Internal Circulation (IC) Anaerobic Reactors: Where the Numbers Come From When No National Standard Exists

1. State Clearly Which Standards Apply

A fact has to be stated at the outset: in the full published text of HJ 2013-2012, Technical specifications of up-flow anaerobic sludge blanket (UASB) reactor for wastewater treatment (approved 19 March 2012, in force 1 June 2012), a page-by-page search finds zero occurrences of the terms internal circulation and IC. Its scope, definitions, tank body, three-phase separator, start-up and biogas cleaning clauses all address UASB only. There is at present no national environmental protection standard covering the IC reactor. Values used in an IC design therefore cannot be presented as compliance with a national standard; they have to be attributed to a group standard, a corporate standard or demonstrated plant data.

The primary source used here is T/SDEPI 047-2024, Technical specification for treatment and reuse of high concentration organic wastewater, issued by the Shandong Environmental Protection Industry Association on 31 December 2024 together with its explanatory memorandum, cross-checked against the UASB clauses of HJ 2013-2012. Clause 6.2.2.1 of T/SDEPI 047-2024 lists UASB, EGSB, IC and CSTR as candidate anaerobic reactors and specifies the design basis for each.

2. The Four Reactor Types in T/SDEPI 047-2024

• 6.2.2.2 UASB: complies with HJ 2013; operated with flocculent sludge; mesophilic; loading below 5 kgCODCr/(m3.d); CODCr removal 80 to 90 percent.
• 6.2.2.3 EGSB: complies with HJ 2023; granular sludge; mesophilic; loading 8 to 20 kgCODCr/(m3.d); CODCr removal 80 to 90 percent.
• 6.2.2.4 IC: operated with granular sludge; mesophilic; loading 10 to 24 kgCODCr/(m3.d); CODCr removal 80 to 90 percent.
• 6.2.2.5 CSTR: complies with HJ 2024; flocculent sludge; suited to high-strength wastewater with high suspended solids; thermophilic or mesophilic; loading below 5 kgCODCr/(m3.d); CODCr removal 80 to 90 percent; mixing required to achieve complete mixing.

The explanatory memorandum adds geometry: height up to 16 to 25 m and height-to-diameter ratio generally 4 to 8, with internals comprising a bottom distribution system, two stages of three-phase separators, an internal circulation system, an effluent system, a biogas collection system and an external circulation loop. It targets high-strength wastewater dominated by hydrocarbons - starch, fermentation, protein and food processing effluents - and gives, for granular sludge under mesophilic conditions, a loading of 10 to 24 kg/(m3.d) with CODCr removal of 70 to 90 percent. The same document puts the EGSB height-to-diameter ratio at 3 to 5.

Cross-check against HJ 2013-2012

• Table 2 of HJ 2013-2012 (at 35 degrees Celsius): for influent CODCr of 2000 to 6000 mg/L, granular sludge 4 to 6 and flocculent sludge 3 to 5 kgCODCr/(m3.d); for 6000 to 9000 mg/L, 5 to 8 and 4 to 6; above 9000 mg/L, 6 to 10 and 5 to 8, with allowance for higher values under thermophilic operation.
• The IC range of 10 to 24 kgCODCr/(m3.d) in T/SDEPI 047-2024 clearly exceeds the granular-sludge UASB ceiling of 10 kgCODCr/(m3.d). The gap comes from the two-stage separator and the internal circulation enhancing mass transfer, but it rests only on a group standard and must not be extrapolated beyond its stated range.

3. Preconditions: Temperature and pH Must Be Right Upstream

Clause 6.2.1 of T/SDEPI 047-2024 requires the wastewater to be brought to 35 to 37 degrees Celsius by steam heating or heat exchange before it enters anaerobic treatment, and pH to be adjusted with acid or alkali to 5 to 7, depending on the industry and the wastewater. This connects to clause 11.3.1.5 of HJ 2013-2012, which requires temperature to be raised stepwise, preferably by 2 degrees Celsius per day, until the design temperature is reached.

4. Volume and Geometry: A Worked Example You Can Do by Hand

Take a flow Q of 2000 m3/d with influent CODCr of 8000 mg/L, giving an organic load of 16000 kgCODCr/d. Choosing 15 kgCODCr/(m3.d) from clause 6.2.2.4 of T/SDEPI 047-2024, mid-range and mesophilic with granular sludge, the required effective volume is V = 16000 / 15, roughly 1067 m3.

Now apply the geometric constraints. With a height-to-diameter ratio H/D of 6 and total height inside the 16 to 25 m window, one tower gives V = (pi.D2/4).H = (pi.D2/4).6D. Solving for 1067 m3 yields D about 6.1 m and H about 36.5 m, far outside the stated height window, so the duty has to be split. With four towers in parallel, each about 267 m3, the back-calculation gives D about 3.8 m and H about 23 m, both within the window and consistent with the memorandum.

Check up-flow velocity separately

For the four-tower arrangement each tower takes 500 m3/d, that is 20.8 m3/h, over a cross-section of about 11.6 m2, giving a superficial up-flow velocity of roughly 1.8 m/h. That is above the UASB limit of less than 0.8 m/h in clause 6.3.1.5 of HJ 2013-2012, but the two are not comparable: in an IC reactor the real liquid velocity in the reaction zone is the superficial velocity plus the circulation driven by gas lift, so superficial velocity is not the actual contact velocity. Good practice is to state both the superficial velocity and the circulation ratio. Circulation depends on gas production and on riser and downcomer design, which are manufacturer-specific; no published standard fixes a uniform value, so the written calculation should be demanded in the equipment specification.

5. Biogas Side: HJ 2013-2012 Remains the Firmest Basis

T/SDEPI 047-2024 clause 6.6 sets out the biogas section, while the binding requirements come from HJ 2013-2012:

• 6.6.1: the biogas train is generally arranged as a desulfurisation unit, a pressure-stabilising unit and a utilisation unit; treatment and utilisation shall comply with HJ 2013; motors shall be explosion-proof and layout shall satisfy fire protection requirements.
• 6.6.2: common desulfurisation processes include biological desulfurisation, chelated iron, wet scrubbing and dry bed desulfurisation, selected to suit the duty.
• 6.6.3: a gas holder is recommended for pressure and flow regulation, either membrane or water-sealed type.
• 6.6.4: utilisation units include steam boilers, hot water boilers, generators and regenerative thermal oxidisers, with a flare provided for combustion during maintenance.
• HJ 2013-2012 clause 6.5.3: biogas cleaning and utilisation design shall comply with NY/T 1220.1, NY/T 1220.2 and GB 50016.
• HJ 2013-2012 clause 6.5.4: biogas shall be dehydrated and desulfurised before entering any utilisation equipment.

One update matters: the national standard information platform records the current edition of NY/T 1220.1, Part 1 Engineering design, as NY/T 1220.1-2019, issued 17 January 2019 and effective 1 September 2019, which wholly replaces NY/T 1220.1-2006. References through clause 6.5.3 of HJ 2013-2012 should therefore be read against the current edition.

Biogas production can be estimated with clause 6.5.1 of HJ 2013-2012, which gives a UASB yield of 0.45 to 0.50 Nm3 per kg CODCr. Continuing the example at 85 percent COD removal gives 13600 kgCODCr destroyed per day and a biogas output of roughly 6120 to 6800 Nm3/d. This is an estimate built on a UASB yield figure; allow margin when sizing desulfurisation towers and gas holders and check it against the actual mass balance.

6. Start-up: Still Governed by HJ 2013-2012

Since the IC reactor runs on granular sludge, the start-up clauses of HJ 2013-2012 section 11.3.2 apply:

• 11.3.2.1: inoculum for granular sludge operation preferably 10 to 20 kgVSS/m3;
• 11.3.2.2: initial loading preferably 3 kgCODCr/(m3.d);
• by comparison, flocculent sludge start-up under 11.3.1.3 needs 20 to 30 kgSS/m3 and under 11.3.1.4 a starting load below 1 kgCODCr/(m3.d) with up-flow velocity below 0.2 m/h;
• 11.3.1.6: once CODCr removal exceeds 80 percent or effluent volatile fatty acids fall below 200 mg/L, the loading may be raised stepwise, each step preferably 20 to 30 percent of the design load, until design load and design removal are reached.

This ramp discipline matters more for IC than for UASB because the design load is more than double. Raising load too quickly acidifies the lower granular bed first, and volatile fatty acids plus removal efficiency are exactly the two release criteria given in HJ 2013-2012.

7. Clauses to Put Straight into the Equipment Specification

1. State explicitly that no national environmental standard covers IC, and list the basis: clause 6.2.2.4 of T/SDEPI 047-2024 for loading, the explanatory memorandum for height 16 to 25 m and H/D 4 to 8, and HJ 2013-2012 for biogas cleaning and start-up control.
2. Require the vendor to supply in writing the calculation of effective volume, diameter, total height, superficial up-flow velocity, internal circulation flow and circulation ratio for each tower.
3. Write inlet conditions into the contract: 35 to 37 degrees Celsius and pH 5 to 7 per clause 6.2.1 of T/SDEPI 047-2024, with heating or heat exchange and pH adjustment provided.
4. Make dehydration and desulfurisation mandatory per clauses 6.5.3 and 6.5.4 of HJ 2013-2012, desulfurisation to follow the current NY/T 1220 series.
5. Run start-up and load ramp per clause 11.3.2 of HJ 2013-2012, with COD removal of 80 percent or volatile fatty acids below 200 mg/L as the gate for each step.

Limits of this article: IC loading, height, height-to-diameter ratio and removal figures come from T/SDEPI 047-2024 and its explanatory memorandum and are group-standard reference values, not mandatory national requirements. UASB comparison values, biogas cleaning and start-up clauses come from the published text of HJ 2013-2012. IC-specific parameters such as circulation ratio and riser velocity have no published uniform value and must come from the vendor's calculation or from testing.