SBR Process Design and Cycle Control: A Clause-by-Clause Reading of HJ 577-2010

1. Scope and Process Definition

HJ 577-2010, the Chinese MEE technical specification for the sequencing batch reactor activated sludge process, was approved on 12 October 2010 and entered into force on 1 January 2011 as a first edition. It sets out requirements for process design, major process equipment, monitoring and control, construction and acceptance, and operation and maintenance of wastewater treatment works using SBR, and applies to both municipal and industrial wastewater projects.

Clause 3.1 defines SBR as an activated sludge method in which the five basic steps of fill, aeration/react, settle, decant and idle are carried out sequentially in time in a single tank. Its principal variants are the cyclic activated sludge system (CASS or CAST), the demand aeration tank - intermittent aeration tank process (DAT-IAT) and the alternating internal cyclic activated sludge system (AICS). Clauses 6.1.2 and 6.1.3 add the two conditions that distinguish SBR from continuous-flow plants: the tank must combine ideal plug flow in time with complete mixing in space, and it must provide quiescent settling with reliable solid-liquid separation.

Clause 4.1 states that SBR is suited to small and medium-sized municipal and industrial wastewater projects. Clause 4.3 requires that the effect of low winter temperature on carbon removal, nitrogen removal and phosphorus removal be fully considered, with measures such as reducing the load, reducing sludge wasting (increasing sludge age), adjusting the anaerobic and anoxic hydraulic retention time shares, and thermal insulation or heating. Clause 4.4 requires that different operating schemes be prepared for the operating conditions that may occur.

2. Influent Quality Thresholds

Clause 5.2.3 sets the quantitative influent conditions that must be checked at design stage. Where they are not met, clause 6.2.3 requires appropriate pre-treatment:

• Water temperature 12 to 35 degrees Celsius, pH 6 to 9, and a BOD5 to CODCr ratio of not less than 0.3.
• Where ammonia removal is required, the ratio of total alkalinity (as CaCO3) to ammonia nitrogen should be not less than 7.14; alkalinity must be supplemented where this is not met.
• Where nitrogen removal is required, the BOD5 to total nitrogen ratio should be not less than 4.0 and the total alkalinity (as CaCO3) to ammonia nitrogen ratio not less than 3.6; carbon source or alkalinity must be added where these are not met.
• Where phosphorus removal is required, the BOD5 to total phosphorus ratio should be not less than 17; where both nitrogen and phosphorus removal are required, both conditions apply.

In the absence of measured data, clause 5.2.1 permits per-capita loadings: BOD5 25 to 50 g, suspended solids 40 to 65 g, total nitrogen 5 to 11 g and total phosphorus 0.7 to 1.4 g per person per day.

3. Design Removal Rates

Table 2 of clause 5.3 gives the removal rates in percent. For municipal wastewater with primary sedimentation plus SBR: suspended solids 70 to 90, BOD5 80 to 95, CODCr 80 to 90, ammonia nitrogen 85 to 95, total nitrogen 60 to 85 and total phosphorus 50 to 85. For industrial wastewater with pre-treatment plus SBR: suspended solids 70 to 90, BOD5 70 to 90, CODCr 70 to 90, ammonia nitrogen 85 to 95, total nitrogen 55 to 85 and total phosphorus 50 to 85. Whether a primary sedimentation tank is provided depends on the wastewater characteristics and the SBR variant.

4. Reactor Volume and Cycle Time Allocation

Clause 6.3.2.1 gives the effective reactor volume: V = 24 x Q' x S0 / (1000 x Ls x X x tR), where V is the effective tank volume in cubic metres, Q' is the influent volume per cycle in cubic metres, S0 is the influent BOD5 in mg/L, Ls is the BOD5 sludge loading in kg BOD5 per kg MLSS per day, X is the average mixed liquor suspended solids concentration in kg MLSS per cubic metre, and tR is the reaction time per cycle in hours.

Clause 6.3.2.2 allocates the cycle:

• Fill time tF = t / n, where t is the length of one cycle in hours and n is the number of tanks in the series.
• Reaction time tR = 24 x m x S0 / (1000 x Ls x X), where m is the fill ratio taken from Tables 3 to 7.
• Settling time tS should be 1 h.
• Decant time tD should be 1.0 to 1.5 h.
• Total cycle time t = tR + tS + tD + tB, where tB is the idle time in hours.

Clause 6.3.2.3 requires the number of cycles per day to be a whole number such as 2, 3, 4, 5 or 6. Clause 6.3.2.4 sets the water depth at 4.0 to 6.0 m and the length to width ratio of rectangular tanks at 1:1 to 2:1. Clause 6.3.2.5 sets the freeboard at 0.5 to 1.0 m, and clause 6.3.2.6 requires not fewer than two tanks arranged in parallel.

5. The Five Design Parameter Tables

Clauses 6.3.3.1 to 6.3.3.5 provide five parameter tables. The standard repeatedly states that where industrial wastewater differs markedly from municipal wastewater, design parameters must be established by testing or by reference to similar projects.

5.1 Carbonaceous removal (Table 3)

• BOD5 sludge loading 0.25 to 0.50 kg BOD5 per kg MLVSS per day, equivalent to 0.10 to 0.25 kg BOD5 per kg MLSS per day.
• Mixed liquor suspended solids 3.0 to 5.0 kg MLSS per cubic metre; mixed liquor volatile suspended solids 1.5 to 3.0 kg MLVSS per cubic metre.
• Sludge yield 0.3 kg VSS per kg BOD5 with primary sedimentation, or 0.6 to 1.0 without.
• Total hydraulic retention time 8 to 20 h; oxygen demand 1.1 to 1.8 kg O2 per kg BOD5; SVI 70 to 100 mL/g; fill ratio 0.40 to 0.50; overall BOD5 removal 80 to 95 percent.

5.2 Ammonia removal (Table 4)

• BOD5 sludge loading 0.10 to 0.30 kg BOD5 per kg MLVSS per day, equivalent to 0.07 to 0.20 kg BOD5 per kg MLSS per day.
• Mixed liquor suspended solids 3.0 to 5.0 kg MLSS per cubic metre; sludge yield 0.4 to 0.8 with primary sedimentation or 0.6 to 1.0 without, in kg VSS per kg BOD5.
• Total hydraulic retention time 10 to 29 h; oxygen demand 1.1 to 2.0 kg O2 per kg BOD5; SVI 70 to 120 mL/g; fill ratio 0.30 to 0.40.
• Overall BOD5 removal 90 to 95 percent and ammonia nitrogen removal 85 to 95 percent.

5.3 Biological nitrogen removal (Table 5)

• BOD5 sludge loading 0.06 to 0.20 kg BOD5 per kg MLVSS per day, equivalent to 0.04 to 0.13 kg BOD5 per kg MLSS per day; total nitrogen loading rate not exceeding 0.05 kg TN per kg MLSS per day.
• Anoxic share of reaction time 20 percent and aerobic share 80 percent; total hydraulic retention time 15 to 30 h.
• Oxygen demand 0.7 to 1.1 kg O2 per kg BOD5; SVI 70 to 140 mL/g; fill ratio 0.30 to 0.35.
• Overall removal: BOD5 90 to 95 percent, ammonia nitrogen 85 to 95 percent, total nitrogen 60 to 85 percent.

5.4 Simultaneous nitrogen and phosphorus removal (Table 6)

• BOD5 sludge loading 0.15 to 0.25 kg BOD5 per kg MLVSS per day, equivalent to 0.07 to 0.15 kg BOD5 per kg MLSS per day; total nitrogen loading rate not exceeding 0.06 kg TN per kg MLSS per day.
• Anaerobic, anoxic and aerobic shares of reaction time 5 to 10 percent, 10 to 15 percent and 75 to 80 percent respectively; total hydraulic retention time 20 to 30 h.
• Sludge return ratio 20 to 100 percent and mixed liquor recycle ratio not less than 200 percent, both applicable to CASS or CAST only.
• Oxygen demand 1.5 to 2.0 kg O2 per kg BOD5; SVI 70 to 140 mL/g; fill ratio 0.30 to 0.35.
• Overall removal: BOD5 85 to 95 percent, total phosphorus 50 to 75 percent, total nitrogen 55 to 80 percent.

5.5 Biological phosphorus removal (Table 7)

• BOD5 sludge loading 0.4 to 0.7 kg BOD5 per kg MLSS per day; mixed liquor suspended solids 2.0 to 4.0 kg per cubic metre; sludge yield 0.4 to 0.8 kg VSS per kg BOD5.
• Anaerobic share 25 to 33 percent and aerobic share 67 to 75 percent of reaction time; total hydraulic retention time 3 to 8 h.
• Oxygen demand 0.7 to 1.1 kg O2 per kg BOD5; SVI 70 to 140 mL/g; fill ratio 0.30 to 0.40; phosphorus content of sludge 0.03 to 0.07 kg TP per kg VSS.
• Sludge return ratio 40 to 100 percent for CASS or CAST; overall total phosphorus removal 75 to 85 percent.

6. Oxygen Demand and Standard-State Conversion

Clause 6.3.4.1 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]. Here a is the oxygen equivalent of carbon, taken as 1.47 when carbonaceous matter is expressed as BOD5; b is the oxygen required per kilogramme of ammonia nitrogen oxidised, taken as 4.57 kg O2 per kg N; c is the oxygen equivalent of bacterial cells, taken as 1.42; 0.12 dXv is the nitrogen content of the micro-organisms wasted from the system; and 0.62 is the oxygen credit recovered by denitrification.

Clause 6.3.4.2 defines the standard state as an atmospheric pressure of 101325 Pa and a temperature of 20 degrees Celsius, with OS = K0 x O2 and K0 = CS / [alpha x (beta x CSW - CO)] x 1.024^(T-20). The standard fixes the clear-water saturation dissolved oxygen CS at 9.17 mg/L, alpha (ratio of the overall oxygen transfer coefficient in mixed liquor to that in clean water) at 0.80 to 0.85, beta (ratio of saturation dissolved oxygen in mixed liquor to that in clean water) at 0.90 to 0.97, the residual dissolved oxygen CO at 2 mg/L, and T as the design water temperature in degrees Celsius.

Clause 6.3.4.3 converts standard-state oxygen demand to standard-state air supply: GS = OS / (0.28 x EA), where 0.28 is the mass of oxygen in one cubic metre of air at standard state in kg O2 per cubic metre, and EA is the oxygen utilisation efficiency of the aeration device in percent. EA must be taken under the test conditions prescribed by the diffuser product standard; values quoted by suppliers for deep-tank conditions must not simply be transferred.

7. Key Equipment and Operational Control

Clause 7.1.1 requires decanting by a decanter, using a rotary, siphon or unpowered floating-weir siphon type, with rotary decanters complying with HJ/T 277. Clause 7.1.2 gives three quantified figures: weir loading 20 to 35 L per metre per second, maximum supernatant withdrawal rate 30 mm per minute, and decant duration 1.0 h. Clauses 6.1.5 and 6.1.6 additionally require a fixed emergency draw-off set at the water level reached at the end of decanting, and a decanter fitted with a scum-retaining device.

On aeration hardware, clauses 7.2.4 to 7.2.8 respectively require single-stage high-speed aeration centrifugal blowers to comply with HJ/T 278, Roots blowers with HJ/T 251, fine-bubble diffusers with HJ/T 252, mechanical surface aerators with HJ/T 247 and submersible aerators with HJ/T 260. Clause 7.2.2 states that at constant water level a fine-bubble blown aeration system should use one blower serving several tanks or mechanical surface aeration, whereas at variable water level it should use a one-to-one blower-to-tank arrangement or a submersible aeration system. Clause 7.3.1 sets mixing power at 2 to 8 W per cubic metre.

Chapter 11 provides the following quantified points suitable for direct inclusion in operating procedures:

• Clause 11.2.4: during decanting the water surface must fall uniformly at a rate not exceeding 30 mm per minute.
• Clause 11.2.6.1: when a blown aeration system starts, water held in the pipework must be drained and the automatic drain valves checked regularly.
• Clause 11.2.6.2: at the end of the aeration step the dissolved oxygen in the main reaction zone should not fall below 2 mg/L.
• Clause 11.2.7.2: the drop of the sludge blanket at the end of settling must be observed, and the distance from the sludge interface to the lowest water level should not be less than 500 mm.
• Clause 11.3.6: fine-bubble diffusers in SBR tanks are prone to clogging; blocking and damage must be inspected regularly and damaged units replaced.
• Clause 11.3.7: a submersible flow pusher must not run dry for more than 3 minutes.

8. CASS and CAST Variants

Clause 6.4.2 for nitrogen removal only: the tank is generally divided into two zones, an anoxic biological selector followed by an aerobic zone; dissolved oxygen in the anoxic zone is below 0.5 mg/L; the anoxic zone should occupy 20 percent of the total effective volume; and the recycle ratio from the aerobic zone to the anoxic zone should be established by testing and should not be less than 20 percent.

Clause 6.4.3 for combined nitrogen and phosphorus removal: the tank is generally divided into three zones, an anaerobic selector, an anoxic zone and an aerobic zone; dissolved oxygen in the anoxic zone is below 0.5 mg/L and it should occupy 20 percent of the total effective volume; dissolved oxygen in the anaerobic selector is zero and it should occupy 5 to 10 percent of the total effective volume; and the recycle ratio from the aerobic zone to the anaerobic selector should not be less than 20 percent. Clause 6.4.6 requires not fewer than two tanks in one system.

Engineering note: for industrial wastewater the standard repeatedly insists that design parameters be established by testing or by reference to similar projects. The values in Tables 3 to 7 therefore apply directly only to municipal wastewater or industrial wastewater of similar character. For high-strength or inhibitory industrial streams, bench or pilot testing must first establish loading and removal data before detailed design begins.