EGSB Reactor Design: Upflow Velocity, Volumetric Loading and Start-up Control

1. A guidance standard, and only for industrial organic wastewater

HJ 2023-2012, Technical specifications of expanded granular sludge bed (EGSB) reactor for wastewater treatment, was approved by the former Ministry of Environmental Protection on 2012-12-24 and became effective on 2013-03-01. Its foreword states that it is a guidance standard and a first edition. Clause 1 limits the scope to the design, construction and operation of projects treating industrial organic wastewater with EGSB reactors, and notes that internal circulation anaerobic reactors and anaerobic fluidised bed reactors may refer to it. Municipal sewage and low strength wastewater are therefore outside its scope.

Clause 3.1 reveals the essential difference from a conventional UASB: an EGSB reactor is one in which recirculation and structural design give the wastewater a relatively high upflow velocity so that the granular sludge inside is in an expanded state. Expansion is not a natural consequence - it is designed in.

2. Influent conditions: eight thresholds as an admission checklist

Clause 4.2.2 sets the conditions the EGSB influent must meet, and reads naturally as an admission checklist:

• pH preferably 6.0 to 8.0;
• Temperature: ambient anaerobic preferably 20 to 25 degrees C, mesophilic 35 to 40 degrees C, thermophilic 50 to 55 degrees C;
• Nutrient ratio COD:N:P preferably 100 to 500:5:1;
• Suspended solids preferably below 2000 mg/L;
• Ammonia nitrogen preferably below 2000 mg/L;
• Sulphate should be below 1000 mg/L, with the COD/SO4 ratio above 10;
• COD preferably above 1000 mg/L;
• Heavy metals, cyanide, phenols and similar substances must be strictly controlled before entering the reactor.

Clause 4.2.3 offers a practical dilution tool: raising the external effluent recirculation ratio lowers the concentration of toxic substances such as heavy metals, cyanide and phenols, mitigating their effect. Table 1 sets removal expectations: for readily biodegradable wastewater (B/C above 0.3), COD 70% to 90%, BOD5 60% to 80% and SS 30% to 50%; for poorly biodegradable wastewater (B/C below 0.1), COD 50% to 70%, BOD5 40% to 60% and SS 20% to 40%.

3. Five key numbers in the main design

Clause 6.3.2.1 gives the volume equation V = Q x S0 / (1000 x NV). Clause 6.3.2.2 requires the volumetric loading to be determined by testing or by reference to similar projects, with Appendix A used where data are lacking, and states that the EGSB volumetric loading range should be 10 to 30 kgCOD/(m3.d). For comparison, HJ 2013-2012 for UASB reactors specifies an effective water depth of 5 to 8 m in clause 6.3.1.4 and an upflow velocity preferably below 0.8 m/h in clause 6.3.1.5, which shows the difference in magnitude between the two technologies.

The EGSB geometry itself is set by:

• Clause 6.3.2.4: effective water depth preferably 15 to 24 m;
• Clause 6.3.2.5: upflow velocity in the reactor preferably 3 to 7 m/h;
• Clause 6.3.2.6: preferably a cylindrical tower with a height to diameter ratio of 3 to 8;
• Clause 6.3.2.3: not fewer than two reactors, connected in parallel, to facilitate sludge cultivation and start-up.

Clause 6.3.6.2 ties upflow velocity directly to recirculation: both external and internal circulation are achieved by pumps, and the recirculation ratio is determined from the upflow velocity using v = (Q + Qr)/A. This is the most substantial design difference from UASB, which normally needs no dedicated forced recirculation pump. Clause 6.3.6.4 requires the recirculation return points to be on the raw water inlet pipe so that the recirculated flow enters the reactor together with the raw water.

4. Distribution, three-phase separation and effluent collection

Clause 6.3.3.2 for single pipe multi-orifice distribution: orifice velocity should exceed 2 m/s, perforated pipe diameter should exceed 100 mm, and the distribution pipe centreline should be kept 150 to 250 mm above the reactor floor. Clause 6.3.3.3 for multi-pipe distribution: each inlet should serve 2 to 4 m2.

Clause 6.3.4.2: the inclined plate angle of an integral three-phase separator should be 55 to 60 degrees; for a split type, the overlap Z1 between the baffle and the gap should be 100 to 200 mm and the inter-layer spacing Z2 should be 100 to 200 mm. Clause 6.3.4.4: with a two-stage separator, the lower stage is placed at mid height covering preferably 50% to 70% of the area and the upper stage near the top. Clauses 6.3.5.3 and 6.3.5.4: the head over the triangular weir on the collection trough should exceed 25 mm with the water level at half the notch height, and the weir loading should be below 1.7 L/(s.m).

Clause 6.3.7.1 gives a sludge yield of 0.05 to 0.10 kgVSS/kgCOD, with the withdrawal frequency set from the sludge concentration profile, which requires sampling ports at different heights; clause 6.3.7.3 requires a sludge pipe diameter above 150 mm, with the bottom sludge pipe also serving as the drain.

5. Three quantitative rules on the biogas side

Clause 6.5.1: biogas production is calculated from a yield, generally 0.45 to 0.50 Nm3 per kg COD. Clause 6.5.5 sets the gasholder volume: 50% to 60% of daily production for domestic cooking use; for boilers, power generation and partial domestic use the volume follows the supply balance curve, and where no curve exists it should not be below 10% of daily production. Clause 6.5.8 sets the utilisation threshold: below 1300 m3 per day the gas is preferably used for cooking, heating or anaerobic heat exchange, and above 1300 m3 per day power generation or the same thermal uses are preferred.

6. Start-up: eight operating clauses

Section 10.2 turns start-up into an executable procedure, and is arguably the most valuable part of the specification:

• Clause 10.2.2: seed with granular sludge at 10 to 20 kgVSS/m3, preferably granular sludge from wastewater of the same type as that to be treated;
• Clause 10.2.3: the granular sludge should be uniform, 0.5 to 3.0 mm in diameter, with a settling velocity of 20 to 100 m/h and high mechanical strength;
• Clause 10.2.4: store granular sludge in a sealed container with nutrient solution at about 4 degrees C and keep the storage time as short as possible;
• Clause 10.2.5: the start-up loading should be below 2 kgCOD/(m3.d) with an upflow velocity below 0.5 m/h, compared with below 1 kgCODCr/(m3.d) and below 0.2 m/h in the UASB specification;
• Clause 10.2.6: raise the temperature gradually, preferably 2 degrees C per day;
• Clause 10.2.7: increase the recirculation flow slowly after seeding, and reduce it if effluent suspended solids rise too quickly;
• Clause 10.2.8: once COD removal stabilises or the effluent volatile fatty acid concentration falls below 200 to 300 mg/L, the volumetric loading may be raised in steps of 20% to 30% of the design loading;
• Clause 10.3.1: during operation the alkalinity (as CaCO3) should be above 2000 mg/L and volatile fatty acids preferably kept within 200 mg/L.

Clause 10.3.2 sets a shutdown red line: when pH falls below 6.0, the plant should be stopped, the cause investigated, and alkaline substances such as sodium bicarbonate added. Clause 10.3.3 requires the sludge concentration in the reaction zone to be not below 30 kgVSS/m3, and clause 10.3.4 requires the sludge blanket to be kept 0.5 to 1.5 m below the three-phase separator.

7. Appendix A: the real distribution of loading rates

Appendix A, an informative annex, compiles design loadings from actual projects and can be used directly for comparison: chemicals average 12.0 with a maximum of 25.8 and a minimum of 5.3 across 14 plants; breweries average 16.6 (30.2 and 13.0) across 12 plants; sugar beet and similar average 20.0 (22.8 and 18.7) across 3 plants; yeast and corn processing average 23.9 (29.0 and 14.6) across 3 plants; citric acid and vegetable processing average 18.0 (25.1 and 12.0) across 6 plants; food processing average 14.2 (18.9 and 10.0) across 7 plants; pharmaceutical plants average 16.2 (22.5 and 12.0) across 8 plants; starch, ethanol and condiment plants average 15.0 (25.0 and 9.0) across 16 plants.

Comparing these averages with the 10 to 30 kgCOD/(m3.d) range of clause 6.3.2.2 shows that most sectors fall between 12 and 24, and that the upper bound of 30 appears only in a few high strength readily biodegradable cases. Adopting 25 to 30 at the design stage is the most common over-optimistic choice in EGSB projects.

8. Parameters that must come from testing

The specification explicitly assigns the volumetric loading to testing or comparable projects in clause 6.3.2.2. Next is the recirculation ratio, which is only defined by back calculation from the upflow velocity with no recommended range. Third is the operating temperature and heat demand after heating, for which clause 6.2.7 only requires the heat exchanger area to be calculated from a heat balance with a 10% to 20% margin. Fourth is the detailed design of secondary pollution control such as deodorisation, which is treated only in principle.

The EGSB design thread can be summarised in one sentence: an effective depth of 15 to 24 m buys the space for effluent recirculation, an upflow velocity of 3 to 7 m/h buys sludge expansion and mass transfer, and a start-up loading below 2 kgCOD/(m3.d) with 2 degrees C of heating per day brings this high rate system online safely.