Hydrolysis-Acidification Followed by Aerobic Treatment of Textile Dyeing Wastewater: Parameter Boundaries in HJ 471-2020 and HJ 2047-2015

1. How the Two Standards Divide the Work

HJ 471-2020, Technical specification for dyeing and finishing wastewater treatment of textile industry, was approved by the Ministry of Ecology and Environment on 13 January 2020, issued and brought into force on 14 January 2020, and replaced HJ 471-2009. It is a guidance standard and its cover page carries the note that the electronic version is the issued draft. HJ 2047-2015, Technical specifications for hydrolysis and acidification reactor in wastewater treatment, was approved on 20 November 2015 and entered into force on 1 January 2016, also as a first-edition guidance standard.

The relationship between them is stated in the text itself. Clause 6.4.2.5.1 of HJ 471-2020 fixes the volumetric loading rate for hydrolysis and acidification and then requires that reactor design refer to the relevant provisions of HJ 2047. In other words, HJ 2047 supplies the generic method for the unit (tank type, distribution, effluent collection, sludge withdrawal, volume calculation) while HJ 471-2020 supplies the sector-specific values. Where the sector clause is stricter, it governs; where it is silent, fall back on HJ 2047.

2. Influent Thresholds: pH First, Then Organics

Appendix A of HJ 471-2020 (informative) lists reference wastewater quality. Table A.1 gives, for woven cotton and cotton-blend dyeing wastewater of pure cotton printed products, pH 10.0 to 12.0, colour 400 to 800 times, BOD5 300 to 500 mg/L, CODCr 1500 to 3000 mg/L and SS 200 to 500 mg/L. Table A.5 gives silk-spinning scouring wastewater at pH 9.0 to 11.0, CODCr 4000 to 5000 mg/L and BOD5 2400 to 3000 mg/L.

Clause 4.2.2 of HJ 2047-2015 sets considerably tighter influent conditions for the hydrolysis reactor:

• pH preferably 5.0 to 9.0;
• COD:N:P preferably 100 to 500:5:1;
• for readily biodegradable wastewater the COD concentration should preferably stay below 1500 mg/L; it may be relaxed where biodegradability is poor.

Because textile wastewater routinely exceeds the pH 9.0 ceiling of HJ 2047, neutralisation is not optional. Clause 6.4.2.2.6 of HJ 471-2020 states that a pH conditioning tank should be provided when the wastewater pH is below 6 or above 9, and clause 6.4.2.2.7 requires that this tank be split into coarse and fine stages with 20 to 30 minutes of detention each, mixed hydraulically, mechanically or with air. Those two clauses connect directly to the influent threshold in HJ 2047: the coarse stage brings pH into the 5.0 to 9.0 window and the fine stage damps the swings.

3. Hydrolysis and Acidification: Two Sets of Loading and Time Values

Clause 6.4.2.5.1 of HJ 471-2020 sets the effective volumetric loading rate at 0.7 to 1.5 kgCOD/(m3.d). Clause 6.4.2.5.2 sets minimum hydraulic retention times by wastewater class, explicitly to be decided according to the nature and concentration of the refractory pollutants:

• denim washing wastewater: not less than 12 h;
• silk, wool and knitted fabric dyeing wastewater: not less than 16 h;
• cotton and polyester dyeing wastewater at higher strength: not less than 24 h.

Table 2 of HJ 2047-2015 addresses wastewater of medium biodegradability with a non-dissolved COD fraction of 30 to 60 percent - a category that explicitly names textile dyeing wastewater and industrial park wastewater - with a reference HRT of 4 to 12 h for up-flow reactors, and gives "above 10 h" for poorly biodegradable wastewater with a non-dissolved COD fraction below 30 percent.

Rule for practice: where the two standards disagree on the lower bound, take the stricter one. In practice the up-flow reactor is sized against the sector minimum in HJ 471 (12, 16 or 24 h) and then cross-checked by back- calculation from Table 2 of HJ 2047, with the loading rate of 0.7 to 1.5 kgCOD/(m3.d) as a second check.

The completely mixed configuration

For a completely mixed reactor, clause 6.5.2 of HJ 2047-2015 gives HRT = C / X, where X is the average sludge concentration, generally 4 to 8 g/L, and C is a constant. Table 3 assigns C = 60 to 150 h.g/L to pulp and paper, coking, coal chemical, petrochemical, tannery, oily and textile dyeing wastewaters including industrial park wastewater. Substitution gives an HRT between roughly 7.5 h (C = 60, X = 8) and 37.5 h (C = 150, X = 4), which is compatible in magnitude with the sector minimum of HJ 471, though HJ 471 remains governing. Clause 6.5.3 additionally requires a mechanical mixer with a power input of not less than 6 W/m3 and prohibits mixing by aeration; clause 6.5.5 requires a sludge return ratio of not less than 100 percent from the downstream clarifier.

Geometric constraints for up-flow reactors

• Clause 3.2 of HJ 2047-2015: average sludge concentration 15 to 25 g/L;
• 6.3.2.5: effective water depth 4 to 8 m, freeboard 0.5 to 1.0 m;
• 6.3.2.6: up-flow velocity 0.5 to 2.0 m/h, reducible or offset by effluent recirculation for refractory wastewater;
• 6.3.3.1: multi-point distribution with no more than 2 m2 per distribution point; 6.3.3.2: inlet points kept 150 to 250 mm above the floor;
• 6.3.4.1: weir outlet with a weir loading not exceeding 2.9 L/(s.m);
• 6.3.5.5: sludge withdrawal header larger than 150 mm.

4. The Aerobic Stage: Loading, Air Demand and Recirculation

• Clause 6.4.2.6.2 of HJ 471-2020: biofilm treatment should use the biological contact oxidation process; where nitrogen removal is required, pre-denitrification (A/O), SBR and its variants, or oxidation ditches are preferred.
• 6.4.2.6.3: activated sludge is designed at 0.30 to 0.50 kgCOD/(kgMLSS.d); biological contact oxidation at 0.4 to 0.8 kgBOD5 per cubic metre of media per day, checked against hydraulic retention time.
• 6.4.2.6.4: oxygen demand should be calculated from the BOD5 of the hydrolysis-acidification effluent and checked against an air-to-water ratio of 15:1 to 30:1.
• 6.4.2.6.5: return activated sludge ratio generally 60 to 100 percent to hold 3 to 5 g/L mixed liquor suspended solids; internal recycle for pre-denitrification preferably 200 to 400 percent.
• 6.4.2.6.6: secondary clarifier at 0.5 to 0.7 m3/(m2.h) surface loading, 60 to 150 kg/(m2.d) solids loading and 2 to 4 h detention.

Note the base chosen in clause 6.4.2.6.4: oxygen demand is derived from the BOD5 of the hydrolysis- acidification effluent, not of the raw wastewater. Hydrolysis converts part of the non-dissolved COD into dissolved small molecules, and the effect is normally tracked with the BOD5/CODCr ratio - clause 6.6.2 of HJ 2047-2015 likewise requires the downstream design to account for the changes in SS, organics and the BOD5/CODCr ratio brought about by the hydrolysis stage.

5. Self-Checking with Appendix B

Clause 6.4.2.8 of HJ 471-2020 requires removal efficiencies to be established by testing or by analogy with similar plants, and refers to Appendix B where no data exist. Table B.1 gives:

• upstream physico-chemical treatment: BOD5 30 to 40 percent, COD 40 to 60 percent, colour 60 to 80 percent;
• hydrolysis and acidification: BOD5 10 to 20 percent, COD 15 to 25 percent, colour 40 to 60 percent;
• aerobic, activated sludge: BOD5 90 to 95 percent, COD 60 to 70 percent, colour 30 to 50 percent;
• aerobic, biofilm: BOD5 85 to 95 percent, COD 55 to 70 percent, colour 30 to 50 percent;
• downstream physico-chemical treatment: BOD5 15 to 25 percent, COD 30 to 50 percent, colour 50 to 70 percent.

Two conclusions follow. First, the job of hydrolysis-acidification is not COD destruction: it removes only 15 to 25 percent of COD, but strips 40 to 60 percent of colour and improves downstream biodegradability, so it cannot carry COD compliance on its own. Second, colour removal is carried mainly by physico-chemical treatment and hydrolysis, with the aerobic stage contributing only 30 to 50 percent, which is why clause 6.4.2.7.1 of HJ 471-2020 prescribes a dedicated oxidative decolourisation step, normally chlorine dioxide or ozone, and warns against chlorine or sodium hypochlorite because of the secondary pollution they can cause.

6. Monitoring and Secondary Pollution

Clause 8.2.3 of HJ 471-2020 requires the hydrolysis unit to be monitored for pH, temperature, oxidation-reduction potential, COD, BOD5 and volatile fatty acids across the inlet and outlet, plus alkalinity, sludge characteristics and sludge concentration inside the reactor; the aerobic unit is monitored for pH, temperature, ammonia nitrogen, total nitrogen, total phosphorus, COD and BOD5 plus dissolved oxygen, sludge characteristics and sludge concentration. Clause 8.2.4 makes flow, dissolved oxygen, pH, temperature, level, ORP and COD candidates for online measurement.

On odour, clause 6.8.2.1 explicitly lists the equalisation basin, the hydrolysis-acidification tank, sludge storage tanks and the dewatering room as sources requiring capture and centralised treatment, and clause 6.8.2.2 sets a ventilation rate of not less than six air changes per hour for structures regularly entered by personnel. Clause 6.7.1 of HJ 2047-2015 matches this by requiring the reactor to be covered and sealed.

On sludge reduction, clause 6.6.1 of HJ 2047-2015 notes that the downstream process is normally activated sludge, an oxidation ditch or SBR, and that aerobic excess sludge may be discharged into the hydrolysis-acidification reactor for digestion and reduction. Clause 6.3.5.1 gives the sludge production equation in which the suspended solids removal fraction comes from Table 1 and the sludge hydrolysis rate is to be determined by testing or by reference to similar works, taking 30 percent as a typical value for municipal wastewater.

7. Design Checklist

1. Obtain design quality from Appendix A or from measurement, check it against the pH 5.0 to 9.0 threshold of HJ 2047-2015, and decide on a pH conditioning tank and its staged detention time.
2. Take the volumetric loading rate of 0.7 to 1.5 kgCOD/(m3.d) per clause 6.4.2.5.1 of HJ 471-2020 and check the HRT floor (12, 16 or 24 h) per clause 6.4.2.5.2.
3. Check up-flow velocity, effective depth, distribution point density, weir loading and sludge pipe diameter against clause 6.3 of HJ 2047-2015.
4. Compute oxygen demand from the BOD5 leaving the hydrolysis stage and verify blower selection against an air-to-water ratio of 15:1 to 30:1.
5. Close the mass balance with the Appendix B ranges; if colour fails, add oxidative decolourisation under clause 6.4.2.7.

Note: every figure above is taken from the two published MEE standard PDFs, with clause and table numbers given in the text. The officially printed editions published by China Environmental Science Press remain the definitive text.