UASB Reactor Design and Start-up: HJ 2013-2012 Parameters, Volumetric Loading Table and Separator Details

1. Scope and Influent Conditions

HJ 2013-2012 covers the design, construction and operation of UASB reactors treating medium and high strength organic wastewater. It was approved on 19 March 2012 and took effect on 1 June 2012. Clause 4.2.2 sets the following influent conditions:

• pH preferably 6.0-8.0.
• Temperature: ambient digestion 20-25 degrees Celsius; mesophilic 35-40 degrees Celsius; thermophilic 50-55 degrees Celsius.
• Nutrient ratio (CODCr to ammonia nitrogen to phosphorus) preferably 100-500:5:1.
• BOD5 to CODCr ratio preferably above 0.3.
• Suspended solids preferably below 1500 mg/L.
• Ammonia nitrogen preferably below 2000 mg/L.
• Sulphate preferably below 1000 mg/L.
• CODCr preferably above 1500 mg/L.
• Heavy metals, cyanide and phenols must be strictly controlled before the reactor.

Where these conditions are not met, pretreatment should be provided: screening, grit removal, equalisation, acidification, nutrient dosing, pH and temperature conditioning, heating and insulation.

2. Baseline Removal Rates

Table 1 of HJ 2013-2012 gives reference removal rates for a UASB reactor: CODCr 80%-90%, BOD5 70%-80% and suspended solids 30%-50%.

3. Volumetric Loading

Reactor volume is normally sized by the volumetric loading method: V = Q x SO divided by (1000 x NV), where V is effective volume in cubic metres, Q is design flow in cubic metres per day, NV is the volumetric loading rate and SO is influent organic concentration in mg CODCr per litre. Loading should be fixed by testing or by reference to comparable projects; where data are lacking, Table 2 of the standard applies:

• Influent CODCr 2000-6000 mg/L: granular sludge 4-6 kg CODCr per cubic metre per day; flocculent sludge 3-5.
• Influent CODCr 6000-9000 mg/L: granular sludge 5-8; flocculent sludge 4-6.
• Influent CODCr above 9000 mg/L: granular sludge 6-10; flocculent sludge 5-8.
• Thermophilic operation allows the loading to be raised appropriately above these figures.

4. Tank and Structure

• Two trains are preferred for operational flexibility and separate sludge cultivation and start-up; the maximum single reactor volume should be below 3000 cubic metres.
• Effective water depth between 5 and 8 m.
• Upflow velocity in the reactor preferably below 0.8 m/h.
• Materials: reinforced concrete, stainless steel or carbon steel. Concrete structures should be protected with epoxy resin for 1.0 m above and below the gas-liquid interface; carbon steel requires reliable corrosion protection.
• Common insulation for steel reactors includes expanded polystyrene, polyurethane foam, glass wool, foamed concrete and expanded perlite.

5. Distribution Devices

• Multi-point distribution is preferred. For granular sludge, a loading of 2-4 kg CODCr per cubic metre per day applies where each inlet serves 0.5-2 square metres, and above 4 where each inlet serves more than 2 square metres. For flocculent sludge, below 1-2 applies at 1-2 square metres per inlet and above 2 at 2-5 square metres per inlet.
• Inlet points should sit 150-250 mm above the reactor floor.
• For perforated single-pipe distribution, orifice velocity should exceed 2 m/s and the pipe diameter exceed 100 mm.
• For branch distribution, outlets face downward 200 mm above the floor, with 15-25 mm orifices, a 45 degree deflector and outlets aimed at the floor.

6. Three-Phase Separator

The separator governs sludge retention and gas capture. Key controls:

• An integral or assembled separator is preferred.
• Surface loading in the settling zone preferably below 0.8 cubic metres per square metre per hour, with total settling zone depth above 1.0 m.
• Gas pipe diameter must allow biogas to leave the collection chamber freely.
• A foam-breaking nozzle should be fitted at the top of the gas collection chamber.
• Materials: HDPE, carbon steel or stainless steel, with corrosion protection where carbon steel is used.

7. Effluent Collection, Sludge Withdrawal and Biogas

• Effluent collection sits at the top of the reactor: parallel weirs for rectangular sections, radial or polygonal troughs for circular sections.
• V-notch weirs on the collection trough with head above 25 mm and water level around half the notch depth; weir loading preferably below 1.7 litres per second per metre.
• Where the wastewater contains protein, fat or heavy suspended solids, a baffle should be installed ahead of the collection device.
• Sludge yield is 0.05-0.10 kg VSS per kg CODCr; gravity multi-point withdrawal is preferred, with points in the upper-middle sludge zone (0.5-1.5 m below the separator) and at the bottom; withdrawal pipe diameter above 150 mm, with the bottom pipe doubling as a drain.
• Biogas yield is 0.45-0.50 normal cubic metres per kg CODCr; biogas must be dewatered and desulphurised before use, with gasholder volume set by end use.
• Reactors producing below 1300 cubic metres of biogas per day are best used for cooking, heating or anaerobic heat exchange; above that threshold, power generation is preferred.

8. Start-up and Monitoring

• The reactor should be instrumented with online pH, temperature and sludge blanket level devices, with online monitoring complying with HJ/T 353.
• Automatic temperature and pH monitoring at the equalisation tank outlet should drive heating and chemical dosing.
• Granular sludge should be stored and, after settling and decanting, reused as inoculum; flocculent sludge is normally dewatered together with waste activated sludge.
• Daily monitoring of pH, volatile fatty acids and alkalinity is required; a rising VFA to alkalinity ratio is the early warning of acidification and calls for immediate load reduction or alkalinity supplementation.

References

• HJ 2013-2012, Technical Specifications of Up-flow Anaerobic Sludge Blanket Reactor for Wastewater Treatment (MEE release copy, in Chinese)
• Technical Specifications for Brewing Industry Wastewater Treatment (UASB design values, in Chinese)