4.5 kg per cubic metre and MLVSS 1.4 to 3.2 kg per cubic metre; the MLVSS to MLSS ratio is 0.7 to 0.8 with primary sedimentation and 0.5 to 0.7 without.
• BOD5 volumetric loading 0.20 to 2.25 kg per cubic metre per day; design sludge age (for reference) 5 to 15 days; sludge yield 0.3 to 0.6 with primary sedimentation or 0.6 to 1.0 without, in kg per kg.
• Total hydraulic retention time 4 to 20 h; sludge return ratio 50 to 100 percent; oxygen demand 1.1 to 1.8 kg O2 per kg BOD5; overall BOD5 removal 75 to 95 percent.

3.2 Biological nitrogen removal (Table 4)

• BOD5 sludge loading 0.07 to 0.21 kg per kg MLVSS per day, equivalent to 0.05 to 0.15 kg per kg MLSS per day; MLSS 2.0 to 4.5 and MLVSS 1.4 to 3.2 kg per cubic metre.
• MLVSS to MLSS ratio 0.65 to 0.75 with primary sedimentation and 0.5 to 0.65 without; BOD5 volumetric loading 0.12 to 0.50 kg per cubic metre per day; total nitrogen loading rate not exceeding 0.05 kg TN per kg MLSS per day.
• Design sludge age (for reference) 12 to 25 days; sludge yield 0.3 to 0.6 with primary sedimentation or 0.5 to 0.8 without.
• Sludge return ratio 50 to 100 percent; anoxic retention 1 to 4 h, aerobic 6 to 14 h, total 7 to 18 h; mixed liquor recycle ratio 100 to 400 percent.
• Oxygen demand 1.1 to 2.0 kg O2 per kg BOD5; overall removal of BOD5 90 to 95 percent, ammonia nitrogen 85 to 95 percent and total nitrogen 60 to 85 percent.

3.3 Simultaneous nitrogen and phosphorus removal (Table 5)

• BOD5 sludge loading 0.10 to 0.21 kg per kg MLVSS per day, equivalent to 0.07 to 0.15 kg per kg MLSS per day; total nitrogen loading rate not exceeding 0.06 kg TN per kg MLSS per day.
• BOD5 volumetric loading 0.20 to 0.7 kg per cubic metre per day; design sludge age (for reference) 12 to 25 days.
• Anaerobic retention 1 to 2 h, anoxic 1 to 4 h, aerobic 6 to 12 h, total 8 to 18 h; sludge return ratio 50 to 100 percent and mixed liquor recycle ratio 100 to 400 percent.
• Oxygen demand 1.1 to 1.8 kg O2 per kg BOD5; overall removal of BOD5 85 to 95 percent, total phosphorus 50 to 75 percent and total nitrogen 55 to 80 percent.

3.4 Extended aeration (Table 6)

• BOD5 sludge loading 0.04 to 0.11 kg per kg MLVSS per day, equivalent to 0.03 to 0.08 kg per kg MLSS per day; BOD5 volumetric loading 0.06 to 0.36 kg per cubic metre per day.
• Design sludge age (for reference) greater than 15 days; sludge yield 0.3 to 0.6 with primary sedimentation or 0.4 to 0.8 without.
• Sludge return ratio 75 to 150 percent and mixed liquor recycle ratio 100 to 400 percent; oxygen demand 1.5 to 2.0 kg O2 per kg BOD5; total hydraulic retention time not less than 16 h; overall BOD5 removal efficiency 95 percent.

4. The Kinetic Route for Denitrification Checking

Clause 6.4.2.3 provides three sets of equations for nitrification and denitrification kinetics, which form the backbone of the denitrification check.

4.1 Anoxic zone volume

Vn = 0.001Q(Nk - Nte) / (Kde(T) x X) - 0.12 dXv / Kde(T), where Kde(T) is the denitrification rate at T degrees Celsius in kg nitrate nitrogen per kg MLSS per day and should be established from test data. Where no test data exist, Kde(T) = Kde20 x 1.08^(T-20) with Kde20 taken as 0.03 to 0.06 kg nitrate nitrogen per kg MLSS per day. The total sludge yield Yt (MLSS per BOD5) should also be established by testing, taken as 0.3 where primary sedimentation is provided and 0.6 to 1.0 where it is not.

4.2 Aerobic zone volume

Vo = Q(So - Se) x theta_co x Yt / (1000 x X), with the aerobic design sludge age theta_co = F / mu, where the safety factor F is 1.5 to 3.0 and the nitrifier growth rate mu = 0.47 x [Na / (KN + Na)] x e^(0.098(T-15)). Here Na is the ammonia concentration in the reactor in mg/L, KN is the half-saturation constant for nitrogen in nitrification, generally taken as 1.0 mg/L, and T is the design temperature in degrees Celsius.

This equation couples temperature, ammonia concentration and the sludge age safety factor, and it is the controlling step for oxidation ditch design in cold regions: as the design temperature falls, mu decreases, theta_co must increase, and the aerobic zone volume has to be enlarged accordingly.

4.3 Mixed liquor recycle flow

QRi = [1000 x Vn x Kde(T) x X / (Nke - Nt)] - QR, and the standard states that the mixed liquor recycle ratio should not exceed 400 percent. When the required ratio exceeds this ceiling, the anoxic and aerobic volumes must be re-apportioned rather than simply installing a larger recycle pump.

4.4 Checking checklist

1. Verify influent alkalinity: clause 5.2.3 requires a BOD5 to total nitrogen ratio of at least 4.0 and a total alkalinity (as CaCO3) to ammonia ratio of at least 3.6 where total nitrogen removal is required, with carbon source or alkalinity dosing where these are not met.
2. Confirm the total nitrogen loading rate does not exceed 0.05 kg TN per kg MLSS per day from Table 4, or 0.06 where simultaneous nitrogen and phosphorus removal applies from Table 5.
3. Back-calculate the anoxic and aerobic volumes from the kinetic equations and cross-check them against the retention time ranges in Table 4.
4. Check the mixed liquor recycle ratio; if it exceeds 400 percent, re-apportion the zone volumes.
5. Verify that the channel bottom velocity is not below 0.3 m per second, and account for any pumping energy where internal recycle is not achieved hydraulically.

5. Oxygen Demand and Air Supply Conversion

Clause 6.6.1 gives the oxygen demand equation for the aerobic zone, identical in form to that of HJ 576-2010: 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 per kg and c as 1.42.

Clause 6.6.2 provides simplified unit values for preliminary estimating or for checking the reasonableness of the equation: 0.7 to 1.2 kg oxygen per kg BOD5 for carbonaceous removal, 1.1 to 1.8 kg where anoxic denitrification is included, and 1.5 to 2.0 kg for extended aeration.

Clause 6.6.3 sets the standard-state demand OS = Ko x O2. For surface aerators Ko = CS / [alpha x (beta x Csw - Co)] x 1.024^(T-20); for blown aeration devices equations 18 to 20 apply, including the depth correction for the submergence of the diffuser and the exit gas oxygen content Ot = 21(1 - EA) / [79 + 21(1 - EA)] x 100, where EA is the oxygen utilisation efficiency in percent. The standard fixes CS at 9.17 mg/L, alpha at 0.80 to 0.85, beta at 0.90 to 0.97, Co generally at 2 mg/L, and the mixed liquor temperature T generally at 5 to 30 degrees Celsius.

Clause 6.6.4 gives the standard-state air supply for blown aeration as GS = OS / (0.28 x EA).

6. Ditch Layout, Guide Walls and Baffles

Clause 6.5.1 requires the straight length of the ditch to be not less than 12 m or twice the surface width, excluding concentric centripetal-flow ditches, with the channel width determined by site constraints and the type and size of the aeration equipment. Clause 6.5.2 sets the freeboard at 0.5 m where brush or disc aerators are used, and 0.6 to 0.8 m near bends where a vertical-shaft surface aerator is installed, with the equipment platform 1.0 to 1.7 m above the design water surface.

Clause 6.5.3 gives guide wall geometry that can be read directly from Table 7:

• Brush length 3.0 m at 1 m diameter: channel width 4.15 m, guide wall eccentricity 0.35 m, guide wall radius 2.25 m.
• Brush length 4.5 m: channel width 5.56 m, eccentricity 0.50 m, radius 3.00 m.
• Brush length 6.0 m: channel width 7.15 m, eccentricity 0.65 m, radius 3.75 m.
• Brush length 7.5 m: channel width 8.65 m, eccentricity 0.60 m, radius 4.50 m.
• Brush length 9.0 m: channel width 10.15 m, eccentricity 0.95 m, radius 5.25 m.

Supporting requirements: a single guide wall may be used where the channel width is below 7.0 m, while two or more are recommended above 7.0 m, the outer channel being half the ditch width where two are provided; the guide wall should extend one channel width downstream; it should stand 0.3 m above the design water level; the upstream baffle is 1.0 to 2.0 m high and installed vertically 2 to 5 m upstream of the brush, while the downstream baffle is placed 2.0 to 3.0 m downstream, inclined at 60 degrees to the horizontal with its top 150 mm below the water surface and its lower part extending beyond 1.8 m water depth; no guide wall should be provided at a bend where a vertical-shaft surface aerator is installed; and oval ditches should not be fitted with baffles.

7. Aeration Equipment and Standby Rules

Clause 7.1.1 requires the standard-state oxygen demand to be calculated from the wastewater characteristics, required removal efficiencies and operating conditions, with equipment then selected on the basis of its oxygenation capacity and power efficiency. Clause 7.1.2 notes that the equipment should ideally combine oxygen supply, propulsion and mixing, and may be a vertical-shaft mechanical surface aerator, brush aerator, disc aerator or submersible blown aerator, which clause 7.1.3 requires to comply with HJ/T 247, HJ/T 259, HJ/T 280 and HJ/T 260 respectively.

Clause 7.1.4 sets the standby rules: vertical-shaft surface aerators may be backed up to not less than 20 percent of the oxygen demand, with not fewer than one unit on variable-frequency drive; brush and disc aerators should have one or two standby units; the blower house must have standby blowers, one standby unit where fewer than four duty blowers are installed and two where four or more are installed, with the standby sized on the largest unit in the design.

Clauses 7.1.5 and 7.1.6 give installation geometry: brushes should be placed on a straight length of the channel width plus 1.6 m before the bend, and 5.0 m downstream of the bend on leaving it; the minimum spacing between brushes on a straight should not be less than 15 m; brush submergence is generally 0.15 to 0.30 m and disc immersion generally 0.40 to 0.55 m. The vertical-shaft surface aerator should be located at a bend with the unit offset towards the outlet, with an impeller lift travel of plus or minus 100 mm and an impeller tip speed of 3.5 to 5 m per second.

8. Chemical Phosphorus Removal and Outlet Weirs

Clause 6.8 provides that chemical phosphorus removal be used as a supplement where the effluent total phosphorus cannot meet the discharge standard, with the optimum chemical, dose and dosing point established by testing. Aluminium salts, iron salts or lime may be used; where aluminium or iron salts are used as coagulants, an ionic polyelectrolyte should be added as a coagulant aid, and the molar ratio of coagulant to total phosphorus should be 1.5 to 3. Chemical storage capacity should cover 4 to 7 days of theoretical dose, with not fewer than two dosing systems, preferably using metering pumps. Clause 7.2.4 states that where the ditch serves as a sedimentation tank outlet weir, the depth of water over the weir should not exceed 50 mm.